Thermal performance of phase change material integrated heat pipe evacuated tube solar collector system: An experimental assessment

Energy Conversion and Management 203 (2020) 112205

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Thermal performance of phase change material integrated heat pipe evacuated tube solar collector system: An experimental assessment

T

K. Chopraa,b, Atin K. Pathaka, V.V. Tyagia, , A.K. Pandeyc, Sanjeev Ananda, 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 University, 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 Kingdom of Saudi Arabia, Saudi Arabia b

ARTICLE INFO

ABSTRACT

Keywords: Phase change material Energy efficiency Evacuated tube collector Solar energy Heat pipe

This manuscript presents an experimental investigation of heat pipe evacuated tube solar collector with and without phase change material for water heating application under the same weather conditions. In this study, a comparative analysis of two systems has been done in the same weather condition. Where evacuated tubes of the first system (evacuated tube collector-A) were left without phase change material and second system (evacuated tube collector-B) was integrated with SA-67 as phase change material. In order to ensure the thermal and chemical stability of the selected phase change material, thermal cycling treatment was carried out. The results showed that SA-67 has excellent chemical and thermal stability even after 1500 thermal cycling treatment. In order to analyze the thermal performance of the designed systems, the experiment was conducted with five different water flow rates (8, 12, 16, 20 and 24 L per hour). The daily thermal efficiency of evacuated tube solar collector with and without phase change material was varied in the range of 42–55% and 79–87% respectively. Although, the daily energy efficiency of evacuated tube collector integrated with phase change material was 37.56%, 35.31%, 36.69%, 32.34%, and 32.73% higher than evacuated tube collector without phase change material for water flow rates of 8, 12, 16, 20 and 24 L per hour respectively. The daily thermal efficiency for both systems was maximum at the flow rate of 20 L per hour. The heat transfer parameters for the designed systems have also been evaluated and compared.

1. Introduction World total hot water demand is exponentially rising due to an increase in population and industrial growth. In the USA and European Union, the energy requirement in the residential sector to accomplish hot water demand is approximately 18% and 14% respectively of total energy demand [1]. The Ministry of New and Renewable Energy (MNRE), Government of India reported that the in the residential sector demand for hot water was about 129 million/day in 2017 and it is forecasted that this consumption will get doubled by 2022 [2]. This large hot water demand is fulfilled by either direct use of fossil fuel or electricity [3]. The evacuated tube collector (ETC) is one of the promising collectors used for water heating applications [4]. The evacuated tubes used in ETC comprised of two concentric glass tubes [5]. In order to

⁎

minimize the heat losses, the vacuum is created in the annulus space between the inner and outer tube [6]. Further, heat absorbed by an absorber is transferred to heat transfer fluid through different techniques [7]. Among various techniques, the heat pipe is one of the latest and efficient technologies that have an extremely high thermal conductivity [8]. The long lifetime, corrosion resistance and controlled operating temperature are the main advantages of heat pipe evacuated tube solar collector [9]. In order to enhance the performance of solar water heating systems with heat pipe, Shafieian et al. [10] used different variable mass flow rate techniques of and working fluids. They operated the proposed system in the same weather conditions for three different cases i.e. constant flow rate of distilled water (case I), constant flow rate of nanofluid (case II), variable flow rate of nanofluid (case III). Compared to case I, improvement in exergy efficiency for case II and III was found to be 1.58% and 2.66% respectively. The results of variable

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

https://doi.org/10.1016/j.enconman.2019.112205 Received 1 July 2019; Received in revised form 14 October 2019; Accepted 16 October 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature A Cp D F Fc GT h h k L m NHP Q QPCM UL Ut

Uman

R T Vw

w wo wi man cond go gi L Avg hp int wk out in l sd v s

area (m2) specific heat (J/kg K) diameter (m) shape factor collector efficiency factor solar radiation (W/m2) heat transfer coefficient (W/m2 K) latent heat (J/kg) thermal conductivity (W/m K) length (m) mass flow rate (kg/s) number of heat pipe heat transfer rate (W) heat stored in phase change material (J) overall heat loss coefficient (W/m2 K) heat loss coefficient among evacuated tubes and ambient (W/m2 K) heat loss coefficient between manifold and ambient (W/ m2 K) thermal resistance (K/W) temperature (K) wind velocity (m/s)

deg m f PCM hf

Greek letters

(

)e

µ d

Superscript

effective transmittance-absorption product emissivity thickness (m) density (kg/m3) dynamic viscosity (m2/s) daily thermal efficiency

conv rad evap cond

convection radiation evaporator condenser

Acronym

Subscript ue ab amb

water water outlet water inlet manifold Condenser outer glass tube inner glass tube loss average heat pipe internal wick outer inner liquid solid vapour saturation metal wall degradation melting freezing phase change material heat transfer fluid between heat pipe & inner tube

ETC PCM LPH

useful energy absorber ambient

mass flow rate techniques were found very effective to enhance the performance of the proposed system. Different strategies to improve the performance of heat pipe solar collector were discussed by the Shafieian et al. [11]. They discussed the novel designs and structure of heat pipe solar collector systems aiming was to enhance its thermal efficiency. They also reviewed the different techniques to store thermal energy in a more effective way to increase the overall efficiency, operation time and heat transfer in the solar system. Huang et al. [12] experimentally found that when evacuated tube collector was equipped with heat shields, the thermal efficiency improved by 11.8%. Besides this, their simulated results revealed that the thermal efficiency of a developed collector in comparison to the conventional collector was higher when operated under weaker solar radiation and lower ambient temperature. The state of art demonstrates that heat pipe ETC system is an attractive option for solar thermal applications. In order to further enhance the performance of the heat pipe ETC system, the integration of phase change material (PCM) is a promising solution. However, there are some critical barriers of using of phase change materials with solar thermal systems: (i) thermal degradation of PCMs with respect of time (ii) PCM’s having low thermal conductivity generally in the range of 0.2–0.7 W/m K (iii) inorganic PCM’s having super-cooling effect during melting and solidification process (iv) promising PCM’s are not cost effective (v) corrosion problem between PCMs and metal containers (vi) volumetric expansion during phase transition from liquid to solid and

evacuated tube collector phase change material liters per hour

vice-versa. It can be seen that lot of research work is ongoing to solve these problems such as mixing of nucleating agent to remove supercooling effect and fins are using to increase the heat transfer rate in different applications. Ongoing research work showed that phase change materials based storage system is the most promising technology for the different applications. In this context, continuous development efforts and research work are ongoing to scaling up this technology. Wu et al. [13] studied the thermal performance of an oscillating heat pipe evacuated solar collector with paraffin wax as PCM. Authors reported that (i) fluctuations in efficiency of solar collector with PCM were 30% less than reference solar collector (ii) the water temperature at the outlet of collector with PCM was less in comparison to solar collector without PCM (iii) coefficient of performance and exit water temperature of the system with PCM reached to 3 °C and 50 °C respectively. Depending on PCM temperature and solar intensity, Feliński et al. [14] found that the annual charging efficiency of evacuated tube solar collector with PCM (paraffin) was in the range of 33–66%. They also found that the annual fraction of evacuated tube solar collector with storage improved by 20.5% in comparison to the collector without storage. Abokersh et al. [15] observed that efficiency of direct flow ETC with paraffin wax as thermal energy storage was 14% higher than the collector without PCM. However, the overall heat loss coefficient of the collector with PCM was found to be 22.5% higher in comparison to the collector without PCM. It was observed that time 2

