Global advancement on experimental and thermal analysis of evacuated tube collector with and without heat pipe systems and possible applications

Applied Energy 228 (2018) 351–389

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Global advancement on experimental and thermal analysis of evacuated tube collector with and without heat pipe systems and possible applications

T

⁎

K. Chopraa,b, V.V. Tyagia, , A.K. Pandeyc, Ahmet Sarid,e a

School of Energy Management, Shri Mata Vaishno Devi University, Katra 182320, Jammu & Kashmir, India School of Mechanical Engineering, Shri Mata Vaishno Devi University, Katra 182320, Jammu & Kashmir, India c Research Centre for Nano-Materials and Energy Technology (RCNMET), School of Science and Technology, Sunway University, No. 5, Jalan Universiti, Bandar Sunway, Petaling Jaya, 47500 Selangor Darul Ehsan, Malaysia d Department of Metallurgical and Material Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey e Center of Research Excellence in Renewable Energy (CORERE), Research Institute, King Fahd University of Petroleum & Minerals (KFUPM), 31261, Saudi Arabia b

H I GH L IG H T S

and ETC-HP system is discussed with possible applications. • ETC of heat pipe ETC is higher compared to direct flow ETC. • Performance application in ETC system proves it more efficient as compared to without PCM. • PCMs Pipe ETC found more suitable for industrial/domestic applications. • Heat • Thermal Analysis of ETC-HP and direct flow ETC is discussed.

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar energy Solar collectors Evacuated tube collector Heat pipe Thermal analysis Water heating system Air heating Thermal energy storage PV/thermal collector

Sun is the prime source of energy. There are two types of technologies available for the harnessing of solar energy i.e. Solar Thermal and Solar photovoltaic. Solar thermal energy having a potential to provide the domestic and industrial energy demand for hot water, air heating, solar cooling, solar drying etc. Among multiple applications of solar energy, water heating, space heating, and cooling are consuming more energy. The energy consumption in production of hot water represents a large contribution of total building energy consumption. The Collector is the important aspect for efficient energy needs for these applications. Among all thermal collectors specifically for low/medium temperature applications, evacuated tube collector is found to have the best efficiency. This paper addresses the advancement, different types of evacuated tube collectors and its low/ medium temperature applications. The use of heat pipe in evacuated tube has been studied by many researchers around the globe to overcome the lower performance issue in direct flow evacuated tube collector. This turns out to be one of the most important advancement in this area. Another, important advancements in this research have been found to be integration of phase change materials with evacuated tube collector which has the great impact on its performance. This makes the evacuated tube technology more efficient, reliable and user-friendly. This review covers the recent research areas of the direct flow and heat pipe evacuated tube collector with different applications and comprehensive knowledge of the theoretical analysis. This paper also provides financial advantages, classification with and without thermal energy storage, advantages and drawbacks of evacuated technology and future recommendation for future improvement and recent research trend have also incorporated in this manuscript for researchers and practice engineers.

1. Introduction The demand of energy is increasing with the expansion of industrialization while, fossil fuels sources are declining with time due to limited reserve. Moreover, global warming and pollution due to the ⁎

Corresponding author. E-mail address: [email protected] (V.V. Tyagi).

https://doi.org/10.1016/j.apenergy.2018.06.067 Received 21 March 2018; Received in revised form 31 May 2018; Accepted 12 June 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

extensive use of fossil fuels are becoming the major problem for society. However, renewable energy sources especially, solar energy is having the potential to meet global energy demand without compromising the environmental pollution [1]. Solar energy is freely available, safe, clean and available in abundance. Solar energy reaching on the earth can be

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Nomenclature A C D F FR h hfg I k l ṁ N NTU P Q̇ U R SF T t V

ab air cr con cond conv en evap f fi fo gi go hd hp i l loss o rad s sat sf v w

area, m2 specific heat, J/kg·K diameter, m efficiency factor heat removal factor convective heat transfer coefficient, W/m2 K latent heat of fusion, J/kg solar isolation, W/m2 thermal conductivity, W/m·K length, m mass flow rate, kg/s number number of transfer unit perimeter, m rate of heat transfer, W overall heat transfer coefficient, W/m2·K thermal resistance, K/W shape factor temperature, K thickness, m velocity, m/s

Greek letters γ β ε ρ ηfin μ

transmittance absorptivity emissivity density, kg/m3 fin efficiency dynamic viscosity, N·s/m2

absorber/absorb air between absorber tube and heat pipe collector condenser conductive convective solar radiations entering through glazing cover evaporator fluid fluid inlet fluid outlet inner glass outer glass hydraulic heat pipe inside liquid heat loss to ambient outlet radiative solid saturated surface vapour wick

Abbreviation ETC THES FPC

evacuated tube collector thermal energy storage flat plate collector

Subscript & superscript a

ambient

Over the recent years, in ETC technology, mainly two developments have occurred which has increased the performance in a profound manner. One is heat pipe based ETC and other is a phase change materials (PCMs) based ETCs for different potential real life applications. Jafarkazemi and Abdi [7] investigated a heat pipe ETC, experimentally and theoretically. The theoretical results such as collector efficiency, mass flow rate of working fluid, collector area and heat gain were compared with results obtained from experiment. The results of the theoretical model showed good agreement with experimental results. Kumar et al. [8] reported that heat pipe collector performance is highly sensitive to outside conditions such as ambient temperature and solar radiation. The evaporator length is a crucial design parameter for heat pipe design. The effect of number of heat pipes on the performance of solar collector was studied by Azad [9]. He concluded that efficiency of heat pipe solar collector can be enhanced firstly by increasing number of heat pipes and secondly by an effective & proper design of condenser. Abokersh et al. [10] compared the forced circulation finned and unfinned U-pipe ETC with phase change material under same external conditions. The system was investigated simultaneously under real water consumption profile as well as on-demand operation. The improvement in heat transfer characteristics and system stability was observed in developed finned system. Owing to crucial properties of heat pipe such as high performance, anti-freezing property, constant temperature level and heat flow transmitter (from evaporator to condenser) enabled heat pipe ETCs to grab mammoth interest for industrial and household applications [11,12]. Ismail and Abogderah [13] compared performance analysis

utilized in two different ways viz. directly (using Photovoltaic panels) and indirectly (using solar thermal collectors). One of the prominent options for harvesting the solar energy is through solar thermal collectors to meet growing energy demands and to minimize the emission of greenhouse gases. Solar collectors are used to collect solar energy by different methods of collection. Solar collectors can be categorized into three groups: Flat plate collectors (FPC), Evacuated tube collector (ETC) and concentrating collector. Over the past few years, owing to cheap and simple design, FPC dominated the market at a temperature range of 30–80 °C applications. On the other hand, concentrating collectors are being used to produce heat above 300 °C to generate electricity. Despite the large number of applications (solar cooling, water/air heating, desalination, food processing and electricity generation), concentrated solar collectors have limited use due to its high cost compared to conventional energy sources and Photovoltaic modules [2]. Although, the ETCs are capable to produce heat above 100 °C at relatively low cost [3]. According to recent market scenario, 77.8% of newly installed solar collectors are ETC due to its cheap cost and high efficiency. IEA (International Energy Agency report) reported that in 2010 more than 50% of total solar collectors installed word widely were ETC [4]. In last 20 years, ETCs overtook the market of flat plate collectors due to the growth of inexpensive sputtering technology for producing twin glass evacuated tubes. The characteristics such as higher performance, easy installation at low cost enabled evacuated tube collectors to be used widely for various applications. In ETC, convective and radiative losses are very minimal. Higher energy and exergy efficiencies are two most crucial design factors for twin glass evacuated tubes [5,6]. 352

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concentration of 0.2 wt% of Al2O3 nanoparticles. Faizal et al. [37] did energetic, environmental and economic analysis of a FPC by using silver oxide nanofluid. They found that silver oxide nanoparticles in flat plate solar collector can save 26.2% energy and 170 kg of carbon dioxide emission. It can be seen that most of the experimental investigation concerning to nanofluids operated solar collectors are the flat plate type collectors [38]. While, in comparison to FPC, ETC is better technology due to its minimum heat losses [39]. Mostly FPC employs for low temperature applications as they operate in the temperature range of 30–80 °C. However, ETC operation temperature range is 50–120 °C which is suitable for high/medium temperature applications [40]. Lu et al. [41] carried out an experimental study on ETC with silver nanoparticles they found that ETC with silver nanoparticles have better efficiency which is due to lower particle size and higher thermal conductivity of silver nanoparticles. Tong et al. [42] used multi-wall CNTwater nanofluids of different concentration and found 8% higher convective heat transfer coefficient between working fluid and tube for 0.24 vol% of nanofluid compared to water. Kim et al. [43] found highest efficiency for MWCNT nanofluid compared to SiO2, CuO, Al2O3 and TiO2. Ozsoy and Corumlu [44] investigated the thermal performance of heat pipe charged with silver-water nanofluid. Experimental outcomes depicted that thermal efficiency of ETC equipped with nanofluid charged thermosyphon heat pipes is 20.7–40% higher than solar collector equipped with pure water charged thermosyphon heat pipes. Hussein et al. [45] observed that nanofluid in heat pipe solar collector plays a vital role in enhancing its thermal performance. Sharafeldin and Grof [46] found highest temperature difference across the evacuated tube collector when CeO2 nanofluid was used. They also reported that temperature difference increases with enhancement of concentration of nanoparticles. Ghaderian and Sidik [47] studied the effect of nanofluid (Al2O3 and distilled water) as working on the thermal efficiency of ETC. Mahbubul et al. [38] enhanced the heating performance of ETC (20 kW heating capacity) by using single walled carbon nanotube water nanofluids. Recently nanofluid is also introduced for heat transfer enhancement for different applications. Sheikholeslami and Ganji [48] reported that platelet shaped nanoparticles produce highest temperature gradient. Hosseini et al. [49] found that water-Al2O3 nanofluid is the best among other nanofluid when it is used as a coolant fluid in solution. Sheikholeslami et al. [50,51] demonstrated the influence of nanofluid when flow over a stretching plate under magnetic field. Results revealed that concentration and velocity of nanofluid increase with rise of melting parameters. Sheikholeslami et al. [52] depicted the behavior of nanofluid (CuOwater) inside porous enclosure with hot obstacle on magneto hydrodynamic forced convection. Theoretical analysis established that convective heat transfer increases with the increase of Reynolds number and decline with augment of Lorentz forces. Sheikholeslami et al. [53] deliberated the behavior of CuO nanofluid in porous media with the help of Darcy model. Sheikholeslami et al. [32,54] studied the effect of thermal radiation on heat transfer of nanofluid in porous media and influence of variation of magnetic field on heat transfer in nanofluid with effect of nanoparticle’s shape in same media. The above literature depicted that nanofluids have potential in enhancement of thermal efficiency of solar collectors. It can be seen that few review papers are available on ETC and its applications. Presently research work is ongoing for new development in ETC based continuous water/air heating systems. Thermal modeling and experimental work collection is a key requirement for new researchers and industries for better understanding of the technology and ongoing research work. This paper provides an extensive research work on heat pipe ETC system with thermal modeling and PCM’s based storage materials. The Latest development in direct flow ETC system and HP-ETC system with potential applications are also discussed in details. This review paper will be beneficial for researchers and practicing engineers working in the thrust area of ETC designing and their

(experimentally and theoretically) of heat pipe based conventional collectors. The result reveals that conventional collectors have poor performance than heat pipe based collectors. Han et al. [14] found that heat pipe ETC has better instantaneous efficiency and paltry heat loss than conventional ETC. Li et al. [15] established the heat transfer model of ETC for water heating in forced mode. Jack et al. [16] studied the different heat transfer parameters of heat pipe in solar collectors. Daghigh and Shafieian [17] worked on heat pipe ETC with heat recovery system for drying applications. Mosleh et al. [18] did experimental investigation on combinations of evacuated tubes, parabolic trough collector and heat pipes for desalination of brackish water. ETC is being used in different potential applications such as building heating, water heating and desalination. Now a day’s applications of ETC have extended to drying of crops, electricity generation and solar cooling. Kumar et al. [19] studied the integrated effect of solar still and evacuated tube collector. Bracamonte et al. [20] did numerical and experimental investigation of effect of tilt angle on thermal stratification and thermal efficiency in twin-glass evacuated tube solar water heater system. Pandey et al. [21] did energy and exergy analysis of evacuated tube based water heating system. Zhang et al. [22] did experimental investigation on simultaneous production of hot water as well as electricity by ETC and thermoelectric modules. Felinski and Sekret [23] presented the use of paraffin as phase change material in annular space between heat pipe and absorber tube of evacuated tube fitted with a CPC. Kabeel et al. [24] did a novel study pertaining to use of concentric heat pipe for air heating purpose. Sokhansefat et al. [25] did thermoeconomical analysis of ETC and FPC using TRNSYS software for cold climate condition. They found that external conditions such as ambient condition and solar radiation highly affect the performance of both collectors. It was also observed that based on economical and thermal analysis the yearly energy gain in ETC is 30% higher compared to FPC and 41% better than that of FPC in cold conditions. They have recommended to use ETC in cold climate conditions. Solar thermal systems can be used for number of applications such as cooling, heating and generation of electricity without any impact on environmental conditions. The coupling of thermal energy storage system also make the solar thermal system more reliable and reduce the gap between demand and supply of energy. However, lower thermal efficiency of solar thermal system is a big challenge in front of solar industry. The changing in structure and design of solar thermal systems are considered for improvement in efficiency which requires huge investment [26,27]. The increasing of surface area of absorption and improving absorptivity of coating on absorber area are the main design factors used as modification in thermal system [28]. The easiest and best way to increase the efficiency is by enhancing the thermal performance of heat transfer fluid and in this regard nanofluids are the promising technology [29]. The traditional fluids disperse with nanoparticles improve the thermal properties of these fluids [30]. Moreover, the Brownian motion of nanoparticles in base fluid also increase the turbulence in fluid [31]. The nanofluid is an innovative way to increase the heat transfer rate [32]. Hence nanofluids are the new type of fluid which replaces the conventional fluids in solar thermal collectors. Nanofluid is a technology that can be used in different kinds of collectors that include but not limited to flat plate, parabolic trough, direct absorption, and ETC. In past literature, it is found that most of the studies related to solar collectors working with nanofluid are flat plate and absorption types [33]. Tyagi et al. [34] reported that efficiency of absorption type solar collector improves accordingly with concentration of nanoparticle size and collector height. They have also found strong relationship between nanoparticle size and height of solar collector. Otanicar et al. [35] studied benefits of using small size nanoparticles in base fluid on direct absorption solar collector. They found that smaller size and higher volume fraction enhances the thermal efficiency of collector. Yousefi et al. [36] performed an experimental investigation of FPC working with Al2O3-water nanofluid. They found maximum improvement (28.3%) in efficiency for nanofluid with 353

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All these components are arranged into casing thermally insulated at lateral and rear sides. FPC has numerous advantages such as durability, reliability and manufacturability. The main disadvantages of FPC are high heat losses and high hydraulic resistance in comparative of ETC system. ETC direct flow system also having drawback for continuous hot water production. This drawback in ETC gets reduced if heat pipes are deployed as passive heat transfer devices in ETC system. Evacuated tubes with heat pipes has very low hydraulic thermal resistance, nearly isothermal heat absorbing surface, uniform flow of working fluid. Heat pipe is durable and reliable device in operation for longer time. Year around study of heat pipe ETC and FPC has done by Ayompe et al. [11] revealed that a unit area of ETC and FPC collector generates 681 kWh/ m2 and 496 kWh/m2 of heat respectively. They found that average annual efficiency for ETC and FPC were 60.7% and 46.1% respectively. Some advantages of heat pipe ETC over FPC are given in Table 4. During unfavorable conditions in cold climatic regions, the advantages of FPC solar collectors become greatly reduced. FPC deteriorates due to condensation in cold climatic condition resulting in declined performance and eventually leads to system failure [63]. The vacuum envelope in ETC decreases conduction and convection losses therefore they can perform better in unfavorable conditions whereas performance of FPC become poor due to heat losses [64]. The FPC of optimum design, mass flow and exposed to high solar radiations can reach to higher outlet water temperature (close to 80 °C) [65]. In most of studies, it is reported that the thermal performance and efficiency of ETCs is better than FPCs. Under same climate condition in Ireland the annual average system efficiencies were 50.3% and 37.9% while collector efficiencies were 60.7% and 46.1% for ETC and FPC respectively [11]. ETC system is more suitable than FPC and found more efficient for high temperature applications [66,67].

applications. Paper has been divided into six different sections where Sections 1 and 2 explains the introduction of ETC technology and its advantages & disadvantages, Section 3 is focuses on classification with thermal energy storage, Sections 4 and 5 provides the thermal analysis of heat pipe system and uncertainty analysis, recent trends on ETC with and without heat pipe system for different solar applications are presented in Section 6 and last section (i.e. in Section 7) provides the conclusions, future recommendations and recent publication trends for the developed technology. 2. Advantages and key challenges of Evacuated Tube Collector (ETC) system Although FPC and ETC technology has their own advantages and disadvantages. However, economic and technology assisted benefits are the key factors to promote ETC system more popular rather than FPC system. 2.1. Financial advantages of the ETC system Many studies [55] revelead that popularization of ETC solar water heating system is due to its low initial cost, low operational cost and short payback period. In today’s era financial aspects has become a basic and crucial factor to establish a technology for commercialization. In the last decade, the solar water heater market gets significantly expanded due to very low manufacturing cost of evacuated tube collector systems. As per survey in 2001, the production rate of evacuated tubes in china was 40 million tubes/year [56]. Mangal et al. [57] conveyed that it is very cheap to replace broken tube, in case any tube is broken in the ETC system. It is quite cheaper than flat plate collector system where whole system has to be repaired. Tang et al. [58] also reported that recently manufacturing cost of evacuated tubes is exponentially decreasing. Arefin et al. [59] compared the cost of an electric heater with ETC based solar water heater and given that lifetime of electric heater is 5 years whereas lifespan of solar water heater is 30 years. Consequently, electric heater has to be replaced after every 5 years which is expensive. Thus it is more cost effective and beneficial to replace electric heater with solar water heating system. Shukla et al. [60] also found that performance of ETC is higher than of flat plate collector. Comparative assessment of initial cost for FPC & ETC system presented in Table 1. The data is gathered from producers in china producing both types of solar water heater (FPC and ETC). It may be observed that ETC systems are much economical than FPC system [61]. Payback period, initial cost and maintenance cost of FPC and ETC are given in Table 2. It showed that payback period for FPC is higher than ETC systems.

