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ScienceDirect Energy Procedia 70 (2015) 41 – 48

International Conference on Solar Heating and Cooling for Buildings and Industry, SHC 2014

Experimental study of the thermal performance for the novel flat plate solar water heater with micro heat pipe array absorber Yuechao Denga, Yaohua Zhaoa*, Zhenhua Quana, Tingting Zhua a

The Department of Building Environment and Facility Engineering, The College of Architecture and Civil Engineering, Beijing University of Technology, No.100 Pingleyuan, Chaoyang District, Beijing 100124, China

Abstract This paper presents a novel flat plate solar water heater (SWH) using micro heat pipe array (MHPA) sprayed solar selective coating and arranged closely as the absorber of collector. The collector has the advantage of resistance to freezing, high heat transfer ability, relatively low heat loss, elimination of welding, and prevention of leakage. The experiments were conducted first to investigate the thermal performance of the novel collector. The test results for the collector's instantaneous efficiency show that the slope and intercept of the instantaneous efficiency curve are 5.6 and 0.85 respectively. Then, the thermal performance monitoring analysis of the forced circulation SWH with 2 m2 novel collector under different weather conditions were carried out using trial installations in Beijing, China. The test data were analyzed from the aspects of water temperature, effective heat gain, and thermal efficiency. The test results show that the daily effective heat gains on the three typical days in different seasons are 13.43MJ/m2, 11.05MJ/m2, and 7.42MJ/m2 respectively, corresponding to the solar irradiation of 18.9MJ/m2, 17.2MJ/m2, and 14.7MJ/m2. The daily average thermal efficiencies are 71.05%, 64.25%, and 50.49%, respectively. These experimental results provide basis and reference for practical application of the novel SWH. © by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2015 2015The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review by the scientific conference committee of SHC 2014 under responsibility of PSE AG. Peer-review by the scientific conference committee of SHC 2014 under responsibility of PSE AG Keywords: Micro heat pipe array; Novel flat plate solar collector; Novel solar water heater; Thermal performance

1. Introduction Currently, conventional flat plate solar collector (FPC) uses water pipes attached to the absorber where water circulates and transfers the heat collected by absorber to a storage tank. However, there are some shortcomings of

* Corresponding author. Tel.: +86 13910794840; fax: +86 010 67391608-801. E-mail address: [email protected]

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review by the scientific conference committee of SHC 2014 under responsibility of PSE AG doi:10.1016/j.egypro.2015.02.095

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Yuechao Deng et al. / Energy Procedia 70 (2015) 41 – 48

this type of collector such as freeze cracking, high requirement for welding, low efficiency, low reliability and high cost, which impede further development of flat plate solar water heater (SWH). Heat pipe (HP) is a device of high thermal conductance, which has been recommended to be used in FPC as a solution to the drawbacks of conventional collectors. Numerous experimental and theoretical investigations have been conducted over the years on HP-FPCs. Hussein H M S et al.[1-5] investigated a wickless HP-FPC through theoretical analysis and experimental research, and the optimal studies were carried out to improve the thermal efficiency. Azad E[6-9] designed and constructed a gravity assisted HP-FPC. The thermal behavior of the collector was investigated theoretically and experimentally, and the optimum methods were also discussed. Rittidech S and Wannapakne S[10] conducted an experimental investigation on the thermal performance of a FPC by closed-end oscillating HP. Results showed that an efficiency of approximately 62% was achieved. Nguyen K B et al.[11] investigated the effect of the working-fluid filling ratio and the cooling-water flow rate on the thermal performance of a FPC equipped with a closed-loop oscillating HP by experiments. Kargarsharifabad H et al.[12] investigated experimentally the performance of a FPC operating in conjunction with a closed-loop pulsating HP. The effect of the HP evaporator length, pulsating HP filling ratio, inclination angle, and flow rate were tested and analyzed. As seen from these literatures, HP usually refers to the gravity assisted HP, the wickless gravity HP, the closedloop oscillating HP or the closed-end oscillating HP. Most of these are circular copper HP, which makes the collectors have shortcomings, such as high cost, high contact thermal resistance, high requirements for the processing technique, and large initial investments. However, the appearance of novel flat plate HP called micro heat pipe array (MHPA) make the structure and processing technology of solar collector changed. The novel flat plate solar collector (FPC) based on MHPA could overcome the shortcomings of the existing solar collectors [13, 14]. In this paper, a novel FPC using MHPA as absorber and the solar water heater (SWH) are designed and manufactured. The characteristics of the novel collector are highlighted. After that the instantaneous efficiency of the collector is investigated. Furthermore, the thermal performance of using SWH with MHPA absorber is investigated in Beijing, China. 2. The structure of novel SWH 2.1. MHPA MHPA is an innovative functional material of powerful heat transfer. As shown in Fig.1, it is a flat aluminum plate with multiple parallel micro heat pipes operating independently inside [15, 16]. Each micro heat pipe has many inner micro-grooves (or micro-¿ns) to enhance the heat transfer by repeated evaporation and condensation of inner working fluid. Due to the special structure, the MHPA bears the advantages of high heat transfer performance, high reliability, high compressive strength, low cost, and small contact thermal resistance [13]. Therefore, MHPA is an excellent heat transport material for the solar energy systems.