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constant for system without PCM was 37.5% higher than system with PCM. They also found that energy gained for the un-finned and finned PCM integrated collector was higher than conventional solar collector by 35.8% and 47.7% respectively. Xue et al. [16] enhanced the efficiency, daily useful thermal efficiency and average thermal efficiency of U-Pipe ETC system by using Ba (OH)2·8H2O as heat storage material. Naghavi et al. [17] observed that the thermal performance of the PCM integrated heat pipe ETC system was much higher than the similar collector of the same specification without PCM. Papadimitratos et al. [18] studied the thermal performance of ETC system with two different phase change materials namely tritriacontane and erythritol during its normal and on-demand operation. The experimental outcomes showed that improvement in the efficiency of the proposed system during normal operation and stagnation operation was 26% and 66% respectively in comparison to the standard ETC system. Feliński and Sekret [19] performed an experimental investigation of PCM (paraffin wax) integrated ETC which was equipped with a CPC (compound parabolic concentrator). It was observed that maximum and average efficiency of charging of ETC system were improved by 40% to 49% and 31% to 36% respectively. Li and Zhai [20] did thermal performance evaluation of the heat pipe ETC with expanded graphite and erythritol as composite PCM. The authors observed that heat storage and average storage efficiency of the proposed system with PCM were 5.17 MJ/m2 and 40.17% respectively when solar intensity was 12.88 MJ/m2. Naghavi et al. [21] performed an experimental investigation of the heat pipe evacuated tube solar collector with paraffin as latent heat storage. They reported that efficiency of the proposed system was varied in the range of 34–36% and 38–42% during cloudy-rainy and sunny days respectively. The authors also observed that the variation of water flow rate directly influenced the efficiency of the proposed system. Essa et al. [22] found that the energy efficiency of the standard solar collector (ETC) in comparison to ETC integrated with PCM (paraffin wax) was 21.9% less at the lowest water flow rate. They observed that this increase in the thermal efficiency of ETC system was due to the complete phase transition of considered PCM. At the highest water flow rate (1.2 L per minute), daily efficiency of PCM integrated collector reached 6.8. Bazri et al. [23] reported that the energy efficiency of the developed system with three different types of PCM was in the range of 36–54% and this thermal efficiency enhanced in the range of 47–58% during rainy/cloudy day. The previous literature showed that in most of the research work paraffin wax as a thermal energy storage material has been used with ETC system. But the major drawbacks of paraffin wax are chemical/ thermal instability, low latent heat capacity, poor thermal conductivity, low density, etc. The outcomes of the latest study showed that after 200 thermal cycles of paraffin wax, its maximum degradation temperature and latent heat capacity decreased by 33.72% and 25.17% respectively [24]. The PCM (SA-67) selected in this study does not only have desirous properties but also cost-effective compared to paraffin wax. The differential scanning calorimetry (DSC) results of SA-67 revealed that its latent heat storage capacity is much higher than the storage capacity of different grades of paraffin wax used in previous studies. The FT-IR (Fourier-transform infrared) spectroscopy analysis of SA-67 showed that a sample of SA-67 has excellent chemical stability even after 1500 thermal cycles. It shows that SA-67 is quite enough to use as a thermal energy storage material for the heat pipe ETC system. Also, per kg cost of SA-67 (1–1.20 USD/kg) is much lower than paraffin wax (1.30–1.50 USD/kg). The integration of SA-67 as PCM with heat pipe ETC system over conventional ETC system (i) eliminated the overheating problem in heat pipe (ii) overcame the immediate impact of the fluctuation of solar radiation on the solar thermal system performance. (iii) enhanced thermal efficiency. Hence SA-67 as PCM plays a significant role in a solar water heating system. Also, till now performance study of heat pipe ETC system with the integration of SA-67 is not reported in the open literature.

2. Selection, characterization and thermal cycle testing of phase change material This section is divided into two parts. In the first part, important aspects based on which, phase change material is selected for heat pipe evacuated tube solar collector are discussed. In order to study the impact of thermal cycle treatment on the properties/stability of the selected phase change material, used instruments with their specifications are discussed in the second part. 2.1. Selection of phase change material The selection of PCM for any application is based on different parameters i.e. temperature in the appropriate range, high thermal conductivity, high latent heat, high chemical stability and thermal stability. The selection of PCM based on these properties is an optimal choice. Among latent heat storage materials, SA-67 has desired properties. In the present study, before the selection of SA-67 as a storage material for the heat pipe ETC system, it has been undergone thermal and chemical stability tests and found to be an optimal option to be integrated with ETC for water heating applications. The results of thermal and chemical stability tests are shown in subsequent sub-section (5.1, 5.2 and 5.3). The thermal properties of SA-67 are given in Table 1. 2.2. Characterization and thermal cycle testing of phase change material To examine the thermal cycling life of SA-67, it was subjected to thermal cycling operation by using a thermal cycler (TC-25/H model; BIOER Company, China). The thermal cycling comprised of 1500 heating/cooling steps. The spectral and DSC measurements were compared for fresh sample (0th cycle) and cycled sample (after 1500th cycle). In DSC, the temperature was maintained in the range of 40–85 °C for the measurement of its latent heat. The thermal properties of SA-67 were measured (at heating and cooling rate of at 3 °C/ min) by DSC (Perkin Elmer-JADE model) before and after thermal cycling. The chemical structure of SA-67 was characterized using FTIR (JASCO 430 model). The thermal stability of cycled and non-cycled SA-67 was examined using thermogravimetry analysis technique (TGA; Perkin–Elmer model) between 30 and 600 °C at 10 °C/min. The thermal degradation of SA-67 was started at a temperature of 225 °C for both non-cycled and 1500-cycled and ended at 297 and 291 °C respectively. The XRD (X-Ray Diffraction) analysis was performed using a powder diffraction meter (PANalytical X'-Pert3 model) with Cu (Kα = 1.5406 Å). The results of these tests have been further discussed in the results and discussion section. 3. Experimental setup and procedure In this study performance of heat pipe ETC has been enhanced by the integration of highly dense SA-67 as thermal energy storage material. The integration of SA-67 not only stored the heat energy but also increased the heat transfer coefficient across heat pipe and inner tube. The aluminum fins were also attached with heat pipes to further Table 1 Thermophysical properties of SA-67.

