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INTEGRAL-TYPE SOLAR WATER HEATER USING LOOP HEAT PIPEB.J. Huang, P.E. Yang , J. H. Wang, J. H. WuNew Energy Center, Department of Mechanical EngineeringTaiwan University, Taipei, Taiwanm, Chinabjhuang@.twABSTRACTTaiwan University has been devoted to the development of low-cost LHP (loop hat pipe). A new manufacturing process for low-cost LHP has been developed that leads to a cost down to less than 10 USD for a 100W LHP. The present study utilizes the low-cost LHP to develop a new type of solar water heater. A thermosyphon which is thermally bonded with a solar absorber plate is used to absorb solar energy and transfer the heat upward to the evaporator of the LHP. From that, the heat is conducted downward to the condenser of the LHP which is immersed in a hot water tank beneath the solar collector. The solar water heater thus can be designed in integral type with streamline shape which is easy to install and has a good outlook as well as high efficiency. A prototype was designed and fabricated in the present study.A preliminary heat transfer test for a single unit of thermosyphon-LHP combination shows that it is capable of transferring the absorbed energy downward to the LHP condenser immersed in a hot water tank. The overall thermal resistance from the absorber plate to water is 0.34o C/W, and the thermal resistance of the LHP is 0.16o C/W. Two prototypes of the solar water heaters with 50 liters and 80 liters hot water tanks were designed according to the results of the feasibility test. I n addition, an automatic monitoring system was designed and set up for the performance test of these two solar water heaters based on the test standard CNS B7277. The measured overall thermal resistance from solar absorber plate to water is 0.369o C/W. The daily solar water heater efficiency is 0.503. The present study has shown that the application of low-cost LHP in solar water heater is feasible and cost competitive.1. INTRODUCTIONSolar hot water heater is important in renewable energy application. There are three types of commonly-used solar water heaters: namely, forced-circulation system, natural-circulation system (known as thermosyphon), and constant temperature-intermittent output system (known as once-through system). Among them, the thermosyphon solar water heater is the most popular one since no circulation pump or regulating valve is needed. However, in order to prevent heat loss during night time or raining days caused by reverse-flow phenomenon, the water tank must be placed upon the collector [1]. I n addition, the whole solar water heater is manufactured at factory in three separate parts: solar collector, tank, and supporting frame (Figure 1). The solar water heater is then installed by assembling the three separate parts together on site, including frame fixing on roof and cold and hot water piping. The installation cost is thus time consuming and at high cost.The design of integral-type solar water heater (I SWH) as shown in Figure 2 can greatly simplify the installation since the whole system can be manufactured in factory with good quality control. The whole system is then moved to the3 SOLAR COLLECTOR TECHNOLOGI ES AND SYSTEMS679rooftop and just placed on the floor without additionalfixing work. A few solar water heaters of this type were in market. However, the thermal efficiency is low since the downward heat transfer design is poor, usually with daily thermal efficiency < 0.4. The poor integral-type solar water heater cannot survive in Taiwan solar market since the government subsidy program asks all products to havedaily solar thermal efficiency larger than 0.5.Fig. 1: Natural-circulation solar water heater.Fig. 2: Integral-type solar water heater design.Two companies in Taiwan had ever marketed the integral- type product about 10 years ago, but failed (Figure 3). Brand A was designed in forced-circulation type, with water capacity 75 liters which is placed under the solar collector. The water pump is driven by a solar PV to circulate the water through solar collector to tank (Figure 4). This product failed due to the reliability problem of solar-pumping system.Brand B is designed in thermosyphon type with water capacity 150 liters. The solar collector is placed nearly on floor in order to keep it below the tank (Figure 4). The vertical distance between the tank inlet and the collector outlet thus becomes negative and causes flow reversal at night or cloudy days. The solar energy collection efficiencyis thus very low, < 0.40, and the product cannot survive on the market. Since then, the integral-type solar water heater disappeared from Taiwanese market. However, the installation process of both brand A and B had been simplified a great deal. Brand A (5 modules, 375 liters) takes only two man-hour to install. Brand B (300 liters, single module) is too heavy (>60kg) and took about 6 man-hour to install on the roof.Fig. 3: Failed ISWH products.Fig. 4: Failed ISWH design.The present study intends to utilize the low-cost LHP to develop a new integral-type solar water heater. Two prototypes were designed and fabricated in the presentstudy.Brand BProceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement6802. PRELIMINARY TEST OF THERMOSYPHON-LHP n order to prove that the low-cost LHP can achieve downward heat transfer to the water tank from solar collector, a thermosyphon-LHP experiment was first carried out. A single unit of thermosyphon-LHP device was designed. The device includes a single strip of tube-in-sheet solar collector (0.12m 2) with a thermosyphon, a LHP and a water tank set beneath the collector. An electric film heater is adhered on the solar collector to simulate solar