2.3. Key challenges associated with ETC system Although ETC based system has their advantages over FPC systems but there are also some challenges is associated with the ETC technology. This section addressed the problems associated with ETC technology. As we know that evacuated tubes made of two concentric cylindrical borosilicate glass tubes in which selectively coated tube (inner tube) is inserted into another borosilicate tube (outer tube). The vacuum is formed by the process of expelling the air within the space between inner and outer tubes. These cylindrical glass tubes are much fragile than toughened glass used in flat plate solar collector. Due to its fragility, tubes require extra care to handle or transport. ETC systems are more beneficial for commercial application rather than residential sector due to the high temperature water production. FPC based water

2.2. Comparative analysis and advantages of FPC and ETC system

Table 1 Comparison of initial cost between FPC and ETC solar water heating system [Source:Tmall.com].

The demand of hot water in residential & commercial sector is increasing day by day. Year wise solar water heating potential in different sectors of India is presented in Table 3. The demand of hot water in residential sector near to 84% of the total hot water demand and this demand is getting double by every three years in the country. In hospitals and hotels having water heating demand by 29% and it is expected by 53% till 2022 [61]. Most of the hot water producing systems are operated by direct use of electricity and fossil fuel. Presently, most of the water heaters use in residential buildings are based on electricity or gas based fuel. In USA nearly 40% houses are using electricity for water heating purpose [62]. The solar collectors used for water heating applications are ETCs, FPCs and compound parabolic collectors. ETC and FPC solar collectors are commonly used for water heating applications [11]. But direct flow and heat pipe ETC are the collectors that are widely used for water heating in residential and commercial sector [60]. Generally, flat plate collector comprised of glazed blackened absorber plate and differently configured highly conductive pipelines for circulation of working fluid.

Producer

Solar collector type

Model

Tank volume [L]

Cost

Huayang

ETC FPC ETC FPC

HY-QZD-1858-18 A HY-YT-100 P Huoxianfeng FPC series [Black chrome] FPC series [Blue film imported] HJH-18-24 PB 100 [Blue Titanium] Huandong1800 FPC1188 QBJ 1-110/1.54-34 PJF2-100/1.82/0.7003 QBJ 1-110/1.62/0.05

145 100 128 100

$679 $1108 $358 $860

100

$958

130 100

$251 $698

122 100 110 100

$437 $961 $368 $1243

110

$323

Jiadele

FPC Sanggao

ETC FPC

Sijimuge

ETC FPC ETC FPC

Linuo Paradigma

ETC

354

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Table 2 Comparison of initial, payback period and maintenance cost of ETC and FPC system. Author

Location

Type of solar collector

Tank volume [L]

Initial cost

Maintenance and operational cost

Payback period (years)

Koroneos and Nanaki [62] Gastli [63] Saxena and Srivastava [64] Han et al. [65]

China China China China

FPC FPC ETC system ETC system

200 200 100 200

$1911 $2000 $377 $330

1–4% of the initial cost $ 20/year Negligible $ 3.2/year

5 19.7 4.41 0.7–3

heat pipe used in heat pipe evacuated tube collector can be illustrated through Fig. 3. The heat pipe is a two phase device that contains low boiling heat transfer fluid. When heat is transfer to evaporator section of heat pipe, the low boiling fluid gets vaporize and rises to condenser section (cooling zone) of heat pipe. Hence flowing fluid in manifold gained heat from condenser sections of heat pipes [68].

Table 3 Solar water heating potential in India under realistic scenario (cumulative million m2) [61]. 2010

2013

2017

2022

Residential Commercial/institutional Hotel Hospital Others Industry

2.58

4.25

7.68

15.74

• • •

0.19 0.10 0.18 0.19

0.35 0.17 0.27 0.33

0.61 0.27 0.39 0.57

0.97 0.43 0.52 1.05

Total

3.24

5.37

9.52

18.70

3.2. ETC without heat pipes or direct flow ETC Direct flow ETC commonly known as U pipe solar collector is different from previous ones, in that heat pipes inserted in the center of evacuated tubes. In this arrangement, one copper pipe acts as the flow pipe while other copper pipe acts as the return pipe. Both pipes are brazed together at the bottom of the evacuated tube with a U bend. As shown in Fig. 4 in this arrangement a U-shaped tube is inserted into evacuated tube. The metal fin is fixed at the U-tube for increasing the heat transfer between the inner surface of evacuated tube and working fluid. Heat Pipe ETC installation is much easier due to “dry” connection between the manifold and the absorber plate in comparison of installation of direct flow collector. Moreover, no dismantling is required. It simply exchange the tube without emptying the whole setup, if an evacuated tube cracks or breaks and if vacuum gets lost. This flexibility offered by heat pipe ETC to extend the number of evacuated tubes makes it ideal for closed loop solar designs.

(1 m2 = 50 L/day)

Table 4 Advantages of heat pipe ETC over FPC. Heat pipe evacuated tube solar collector

Flat plate solar collector

Quick heat generation Collector efficiency is higher on high temperature Working temperature range is 40–100 °C Heat pipe ETC is flexible as if a tube cracks or breaks it can be easily replaced The maintenance cost is low Satisfactory performance even in extreme cold condition due to anti-freeze liquid in heat pipes

Slow heat generation On high temperature collector efficiency is poor Temperature range is 40–80 °C Difficult to replace glass over It requires high maintenance cost At high altitude freezing of water will take place causing damage to the collector

3.3. ETC with thermal energy storage heating systems are using for residential demand for hot water in the range of 50–60 °C and ETC is very useful for different industrial applications. The main benefit of evacuated tube is its minimum convective and radiative losses. However, in cold regions, this causes a problem. One of the major concerns of using of ETC system in cold region is the deposition of snow on outer surface of evacuated tubes. As tubes are insulator in nature, the snow collect on evacuated tubes also gets stuck in the gaps between the tubes of an evacuated tube collector.

Thermal energy storage plays an important role in energy conservation and reduction of peak load. [70]. Periodic and intermittent nature in solar radiation is the innate drawbacks of solar energy based technologies. Thermal energy storage is the one of the solution. The coupling of thermal energy storage material with solar collectors reduces the gap between demand and supply of energy by absorbing extra heat during sunshine hours and release during off sunshine hours [71]. Thermal energy storage has wide range of applications in heating, building, air conditioning, drying, air/water heating etc. Thermal energy in the form of heat may be stored either in the form of latent heat or sensible heat or through a combination of both. Many studies have been carried out to investigate the thermal energy storage based solar collectors. In this section, our main focus is on the integration of thermal energy storage with direct flow or heat pipe ETC. The arrangement of integration of thermal energy storage material with evacuated tube can be seen in Fig. 5.

3. Classification of evacuated tube solar collector According to design of ETC, it may be broadly classified into two categories: ETC with Heat pipe and ETC without heat pipe which can be further divided into with and without thermal energy storage as shown in Fig. 1. The evacuated tube consists of selectively coated tube further inserted into another borosilicate tube (outer tube). The vacuum is formed by the process of expelling the air within the space between inner and outer tubes. Evacuated tubes are able to operate very efficiently because vacuum is an excellent insulator for heat loss [69]. Both types of ETC can be used for different applications such as water heating, air heating, desalination and other applications.

3.3.1. ETC with latent heat storage Latent heat is the energy stored or released by material during its phase change at constant temperature. The peculiar isothermal nature of heat addition and rejection associated with latent heat makes it an ideal candidate for thermal management in various energy systems. In recent years, latent heat storage materials has received remarkable attention. It has the benefits such isothermal operation, large energy density during solidification and melting. In last decades research is focused mainly on preparation of novel type PCMs and heat exchanger design [73]. The existing and developed PCMs integrated with solar collectors to increase their reliability and constant supply of energy.

3.1. ETC with heat pipes In heat pipe ETC, heat pipes are inserted into evacuated tubes. As shown in Fig. 2 the aluminum/copper fins along with heat pipe use for increasing the heat transfer and a set of spring clips strongly held the heat pipes against the selectively coated inner tubes. The principle of 355

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Fig. 1. Classification of evacuated tube solar collector.

heating system improve by 26% and 66% in normal and stagnation mode respectively compared to system without PCM. The charging and discharging of PCM used in heat pipe ETC was studied by Naghavi et al. [77]. Theoretical investigation showed, thermal performance of latent heat storage based heat pipe ETC is higher than system without latent heat energy storage material. Authors suggested to use designed system as complementary part to conventional heat pipe ETC to provide hot water during night or week radiation hours. Tyagi et al. [72] reported that energy and exergy efficiency of ETC with thermal energy storage (THES) higher than system without THES. Besides this, both efficiencies higher in case of system with paraffin wax. Tables 5 and 6 enlisted some phase change materials and their eutectics in the temperature range of 40–80 °C, these PCMs can be used with ETC.

The energy stored within the PCM can be used at any time even when solar radiations are not available [74]. The first study regarding evacuated tube collector integrated with PCM was carried out in 2006 [75]. Papadimitratos et al. [69] investigated the performance of heat pipe equipped ETC with PCM. This arrangement is shown in Fig. 6. The stored energy in PCM can be used for an extended period of time or when solar radiations are insufficient. Li et al. [76] enhanced the thermal conductivity of Erythritol by adding expanded graphite in mass fraction of 3%. The composite PCM when used as energy storage with ETC, daily storage efficiency reached to 39.8%. System also completely finished the melting process of composite PCM when daily solar irradiance higher than 15.23 MJ/m2. Papadimitratos et al. [69] used two different PCMs namely Erythritol and Tritriacontane, results revealed that efficiency of solar water

Fig. 2. Schematic diagram of heat pipe ETC [69]. 356

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Fig. 3. Schematic diagram of heat pipe [69].

energy storage with heat pipe ETC was studied by Riffat et al. [75]. Evacuated tubes equipped with heat pipes partially filled with water produced highest thermal efficiency. Tyagi et al. [72] reported that hytherm oil as energy storage in direct flow ETC system gives better energy and exergy efficiency than system without thermal energy storage. Some sensible storage materials are provided in Table 7 that can be used as sensible heat storage for heat pipe ETC or direct flow ETC. 4. Thermal analysis of evacuated tube collector systems This section presents the thermal analysis of direct flow and heat pipe ETC. There are two different methods for thermal analysis of solar thermal collectors: steady state and transient test method. The boundary conditions such as ambient temperature, solar irradiation, and inlet temperature to collectors are maintained constant during steady state test method and for transient test method, the boundary is free to change. In order to simplify the analysis, the steady state test method for thermal analysis of ETC systems is being adopted [80].

Fig. 4. Elevation view of evacuated tube solar collector without heat pipe [68].

4.1. Thermal analysis of heat pipe evacuated tube collector system In this section following assumption are made concerning to thermal modeling of heat pipe ETCs:

• The temperature variation occurred only in the radial direction and • • •

the variation of temperature in the longitudinal direction of the collector can be ignored. The manifold can absorb all the heat delivered by the evaporator. The heat loss from the manifold and surrounding is negligible. The loss of heat between the collector and ambient is constant.

The Fig. 7 presented the sectional view of heat pipe ETC. The out of total solar energy entered into inner chambers of pipe array, part of energy absorbed by absorber and rest of the energy loss to the surrounding due to difference in temperature between absorber and surrounding [81]. Thus the useful or absorbed energy can be obtained by subtracting total energy entered into inner chamber of heat pipe array from energy loss to ambient [82].

Fig. 5. Cross-sectional view of evacuated tube with thermal energy storage [72].

3.3.2. ETC with sensible heat storage The sensible heat is a phenomena which is related with change in temperature of substance without its phase change. The most common examples of sensible heat storage in solid media include rock bed, pebbles, bricks, metals, concrete, sand etc., while liquid media consists of salty water, water, petroleum based oil, therminol and storage in other fluids. Small energy density and non-isothermal operation are the main drawbacks of sensible heat storage material. However some remarkable efficiency improvement by using of sensible heat storage materials in solar collectors are observed from various studies. Elhady et al. [79] experimentally investigated the combined effect of oil as heat storage and foamed metal as fins to transfer heat from inner surface of evacuated tube to heat pipe. Authors observed improvement in heating efficiency and bulb temperature of evacuated tubes. Water as thermal

̇ = Qen ̇ −Qloss ̇ Qab

(1)

The amount of energy entered into array of heat pipe evacuated tubes as [82]:

̇ = γgo γgi βab Aab Nhp I Qen

(2)

The loss of heat to ambient due to difference in temperature between absorber and surrounding may also be expressed as [83]:

̇ = Qloss

Tab−Ta R

∑loss

(3)

Thermal resistance network between absorber and ambient can be 357

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Table.6 List of organic and inorganic eutectics materials [78]. Material

Composition (wt%)

Melting temperature (°C)

Latent heat (kJ/ kg K)

NH2CONH2 + NH4NO3 Mg(NO3)3·6H2O + NH4NO3 Mg(NO3)3·6H2O + MgCl2·6H2O Mg(NO3)3·6H2O + MgCl2·6H2O Mg(NO3)3·6H2O + Al(NO3)2·9H2O CH3CONH2 + C17H35COOH Mg(NO3)2·6H2O + MgBr2·6H2O Napthalene + benzoic acid NH2CONH2 + NH4Br

53 + 47 61.5 + 38.5 58.7 + 41.3 50 + 50 53 + 47 50 + 50 59 + 41 67.1 + 32.9 66.6 + 33.4

46 52 59 59.1 61 65 66 67 76

95 125.5 132.2 144 148 218 168 123.4 151

Fig. 6. Schematic diagram of evacuated tube filled with PCM [69]. Table.7 List of some sensible heat storage materials [78].

Table.5 List of latent heat storage materials [78]. Material

Melting point (°C)

Latent heat (kJ/kg K)

Lauric acid Pentadecanoic acid Tristearin Myristic acid Palmatic acid Stearic acid Eladic acid Methyl eicosanate 3-Heptadecanone 2-Heptadecanone Hydrocinnamic acid Cetyl alcohol a-Nepthylamine Camphene O-Nitroaniline 9-Heptadecanone Thymol Methyl behenate Diphenyl amine p-Dichlorobenzene Oxolate Hypophosphoric acid O-Xylene dichloride b-Chloroacetic acid Chloroacetic acid Nitro naphthalene Trimyristin Heptaudecanoic acid a-Chloroacetic acid Bee wax Bees wax Glyolic acid Glycolic acid p-Bromophenol Azobenzene Acrylic acid Dinto toluent (2,4) Phenylacetic acid Thiosinamine Bromcamphor Durene Benzylamine

49 52.5 56 58 55 69.4 47 45 48 48 48 49.3 50 50 50 51 51.5 52 52.9 53.1 54.3 55 55 56 56 56.7 33–57 60.6 61.2 61.8 61.8 63 63 63.5 67.1 68 70 76.7 77 77 79.3 78

178 178 191 199 163 199 218 230 218 218 118 – 141 93 238 93 213 115 234 107 121 178 213 121 147 130 103 201–213 189 130 177 177 109 109 86 121 115 111 102 140 174 156 174

Temperature range

Density (kg/m3)

Specific heat (J/kg K)

Water Caloriea HT43 Engine oil Ethanol Proponal Butanol Isotunaol Isopentanol Octane Rock Brick Concrete

0–100 12–260 Up to 160 Up to 78 Up to 97 Up to 118 Up to 100 Up to 148 Up to 126 20 20 20

1000 867 888 790 800 809 808 831 704 2560 1600 1900–2300

4190 2200 1880 2400 2500 2400 3000 2200 2400 879 840 880

Fig. 7. Sectional view of heat pipe evacuated tube solar collector [81].

rad Rab − gi

⎡ = ⎣

(

1 − εab εab Aab Nhp

)+

1 Aab Nhp SFab − gi

σ (Tab +

2 Tgi )(Tab

+

+

( ) ⎤⎦ 1 − εgi Agi εgi

Tgi2 )

(5)

Thermal resistance between inner and absorber tube due to convection may be written by following equation. It may be noted that during evaluation of convective thermal resistance, fluid properties are measured at an average temperature of absorber and of inner glass tube [86].

illustrated from Fig. 8 and evaluated through following equation [81,84].

∑

Material

1

conv Rab − gi =

conv rad conv cond cond rad Rloss = Rab + Rgirad + Rgo − gi + Rab − gi + R gi − go + R go − a + R go − a

hair ⎛ ⎝

(4)

Nhp Aab + Agi 2

⎞ ⎠

(6)

Thermal resistance of inner glass may be evaluated as [87]:

The radiative thermal resistance between inner glass surface and absorber can be evaluated as follow [85].

Rgicond = 358

tgi Agi k gi

(7)

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Fig. 8. Thermal resistance network between ambient and absorber.

of the heat pipes is thermally developed; hence Nusselt number may be calculated under constant heat flux for different ration of external and internal diameter of heat exchanger [67]. The useful heat gained by single heat pipe solar collector, is calculated by subtraction of heat loss to surrounding from solar energy absorbed by heat pipe. It can also be evaluated as follow [90].

As shown in Fig. 7 there is vacuum between inner and outer glass surface so thermal resistance is only due to radiation. Hence radiative thermal resistance between these surfaces may be calculated as [85]:

Rgirad − go

⎡ = ⎣

( )+ 1 − εgi εgi Agi

1 Agi SFgi − go

+

( ) ⎤⎦ 1 − εgo

Ago εgo

2 σ (Tgi + Tgo)(Tgi2 + Tgo )

(8)

̇ 1 = Fcr Aab1 [I (γβ )evap−Uloss (Thp1−Ta)] Qul

Similarly thermal resistance of outer glass may be evaluated as [88]:

The useful heat gained from condensing section by fluid flowing through flow channel can also be expressed as [83]

tgo

cond Rgo =

Ago k go

̇ 1 = ṁ f Cf (To1−Tfi ) Qul

Thermal resistance between outer tube and surrounding is due to radiation that may be formulated by Eq. (9). Since surrounding is assumed to be a black body, therefore emissivity of sky is assumed one [89]. rad Rgo −a =

1 2 + Ta2) σεgo Ago (Tgo + Ta)(Tgo

1 Ago ha

Thp1 = Ta +

∅1 =

(10)

1 hf

NTUhp1

(19)

In other words effectiveness may also be written as the ratio of actual heat transfer to maximum heat transfer. This can be given by

̇ 1 = ṁ f Cf (Tcon, o1−Tfi ) = Qhp (12)

Tfi + BThp1 1+B

(13)

Uhp1 Ahp1

. where B = A U con1 con, o1 The value of hf is highly influenced by variation in velocity of fluid, geometry and cross-sectional area of heat exchanger. In order to evaluate the value of hf, the flow channel considered to be an annular geometry rather than semi-annular one. To fulfill the above conditions, heat flow from inner wall should be doubled. For above conditions, evaluation of hf can be carried out through following equation [75].

hf =

k ·Nu Dhd

(18)

Acon1 Ucon, o1 . ṁ f Cf

By equating (11) and (12) [83] and we get the value of Tcon,o1

Tcon, o1 =

(17)

Fcr Aab1 Uloss . ṁ f Cf

∅1 = 1−e−(NTU )con1

(11)

+

(To1−Tfi )

Since working fluid of heat pipe is assumed at constant temperature. Therefore heat capacity ratio of fluid inside heat pipe and fluid flowing through heat exchanger (manifold) will be equal to zero. To Comply the above condition the value of Cr = 0. Hence effectiveness may be as follow [88,91].