3

Appearance

White

Degree of purity Latent heat Melting temperature Density @ 40 °C Specific heat @ 40 °C Density @ 80 °C Specific heat @ 80 °C

99.92% 244.21 (J/g) 67.10 °C 1.10 (g/cm3) 2.01 (kJ/kg K) 1.19 (g/cm3) 2.47 (kJ/kg K)

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enhance the heat transfer between heat pipe and inner tube. Furthermore, the integration of PCM with heat pipe equipped ETC is more convenient because dry connection makes rapid and simple assembly of evacuated tubes with manifold (without dismantling of hydraulic components of the test stand). Based on these advantages, an experimental study was conducted in the outdoor condition of Shri Mata Vaishno Devi University (SMVDU) (Longitude: 74.9318°E, Latitude: 32.9915°N), India. Both solar collectors (ETC-A and ETC-B) were inclined at 45° with reference to the horizontal plane and directed towards the south direction. The photograph of the experimental unit is presented in Fig. 1. As presented in Fig. 2, each evacuated tube of ETC-B is filled with SA-67 (2.25 kg/tube) as thermal storage material by 75% of the total volume of the tube. While evacuated tubes collector without PCM (ETCA) considered as reference collector. The present study was done with five different flow rates of water during sunny days. As shown in Fig. 3, water was supplied to both of the collectors through their individual 18 W submersible pump from 1000 L cold water tank. The water flow through ETC-A and ETC-B was measured by their corresponding rotameter and controlled by valve V2 and V3 respectively. As shown in Fig. 3 water inlet, water outlet, heat pipe and PCM temperature of ETC-B were measured by temperature sensors T1, T2, T3-4, and T5-6 respectively. Whereas T7, T8, T9-10, and T11-12 measured water inlet, water outlet, heat pipe and air (between the inner tube and heat pipe) temperature for ETC-A. The ambient temperature was measured by temperature sensor T13. All measurements were recorded after every 5 min interval by Masibus 85x++ data logger. The metrological data was collected from SRRA (solar radiation resource assessment) station. The specification of collectors is given in Table 2. The uncertainties in the instruments used were also taken into consideration. The instruments used for the recording of temperature measurement were Masibus 85xx + data logger with temperature sensors of RTD type in the temperature range of −50 to 600 °C (maximum uncertainty of ± 1.3 °C). The water rotameter with a needle valve in the range of 0–100 LPH for each solar collector (maximum uncertainty of ± 2LPH (FSD)). Solar radiation data was collected from SRRA station (maximum uncertainty of < 10 W/m2). The accuracy in temperature and latent heat measurements were determined as ± 0.11 °C and ± 1.42%, respectively. The uncertainty in the measurements is used to evaluate the uncertainty in daily thermal efficiency. The highest value of uncertainty in daily thermal efficiency was found to be ± 12.7%.

incident solar radiation; ( )e , effective transmittance-absorption product; UL (W/m2 K), heat loss coefficient; Tab (K), absorber temperature; Tamb (K), ambient temperature. The useful heat gained by heat transfers fluid (water) can be estimated as [26]:

Que = m w × Cp, w × (Two

(2)

Twi )

m w (kg/s), water flow rate; Cpw (J/kg K), specific heat of water at constant pressure; Two (K), water outlet temperature, Twi (K), water inlet temperature. If the thermal resistance owing to the transportation of liquid–vapor within the heat-pipe is neglected, the working fluid temperature of heat is uniform at all sections. The heat gained by working fluid (water) per unit length of the heat-pipe condenser is expressed as [27]: m w × Cpw × L ×

dTw = Aman × hman × (Tcond dx

Tw )

2

Aman (m ), area of manifold; hman (W/m K), convection heat transfer coefficient between water and condenser; Tcond (K), surface temperature of condenser; Tw (K), average water temperature. From Fig. 5 condenser temperature can be calculated by solving Eq. (4) which is expressed as [28]: Two = e

w

× (Twi

(4)

Tcond) + Tcond

where w

=

Aman × hman m w × Cp, w

For an ETC with the ‘N’ number of heat pipes, the outlet temperature of water from the first condenser becomes inlet temperature of the second condenser and so on. The condenser temperature of the collector with N number of heat pipe can be calculated as [29]:

Two, N = e

w, N

× (Twi, N

(5)

Tcond, N ) + Tcond, N

The overall heat loss coefficient is a very crucial factor that affects the performance of the collector. The overall heat loss is calculated with the help of the thermal network of systems which is shown in Fig. 5. The factor UL is the summation of two losses namely losses from evacuated tubes and losses through manifold of collector. The overall coefficient of heat loss can be expressed as [30]: (6)

UL = Ut + Uman 2

Ut (W/m K), heat loss coefficient among vacuum tubes and ambient; Uman (W/m2 K), heat loss coefficient among manifold and ambient [31].

4. Thermal performance analysis

Ut =

In this section, thermal performance analysis of heat pipe ETC is presented. The thermal network of each component of heat pipe ETC is given in Fig. 4. The following assumptions were made while doing the thermal analysis of the systems:

1 conv Rgo amb

+

rad Rgo amb

+ Rgiradgo + R

conv ab gi

+R

rad ab gi

itudinal direction can be neglected.

An evacuated tube solar collector consists of an array of evacuated tubes equipped with finned heat pipe commonly connected to a manifold. The useful energy (Que ) collected by collector with single heat pipe is the difference of energy absorbed by absorber and energy loss to the surrounding which can be expressed by following equation [25]:

)e)

(UL × (Tab

Tamb))]

(7)

(K/W), thermal resistance for convective heat transfer rad among outer tube and ambient; Rgo amb (K/W), thermal resistance for radiative heat transfer among outer tube and ambient; Rgiradgo (K/W), conv Rgo amb

• Thermal resistance between vapour-liquid interface in condenser and evaporator is neglected. • The temperature of both inner tube and aluminum fin is assumed to be the same. • The variation of temperature in collector occurred in the long-

Que = Aab × Fc × [(GT ×(

(3)

2

(1) Fig. 1. Photograph of an experimental setup.

2

Aab (m ), absorber area; Fc , collector efficiency factor; GT (W/m2), 4

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K. Chopra, et al.