a) Photograph of MHPA

b) Cross-section photo of MHPA Fig. 1. Photos of MHPA


2.2. FPC with MHPA absorber The novel FPC with MHPA absorber is shown in Fig. 2. The rectangular frame of the collector is 2000×1000×90mm. The top is covered by 3.2mm low iron tempered glass cover with solar energy transmittance of 92%. Spacing between the glass cover and the MHPA absorber is 30mm. The MHPA absorber is shown in Fig. 3. It is consisted of 32 MHPAs(930mm×60mm×3mm, 20% acetone working fluid), which is sprayed solar selective coating with 95% absorbility and 30% emissivity and closely arranged one by one. The small gap between the MHPA condenser and the top surface of water tube is filled with conductive grease, then connecting closely by riveting. A glass wool thermal insulation with a thickness of 60 mm is attached underneath the MHPAs and the water tube. The aluminum alloy frame is used to house all the parts of the collector. When it is in use, the MHPAs transfer the heat from its top surface to the water inside the water tube through the phase change of the working fluid.

Fig. 2. Structure schematic of novel collector

Fig. 3. Structure schematic of MHPA absorber

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Owing to the special structure, the collector possesses the following advantages. 1) The MHPA has a good property of low temperature bearing (-100°C). The extruded-forming water tube provides strong pressure-bearing capability, which could bear temperature of -15°C when it is full of water, and could also easily drain down. Therefore, the MHPA absorber has a perfect frost-resistant character. 2) The contact thermal resistance between the MHPA condenser and water tube is smaller, thus its thermal transfer capability is improved. 3) Due to the MHPA with excellent isothermal characteristic, the MHPA absorber has uniform temperature field than that of the traditional one, thus the heat loss by natural convection in the air gap between the glass cover and the absorber is reduced. 4) The system is of no problem of water leakage because of no soldering on any part of the collector. 2.3. SWH with MHPA absorber The novel flat plate SWH consists of a 155 L water tank, a 2m2 FPC with MHPA absorber, a water pump, valves and piping system. 3. Description of the experiments Taking the FPC with MHPA absorber and the SWH for research objects, the instantaneous efficiency for the FPC and the daily thermal performance for the SWH were tested and analyzed. 3.1. Instantaneous efficiency of the FPC with MHPA absorber The thermal performance of the solar collector is determined by the value of instantaneous efficiency. The experiments on the collector instantaneous efficiency were conducted at different inlet cooling water temperatures. The incident irradiance, ambient temperature, inlet and outlet water temperatures, and water flow rate were recorded. The detailed instruments, process and methods of the experiment were presented in reference [13]. The thermal efficiency of the collector under steady-state conditions can be calculated from:

Kc

mw C w (Two Twi )

(1)

GAc

The instantaneous efficiency of the collector can be written as:

Kc

FR (WD ) e FR U L

(Twi Ta )

