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Performance study on a passive solar seawater desalination system using multi-effect heat recovery
Applied Energy 213 (2018) 343–352
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Performance study on a passive solar seawater desalination system using multi-effect heat recovery
T
⁎
Shuang-Fei Lia, Zhen-Hua Liua, , Zhi-Xiong Shaob, Hong-shen Xiaoc, Ning Xiab a
School of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240, China Sunshore Solar Energy Co., Ltd, Nantong 226000, China c School of Mechanical Engineering, Nantong Vocational University, Nantong, China b
H I G H L I G H T S solar seawater desalination system with multi-effect heat recovery processes. • ASystem uses all-glass evacuated tube absorber as heat collector with a simplified CPC. • Mechanism of its operation are multi-stage distillation and heat recovery. • The system operated under barotropic or atmospheric pressure without power consumption. •
A R T I C L E I N F O
A B S T R A C T
Keywords: Solar energy Seawater desalination Heat recovery CPC
A novel small-sized solar seawater desalination system with multi-effect heat recovery processes using all-glass evacuated tube absorber as heat collector, in which there is no power pump and the steam and freshwater flow are driven only by pressure drop, was designed and tested. The whole system consists of 7 heat collecting/heat recovery integration units, which were divided into 7 temperature/pressure states. Each unit has a heat collector which consists of a simplified CPC panel, an all-glass evacuated tube absorber, a seawater tank and a bar heat pipe that connects the absorber and seawater tank to transfer heat from the absorber to the seawater tank. Every unit operated under barotropic or atmospheric pressure. Meanwhile, a stepwise heat recovery method was adopted to recycle the sensible heat and latent heat of the steam generated. In order to investigate the effects of operating parameters on system performance, including freshwater yield, solar collecting performance and heat recovery performance, a series of experiments were conducted under different weather condition. It can be found that the all-day freshwater yield of unit area can reach 4.23 kg/m2 on the sunny day and 3.03 kg/m2 on the cloudy day. Meanwhile the collecting efficiency and comprehensive thermal coefficient can reach 0.41 and 1.39 respectively. The experiment results confirm that the designed system has a superior performance in seawater desalination without power consumption.
1. Introduction Water is the most active and influential factor in the ecological environment and it is also one of the most valuable resources in the world. However, about 97% of the Earth’s water is salt water in the ocean, and a tiny 3% is fresh water [1]. At present, about 1.5 billion people in more than 80 countries around the world face the shortage of fresh water. By 2025, it is projected that there will be 3 billion people lack of water, involving more than 40 countries and regions. Water resource is becoming a valuable resource in the twenty-first Century; as a result, the problem of water resource is not only a resource problem, but also a major strategic issue related to the national economy, social ⁎
sustainable development and long term stability. Therefore, it is very urgent to solve the problem of water shortage. Desalinating salt water from the ocean, river, lakes is the best way to supply fresh water to growing population. Seawater desalination techniques are mainly divided into two categories on the basis of different elements of fresh water production, namely, the membrane processes, for which reverse osmosis (RO) and electrodialysis (ED) were utilized [2,3] and the thermal processes whose thermal energy may be obtained from a conventional fossil-fuel source, nuclear energy or from a non-conventional solar energy, etc. [4–6]. Moreover, the thermal processes are broadly classified into two major categories, direct and indirect systems. The indirect method of
Corresponding author. E-mail address: [email protected] (Z.-H. Liu).
https://doi.org/10.1016/j.apenergy.2018.01.064 Received 10 November 2017; Received in revised form 2 January 2018; Accepted 22 January 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.
Applied Energy 213 (2018) 343–352
S.-F. Li et al.
Nomenclature
Acpc Gm Gm,all Gm′ Δh n PR qr Q Q̇
ηre ηt ε
2 3 4 5 6 7 Sys Ac Th
horizontal projection area of CPC panel (m2) freshwater yield (kg/h) all-day freshwater yield of the system (kg) freshwater yield of unit collecting area (kg/(h m2)) enthalpy difference (J/kg) number of collecting units in the system performance ratio of desalination irradiance (W/m2) power (W) useful power extracted from collector (W) heat recovery efficiency heat collecting efficiency comprehensive thermal coefficient
Unit 2 Unit 3 Unit 4 Unit 5 Unit 6 Unit 7 system actual theoretical
Acronyms and abbreviations CPC MED MEH/D
compound parabolic concentrator multi-effect distillation multi-effect humidification/dehumidification
Subscripts 1
Unit 1
both transient and steady states. Meanwhile, a mathematical model of heat and mass transfer was developed. Patricia Palenzuela et al. [21] conducted work to analyze whether the integration of Multi-Effect Distillation (MED) process into Concentrating Solar Power (CSP) plants can be more competitive, under certain conditions, than the independent freshwater and power production by connecting a Reverse Osmosis (RO) system to a CSP plant. Guo Xie et al. [22] presented a novel conceptual design of a low temperature multi-effect desalination (LT-MED) system and is composed of several modules of tubular solar still (TSS) cells. Reddy et al. [23] developed a novel multi-effect evacuated solar desalination system utilizing latent heat recovery and investigated the effect of various design and operating parameters on the system performance to optimize the configuration. Estahbanati et al. [24] conducted experiment to investigate the effect of the number of stages on the productivity of a multi-effect active solar still. Dayem [25] conducted work to demonstrate experimentally and numerically the performance of a simple solar distillation unit that is based on the multiple condensation-evaporation cycle. Chorak et al. [26] presented and discussed an experimental characterization of the solar MED system under off-design conditions. So far, a lot of research relate to the MED systems have been published, however, many of them need auxiliary energy consumption during runtime. In addition, most of them have large floor space and high cost. In order to design a MED system that is suitable for the island and small fishing boats where the solar energy is rich, but electricity, fresh water and fossil fuel are rare, our research team developed a principle passive solar desalination system with multi-effect evaporation/heat recovery processes and verified the feasibility of the device [27], which is the mechanism research for the system presented in this paper. Furthermore, Liu ZH carried out some modification measures to optimize the principle system and considered more operating parameters for analyzing the system performance [28]. This system integrated solar collecting, seawater evaporation, heat recovery and condensation processes into the common evacuated tubular solar collector to produce freshwater directly. The system operated under barotropic and atmospheric pressure and adopted a stepwise heat recovery method to recycle the sensible heat and latent heat of the steam generated. However, for this principle system, it is difficult to develop a commercial product due to its especial fabrication. In this research, based on the principle system, a small-sized solar seawater desalination system with multi-effect heat recovery processes using all-glass evacuated tube absorber as heat collector, in which there is no power pump and the steam and freshwater are driven only by pressure drop, is designed and tested. The originality of this system
solar desalination plant consists of two subsystems, solar collector and desalination unit. The different types of solar collector such as evacuated tube, flat plate and heat pipe can be used along with the thermal desalination processes such as multiple effect evaporation (MEV), vapor compression (VC) and multi-stage distillation (MSD) [7]. In addition, the solar distillation systems can be classified as passive solar still and active solar still. In a passive solar still, the solar radiation is received directly by the basin water and is the only source of energy for raising the water temperature. However, in an active solar still, an extra thermal energy is supplied to the basin through an external mode to increase the evaporation or a vacuum pump is used to maintain the vacuum working condition. The water demand is increasing rapidly than the sustainable level and desalination is the best method to provide the shortfall of water. From the perspective of energy consumption, most of the desalination systems [8–11] are energy intensive, which consume high grade energy like gas, electricity, oil and fossil fuels accelerating the global warming that is the burning topic and becomes threat to life sustainability. Renewable energy is the alternative solution to decreasing consumption of fossil fuels [12]. So far, there are already several kinds of renewable energy applied to seawater desalination such as wind energy [13,14] and solar energy. Considering that the solar energy can be efficiently harvested for solar to implement the heat application for green and environmental sustainability, it is one of the most promising applications of renewable energies in thermal desalination processes. So far, there have been many solar desalination researches carried out as follows. Many researchers have carried out research on solar humidification/dehumidification desalination system (HDDS) [15,16]. However, the performance of HDDS seriously depends on the thermal properties of working medium and the humidification/dehumidification ability is weak at low temperature difference. Besides the HDDS, Khamid Mahkamov et al. [17] developed and tested an innovative small dynamic water desalination plant that is a combination of a heat pipe evacuated tube solar collector, conventional condenser and novel fluid piston converter. Young-Deuk Kim et al. [18] proposed a hybrid desalination system consisting of vacuum membrane distillation (VMD) and absorption desalination (AD) units, designated as VMD-AD cycle. Nematollahi et al. [19] carried out an experimental and theoretical energy and exergy analysis for a solar desalination system consisting of a solar collector and a humidification tower. In addition, the multi-effect desalination system attracts many researchers due to its high-energy efficiency. Zhili Chen et al. [20] designed a multi-stage stacked-tray solar seawater desalination still and test water production performance in 344
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consists of the following aspects: (1) The most originality is that this is a novel passive system without pumps. (2) The heat pipe was utilized as the heat transfer element, which realizes the rapid heat transfer between the collector section and the desalination unit. (3) A stepwise heat recovery method was adopted to recycle the sensible heat and latent heat of the steam generated, which is realized by several pressure regulating valves. In order to investigate the effects of operating parameters on system performance, including freshwater yield, solar collecting performance and heat recovery performance, a series of experiments were conducted under different weather and irradiance conditions. Through the experiments, the heat collection performance, heat recovery performance and desalination performance have been discussed and analyzed. The experiment results confirm that the designed system has a superior performance in seawater desalination without power consumption. The experimental research has a significant reference value for the development of a passive small-sized solar desalination system.
Fig. 2. A photograph of solar seawater desalination system with 7 collecting units.
2. Experimental apparatus and procedure 2.1. Experimental apparatus The designed seawater desalination system is made up of 7 heat collecting/heat recovery integration units, which were divided into 7 temperature/pressure states. Fig. 1 gives out a schematic view of the whole system for every units and connection relationship in which the saturated temperature of steam generated at every unit is illustrated, and Fig. 2 shows a photograph of the actual experimental apparatus. As shown in Fig. 1, the designed system mainly consists of seven linked units for heat collecting/recovery, air cooler and freshwater collector. The apparatus is located in Tongzhou District, Nantong, Jiangsu, China. The precise location is “N32°06′45.38″, E121°0′17.36″”. In addition, the average local irradiance is above 500 W/m2 during work time (from 9:00 to 15:00). In the first unit, the energy used to evaporate seawater is only obtained from solar energy. However, in the remaining 6 units, the steam produced at the upper unit releases the latent heat to the seawater in the next unit through the heat recovery coil tube. The steam/water mixture outlet of the last unit is connected with the naturally convective air cooler that is a passive cooler and has no energy consumption to condense the final steam-water mixture. And then, the condensed freshwater was stored in the freshwater collector. The Units 2–7, as depicted in Fig. 3, have the same structure containing a seawater tank, a pressure reducing valve, a simplified CPC panel, a bar heat pipe, an all-glass evacuated tube, a seawater inlet, a steam/water mixture inlet, a steam outlet tube, a steam/water mixture outlet and a heat recovery coil tube. In every unit, evaporation section of heat pipe is concentrically mounted into the all-glass evacuated tube, meanwhile, the condensation section of heat pipe is concentrically mounted into the heating tube fixed in the center of the seawater tank. The first unit (Unit 1), as a simple heat collector without heat recovery process, doesn’t have a heat recovery coil tube installed inside.