Fig. 2. Cross-section of evacuated tube (a) with PCM (b) without PCM.

Data Processing Unit

T1

T2

Data logger Hot water

R

T3 T4

V3 P

T5 T6

ETC-B (With PCM) T7

T13 T8

V1 Cold water tank

Hot water R

T9 T10

V2 P

T11

ETC-A (Without PCM)

T12

T1----Water inlet temperature of ETC-B T2----Water outlet temperature of ETC-B T3-T4----Heat pipe temperature of ETC-B T5-T6----PCM temperature of ETC-B T7----Water inlet temperature of ETC-A T8----Water outlet temperature of ETC-A T9-T10---Heat pipe temperature of ETC-A T11-T12--Air temperature of ETC-A T13—Ambient temperature V---Valve R---Rotameter P---Pump

Fig. 3. Schematic diagram of an experimental unit and location of temperature sensors.

thermal resistance for radiative heat transfer among inner and outer conv tube; Rab gi (K/W), thermal resistance for convective heat transfer rad among absorber and inner tube; Rab gi (K/W), thermal resistance for radiative heat transfer among absorber and inner tube.

Table 2 Specifications of developed system. Item

Specification/Value

Number of evacuated tubes Collector area Material of outer/inner tube Thickness of outer/inner glass tube Length of inner/outer glass tube Diameter of outer/inner glass tube Diameter of evaporator Diameter of condenser Length of condenser Length of evaporator

10 0.752 m2 Borosilicate glass 2 mm 1800 mm 58/47 mm 9.5 mm 14 mm 63 mm 1600 mm

conv Rgo amb =

1 conv {hgo amb = 8.55 + 2.56Vw conv hgo amb × A go

(8)

2

(W/m K), convection heat transfer coefficient between outer tube & ambient; Ago (m2), surface area of outer tube; Vw (m/s), wind velocity [12]. conv hgo amb

rad Rgo amb =

1 rad hgo amb × Ago

rad {hgo amb =

go

2 2 × (Tgo + Tsky ) × (Tgo + Tsky )

(9) 5

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outer tube; Agi (m2), area of inner tube ; Tgi (K), temperature of inner tube; gi , emittance of inner tube; Fgi go , shape factor across inner and outer tube; Dgi (m2), diameter of inner tube; Dgo (m2), diameter of outer tube.

1

conv Rab gi =

conv hab gi

(

×

Aab + Agi 2

conv hab gi =

)

khf Dhf

×Nuhf (11)

2

(W/m K), convection heat transfer coefficient between absorber & inner tube. conv hab gi

1

rad Rab gi =

(

rad hab gi ×

Aab + Agi 2

rad hab gi =

)

2 × (Tgi + Tab) × (Tgi2 + Tab ) (1

ab ) ab

+

1 Fab gi

+

(1

gi ) gi

×

Dab Dgi

(12) 2 rad hab gi (W/m K), radiation heat transfer coefficient between absorber & inner tube; Fab gi , shape factor across absorber and inner tube; Dab (m2), diameter of absorber. The theoretical value of Tgi and Tgo can be evaluated by following heat balance Eqs. (13) and (14) [32]:

QL, go

amb

conv = hgo amb × Ago × (Tgo

rad Tsky ) + hgo amb × A go × (Tgo

Tsky ) (13)

QL, go amb (W), heat loss due to temperature difference across the outer tube and ambient. QL, gi

go

= hgiradgo × Agi × (Tgi

(14)

Tgo)

QL, gi go (W), heat loss due to temperature difference across inner and outer tube. The heat losses between water flowing through manifold and ambient can be computed as [28]: QL, man

amb

=

TAvg

Tamb (15)

Rman

QL, man amb (W), heat loss between manifold and ambient; TAvg (K), the average temperature of manifold; Rman (K/W), thermal resistance of manifold. The total heat pipe thermal resistance of heat pipe can be calculated by Eq. (16) which is the summation of evaporator wall resistance evap (Rhp ), the evaporator internal resistance (Rievap ), the evaporator wick resistance (Rwevap ), the condenser internal resistance (Ricond ), and the cond condenser wall resistance (Rhp ) [33].

Fig. 4. Thermal resistance network of solar collector.

cond

evap evap evap cond Rhp = Rhp + Rint + Rwk + Rint + Rhp

(16)

Thermal resistance due to evaporator wall, internal fluid, and wick conduction can be calculated by following Eqs. (17)–(19) [34]. evap Rhp =

evap ln (Dout / Dinevap ) 2 × ×NHP × khp × Levap

(m), outer diameter of evaporator; (m), inner diameter of evaporator; NHP , number of heat pipes; khp (W/m K), thermal conductivity of heat pipe surface; Levap (m), length of evaporator. The thermal resistance produced by internal fluid in evaporator may be calculated by the following equation [35].

Fig. 5. A sectional view of the manifold. 2 rad hgo amb (W/m K), radiation heat transfer coefficient between outer tube & ambient; (W/m2 K4), Stefan Boltzmann constant; go , emittance of outer tube; Tgo (K), temperature of outer tube; Tsky (K), sky temperature.

Rgiradgo =

1 × Agi

hgiradgo

hgiradgo =

evap Rint =

gi ) gi

+

1 Fgi go

+

(1

go ) go

×

Dgi Dgo

2 × wk ×Dinevap × kl ×NHP × Levap

(18)

wk (m), thickness of wick; kl (W/m K), thermal conductivity of heat pipe fluid in a liquid state. The evaporator of heat pipe uses a screen mesh structure known as wick. The thermal resistance due to wick may as follows [35].

2 × (Tgo + Tgi) × (Tgo + Tgi2 ) (1

(17)

Dinevap

evap Dout

(10)

evap Rwk =

hgiradgo (W/m2K), radiation heat transfer coefficient between inner & 6

ln (Dinevap /(Dinevap 2 wk )) 2 × × NHP × k wk × Levap

(19)

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K. Chopra, et al.

The effective thermal conductivity of saturated (k wk ) can be calculated as [35].

k wk =

kl × [kl + ksd (1 kl + ksd + (1

wk ) wk )

× (kl ksd)] × (kl ksd )

excellent chemical stability even after 1500 thermal cycling treatment. On the other hand, the influence of thermal cycling treatment on the crystalline structure of SA-67 was investigated by comparing the XRD diffractograms of the 0th and 1500th cycled samples. As shown in Fig. 7, the main characteristic peaks (6.57°, 11.04°, 21.52°, and 24.07°) were noticed for same 2(theta) values for both (fresh and 1500th cycled) samples. These results clearly show that the thermal cycling process did not put any damaging effect on the crystalline structure of SA-67.