(2) G Where mw is the flow rate of water, kg/s; Cw is the specific heat of water, J/kgǜK; Two is the outlet water temperature of the collector, °C; Twi is the inlet water temperature of the collector, °C; G is the solar irradiance in the aperture plane, W/m2; Ac is the aperture area, m2; FR is the heat removal factor; (ĲĮ)e is the collector effective transmittance-absorptance product; UL is the heat loss coefficient, W/(m2ǜK). According to the experimental thermal efficiency under steady-state conditions, the instantaneous efficiency of FPC can be obtained by using a curve fitting technique. The values of FR(ĲĮ)e which is the maximal thermal efficiency and FRUL which is the total heat loss factor are also obtained. 3.2. Thermal performance of the SWH To investigate the thermal performance of the SWH under different weather conditions in Beijing, China, a test system was built as shown in Fig. 4. The SWH was installed in Beijing (39.8° north latitude, 116.47° east longitude) and the FPC with MHPA absorber was installed south facing with an inclined angle of 45°. Three T-type thermocouples with an accuracy of ±0.1°C were used to measure the water temperatures in the water tank. Their average value was taken as the average water temperature of water tank. A global pyranometer with an accuracy of ±2% mounted on a surface parallel to the collector surface was used to measure the global solar irradiance. A temperature sensor with an accuracy of ±0.1°C was used to test ambient temperature. An electronic

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balance(NVT3201B) with an accuracy of ±0.4 g was used to test the water quantity. All the data were collected realtimely at the same time at 1 minute interval. The detailed test parameters, instruments, and test methods were shown in reference [17]. Based on the measured parameters, the effective heat gain of the SWH can be calculated as: Qu MCw 'Tw (3) The thermal efficiency of the SWH can be calculated as:

K

Qu Ac G

(4)

Where Qu is the effective heat gain, MJ; M is the water mass in the water tank, kg; Cw is the specific heat of water, J/kgǜK; ǻTw is the change of water tank temperature, °C; Ș is the thermal efficiency; Ac is the aperture area, m2; G is the solar irradiance, W/m2.

Fig. 4. Test system of SWH with MHPA absorber

4. Results and discussion

4.1. Instantaneous efficiency for the FPC with MHPA absorber The tests took place for an average solar irradiance and ambient temperature of about 790 W/m2 and 37.6°C, respectively. These conditions were encountered around solar noon during summer seasons in Beijing. The experiments were conducted for the inlet water temperatures from about 38 to 78°C. The fitting curve of experimental results is presented in Fig. 5. The test results show that the slope and intercept of the instantaneous efficiency curve are 5.62 and 0.854 respectively, which are 6.3% and 18.6% superior to the technical data of the Chinese national standard [18]. The FPC with MHPA absorber shows excellent thermal performance, particularly the maximal thermal efficiency. The excellent maximal efficiency can be attributed to high transmittance of glass cover, high absorbility of selective coating and excellent heat transfer ability of the collector.

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Fig. 5. Instantaneous efficiency curve of FPC with MHPA absorber