Owing to continuous heat exchange between heat pipe condenser and fluid flowing through heat exchanger, the fluid temperature increases progressively in downstream. Heat exchange between condenser and cooling fluid flowing through heat exchanger may be expressed as follow [88,67]. tcon k con

−

1−exp [−NTUcon1 (1−Cr )] 1−Crexp [−NTUcon1 (1−Cr )]

where NTUcon1 =

Rhp1

Acon1 (Tcon, o1 −Tfi )

Uloss

As shown in Fig. 9 single heat pipe manifold, fluid to be heated in counter flow with fluid flow inside the heat pipe. Therefore effectiveness for counter flow heat exchanger can be written as [88]:

(Thp1−Tcon, o1)

̇ 1= Qcon

I (γβ )evap

where NTUhp1 =

(9)

4.1.1. Thermal analysis of single heat pipe evacuated tube solar collector In order to attempt the thermal modeling of heat pipes in evacuated tubes, initially we considered the single heat pipe in evacuated tube assembly attached to manifold which is shown in Fig. 9. The heat transfer from evaporator to condenser section may be written as [83].

̇ 1= Qhp

(16)

By solving Eqs. (15) and (16) [83] we may evaluate the value of Thp1

Convective thermal resistance between outer glass tube and surrounding can be evaluated from following equation [8]: conv Rgo −a =

(15)

(14)

where Dhd = Di−do . Where Di and do are the inside diameter of manifold and outside diameter of heat pipe respectively. It is assumed that flow of fluid through manifold is completely developed laminar flow. It is assumed that the flow inside the condenser

Fig. 9. ETC with single heat pipe [83]. 359

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∅ = 1−(1−∅1) N

following equation [88].

∅1 =

To1−Tfi Tcon, o1−Tfi

In terms of temperature overall effectiveness may be calculated as [83]:

(20)

By inserting the value of Tcon1 from Eq. (13) in Eq. (20) and we may get the value of outlet temperature from single heat pipe solar collector.

To1 = Tfi + ∅1

{

}

B (Thp1−Tfi ) 1+B

∅=

Thp1 =

Uloss

+ Ta +

1+

To = Tfi + ∅

Tfi ∅1 B NTUhp1 1 + B

{ } { }

(22)

I(γβ )evap

Thp

As shown in Fig. 10 the solar collector with N number of heat pipes, water is flow from condenser of one heat pipe to another heat pipe. In other words the exit temperate of first condenser will be entry temperature of second condenser and so on. The final temperature may be calculated through following equation of collector with N heat pipes [83].

{

}

B (Thp (N )−TO (N − 1) ) 1+B

{ }

B (Thp−Tfi) 1+B

(27)

Uloss

+ Ta +

1+

{ } { } Tfi ∅ B NTUhp 1 +B

B ∅ NTUhp 1 +B

(28)

where

NTUhp = N(NTU) hp1 and NTUcon = N(NTU)con1

4.1.3. Heat pipe thermal resistance The total thermal resistance of heat pipe can be modeled by considering the thermal resistance due to internal fluid, condenser wall, evaporator wall and wick conduction [92]. To illustrate the thermal resistance due to heat pipe, a thermal resistance network is shown in Fig. 11.

(23)

4.1.2. Thermal analysis of array of heat pipe The final outlet temperature of water from array of N number of heat pipe may be calculated through different method by without going in calculation of each heat pipes. As shown in Fig. 10 like single heat pipe, overall effectiveness and final outlet may be evaluated as: The overall effectiveness may be written as [83]

∅ = 1−(1−∅1)(1−∅2)(1−∅3)…(1−∅N )

(26)

and

B ∅1 NTUhp1 1 + B

TO (N ) = TO (N − 1) + ∅N

Tfo−Tfi Thp−Tfi

The outlet temperature of fluid from collector with array of heat pipes can be evaluated as [83].

(21)

By combining Eqs. (17) and (21), we get [83] I (γβ )evap

(25)

∑

Rhp = Rpevap + Rwevap + Rievap + Rv + Rpcon + Ricon

(29)

Thermal resistance due to evaporator wall, wick conduction and internal fluid can be expressed by following equations [93].

(24)

Rpevap =

Considering the effectiveness of all condensers are equal, Eq. (24) become [83].

ln(Doevap / Dievap) 2πNhp kp Levap

Fig. 10. Evacuated tube collector with N number of heat pipe [83]. 360

(30)

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kl3 gρl (ρl −ρv ) hfg ⎤ hcon, i = 0.72 ⎡ ⎢ Nhp μ (Tsat −Tsf ) Dicon ⎥ l ⎣ ⎦

(36)

4.2. Direct flow evacuated tube solar collector The useful heat available for direct flow evacuated tube collector can be expressed by following expression [98].

̇ = FR A cr ⎡ (γβ ) I −Uloss ⎛ Aloss ⎞ (Tfi−Ta )⎤ = ṁ f Cf (Tfo−Tfi ) Qul ⎢ ⎥ ⎝ Acr ⎠ ⎣ ⎦ ⎜

⎟

(37)

The dimensionless factor i.e. heat removal factor can be determined experimentally or calculated theoretically from fundamental principles [98].

FR = (sinhω2 £1{ω2 £1[coshω2 £1 + (ω1 ω2) sinhω2 £1]}) ηfin

(38)

In order to evaluation of flow rate factor one must compute the following parameters [68]

ω1 = ηfin Uloss Ploss /2h3 P3

(39)

ω2 = ηfin Uloss Ploss [1 + 4(h1P1/ ηfin Uloss Ploss )]0.5 /2h3P3

(40)

£1 = lh3 P3/ṁ f Cf And ηfin = 1/(1 + Uloss Ploss / h3 P3)

(41)

The overall heat transfer coefficient due to loss of heat to surrounding owing to temperature difference between absorber and ambient temperature may also be expressed as [99]. −1

1 1 1 1 1 Uloss = ⎡ rad + cond + cond + rad + conv ⎤ ⎢h ⎥ h h h h −a ⎦ 6 a − − − − 4 5 5 6 3 4 6 ⎣

As shown in Fig. 12 there is vacuum between inner and outer glass surface (4 & 5) so thermal resistance is only due to radiation. Hence radiative resistance between these surfaces may be calculated as [94,100].

Fig. 11. Thermal resistance network of heat pipe.

Rwevap =

ln(Dievap /(Dievap−2tw )) 2πNhp k w levap

(42)

(31)

h4rad −5 =

Evaporator section used screen mesh structure. The effective thermal conductivity due to saturated wick may be evaluated as [94].

σ (T4−T5)(T42 + T52) [(1−ε4 )/ ε4] + [(1−ε5)/ ε5 (A 4 /A5 )] + 1/SF4 − 5

(43)

The conductive resistance due to outer borosilicate glass tube [68].

kw =

kf [kf + ks−(1−εw )(kf −ks )] kf + ks + (1−εw )(kf −ks )

h5cond −6

(32)

2tw evap Dhp , i k f πlevap

(44)

Similarly conductive resistance due to inner selective coating glass tube [68].

where εw is the voidage fraction which is the ratio of volume of working fluid to the total volume of the wick. A wick lined wall in evaporator the film coefficient is almost equal to the ratio of thermal conductivity of the fluid to the wick thickness Thermal resistance inside evaporator section may be evaluated through following equation [95].

Rievap =

= k /[(D5 /2) ln (D6 / D5)]

h3cond − 4 = k /[(D3 /2) ln (D4 / D3)]

(45)

Since surrounding is assumed to be black body, therefore εsky is equal to unity [101], so radiative heat transfer coefficient between surrounding and outer glass tube can be reduced as follow:

(33)

The latent heat released by condensation of vapour on the inner wall of the condenser must be conducted to outer surface of condenser. The thermal resistance due to conduction process may be evaluated as [95].

Rpcon =

ln(Docon / Dicon ) 2πNhp kp lcon

(34)

Thermal resistance due to internal fluid during condensation process is [96]

Ricon =

1 Nhp hcon, i Dicon πlcon

(35)

Fig. 12. Cross-sectional view of direct flow evacuated tube solar collector [1: inner surface of the riser tube, 2: outer surface of the riser tube, 3: inner surface of the absorber tube, 4: outer surface of the absorber tube, 5: inner surface of the cover tube, 6: outer surface of the cover tube] [98].

The condensation in condenser is assumed to be film wise condensation, so heat transfer coefficient by Nusselt analysis may be calculated as [97]. 361

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uncertainty [106]. Under the same condition, statistical determination of repeated measurement uncertainty is termed as Type-A whereas Type-B exists throughout the measured results by considering available data such as data logger uncertainty, sensor uncertainty and the preciousness of instruments or sensor [107]. Type-A uncertainty from m number of observations for a quantity Y is expressed as:

(46)

Heat transfer coefficient due to convection between outer glass tube and surrounding may be evaluated from following equation [101].

h6conv − a = 5.7 + 3.8V

(47)

The experimental data for convective heat transfer is often represented with reasonable accuracy by a simple power-law relation may be expressed as [89].

D 0.055 l Nu = 0.036Re 0.8 Pr 1/3 ⎛ ⎞ for 10 < < 400 D ⎝l⎠

2

⎛ ∑ j (yj −y ) ⎞ ⎟ ⎝ m (m−1) ⎠

wA, Y =

⎜

(48)

The convective heat transfer coefficient inside riser tube and space between inner tube and riser tube can be expressed as [89].

h1conv =

Nu·k D1

(49)

conv h23 =

Nu·k D3

(50)

(51)

where parameters wA, Y , yj , y and m are the Type-A uncertainty, individual measurement, arithmetic mean of yj and total number of observations during test respectively. For an instrument or sensor having “accuracy” c, which is the highest deviation from the true value, Type-B uncertainty may be evaluated. It is presumed that all measured values within the interval 2c are equally probable. The uncertainty in this can be evaluated as follow:

vB =

5. Uncertainty analysis

c 3

(52)

If z is the measurement, depend on measured values yi the uncertainty vi (z) from yi to z is given by

While summarizing the measurement results of a physical parameters, it’s required that some measurable variation in result reliability be specified. In the absence of such an indication, measured results can’t be compared, with standardized reference values or with themselves. Therefore it’s necessary to express and evaluate the uncertainty an instantly implementable, simply understandable, and usually acceptable procedure for characterization of quality of measurement results are required. Shafii et al. [102] performed uncertainty analysis of experimental results of solar still coupled with evacuated tubes and thermoelectric modules. They measured the expanded and resolution uncertainties of experimental results. Sheikholeslami and Ganji [103–105] did uncertainty analysis of experimental measured parameters such as tube diameter, tube length, temperature, pressure, air flow rate and water flow rate in a double pipe heat exchanger. Generally, uncertainties may be categorized into Type-A and Type-B

vi (z ) =

∂z vB (yi ) ∂yi

(53)

The global uncertainty for the parameters which are uncorrelated may be evaluated by Schultz and Cole method [103,105]. 2

v (z ) =

∑i

⎡ ⎛ ∂z ⎞ v (y ) ⎤ ⎢ ⎜ ∂y ⎟ i ⎥ ⎣⎝ i ⎠ ⎦

(54)

It may be noted that it is essential to consider all possible sources of error to calculate the global uncertainty in a system.

Fig. 13. Schematic and cross section view of THPETC [108]. 362

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6. Recent trends in evacuated tube solar collector systems for potential applications An evacuated tube collector system is a technology that can operate over wide range of temperatures (low to medium temperature). There are number of applications such as air heating, water heating, desalination and other applications where ETC can be used. The following sections provide the latest review of ETC systems in these applications. 6.1. Air heating In solar air heater, solar radiations fall on absorber plate converted into thermal energy then air is flows over that absorbing surface to get heated air. When ETC system is used for air heating then it is called evacuated tube solar air heater. There are mainly two type of solar collector used for air heating purpose which are flat plate solar air heater and evacuated tube solar air heater. In market, ETC based air heaters preferred over flat plate solar air heater due to its lower cost and high performance. Mustafa Ali Ersoz [108] did an exergy and energy evaluation of thermosyphon HP-ETC as shown in Fig. 13 with six different working fluids (ethanol, acetone, methanol hexane, chloroform, and petroleum ether), for same filling rate and inclination angles. Author found that hexane showed poor performance in the terms of energy, optical exergy efficiency and concluded that chloroform and acetone gave the best performance under considered conditions. Daghigh and Shafieian [17]in this study authors observed: (i) the large temperature variation in working fluid of considered system due to continuous exchange of heat between working fluid and air to be heated for drying (ii) ambient temperature air puts a great influence on the performance of developed system (iii) due to fall in ambient temperature exergetic efficiency increases with time and got maximum (11.7%) value at the end of the day (iv) Maximum 44.3 °C outlet temperature of dryer was observed at 0.0328 m3/s volumetric flow rate (v) Solar energy intake increases with increase in solar radiation. The presented setup is shown in Fig. 14. In this system working fluid (water) after passing through condenser area of collector gets heated. This heated fluid entered the copper coil and exchange its heat with water in the tank. Hot water in tank is pumped to heat exchanger where its heat is transferred via fan. Further hot air is supplied to drying chamber for desired applications for compensating low solar radiations available in the early morning hours, a 2 kW auxiliary heater was also installed in the tank. In order to get the maximum utilization of heat carried by water, a heat recovery concept was also implemented in

Fig. 15. Schematic of the air heating system [109].

experimental unit. In this connection, if the water temperature is higher than temperature of air, it is poured on the walls and roof of drying chamber, otherwise return to the water tank. Daghigh and Shafieian [109] observed that: rate of increase of temperature difference across solar collector and efficiency become negligible by employing more than 30 heat pipes in solar collector and in month of January collector got maximum energy efficiency (56.8%) and exergy efficiency (7.2%). The system is shown in Fig. 15 illustrated that air mixture (air coming from space and fresh air) is heated by heat pipe ETC system meant for heating of air mixture and supplied to space to be heated. PCM is used in presented system to store extra energy delivered by solar collector. A novel study pertaining to use of the concentric heat pipe for air heating purpose has done by Kabeel et al. [24]. The working fluid (air) at different mass flow rates (0.009, 0.007, 0.0062 and 0.0051 kg/s) is flow through inner tube of heat pipe and annulus space between heat pipe and absorber tube. On the other hand annulus space between inner and outer tube of heat pipe was filled with refrigerants

Fig. 14. Schematic diagram [17]. 363

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(R22 or R134a) at different filling ratios (in the range of 30–60%). At mass flow rate of 0.009 kg/s they have found 67% increment in thermal efficiency without heat pipe. They have also optimized the tilt angle for all glass tubes during experiment. Lamnatou et al. [110] fabricated and designed an experimental unit for drying apples, carrots and apricots samples. Experimental results revealed that without preheating of outlet air designed solar collector is appropriate for solar drying applications. Different performance parameters-optical efficiency, pick up efficiency, energy utilization ratio, moisture ratio, drying rates, energy and exergy efficiency were evaluated for different configurations such as single and double-trays for different air velocities. Authors recommended to adopt proposed system for drying of large quantity of products. The experimental unit is shown in Fig. 16 comprised of mainly ETC system for air heating, drying chamber for placement of sample in wire-mesh metal plate, fans for discharging or suction of working fluid and dampers employed for mixing of collector outlet air with ambient air. A drying experimental unit for drying of bitter gourd was presented by Sundari et al. [111]. The hot air from ETC system passed to drying chamber through air pump. The drying chamber as shown in Fig. 17 consists of three stacks (Tray 1, Tray 2 and Tray 3) for holding of bitter gourd samples. Drying chamber also connected with chimney to increase the air flow rate within drying chamber. Authors compared various performance parameters of drying process through solar dryer with natural sun drying process. They observed that moisture content of considered samples decreases from 91% to 6.25% in 6 h with proposed system and in 10 h through natural sun drying. They also found that initially, removal of moisture content from considered samples was

Fig. 17. Experimental Unit (1: Temperature sensor, 2: Blower, 3: ETC, 4: Drying chamber, 5: Chimney) [111].

Fig. 16. Overall view of experimental solar drying system [110].