(20)

ksd (W/m K), thermal conductivity of wick; wk , voidage fraction of wick. The heat released due to the condensation of heat pipe fluid on inner wall of the condenser conducted to outer surface of the condenser. Due to conduction, thermal resistance can be calculated as [36]. cond Rhp

cond ln (Dout / Dincond ) = 2 × × NHP × khp × Lcond

5.2. Effect of thermal cycling treatment on the latent energy storage of SA67

(21)

The latent heat energy storage life of PCM with respect to thermal cycling treatment is the main criterion to be taken into consideration for thermal energy storage applications. In this regard, in order to determine the effect on latent heat storage after thermal cycling treatment, the DSC thermograms of 0th and 1500th cycle of SA-67 samples were taken and presented in Fig. 8. From the heating and cooling DSC curves, it was found that the fresh SA-67 melted at 67.30 °C and solidified at 66.10 °C while after 1500 cycles it melted at 67.10 °C and solidified at 65.74 °C. Moreover, at 0th cycle, SA-67 has a latent heat capacity of melting and freezing were 244.21 and 244.14 J/g respectively while the value of these parameters after 1500 cycles was 228.65 and 233.87 J/g, respectively. These results showed that melting temperature was not greatly influenced (as 0.2 °C) after 1500 thermal cycling treatment of SA-67. But its latent heat capacity decreased by 6.37% after 1500 thermal cycling treatment. Based on these findings, it can be concluded that the SA-67 has reliable latent heat energy storage life for real thermal energy storage applications after a long term thermal cycling period.

(m), outer diameter of condenser; (m), inner diameter of condenser; Lcond (m), length of the condenser. The thermal resistance of heat pipe working fluid during the condensation process can be evaluated as [36]. cond Dout

Dincond

cond cond Rint = ( ×NHP × Dincond ×Lcond × hint )

1

(22)

The condensation process in the condenser is considered to be filmwise condensation, hence coefficient of heat transfer due to film condensation can be calculated as [36]. cond hint =C

kl3 × g ×

l

×(

NHP × µl × (Ts

l

v )×

hm

Tmw ) × Dincond

(23)

(kg/m ), density of heat pipe fluid in liquid state; v (kg/m3), density of heat pipe fluid in vapor state; hfg (J/kg), latent heat of vaporization of heat pipe fluid; µl (Ns/m2), dynamic viscosity of heat pipe fluid in liquid state; Ts (K), saturation temperature of heat pipe fluid; Tmw (K), metal wall temperature of condenser. The daily thermal efficiency ( d ) of an evacuated tube collector is expressed by the following equation can be calculated as the ratio of total heat gained from collector to the total incident solar insolation on solar collector [37]. l

d

=

3

Que dt GT ×NHP × Aab ×

day

5.3. Effect of thermal cycling treatment on the thermal stability of SA-67 It is expected from an ideal PCM that it should be thermally stable or should not be degraded around its servicing temperature. Fig. 9 shows the TGA curves of fresh and 1500th cycled SA-67. From the weight loss-temperature curves, it was found that the thermal degradation of SA-67 was started at the same temperature i.e. 225 °C for both non-cycled and 1500-cycled and ended at 297 °C and 291 °C, respectively. From these results, it can be concluded that the SA-67 has outstanding thermal stability because its initial thermal degradation temperature was much over its working temperature (about 67 °C).

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5. Results and discussion This section presents a detailed discussion of results. Initially, the variation in properties with and without thermal cycling treatment is discussed then the discussion has been carried out on the performance parameters of ETC-A and ETC-B for different water flow rates. In this investigation, the phase change material in evacuated tubes of ETC-B (with PCM) was allowed to charge until it achieved its stable temperature. Then hot water was taken from ETC-B till PCM lost its stored heat. While from ETC-A, the water was taken between the period of sunrise to sunset. The test-1, test 2, test 3, test 4 and test 5 were assigned for the water flow rate of 8, 12, 16, 20 and 24 LPH (liters per hour) respectively. The detail of the experimentation is given in Table 3.

5.4. Variation of solar radiation with time during different tests In the present study, five tests had been performed. Each test was carried out on different days with different solar radiation. Table 3 summarizes the details of each experiment for each test. The variation of global solar radiation with time for five considered days to perform the five tests is presented in Fig. 10. It was found that the trend of solar radiation was almost similar for the five selected clear-sky-days. Table 3 Detail of experimentation.

5.1. Effect of thermal cycling treatment on the chemical stability of SA-67 The FT-IR spectrums of the fresh and 1500-cycled SA-67 are presented in Fig. 6. From the spectral results, the stretching and bending vibration bands of the characteristic peaks were placed in the range of 3600–3750 cm−1 for eOH band, 2850–3990 cm−1 for eCeH band, 1718 cm−1 for eC]O band, 1425 cm−1 for eCeH band, 986 cm−1 for eCeO and 647 cm−1 for CeH (bending). When spectral results of fresh and cycled SA-67 were compared, it was remarkably noted that no alteration took place in the wavenumbers of the bands. It was also found that at 1500th cycle of SA-67 any new peak did not arise and any deterioration did not occur in the shapes. This shows that the SA-67 has 7

Test No

Flow rate (LPH)

Date of experiment

System Type

No. of runs

Test 1

8

10 April 2019

2

Test 2

12

22 April 2019

Test 3

16

04 May 2019

Test 4

20

21 April 2019

Test 5

24

05 May 2019

ETC-A ETC-B ETC-A ETC-B ETC-A ETC-B ETC-A ETC-B ETC-A ETC-B

2 2 2 2

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Fig. 8. DSC results of fresh and cycled SA-67 samples.

Fig. 6. The FT-IR spectrums of the fresh (0th cycle) and 1500-cycled SA-67.

Fig. 9. Percentage of weight loss with temperature.

5.5. Variation of inlet and outlet temperature of water The variation in the water temperature for different flow rates has been illustrated by Fig. 11. As the supply water tank was not insulated so the large fluctuations in the inlet temperature of water were observed. The average values of inlet temperature were 28, 31, 33.4, 30.5 and 34.8 °C during test-1, test-2, test-3, test-4, and test-5 respectively. As shown in Fig. 11(a), the ETC-A and ETC-B achieved the maximum outlet temperature of 72.3 °C (at 2:10 P.M.) and 846 °C (at 3:05 P.M.) respectively. For a short period of time (11:00 A.M. to 12:20 P.M.), the outlet temperature of ETC-B was found to be lower than ETC-A but after 12:00 P.M., the temperature at the outlet of ETC-B started to rise. Moreover, hot water supply from ETC-B remained for a longer period (till 7:40 A.M. of next day morning) in comparison to ETC-A (till 6:30 P.M.). This was due to the integration of PCM as a thermal energy storage material with heat pipe ETC. It was also found that the highest difference in temperature between outlet and inlet for ETC-A was 30 °C, whereas for ETC-B highest difference in temperature between outlet and inlet was 43 °C. The water maximum temperature at the outlet of ETC-B for the lowest flow rate was near to the boiling point of water. Though the water of this temperature range is beneficial to be used for medium temperature applications yet, this condition requires safety regulations for the system. Fig. 11(b) depicts the effect on hot water temperature when the water flow rate was increased from 8 to 12 LPH. At a flow rate of 12 LPH, water maximum temperature at the outlet of ETC-A and ETC-B

Fig. 7. The XRD spectrums of the fresh and 1500-cycled SA-67.