4.2. Daily thermal performance for the SWH with MHPA absorber To evaluate the thermal performance of the SWH, three typical days in summer, autumn, and winter were chose to analyze. Fig. 6a) presents the global solar irradiance in the aperture plane and the ambient temperature for the three typical days on the 17/08/2013, 10/10/2013 and 20/12/2013 measured at 1 minute interval. During the testing time(8:00̚ 16:00), the solar irradiances firstly increase and then decrease, reaching maximum before and after midday. the solar irradiance ranges for the three days were 99̚880 W/m2, 61̚855W/m2 and 12̚767W/m2, respectively. The average irradiances were 655.7W/m2, 595.4W/m2 and 532W/m2, respectively. The daily irradiations were 18.9 MJ/m2, 17.2 MJ/m2 and 14.7 MJ/m2, respectively. During the testing time(8:00̚16:00), the ambient temperature ranges for the three days were 33.3̚38.5°C, 17.2̚22.0°C and -3.3̚5.1°C, respectively. The average ambient temperatures were 36.4°C, 20.7°C and 2.1°C, respectively. Fig. 6b) presents the water temperature in the tank for the three typical days. It shows that the water temperatures increased from 24.8 to 65.0°C, 19.1 to 52.2°C and 11.8 to 34.0°C on the three days, respectively. And the temperature rises were 40.2°C, 33.1°C and 22.2°C, respectively. On 17/08/2013, the increase of water temperature was nearly linear from 8:00 to 15:00 and became slow after 15:00. On 10/10/2013 and 20/12/2013, the water temperatures increased slowly from 8:00 to 10:00 because of the lower solar irradiance and ambient temperature. Then, the water temperatures increased linearly from 10:00 to 15:00. After 15:00, the ascending velocities of water temperature slowed down with the decreasing solar irradiance, ambient temperature and increasing heat loss. The comparison of the water temperatures on the three days shows that the growth rates of the water temperature decrease gradually owing to the decreasing ambient temperature. Fig. 6c) presents the effective heat gain for the three typical days at 1 minute interval. It can be seen that the effective heat gain increased initially and then decreased, and peaked at around solar noon. The change conformed to the solar irradiance. The negative effective heat gain occurred between 15:00 and 16:00 on the three days, which meant SWH only lose heat without collecting solar energy owing to the decreasing solar irradiance, ambient temperature and increasing water temperature. In the three typical days, the daily effective heat gains were 13.43, 11.05, and 7.42 MJ/m2, respectively. Fig. 6d) presents the hourly efficiency for the three typical days. As shown, the hourly thermal efficiency almost decreased on 17/08/2013 and 10/10/2013, whereas it increased at first and then decreased on 20/12/2013. On 17/08/2013, the water temperature was below ambient temperature at the initial stage of the test, which made the


SWH almost collect heat without heat loss. Therefore, the hourly thermal efficiency reached 88.9%. Then, the hourly thermal efficiency increased due to the sudden decline of solar irradiance and accumulation of heat. Afterwards, the hourly thermal efficiency reduced gradually with the increase of heat loss. The change rule of hourly thermal efficiency on 10/10/2013 was somewhat similar to that on 17/08/2013. On 20/12/2013, the hourly system thermal efficiency was 23.29% at 8:00 owing to the low ambient temperature. Then, it increased and reached 63.09% at 12:00 with increase of solar irradiance and ambient temperature. Finally, the hourly efficiency was negative value at 16:00 owing to the high operation temperature of the SWH lead to high heat loss. The average thermal efficiencies of the three days were 71.05%, 64.25%, and 50.49%, respectively.

a) Global solar irradiance and ambient temperature

b) Water temperature

c) Effective heat gain by the SWH system

d) Efficiency by the SWH system

Fig. 6. Thermal performance for three typical days

5. Conclusions In order to overcome the shortcomings of the existing solar collectors, a novel FPC with MHPA absorber and the SWH were designed and built in this study. An instantaneous efficiency experiment for the FPC with MHPA absorber and a thermal performance experiment for the forced circulation SWH were conducted to investigate the