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of 15 evacuated tubes inserted with aluminum tubes and manifold comprising of two concentric squared pipes, one being the inner squared mild steel pipe and others is outer insulated stainless steel squared pipe. As shown in Fig. 20 air is to be cooled, first it is entered through inner squared pipe and then flown through the inserted aluminum tubes into evacuated tubes. In all considered cases (with THES, without THES and with hytherm oil) both efficiencies increase with time. After gaining heat air is exit through outer square pipe. In this experimental setup authors have investigated the effect of variable length of aluminum tubes by considering different performance parameters. For considered length, thermal efficiency increases and temperature difference decreases with increase of air flow rate. Further for selected air flow rates, temperature difference and efficiency of system with 0.83 m aluminum tubes was higher compared to system with 0.415 m aluminum tube. Finally, Authors has concluded that system without insertion of aluminum tubes showed poor results in term of temperature difference and efficiency. The system presented by Tyagi et al. [72] comprised of solar collector consists of 12 evacuated tubes out of which 4 tubes are filled with temporary heat energy storage (paraffin wax), 4 tubes with hytherm oil and rest of the tubes without THES. As shown in Fig. 21 in this arrangement evacuated tubes inserted with U-shape copper tube of

faster and then gets reduced exponentially. Wang et al. [112] designed and fabricated a novel concentric annular tube heat exchanger based ETC system with simplified compound parabolic trough collector. A concentric annular tube heat exchanger is shown in Fig. 18(a) constructed with two welded copper whose one tube has smaller diameter than another. As cleared from Fig. 18(b) the compressed air through compressor is entered into air tank, which stabilized the steady initial temperature and pressure of air. This compressed air is passed through 30 collector units to get the desired temperature. Designed and fabricated system can be used for industrial applications in the temperature range of 150–200 °C. It has been observed that thermal efficiency is 52% at 70 °C, 35% at 150 °C and 21% at 220 °C respectively. Mehla and Yadav [113] experimentally studied the ETC based air heating system integrated with acetamide PCM. System is shown in Fig. 19 comprised of water filled 40 evacuated tubes with aluminum sheet as reflector and concentric rectangular manifold. Authors have analyzed that for considered mass flow rate of air, squared shape obstruction produced more promising results in term of efficiency and outlet temperature of air during sunshine and off shine hours. Further, they found that efficiency also increases with enhancement of air flow rate. The experimental setup investigated by Kumar et al. [114] consist

Fig. 18. (a) Structure of concentric tube (b) Schematic diagram of experimental setup [112]. 365

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Fig. 19. Schematic diagrams of evacuated tube solar air collector based on an air heating system with PCM unit [113].

Fig. 20. Schematic diagram of the evacuated tube solar air collector [114].

theoretical and overall COPs were 6.38, 6.33 and 5.56 respectively. Yadav and Bajpai [116] discussed the usage of heat collected by ETC for regeneration of activated alumina, activated charcoal and silica gel. The heat collected by ETC is transferred to air through heat transfer fluid (water). Experimental outcomes revealed that: (i) desiccant regeneration performance was highly affected by initial moisture content, flow rate for regeneration and regeneration temperature (ii) temperature difference of air across manifold was higher at low flow rate of air flowing through manifold (iii) selected variation of air flow rate through manifold put no effect on adsorption time of considered desiccants (iv) adsorption time taken by silica get is more among selected desiccants and (v) regeneration time of silica gel and activated alumina is higher than activated charcoal at low flow rate of air (vi) regeneration time at high flow rate of air for activated charcoal and activated alumina is lower than silica gel. According to Indian climate conditions, authors recommended for adoption of silica gel for adsorption and ETC for regeneration of silica gel. Kumar et al. [117] did experiment to observe performance parameters of ETC with and without reflector. They found that at low flow rate of air (6.70 kg/h), maximum outlet temperature and temperature difference with reflector were 97.4 °C, 74.4 °C respectively. However

12 mm diameter for circulation of air compressed by compressor through rotameter. Authors did comparative thermodynamic analysis of air heater for different mass flow rate of air. As the results of analysis, it is revealed that They observed that exergy efficiency graph is smoother than energy efficiency graph, besides this energy efficiency is higher than exergy efficiency for selected time range. It was also found that efficiencies of collector without THES lower than that of with THES. Caglar and Yamali [115] studied a test house heated by combined cycle of vapour compression heat pump system and solar heating system. Helical coiled evaporator of VCR is shown in Fig. 22. It was immersed in storage tank of hot water and building area was heated by heat rejected from condenser of heat pump. The water in well insulated storage tank was heated (between 5.2 and 20.7 °C due to operating limits of compressor) by ETC system and immersed evaporator coil was used as heat source for heat pump. The experimental results indicated that increase in evaporation temperature increases COP of heat pump and heat transfer rate in evaporator and condenser of heat pump. The maximum experimental values of heat transfer rate in evaporator, condenser were 4.95 kW, 5.87 kW respectively whereas corresponding theoretical values were 5.10 kW, 6.06 kW respectively. Experimental, 366

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Fig. 21. Schematic of the experimental setup [72].

with storage and natural sun drying were 22.03, 34.23 and 9.32% respectively. In addition to this moisture content of chilies is reduced in10 and 12 h (from 87.36% to 3.4%) with and without heat storage materials respectively and natural sun drying took 32 h for same. The latest research in the area of air heating system with possible outcomes by the different researchers is given in Table 8. Solar air collectors are suitable for applications that require direct use of hot air such as space heating and drying. In the discussed literature survey, ETCs are extensively used for drying and space heating. Evacuated tube air collector is the promising advanced technology that can produce high temperature air in comparative to FPC. However drying of fruits, vegetables, spices etc. requires hot air for drying in the temperature range of 50–70 °C and temperate limit for space heating is 30 °C, FPCs are most appropriate for this temperature range.

concerned values without reflector were 79.9 °C and 52.9 °C respectively. Authors have also evaluated the thermal efficiency for different flow rate of air and its maximum values were found to be 58% with reflector whereas 50% without reflector at high flow rate of air (13.28 kg/h). Authors have suggested to use reflector with ETC to enhance its performance. Sundari et al. [118] fabricated an experimental unit for drying of green chilli with or without energy storage material (rock bed). In this system hot air from ETC system passed to drying chamber through air pump. The drying chamber consists of three stacks (Tray 1, Tray 2 and Tray 3) for holding of sample of green chilli and drying chamber also connected with chimney to increase the air flow rate within drying chamber. Developed solar dryer performance in term of loss of moisture content was better with heat storage material (rock bed). Designed experimental unit reduced the drying period by 66%. Authors have observed that efficiency of the dryer: without storage,

Fig. 22. The schematic representation of the experimental setup [115]. 367

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Table 8 Type of investigation and outcomes for air heating applications. Author

Type of investigation

Type of ETC

Outcomes

Mustafa Ali Ersoz [108]

Experimental

HP-ETC

Daghigh and Shafieian [17] Daghigh and Shafieian [109] Kabeel et al. [24]

Experimental

HP-ETC

Experimental and theoretical Experimental

HP-ETC

– – – – –

Lamnatou et al. [110]

Experimental

WHP-ETC

Sundari et al. [111] Wang et al. [112]

Experimental Experimental

WHP-ETC WHP-ETC

Mehla and Yadav [113]

Experimental

WHP-ETC

Kumar et al. [114]

Experimental

WHP-ETC

Tyagi et al. [72]

Experimental

WHP-ETC

Caglar and Yamali [115]

Experimental and theoretical

WHP-ETC

Yadav and Bajpai [116]

Experimental

WHP-ETC

Kumar et al. [117] Sundari et al. [118]

Experimental Experimental

WHP-ETC WHP-ETC

HP-ETC

Hexane as working fluid in heat pipe showed poor performance. Chloroform and acetone gave best performance under same conditions. Maximum exergetic efficiency was 11.7% at the end of the day Maximum 44.3 °C outlet temperature at 0.0328 m3/s volumetric flow rate Maximum collector energy and exergy efficiency were 56.8% and 7.2 respectively

– Increase in thermal efficiency. – Optimized angle of tilt all glass tubes – Designed solar collector is appropriate for solar drying applications without preheating the outlet air. – Performance parameters such as optical efficiency, pick up efficiency, energy utilization ratio, moisture ratio, drying rates, energy and exergy efficiency were evaluated for different configurations. Moisture of bitter gourd samples decreased from 91% to 6.25% in 6 h – Used for industrial applications in the temperature range of 150–200 °C – Thermal efficiency is 52% at 70 °C, 35% at 150 °C and 21% at 220 °C respectively – Maximum 37 °C and 20.2 °C temperature difference observed across manifold during sunshine and off-sunshine hours respectively. – Efficiency increased with increase of air flow rate – Studied the effect of inserted aluminum tubes into evacuated tubes. – Maximum temperature difference of air with a flow rate of 5.06 kg/h across manifold was 72.7 °C. – Energy efficiency higher than exergy efficiency. – Energy and exergy efficiencies of ETC system without THES is lower than that of with THES. – Increase in evaporation temperature increases COP of heat pump – The maximum experimental value of heat transfer rate in evaporator, condenser were 4.95 kW, 5.87 kW respectively. – Desiccant regeneration performance was highly affected by initial moisture content – Temperature difference of air across manifold was higher at low flow rate of air flowing through manifold – Adsorption time taken by silica get is more among selected desiccants – Authors suggested to use reflector with ETC to enhance its performance. – Efficiency of the dryer without storage, with storage and natural sun drying were 22.03, 34.23 and 9.32% respectively – Moisture content of chilli reduced in10 and 12 h from 87.36% to 3.4% with and without heat storage materials respectively.

There are different types of solar collector (flat plate, evacuated tube, concentrated etc.) available in the market which are used for water heating applications. Out of these ETC have captured the solar market due to its higher performance and low cost. In this section authors have discussed the contribution of various researchers in ETC system for water heating. Felinski and Sekret [23] explored the use of paraffin as PCM in annular space between heat pipe and absorber tube of evacuated tube fitted with a CPC (concentration ratio of 1.2×). The compound parabolic concentrator used to prevent the uneven melting of paraffin as PCM, as only exposed area of glass tubes get the direct solar

6.2. Water heating There is huge demand of hot water for domestic and industrial purpose consequently the large amount of energy is required for the production of hot water. The solar water heater is one of the solution to avoid use of conventional energy for hot water production. The working principle of solar water heater is, that solar radiations fall on the absorbing surface plate is converted into thermal energy then thermal energy is transferred to water. The evacuated tube solar collector used for water heating is referred to as evacuated tube solar water heater.

Fig. 23. Schematic diagram of experimental water heating system [119]. 368

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radiations without fitting of CPC. In addition to this evacuated tubes equipped with CPC enhanced the average gross charging efficiency from 31% to 36% and maximum charging efficiency from 40% to 49%. On average usage of European household; a solar water heating system was designed by Ayompe and Duffy [119]. The Thermomax HP 200 solar collector (comprising of 30 evacuated heat pipes and well insulated water manifold), insulated stainless steel tank equipped with 3 kW capacity electric heater and an automatic hot water dispensing unit are the main parts of the designed experimental unit. An electric heater will activate during peak usage of hot water and will turn it off when temperature of tank exceeds the threshold value (60 °C). As shown in Fig. 23 solenoid valve is used to dispense the hot water which is controlled by programmable logic controller (PLC) and pulse flow meter to measure the volume of water extracted from hot water tank. An average total of 20.4 MJ/day energy collected by solar collector and energy supplied by solar coil was 16.8 MJ/day. Experimental findings revealed that 33.8% of total annual hot water demand was fulfilled by proposed experimental unit and rest through auxiliary energy source. Maximum efficiency of collector and system were 63.2 and 52% respectively. In addition to this maximum 70.3 and 59.5 °C solar fluid temperatures were recorded at outlet of collector and bottom of water tank respectively. Daghigh and Shafieian [120] fabricated an experimental unit used for extraction of hot water as per the consumption pattern of dormitory hall. Theoretical and experimental outcomes revealed that water temperature of tank increases as intensity of solar radiation intensifies. Authors have also observed the large variation in tank temperature due to consumption of hot water and injection of cold water from the bottom of the tank. It was also analyzed that exergetic efficiency ascends with time and has maximum value (5.4%) at the end of the day. A

study was given by Ayompe et al. [121]which is based on the European Union mandate, 200 L hot water at 60 °C equivalent to 11.7 kWh dispensed off using solar collector (either flat plate or ETC). They have observed that mass flow rate of solar fluid has a great dependency on the level of solar intensity. Authors have also measured the percentage mean absolute errors (PMAE) for FPC and ETC found that PMAE at exit temperature is 16.9%-FPC/18.4%-ETC, heat delivered to the load is 14.1%-FPC/16.8%-ETC and heat collected is 6.9%-FPC/7.6%-ETC. Model underestimated the collector outlet temperature, heat delivered to load and overestimated the heat collected by −9.6%, 6.9% and 7.6% respectively for FPC model. The model also overestimated the considered parameters by 13.7, 12.4 and 7.6% respectively for ETC system. The schematic arrangement developed by Singh et al. [122] consists of ETC based solar cooker. The system in Fig. 24 consists of 15 evacuated tubes whose open ends connected with squared shape header and closed ends are supported by frame. In order to reflect the solar radiations on evacuated tubes, a reflector was used under vacuum tubes. Outlet and inlet of header are connected to solar cooker through inlet gate valve and exit gate valve respectively. Solar cooker consists of three hollow concentric aluminum cylinders. The middle and outer vessel of solar cooker was filled with acetamide as PCM and water as working fluid respectively. Experimental results revealed that PCM used in solar cooker stores 18.8% more energy as compared to thermal oil. Maximum temperature inside solar cooker for same cooking load was higher when thermal oil was used as heat transfer fluid in comparison water. It was also found that for considered working fluids when both gate valves of solar cooker were closed, PCM got its highest temperature. Authors recommended to use of thermal oil as heat transfer fluid for cooking purpose according to the Indian climate conditions. Felinski and Sakret [123] did

Fig. 24. Photograph of the experimental setup [122]. 369

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comparative analysis of partially filled ETC tubes with paraffin and tubes without paraffin. The experimental study showed that operating temperature of partially filled ETC tubes with PCM was lowered as compared conventional system. In comparison of conventional ETC unit heat losses were reduced (31–32%) and heat gain per day has increased (45–79%) in ETC/S unit. An experimental unit used in study shown in Fig. 25 comprised of manifold of ETC collector unit connected with heat collection unit through pump. Both water meter and flow meter installed before heat collection unit used to calculate water volume and water flow rate respectively. In case of excessive pressure condition, expansion vessel and safety valve were also installed in this system. Rybar et al. [124] compared standard manifold header (ETC-1-S) and manufactured metal foam structural manifold (ETC-2-MF). As shown in Fig. 26 cold water pumped to ETC-1S and ETC-2-MF through T-distribution valve. Hot water at outlets of both manifolds was mixed and then supplied to counter flow heat exchanger. In case of excessive pressure condition, an expansion vessel installed between heat exchanger and pump. Various control and on/off valves, thermocouples and flow meters were installed at desired locations. Authors did performance analysis of simultaneous operation of standard manifold header and innovative manifold. Performance enhancement factor and thermal performance of solar collector with innovative manifold were much greater than solar collector with standard manifold. Authors also did pressure drop analysis of innovative manifold. Thermal power and enhancement factor of innovative manifold increases from 85.2 to 210.8 W and 1.14 to 3.20 respectively. Zhang et al. [22] discussed the simultaneous production of hot water as well as electricity by using of evacuated tube collector and thermoelectric modules system. The hot side of TEM as shown in Fig. 27 attached to hot side of heat pipe which is heated by electric heater and water jacket cool the cold side of thermoelectric module. Henceforth electricity will be generated due to temperature difference; also water passing through water jacket gets heated up to 55 °C. So Proposed system can be produced 0.19 kWh electric energy and 300 L hot water (up to 55 °C) per day when solar radiation intensity less than 1000 W/m2 and ZTM (figure of merit) is 0.59. Experimental findings revealed that both collector efficiency & electric output increases with increase in solar isolation and decreases with increases in difference of hot water and ambient temperature. When the figure of merit for thermoelectric module is unity, collector efficiency, output electrical power and electrical efficiency were 47.54%, 64.80 W and 1.59%, respectively at 1000 W/m2, 1.3 m/s, 25 °C and 25 °C solar intensity, air

velocity, surrounding temperature and water temperature respectively. It was found the increase in figure of merit (ZTM) also enhances the electrical efficiency of the system. Authors also worked on the cost analysis of the presented system. Authors calculated that payback period of 36 evacuated tubes and thermoelectric modules are 8 years with initial investment of US$2380.86. The numerical and experimental investigation did by Bracamonte et al. [20] discussed the change of tilt angle on thermal stratification and thermal efficiency in glass evacuated tube solar water heater system. Effect of three tilt angles (10°, 27° and 45°) on daily solar energy gain, the flow pattern inside storage tanks and stratification was studied. Lowering of tilt angle increases daily solar heat gain and uniformity of temperature above the tubes openings also non-uniformity of temperature in storage tank. It was also observed that variation in tilt angle does not produce much effect on thermal efficiency of system. Ghaderian and Sidik [47] deliberated on the effect of nanofluid (Al2O3 and distilled water) as working fluid with different volume fraction (0.03% and 0.06%) of nanoparticles on thermal efficiency of ETC. Triton X-100 used as surfactant in present study. The impact of variation in mass flow rate (20–60 LPH) of water and Al2O3 nanofluid also investigated experimentally. It was found that for 0.06 vol%, maximum thermal efficiency observed to be 57.63% at mass flow rate of 60 LPH. It was also observed that collector efficiency increase with mass flow rate and enhancement in volume fraction of nanoparticles (Al2O3) in water. They concluded and suggested that nanofluids may be used as working fluid to enhance the thermal efficiency of evacuated tube solar collectors. Pandey et al. [21] did thermodynamic analysis of evacuated tube based water heating system. As presented in Fig. 28 system consist of nine ETC tubes inserted with U-shape copper tubes for continuous flow of water in the whole arrangement. The cold water is supplied through overhead tank and hot water is collected in insulated tank. Both energy and exergy efficiency of ETC based water heating system for selected volume flow rates (10, 15, 20, 25, 30 LPH) increased with increase of solar radiations. They found that outlet temperature of considered system is the function of solar radiation, which is an increase with enhancement in intensity of solar radiations. Maximum outlet temperature, energy efficiency and exergy efficiency were 79 °C, 66.57% and 13.38% respectively at 15 LPH. Corresponding lowest values were at 30 LPH. Faraji et al. [125] proposed a cogeneration system for simultaneous production of electricity and hot water by using of renewable energy source was presented. As shown in Fig. 29 hot water produced from series-parallel combination of evacuated solar collector system passed through hot junction of thermoelectric modules. Then this hot water

Fig. 25. Schematic diagram of the test stand [123]. 370

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Fig. 26. Schematic diagram of experimental setup [124].