Fig. 3. Structures of the unit (red point: measuring point of thermocouples). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
By such arrangement, the collected solar energy can be rapidly transferred into the seawater tank by using a bar heat pipe to heat the seawater under small temperature difference. The steam generated from unit 1 flows into the heat recovery coil tube fixed in the seawater tank of unit 2 through the steam/water mixture inlet. Similarly, from
Fig. 1. Designed system for every units and connection chart Blue line: seawater, red line: saturated water, green line: vapor. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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experiment, the steam temperature and pressure were changing all the time with this control method. As a compromise, this control method is feasible and has the advantage of low cost and simple operation. In the present stage, a naturally convective air cooler was used for final steam-water mixture condensation and there is not energy consumption in this passive heat exchanger. In this experimental system, the steam temperatures, the wall temperatures of heat pipe, the temperatures of steam/water mixture inlet and outlet were measured for investigating both the solar collecting and heat recovery performances. The measuring positions of thermocouples are displayed in Fig. 3. For each heat pipe, the temperatures at evaporation section, adiabatic section and condensation section were monitored to ensure the heat pipe is in normal working condition. For each unit, the steam temperatures were measured to monitor its operating temperatures. From unit 2 to unit 7, the temperatures of steam/water mixture inlet and outlet were measured to study the heat recovery performance. In addition, a TES-133R solar radiometer was utilized to measure the solar irradiance. As long as the measuring surface of the solar radiometer was placed in parallel with the heat collecting unit, the real-time solar irradiance perpendicular to the CPC plate can be obtained.
unit 2 to unit 7, in each seawater tank, there is a steam/water mixture inlet and a steam/water mixture outlet to conduct both the steam generated and the water condensed into the heat recovery coil tube fixed in the seawater tank of next unit. Therefore all latent heat of the steam can be recovered in the last six units for the evaporation of seawater. In this designed system, a steam condenser that consists of coil tubes and use natural convective heat transfer of air to cool fluid in tube, is installed at the final stage in the system to fully condense the rest steam from the last unit and the freshwater is finally stored in the freshwater collector. In addition, the cumulative freshwater influx was measured every 15 min to get the mean freshwater yield. The irradiance, steam temperatures and wall temperatures of the tubes in each unit were measured, collected and processed by the data acquisition system. Data were logged using Agilent 34970A data-logger connected to the computer. 2.2. Geometry design of simplified CPC For obtaining a relatively high collecting temperature and steam pressure, a CPC-type concentrator was utilized in this study which is the same as that in the authors’ principle study [28]. The design method of standard CPC for tubular all-glass evacuated tube comes from Winston [29] and Rabl [30–32]. In addition, Alaydi [33] and Kalogirou [34] applied parabolic solar-collector for seawater desalination, and Lillo et al. [35] evaluates the potential of solar concentration technologies as an alternative to conventional sources of energy for industrial processes in Latin America. For reducing CPC material and increasing collecting time, the standard CPC was cut to a truncated CPC. Then, for further reducing machining cost, bottom involute shape was cut to flat shape. Fig. 4 presents a finally simplified CPC profile after truncation and involute shape cutting. According to the calculation and test results, the concentration efficiency of the simplified CPC with a flat curve bottom decreases by about 15% compared with the truncated CPC with an involute bottom [36]. The CPC is made of a stainless steel mirror plate welded on the steel shelf directly. For the present tubular absorber, the concentration ratio of the CPC is defined as the ratio of the aperture area of the CPC to the peripheral area of the tubular absorber. According to this definition, the concentration ratio of the present CPC is 2.16 which does not depend on the acceptance half angle of standard CPC. The characteristics of the presented collector part including CPC, all-glass evacuated tube and bar heat pipe are shown in Table 1.
2.4. Uncertainty analysis Five main parameters are considered during the test, including instantaneous freshwater yield, performance ratio, heat collecting efficiency and heat recovery efficiency, which are calculated as follows:
Gm =
m t
(1)
PR =
Gm Δh Q̇
(2)
ηt ,exp =
ε=
Gm′ h Acpc qr
(3)
Gm Δh ACPC nqr
ηre =
(4)
Gm,ac Gm,all−7Gm,1 = Gm,th 21Gm,1
(5)
where m is the freshwater yield within 15 min, t is the time interval, t = 15 min = 0.25 h. According to the relative standard deviation of the transfer function theory, the maximum relative deviation of instantaneous freshwater yield, performance ratio, heat collecting efficiency and heat recovery efficiency can be derived from Eqs. (1)–(4) as follows:
2.3. Experimental procedure In the experimental process, NaCl solution with a mass fraction of 3.5% and temperature of 20 °C–25 °C is used to substitute sea water. In addition, the ion level of Na in the freshwater and NaCl solution have been measured by Inductively Coupled Plasma Optical respectively and the value is 0.0138% for the freshwater and 3.414% for the NaCl solution. Then, the temperature and pressure of the saturated steam in each unit were controlled by the pressure reducing valves. All these valves were regulated only once at noon when the direct irradiance was approximately strongest. And then, the highest temperatures of the steam in each unit were dependent on their own without the further adjustment of the valves. Because the designed solar desalination system was demanded to run all year round, the highest collecting temperature should not be too high and could be achieved even in the winter. Meanwhile, it should achieve six-effect heat recovery, and the temperature gradient of two adjacent units should not be too large. Since the steam temperature in unit 7 is 100 °C corresponding to the atmospheric pressure, the ideal distribution of the steam temperature is that the steam temperature in unit 1 is controlled at about 150 °C and decreased by 8 °C at each next unit. According to the preparatory adjustment, the maximum difference between the two adjacent units was set at 8 °C. As a matter of fact, with the irradiance changing during the
σGm = Gm
2
2
⎛ ∂lnGm ⎞ σm2 + ⎛ ∂lnGm ⎞ σt2 ⎝ ∂m ⎠ ⎝ ∂t ⎠
Fig. 4. Simplified CPC profile for an all-glass evacuated tube (unit: mm).
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(6)
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long term test data of all-day freshwater yield of unit collecting area where the temperature represents the average environment temperature during the test process. For the laboratory prototype stage, even if the device runs continuously, it is not necessary to carry out performance tests every day. The performance tests were conducted randomly according to the weather condition. In addition, the latest 5 sets of test data are adopted to further analyze the performance of freshwater production including freshwater yield and performance ratio, the steam temperatures and the system efficiency including heat collecting efficiency, comprehensive coefficient and heat recovery efficiency have been discussed.
Table 1 Characteristics of the collector system. Parameter
Value
CPC Width of CPC: Effective length of CPC Height of CPC: Aperture area of a CPC plate Radius of absorber Distance of CPC cusp and absorber Acceptance half angle of standard CPC Edge-ray angle of truncated CPC Reflectivity of CPC Concentrating ratio All-glass evacuated tube Cover glass tube diameter Inner glass tube diameter Effective length glass tube Bar heat pipe Outer dia. of bar heat pipe Inner dia. of bar heat pipe Effective length Wall thickness Working fluid Stainless steel screen Material Wire diameter Mesh number
ηt ,exp
σε = ε σηre ηre
=
⎜
⎟
2 ⎛ ∂lnηt ,exp ⎞
⎜
⎟
⎝ ∂Gm′ ⎠
σG2m′ +
Gm∗ =
304 stainless steel 0.12 mm 150 meshes
(11)
⎟
(7)
2 ⎛ ∂lnηt ,exp ⎞
⎜
⎝
⎟
∂qr
⎠
σq2r
3.1.1. Analysis of freshwater yield Fig. 6 shows the data summary of all-day freshwater yield of unit area, all-day freshwater yield and the average irradiance for unit 1 under different weather conditions to shows the effect of alone heat colleting capacity on freshwater yield. Meanwhile, Fig. 7 shows the data summary of all-day freshwater yield of unit area, all-day freshwater yield and average irradiance for the whole system to show the multiple effect of both heat collecting and heat recovery on freshwater yield. As manifested in Figs. 6 and 7, the solar radiation intensity was closely related to weather conditions. The maximum average irradiance during sunny days was about 769.69 W/m2, 26.7% higher than that during the cloudy day (607.52 W/m2). Furthermore, it is obvious that the all-day freshwater yield depends on the average irradiance and is basically proportional to the average irradiance. For unit 1, there is no heat recovery process, that is to say, the freshwater was only produced by the effective heat collection of CPC panel. According to Fig. 6, it can be found that the maximum all-day freshwater yield can reach 1.2 kg on the sunny day and 0.95 kg on the cloudy day for one collecting glass tube. Correspondingly, the maximum all-day freshwater yield of unit area can reach 2.3 kg/m2 on the sunny day and 1.98 kg/m2 on the cloudy day. It can be estimated by this data that the maximum heat collecting efficiency can reach 41% on the sunny day and 34% on the cloudy day for even a high collecting temperature of 140 °C as shown in Fig. 13. It is obvious that the designed system has a good heat collecting performance. For the whole system, there is a heat recovery process in each unit from unit 2 to unit 7 to recover the latent heat of steam generated in the previous unit. As manifested in Fig. 7, the maximum all-day freshwater yield can reach
(8)
2
2
Gm Acpc × n
The calculation of all-day freshwater yield and all-day freshwater yield of unit collecting area is similar to Eqs. (1) and (11), the difference is that m is the value of all-day freshwater production; t is the all-day working hours.
⎟
2
=
45 mm 41 mm 2800 mm 2 mm Water
⎛ ∂lnε ⎞ σ 2 + ⎜⎛ ∂lnε ⎟⎞ σ 2 Gm qr ⎝ ∂Gm ⎠ ⎝ ∂qr ⎠ ⎜
The amount of water production is obtained by weighing the freshwater collected in a bucket. In order to improve measurement accuracy, the actual measurement time interval is 15 min. Therefore, the instantaneous value of the freshwater yield is the average value within 15 min and calculated by Eq. (1). The instantaneous value of the freshwater yield of unit collecting area is obtained by Eq. (11).
2
2
⎜
3.1. Freshwater production performance
58 mm 47 mm 1600 mm
⎛ ∂lnPR ⎞ σ 2 + ⎛ ∂lnPR ⎞ σ 2 Gm Q̇ ⎝ ∂Gm ⎠ ⎝ ∂Q̇ ⎠
σPR = PR σηt,exp
330 mm 1600 mm 146 mm 0.512 m2 24 mm 6 mm 18o 65o 0.85 2.16
(9) 2
⎛⎜ ∂lnηre ⎞⎟ σ 2 ⎛ ∂lnηre ⎞⎟ σ 2 Gm,all + ⎜ Gm,1 ∂ G ⎝ m,all ⎠ ⎝ ∂Gm,1 ⎠
(10)
In the experiment process of the designed system, the parameters that need to be measured include temperature, fresh water mass and solar irradiance. In addition, the time also need to be recorded. For the measurement of temperature, GG-K-30 thermocouples were utilized with ± 0.1 K measurement errors. An electronic scale with ± 0.005 kg measurement errors was adopted to measure the mass of freshwater obtained in a measuring interval of 15 min. A TES-133R solar radiometer was utilized to measure the solar irradiance. As long as the measuring surface of the solar radiometer was placed in parallel with the heat collecting unit, the real-time solar irradiance perpendicular to the CPC plate can be obtained and the measurement error was ± 4%. For the record of time, it was achieved by the system time of computer which was consistent with “Beijing time”. In addition, because the record of fresh water mass was manual operation consuming time, there existed a time measurement error with about ± 3 s. The measurement errors of the parameters in the experiment are listed in Table 2. According to Eqs. (5)–(8), the relative errors of instantaneous freshwater yield, performance ratio, heat collecting efficiency and heat recovery efficiency are less than 4.3%, 6.1%, 7.4%, 5.9% and 9.7%, respectively.
Table 2 Measurement errors for experimental parameters. Parameters
Measurement errors
Temperature Fresh water Solar irradiance Time
± 0.1 K ± 0.02 kg/h ± 4% ±3s
3. Results and discussion In order to verify the superior performance of the designed solar seawater desalination system, a series of experiments have been conducted under different weather conditions. Fig. 5 shows some typical 347
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800
7
700
6
600
Fig. 5. The long term test data of all-day freshwater yield of unit collecting area.