However, due to the cloudy effect, some variation in solar radiation was observed between 4:00 P.M. and 5:00 P.M. during days selected for test2 and test-4. It was also found that the highest global solar radiation for test-1, test-2, test-3, test-4 and test-5 was reached to 918 W/m2 (at 12:10 P.M.), 933.15 W/m2 (at 12:00 P.M.), 992.76 W/m2 (at 12:10 P.M.), 919 W/m2 (at 12:05 P.M.), 982.21 W/m2 (at 12:10 P.M.) respectively. 8

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Fig. 10. Variation of solar radiation for five days selected on (a) 10 April 2019 for 8 LPH (b) 2 April 2019 for 12 LPH (c) 4 May 2019 for 16 LPH (d) 21 April 2019 for 20 LPH and (e) 5 May 2019 for 24 LPH.

was 62.7 °C (at 12:10 P.M.) and 72 °C (at 3:05 P.M.) respectively. When ETC-A and ETC-B tested with a flow rate of 12 LPH, then the highest difference in temperature between outlet and inlet for ETC-A was 23 °C, whereas for ETC-B highest difference in temperature between outlet and inlet was 31 °C. It shows that the highest difference in temperature between outlet and inlet for both systems attained at 12 LPH flow rate was lower in comparison of 8 LPH flow rate. It was due to the fact that by increasing the water flow rate, the contact period between water and condensers gets reduced. It was also noticed that the difference in temperature between outlet and inlet for ETC-B was higher than 5 °C till 3:50 A.M., which validated the storage advantage of SA-67. Thus heat supplying period from ETC-B at this flow rate (12 LPH) was approximately 3.5 h. less in comparison to the system worked with 8 LPH water flow rate. The variation of water temperature for 16 LPH is presented by Fig. 11(c) for both systems. At a flow rate of 16 LPH, the ETC-A and ETC-B achieved the maximum outlet temperature of 60.7 °C (at 12:40 P.M.) and 71.8 °C (at 2:50 P.M.) respectively. These highest values for both systems were equivalent to the corresponding values

attained at a water flow rate of 12 LPH. This was due to the high difference in solar radiation (the daily average solar intensity for test-2 and test-3 were 24.96 and 27.65 MJ/m2 respectively). For a flow rate of 16 LPH, then the highest difference in temperature between outlet and inlet for ETC-A was 18 °C, whereas for ETC-B highest temperature difference between outlet and inlet was 28 °C. It shows that the highest difference in temperature between outlet and inlet for both systems attained at 16 LPH flow rate was much lower in comparison of 8 LPH flow rate. It was also observed that hot water supply (temperature difference of water up to 5 °C) from ETC-B remained till 02:00 A.M. Thus hot water supply from ETC-B tested with 16 LPH flow rate was approximately 2 h less in comparison to the supply of hot water from the system run with a water flow rate of 12 LPH. Fig. 11(d) illustrates the variation of water temperature for a water flow rate of 20 LPH. At this flow rate, ETC-A and ETC-B achieved the maximum temperature of 53.2 °C (at 2:10 P.M.) and 60.5 °C (at 2:50 P.M.) respectively. It was found that ETC-B with a flow rate of 20 LPH can supply hot water (with water temperature difference up to 5 °C) till 12:50 A.M. Thus at a flow rate of 20 LPH, ETC-B can supply hot 9

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Fig. 11. (continued)

water approximately up to 6 h after sunset. Fig. 11(e) presents the variation of water temperature when the water flow rate was increased to 24 LPH. At this flow rate, the ETC-A and ETC-B achieved the maximum outlet temperature of 55 °C (at 12:35 P.M.) and 61 °C (at 3:15 P.M.) respectively. At this flow rate, the PCM discharge period was found to be lowest when the system operated at a water flow rate of 24 LPH. Hence it is clear from the above discussion that the water maximum temperature at the outlet of ETC-B was much greater in comparison to the respective value of water temperature at the outlet of ETC-A for corresponding flow rates. Moreover, compared to ETC-A, the supply of hot water from ETC-B continued even after sunset. 5.6. Variation of phase change material temperature, air temperature, ambient temperature and charging/discharging process with time The variation of process of phase change with time for the water flow rate of 8, 12, 16, 20 and 24 LPH has been illustrated by Fig. 12(a)–(e) respectively. However, solar radiation incident during test-1 was lowest but due to the domination of flow rate, the heat stored in PCM for this test was maximum. The maximum heat stored within

Fig. 11. Variation of inlet and outlet temperature over the period of time for the flow rate of (a) 8 LPH (b) 12 LPH (c) 16 LPH (d) 20 LPH (e) 24 LPH.

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Fig. 12. (continued)

Fig. 13. Effect of mass flow rate on the variation of daily thermal efficiency.

temperature sensors T5 and T6 installed at different locations in PCM showed that its temperature was uniform. It was due to the incident of the uniform solar insolation on each tube. Therefore, the average values of T5 and T6 are presented in Fig. 12(a–e). As soon as the PCM temperature reached to its melting point, the phase change process in PCM was observed. But for all tests, PCM took approximately 4 h time to reach its melting point. As clear from Fig. 12(a–e) that, a linear relationship between PCM temperature and time was observed for all tests.

Fig. 12. Variation of PCM stored energy, PCM temperature, air temperature and ambient temperature with time for (a) 8LPH (b) 12LPH (c) 16LPH (d) 20LPH and (e) 24LPH.

the PCM for test-3 was higher than that maximum heat stored within PCM for test-2 due to the domination of solar radiation. The increasing and decreasing trend of PCM temperature during its charging and discharging mode for different tests are shown by Fig. 12(a–e). The small difference in temperature values obtained from 11

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enhancement of the convective heat transfer coefficient, the heat transfer fluid did not get enough time to absorb heat from the collector. The experimental results of the present study showed that the daily thermal efficiency of heat pipe ETC improved in the range of 32–37% by the integration of SA-67 as thermal energy storage material. However, the results of the experimental study done by Essa et al. [22] revealed that the energy efficiency of ETC with paraffin wax as phase change material was improved in the range of 6–21%. It can be seen that heat pipe ETC system with SA-67 as a storage medium having good potential to replace the conventional ETC system for different commercial and residential applications.