purpose as well as floor heating. The water circulation maintained at desired temperature on first and second floor of modeled building. On the other hand hot water at constant temperature i.e. 45 °C (by mixing of hot water in tank and cold water) also supplied for domestic purpose to four persons with average daily consumption of 384.4 L between 6:00 and 8:00 am every day. It was collected that total initial cost and return on investment by using of evacuated tube solar collectors for proposed system higher than using of flat plate collectors. Bin Du et al. [127] did study on water heating system shown by Fig. 31, in which water circulation circuit comprised of main water tank, pump run on variable frequency, valves, filter, sight glass and flow meter. The water as heat transfer fluid pumped to solar collector through valve, filter, sight glass, flow meter and second stage heater. Its temperature enhanced due to absorption of heat in solar collector and then flow back to the main tank through cooling water heat exchanger. All pipe lines of the system were well insulated to decrease the thermal losses and to increase the accuracy of temperature measuring and controlling system. The exhaust valves are provided at the outlet of solar collector and in the main tank so that hot water can be discharged through these valves when measurement procedure is in the high temperature state. The controlling system to control the water temperature consist of heater in main water tank, second stage heater and cooling water heat exchanger. The heater in the main tank maintained the constant inlet temperature of water. The PID controlled second stage heater measured the temperature of water at inlet of solar collector then it sends the error (deviation) signal to the control system, if the temperature at inlet is lower than the set temperature, then second stage heater used to heat the water. The present investigation focused on the evaluation of instantaneous efficiency, its correlation with absorber and receiver areas, the effective heat capacity, pressure drop and angle modifier by theoretically and experimentally. Authors suggested that same experimental investigation can be done with other types of solar collectors. Joo and Kwak [128] experimentally evaluated the thermal performance of indoor heat pipe ETC unit with four (water, ethanol, flutecpp9 and methyl acetate) different working fluids. It was found that

Fig. 27. Structure of ETC with TEM (a) evacuated tube with TEM; (b) crosssection of glass tube; and (c) top section of [22].

was stored in well insulated tank for domestic purpose and stored as thermal storage source for off shine hours. On the other hand, cold water is passed through cold junction of thermoelectric modules from non-insulated tank. Authors found that electric generation increases with increase of temperature difference across thermoelectric modules. Authors concluded that electric output was highest (57 V DC) for maximum solar radiation intensity and minimum at minimum solar radiations impingement. Najera-Trejo et al. [126] discussed solar collector system (either flat plate or evacuated tube solar collector) with tank less boiler acted as an auxiliary support shown in Fig. 30. Hot water from solar collector used for domestic 371

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Fig. 28. Schematic arrangement of water heating system [21].

among four selected working fluid, water showed the fastest response performance and maximum thermal performance. For fixed inclination angle (40°), the values of FR (τα) for water, ethanol, flutec-pp9 and methyl acetate as working fluid were 0.6636, 0.6147, 0.525 and 0.6572 respectively and values of FR UL were −1.8457, −0.6365, −3.2313 and −2.0086 respectively. It was also observed that when flutec-pp9 used as working fluid, the collector performance was greatly affected by the inclination angle. Naghavi et al. [129] designed the heat pipe ETC with PCM as latent heat storage. The system comprised of array of heat pipe in evacuated tube, connected to tank filled with PCM demonstrated by Fig. 32. The heat absorbed by incident solar radiations on evaporator of heat pipe is transferred to the condensers of heat pipe which were connected to the PCM filled tank. The working fluid (water) was heated by passing through finned pipe line inside the PCM tank.

Authors experimentally evaluated that range of thermal efficiency of presented system in sunny and cloudy rainy days were 38–42% and 34–36% respectively. It was found that variation in flow rate of working fluid has great influence on the thermal efficiency of system. Authors reported that by this novel design they completely removed the thermal stratification in energy storage tank and system could be used for hot water demand during nigh time. Research work by the different researchers in the in the area of solar water heating system and possible outcomes are given in the Table 9. The world market is speedily developing for solar water heater, therefore high scale development for high quality solar products is the market requirement. The solar collectors used for water heating applications are concentrated solar water heater, evacuated tube solar water heater, flat plate solar water heater. A concentrated solar water

Fig. 29. Schematic diagram of proposed system [125]. 372

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Fig. 30. Diagram of Proposed system [126].

heater used for steam or high temperature water generation and flat plate/evacuated tube solar water heater for low and medium temperature applications. Now a days evacuated tube water heater having dominancy over flat plate water heater due to its several advantages. Particularly for high temperature operations, ETC have high efficiency than FPCs and manufacturing cost of ETC is also lower than FPC. The most important characteristic of ETC is that they are much hotter than FPC and supply high temperature water (up to 100 °C). Heat pipe ETC system is the latest advancement in the solar water heating market. Continuous hot water supply and higher efficiency compare to direct ETC are the salient features of the heat pipe ETC water heater. The high temperature in evacuated tubes can cause a significant concern in domestic hot water system such as cracking and overheating of evacuated tubes. However this issue does not arise in FPC.

eliminate such problems solar desalination is the most promising technology for the production of fresh water. In this section desalination of brackish water with evacuated tube solar collector and coupling of ETC with other type of solar collector is discussed. Mosleh et al. [18] found that using of oil instead of aluminum foil between heat pipe and evacuated tube increases efficiency and rate of production of pure water from 21.7% to 65.2% and 0.48 kg/h to 1.68 kg/h respectively. They evaluated that cost of production of pure water/liter is 0.0450 $/1/m2 for 25 years of system operation. They developed and tested an experimental facility for desalination in Tehran, Iran. As shown in Fig. 33 evacuated tube equipped with heat pipes (partially filled by ethanol) located at the focal line of the collector to absorb the solar radiations. Authors suggested that sun tracking system in proposed experimental unit is necessary to get the higher solar heat gain. A desalination experimental unit discussed by Omara et al. [130] installed at Kafrelsheikh University, Egypt. The Fig. 34 illustrated that brackish water heated by passing through condenser of heat pipe based ETC collector. This heated water is supplied to wick types and conventional solar stills through well insulated pipes. Brackish water feeding tank used to compensate water on a daily basis. In this

6.3. Solar desalination The conventional desalination technology might solve the shortage of fresh water but this technology consumes huge amount of fossil fuels and thus a critical factor for environmental problems. In order to

Fig. 31. Schematic diagram of experimental setup [127]. 373

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Fig. 32. The schematic diagram of experimental unit [129].

also illustrated that for selected tilt angles (20 and 30°) distillate production of SLPW, SLLW, SLSW, DLLW and DLSW solar stills were 90, 98, 114, 107 and 104% respectively comparatively higher than CSS. This may be due to higher evaporation rate in wick type solar stills. However mass flow rate in wick type solar still is lower than conventional solar still, due to higher amount of water absorption by selected wicks in solar still. Productivity of distilled water is enhanced by 215%, by feeding of hot brackish water during off-sunshine period. Shafii et al.

investigation comparative analysis of wick types (single layers, double layers, plane, lined and square thick linen woven fabrics wick) and conventional solar still was done. Ambient air, basin water, water in wick base, water vapour and glass cover temperatures were the performance parameters evaluated at an hourly interval for different arrangement of experiment. The experimental results showed that when vertical walls and base of conventional solar still covered with developed wick, it will increase the daily average productivity. The results Table 9 Type of investigation and outcomes for water heating applications. Author

Type of investigation

Type of ETC

Outcomes

Felinski and Sekret [23]

Experimental

HP-ETC

Ayompe and Duffy [119]

Experimental

HP-ETC

Daghigh and Shafieian [120]

Experimental and theoretical

HP-ETC

Singh et al. [122]

Experimental

WHP-ETC

Felinski and Sakret [123]

Experimental

HP-ETC

Rybar et al. [124]

Experimental

HP-ETC

Zhang et al. [22]

Experimental

HP-ETC

Bracamonte et al. [20]

Experimental and theoretical

WHP-ETC

Ghaderian and Sidik [47]

Experimental

WHP-ETC

Pandey et al. [21]

Experimental

WHP-ETC

Faraji et al. [125]

Experimental

WHP-ETC

Bin Du et al. [127]

Experimental and theoretical

HP-ETC

Naghavi et al. [129]

Experimental

HP-ETC

– Enhanced the average gross charging efficiency from 31 to 36%. – Increased maximum charging efficiency from 40% to 49%. – Approximately one-third of total annual hot water demand was fulfilled by designed experimental unit – Water temperature in tank increased by increasing of solar radiation intensity. – Exergetic efficiency ascending with time and had maximum value (5.4%) at the end of the day – Solar cooker with PCM stored 18.8% more energy when thermal oil compared to water used as working fluid – Heat losses were reduced by 31–32% – Heat gain per day has increased by 45–79% in ETC/S unit – Thermal power and enhancement factor of innovative manifold increases from 85.2 to 210.8 W and 1.14 to 3.20 respectively – System simultaneously produced 0.19 kWh electric energy and 300 L hot water (up to 55 °C) per day – Lowering of tilt angle increases daily solar heat gain – Variation in tilt angle does not produce much effect on thermal efficiency of system – Maximum thermal efficiency observed to be 57.63% at mass flow rate of 60 LPH with 0.06 vol% naofluid. – Collector efficiency increase with mass flow rate and enhancement in volume fraction of nanoparticals. – The outlet temperature of collector increase with increase in intensity of solar radiations. – Maximum outlet temperature, energy efficiency and exergy efficiency were 79 °C, 66.57% and 13.38% respectively at 15 LPH. – Electric generation increases with increase of temperature difference across thermoelectric modules. – Electric output was highest (57 V DC) for maximum solar radiation intensity and minimum at minimum solar radiations impingement – Evaluated instantaneous efficiency, its correlation with absorber and receiver areas, the effective heat capacity, pressure drop and angle modifier – Variation in flow rate of working fluid has great influence on the thermal efficiency of system. – Completely removed the thermal stratification

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Fig. 33. Schematic diagram of desalination system [18].

yield and efficiency by 21 and 13.7% respectively. Solar collector at an angle of 45°, 35° and 25° observed 0.82 kg/m2 h, 0.83 kg/m2 h and 0.61 kg/m2 h yield respectively. It was found that 35° inclination angle is the optimum angle for experiment. Kumar et al. [19] reported the integrated effect of solar still and ETC on desired performance parameters. As shown in Fig. 37 ETC collector connected with solar still through insulated pipes and water pump. In order to avoid vapour leakage, solar still is covered with glass cover inclined 15° to the horizontal. Blackened inner envelop of solar still absorb 95% solar radiations. A check valve was also installed to prevent the reverse flow during off-sunshine hours. An experimental unit is faced toward south direction to receive maximum solar radiations. Authors observed that lowest water depth in solar still basin (0.01 m) and lowest mass flow rate of working fluid (0.001 kg/s) at 14:00 h gave maximum water temperature, highest hourly yield (0.485 kg/m2 h) and maximum glass cover temperature. The total productivity decreases with increase of water depth in solar still. It was found that at highest water mass flow rate (0.006 kg/s) and solar still water depth (0.03 m) thermal efficiency, exergy efficiency, daily yield and maximum water temperature get the optimum values. Optimum energy efficiency, daily yield and exergy efficiency were 33.8%, 3.9 kg and 2.6% respectively. It was found that higher mass flow rate leads to higher power consumption but increase in power consumption is compensated by increase in thermal efficiency. It was also observed that performance parameters i.e. Yield, energy efficiency and exergy efficiency are higher in summer months compared to winter months due to difference in solar radiation. Authors also worked on the comparative analysis of evacuated tube integrated solar still (EISS) in natural mode, forced mode and hybrid (PV-T) active solar still in the terms of average annual yield per m2 of solar radiation

[102] experimentally found the effect of propeller fan on the production of distilled water in presented experimental unit. The Fig. 35 shows that propeller fan is driven by electricity generated from 20 installed thermoelectric modules on the ceiling and outer walls of the chamber. In order to run propeller fan, TEM was used to utilize the heat from the vapour condensation. Enhancement in evaporation rate increase daily water yield, maximum hourly water productivity and hourly efficiency by 14, 12.61 and 11.76% respectively. Discussed performance parameters also increased by 27% percentage with full glass tubes compared to half-full case. Shafii et al. [131] worked on experimental setup placed in Tehran, Iran (latitude: 35°42′, longitude: 51°35′ and altitude of 1172 m above mean sea level) between 8:30 AM to 5:00 PM during summer season. As shown in Fig. 36 the experimental unit comprising of two sections. First section consists of twin glass evacuated tube in which inner tube of evacuated tube was partially or completely filled with saline water. Condenser, which condenses and accumulates condenses of water vapour, is the second section of the experimental setup. Condenser is made up of steel tube, connector and cap. Steel tube is connected with glass tube through connector and sealed connection between steel tube and glass tube is provided using Teflon poly tetraflouroethylene (PTFE). This improved design leads to increase the production rate of distilled water up to 0.83 kg/m2 h. Variation in efficiency and rate of production of distilled water for different volume fraction (20, 40 and 80%) of brackish water was studied. Experimental outcomes showed that 80 and 20% of the total filled volume of evacuated tubes give highest and lowest performance (yield and efficiency) respectively. It was found that using steel wool between heat pipe and absorber tube increase the conductive heat transfer rate and consequently enhance 375

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(a) Conventional Still

(b) Double layer wick still

(c) Single layer wick still

Fig. 34. Schematic diagram of experimental setup [130].

humidifier to get hot and saturated air. The exit warm water pumped to solar still to take benefit during daytime as well as nighttime. On the other hand, warm and saturated air condensed by passing through condenser and after this unsaturated cold air entered to humidifier (closed loop). Inlet water to solar collector is pre-heated by getting latent heat of air. It was examined that productivity is enhanced by increase in solar radiation intensity. Highest inlet temperature to evaporator also leads to increase in humidifier outlet temperature,

collector area. Authors concluded that annual productivity per m2 of solar radiation collector area of EISS forced mode is much higher than that of hybrid solar still. Sharshir et al. [132] fabricated and tested an experimental unit for production of continuous supply of distilled water using humidification-dehumidification at Kafrelsheikh University, Egypt in the period of May to June. The Fig. 38 shows that hot brackish water through hot water pump sprayed over the cellulose packing in the

Fig. 35. Schematic diagram of process unit [102]. 376

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Fig. 36. Schematic of the tube [131].

Fig. 37. Schematic diagram of experimental setup [19].

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Fig. 38. Diagram of the experimental unit [132].

solar still. It was found that distill output increase by 56% by coupling of evacuated tube with solar still. Black granite gravels with average size of 10, 20 and 30 mm will enhance the distill output by 60, 63 and 67% respectively. The system meant for production of distill water by coupling of solar still with ETC was studied by Sampath Kumar et al. [135] The Fig. 41 presented that gate valves mounted across manifold of ETC collector used for controlling of flow rate of saline water. Heat collected by solar still and ETC used to heat up the saline water and produced distilled water collected in plastic measuring jar through condensing glass cover. Experimental results validated by theoretical results of considered system under natural circulation mode with 0.05 m water depth. Experimental results revealed that maximum 79 °C temperature of water was achieved through active solar still (Coupling of solar still with ETC) which is 12 °C higher than passive solar still. Inner surface temperature of condensing glass is higher than outer surface after getting steady state condition and vice versa during morning hours. Among convective, evaporative, radiative and total heat transfer coefficient, evaporative and total heat transfer coefficient increases with time and rest showed almost constant behavior. It was also observed that maximum value of evaporative heat transfer coefficient is much higher than convective and radiative heat transfer coefficient. Authors concluded that distill output increase with increase of solar intensity. Panchal and Shah [136] studied the coupling of evacuated tubes with inner basin of solar still and utilization of condensation heat of inner basin increased distillate output. Comparative study of pebbles, granite gravel and calcium stones in term of distill output by double basin still with evacuated tubes was done. It was concluded that calcium stones showed 74% higher output than that of black granite gravel and pebbles. Dev and Tiwari [137] performed on same experimental unit as

condenser inlet temperature of air & condenser outlet air temperature. It was also observed that productivity enhances with increase of water mass flow rate but limited to 2.5 L/min. The incorporation of solar still in system enhances the efficiency of system by 242%. The production of high temperature air by heat pipe ETC is deliberated by Xing Li et al. [133]. In this system air is passed through humidifier to increase the humidity level. The Fig. 39 presented that in humidifier water from Tank-1 is sprayed through pump1 and warm sea water was collected in the basin of humidifier which was returned back to spray again on hot air. The hot and humid air from humidifier entered in condenser. In order to collect the condensate in tank-3 from humid air, cold sea water from pump circulated through condenser tubes. It was found that thermal efficiency of ETC enhanced by increasing the mass flow rate of air. At 140 and 33 m3/h air flow rate, thermal efficiencies were 47% and 37% respectively. Authors recommended that for the better thermal efficiency of solar collector air leakage ratio (inlet flow rate/ outlet flow rate) should be as lowest as possible. They also found that to increase the production of fresh water, hot air from solar collector should be humidified at higher temperature. It leads to increase the productivity for same cooling condition. Panchal and Shah [134] worked on experimental unit comprised of double basin solar still and twin glass type solar collector. The Fig. 40 demonstrated that design of double basin solar still in which inner basin is connected with 14 evacuated tubes and outer basin is exposed to sun. Evaporation of brackish water in inner basin is done by supplying of heat collected by ETC. On the other hand distillate output from outer basin is done by evaporation of brackish water through heat collected by black painted absorber plate. Both basins were covered by toughened glass sheet. Black granite gravels of different sizes enhance the productivity by reduction in quantity of brackish water in double basin 378

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Fig. 39. Schematic diagram of solar desalination pilot plant [133].

