SCIENCE CHINA Technological Sciences

SCIENCE CHINA Technological Sciences RESEARCH PAPER May 2013 Vol.56 No.5: 1177 1185 doi: 10.1007/s11431-013-5204-7 Experimental investigations on the heat transfer characteristics of micro heat pipe array applied to flat plate solar collector DENG YueChao, QUAN ZhenHua *, ZHAO YaoHua & WANG LinCheng Department of Building Environment and Facility Engineering, College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, China Received October 19, 2012; accepted March 18, 2013; published online March 31, 2013 This paper introduces a novel flat plate solar collector (FPC) using micro heat pipe array (MHPA) as a key element. To analyze the thermal transfer behavior of flat plate solar collector with micro heat pipe array (MHPA-FPC), an indoor experiment for thermal transfer characteristic of MHPA applied to FPC was conducted by using an electrical heating film to simulate the solar radiation. Different cooling water flow rates, cooling water temperatures, slopes, and contact thermal resistances between the condenser of MHPA and the heat exchanger were tested at different heating powers. The experimental results indicate that MHPA-FPC exhibits the enhanced heat transfer capability with increased cooling water flow rate and temperature. Total thermal resistance has a maximum decline of approximately 10% when the flow rate increases from 180 to 360 L h 1 and 38% when the cooling water temperature increases from 20 C to 40 C. When the inclination angle of MHPA-FPC exceeds 30, the slope change has a negligible effect on the heat transfer performance of MHPA-FPC. In addition, contact thermal resistance significantly affects the heat transfer capability of MHPA-FPC. The total thermal resistances lowers to nearly half of the original level when contact material between the condenser of MHPA and the heat exchanger changes from conductive silicone to conductive grease. These results could provide useful information for the optimal design and operation of MHPA-FPC. micro heat pipe array, novel flat plate solar collector, heat transfer characteristic, cooling water flow rate, cooling water temperature, slope, contact thermal resistance Citation: Deng Y C, Quan Z H, Zhao Y H, et al. Experimental investigations on the heat transfer characteristics of micro heat pipe array applied to flat plate solar collector. Sci China Tech Sci, 2013, 56: 1177 1185, doi: 10.1007/s11431-013-5204-7 1 Introduction Heat pipes have various benefits such as high thermal transfer ability, isothermal characteristic, the variable heat flux, operating like a thermal diode, which make heat pipes have a widespread prospect in the application of solar collectors. Numerous studies on heat pipe flat plate solar collector (HP-FPC) have been conducted. The wickless HP- FPC was studied through theoretical analysis and experimental research [1 5], and optimal studies were performed *Corresponding author (email: quanzh@126.com) to improve thermal efficiency. Emmanouil and Vassilis [6] investigated the performance of a new solar hot water system with an integrated wickless gravity-assisted loop HP solar collector. Results showed that the system could achieve satisfactory work efficiency. Samuel and Sergio [7] focused on the experimental analysis of the thermal behavior of a two-phase closed thermosyphons with an unusual geometry, which was characterized by a semicircular condenser and a straight evaporator. Esen and Esen [8] conducted an experimental investigation on the effects of different refrigerants (R-134a, R407C, and R410A) on the thermal performance of HP-FPC. Among the working fluids used in the experi- Science China Press and Springer-Verlag Berlin Heidelberg 2013 tech.scichina.com www.springerlink.com