Average solar radiation intensity
5
500
All-day freshwater yield of unit collecting area
400
3
300
2
200
1
100
2
4
Average solar radiation intensity /W/m
14-3-16 21-4-16 17-8-16
21-7-16
20-7-16 23-7-16 22-7-16
17-10-15
7-12-15
8-1-16
Autumn Sunny 20.7 °C
Winter Sunny 8.2 °C
Winter Spring Spring Summer Summer Summer Summer Summer Sunny Sunny Cloudy Cloudy Sunny Cloudy Cloudy Sunny 3.5 °C 7.6 °C 19.8 °C 30.3 °C 31.1 °C 31.8 °C 32.3 °C 31.5 °C
900
20
All-day freshwater yield of unit area (kg/m ) 800 All-day freshwater yield (kg)
18
Average solar irradiance 2
700 600 500 400 300 200
17-8-16
21-7-16
20-7-16
23-7-16
22-7-16
2
All-day freshwater yield of unit area (kg/m )
16
800
Average solar irradiance 700
14
600
12
500
10
400
8
300
6 4
200
2
100
0
0
900
All-day freshwater yield (kg)
17-8-16
21-7-16
20-7-16
23-7-16
22-7-16
2
100
0
Average solar irradiance /(W/m )
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Average solar irradiance /(W/m 2 )
All-day freshwater yield of Unit 1
0
All-day freshwater yield
All-day freshwater yield of unit collecting area /kg/m
2
8
0
Summer Summer Summer Summer Summer Sunny Sunny Sunny Cloudy Cloudy
Summer Summer Summer Summer Summer Sunny Sunny Sunny Cloudy Cloudy
Fig. 7. Data summary of all-day freshwater yield (black column), all-day freshwater yield of unit area (red column) and the average irradiance for the whole system. summary of all-day freshwater yield of unit area (black col
Fig. 6. Data summary of all-day freshwater yield of unit area (black column), all-day freshwater yield (red column) and the average irradiance for unit 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
relatively higher average irradiance. It is also found that the change of instantaneous freshwater yield was basically in accordance with that of irradiance, which illustrates that the instantaneous freshwater yield is very sensitive to the irradiance. The change of irradiance not only causes change of the heat collecting efficiency, but also causes change of the heat recovery efficiency which is more important for freshwater yield. In addition, there was no steam generation before 11:15 whether the weather was sunny or cloudy, because a large amount of seawater accumulated in the seawater tank was being heated form a relatively low temperature to the saturated temperature before noon. When the weather was sunny, the maximum instantaneous freshwater yield appeared at noon and the value was about 1.51 kg/(h m2). In the afternoon, the instantaneous freshwater yield decreased gradually as the time goes on until there was no freshwater generation. When the weather was cloudy, the instantaneous freshwater yield fluctuated with the change of irradiance and the maximum value was 1.5 kg/(h m2) that was close to the value (1.51 kg/(h m2)) of sunny day, even so, it is obvious that the average value of instantaneous freshwater yield in cloudy day was much lower than that in sunny day.
17 kg on the sunny day and 12 kg on the cloudy day. Correspondingly, the maximum all-day freshwater yield of unit area can reach 4.23 kg/ m2 on the sunny day and 3.03 kg/m2 on the cloudy day. Based on the comparison of the freshwater yield of unit 1 and whole system, it can be found that the maximum all-day freshwater yield of unit area can be increased by about 2 times, the mean all-day freshwater yield of unit area of whole system can be increased by 1 times for all days, which is attributed to the heat recovery process. It can be confirmed that the heat recovery effect was relatively ideal whether the weather was sunny or cloudy. 3.1.2. Analysis of instantaneous freshwater yield Fig. 8 depicts the instantaneous freshwater yield of unit collecting area and irradiance along with time under different weather conditions to show the effect of instantaneous irradiance on the freshwater yield. It is clear from Fig. 8 that the irradiance fluctuated violently in cloudy weather, which leads to the lower average irradiance. On the contrary, the irradiance fluctuated gently in sunny weather, which leads to 348
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2.0
1400
3.0
1.5
1200
2.5
1000
0.0
800
-0.5
600
-1.0
400
-1.5
-2.5 -3.0 11:15
11:45
12:15
12:45
17-8-2016 21-7-2016 20-7-2016 23-7-2016 22-7-2016
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13:45
200
0.5 0.0 -0.5
17-8-2016 sunny 21-7-2016 sunny 20-7-2016 sunny 23-7-2016 cloudy 22-7-2016 cloudy
-1.0
0 14:15
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Time /hr
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17-8-2016 21-7-2016 20-7-2016 23-7-2016 22-7-2016
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13:45
14:15
2
17-8-2016 sunny 21-7-2016 sunny 20-7-2016 sunny 23-7-2016 cloudy 22-7-2016 cloudy
-2.0
1.5
1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 -100 -200 14:45
Solar irradiance (W/m )
0.5
2.0
Performance ratio
2
1.0
Solar irradiance (W/m 2 )
Freshwater yield /(kg/h · m )
S.-F. Li et al.
Time /hr
Fig. 8. Instantaneous freshwater yield of unit collecting area and irradiance changes with time.
Fig. 9. Performance ratio and irradiance changes with time.
the solar radiation was stronger, the temperature difference between adjacent units became larger, meanwhile, the steam flow rate was also larger, the heat recovery performance was better, and therefore, the value of PR was relatively large. On the contrary, when the solar radiation was weaker, the temperature difference between adjacent units became smaller, meanwhile, the steam flow rate was smaller, the heat recovery performance deteriorated, which led to the rapid decrease of PR. According to the data from Fig. 9, it is found that the actual PR is much less than the theoretical value and there is great improvement space for the heat recovery capacity by increasing the heat transfer area of coil heat exchanger installed in the seawater tank which will only cause small cost increase. If the actual PR value can increase 50%, that means the maximum instantaneous value reaching 4 and mean value of all-day reaching 3, then, the freshwater yield will increase 50%.
3.1.3. Analysis of performance ratio The performance ratio (PR) is an important performance factor to evaluate the performance of desalination system and it is defined as the ratio of the total energy used to evaporate seawater into freshwater including the energy recovered to the net energy absorbed from the collector. PR represents the heat recovery performance of the system and it would be 1 if there is no heat recovery effect. The theoretical value of PR is determined by the system structure and calculated by the Eq. (2). Assuming that the collecting power of each panel, Q̇ , is the same and the latent heat of vaporization of water can be fully recovered, then the power used to evaporate seawater in each unit is as follows:
Q1 = Q̇
(12a)
Q2 = Q̇ + Q1 = 2Q̇
(12b)
Q3 = Q̇ + Q2 = 3Q̇
(12c)
3.2. Analysis of steam temperature difference between each unit
Q4 = Q̇ + Q3 = 4Q̇
(12d)
Q5 = Q̇ + Q4 = 5Q̇
(12e)
Q6 = Q̇ + Q5 = 6Q̇
(12f)
Q7 = Q̇ + Q6 = 7Q̇
(12g)
The designed system operates in passive mode and both freshwater and steam flow depends on the steam temperature difference (the saturated steam pressure difference) between each unit. Therefore, it is of vital importance to understand the steam temperature difference between each unit. Figs. 10 and 11 show the steam temperature curves of each unit and irradiance curves along with time on sunny and cloudy day in summer, respectively. As depicted in Figs. 10 and 11, in the morning, the water accumulated in the system was being heated from a relatively low temperature to the saturated temperature, hence there was no steam generation during this period and the steam temperature was below 100 °C. The amount of heat required to heat the total seawater with a maximum value of 34 kg in seven seawater tanks from initial 20 °C to a mean saturated temperature of 120 °C of the whole system takes a preheating time of about 2.4 h. This preheating time is too long for an actual application. Since this study focused on the collecting and heat recovery performance, thence had not used auxiliary preheating device. When the steam temperatures of each unit reached above 100 °C at about 11:15, the steam started to emerge. At noon, the steam temperatures and steam pressures in each unit reached the maximum set value and the temperature difference between two adjacent units can reach the designed value of 8 K on sunny day. In the afternoon, the temperature difference gradually narrowed with the decrease of solar radiation. In the next study, the system will install a passive type of seawater preheater to preheat the cold seawater to hot seawater with about 90 °C–95 °C for experiment in the following day. If the temperature of seawater in the preheater can hold at about 70 °C in the morning of the following day, then about 1.2 h preheating time will be saved and the
where, Q1 ∼ Q7 are the total power used to evaporate seawater in unit 1 ∼ unit 7, respectively. Q̇ is the net power from collector in each unit. Hence, the maximum theoretical value of the PR for the system is calculated as follows:
PR =
Q1 + Q2 + Q3 + Q4 + Q5 + Q6 + Q7 28Q̇ = =4 7Q̇ 7Q̇
(12h)
Due to the existence of heat recovery process, the PR will be bigger than 1, and the bigger the value is, the better the heat recovery effect will be. In this designed system, the maximum theoretical value of PR is 4 according to Eq. (12a-h). Fig. 9 shows the actual performance ratio curves and irradiance curves along with time under different weather conditions. It can be noticed that the fluctuation of instantaneous value of PR along with time was similar to that of instantaneous freshwater yield that is closely related to the irradiance. According to Fig. 9, on the sunny day, the actual PR value increased gradually with the increase of irradiance before 12:15 and almost remained stable during the 12:00 – 13:15 period, and then decreased gradually with the decrease of irradiance after 13:15. The maximum instantaneous value of PR can reach 2.5 at the noon, and mean value of PR for all-day is close to 2. The change of PR mainly depends on the change of heat recovery performance. When 349
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Unit 6 Unit 7
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300 0
120
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Solar irradiance (W/m )
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efficiency of the unit 1 which is a simple heat collector without heat recovery part was used for representing the system collecting efficiency and its efficiency may be slightly lower than the actual system efficiency due to its higher collecting temperature compared with other units. The heat collecting efficiency is defined as the ratio of the net energy absorbed by the seawater in seawater tank to the total energy incident on collector, and its instantaneous value is calculated by Eq. (3). Fig. 12 plots the instantaneous heat collecting efficiency curves of unit 1, which is a mean measured value in 15 min and may represent mean heat collecting efficiency of system, and instantaneous irradiance curves along with time under different weather conditions. According to Fig. 12, on the sunny day, the heat collecting efficiency increased gradually in the morning, on the contrary, it decreased gradually in the afternoon. On the cloudy day, the heating collecting efficiency fluctuated acutely with the fluctuation of irradiance. The maximum value of the instantaneous heat collecting efficiency can reach 0.42 on the sunny day and 0.34 on the cloudy day. It is obvious that the designed system possesses superior instantaneous heat collecting performance during a short noon time. However, the heat collecting efficiency decreased rapidly in the afternoon. One of the reasons is that the solar radiation decreased in the afternoon. The second reason is the structure of CPC can’t make all the solar energy be projected to the CPC panel, which reduces the collecting efficiency and it is also a limitation of nontracking type collectors. But in terms of practical and civilian applications, this limitation is acceptable. In the present system, the acceptance half angle of standard CPC and the edge-ray angle of truncated CPC are 18° and 65°, respectively. If the acceptance half angle of standard CPC increase by 1 time with the same concentration ratio, the highest collecting temperature at noon will have a decline, but, both the all-day collecting time and the mean heat collecting efficiency of all-day will significantly increase. According to the experimental results, the highest collecting temperature was not the key issue for the designed system; therefore, it’s meaningful to further improve CPC design.
900
-300
100
90 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00
Time /hr Fig. 10. The curves of steam temperatures in each unit changing with time on the sunny day.