Fig. 40. Diagram of experimental set up [134]. 379

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Fig. 41. Schematic diagram of solar still [135].

remarkable distill output compared to conventional solar still. Experimental outcomes had good match with theoretical results. Sampath and Senthil [139] designed and fabricated an experimental unit shown in Fig. 42. It comprised of solar still, storage tank, evacuated tube solar collector, overhead water tank and valves. The rear side of solar still connected to inlet pipe for supplying of brackish water. Two additional pipes were also connected at the front and bottom of solar still which are further connected to outlet and inlet of solar collector. In order collect the distillate the trough was fitted on the lower side of solar still. The solar collector shown in Fig connected to overhead tank through non-returning valve V1 for supply of cold water. The T joint between overhead tank and inlet of solar collector helped to supply water through valve V4 from solar still to solar collector by closing the (V1)

discussed in Fig. 37, among hourly variation of EISS in January, February, May and June months at New Delhi, India, maximum ambient (39 °C) and water temperature (90.8 °C) was found in month of June. While maximum inner surface temperature of glass cover (81.2 °C) and yield (0.410 kg/m2) of EISS obtained in May. Yield of considered months was much higher for EISS compared to conventional SS. Because of less rate of heat loss yield amount was higher in summer months. Maximum 29.9% and average 21.3% thermal efficiency was calculated by authors. EISS (630 kg/m2) produced higher yield than yield produced by single slope SS (327 kg/m2). Highest overall thermal efficiency of EISS observed to be 30.1% on 16 May 2008. Annul production cost of distilled water was US $ 0.128 per kg. Panchal and Shah [138] discussed that coupling of vacuum tubes with solar still increased

Fig. 42. Schematic view of experimental setup [139]. 380

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cold water supply from water tank. Another T joint was fixed between header and storage tank with gate valve (V2) for supplying of hot water from solar collector to storage tank. Valves V1 and V2 meant for the operation of solar collector and valve V3 and V4 were used for the operation of solar still. Valves V5, V6 and V7 were used for the cleaning of solar still supply the raw water for passive mode of operation and control the hot water for human use respectively. Authors did comparative study on the productivity of simple solar still and coupled solar still. They found that productivity of coupled solar still was just doubled when compared to simple solar still. Also it was observed that thermal efficiency of passive solar still is greater than active solar still. Authors suggested using of proposed system in rural area where quality of water is very poor. A desalination unit presented by Nabil et al. [140] shown in Fig. 43. In this sea water to be treated (which was coming from the treatment plant) pumped firstly into condenser to condense saturated steam coming from evaporator then into the storage tank. The solar collector (ETC) used to maintain the desired temperature inside the storage tank. The preheated saline water from storage tank entered directly into evaporator where it was further heated by induced air. The hot air was forced inside evaporator by using of blower and heated by an electric heater. A control panel connected to blower used to control the temperate of air at the inlet of evaporator. The vapours from evaporator were then directed via a tube with control valve to water condenser. Thus the water produced after condensation collected at the condenser outlet in a beaker. Authors studied the effect of flow rate of hot air and inlet temperature of water to evaporator on daily distilled water productivity. It was found the increase in flow rate of hot air increased the productivity up to critical flow rate after which productivity started decline. Distilled Water was produced at the rate of 0.014 USD/L. Behnam and Shafii [141] investigated experimental setup shown in Fig. 44 consists of solar collector (heat pipe ETC), dehumidifier and humidifier. In proposed system the air is injected in the humidifier thorough an air stone and discharged saturated air from humidifier entered the dehumidifier through duct. In order to cool the saturated air it passed through cooling coil. Thus the distilled water was produced and collected in the jar at the bottom of dehumidifier. The discharged air of the humidifier is pumped back into the humidifier and cycle was continued. Authors experimentally examined the effect of considered performance parameters such as air flow rate, initial water depth in humidifier and using fluids (oil or water) in the space between absorber tube of evacuated tube and heat pipe. The experimental investigation revealed that productivity of fresh water enhanced and daily efficiency reached to 6.275 kg/day·m2 and 65% respectively by adding of oil between heat pipe and absorber tube. The productivity of fresh water was slightly enhanced by increase in air flow rate. The calculated cost of production of fresh water

through the HDH system was 0.028 $/L. Li et al. [142] designed and tested a multi-effect heat recovery processing desalination system using absorber evacuated tube as heat collector. It was observed that yield of freshwater of unit area reached to 3.03 kg/m2 and 4.23 kg/m2 on cloudy and sunny days respectively. Meanwhile, the comprehensive thermal coefficient and collecting efficiency reached to 1.39 and 41% respectively. Authors ensure that designed system has higher performance and can be used for desalination of seawater without power consumption. Research work by the different researchers in the in the area of desalination and outcomes are given in Table 10. Desalination refers to the separation of minerals and salt from brackish water, this requires high temperature (80–120 °C). The ETC has more advantages than FPC for solar desalination application because it can produce temperature in this range. In FPC, only at noon solar radiations are perpendicular to the collector and thus a proportion of solar radiations striking the collector surface. However in ETC, owing to cylindrical shape of evacuated tube, most of the day sun radiations always perpendicular to the surface of glass. Most of the studies reveals that coupling of ETC with solar still enhance the distill output. Most of the studies reveals that FPC collectors are facing comparatively more choking problem compared to ETC by deposition of salts and minerals. 6.4. Solar cooling Cooling is the major requirement to maintain comfort zone in buildings and long term storage of fruits and vegetables in cold storage. Presently 40% of total energy consumption used for space cooling and heating requirement. Cooling through solar energy is a viable phenomenon to reduce the energy consumption in residential and commercial sector. Solar Absorption cooling systems having potential to reduce energy demand for different cooling applications in residential and industrial sector. Ozgoren et al. [143] studied the variation of different COPs (ideal cooling, actual cooling and actual heating) at constant generator temperature (110 °C) with hourly atmospheric temperature and found that all COPs get maximized during off sun shine hours (morning and evening hours). As COPs is inversely proportional to the ambient temperature. It was further examined that COPs increases with increase of generator temperature. Actual cooling and heating COP vary in the range of 0.243–0.454 and 1.243–1.454 respectively. It was found that condenser capacity, heat transfer rate (in generator & absorber) and Collector surface area increases with increase in cooling capacity at constant generator temperature. Collector efficiency found to be maximum (0.786) at 13:00 at maximum solar intensity (0.719 W/m2). ETC area for a 3.5 kW cooling load capacity estimated to be 19.85 m2 at 12:00 and 35.95 m2 for the region at 16:00. Anand et al. [144] presented NH3-H2O vapour absorption refrigeration system. As shown in Fig. 45, ETC was used as heat source to supply heat

Fig. 43. Schematic diagram of experimental test rig [140]. 381

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Fig. 44. Schematic diagram of experimental test rig [141].

in temperature and continuous generation of vapour of lithium bromide solution in evacuated tube when tube directly absorb the solar energy. The solar coefficient of performance of ETC driven Li-Br vapour absorption refrigeration system was found to be 0.1725 which is near to obtained highest value with current technologies. Heat pipe ETC systems can maintain the temperature up to 120 °C. The temperature required to run solar cooling systems is lie in the range of 90–120 °C. Thus heat pipe ETC systems are suitable to run single stage solar cooling systems.

to ammonia water mixture in generator. Owing to vapour pressure difference, ammonia gets vaporized and entered into condenser where vaporized ammonia gets condensed by rejection of heat to cooling water flowing through condenser. On the other hand weak solution left in generator, entered in absorber through heat exchanger and pressure reducing valve. Aqueous ammonia from condenser is moved to evaporator through throttling valve for producing refrigeration effect in evaporator chamber. Vaporized ammonia from evaporator entered to absorber and mixed with weak solution. Then strong solution pumped to generator through solution heat exchanger. The above process continued and produced cooling effect in evaporator. Exergy efficiency, COPHEATING and COPCOOLING of system increases whereas effectiveness of solution heat exchanger decreases with enhancement in generator temperature. Corresponding to 0.9 kW/m2 solar intensity, 90 °C hot water temperature and 431.7 m2 collector area, COPHEATING and COPCOOLING values lies in the range of 0.012–0.498 and 1.012–1.498 respectively. Exergy losses were maximum and minimum in generator and condenser respectively. COPCOOLING and exergy efficiency increases with increase of evaporator temperature whereas COPHEATING decreases with increase of condenser temperature. Assilzadeh [100] did simulation of Li-Br vapour absorption unit driven by ETC for Malaysia and other similar tropical regions. They observed that cooling demand is increasing with increases of intensity of solar radiation. As per according to metrological condition of Malaysia 35 m2 area of ETC is required for running of 3.5 kW system. Bellos et al. [145] tested Li-Br sorption refrigeration with four different solar collector (parabolic trough collectors, flat plate solar collector, ETC, compound parabolic collectors) under same condition. The capacity of sorption machine is equal to 100 kW (cooling demand of Athens (Greece)) during summer season. Financial comparison among four selected solar collector showed that ETC is most optimum one while parabolic trough technology is most exergetic. Nkwetta and Smyth [146] reported that concentrator augmented heat pipe ETC is an attractive solution to be used in the temperature range of 70–120 °C for powering the solar air-conditioning system. The results depicted that heat losses in heat pipe ETC are higher compared to concentrator augmented heat pipe ETC. Chen et al. [147] observed the rapid increase

6.5. Photovoltaic thermal system with heat pipes Presently, PV/Thermal system has potential to gain electricity and heat in a single device. Most of the researchers are working to reduce PV temperature at certain level for improving the performance of solar cell. Use of the heat pipe with PV module is also a new research work which having improving potential to convert a hybrid system (electrical + thermal) in a single device. In this section, work is focus to provide the use of heat pipe based systems for solar PV module with potential designs. Gang et al. [148] performed work on heat pipe PV/T system for simultaneous production of hot water and electricity shown in Fig. 46. It consists of aluminum plate that act s as base panel and evaporator of the heat pipe was brazed on back of the aluminum plate. The condenser sections of heat pipe inserted into manifold. The single crystalline silicon PV cells were laminated in the gaps between heat pipes. A tempered glass sheet with low iron content used as upper glaze to prevent thermal losses and entry of dust particles. An insulation layer was also fixed on back of aluminum plate to prevent thermal losses. Experimental results revealed that output of PV/T collector in the terms of electrical and thermal energy which depends on the intensity of solar radiation. The proposed system was used in three different climate conditions of china (Hong Kong, Lhasa and Beijing) without assistant of auxiliary heating equipment. The proposed system was also annually studied with four different hot-water load per unit collecting area (103.2, 90.3, 77.4 and 64.5 kg/m2). Wu et al. [149] proposed a building-integrated heat pipe PV/T system. In this proposed system, the 382

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Table 10 Type of investigation and outcomes in desalination system. Author

Type of investigation

Type of ETC

Outcomes

Mosleh et al. [18]

Experimental

HP-ETC

Omara et al. [130]

Experimental

HP-ETC

Shafii et al. [119]

Experimental

WHP-ETC

Shafii et al. [131]

Experimental

WHP-ETC

Kumar et al. [19]

Experimental

WHP-ETC

Sharshir et al. [132]

Experimental

WHP-ETC

– Oil instead of aluminum foil between heat pipe and evacuated tube increases efficiency and rate of production of pure water – Solar still Vertical walls and base covered with developed wick increase the daily average productivity. – Distillate production of SLPW, SLLW, SLSW, DLLW and DLSW solar stills were 90, 98, 114, 107 and 104% respectively. – Enhancement in evaporation rate to increase daily water yield, maximum hourly water productivity and hourly efficiency by 14, 12.61 and 11.76% respectively. – Performance parameters also increased by 27% percentage with full glass tubes compared to half-full case. – It showed that 80 and 20% of the total filled volume of evacuated tubes give highest and lowest performance (yield and efficiency) respectively. – Steel wool between heat pipe and absorber tube increase the conductive heat transfer rate and consequently enhance yield and efficiency – The 35° inclination angle is the optimum angle for experiment. – The total productivity decreases with increase of water depth in solar still – Higher mass flow rate leads to higher power consumption but increase in power consumption is compensated by increase in thermal efficiency. – Performance parameters are higher in summer months compared to winter months – Annual productivity per m2 of solar radiation collector area of EISS forced mode is much higher than that of hybrid solar still. – Productivity enhances with increase of water mass flow rate – The incorporation of solar still with ETC enhances the efficiency of system by 242%.

Xing Li et al. [133]

Experimental

WHP-ETC

Panchal and Shah [58]

Experimental

WHP-ETC

Kumar et al. [59]

Theoretical and Experimental

WHP-ETC

Panchal and Shah [134]

Experimental

WHP-ETC

Dev and Tiwari [137]

Experimental

WHP-ETC

Sampath and Senthil [139] Nabil et al. [140]

Experimental

WHP-ETC

Theoretical and Experimental Experimental

WHP-ETC

Behnam and Shafii [141]

HP-ETC

– Thermal efficiency of ETC enhanced by increasing the mass flow rate of air. – For better thermal efficiency of solar collector air leakage ratio should be as lowest as possible. – Black granite gravels of different sizes enhance the productivity by reduction in quantity of brackish water in double basin solar still. – Distill output increase by 56% by coupling of evacuated tube with solar still. – Maximum 79 °C temperature of water was achieved through Coupling of solar still with ETC collector – Evaporative and total heat transfer coefficient increases with time – Maximum value of evaporative heat transfer coefficient is much higher than convective and radiative heat transfer coefficient. – Distill output increased by 56% by coupling of evacuated tubes with solar still. – Black granite gravels of different sizes enhance the distill output. – Yield of distill output was much higher for EISS compared to conventional SS. – Yield amount was higher in summer months. EISS produced higher yield than yield produced by single slope SS – Productivity of coupled solar still was just doubled when compared to simple solar still. – Thermal efficiency of passive solar still is greater than active solar still. – Increase in flow rate of hot air increased the productivity up to critical flow rate after which productivity started decline. – Productivity of fresh water enhanced and daily efficiency reached to 6.275 kg/day·m2 and 65% respectively by adding of oil between heat pipe and absorber tube. – The productivity of fresh water was slightly enhanced by increase in air flow rate

thermal performance of heat pipe PV/T system. Authors have also determined that unlike wickless heat pipe PV/T system, wire meshed heat pipe PV/T system are not sensitive to inclination angle. The optimum results of PV/T system with two types of heat pipe were obtained at an inclination angle of 40°. The maximum thermal efficiency of wire meshed heat pipe PV/T system and wickless heat pipe PV/T system were 51.8% and 52.8% respectively, at an inclination angle of 40°. Authors extended their conclusion pertaining to applications of selected heat pipes in solar collectors that wire-meshed heat pipe PV/T system and wickless heat pipe PV/T system can be used at altitude lower and higher than 20° respectively. Zhang et al. [152] demonstrated heat pipe photovoltaic system for simultaneous production of electricity as well as thermal energy shown in Fig. 47, the hot water used for building heating through radiant floor heating method and electric energy was stored in batteries or merged in state grid. The hot water produced through heat pipe PV/T stored in hot storage water tank (shown by dashed box (a)) when water reached to temperature of 45 °C. The electrical energy supplied to batteries or merged in state grid (shown by dashed box (b)). In present study tilt angle and water tank volume of heat pipe PV/T system were optimized and considered as key parameters. The same experimental unit also simulated using TRNSYS and theoretical results validated with experimental outcomes. Authors found that electric energy and hot water generation initially decreased

PV panel, glass cover and glass seal formed a closed space. Solar modules were fixed on PV panel and evaporator of heat pipe attached to the back of PV panel through highly thermal conductive material. The evaporator sections were also coated with insulated material to minimize the thermal losses. The outer surface of heat pipe condenser is also equipped with fins for enhancement of heat transfer rate. The fluid is to be heated flow through fluid channel in which condenser section of heat pipe were fixed. Experimental outcomes revealed that exergy, thermal and electrical efficiency of proposed system are 10.26%, 63.65% and 8.45% respectively, which is obtainable under considered operating conditions. Authors have also found that the variation of operating temperature of solar cell on absorber plate is less than 2.5 °C. Gang et al. [150] did novel study on heat pipe PV/T for simultaneous production of thermal and electrical energy. The proposed system can be used in cold regions due to the anti-freezing property of fluid in heat pipe. The average electrical and thermal gain were found to be 62.3 and 279.9 W/m2 respectively while electrical and thermal efficiencies were 9.4% and 41.9% respectively. In addition to this authors also evaluated the average exergy efficiency (6.8%) of system. Hu et al. [151] presented an experimental study of photovoltaic collector was carried out with two types of heat pipe (wickless and wire meshed) in enthalpy difference laboratory with a solar simulator. The proposed study gives the information regarding effect of change in inclination angle on 383

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Fig. 45. Schematic line diagram of an ETC based ammonia-water vapour absorption refrigeration system [144].

compared the experimental results of building integrated heat pipe PV/ T with results obtained through simulation model. An experimental test rig was installed on the roof of the building in Dongguan city, Guangdong province of China.

and then enhanced with volume of water circulating tank. The maximum integrated efficiency reached to 67.5% while volume of water circulating tank was 80 L. The heat collection and mean annual efficiency of system was 2328.16 MJ/year and 34.37%. Hui et al. [153]

Fig. 46. Schematic view of heat pipe PV/T solar collector [148]. 384

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Fig. 47. Diagram of system principle [152].

Whereas COPPV/T decreases when supply temperature and ambient temperature in condenser & heat pipe pitch decreases. The absorptivity of PV backboard had little effect on the COPPV/T of heat pipe PV/T heat pump system. Available literature for the use of heat pipe system to reduce PV module temperature and its outcomes are tabulated in Table 11. Heat Pipe can be easily integrated with most types of solar collectors. The application of heat pipes to solar collectors has several advantages. They can transfer heat without losses and increase the efficiency of collector. Photovoltaic module (PV) having efficiency in the range of 16–18% depends on the availability of solar radiations. The maximum efficiency of PV module can be achieved when its surface temperature is maintained at 25 °C. However, increase in the module temperature of 1 °C cause decrease of 0.5% of the module efficiency. In most of the hot climatic countries temperature of the module lies in the range of 50–55 °C. Thermal collector with PV module is a good solution to increase the efficiency of the module and extract heat for water/air heating applications. As per latest research work, it can be seen that heat pipes having potential to extract heat from PV module efficiently as well as maintain its electrical efficiency in hot climatic zone.

The outcomes revealed that heat transmission over a year in Hong Kong through HP-PVT wall can be reduced by 23.2% compared to common brick building façade. Authors found that thermal and electrical efficiency of proposed system were 34.7% and 9.8% respectively. It is recommended to use BiHP-PVT system due to its higher performance and climate adaptability over Bi-PVT/w system. Chen et al. [154] proposed a heat pipe PV/T collector combined with heat pump shown in Fig. 48. The hot water is produced through HP-PV/T system circulated through evaporator of heat pump in closed cycle and electric energy was stored in batteries. A temperature controlled water bath where hot water could be cooled and cold water could be heated also connected to condenser of heat pump. The pressure and temperature sensors were also installed at desired locations of system. They found that experimental results well agreed with simulation results with a relative error within ± 15%. It was observed that the increase of ambient temperature, solar radiation and PV back board absorptivity results in increase of COP While increase of water supply temperature to condenser, heat pipe pitch and PV packing factor leads to decreased of COPth. Moreover increase of packing factor and solar radiations lead to increase of advanced COP (COPPV/T) based on electrical and thermal.