1178 Deng Y C, et al. Sci China Tech Sci May (2013) Vol.56 No.5 ment, R410A presented the highest solar energy collection performance. Riffat et al. [9] designed and constructed a thin membrane HP-FPC. A theoretical model incorporating a set of heat balance equations was developed to analyze the heat transfer occurring in the collector. Rittidech and Wannapakne [10] experimentally investigated the performance of FPC with the closed-end oscillating heat pipe, and results showed that the maximum efficiency of the collector was approximately 62%. Azad [11, 12] developed a theoretical model based on the effectiveness-number of transfer units method to evaluate the thermal efficiency of HP-FPC. The same author subsequently constructed and tested three different types of HP-FPC with different condenser designs to compare their instantaneous efficiency [13]. Xiao et al. [14] proposed a new theoretical method for analyzing the effect of relevant parameters on the thermal performance of HP-FPC. In the above studies, heat collection components generally comprised circular copper heat pipes and aluminum fins, which make HP-FPC have a high cost. The contact surface between the absorber and the circular heat pipes is extremely small, such that the thermal resistance increases and consequently degrades heat transfer efficiency. At the same time, the direct insertion of the heat pipe condenser section into the manifold causes scale formation and leakage on the surface of the condenser section during operation, which degrades the performance of the heat pipes and eventually leads to failure. The results of the existing studies suggest that a novel structure for a highly efficient HP- FPC, which could overcome the disadvantages of the conventional devices, is needed. Several new types of heat pipes have been developed [15 17]. In this paper, a novel type of flat plate heat pipe [18] called micro heat pipe array (MHPA) has been applied to FPC [19]. This paper introduces the structure and characteristics of the novel collector, which uses MHPA as the high-efficiency heat transfer element. An indoor experiment was carried out to investigate the effect of basic design parameters on the heat transfer performance of MHPA applied to FPC for the further improvement of the MHPA-FPC thermal performance. Different cooling water flow rates, cooling water temperatures, slopes, and contact thermal resistances between the condenser of MHPA and the heat exchanger were tested for different heat fluxes. The effects of these parameters on MHPA-FPC performance were analyzed to provide information for the design and operation of MHPA-FPC. 2 MHPA-FPC 2.1 MHPA characteristics The schematic of MHPA is shown in Figure 1. The appearance of MHPA is similar to a thin aluminum plate, which consists of aluminum and working fluid. Common working fluids include R141b, acetone, methanol, and ethanol. A MHPA has a typical thickness of 3 mm, whereas the length and width can be customized. MHPA is a device with high thermal conductance that transfers thermal energy through a two-phase working fluid circulation. This device has an evaporator and a condenser, where the working fluid evaporates and condenses, respectively. As shown in Figure 1(b), MHPA contains a large number of micro heat pipes that work independently. Within each micro heat pipe are inner micro-grooves (or micro fins) that increase the heat exchange area. The clapboards between two adjacent micro heat pipes work as a support for MHPA and the hydraulic diameter of each micro heat pipe is only 0.4 1.0 mm. Therefore, MHPA has strong pressure-bearing capability. The number of independent micro heat pipes is extremely large, such that the failure of any individual micro heat pipe has a negligible effect on overall performance, thus improving the reliability of the system significantly. MHPA is produced from extruded aluminum with micro grooves (or micro fins). Thus, the cost is significantly lower than that of traditional copper heat pipes. Figure 1 Schematic diagram of MHPA.

Deng Y C, et al. Sci China Tech Sci May (2013) Vol.56 No.5 1179 As shown in Figure 2, the working principles of MHPA are as follows. When the outer surface of one side of evaporator section is heated, heat transfers to the inner surface of evaporator section by conduction. Because of the existence of the clapboards between the micro heat pipes, a part of heat transfers from heating surface to the opposite inner surface by conduction, which makes the whole inner surface of each micro heat pipe to be heated simultaneously. Therefore, the heat transfer at the evaporator section is enhanced. When the inner surface of the evaporator section is heated, the working fluid inside micro heat pipes evaporates to the saturated vapor forming vapor-liquid interface. The working fluid inside the micro groove of the evaporator section forms meniscus under the capillary force [20]. The pressure difference causes the saturated