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novel SWH. The results of this research provide basis and reference for the actual application of the novel solar collector technology. The main conclusions are drawn as follows: (1) The novel collector adopts MHPA as the absorber with the advantage such as resistance to freezing, high heat transfer ability, relatively low heat loss, elimination of welding, prevention of leakage. (2) The test results for the collector's instantaneous efficiency show that the slope and intercept of the instantaneous efficiency curve are 5.6 and 0.85 respectively, which are 6.3% and 18.6% superior to the technical data of the Chinese national standard. (3) The thermal performance monitoring results of the forced circulation SWH with MHPA absorber under three different weather conditions (17/08/2013, 10/10/2013, and 20/12/2013) in Beijing show that the daily effective heat gains are 13.43MJ/m2, 11.05MJ/m2, and 7.42MJ/m2 respectively, corresponding to the solar irradiation of 18.9MJ/m2, 17.2MJ/m2, and 14.7MJ/m2. The daily average thermal efficiencies are 71.05%, 64.25%, and 50.49%, respectively. The test results show that the novel SWH has excellent thermal performance. Acknowledgements The project was financially supported by Beijing Postdoctoral Research Foundation (2014ZZ-34) and China Postdoctoral Science Foundation funded project (2014M550578). The authors are grateful for the support of these sponsors. References [1]Hussein H M S, Mohamad M A. Optimization of a wickless heat pipe flat plate solar collector. Energy Conversion and Management, 1999, 40(18): 1949-1961. [2]Hussein H M S, Mohamad M A, El-Asfouri A S. Transient investigation of a thermosyphon flat-plate solar collector. Applied Thermal Engineering, 1999, 19(7): 789-800. [3]Nada S A, El-Ghetany H H, Hussein H M S. Performance of a two-phase closed thermosyphon solar collector with a shell and tube heat exchanger. Applied Thermal Engineering, 2004, 24(13): 1959-1968. [4]Hussein H M S, El-Ghetany H H, Nada S A. Performance of wickless heat pipe flat plate solar collectors having different pipes cross sections geometries and filling ratios. Energy Conversion and Management, 2006, 47(11/12): 1539-1549. [5]Hussein H M S. Theoretical and experimental investigation of wickless heat pipes flat plate solar collector with cross flow heat exchanger. Energy Conversion and Management, 2007, 48(4): 1266-1272. [6]Azad E. Theoretical and experimental investigation of heat pipe solar collector. Experimental Thermal and Fluid Science, 2008, 32(8): 16661672. [7]Azad E. Performance analysis of wick-assisted heat pipe solar collector and comparison with experimental results. Heat Mass Transfer, 2009, 45(5): 645-649. [8]Azad E. Theoretical analysis to investigate thermal performance of co-axial heat pipe solar collector. Heat Mass Transfer, 2011, 47(12): 1651 ˉ1658. [9]Azad E. Assessment of three types of heat pipe solar collectors. Renewable and Sustainable Energy Reviews, 2012, 16(5): 2833-2838. [10]Rittidech S, Wannapakne S. Experimental study of the performance of a solar collector by closed-end oscillating heat pipe(CEOHP). Applied Thermal Engineering, 2007,27(11/12): 1978-1985. [11]Nguyen K B, Yoon S, Choi J H. Effect of working-fluid filling ratio and cooling-water flow rate on the performance of solar collector with closed-loop oscillating heat pipe.Journal of Mechanical Science and Technology, 212, 26 (1): 251-258 [12]Kargarsharifabad H, Jahangiri Mamouri S, Shafii M B, Taeibi Rahni M. Experimental investigation of the effect of using closed-loop pulsating heat pipe on the performance of a flat plate solar collector. Journal of Renewable and Sustainable Energy, 2013,5(1):1-12 [13]Deng Yuechao, Zhao Yaohua, Wang Wei, Quan Zhenhua, Wang Lincheng, Yu Dan. Experimental investigation of performance for the novel flat plate solar collector with micro-channel heat pipe array (MHPA-FPC). Applied Thermal Engineering, 2013, 54 (2): 440-449. [14] Deng Yuechao, Quan Zhenhua, Zhao Yaohua, Wang Lincheng. Experimental investigations on the heat transfer characteristics of micro heat pipe array applied to flat plate solar collector. SCIENCE CHINA Technological Sciences, 2013, 56(5): 1177-1185. [15]Zhao Yaohua, Wang Hongyan, Diao Yanhua, Wang Xinyue, Deng Yuechao, Heat transfer characteristics of flat micro heat pipe array, CIESC Journal,2011,62(2): 336-343. [16]Wang Hongyan, Zhao Yaohua. Numerical investigation on heat transfer of vertical micro-heat pipe arrays. CIESC Journal, 2014,65(2): 508515. [17]Deng Yuechao, Wang Wei, Zhao Yaohua, Yao Liang, Wang Xinyue. Experimental study of the performance for a novel kind of MHPA-FPC solar water heater. Applied Energy, 2013,112: 719-726. [18]GB/T6424-2007, 2007. Flat plate solar collectors. Standard press of China.