Fig. 48. Heat pipe PV/T heat pump system [154]. 385

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Table 11 Latest research work in the area of heat pipe PV/T system. Author

Type of investigation

Outcomes

Gang et al. [148] Wu et al. [149]

Experimental Experimental

Gang et al. [150]

Experimental

Hu et al. [151]

Experimental

Zhang et al. [152]

Experimental and theoretical

Hui et al. [153]

Experimental and theoretical

Chen et al. [154]

Experimental and theoretical

– Output of PV/T collector in the terms of electrical and thermal energy depends on the intensity of solar radiation. – Exergy, thermal and electrical efficiency of proposed system are 10.26%, 63.65% and 8.45% respectively – Variation of operating temperature of solar cell on absorber plate is less than 2.5 °C. – Due to the anti-freezing property of fluid in heat pipe system can be used in cold regions. – The average electrical and thermal gain were found to be 62.3 and 279.9 W/m2 respectively while electrical and thermal efficiencies were 9.4% and 41.9% respectively. – Unlike wickless heat pipe PV/T system, wire meshed heat pipe PV/T system are not sensitive to inclination angle. – The optimum results of PV/T system with two types of heat pipe were obtained at an inclination angle of 40°. – Wire-meshed and wickless heat pipe PV/T system can be used at altitude lower and higher than 20° respectively. – Electric energy and hot water generation initially decreased and then enhanced with volume of water circulating tank. – The maximum integrated efficiency reached to 67.5% while volume of water circulating tank was 80 L. – The heat collection and mean annual efficiency of system was 2328.16 MJ/year and 34.37%. – Heat transmission over a year in Hong Kong through HP-PVT wall can be reduced by 23.2% compared to common brick building façade. – Thermal and electrical efficiency of proposed system were 34.7% and 9.8% respectively. – Recommended to use BiHP-PVT system due to its higher performance and climate adaptability over Bi-PVT/w system. – Increase of ambient temperature, solar radiation and PV back board absorptivity results in increase of COP – Increase of water supply temperature to condenser, heat pipe pitch and PV packing factor leads to decrease of thermal COP. – Increase of packing factor and solar radiations lead to increase of electrical and thermal COP. – The absorptivity of PV backboard had little effect on the COP of heat pipe.

450 400

•

350 300 250 200 150

Finally, it can be concluded that extensive research work is being carried out globally on different aspects of ETCs for low/medium temperature applications and this comprehensive review will help researchers and practice Engineers to find the way forward and get an insight view of work done.

100 50 0 2009

2010

2011

2012

Water heating

2013

2014

Air heating

2015

2016

2017

be more efficient as compared to without PCM and can be utilized even in the absence of solar energy due to thermal energy storage. The performance of evacuated tube collector as compared to flat plate collector in the temperature range of 50–100 °C is very high even though its long term reliability is low, but more economical due to the use of low cost sputtering technology in evacuated tubes.

2018

Desalination

7.2. Future recommendations

Fig. 49. Number of articles published for ETC applications.

It is discovered that evacuated technology has got excellent prospects to be used as green technology and should be addressed meticulously. Some of the recommendations for future studies are given as below:

7. Conclusions, future recommendations and publication trends This section presents the main concussions obtained from the review, the way forward and current publication trends.

• The major drawback of evacuated tube collector technology is that

7.1. Conclusions Evacuated tube collector is a simple device which collects the solar energy and converts it into useful applicable forms. A manifest classification of evacuated tube collector, their types, applications and recent developments are summarized in this paper. The main conclusions derived from the study are given as below:

•

• The influencing applications of evacuated tube collector are found •

•

to be water heating, air heating and desalination. The compatibility of heat pipes with other solar technologies such as PV/T, flat plate collector makes it more potential technology in terms of higher outlet temperature and higher efficiencies. The efficiency and temperature output have been reported to be higher by using heat pipes in evacuated tube collectors. In this comprehensive review, it’s been manifested that the heat pipe evacuated tube collector has several advantages such as flexibility in operation for different applications and effective in transferring heat even when temperature difference is trifling. Photovoltaic integrated heat pipes have also been found to be economical solution for simultaneous production of electricity and hot water. It has been found that evacuated tube collector with PCMs proves to

•

•

386

evacuated tubes are fragile in nature and that can be damaged very easily during transportation and installation process. To overcome this problem research should be carried out to improve tubes materials, its hardness for better reliability without compromising the performance. After the detailed review, it is found that in the most of research work water is used as working fluid and very few studies were conducted with nanofluids in evacuated tube collector. Use of nanofluids as working fluid can be more efficient due to its excellent thermal characteristics. Thus it is recommended that more research work should be focus on the incorporation of nanofluids in evacuated tube collector as a working fluid. Heat pipe evacuated tube collector is a new technology having high efficiency in comparison of direct flow evacuated tube collector. Nanofluids in heat pipes will further enhance their efficiency. The use of heat pipes evacuated tube collector with reflector can also extend the temperature range which can be used it from medium to high temperature applications. Thermal energy storage is a necessary requirement for each solar energy application due to intermittence nature of solar energy. Presently extensive research work has been done for PCM’s development in the temperature range of 50–90 °C. These materials

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•

•

• • •

having high latent heat which can be used with evacuated tube collector system to store heat during day time and provide hot water/air in night time for commercial and residential applications. Presently, in heat pipe evacuated tube collector air between absorber tube and heat pipe act as a heat transfer media to transfer heat from absorber tube to heat pipe. Since air having low thermal conductivity comparative to liquid fluids/phase change materials so fluids having high thermal conductivity should be filled in evacuated tubes to enhance the heat transfer for better performance. The regions where water having high mineral content, the evacuated U-tube solar collector is not suggested to use, as continuous flow of water added with minerals and salts through U-shaped copper tubes inserted in evacuated tubes results in its deposition in copper tubes causing choking problem. In such areas it is recommended to use heat pipe evacuated tube collector to avoid such problem. The heat pipe evacuated tube collectors are recommended to install in industries like food, paper, textile, beverages, chemical, distillery etc. where hot water requirement in the temperature range of 60–90 °C. The main disadvantage of heat pipe evacuated tube collector is small surface area of condenser section of heat pipe for heat transfer, so further research work is required to enhance the efficiency of heat pipe evacuated tube collector. The surface area of the condenser section can be increased with new fins design. The life of coating on absorber tube of evacuated tube is 5–6 years, further research should be done to increase the stability of coating.

[9] Azad E. Experimental analysis of thermal performance of solar collectors with different numbers of heat pipes versus a flow through solar collector. Renew Sustain Energy Rev 2018;82:4320–5. [10] Abokersh Mohamed Hany, El-Morsi Mohamed, Sharaf Osama, Abdelrahman Wael. On-demand operation of a compact solar water heater based on U-pipe evacuated tube solar collector combined with phase change material. Sol Energy 2017;155:1130–47. [11] Ayompe Lacour, Duffy Aidan, McCormack Sarah, Mc Keever Mick, Conlon Michael. Comparative field performance study of flat plate and heat pipe evacuated tube collectors (ETCs) for domestic water heating systems in a temperate climate. Energy 2011;36:3370–8. [12] Reay DA, Kew PA. Heat pipes. Elsevier Science Ltd; 2013. [13] Ismail KAR, Abogderah MM. Performance of a heat pipe solar collector. Solar Energy Eng 1998;120:51–9. [14] Han J, Tian R, Yan S. Comparative analysis of the instantaneous efficiency about two types of solar collector. Energy Eng 2009;2:25–7. [15] Li Zhiyong, Chen Chao, Luo Hailiang, Zhang Ye, Xue Yaning. All-glass vacuum tube collector heat transfer model used in forced-circulation solar water heating system. Sol Energy 2010;84:1413–21. [16] Jack Steffen, Katenbrink Nils, Schubert Felix. Evaluation methods for heat pipes in solar thermal collectors-test equipment and first results. In: ISES solar world congress; 2011. [17] Daghigh Roonak, Shafieian Abdellah. An experimental study of a heat pipe evacuated tube solar dryer with heat recovery system. Renew Energy 2016;96:872–80. [18] Jafari Mosleh H, Jahangiri Mamouri S, Shafii MB, Hakim Sima A. A new desalination system using a combination of heat pipe, evacuated tube and parabolic trough collector. Energy Convers Manage 2015;99:141–50. [19] Kumar Shiv, Dubey Aseem, Tiwari GN. A solar still augmented with an evacuated tube collector in forced mode. Desalination 2014;347:15–24. [20] Bracamonte Johane, Parada José, Dimas Jesús, Baritto Miguel. Effect of the collector tilt angle on thermal efficiency and stratification of passive water in glass evacuated tube solar water heater. Appl Energy 2015;155:648–59. [21] Pandey AK, Tyagi VV, Rahim NA, Kaushik SC, Tyagi SK. Thermal performance evaluation of direct flow solar water heating system using exergetic approach. Therm Anal Calorim 2015;121:1365–73. [22] Zhang Ming, Miao Lei, Kang Yi Pu, Tanemura Sakae, Fisher Craig AJ, Xu Gang, et al. Efficient low-cost solar thermoelectric cogenerators comprising evacuated tubular solar collectors and thermoelectric modules. Appl Energy 2013;109:51–9. [23] Felinski Piotr, Sekret Robert. Effect of a low cost parabolic reflector on the charging efficiency of an evacuated tube collector/storage system with a PCM. Sol Energy 2017;144:758–66. [24] Kabeel AE, Khairat Dawood Mohamed M, Shehata Ali I. Augmentation of thermal efficiency of the glass evacuated solar tube collector with coaxial heat pipe with different refrigerants and filling ratio. Energy Convers Manage 2017;138:286–98. [25] Sokhansefat Tahmineh, Kasaeian Alibakhsh, Rahmani Kiana, Heidari Ameneh Haji, Aghakhani Faezeh, Mahian Omid. Thermoeconomic and environmental analysis of solar flat plate and evacuated tube collectors in cold climatic conditions. Renew Energy 2018;115:501–8. [26] Mahbubul IM, Kamyar A, Saidur R, Amalina MA. Migration properties of TiO2 nanoparticles during the pool boiling of nano refrigerants. Ind Eng Chem Res 2013;52:6032–8. [27] Suman S, Khan MK, Pathak M. Performance enhancement of solar collectors: a review. Renew Sustain Energy Rev 2015;49:192–210. [28] Sabiha MA, Saidur R, Mekhilef S, Mahian O. Progress and latest developments of evacuated tube solar collectors. Renew Sustain Energy Rev 2015;51:1038–54. [29] Saidur R, Leong KY, Mohammad HA. A review on applications and challenges of nanofluids. Renew Sustain Energy Rev 2011;15:1646–68. [30] Yu W, France DM, Routbort JL, Choi SUS. Review and comparison of nanofluid thermal conductivity and heat transfer enhancements. Heat Trans Eng 2008;29:432–60. [31] Wang P-Y, Chen X-J, Liu Z-H, Liu Y-P. Application of nanofluid in an inclined mesh wicked heat pipes. Thermochim Acta 2012;539:1000–108. [32] Sheikholeslami M, Ganji DD. Numerical approach for magnetic nanofluid flow in a porous cavity using CuO nanoparticles. Mater Des 2017;120:382–93. [33] Hussein AK. Applications of nanotechnology to improve the performance of solar collectors recent advances and overview. Renew Sustain Energy Rev 2016;62:767–92. [34] Tyagi H, Phelan P, Prasher R. Predicted efficiency of a low-temperature nanofluidbased direct absorption solar collector. J Sol Energy Eng 2009;131(4):041004. [35] Otanicar TP, Phelan PE, Prasher RS, Rosengarten G, Taylor RA. Nanofluid based direct absorption solar collector. J Renew Sustain Energy 2010;2(3):033102. [36] Yousefi T, Veysi F, Shojaeizadeh E, Zinadini S. An experimental investigation on the effect of Al2O3-H2O nanofluid on the efficiency of flat-plate solar collectors. Renew Energy 2012;39(1):293–8. [37] Faizal M, Saidur R, Mekhilef S, Hepbasli A, Mahbubul IM. Energy, economic, and environmental analysis of a flat-plate solar collector operated with SiO2 nanofluid. Clean Technol Environ Policy 2015;17(6):1457–73. [38] Mahbubul IM, Khan Mohammed Mumtaz A, Ibrahim Nasiru I, Muhammad Ali Hafiz, Al-Sulaiman Fahad A, Saidur R. Carbon nanotube nanofluid in enhancing the efficiency of evacuated tube solar collector. Renew Energy 2018;121:36–44. [39] Sabiha MA, Saidur R, Hassani S, Said Z, Mekhilef S. Energy performance of an evacuated tube solar collector using single walled carbon nanotubes nanofluids. Energy Convers Manage 2015;105:1377–88. [40] Kalogirou SA. Solar thermal collectors and applications. Prog Energy Combust Sci 2004;30:231–95. [41] Lu L, Liu Z-H, Xiao H-S. Thermal performance of an open thermosyphon using

7.3. Recent publication trends The number of articles published on ETC technology and its application between 2009 and 2018 is shown in Fig. 49. It included all published articles on ETC technology and its various important applications such as air heating, water heating and desalination. The Fig. 49 shows that research on this technology took the momentum in 2009 with continuous increasing up to 2018. From the year 2013 onwards the significant growth is observed in this area which made this topic a thrust area of research. Water heating through ETC is more researches followed by air heating while desalination is less focused. Acknowledgement Mr. Kapil Chopra is grateful to Shri Mata Vaishno Devi University, Katra (J&K) for the Professional Development Assistance Grant for this work. One of the author, Dr. V. V. Tyagi is also highly thankful to University Grant Commission (Govt. of India) for providing startup research grant at Shri Mata Vaishno Devi University, Katra (J&K). References [1] Zhang HF. Thermal utilization principles and computer simulation of solar energy. 2nd ed. Xi An: Northwestern Polytechnic University Press; 2003. [2] Esen Mehmet. Thermal performance of a solar cooker integrated vacuum-tube collector with heat pipes containing different refrigerants. Sol Energy 2004;76:751–7. [3] Kim Yong Sin, Balkoski Kevin, Jiang Lun, Winston Roland. Efficient stationary solar thermal collector systems operating at a medium-temperature range. Appl Energy 2013;111:1071–9. [4] Weiss W, Mauthner M. A solar heat worldwide. A markets contribution energy supply; 2010. [5] Hossain MS, Saidur R, Fayaz H, Rahim NA, Islam MR, Ahameda JU, et al. Review on solar water heater collector and thermal energy performance of circulating pipe. Renew Sustain Energy Rev 2011;15:3801–12. [6] Kim Yong, Seo Taebeom. Thermal performances comparisons of the glass evacuated tube solar collectors with shapes of absorber tube. Renew Energy 2007;32:772–95. [7] Jafarkazemia Farzad, Abdi Hossein. Evacuated tube solar heat pipe collector model and associated tests. Renew Sustain Energy. 2012;4:1–13. [8] Siva Kumar S, Mohan Kumar K, Sanjeev Kumar SR. Design of evacuated tube solar collector with heat pipe materials today: proceedings, vol. 4; 2017. p. 12641–6.

387

Applied Energy 228 (2018) 351–389

K. Chopra et al.

[42]

[43] [44]

[45]

[46] [47]

[48]

[49]

[50]

[51] [52]

[53]

[54]

[55] [56] [57] [58]

[59] [60] [61]

[62] [63] [64] [65]

[66]

[67]

[68] [69]

[70]

[71]

[72]

[73]

[74] Pandey AK, Hossain MS, Tyagi VV, Rahim Nasrudin Abd, Selvaraj Jeyraj AL, Sari Ahmet. Novel approaches and recent developments on potential applications of phase change materials in solar energy. Renew Sustain Energy Rev 2018;82:281–323. [75] Riffat S, Jiang L, Zhu J, Gan G. Experimental investigation of energy storage for an evacuated solar collector. Int J Low Carbon Technol 2006;1:139–48. [76] Li Bin, Zhai Xiaoqiang, Cheng Xiwen. Experimental and numerical investigation of a solar collector/storage system with composite phase change materials. Sol Energy 2018;164:65–76. [77] Naghavi MS, Ong KS, Badruddin IA, Mehrali M, Silakhori M, Metselaar HSC. Theoretical model of an evacuated tube heat pipe solar collector integrated with phase change material. Energy 2015;91:911–24. [78] Sharma Atul, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sustain Energy Rev 2009;13:318–45. [79] Abd-Elhady MS, Nasreldin M, Elsheikh MN. Improving the performance of evacuated tube heat pipe collectors using oil and foamed metals. Ain Shams Eng J; 2017 [in press]. [80] Kong Weiqiang, Wang Zhifeng, Fan Jianhua, Perers Bengt, Chen Ziqian, Furbo Simon, et al. Investigation of thermal performance of flat plate and evacuated tubular solar collectors according to a new dynamic test method. Energy Proc 2012;30:152–61. [81] Wang Z, Duan Z, Zhao X, Chen M. Dynamic performance of a facade-based solar loop heat pipe water heating system. Sol Energy 2012;86:1632–47. [82] Daghigh Roonak, Shafieian Abdellah. Theoretical and experimental analysis of thermal performance of a solar water heating system with evacuated tube heat pipe collector. Appl Therm Eng 2016;103:1219–27. [83] Azad E. Theoretical and experimental investigation of heat pipe solar collector. Exp Therm Fluid Sci 2008;32:1666–72. [84] Dunn PD, Reay DA. Heat pipes. Elsevier Science Ltd.; 1994. [85] Iv Lienhard JH, Lienhard JHV. DOE fundamentals handbook: thermodynamics, heat transfer, and fluid flow. Washington DC: US Department of Energy; 2008. [86] Siva Kumar S, Mohan Kumar K, Sanjeev Kumar SR. Design of evacuated tube solar collector with heat pipe. Mater Today: Proc 2017;4:12641–6. [87] Souayfane Farah, Biwole Pascal Henry, Fardoun Farouk. Thermal behavior of a translucent superinsulated latent heat energy storage wall in summertime. Appl Energy 2018;217:390–408. [88] Yunus Cengel A. Heat transfer-a practical approach. McGraw-Hill; 2007. [89] Incropera Frank P, Dewitt David P. Fundamentals of heat and mass transfer. John Wiley; 2011. [90] Hottel HC, Willier A, Evaluation of flat plate solar collector performance. In: Transactions of the conference on solar energy. Thermal processes, vol. 2; 1955. p. 74–104. [91] Kays MW, London AC. Compact heat exchanger design. 3rd ed. New York: McGraw-Hill; 1984. [92] Faghri Amir. Heat pipe science and technology. Taylor and Francis Group; 1995. [93] Incropera F, DeWitt D. Introduction to heat transfer. New York: John Wiley; 2002. [94] Chi SW. Heat pipe theory and practice. New York: McGraw-Hill; 1976. [95] Javed Saqib, Spitler Jeffrey. Accuracy of borehole thermal resistance calculation methods for grouted single U-tube ground heat exchangers. Appl Energy 2017;187:790–806. [96] Tong Y, Kim HM, Cho HH. Theoretical investigation of the thermal performance of evacuated heat pipe solar collector with optimum tilt angle under various operating conditions. J Mech Sci Technol 2016;30:903–13. [97] Chen MM. An analytical study of laminar film condensation. J Heat Trans ASME 1961;83(1):48–60. [98] Beekley DC, Mather Jr GR. Analysis and experimental test of high performance evacuated tubular collector. DOE/NASA, CR-/50874; 1978. [99] Ghosh Aritra, Norton Brian, Duffy Aidan. Measured overall heat transfer coefficient of a suspended particle device switchable glazing. Appl Energy 2015;159:362–9. [100] Assilzadeh F, Kalogirou SA, Alia Y, Sopian K. Simulation and optimization of a LiBr solar absorption cooling system with evacuated tube collectors. Renew Energy 2005;30:1143–59. [101] Duffie John A, Beckman William A. Solar engineering of thermal processes. John Wiley; 2013. [102] Shafii Mohammad Behshad, Shahmohamadi Mojtaba, Faegh Meysam, Sadrhosseini Hani. Examination of a novel solar still equipped with evacuated tube collectors and thermoelectric modules. Desalination 2016;382:21–7. [103] Sheikholeslami M, Ganji DD. Heat transfer enhancement in an air to water heat exchanger with discontinuous helical turbulators; experimental and numerical studies. Energy 2016;116:341–52. [104] Sheikholeslami Mohsen, Ganji Davood Domiri. Turbulent heat transfer enhancement in an air-to-water heat exchanger. J Process Mech Eng Proc IMechE Part E 2016:1–14. [105] Sheikholeslami M, Ganji DD. Heat transfer enhancement in an air to water heat exchanger with discontinuous helical turbulators; experimental and numerical studies. Energy 2016;116:341–52. [106] BIPM, IEC, IFCC, ILAC, ISO, IUPAC, IUPAP, OIML. Evaluation of measurement data—Guide to the expression of uncertainty in measurement; 2008. [107] Tang R, Li Z, Zhang H, Lan Q. Assessment of uncertainty in mean heat loss coefficient of all glass evacuated solar collector tube testing. Energy Convers Manage 2006;47:60–7. [108] Ersoz Mustafa Ali. Effects of different working fluid use on the energy and exergy performance for evacuated tube solar collector with thermosyphon heat pipe. Renew Energy 2016;96:244–56.