vapor at the liquid-vapor interface to flow upward from the evaporator section to the condenser section. In the condenser section, the vapor condenses and forms liquid film on the inner surface of the condenser section. Through surface tension, the condensate then flows from the micro fin top to the micro groove, which spreads the condensing liquid film to a larger surface. Thereafter, the liquid film thickness at the micro fin top decreases, thus enabling rapid heat transfer through the thinner condensing liquid film. Finally, through heat conduction, heat transfers from the vapor-liquid interface, condensing liquid film, internal surface of the condenser section, and outer surface of the condenser section to the cold source. The condensing liquid film at the micro grooves promptly returns to the evaporator section liquid pool by gravity. The cycle facilitates heat transfer from the evaporator section to the condenser section of MHPA. The working principles show that aluminum partitions enable the phase transition to occur at the peripheral surface throughout the micro heat pipe. Therefore, the heat transfer ability of the vapor flux is significantly strengthened. Meanwhile, the micro grooves and micro fins on the inner wall of each micro heat pipe could increase the heat exchange areas to enhance the heat transfer capability. More importantly, a large number of thin films in the micro grooves significantly improve phase-change heat transfer performance for both evaporation and condensation. In previous studies by the authors, several thermal performance tests of MHPA have been conducted to verify that MHPA has satisfactory heat transfer capability. Optimal working fluid, filling rate, and interior structure of micro channels were achieved [21]. These results were helpful to the further application of MHPA. 2.2 Structure of novel MHPA-FPC Given its unique characteristics, MHPA is used to develop a novel type of FPC. Two different structures of the novel MHPA-FPC have been developed: MHPAs with solar selective film, as shown in Figure 3(a), and MHPAs with solar selective coating, as shown in Figure 3(b). The users select suitable types based on local climate characteristics and the required hot water temperature. The cross section of a MHPA-FPC is shown in Figure 3(c). The top is covered by the low iron and tempered glass with the thickness of 3.2 mm. Spacing between the glass cover and the absorber plate is 30 mm. Figure 3(a) shows that the absorber plate is made of aluminum with a thickness of 0.4 mm and covered by a solar selective film. Under the absorber plate, 17 MHPAs are arranged in a line. In Figure 3(b), 32 MHPAs with solar selective coating are arranged in a line. Each MHPA is divided into two sections: an evaporator section with a length of 830 mm and a condenser section with a length of 100 mm. Moreover, an airfoil-shape heat exchanger with an inner diameter of 24.5 mm is designed for this novel FPC. The heat exchanger is produced by extrusion, which provides strong pressurebearing capability. A polyurethane thermal insulation with a thickness of 60 mm is attached underneath the MHPA and the heat exchanger. The aluminum alloy profiles are used to frame all the parts of the collector, and the gaps between the joints of profiles are sealed with silicone glue. When MHPA-FPC is in use, MHPA is used to transfer the heat from the absorber plate to the water inside the heat exchanger through the phase change of the working fluid inside MHPA. The novel collector has the following advantages: 1) The rectangular cross section of MHPA significantly reduces the contact thermal resistance compared with the circular heat pipe, thus improving thermal transfer capability Figure 2 Schematic diagram of working principle for MHPA.