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170 150 140 130
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Unit 2 Unit 3 Unit 4 Unit 5
Unit 1
Unit 6 Unit 7 300
120 110
0
2
100
Solar irradiance (W/m )
Steam temperature (
160
900
90 80 -300 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00
Time /hr Fig. 11. The curves of steam temperatures in each unit changing with time on the cloudy day.
3.3.2. Comprehensive thermal coefficient Besides the heat collecting efficiency, the comprehensive thermal coefficient is also an important parameter for the solar desalination system with heat recovery process which reveals the total thermal performance of the system and is defined as the ratio of the total energy used to evaporate seawater into freshwater to the total energy incident on the solar collector. The parameter can be seen as an integrated indicator to judge the solar energy collection and heat recovery performance of the system, in addition, the value of comprehensive thermal coefficient may be greater than 1 due to the existence of heat recovery process and is calculated by Eq. (4). Fig. 13 shows the comprehensive thermal coefficient curves and irradiance curves along with time under different weather conditions. It 1400
0.40
1300 1200
Collecting efficiency
0.35
3.3. Analysis of system efficiency
1000
0.25
900
0.20
800 700
0.15
0.05 0.00 11:15
600 17-8-2016 21-7-2016 20-7-2016 23-7-2016 22-7-2016
11:45
17-8-2016 exp. 21-7-2016 exp. 20-7-2016 exp. 23-7-2016 exp. 22-7-2016 exp.
12:15
12:45
13:15
500 400 300 13:45
14:15
Time /hr Fig. 12. The collecting efficiency changing with time.
350
200 14:45
2
0.10
3.3.1. Heat collecting efficiency The system heat collecting efficiency is an important indicator to judge the solar energy collecting performance of a solar collecting system. Because the present system has heat recovery process in the whole heat transfer process, the system collecting efficiency cannot be simply measured by using the final steam yield. The collecting
1100
0.30
Solar irradiance (W/m )
effective steam generating time will prolong 1.2 h, which means the freshwater yield will increase about 30% according to the present system performance. By comparing Fig. 8 with Fig. 10 and Fig. 11, it is noticed that the changing regularity of steam temperature difference was similar to that of instantaneous freshwater yield. When the steam temperature difference kept stable, the instantaneous freshwater yield was also basically stable. And when the steam temperature difference fluctuated, the fluctuation also occurred in the instantaneous freshwater yield. This is because the instantaneous freshwater yield is closely related to the heat recovery capacity, which strongly depends on the steam temperature difference. Furthermore, the collecting power reached the maximum at noon when the irradiance reached peak and at this time, the steam yield was the largest and the steam flow rate was large enough to ensure the set pressure difference. However, in the afternoon, with the irradiance gradually decreasing, the steam temperatures in the upper collecting units gradually decreased, which led to the rapid decrease in the steam temperature differences between the two adjacent units. Therefore, it is very difficult that actual mean PR of all-day reaches the theoretical value.
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12:00
12:30
13:00
17-8-2016 21-7-2016 20-7-2016 23-7-2016 22-7-2016 13:30
14:00
14:30
1000 900 800 700 600 500 400 300 200 100 0 -100 -200 -300 -400 -500 -600 15:00
transfer area and improvement of CPC design will be our priority in the following researches. 3.4. The comparison of performance between the tested system and conventional system Table 3 shows the comparison of performance between the tested system and conventional system and Table 4 lists the main efficiencies of the tested system. It can be noted from Table 3 that the conventional system have the disadvantages of complex structure, high energy-consumption, large size and cost, etc. Though most of them possess high freshwater capacity, they are not suitable for the area where energy is only solar energy. For the MED system presented in this paper, it is designed to serve island and small fishing boats where solar energy is rich, but electricity, fresh water and fossil fuel are rare. Though the freshwater capacity of the presented system is lower than that of conventional system, it possesses more practical value for island and small fishing coats due to its simple structure, low cost, small floor space and no auxiliary energy consumption. In addition, the performance improvement and structure optimization will be conducted in the further study to promote its commercialization.
2
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
Solar irradiance (W/m )
Comprehensive thermal coefficient
S.-F. Li et al.
Time /hr Fig. 13. The comprehensive thermal coefficient for the system.
is affected by heat collection and heat recovery performance of the system, and the irradiance. For the designed system in this study, the temperatures of each unit were getting lower and lower with the decrease of solar irradiance as the time went on (as shown in Figs. 10 and 11). As a result, the total heat used to evaporate the seawater and the amount of steam decreased, which lead to the decrease of latent heat of steam and the deterioration of heat recovery performance. However, it is worth mentioning that the comprehensive thermal coefficient is not proportional to the irradiance or the collecting efficiency as shown in Fig. 13, and it holds a long stable period for 90 min in which the maximum value is 1.32 on the sunny day and 1.39 on the cloudy day. Though the maximum comprehensive thermal coefficient on the sunny day is smaller than that on the cloudy day, it is obvious the average comprehensive thermal coefficient on the sunny day is much larger than that on the cloudy day. In addition, compared with the maximum heat collecting efficiency of 0.42 on the sunny day shown in Fig. 12, the comprehensive thermal efficiency increases 2 times.
4. Conclusions A passive type of small-sized solar seawater desalination system with multi-effect heat recovery processes using all-glass evacuated collector was designed and tested in this study. According to the experimental results, the designed seawater desalination system reached primary demand and showed good freshwater production performance. (1) The all-day freshwater yield of unit area can reach 4.23 kg/m2 on the sunny day and 3.03 kg/m2 on the cloudy day without power consumption. Furthermore, the instantaneous system collecting efficiency can reach 0.41 at the collecting temperature of 140 °C on the sunny day and 0.34 on the cloudy day. In addition, the performance ratio can reach 2.5 on the sunny day and 2.3 on the cloudy day. The comprehensive thermal coefficient can reach 1.32 on the sunny day and 1.39 on the cloudy day. In addition, the allday mean recovery efficiency can reach 0.38 on the sunny day and 0.29 on the cloudy day. (2) Experimental study provides useful guidelines for the development of passive type of high effective solar desalination system. The present system can still be improved a lot due to the various design defects found in the experiment. The freshwater yield may be greatly increased through the increase of the heat transfer area in the heat recovery exchanger. In addition, design improvement of the CPC to increase collecting time. As mentioned above, preheat of the seawater in the seawater tank using passive steam condenser in All-day average efficiency of heat recovery
800 700 Average solar radiation intensity All-day average efficiency of heat recovery
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17-08-16
21-07-16
20-07-16
23-07-16
22-07-16
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Summer Sunny
Summer Sunny
Summer Cloudy
Summer Cloudy
0
Fig. 14. All-day average efficiency of heat recovery and average irradiance.
351
2
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
Average solar irradiance (W/m )
3.3.3. Heat recovery efficiency The heat recovery efficiency is an important indicator to judge the heat recovery performance of a solar seawater desalination system. For the designed system in this research, the heat used to evaporate seawater comes from two parts including heat collection and heat recovery in each unit. The heat recovery efficiency is defined as the ratio of actual freshwater yield (Gm,ac ) generated by heat recovery to the theoretical freshwater yield (Gm,th ) generated by heat recovery and calculated by Eq. (5). For instantaneous heat recovery efficiency, Gm,all and Gm,1 denote the freshwater yields of the whole system and the unit 1, respectively, in unit time. For all-day mean value, Gm,all and Gm,1 denote the freshwater yields of the whole system and the unit 1, respectively, in all-day. In Eq. (7), the freshwater yield of each unit is similarly substituted by that of unit 1. The all-day freshwater yields of unit area for the unit 1 which has not heat recovery equipment and the whole system (7 units) have been shown in Figs. 7 and 8; therefore, the all-day mean value of heat recovery efficiency may be obtained from the data management. Fig. 14 illustrates the all-day mean value of heat recovery efficiency and the mean solar irradiance for five typical weather conditions. According to Fig. 14, the maximum heat recovery efficiency can reach 0.38 on the sunny day and 0.29 on the cloudy day, which are much less than the theoretical value of 1. It is obvious that there is a great gap between the actual value and the ideal value due to the designed heat transfer area of coil heat exchanger for heat recovery is much less than the needed heat transfer area. By doubling the heat transfer area, the all-day mean values of heat recovery efficiency will be approximately doubled without great cost increase. How to improve the all-day mean values of heat recovery efficiency through the increase of the heat
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Table 3 The comparison of performance between the tested system and conventional system. Equipment type
Advantages
Disadvantages
Fresh-water production rate
Auxiliary energy consumption
MEH/D [25]
Keep running all day
7.74 L/(d m2)
2000–14,000 kJ/h
MED-14 effects [26]
High freshwater yield
Complex structure; need auxiliary energy; H/D ability is weak at low temperature difference High energy-consumption; equipment with large size and cost High energy-consumption; equipment with large size and cost Short effective working time; sensitive to weather conditions
127.29 L/(d m2)
190 kW
Hybrid solar/gas MED [37] The present study
24-h operation; operate in different modes; high freshwater yield Simple structure; low cost; no auxiliary energy consumption
Comprehensive thermal coefficient
Heat recovery efficiency
42%
139%
38%
5.6 L/(h m )
150 kW
4.23 L/(d m2)
0
Hydrogen Energy 2016;41(45):20818–22. [17] Mahkamov K, Orda E, Belgasim B, et al. A novel small dynamic solar thermal desalination plant with a fluid piston converter. Appl Energy 2015;156:715–26. [18] Kim YD, Thu K, Ng KC, et al. A novel integrated thermal-/membrane-based solar energy-driven hybrid desalination system: Concept description and simulation results. Water Res 2016;100:7–19. [19] Nematollahi F, Rahimi A, Gheinani TT. Experimental and theoretical energy and exergy analysis for a solar desalination system. Desalination 2013;31(7):23–31. [20] Chen Z, Peng J, Chen G, et al. Analysis of heat and mass transferring mechanism of multi-stage stacked-tray solar seawater desalination still and experimental research on its performance. Sol Energy 2017;142:278–87. [21] Palenzuela P, Zaragoza G, Alarcón-Padilla DC. Characterisation of the coupling of multi-effect distillation plants to concentrating solar power plants. Energy 2015;82:986–95. [22] Xie G, Sun L, Mo Z, et al. Conceptual design and experimental investigation involving a modular desalination system composed of arrayed tubular solar stills. Appl Energy 2016;179:972–84. [23] Reddy KS, Kumar KR, O’Donovan TS, Mallick TK. Performance analysis of an evacuated multi-stage solar water desalination system. Desalination 2012;288:80–92. [24] Estahbanati MRK, Feilizadeh M, Jafarpur K, et al. Experimental investigation of a multi-effect active solar still: The effect of the number of stages. Appl Energy 2015;137:46–55. [25] Dayem AMA. Experimental and numerical performance of a multi-effect condensation–evaporation solar water distillation system. Energy 2006;31(14):2710–27. [26] Chorak A, Palenzuela P, Alarcón-Padilla DC, et al. Experimental characterization of a multi-effect distillation system coupled to a flat plate solar collector field: empirical correlations. Appl Therm Eng 2017;120:298–313. [27] Liu ZH, Hu RL, Chen XJ. A novel integrated solar desalination system with multistage evaporation/heat recovery processes. Renew Energy 2014;64:26–33. [28] Liu ZH, Guan HY, Wang GS. Performance optimization study on an integrated solar desalination system with multi-stage evaporation/heat recovery processes. Energy 2014;76:1001–10. [29] Winston R, Hinterberger H. Principles of cylindrical concentrations for solar energy. Sol Energy 1975;17:255–60. [30] Rab IA. Solar concentrators with maximal concentration for cylindrical absorbers. Appl Opt 1976;15(7):1871–3. [31] Rabl A. Comparison of solar concentrators. Sol Energy 1976;18:93–111. [32] Rabl A, O'GaUagher J, Winston R. Design and test of non-evacuated solar collectors with compound parabolic concentrators. Sol Energy 1980;25:335–51. [33] Alaydi JY. Modeling of a parabolic solar-collector system for water desalination. Global J Res Eng 2013. [34] Kalogirou S. Use of parabolic trough solar energy collectors for sea-water desalination. Appl Energy 1998;60(2):65–88. [35] Lillo I, Pérez E, Moreno S, et al. Process heat generation potential from solar concentration technologies in Latin America: the case of Argentina. Energies 2017;10(3):383. [36] Khonkar HEI, Sayigh AAM. Optimization of the tubular absorber using a compound parabolic concentrator. Renew Energy 1995;6(1):17–21. [37] Alarcón-Padilla DC, Blanco-Gálvez J, García-Rodríguezz L, et al. First experimental results of a new hybrid solar/gas multi-effect distillation system: the AQUASOL project. Desalination 2008;220(1–3):619–25.