Table 4 Variation of heat transfer parameters for different tests. Test No

Test-1 Test-2 Test-3 Test-4 Test-5

Heat transfer parameters Type of System

R go

ETC-A ETC-B

0.057 0.055

ETC-A ETC-B ETC-A ETC-B ETC-A ETC-B ETC-A ETC-B

amb

(m2K/W)

0.056 0.054 0.054 0.052 0.053 0.057 0.058 0.058

rad R gi go

(m2K/W) 1.70 1.65

1.80 1.76 1.74 1.71 1.85 1.78 1.71 1.70

Rhp gi (m2K/W)

UL (W/ m2K)

(K/W)

0.313

0.76 0.86

4.01 3.92

0.74 0.84

3.98 3.94

3.9 × 10 0.376 4.4 × 10 0.373 4.3 × 10 0.322

4 4 4

4.1 × 10 0.321

4

3.9 × 10

4

0.73 0.83

0.73 0.82 0.76 0.85

Rhp

4.05 3.99

4.16 4.05

5.8. Heat transfer parameters

4.01 3.99

The heat transfer parameters for both systems were also evaluated which are shown in Table 4. It was found that thermal resistances such conv rad rad Rhp were as Rgo amb (summation of Rgo amb and R go amb ), R gi go and not significantly varied with the flow rate. However, the values of Rhp gi conv rad (summation of Rhp gi and Rhp gi ) measured for ETC-B was much lower than the same thermal resistances measured for ETC-A. It was due to an increase in the convective heat transfer coefficient between PCM and heat pipe. But the overall heat transfer coefficient was not significantly increased for ETC-A in comparison to ETC-B.

However, depending on the solar insolation and water flow rate, the maximum value of PCM temperature was different for different tests. The PCM maximum temperature for test-1, test-2, test-3, test-4, and test-5 were 149 °C, 141 °C, 144 °C, 137 °C and 141 °C respectively. The temperature of air between the heat pipe and inner glass tube for collector without PCM was measured by two temperature sensors (T11 and T12). During the measurement of air temperature, a small difference between temperature values of T11 and T12 was observed. It was due to the incident of equal flux on all evacuated tubes. Therefore, the average values of T11 and T12 are presented in Fig. 12(a–e). Fig. 12(a)–(e) shows the behavior of air temperature with time for test1, test-2, test-3, test-4 and test-5 respectively. As soon as the air temperature increased, the heat transfer from air to heat pipe was taken place. Although thermal conductivity of air is poor but owing to the large surface area of heat pipe fins, transfer of heat from the air to heat pipe was taken place. The maximum temperature of air for test-1, test2, test-3, test-4 and test-5 were found to be 185 °C (at 1:55 P.M.), 175 °C (at 1:10 P.M.), 178.6 °C, 167.5 °C (at 1:35 P.M.) and 175.6 °C (at 2:10 P.M.) respectively. It was also observed that the air maximum temperature for each test was highly depended on the flow rate and solar intensity. The value of maximum air temperature for test-1, test-2, test-4 was inversely proportional to the flow rate except for test-3 and test-5. Because solar intensity during days chosen for test-3 and test-5 was much higher than the days selected for other tests.

6. Conclusions In this study, an experimental investigation of two types of heat pipe evacuated tube solar collectors were tested in the composite climate zone of India. In order to study the impact on the system performance, the designed collectors were investigated with five different water flow rates (8, 12, 16, 20 and 24 LPH). On the basis of the performance of designed collectors, the following conclusions can be drawn:

• The results obtained by the characterization of SA-67 after 1500 • •

5.7. Effect of water flow rate on daily thermal efficiency

•

Based on collected experimental and metrological data, the systems' daily thermal efficiency has been evaluated for each test by calculating useful energy gain and incident solar radiation. Fig. 13 depicts the variation in daily thermal efficiency for the different tests. The maximum daily thermal efficiency of 87.80% and 55.46% for ETC-B and ETC-A respectively was achieved for test-4. The second highest value of daily thermal efficiency of 84.90% and 52.17% for ETC-B and ETC-A respectively was attained for test-5. Whereas for test-1, the minimum value of daily thermal efficiency (79.98% and 42.42% for ETC-B and ETC-A respectively) was achieved. Thus 20 LPH is an optimum water flow rate in terms of highest efficiency for heat pipe ETC system. It may also be noted that ETC-B is highly efficient in comparison to ETC-A in respect of all selected water flow rates. This is due to the higher thermal conductivity of PCM and its storage capacity. Therefore, the integration of PCM of high thermal conductivity and storage capacity with heat pipe ETC is highly beneficial with respect to increasing its efficiency. Fig. 13 also depicts that efficiency increased with flow rate but up to a certain limit. Because with the increase of flow rate convective heat transfers between condensers and heat transfer fluid was also increased, hence the enhancement in daily thermal efficiency was observed. But after a certain limit (at 24 LPH), along with the

• •

• •

thermal cycling treatment showed (i) that melting temperature and latent heat capacity was decreased by 0.2 °C and 6.37% respectively (ii) excellent chemical stability and found no damaging effect on its crystalline structure. (ii) outstanding thermal stability. The maximum daily thermal efficiency of 87.80% and 55.46% for ETC-B and ETC-A respectively was achieved at the flow rate of 20 LPH. The minimum daily thermal efficiency of 79.98% and 42.42% for ETC-B and ETC-A respectively was achieved at the flow rate of 8 LPH. The experimental results showed that the daily thermal efficiency of heat pipe ETC improved in the range of 32–37% by the integration of SA-67 as thermal energy storage material. Daily thermal efficiency linearly increased till 20 LPH flow rate but after that, it started to fall down. The maximum difference in temperature between outlet and inlet of ETC-B for the flow rate of 8, 12, 16, 20 and 24 LPH were 43 °C, 31 °C, 28 °C, 23.5 °C, and 21 °C respectively. On the other hand, for ETC-A, the maximum difference in temperature between outlet and inlet for the corresponding flow rate was 30 °C, 23 °C, 18 °C, 16.5 °C, and 15 °C. The charging/discharging process of PCM was greatly influenced by water flow rate and incident solar radiation. The thermal resistances such as Rgo amb , Rgiradgo and Rhp were not significantly varied with water flow rate. Moreover, thermal resistance Rhp gi (between heat pipe and the inner tube) for ETC-B was much lowered than ETC-A. But thermal resistance between the heat pipe and inner tube for ETC-B was much lower than ETC-A.