1180 Deng Y C, et al. Sci China Tech Sci May (2013) Vol.56 No.5 Figure 3 Schematic diagram of MHPA-FPC. significantly. 2) The condenser section of MHPA is closely connected to the dry-type heat exchanger. MHPA does not come in contact with water, thus preventing scaling and leakage in the collector. 3) No soldering is used in the processing of MHPA-FPC, which makes fabrication easy. 4) MHPA uses low freezing points of the refrigerants, which makes the collector more suitable for extremely cold areas. Moreover, the unique heat exchanger could exclude water more easily than the traditional copper tube-sheet FPC, which could prevent the freeze cracking of the collector during winter. In summary, the novel collector has several advantages including elimination of welding, prevention of scale formation and leakage on the heat pipe condenser, and resistance to freezing. Previous test results [22] show that the slope and intercept of the instantaneous efficiency curve are 4.7 and 0.80, respectively, which are 11.0% and 22.3% superior to the technical data of the Chinese national standard [23]. Therefore, MHPA-FPC has a broad application prospect because of its excellent performance. 3 Experimental setup 3.1 System introduction An indoor experiment was designed to improve the thermal performance of MHPA-FPC further. Figure 4 shows that an experimental system primarily comprising MHPA, cold/heat source systems, and a data acquisition system was constructed to test the heat transfer characteristics of MHPA when applied to FPC. According to the previous research [21] and actual design of the collector, a MHPA (930 mm 60 mm 3 mm), which was filled with acetone with 20% liquid filling ratio, was selected as the study subject. The MHPA was insulated with rock wool and fixed to a bracket at a certain inclination degree. An electrical heating film that produces the controlled heat input by monitoring the voltage and current was placed over the evaporator section of the MHPA. The condenser section was closely connected to the heat exchanger by conductive silicone. The temperature of the cooling water flowing through the heat exchanger was controlled by a constant temperature bath with an accuracy of ±0.1 C. The flow rate of cooling water was recorded by a metal rotameter with an accuracy of 1%. Using copper-constantan thermocouples with an accuracy of ±0.1 C as the temperature sensor, the position of measurement points along MHPA is shown in Figure 5. 3.2 Experimental parameters Table 1 shows several selected design parameters. The influence of these parameters on the performance of MHPA- FPC was determined. The following parameters were analyzed: cooling water flow rates, cooling water temperatures, slopes, and contact thermal resistances between the condenser of MHPA and the heat exchanger. The settings of the cooling water flow rate value and cooling water temperature value depended on the actual operation condition of MHPA-FPC. The value of the slope

Deng Y C, et al. Sci China Tech Sci May (2013) Vol.56 No.5 1181 Figure 4 Experimental system for heat transfer characteristics of MHPA for FPC. Figure 5 Position of measurement points (unit: mm). Table 1 Values of the experimental parameter settings Parameters Slope ( ) 30, 45, 60, 90 Cooling water temperature (T w ) 20, 40 C Cooling water flow rate (m) Contact material Heating power (Q)/Heating flux density (q) 180, 360 L h 1 Setting values conductive silicone (thermal conductivity 1.0 W (m K) 1 ); conductive grease (thermal conductivity 4.0 W (m K) 1 ) 20/402, 30/602, 40/803, 50/1004, 60/1205, 70/1406, 80/1606, 90/1807, 100/2008, 110/2209, 120/2410 W/(W m 2 ) was determined based on the actual placed angle of the collector. Meanwhile, the selection of contact materials was based on the actual design of MHPA-FPC, and the value of the heating power was determined according to solar irradiance, collector area, and the quantity of MHPA for each MHPA-FPC. 3.3 Test method All the arrangements were tested at 11 different heating powers from 20 to 120 W. At the beginning of each test, the cooling water was circulated before power was supplied. The input power was then regulated to a desired level. The system is considered to reach steady state when each thermocouple temperature fluctuation is no more than 0.5 C within 5 min. In the steady state, the heating powers and temperatures at various locations were recorded. Thereafter, the heating power was increased in steps of 10 W, and the same procedure was repeated until the heating power reached 120 W. 3.4 Data processing The electrical power (P) applied to MHPA is calculated by measuring the current and the voltage: P U I, (1) where P is the electrical power, W; U is the voltage, V; and I is the current, A. Assuming the lack of heat loss attributed to sufficient insulation thickness of the heating section, the electrical power is taken as the effective heat supplied to MHPA (Q).