Table 4 The main efficiencies of the tested system. Heat collecting efficiency
2
the final stage of the system can also significantly increase freshwater yield. These design improvements will be our priority in the following researches. References [1] Glenn EP, Jed Brown J, O'Leary JW. Irrigating crops with seawater. Sci Am-Am Ed 1998;279:76–81. [2] Garcia-Rodriguez L, Romero-Ternero V, Gomez-Camacho C. Economic analysis of wind-powered desalination. Desalination 2001;137:259–65. [3] Miranda MS, Infield DA. wind-powered seawater reverse-osmosis system without batteries. Desalination 2003;153:9–16. [4] El-Ghonemy AMK. Water desalination systems Powered by Renewable energy sources: Review. Renew Sustain Energy Rev 2012;16(3):1537–56. [5] Li CN, Goswami Y, Stefanakos E. Solar assisted sea water desalination: A review. Renew Sustain Energy Rev 2013;19:136–63. [6] Sagie D, Feinerman E, Aharoni E. Potential of solar desalination in Israel and in its close vicinity. Desalination 2001;139(1–3):21–33. [7] Shatat M, Worall M, Riffat S. Opportunities for solar water desalination worldwide: review. Sustain Cities Soc 2013;9:67–80. [8] Li Weiyi, Krantz William B, Cornelissen Emile R, et al. A novel hybrid process of reverse electrodialysis and reverse osmosis for low energy seawater desalination and brine management. Appl Energy 2013;104(2):592–602. [9] Li C, Besarati S, Goswami Y, et al. Reverse osmosis desalination driven by low temperature supercritical organic rankine cycle. Appl Energy 2013;102(2):1071–80. [10] Sayyaadi H, Saffari A. Thermoeconomic optimization of multi effect distillation desalination systems. Appl Energy 2010;87(4):1122–33. [11] Al-Sulaiman FA, Narayan GP, John HLV. Exergy analysis of a high-temperaturesteam-driven, varied-pressure, humidification–dehumidification system coupled with reverse osmosis. Appl Energy 2013;103(1):552–61. [12] Delyannis E. Historic background of desalination and renewable energies. Sol Energy 2003;75(5):357–66. [13] Miranda MS, Infield D. A wind-powered seawater reverse-osmosis system without batteries. Desalination 2003;153:9–16. [14] Garcia-Rodriguez L, Romero-Ternero V, Comez-Camacho C. Economic analysis of wind-powered desalination. Desalination 2001;137:259–65. [15] Rajaseenivasan T, Shanmugam RK, Hareesh VM, et al. Combined probation of bubble column humidification dehumidification desalination system using solar collectors. Energy 2016;116:459–69. [16] Moumouh J, Tahiri M, Salouhi M, et al. Theoretical and experimental study of a solar desalination unit based on humidification–dehumidification of air. Int J
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Performance study on a passive solar seawater desalination system using multi-effect heat recovery
T
⁎
Shuang-Fei Lia, Zhen-Hua Liua, , Zhi-Xiong Shaob, Hong-shen Xiaoc, Ning Xiab a
School of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240, China Sunshore Solar Energy Co., Ltd, Nantong 226000, China c School of Mechanical Engineering, Nantong Vocational University, Nantong, China b
H I G H L I G H T S solar seawater desalination system with multi-effect heat recovery processes. • ASystem uses all-glass evacuated tube absorber as heat collector with a simplified CPC. • Mechanism of its operation are multi-stage distillation and heat recovery. • The system operated under barotropic or atmospheric pressure without power consumption. •
A R T I C L E I N F O
A B S T R A C T
Keywords: Solar energy Seawater desalination Heat recovery CPC
A novel small-sized solar seawater desalination system with multi-effect heat recovery processes using all-glass evacuated tube absorber as heat collector, in which there is no power pump and the steam and freshwater flow are driven only by pressure drop, was designed and tested. The whole system consists of 7 heat collecting/heat recovery integration units, which were divided into 7 temperature/pressure states. Each unit has a heat collector which consists of a simplified CPC panel, an all-glass evacuated tube absorber, a seawater tank and a bar heat pipe that connects the absorber and seawater tank to transfer heat from the absorber to the seawater tank. Every unit operated under barotropic or atmospheric pressure. Meanwhile, a stepwise heat recovery method was adopted to recycle the sensible heat and latent heat of the steam generated. In order to investigate the effects of operating parameters on system performance, including freshwater yield, solar collecting performance and heat recovery performance, a series of experiments were conducted under different weather condition. It can be found that the all-day freshwater yield of unit area can reach 4.23 kg/m2 on the sunny day and 3.03 kg/m2 on the cloudy day. Meanwhile the collecting efficiency and comprehensive thermal coefficient can reach 0.41 and 1.39 respectively. The experiment results confirm that the designed system has a superior performance in seawater desalination without power consumption.
1. Introduction Water is the most active and influential factor in the ecological environment and it is also one of the most valuable resources in the world. However, about 97% of the Earth’s water is salt water in the ocean, and a tiny 3% is fresh water [1]. At present, about 1.5 billion people in more than 80 countries around the world face the shortage of fresh water. By 2025, it is projected that there will be 3 billion people lack of water, involving more than 40 countries and regions. Water resource is becoming a valuable resource in the twenty-first Century; as a result, the problem of water resource is not only a resource problem, but also a major strategic issue related to the national economy, social ⁎
sustainable development and long term stability. Therefore, it is very urgent to solve the problem of water shortage. Desalinating salt water from the ocean, river, lakes is the best way to supply fresh water to growing population. Seawater desalination techniques are mainly divided into two categories on the basis of different elements of fresh water production, namely, the membrane processes, for which reverse osmosis (RO) and electrodialysis (ED) were utilized [2,3] and the thermal processes whose thermal energy may be obtained from a conventional fossil-fuel source, nuclear energy or from a non-conventional solar energy, etc. [4–6]. Moreover, the thermal processes are broadly classified into two major categories, direct and indirect systems. The indirect method of
Corresponding author. E-mail address: [email protected] (Z.-H. Liu).
https://doi.org/10.1016/j.apenergy.2018.01.064 Received 10 November 2017; Received in revised form 2 January 2018; Accepted 22 January 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
Acpc Gm Gm,all Gm′ Δh n PR qr Q Q̇
ηre ηt ε
2 3 4 5 6 7 Sys Ac Th
horizontal projection area of CPC panel (m2) freshwater yield (kg/h) all-day freshwater yield of the system (kg) freshwater yield of unit collecting area (kg/(h m2)) enthalpy difference (J/kg) number of collecting units in the system performance ratio of desalination irradiance (W/m2) power (W) useful power extracted from collector (W) heat recovery efficiency heat collecting efficiency comprehensive thermal coefficient
Unit 2 Unit 3 Unit 4 Unit 5 Unit 6 Unit 7 system actual theoretical
Acronyms and abbreviations CPC MED MEH/D
compound parabolic concentrator multi-effect distillation multi-effect humidification/dehumidification
Subscripts 1
Unit 1
both transient and steady states. Meanwhile, a mathematical model of heat and mass transfer was developed. Patricia Palenzuela et al. [21] conducted work to analyze whether the integration of Multi-Effect Distillation (MED) process into Concentrating Solar Power (CSP) plants can be more competitive, under certain conditions, than the independent freshwater and power production by connecting a Reverse Osmosis (RO) system to a CSP plant. Guo Xie et al. [22] presented a novel conceptual design of a low temperature multi-effect desalination (LT-MED) system and is composed of several modules of tubular solar still (TSS) cells. Reddy et al. [23] developed a novel multi-effect evacuated solar desalination system utilizing latent heat recovery and investigated the effect of various design and operating parameters on the system performance to optimize the configuration. Estahbanati et al. [24] conducted experiment to investigate the effect of the number of stages on the productivity of a multi-effect active solar still. Dayem [25] conducted work to demonstrate experimentally and numerically the performance of a simple solar distillation unit that is based on the multiple condensation-evaporation cycle. Chorak et al. [26] presented and discussed an experimental characterization of the solar MED system under off-design conditions. So far, a lot of research relate to the MED systems have been published, however, many of them need auxiliary energy consumption during runtime. In addition, most of them have large floor space and high cost. In order to design a MED system that is suitable for the island and small fishing boats where the solar energy is rich, but electricity, fresh water and fossil fuel are rare, our research team developed a principle passive solar desalination system with multi-effect evaporation/heat recovery processes and verified the feasibility of the device [27], which is the mechanism research for the system presented in this paper. Furthermore, Liu ZH carried out some modification measures to optimize the principle system and considered more operating parameters for analyzing the system performance [28]. This system integrated solar collecting, seawater evaporation, heat recovery and condensation processes into the common evacuated tubular solar collector to produce freshwater directly. The system operated under barotropic and atmospheric pressure and adopted a stepwise heat recovery method to recycle the sensible heat and latent heat of the steam generated. However, for this principle system, it is difficult to develop a commercial product due to its especial fabrication. In this research, based on the principle system, a small-sized solar seawater desalination system with multi-effect heat recovery processes using all-glass evacuated tube absorber as heat collector, in which there is no power pump and the steam and freshwater are driven only by pressure drop, is designed and tested. The originality of this system
solar desalination plant consists of two subsystems, solar collector and desalination unit. The different types of solar collector such as evacuated tube, flat plate and heat pipe can be used along with the thermal desalination processes such as multiple effect evaporation (MEV), vapor compression (VC) and multi-stage distillation (MSD) [7]. In addition, the solar distillation systems can be classified as passive solar still and active solar still. In a passive solar still, the solar radiation is received directly by the basin water and is the only source of energy for raising the water temperature. However, in an active solar still, an extra thermal energy is supplied to the basin through an external mode to increase the evaporation or a vacuum pump is used to maintain the vacuum working condition. The water demand is increasing rapidly than the sustainable level and desalination is the best method to provide the shortfall of water. From the perspective of energy consumption, most of the desalination systems [8–11] are energy intensive, which consume high grade energy like gas, electricity, oil and fossil fuels accelerating the global warming that is the burning topic and becomes threat to life sustainability. Renewable energy is the alternative solution to decreasing consumption of fossil fuels [12]. So far, there are already several kinds of renewable energy applied to seawater desalination such as wind energy [13,14] and solar energy. Considering that the solar energy can be efficiently harvested for solar to implement the heat application for green and environmental sustainability, it is one of the most promising applications of renewable energies in thermal desalination processes. So far, there have been many solar desalination researches carried out as follows. Many researchers have carried out research on solar humidification/dehumidification desalination system (HDDS) [15,16]. However, the performance of HDDS seriously depends on the thermal properties of working medium and the humidification/dehumidification ability is weak at low temperature difference. Besides the HDDS, Khamid Mahkamov et al. [17] developed and tested an innovative small dynamic water desalination plant that is a combination of a heat pipe evacuated tube solar collector, conventional condenser and novel fluid piston converter. Young-Deuk Kim et al. [18] proposed a hybrid desalination system consisting of vacuum membrane distillation (VMD) and absorption desalination (AD) units, designated as VMD-AD cycle. Nematollahi et al. [19] carried out an experimental and theoretical energy and exergy analysis for a solar desalination system consisting of a solar collector and a humidification tower. In addition, the multi-effect desalination system attracts many researchers due to its high-energy efficiency. Zhili Chen et al. [20] designed a multi-stage stacked-tray solar seawater desalination still and test water production performance in 344
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consists of the following aspects: (1) The most originality is that this is a novel passive system without pumps. (2) The heat pipe was utilized as the heat transfer element, which realizes the rapid heat transfer between the collector section and the desalination unit. (3) A stepwise heat recovery method was adopted to recycle the sensible heat and latent heat of the steam generated, which is realized by several pressure regulating valves. In order to investigate the effects of operating parameters on system performance, including freshwater yield, solar collecting performance and heat recovery performance, a series of experiments were conducted under different weather and irradiance conditions. Through the experiments, the heat collection performance, heat recovery performance and desalination performance have been discussed and analyzed. The experiment results confirm that the designed system has a superior performance in seawater desalination without power consumption. The experimental research has a significant reference value for the development of a passive small-sized solar desalination system.