Declaration of Competing Interest The authors declare that they have no known competing financial 12

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interests or personal relationships that could have appeared to influence the work reported in this paper.

[15] 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. [16] Sheng Xue H. Experimental investigation of a domestic solar water heater with solar collector coupled phase-change energy storage. Renew Energy 2016;86:257–61. [17] 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. [18] 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. [19] Feliński 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. [20] 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. [21] Naghavi MS, Ong KS, Badruddin IA, Mohammad Mehrali, 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. [22] Essa Mohamed A, Mostafa Nabil H, Ibrahim Mostafa M. An experimental investigation of the phase change process effects on the system performance for the evacuated tube solar collectors integrated with PCMs. Energy Convers Manage 2018;177:1–10. [23] Bazri Shahab, Badruddin Irfan Anjum, Naghavi Mohammad Sajad, Seng Ong Kok, Wongwises Somchai. An analytical and comparative study of the charging and discharging processes in a latent heat thermal storage tank for solar water heater system. Sol Energy 2019;185:424–38. [24] Vasu Anusuiah, Hagos Ftwi Y, Mamat R, Jesbains Kaur, Noor MM. The effect of thermal cyclic variation on the thermophysical property degradation of paraffin as a phase changing energy storage material. Appl Therm Eng 2019;149:22–33. [25] Abokersh Mohamed Hany, El-Morsi Mohamed, Sharaf Osama, Abdelrahman Wael. An experimental evaluation of direct flow evacuated tube solar collector integrated with phase change material. Energy 2017;139:1111–25. [26] Shafieian Abdellah, Khiadani Mehdi, Nosrati Ataollah. Thermal performance of an evacuated tube heat pipe solar water heating system in cold season. Appl Therm Eng 2019;149:644–57. [27] Incropera F, DeWitt D. Introduction to Heat Transfer. New York: John Wiley; 2002. [28] Yunus A. Cengel. Heat Transfer-A Practical Approach: McGraw-Hill; 2007. [29] Elsheniti Mahmoud B, Kotb Amr, Elsamni Osama. thermal performance of a heatpipe evacuated-tube solar collector at high inlet temperatures. Appl Therm Eng 2019;154:315–25. [30] 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. [31] Chopra K, Tyagi VV, Pathak Atin K, Pandey AK, Sari Ahmet. Experimental performance evaluation of a novel designed phase change material integrated manifold heat pipe evacuated tube solar collector system. Energy Convers Manage 2019;198:111896. [32] Korres Dimitrios N, Tzivanidis Christos, Koronaki Irene P, Nitsas Michael T. Experimental, numerical and analytical investigation of a U-type evacuated tube collectors' array. Renewable Energy 2019;135:218–31. [33] Faghri Amir. Heat pipe science and technology. Taylor and Francis Group; 1995. [34] 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. [35] Chi SW. Heat pipe theory and practice. New York: McGraw-Hill; 1976. [36] Chen MM. An analytical study of laminar film condensation. J Heat Transfer 1961;83:48–60. [37] Sharafeldin MA, Gróf Gyula. Experimental investigation of flat plate solar collector using CeO2-water nanofluid. Energy Convers Manage 2018;155:32–41.

Acknowledgment One of the authors V. V. Tyagi is highly thankful to the University Grant Commission (Govt. of India) for providing a startup research grant (F.30-325/2016 (BSR)) at Shri Mata Vaishno Devi University, Katra (J&K). Financial assistance in the form of a Junior Research Fellowship (JRF) to Atin Kumar Pathak under the inspire fellowship scheme by the Department of Science and Technology, Government of India is gratefully acknowledged. References [1] Fuentes E, Arce L, Salom J. A review of domestic hot water consumption profiles for application in systems and buildings energy performance analysis. Renew Sustain Energy Rev 2018;81:1530–47. [2] 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. [3] Shirinbakhsh Mehrdad, Mirkhani Nima, Sajadi Behrang. A comprehensive study on the effect of hot water demand and PCM integration on the performance of SDHW system. J Solar Energy 2018;159:405–14. [4] Li-Chao Xu, Liu Zhen-Hua, Li Shuang-Fei, Shao Zhi-Xiong, Xia Ning. Performance of solar mid-temperature evacuated tube collector for steam generation. Sol Energy 2019;183:162–72. [5] Mrinal Bhowmik, Muthukumar P, Anandalakshmi R. Experimental based multilayer perceptron approach for prediction of evacuated solar collector performance in humid subtropical regions. Renew Energy 2019;143:1566–80. [6] Mao Chunliu, Li Muran, Li Na, Shan Ming, Yang Xudong. Mathematical model development and optimal design of the horizontal all-glass evacuated tube solar collectors integrated with bottom mirror reflectors for solar energy harvesting. Appl Energy 2019;238:54–68. [7] Jowzi Mohammad, Veysi Farzad, Sadeghi Gholamabbas. Experimental and numerical investigations on the thermal performance of a modified evacuated tube solar collector: effect of the bypass tube. Sol Energy 2019;183:725–37. [8] Shafieian Abdellah, Khiadani Mehdi, Nosrati Ataollah. A review of latest developments, progress, and applications of heat pipe solar collectors. Renew Sustain Energy Rev 2018;95:273–304. [9] Chopra K, Tyagi VV, Pandey AK, Sari Ahmet. Global advancement on experimental and thermal analysis of evacuated tube collector with and without heat pipe systems and possible applications. Appl Energy 2018;228:351–89. [10] Shafieian Abdellah, Osman Junaid Jaffer, Khiadani Mehdi, Nosrati Ataollah. Enhancing heat pipe solar water heating systems performance using a novel variable mass flow rate technique and different solar working fluids. Sol Energy 2019;186:191–203. [11] Shafieian Abdellah, Khiadani Mehdi, Nosrati Ataollah. Strategies to improve the thermal performance of heat pipe solar collectors in solar systems: a review. Energy Convers Manage 2019;183:307–31. [12] Huang Xiaona, Wang Qiliang, Yang Honglun, Zhong Shuai, Jiao Dongsheng, Zhang Kaili, et al. Theoretical and experimental studies of impacts of heat shields on heat pipe evacuated tube solar collector. Renew Energy 2019;138:999–1009. [13] Wei Wu, Dai Suzhou, Liu Zundi, Dou Yiping, Hua Junyenm, Li Mengyang, et al. Experimental study on the performance of a novel solar water heating system with and without PCM. Sol Energy 2018;171:604–12. [14] Feliński Piotr, Sekret Robert. Effect of PCM application inside an evacuated tube collector on the thermal performance of a domestic hot water system. Energy Build 2017;152:558–67.

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