1182 Deng Y C, et al. Sci China Tech Sci May (2013) Vol.56 No.5 Figure 6 illustrates the thermal network of the system, where Q is the heating power, W; T e and T c are the average temperatures of the evaporator and condenser sections, respectively, C; T w is the average temperature of cooling water temperature, C; R MHPA is the thermal resistance of MHPA, K W 1 ; R cont is the contact thermal resistance between the condenser of MHPA and the heat exchanger, K W 1 ; R cond is the thermal resistance of thermal conduction of the heat exchanger, K W 1 ; R conv is the thermal resistance of the cooling water convective heat transfer, K W 1 ; and R tot is the total thermal resistance, K W 1. The temperature difference between the evaporator and condenser sections ( T) is defined as T T T. (2) The thermal resistance of MHPA is calculated as e c R MHPA. e c T T Q (3) Figure 7 Steady-state wall temperature for MHPA at =45, T w =20 C and m=180 L h 1. The total thermal resistance of the system is calculated as T T Q e w R tot. (4) 3.5 Measurement uncertainties The test uncertainty of individual temperature is ±0.1 K. The accuracy of the voltage and current measurements is 0.5%. Based on uncertainty propagation theory, the maximum uncertainties in Q measurement, R MHPA measurement, and R tot measurement are ±0.8 W, ±0.005 K W 1, and ±0.007 K W -1, respectively. 4 Results and analysis Figures 7 and 8 show the steady-state wall temperature distribution along the axis of MHPA under different heating powers. The behavior of the temperatures along MHPA is almost the same, with small decreases at the bottom of the evaporator section and at the end of the evaporator up to the condenser. The temperatures for the other parts of MHPA are almost the same. The increasing heating power corresponds to a gradual increase in the exterior wall temperatures of MHPA. In Figure 7, the temperature differences Figure 6 Thermal network. Figure 8 Steady-state wall temperature for MHPA at =45, T w =20 C and m=360 L h 1. between the evaporator and condenser sections are equal to 8.0, 6.6, 5.1, 3.8, 3.1, 2.4, 1.2, 1.2, 1.1, 1.0, and 0.6 C, respectively (20 120 W). In Figure 8, the temperature differences are equal to 8.0, 6.7, 5.1, 3.1, 2.3, 1.7, 1.5, 1.5, 1.4, 1.0, and 1.0 C, respectively (20 120 W). These results show that MHPA has excellent isothermal performance, especially at higher heating powers. As shown in Figure 9, with the increase of the heating power, R MHPA decreases gradually. When the heating power exceeds 80 W, R MHPA is lower than 0.01 K W 1. When the heating power is lower, working fluid evaporates to form a litter vapor at the liquid pool under MHPA. The temperature difference from the top to the bottom of the evaporator section becomes relatively large. With the increase of the heating power, the working fluid boils at the liquid pool, which produces a great deal of vapor. The ascending velocity of the vapor and the descending velocity of the condensate gradually increase. Boiling occurs in the whole heating section. The temperatures of the whole MHPA tend to be the

Deng Y C, et al. Sci China Tech Sci May (2013) Vol.56 No.5 1183 Figure 9 Thermal resistances at different cooling flow rates at =45 and T w =20 C. Figure 11 Thermal resistances at different cooling water temperatures at =45 and m=360 L h 1. same and R MHPA gradually decreases. Comparing Figure 7 with Figure 8, the wall temperature of MHPA at the flow rate of 360 L h 1 is lower than that of 180 L h 1, and the maximum descent is approximately 7 C. The thermal resistance at different flow rates is shown in Figure 9. The test results indicate that the maximum descent of R tot is approximately 10% when the flow rate increases from 180 to 360 L h 1. This observation shows that the heat transfer ability of MHPA-FPC improves with increased cooling water flow rate because such increase corresponds to an enhanced convective heat transfer coefficient between the inner surface of the heat exchanger and the cooling water, which consequently decreases R conv. The cooling water takes away more heat and decreases the MHPA temperature. Therefore, the cooling water flow rate should be improved in the operation of MHPA-FPC. Figure 10 shows the steady-state wall temperature for MHPA at a cooling water temperature of 40 C. Figure 11 shows the thermal resistances at different cooling water Figure 10 Steady-state wall temperature for MHPA at =45, m=360 L h 1 and T w =40 C. temperatures. Figure 11 also shows that R tot ranges from 0.67 to 0.45 K W 1 when heating powers increase from 20 to 120 W at a cooling water temperature of 40 C, which is smaller than that at a cooling water temperature of 20 C. R tot declines 38% to 7% at different heating powers. That is, R tot decreases with increasing cooling water temperature because higher cooling water temperature