Fig. 2. A photograph of solar seawater desalination system with 7 collecting units.
2. Experimental apparatus and procedure 2.1. Experimental apparatus The designed seawater desalination system is made up of 7 heat collecting/heat recovery integration units, which were divided into 7 temperature/pressure states. Fig. 1 gives out a schematic view of the whole system for every units and connection relationship in which the saturated temperature of steam generated at every unit is illustrated, and Fig. 2 shows a photograph of the actual experimental apparatus. As shown in Fig. 1, the designed system mainly consists of seven linked units for heat collecting/recovery, air cooler and freshwater collector. The apparatus is located in Tongzhou District, Nantong, Jiangsu, China. The precise location is “N32°06′45.38″, E121°0′17.36″”. In addition, the average local irradiance is above 500 W/m2 during work time (from 9:00 to 15:00). In the first unit, the energy used to evaporate seawater is only obtained from solar energy. However, in the remaining 6 units, the steam produced at the upper unit releases the latent heat to the seawater in the next unit through the heat recovery coil tube. The steam/water mixture outlet of the last unit is connected with the naturally convective air cooler that is a passive cooler and has no energy consumption to condense the final steam-water mixture. And then, the condensed freshwater was stored in the freshwater collector. The Units 2–7, as depicted in Fig. 3, have the same structure containing a seawater tank, a pressure reducing valve, a simplified CPC panel, a bar heat pipe, an all-glass evacuated tube, a seawater inlet, a steam/water mixture inlet, a steam outlet tube, a steam/water mixture outlet and a heat recovery coil tube. In every unit, evaporation section of heat pipe is concentrically mounted into the all-glass evacuated tube, meanwhile, the condensation section of heat pipe is concentrically mounted into the heating tube fixed in the center of the seawater tank. The first unit (Unit 1), as a simple heat collector without heat recovery process, doesn’t have a heat recovery coil tube installed inside.
Fig. 3. Structures of the unit (red point: measuring point of thermocouples). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
By such arrangement, the collected solar energy can be rapidly transferred into the seawater tank by using a bar heat pipe to heat the seawater under small temperature difference. The steam generated from unit 1 flows into the heat recovery coil tube fixed in the seawater tank of unit 2 through the steam/water mixture inlet. Similarly, from
Fig. 1. Designed system for every units and connection chart Blue line: seawater, red line: saturated water, green line: vapor. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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experiment, the steam temperature and pressure were changing all the time with this control method. As a compromise, this control method is feasible and has the advantage of low cost and simple operation. In the present stage, a naturally convective air cooler was used for final steam-water mixture condensation and there is not energy consumption in this passive heat exchanger. In this experimental system, the steam temperatures, the wall temperatures of heat pipe, the temperatures of steam/water mixture inlet and outlet were measured for investigating both the solar collecting and heat recovery performances. The measuring positions of thermocouples are displayed in Fig. 3. For each heat pipe, the temperatures at evaporation section, adiabatic section and condensation section were monitored to ensure the heat pipe is in normal working condition. For each unit, the steam temperatures were measured to monitor its operating temperatures. From unit 2 to unit 7, the temperatures of steam/water mixture inlet and outlet were measured to study the heat recovery performance. In addition, a TES-133R solar radiometer was utilized to measure the solar irradiance. As long as the measuring surface of the solar radiometer was placed in parallel with the heat collecting unit, the real-time solar irradiance perpendicular to the CPC plate can be obtained.
unit 2 to unit 7, in each seawater tank, there is a steam/water mixture inlet and a steam/water mixture outlet to conduct both the steam generated and the water condensed into the heat recovery coil tube fixed in the seawater tank of next unit. Therefore all latent heat of the steam can be recovered in the last six units for the evaporation of seawater. In this designed system, a steam condenser that consists of coil tubes and use natural convective heat transfer of air to cool fluid in tube, is installed at the final stage in the system to fully condense the rest steam from the last unit and the freshwater is finally stored in the freshwater collector. In addition, the cumulative freshwater influx was measured every 15 min to get the mean freshwater yield. The irradiance, steam temperatures and wall temperatures of the tubes in each unit were measured, collected and processed by the data acquisition system. Data were logged using Agilent 34970A data-logger connected to the computer. 2.2. Geometry design of simplified CPC For obtaining a relatively high collecting temperature and steam pressure, a CPC-type concentrator was utilized in this study which is the same as that in the authors’ principle study [28]. The design method of standard CPC for tubular all-glass evacuated tube comes from Winston [29] and Rabl [30–32]. In addition, Alaydi [33] and Kalogirou [34] applied parabolic solar-collector for seawater desalination, and Lillo et al. [35] evaluates the potential of solar concentration technologies as an alternative to conventional sources of energy for industrial processes in Latin America. For reducing CPC material and increasing collecting time, the standard CPC was cut to a truncated CPC. Then, for further reducing machining cost, bottom involute shape was cut to flat shape. Fig. 4 presents a finally simplified CPC profile after truncation and involute shape cutting. According to the calculation and test results, the concentration efficiency of the simplified CPC with a flat curve bottom decreases by about 15% compared with the truncated CPC with an involute bottom [36]. The CPC is made of a stainless steel mirror plate welded on the steel shelf directly. For the present tubular absorber, the concentration ratio of the CPC is defined as the ratio of the aperture area of the CPC to the peripheral area of the tubular absorber. According to this definition, the concentration ratio of the present CPC is 2.16 which does not depend on the acceptance half angle of standard CPC. The characteristics of the presented collector part including CPC, all-glass evacuated tube and bar heat pipe are shown in Table 1.
2.4. Uncertainty analysis Five main parameters are considered during the test, including instantaneous freshwater yield, performance ratio, heat collecting efficiency and heat recovery efficiency, which are calculated as follows:
Gm =
m t
(1)
PR =
Gm Δh Q̇
(2)
ηt ,exp =
ε=
Gm′ h Acpc qr
(3)
Gm Δh ACPC nqr
ηre =
(4)
Gm,ac Gm,all−7Gm,1 = Gm,th 21Gm,1
(5)
where m is the freshwater yield within 15 min, t is the time interval, t = 15 min = 0.25 h. According to the relative standard deviation of the transfer function theory, the maximum relative deviation of instantaneous freshwater yield, performance ratio, heat collecting efficiency and heat recovery efficiency can be derived from Eqs. (1)–(4) as follows:
2.3. Experimental procedure In the experimental process, NaCl solution with a mass fraction of 3.5% and temperature of 20 °C–25 °C is used to substitute sea water. In addition, the ion level of Na in the freshwater and NaCl solution have been measured by Inductively Coupled Plasma Optical respectively and the value is 0.0138% for the freshwater and 3.414% for the NaCl solution. Then, the temperature and pressure of the saturated steam in each unit were controlled by the pressure reducing valves. All these valves were regulated only once at noon when the direct irradiance was approximately strongest. And then, the highest temperatures of the steam in each unit were dependent on their own without the further adjustment of the valves. Because the designed solar desalination system was demanded to run all year round, the highest collecting temperature should not be too high and could be achieved even in the winter. Meanwhile, it should achieve six-effect heat recovery, and the temperature gradient of two adjacent units should not be too large. Since the steam temperature in unit 7 is 100 °C corresponding to the atmospheric pressure, the ideal distribution of the steam temperature is that the steam temperature in unit 1 is controlled at about 150 °C and decreased by 8 °C at each next unit. According to the preparatory adjustment, the maximum difference between the two adjacent units was set at 8 °C. As a matter of fact, with the irradiance changing during the
σGm = Gm
2
2
⎛ ∂lnGm ⎞ σm2 + ⎛ ∂lnGm ⎞ σt2 ⎝ ∂m ⎠ ⎝ ∂t ⎠
Fig. 4. Simplified CPC profile for an all-glass evacuated tube (unit: mm).
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long term test data of all-day freshwater yield of unit collecting area where the temperature represents the average environment temperature during the test process. For the laboratory prototype stage, even if the device runs continuously, it is not necessary to carry out performance tests every day. The performance tests were conducted randomly according to the weather condition. In addition, the latest 5 sets of test data are adopted to further analyze the performance of freshwater production including freshwater yield and performance ratio, the steam temperatures and the system efficiency including heat collecting efficiency, comprehensive coefficient and heat recovery efficiency have been discussed.
Table 1 Characteristics of the collector system. Parameter
Value
CPC Width of CPC: Effective length of CPC Height of CPC: Aperture area of a CPC plate Radius of absorber Distance of CPC cusp and absorber Acceptance half angle of standard CPC Edge-ray angle of truncated CPC Reflectivity of CPC Concentrating ratio All-glass evacuated tube Cover glass tube diameter Inner glass tube diameter Effective length glass tube Bar heat pipe Outer dia. of bar heat pipe Inner dia. of bar heat pipe Effective length Wall thickness Working fluid Stainless steel screen Material Wire diameter Mesh number
ηt ,exp
σε = ε σηre ηre
=
⎜
⎟
2 ⎛ ∂lnηt ,exp ⎞
⎜
⎟
⎝ ∂Gm′ ⎠
σG2m′ +
Gm∗ =
304 stainless steel 0.12 mm 150 meshes
(11)
⎟
(7)
2 ⎛ ∂lnηt ,exp ⎞
⎜
⎝
⎟
∂qr
⎠
σq2r
3.1.1. Analysis of freshwater yield Fig. 6 shows the data summary of all-day freshwater yield of unit area, all-day freshwater yield and the average irradiance for unit 1 under different weather conditions to shows the effect of alone heat colleting capacity on freshwater yield. Meanwhile, Fig. 7 shows the data summary of all-day freshwater yield of unit area, all-day freshwater yield and average irradiance for the whole system to show the multiple effect of both heat collecting and heat recovery on freshwater yield. As manifested in Figs. 6 and 7, the solar radiation intensity was closely related to weather conditions. The maximum average irradiance during sunny days was about 769.69 W/m2, 26.7% higher than that during the cloudy day (607.52 W/m2). Furthermore, it is obvious that the all-day freshwater yield depends on the average irradiance and is basically proportional to the average irradiance. For unit 1, there is no heat recovery process, that is to say, the freshwater was only produced by the effective heat collection of CPC panel. According to Fig. 6, it can be found that the maximum all-day freshwater yield can reach 1.2 kg on the sunny day and 0.95 kg on the cloudy day for one collecting glass tube. Correspondingly, the maximum all-day freshwater yield of unit area can reach 2.3 kg/m2 on the sunny day and 1.98 kg/m2 on the cloudy day. It can be estimated by this data that the maximum heat collecting efficiency can reach 41% on the sunny day and 34% on the cloudy day for even a high collecting temperature of 140 °C as shown in Fig. 13. It is obvious that the designed system has a good heat collecting performance. For the whole system, there is a heat recovery process in each unit from unit 2 to unit 7 to recover the latent heat of steam generated in the previous unit. As manifested in Fig. 7, the maximum all-day freshwater yield can reach
(8)
2
2
Gm Acpc × n
The calculation of all-day freshwater yield and all-day freshwater yield of unit collecting area is similar to Eqs. (1) and (11), the difference is that m is the value of all-day freshwater production; t is the all-day working hours.