nanofluids for high-temperature evacuated tubular solar collectors: part 1: indoor experiment. Sol Energy 2011;85:379–87. Tong YJ, Kim J, Cho H. Effects of thermal performance of enclosed-type evacuated U-tube solar collector with multi-walled carbon nanotube/water nanofluid. Renew Energy 2015;83:463–73. Kim H, Ham J, Park C, Cho H. Theoretical investigation of the efficiency of a Utube solar collector using various nanofluids. Energy 2016;94:497–507. Ozsoy Ahmet, Corumlu Vahit. Thermal performance of a thermosyphon heat pipe evacuated tube solar collector using silver-water nanofluid for commercial applications. Renew Energy 2018;122:26–34. Hussein Ahmed Kadhim, Lib Dong, Kolsic Lioua, Katad Sanatana, Sahooe Brundaban. A review of nano fluid role to improve the performance of the heat pipe solar collectors. Energy Proc 2017;109:417–24. Sharafeldin MA, Grof Gyula. Evacuated tube solar collector performance using CeO2/water nanofluid. J Clean Prod 2018;185:347–56. Ghaderian Javad, Sidik Nor Azwadi Che. An experimental investigation on the effect of Al2O3/distilled water nanofluid on the energy efficiency of evacuated tube solar collector. J Heat Mass Transfer 2017;108:972–87. Sheikholeslami M, Ganji DD. Influence of electric field on Fe3O4-water nanofluid radiative and convective heat transfer in a permeable enclosure. Mol Liq 2018;250:404–12. Hosseini SR, Sheikholeslami M, Ghasemian M, Ganji DD. Nanofluid heat transfer analysis in a microchannel heat sink (MCHS) under the effect of magnetic field by means of KKL model. Powder Technol 2018;324:36–47. Sheikholeslami M, Jafaryar M, Bateni K, Ganji DD. Two phase modeling of nanofluid flow in existence of melting heat transfer by means of HAM. Indian J Phys 2018;92:205–14. Sheikholeslami M, Nimafar M, Ganji DD. Analytical approach for the effect of melting heat transfer on nanofluid heat transfer. Eur Phys J Plus 2017;132:385–97. Sheikholeslami M, Ganji Davood Domairry, Moradi R. Forced convection in existence of Lorentz forces in a porous cavity with hot circular obstacle using nanofluid via Lattice Boltzmann method. J Mol Liq 2017;246:103–11. Sheikholeslami M, Ganji DD. Numerical analysis of nanofluid transportation in porous media under the influence of external magnetic source. J Mol Liq 2017;233:499–507. Sheikholeslami M, Ganji DD, Moradib R. Heat transfer of Fe3O4–water nanofluid in a permeable medium with thermal radiation in existence of constant heat flux. Chem Eng Sci 2017;174:326–36. Chang Keh-Chin, Lee T song-Sheng, Lin Wei-Min, Chung Kung-Ming. Outlook for solar water heaters in Taiwan. Energy Policy 2008;36:66–72. Morrison G, Budihardjo I, Behnia M. Measurement and simulation of flow rate in a water-in-glass evacuated tube solar water heater. Sol Energy 2005;78:257–67. Mangal D, Lamba DK, Gupta T, Jhamb K. Acknowledgement of evacuated tube solar water heater over flat plate solar water heater. Int J Eng 2010;4:279–84. Tang R, Yang Y, Gao W. Comparative studies on thermal performance of water inglass evacuated tube solar water heaters with different collector tilt-angles. Sol Energy 2011;85:1381–90. Arefin M, Hasan M, Azad A. Characteristics and cost analysis of an automatic solar hot water system in Bangladesh. Int Proc Chem Biol Environ Eng 2011;6:179–83. Shukla R, Sumathy K, Erickson P, Gong J. Recent advances in the solar water heating systems: a review. Renew Sustain Energy Rev 2013;19:173–90. Solar water heaters in India: market assessment studies and surveys for different sectors and demand segments submitted to Project Management Unit Global Solar Water Heating Project Ministry of New and Renewable Energy; 2010. Hepbasli A, Yildiz K. A review of heat pump water heating systems. Renew Sustain Energy Rev 2009;13:1211–29. Kalogirou SA. Solar thermal collectors and applications. Prog Energy Combust Sci 2004;30:231–95. Alghoul MA, Sulaiman MY, Azmi BZ, Wahab MA. Review of materials for solar thermal collectors. Anti-Corros Methods Mater 2005;52:199–206. Hamed M, Fellah A, Ben Brahim A. Parametric sensitivity studies on the performance of a flat plate solar collector in transient behavior. Energy Convers Manage 2014;78:938–47. Allouhi A, Jamil A, Kousksou T, El Rhafiki T, Mourad Y, Zeraouli Y. Solar domestic heating water systems in Morocco: an energy analysis. Energ Convers Manage 2015;92:105–13. Riffat SB, Zhao X, Doherty PS. Developing a theoretical model to investigate thermal performance of a thin membrane heat-pipe solar collector. Appl Therm Eng 2005;25:899–915. Tiwari GN, Shyam Arvind Tiwari. Handbook of solar energy. Springer Science; 2016. Papadimitratos Alexios, Sobhansarbandi Sarvenaz, Pozdin Vladimir, Zakhidov Anvar, Hassanipour Fatemeh. Evacuated tube solar collectors integrated with phase change materials. Sol Energy 2016;129:10–9. Sharma RK, Ganesan P, Tyagi VV. Long-term thermal and chemical reliability study of different organic phase change materials for thermal energy storage applications. J Therm Anal Calorim 2016;124:1357–66. Pelay Ugo, Luo Lingai, Fan Yilin, Stitou Driss, Rood Mark. Thermal energy storage systems for concentrated solar power plants. Renew Sustain Energy Rev 2017;79:82–100. Tyagi VV, Pandey AK, Giridhar G, Bandyopadhyay B, Park SR, Tyagi SK. Comparative study based on exergy analysis of solar air heater collector using thermal energy storage. J Energy Res 2012;36:724–36. Li Bin, Zhai Xiaoqiang. Experimental investigation and theoretical analysis on a mid-temperature solar collector/storage system with composite PCM. Appl Therm Eng 2017;124:34–43.

388

Applied Energy 228 (2018) 351–389

K. Chopra et al.

desalination system using evacuated tube collector. Desalination 2016;396:30–8. [132] Sharshir SW, Peng Guilong, Yang Nuo, El-Samadony MOA, Kabeel AE. A continuous desalination system using humidification-dehumidification and a solar still with an evacuated solar water heater. Appl Therm Eng 2016;104:734–42. [133] Li Xing, Yuan Guofeng, Wang Zhifeng, Li Hongyong, Zhibin Xu. Experimental study on a humidification and dehumidification desalination system of solar air heater with evacuated tubes. Desalination 2014;351:1–8. [134] Hitesh N, Panchal Shah PK. Performance analysis of double basin solar still with evacuated tubes. Appl Solar Energy 2013;49:174–9. [135] Sampathkumar K, Arjunan TV, Eswaramoorthy M, Senthilkumar P. Thermal modeling of a solar still coupled with evacuated tube collector under natural circulation mode—an experimental validation. Energy Sour Part A Recov Utiliz Environ Effects 2013;35:1441–55. [136] Hitesh N, Panchal Shah PK. Enhancement of distillate output of double basin solar still with vacuum tubes. Front Energy 2014;8:101–9. [137] Dev Rahul, Tiwari GN. Annual performance of evacuated tubular collector integrated solar still. Desalin Water Treat 2012;41:204–23. [138] Panchal HN, Thakkar H. Theoretical and experimental validation of evacuated tubes directly coupled with solar still. Ther Eng 2016;63:825–31. [139] Sampathkumar K, Senthilkumar P. Utilization of solar water heater in a single basin solar still—an experimental study. Desalination 2012;297:8–19. [140] Nabil Elminshawy AS, Siddiqui Farooq R, Sultan Gamal I. Development of a desalination system driven by solar energy and low grade waste heat. Energy Convers Manage 2015;103:28–35. [141] Behnam Pooria, Behshad Shafii Mohammad. Examination of a solar desalination system equipped with an air bubble column humidifier, evacuated tube collectors and thermosyphon heat pipes. Desalination 2016;397:30–7. [142] Li Shuang-Fei, Liu Zhen-Hua, Shao Zhi-Xiong, Xiao Hong-shen, Xia Ning. Performance study on a passive solar seawater desalination system using multieffect heat recovery. Appl Energy 2018;213:343–52. [143] Ozgoren Muammer, Bilgili Mehmet, Babayigit Osman. Hourly performance prediction of ammonia water solar absorption refrigeration. Appl Therm Eng 2012;40:80–90. [144] Anand S, Gupta A, Tyagi SK. Renewable energy powered evacuated tube collector refrigerator system. Mitig Adapt Strateg Glob Change 2014;19:1077–89. [145] Bellos Evangelos, Tzivanidis Christos, Antonopoulos Kimon A. Exergetic, energetic and financial evaluation of a solar driven absorption cooling system with various collector types. Appl Therm Eng 2016;102:749–59. [146] Nkwetta Dan Nchelatebe, Smyth Mervyn. The potential applications and advantages of powering solar air-conditioning systems using concentrator augmented solar collectors. Appl Energy 2012;89:380–6. [147] Chen Guansheng, Liu Chongchong, Li Nanshuo, Li Feng. A study on heat absorbing and vapor generating characteristics of H2O/LiBr mixture in an evacuated tube. Appl Energy 2017;185:294–9. [148] Gang Pei, Huide Fu, Jie Ji, Tin-tai Chow, Tao Zhang. Annual analysis of heat pipe PV/T systems for domestic hot water and electricity production. Energy Convers Manage 2012;56:8–21. [149] Wu Shuang-Ying, Zhang Qiao-Ling, Xiao Lan, Guo Feng-Hua. A heat pipe photovoltaic/thermal (PV/T) hybrid system and its performance evaluation. Energy Build 2011;43:3558–67. [150] Gang Pei, Huide Fu, Tao Zhang, Jie Ji. A numerical and experimental study on a heat pipe PV/T system. Sol Energy 2011;85:911–21. [151] Renchun MingkeHu, Zheng, Pei Gang, Wang Yunyun, Li Jing, Ji Jie. Experimental study of the effect of inclination angle on the thermal performance of heat pipe photovoltaic/thermal (PV/T) systems with wickless heat pipe and wire-meshed heat pipe. Appl Therm Eng 2016;106:651–60. [152] Zhang Bingzhi, Jianlv, Yang Hongxing, Li Tailu, Ren Shengfeng. Performance analysis of a heat pipe PV/T system with different circulation tank capacities. Appl Therm Eng 2015;87:89–97. [153] Hui Long, Tin-Tai Chow, Jie Ji. Building-integrated heat pipe photovoltaic/ thermal system for use in Hong Kong. Sol Energy 2017;155:1084–91. [154] Chen Hongbing, Zhang Lei, Jie Pengfei, Xiong Yaxuan, Xu Peng, Zhai Huixing. Performance study of heat-pipe solar photovoltaic/thermal heat pump system. Appl Energy 2017;190:960–80.

[109] Shafieian Abdellah, Daghigh Roonak. Energy and exergy evaluation of an integrated solar heat pipe wall system for space heating. Sadhana 2016;41:877–86. [110] Lamnatou Chr, Papanicolaou E, Belessiotis V, Kyriakis N. Experimental investigation and thermodynamic performance analysis of a solar dryer using an evacuated-tube air collector. Appl Energy 2012;94:232–43. [111] Umayal Sundari AR, Neelamegam P, Subramanian CV. An experimental study and analysis on solar drying of bitter gourd using an evacuated tube air collector in Thanjavur, Tamil Nadu, India. In: Hindawi publishing corporation conference papers in energy; 2013. [112] Wang Ping-Yang, Li Shuang-Fei, Liu Zhen-Hua. Collecting performance of an evacuated tubular solar high-temperature air heater with concentric tube heat exchanger. Energy Convers Manage 2015;106:1166–73. [113] Mehla Neeraj, Yadav Avadhesh. Experimental analysis of thermal performance of evacuated tube solar air collector with phase change material for sunshine and offsunshine hours. J Ambient Energy 2015;38:130–45. [114] Kumar Ashish, Kumar Sanjeev, Yadav Avadhesh. Thermal performance analysis of evacuated tubes solar air collector in Indian climate conditions. J Ambient Energy 2014;37:162–71. [115] Caglar Ahmet, Yamali Cemil. Performance analysis of a solar-assisted heat pump with an evacuated tubular collector for domestic heating. Energy Build 2012;54:22–8. [116] Yadav Avadhesh, Bajpai VK. Experimental comparison of various solid desiccants for regeneration by evacuated solar air collector and air dehumidification. Drying Technol 2012;30:516–25. [117] Kumar Ashish, Kumar Sanjeev, Nagar Utkarsh, Yadav Avadhesh. Experimental study of thermal performance of one-ended evacuated tubes for producing hot air. J Solar Energy 2013:1–6. [118] Umayal Sundari AR, Neelamegam P, Subramanian CV. Performance of evacuated tube collector solar dryer with and without heat sources. Iran J Energy Environ 2013;4:336–42. [119] Ayompe LM, Duffy A. Thermal performance analysis of a solar water heating system with heat pipe evacuated tube collector using data from a field trial. Sol Energy 2013;90:17–28. [120] Daghigh Roonak, Shafieian Abdellah. Theoretical and experimental analysis of thermal performance of a solar water heating system with evacuated tube heat pipe collector. Appl Therm Eng 2016;103:1219–27. [121] Ayompe LM, Duffy A, McCormack SJ, Conlon M. Validated TRNSYS model for forced circulation solar water heating systems with flat plate and heat pipe evacuated tube collectors. Appl Therm Eng 2011;31:1536–42. [122] Singh Harvinder, Gagandeep, Saini Karamjeet, Yadav Avadhesh. Experimental comparison of different heat transfer fluid for thermal performance of a solar cooker based on evacuated tube collector. Environ Dev Sustain 2015;17:497–511. [123] Felinski P, Sekret R. Experimental study of evacuated tube collector/storage system containing paraffin as a PCM. Energy 2016;114:1063–72. [124] Rybár Radim, Beer Martin, Cehlár Michal. Thermal power measurement of the novel evacuated tube solar collector and conventional solar collector during simultaneous operation. Measurement 2016;88:153–64. [125] Faraji Amir Yadollah, Date Abhijit, Singh Randeep, Akbarzadeh Aliakbar. Baseload thermoelectric power generation using evacuated tube solar collector and water storage tank. Energy Proc 2014;57:2112–20. [126] Nájera-Trejo Mario, Martin-Domínguez Ignacio R, Escobedo-Bretado Jorge A. Economic feasibility of flat plate vs evacuated tube solar collectors in a combisystem. Energy Proc 2016;91:477–85. [127] Bin Du, Eric Hu, Kolhe Mohan. An experimental platform for heat pipe solar collector testing. Renew Sustain Energy Rev 2013;17:119–25. [128] Joo Hong-Jin, Kwak Hee-Youl. Experimental analysis of thermal performance according to heat pipe working fluids for evacuated tube solar collector. Heat Mass Transf 2017;53(11):3267–75. [129] Naghavi MS, Ong KS, Badruddin IA, Mehrali Mohammad, Metselaar HSC. Thermal performance of a compact design heat pipe solar collector with latent heat storage in charging/discharging modes. Energy 2017;127:101–15. [130] Omara ZM, Eltawil Mohamed A, El Nashar ElSayed A. New hybrid desalination system using wicks/solar still and evacuated solar water heater. Desalination 2013;325:56–64. [131] Shafii MB, Jahangiri Mamouri S, Lotfi MM, Jafari Mosleh H. A modified solar

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