heating in both summer and winter. The thermosyphon is thermally bonded with the solar absorber plate and transfers the heat upward to the evaporator of the LHP. The heat is then conducted downward to the condenser of the LHP which is immersed in a hot water tank beneath the solar collector. The test results (Figure 5) shows that the thermosyphon- LHP configuration is capable of transferring the absorbed energy downward to the LHP condenser immersed in a hot water tank beneath the collector. The overall thermal resistance from the absorber plate to water is 0.34o C/W, and the thermal resistance of the LHP is 0.16o C/W. 00.10.20.30.40.5050100150200Power(W)T h e r m a l r e s i s t a n c e (℃/W )HP +R LHP )LHP )HP )Fig. 5: Thermal resistance measurement in a singlethermosyphon-LHP device.3. LHP-BASED SOLAR WATER HEATERApplying the low-cost LHP (Figure 6) [2] to solar water heater, we can effectively transfer the heat downward to the tank from the solar collector. A thermosyphon which is thermally bonded with a solar absorber plate is used to absorb solar energy and transfer the heat upward to the evaporator of the LHP. From that, the heat is conducteddownward to the condenser of the LHP which is immersed in a hot water tank beneath the solar collector. The solar water heat thus can be designed in integral type with streamline shape which is easy to install and has a good outlook as well as high efficiency. Figure 7 shows the design concept. Two models, 80L(Figure 8) and 50L (Figure 9), were designed.ISWH-1 was designed for feasibility study to validate the downward heat transfer capability. ISWH-1 uses 1m 2solar collector and 4 sets of LHP. The tank is of closed-type which can sustain the tape water pressure. I SWH-2 is designed with compact and rectangular configuration with open-type tank.Fig. 6: Low-cost LHP.Fig. 7: LHP-based solar water heater.3 SOLAR COLLECTOR TECHNOLOGI ES AND SYSTEMS681Fig. 8: ISWH-1.The solar collector area of ISWH-2 is 0.5m2. ISWH-2 uses 2 sets of LHP. The daily solar energy collection efficiency is measured according to the outdoor test standard CNSB7277 [3].Fig. 9: ISWH-2.The daily solar energy collection efficiency is 0.503 (Fig.10) and the heat releasing efficiency of hot water supply is 0.77 as shown in Figure 11. Both exceed the requirement of the subsidy program. The measured overall thermal resistance from solar absorber plate to water is 0.369o C/W.ISWH has been proved at the first time to be able to utilize low-cost LHP as the downward heat transfer element and obtain a good efficiency.n order to understand whether the LHP can perform normally in the long run, we have carried out a long-term outdoor experiment. The result indicates that LHP can function normally; moreover, the daily solar energy collection efficiency is 0.501 (Fig 12) for the test period of3 months.Fig. 10: ISWH-2 thermal efficiency.Fig. 11: Heat releasing test of the tank.0.10.20.30.40.50.60.7-0.5-0.4-0.3-0.2-0.100.10.2X=(Ti-Ta)/Ht (℃m2day/MJ)EfficiencyηFig. 12: ISWH-2 thermal efficiency in the long run.Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement 6824. CONCLUSIONTaiwan University has been devoted to the development of low-cost LHP, in cooperation with local industry. A new manufacturing process for low-cost LHP has been developed that leads to a cost down to less than 10USD for a 100W LHP. The present study utilizes the low-cost LHP to develop a new type of solar water heater. A thermosyphon which is thermally bonded with a solar absorber plate is used to absorb solar energy and transfer the heat upward to the evaporator of the LHP. From that, the heat is conducted downward to the condenser of the LHP which is immersed in a hot water tank beneath the solar collector. The solar water heat thus can be designed in integral type with streamline shape which is easy to install and has a good outlook as well as high efficiency. A prototype was designed and fabricated in the present study.A preliminary heat transfer test for a single unit of thermosyphon-LHP combination shows that it is capable of transferring the absorbed energy downward to the LHP condenser immersed in a hot water tank. The overall thermal resistance from the absorber plate to water is 0.34o C/W, and that of the LHP part is 0.16o C/W. Two prototypes of the solar water heaters with 50 liters and 80 liters hot water tanks were designed according to the results of the feasibility test. In addition, an automatic monitoring system was designed and set up for the performance test of these two solar water heaters based on the test standard CNS B7277. The measured overall thermal resistance from solar absorber plate to water is 0.369o C/W. The daily solar water heater efficiency is 0.503. The present study has shown that the application of low-cost LHP in solar water heater is feasible and cost competitive. Figure 13 shows the outlook of the new product design which is in mass production. The capacity of water storage is 80L, and the solar collector area is 1m2.CFD analysis has also shown that this product can stand in a wind speed >60 m/s without tilt-over or sliding.Fig. 13: Product design of ISWH.5. ACKNOWLEDGEMENTThe present study was supported by Energy Bureau, “Ministry of Economic Affairs”, Taiwan, China, KongLin Group, and Advanced Thermal Devices, Inc.6. REFERENCES(1) B.J.Huang: "Similarity theory of solar water heaterwith natural circulation". Solar Energy, V ol.25(2), pp.105-116 (1980).(2) B.J. Huang: “Development of a low-cost LHP forcommercial application”. 13th I nternational Heat Pipe Conference (13th I HPC), Shanghai, China, September 21-25, 2004.(3) Bureau of Standards, Metrology＆Inspection,M.O.E.A., R.O.C. CNS B7277: Method of Test for Solar Water Heater System1989.。