⎟
2
=
45 mm 41 mm 2800 mm 2 mm Water
⎛ ∂lnε ⎞ σ 2 + ⎜⎛ ∂lnε ⎟⎞ σ 2 Gm qr ⎝ ∂Gm ⎠ ⎝ ∂qr ⎠ ⎜
The amount of water production is obtained by weighing the freshwater collected in a bucket. In order to improve measurement accuracy, the actual measurement time interval is 15 min. Therefore, the instantaneous value of the freshwater yield is the average value within 15 min and calculated by Eq. (1). The instantaneous value of the freshwater yield of unit collecting area is obtained by Eq. (11).
2
2
⎜
3.1. Freshwater production performance
58 mm 47 mm 1600 mm
⎛ ∂lnPR ⎞ σ 2 + ⎛ ∂lnPR ⎞ σ 2 Gm Q̇ ⎝ ∂Gm ⎠ ⎝ ∂Q̇ ⎠
σPR = PR σηt,exp
330 mm 1600 mm 146 mm 0.512 m2 24 mm 6 mm 18o 65o 0.85 2.16
(9) 2
⎛⎜ ∂lnηre ⎞⎟ σ 2 ⎛ ∂lnηre ⎞⎟ σ 2 Gm,all + ⎜ Gm,1 ∂ G ⎝ m,all ⎠ ⎝ ∂Gm,1 ⎠
(10)
In the experiment process of the designed system, the parameters that need to be measured include temperature, fresh water mass and solar irradiance. In addition, the time also need to be recorded. For the measurement of temperature, GG-K-30 thermocouples were utilized with ± 0.1 K measurement errors. An electronic scale with ± 0.005 kg measurement errors was adopted to measure the mass of freshwater obtained in a measuring interval of 15 min. A TES-133R solar radiometer was utilized to measure the solar irradiance. As long as the measuring surface of the solar radiometer was placed in parallel with the heat collecting unit, the real-time solar irradiance perpendicular to the CPC plate can be obtained and the measurement error was ± 4%. For the record of time, it was achieved by the system time of computer which was consistent with “Beijing time”. In addition, because the record of fresh water mass was manual operation consuming time, there existed a time measurement error with about ± 3 s. The measurement errors of the parameters in the experiment are listed in Table 2. According to Eqs. (5)–(8), the relative errors of instantaneous freshwater yield, performance ratio, heat collecting efficiency and heat recovery efficiency are less than 4.3%, 6.1%, 7.4%, 5.9% and 9.7%, respectively.
Table 2 Measurement errors for experimental parameters. Parameters
Measurement errors
Temperature Fresh water Solar irradiance Time
± 0.1 K ± 0.02 kg/h ± 4% ±3s
3. Results and discussion In order to verify the superior performance of the designed solar seawater desalination system, a series of experiments have been conducted under different weather conditions. Fig. 5 shows some typical 347
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800
7
700
6
600
Fig. 5. The long term test data of all-day freshwater yield of unit collecting area.
Average solar radiation intensity
5
500
All-day freshwater yield of unit collecting area
400
3
300
2
200
1
100
2
4
Average solar radiation intensity /W/m
14-3-16 21-4-16 17-8-16
21-7-16
20-7-16 23-7-16 22-7-16
17-10-15
7-12-15
8-1-16
Autumn Sunny 20.7 °C
Winter Sunny 8.2 °C
Winter Spring Spring Summer Summer Summer Summer Summer Sunny Sunny Cloudy Cloudy Sunny Cloudy Cloudy Sunny 3.5 °C 7.6 °C 19.8 °C 30.3 °C 31.1 °C 31.8 °C 32.3 °C 31.5 °C
900
20
All-day freshwater yield of unit area (kg/m ) 800 All-day freshwater yield (kg)
18
Average solar irradiance 2
700 600 500 400 300 200
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All-day freshwater yield of unit area (kg/m )
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800
Average solar irradiance 700
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600
12
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400
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300
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100
0
0
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22-7-16
2
100
0
Average solar irradiance /(W/m )
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Average solar irradiance /(W/m 2 )
All-day freshwater yield of Unit 1
0
All-day freshwater yield
All-day freshwater yield of unit collecting area /kg/m
2
8
0
Summer Summer Summer Summer Summer Sunny Sunny Sunny Cloudy Cloudy
Summer Summer Summer Summer Summer Sunny Sunny Sunny Cloudy Cloudy
Fig. 7. Data summary of all-day freshwater yield (black column), all-day freshwater yield of unit area (red column) and the average irradiance for the whole system. summary of all-day freshwater yield of unit area (black col
Fig. 6. Data summary of all-day freshwater yield of unit area (black column), all-day freshwater yield (red column) and the average irradiance for unit 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
relatively higher average irradiance. It is also found that the change of instantaneous freshwater yield was basically in accordance with that of irradiance, which illustrates that the instantaneous freshwater yield is very sensitive to the irradiance. The change of irradiance not only causes change of the heat collecting efficiency, but also causes change of the heat recovery efficiency which is more important for freshwater yield. In addition, there was no steam generation before 11:15 whether the weather was sunny or cloudy, because a large amount of seawater accumulated in the seawater tank was being heated form a relatively low temperature to the saturated temperature before noon. When the weather was sunny, the maximum instantaneous freshwater yield appeared at noon and the value was about 1.51 kg/(h m2). In the afternoon, the instantaneous freshwater yield decreased gradually as the time goes on until there was no freshwater generation. When the weather was cloudy, the instantaneous freshwater yield fluctuated with the change of irradiance and the maximum value was 1.5 kg/(h m2) that was close to the value (1.51 kg/(h m2)) of sunny day, even so, it is obvious that the average value of instantaneous freshwater yield in cloudy day was much lower than that in sunny day.
17 kg on the sunny day and 12 kg on the cloudy day. Correspondingly, the maximum all-day freshwater yield of unit area can reach 4.23 kg/ m2 on the sunny day and 3.03 kg/m2 on the cloudy day. Based on the comparison of the freshwater yield of unit 1 and whole system, it can be found that the maximum all-day freshwater yield of unit area can be increased by about 2 times, the mean all-day freshwater yield of unit area of whole system can be increased by 1 times for all days, which is attributed to the heat recovery process. It can be confirmed that the heat recovery effect was relatively ideal whether the weather was sunny or cloudy. 3.1.2. Analysis of instantaneous freshwater yield Fig. 8 depicts the instantaneous freshwater yield of unit collecting area and irradiance along with time under different weather conditions to show the effect of instantaneous irradiance on the freshwater yield. It is clear from Fig. 8 that the irradiance fluctuated violently in cloudy weather, which leads to the lower average irradiance. On the contrary, the irradiance fluctuated gently in sunny weather, which leads to 348
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1400
3.0
1.5
1200
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1000
0.0
800
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1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 -100 -200 14:45
Solar irradiance (W/m )
0.5
2.0
Performance ratio
2
1.0
Solar irradiance (W/m 2 )
Freshwater yield /(kg/h · m )
S.-F. Li et al.
Time /hr
Fig. 8. Instantaneous freshwater yield of unit collecting area and irradiance changes with time.
Fig. 9. Performance ratio and irradiance changes with time.
the solar radiation was stronger, the temperature difference between adjacent units became larger, meanwhile, the steam flow rate was also larger, the heat recovery performance was better, and therefore, the value of PR was relatively large. On the contrary, when the solar radiation was weaker, the temperature difference between adjacent units became smaller, meanwhile, the steam flow rate was smaller, the heat recovery performance deteriorated, which led to the rapid decrease of PR. According to the data from Fig. 9, it is found that the actual PR is much less than the theoretical value and there is great improvement space for the heat recovery capacity by increasing the heat transfer area of coil heat exchanger installed in the seawater tank which will only cause small cost increase. If the actual PR value can increase 50%, that means the maximum instantaneous value reaching 4 and mean value of all-day reaching 3, then, the freshwater yield will increase 50%.
3.1.3. Analysis of performance ratio The performance ratio (PR) is an important performance factor to evaluate the performance of desalination system and it is defined as the ratio of the total energy used to evaporate seawater into freshwater including the energy recovered to the net energy absorbed from the collector. PR represents the heat recovery performance of the system and it would be 1 if there is no heat recovery effect. The theoretical value of PR is determined by the system structure and calculated by the Eq. (2). Assuming that the collecting power of each panel, Q̇ , is the same and the latent heat of vaporization of water can be fully recovered, then the power used to evaporate seawater in each unit is as follows:
Q1 = Q̇
(12a)
Q2 = Q̇ + Q1 = 2Q̇
(12b)
Q3 = Q̇ + Q2 = 3Q̇
(12c)
3.2. Analysis of steam temperature difference between each unit
Q4 = Q̇ + Q3 = 4Q̇
(12d)
Q5 = Q̇ + Q4 = 5Q̇
(12e)
Q6 = Q̇ + Q5 = 6Q̇
(12f)
Q7 = Q̇ + Q6 = 7Q̇
(12g)
The designed system operates in passive mode and both freshwater and steam flow depends on the steam temperature difference (the saturated steam pressure difference) between each unit. Therefore, it is of vital importance to understand the steam temperature difference between each unit. Figs. 10 and 11 show the steam temperature curves of each unit and irradiance curves along with time on sunny and cloudy day in summer, respectively. As depicted in Figs. 10 and 11, in the morning, the water accumulated in the system was being heated from a relatively low temperature to the saturated temperature, hence there was no steam generation during this period and the steam temperature was below 100 °C. The amount of heat required to heat the total seawater with a maximum value of 34 kg in seven seawater tanks from initial 20 °C to a mean saturated temperature of 120 °C of the whole system takes a preheating time of about 2.4 h. This preheating time is too long for an actual application. Since this study focused on the collecting and heat recovery performance, thence had not used auxiliary preheating device. When the steam temperatures of each unit reached above 100 °C at about 11:15, the steam started to emerge. At noon, the steam temperatures and steam pressures in each unit reached the maximum set value and the temperature difference between two adjacent units can reach the designed value of 8 K on sunny day. In the afternoon, the temperature difference gradually narrowed with the decrease of solar radiation. In the next study, the system will install a passive type of seawater preheater to preheat the cold seawater to hot seawater with about 90 °C–95 °C for experiment in the following day. If the temperature of seawater in the preheater can hold at about 70 °C in the morning of the following day, then about 1.2 h preheating time will be saved and the
where, Q1 ∼ Q7 are the total power used to evaporate seawater in unit 1 ∼ unit 7, respectively. Q̇ is the net power from collector in each unit. Hence, the maximum theoretical value of the PR for the system is calculated as follows:
PR =
Q1 + Q2 + Q3 + Q4 + Q5 + Q6 + Q7 28Q̇ = =4 7Q̇ 7Q̇
(12h)
Due to the existence of heat recovery process, the PR will be bigger than 1, and the bigger the value is, the better the heat recovery effect will be. In this designed system, the maximum theoretical value of PR is 4 according to Eq. (12a-h). Fig. 9 shows the actual performance ratio curves and irradiance curves along with time under different weather conditions. It can be noticed that the fluctuation of instantaneous value of PR along with time was similar to that of instantaneous freshwater yield that is closely related to the irradiance. According to Fig. 9, on the sunny day, the actual PR value increased gradually with the increase of irradiance before 12:15 and almost remained stable during the 12:00 – 13:15 period, and then decreased gradually with the decrease of irradiance after 13:15. The maximum instantaneous value of PR can reach 2.5 at the noon, and mean value of PR for all-day is close to 2. The change of PR mainly depends on the change of heat recovery performance. When 349
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Unit 2
Unit 3
Unit 5
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Unit 4
Unit 1
Unit 6 Unit 7
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120
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Solar irradiance (W/m )
Steam temperature (
170
efficiency of the unit 1 which is a simple heat collector without heat recovery part was used for representing the system collecting efficiency and its efficiency may be slightly lower than the actual system efficiency due to its higher collecting temperature compared with other units. The heat collecting efficiency is defined as the ratio of the net energy absorbed by the seawater in seawater tank to the total energy incident on collector, and its instantaneous value is calculated by Eq. (3). Fig. 12 plots the instantaneous heat collecting efficiency curves of unit 1, which is a mean measured value in 15 min and may represent mean heat collecting efficiency of system, and instantaneous irradiance curves along with time under different weather conditions. According to Fig. 12, on the sunny day, the heat collecting efficiency increased gradually in the morning, on the contrary, it decreased gradually in the afternoon. On the cloudy day, the heating collecting efficiency fluctuated acutely with the fluctuation of irradiance. The maximum value of the instantaneous heat collecting efficiency can reach 0.42 on the sunny day and 0.34 on the cloudy day. It is obvious that the designed system possesses superior instantaneous heat collecting performance during a short noon time. However, the heat collecting efficiency decreased rapidly in the afternoon. One of the reasons is that the solar radiation decreased in the afternoon. The second reason is the structure of CPC can’t make all the solar energy be projected to the CPC panel, which reduces the collecting efficiency and it is also a limitation of nontracking type collectors. But in terms of practical and civilian applications, this limitation is acceptable. In the present system, the acceptance half angle of standard CPC and the edge-ray angle of truncated CPC are 18° and 65°, respectively. If the acceptance half angle of standard CPC increase by 1 time with the same concentration ratio, the highest collecting temperature at noon will have a decline, but, both the all-day collecting time and the mean heat collecting efficiency of all-day will significantly increase. According to the experimental results, the highest collecting temperature was not the key issue for the designed system; therefore, it’s meaningful to further improve CPC design.