corresponds to MHPA operating temperature that is closer to the boiling point of the working fluid, thus decreasing R MHPA and R tot. Comparisons of the MHPA average temperatures for different slopes are shown in Figures 12(a) and 12(b). When the heating powers are within the scope of 20 120 W, the maximum and minimum discrepancies for the average temperature of the evaporator and condenser sections are 1.9 C, 0.7 C, 1.9 C and 0.3 C respectively. Figure 12(c) shows that the average temperature differences between the evaporator and condenser sections ranging from 0.1 C to 1.5 C. Figure 12(d) shows that the maximum and minimum discrepancies for R MHPA are 0.033 and 0.0014 K W 1, respectively. The small changes in temperature and thermal resistance show that a slight difference can be observed in terms of the thermal performance of MHPA for slopes within the range of 30 90 primarily because the condensate could easily return to the liquid pool to ensure that the inner surface of MHPA is coated with the complete liquid membrane at different slopes. Therefore, the two-phase flow inside MHPA could reach a dynamic balance, which guarantees normal MHPA operation. All the aforementioned results show that the temperature difference between the condenser section of MHPA and the cooling water in the heat exchanger is large. This observation is mainly attributed to the high R cont. Figure 13 shows the steady-state wall temperature distribution of MHPA when the contact material between the condenser of MHPA and the heat exchanger is changed from conductive silicone to conductive grease. As shown in Figure 13, the behavior of the temperatures along MHPA is that there is a larger decrease

1184 Deng Y C, et al. Sci China Tech Sci May (2013) Vol.56 No.5 Figure 12 Comparison of different slopes at T w =20 C and m=360 L h 1. (a) Average temperature of evaporator section; (b) average temperature of condenser section; (c) average temperature difference between the evaporator and condenser sections; (d) thermal resistance of MHPA. Figure 13 Steady-state wall temperature for MHPA with conductive grease at =45, m=360 L h 1, and T w =20 C. Figure 14 Thermal resistances of different contact materials. at the bottom of the evaporator and at the end of the evaporator up to the condenser. Comparing Figure 13 with Figure 8, when conductive grease is used as contact material, the wall temperature of MHPA becomes lower than that of conductive silicone, and the maximum decline of the average temperature reaches nearly 35 C. Figure 14 shows that R tot is reduced to nearly half of the original level when the contact material is conductive grease. Thus, the contact thermal resistance is an important factor in the heat transfer characteristics of MHPA-FPC. Therefore, the use of conductive grease instead of conductive silicone is necessary to decrease the contact thermal resistance between the condenser of MHPA and heat exchanger and to improve the

Deng Y C, et al. Sci China Tech Sci May (2013) Vol.56 No.5 1185 thermal transfer capability of MHPA-FPC. 5 Conclusions This paper focuses on the experimental analysis of the thermal behavior of MHPA applied to FPC. The structure and characteristics of the novel collector, which uses MHPA as the highly efficient heat transfer element, are introduced. Thereafter, an indoor experiment was conducted to investigate the effect of the basic design parameters on the heat transfer characteristic of MHPA-FPC. The results could be applied for the design improvement and optimal operation of MHPA-FPC. The following conclusions can be drawn from this study: 1) MHPA applied to FPC has excellent isothermal performance, especially at high heat transfer rates. 2) Cooling water flow rate and temperature have a number of effects on the heat transfer characteristics of MHPA-FPC. The tests indicate that the wall temperature of MHPA at a flow rate of 360 L h 1 is lower than that at 180 L h 1, and the maximum decline is approximately 7 C. When the flow rate increases from 180 L h 1 to 360 L h 1, the maximum decline of R tot is approximately 10%. R tot decreases 7% to 38% when the cooling water temperature increases from 20 C to 40 C. All these results show that MHPA-FPC exhibits enhanced heat transfer capability with increased cooling water flow rate and temperature. 3) When the inclination angle of MHPA-FPC exceeds 30, the change of slopes has a negligible effect on the heat transfer characteristics of MHPA-FPC. 4) Contact thermal resistance is an important factor in the heat transfer characteristics of MHPA-FPC. When conductive grease is used as contact material, the wall temperature of MHPA becomes significantly lower than that of conductive silicone, and the maximum decline of the average temperature for MHPA reaches nearly 35 C. 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