900
-300
100
90 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00
Time /hr Fig. 10. The curves of steam temperatures in each unit changing with time on the sunny day.
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170 150 140 130
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Unit 2 Unit 3 Unit 4 Unit 5
Unit 1
Unit 6 Unit 7 300
120 110
0
2
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Solar irradiance (W/m )
Steam temperature (
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900
90 80 -300 11:00 11:30 12:00 12:30 13:00 13:30 14:00 14:30 15:00
Time /hr Fig. 11. The curves of steam temperatures in each unit changing with time on the cloudy day.
3.3.2. Comprehensive thermal coefficient Besides the heat collecting efficiency, the comprehensive thermal coefficient is also an important parameter for the solar desalination system with heat recovery process which reveals the total thermal performance of the system and is defined as the ratio of the total energy used to evaporate seawater into freshwater to the total energy incident on the solar collector. The parameter can be seen as an integrated indicator to judge the solar energy collection and heat recovery performance of the system, in addition, the value of comprehensive thermal coefficient may be greater than 1 due to the existence of heat recovery process and is calculated by Eq. (4). Fig. 13 shows the comprehensive thermal coefficient curves and irradiance curves along with time under different weather conditions. It 1400
0.40
1300 1200
Collecting efficiency
0.35
3.3. Analysis of system efficiency
1000
0.25
900
0.20
800 700
0.15
0.05 0.00 11:15
600 17-8-2016 21-7-2016 20-7-2016 23-7-2016 22-7-2016
11:45
17-8-2016 exp. 21-7-2016 exp. 20-7-2016 exp. 23-7-2016 exp. 22-7-2016 exp.
12:15
12:45
13:15
500 400 300 13:45
14:15
Time /hr Fig. 12. The collecting efficiency changing with time.
350
200 14:45
2
0.10
3.3.1. Heat collecting efficiency The system heat collecting efficiency is an important indicator to judge the solar energy collecting performance of a solar collecting system. Because the present system has heat recovery process in the whole heat transfer process, the system collecting efficiency cannot be simply measured by using the final steam yield. The collecting
1100
0.30
Solar irradiance (W/m )
effective steam generating time will prolong 1.2 h, which means the freshwater yield will increase about 30% according to the present system performance. By comparing Fig. 8 with Fig. 10 and Fig. 11, it is noticed that the changing regularity of steam temperature difference was similar to that of instantaneous freshwater yield. When the steam temperature difference kept stable, the instantaneous freshwater yield was also basically stable. And when the steam temperature difference fluctuated, the fluctuation also occurred in the instantaneous freshwater yield. This is because the instantaneous freshwater yield is closely related to the heat recovery capacity, which strongly depends on the steam temperature difference. Furthermore, the collecting power reached the maximum at noon when the irradiance reached peak and at this time, the steam yield was the largest and the steam flow rate was large enough to ensure the set pressure difference. However, in the afternoon, with the irradiance gradually decreasing, the steam temperatures in the upper collecting units gradually decreased, which led to the rapid decrease in the steam temperature differences between the two adjacent units. Therefore, it is very difficult that actual mean PR of all-day reaches the theoretical value.
Applied Energy 213 (2018) 343–352
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11:30
12:00
12:30
13:00
17-8-2016 21-7-2016 20-7-2016 23-7-2016 22-7-2016 13:30
14:00
14:30
1000 900 800 700 600 500 400 300 200 100 0 -100 -200 -300 -400 -500 -600 15:00
transfer area and improvement of CPC design will be our priority in the following researches. 3.4. The comparison of performance between the tested system and conventional system Table 3 shows the comparison of performance between the tested system and conventional system and Table 4 lists the main efficiencies of the tested system. It can be noted from Table 3 that the conventional system have the disadvantages of complex structure, high energy-consumption, large size and cost, etc. Though most of them possess high freshwater capacity, they are not suitable for the area where energy is only solar energy. For the MED system presented in this paper, it is designed to serve island and small fishing boats where solar energy is rich, but electricity, fresh water and fossil fuel are rare. Though the freshwater capacity of the presented system is lower than that of conventional system, it possesses more practical value for island and small fishing coats due to its simple structure, low cost, small floor space and no auxiliary energy consumption. In addition, the performance improvement and structure optimization will be conducted in the further study to promote its commercialization.
2
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4
Solar irradiance (W/m )
Comprehensive thermal coefficient
S.-F. Li et al.
Time /hr Fig. 13. The comprehensive thermal coefficient for the system.
is affected by heat collection and heat recovery performance of the system, and the irradiance. For the designed system in this study, the temperatures of each unit were getting lower and lower with the decrease of solar irradiance as the time went on (as shown in Figs. 10 and 11). As a result, the total heat used to evaporate the seawater and the amount of steam decreased, which lead to the decrease of latent heat of steam and the deterioration of heat recovery performance. However, it is worth mentioning that the comprehensive thermal coefficient is not proportional to the irradiance or the collecting efficiency as shown in Fig. 13, and it holds a long stable period for 90 min in which the maximum value is 1.32 on the sunny day and 1.39 on the cloudy day. Though the maximum comprehensive thermal coefficient on the sunny day is smaller than that on the cloudy day, it is obvious the average comprehensive thermal coefficient on the sunny day is much larger than that on the cloudy day. In addition, compared with the maximum heat collecting efficiency of 0.42 on the sunny day shown in Fig. 12, the comprehensive thermal efficiency increases 2 times.
4. Conclusions A passive type of small-sized solar seawater desalination system with multi-effect heat recovery processes using all-glass evacuated collector was designed and tested in this study. According to the experimental results, the designed seawater desalination system reached primary demand and showed good freshwater production performance. (1) The all-day freshwater yield of unit area can reach 4.23 kg/m2 on the sunny day and 3.03 kg/m2 on the cloudy day without power consumption. Furthermore, the instantaneous system collecting efficiency can reach 0.41 at the collecting temperature of 140 °C on the sunny day and 0.34 on the cloudy day. In addition, the performance ratio can reach 2.5 on the sunny day and 2.3 on the cloudy day. The comprehensive thermal coefficient can reach 1.32 on the sunny day and 1.39 on the cloudy day. In addition, the allday mean recovery efficiency can reach 0.38 on the sunny day and 0.29 on the cloudy day. (2) Experimental study provides useful guidelines for the development of passive type of high effective solar desalination system. The present system can still be improved a lot due to the various design defects found in the experiment. The freshwater yield may be greatly increased through the increase of the heat transfer area in the heat recovery exchanger. In addition, design improvement of the CPC to increase collecting time. As mentioned above, preheat of the seawater in the seawater tank using passive steam condenser in All-day average efficiency of heat recovery
800 700 Average solar radiation intensity All-day average efficiency of heat recovery
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17-08-16
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0
Fig. 14. All-day average efficiency of heat recovery and average irradiance.
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0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
Average solar irradiance (W/m )
3.3.3. Heat recovery efficiency The heat recovery efficiency is an important indicator to judge the heat recovery performance of a solar seawater desalination system. For the designed system in this research, the heat used to evaporate seawater comes from two parts including heat collection and heat recovery in each unit. The heat recovery efficiency is defined as the ratio of actual freshwater yield (Gm,ac ) generated by heat recovery to the theoretical freshwater yield (Gm,th ) generated by heat recovery and calculated by Eq. (5). For instantaneous heat recovery efficiency, Gm,all and Gm,1 denote the freshwater yields of the whole system and the unit 1, respectively, in unit time. For all-day mean value, Gm,all and Gm,1 denote the freshwater yields of the whole system and the unit 1, respectively, in all-day. In Eq. (7), the freshwater yield of each unit is similarly substituted by that of unit 1. The all-day freshwater yields of unit area for the unit 1 which has not heat recovery equipment and the whole system (7 units) have been shown in Figs. 7 and 8; therefore, the all-day mean value of heat recovery efficiency may be obtained from the data management. Fig. 14 illustrates the all-day mean value of heat recovery efficiency and the mean solar irradiance for five typical weather conditions. According to Fig. 14, the maximum heat recovery efficiency can reach 0.38 on the sunny day and 0.29 on the cloudy day, which are much less than the theoretical value of 1. It is obvious that there is a great gap between the actual value and the ideal value due to the designed heat transfer area of coil heat exchanger for heat recovery is much less than the needed heat transfer area. By doubling the heat transfer area, the all-day mean values of heat recovery efficiency will be approximately doubled without great cost increase. How to improve the all-day mean values of heat recovery efficiency through the increase of the heat
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Table 3 The comparison of performance between the tested system and conventional system. Equipment type
Advantages
Disadvantages
Fresh-water production rate
Auxiliary energy consumption
MEH/D [25]
Keep running all day
7.74 L/(d m2)
2000–14,000 kJ/h
MED-14 effects [26]
High freshwater yield
Complex structure; need auxiliary energy; H/D ability is weak at low temperature difference High energy-consumption; equipment with large size and cost High energy-consumption; equipment with large size and cost Short effective working time; sensitive to weather conditions
127.29 L/(d m2)
190 kW
Hybrid solar/gas MED [37] The present study
24-h operation; operate in different modes; high freshwater yield Simple structure; low cost; no auxiliary energy consumption
Comprehensive thermal coefficient
Heat recovery efficiency
42%
139%
38%
5.6 L/(h m )
150 kW
4.23 L/(d m2)
0
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Table 4 The main efficiencies of the tested system. Heat collecting efficiency
2
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