PERFORMANCE OF A SINGLE PASS AIR BASE PHOTOVOLTAIC/THERMAL SOLAR COLLECTOR WITH AND WITHOUT HEXAGONAL HONEYCOMB HEAT EXCHANGER F. Hussain Z.Anuar S. Khairuddin National Metrology Laboratory, SIRIM Berhad, 43900 Sepang, Selangor, Malaysia faridahh_hussain@sirim.my khairiz@sirim.my ksuhaila@sirim.my M.Y.H. Othman B. Yatim H. Ruslan K. Sopian Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia myho@ukm.my baha@ukm.my hafidz@ukm.my ksopian@eng.ukm.my ABSTRACT A single pass photovoltaic/thermal (PV/T) solar collector with hexagonal honeycomb heat exchanger was studied. It is a combination of photovoltaic panel and solar thermal components in one integrated system. The honeycomb was installed horizontally into the channel located at the back side of the PV module. Air, as heat removing fluid is made to flow through the honeycomb. The system was tested with and without the honeycomb at irradiance of 828 W/m 2 and mass flow rate spanning from 0.02 kg/s to 0.13 kg/s. It was observed that the aluminum honeycomb is capable of enhancing the thermal efficiency of the system efficiently. At mass flow rate of 0.11 kg/s, the thermal efficiency of the system with the honeycomb is 87 % and without is 27%. The electrical efficiency of the PV module remains almost the same for both systems. This design is suitable to be further investigated as solar drying system and space heating. Key words: Photovoltaic/ thermal, Honeycomb heat exchanger, Thermal efficiency, Electrical efficiency 1. INTRODUCTION One of the major factors that affect the popularity of photovoltaic module is that, it is relatively low in efficiency. Until today, commercially available photovoltaic module efficiency claim by manufacturers is between 6% to 16%. However the claimed efficiency is at the temperature of 25 C. In reality, especially for countries with hot weather, their ambient temperature during sunny day will be more than 35 C. The rising of the PV temperature will result in the drop of the module efficiency. In order to minimize PV module losing its efficiency, a simultaneous cooling system using air or water as the heat transfer liquid can be implemented. The heat output from the system can be collected and stored as thermal energy. This advance system is known as PV/T technology. A PV/T solar collector is a combination of photovoltaic panel (PV) and solar thermal components or system which is capable of producing both electrical energy and thermal energy simultaneously in one integrated system. The photovoltaic panel could be from the selection of monocrystalline silicon, c-si, polycrystalline silicon, pc-si, amorphous silicon, a-si or thin-film solar cell. The solar thermal components are various designs of a solar thermal collector which collects heat from the back of the PV panel (1). For the last 35 years there have been significant amounts of research and development work on PV/T technology (1). PV/T solar collector system can be classified as PV/T water system and PV/T air system. PV/T/water systems are more efficient compared to PV/T/air due to its high thermo-physical properties (2). However PV/T/air systems are more extensively studied due to their low constructions and operational costs and cost effective solution for building integrated PV system. Early research work for PV/T started in 1970s. The performance of a combination solar photovoltaic and solar heating system (PV/T) of a two story structure, single family residence in Boston, U.S.A had been analyzed (3). The well known Hottel Whillier model had been modified to analyze the photovoltaic/thermal flat plate collectors (4). The modifications were based on the cell array efficiency and the decrease of cell efficiency with temperature. A combination consideration of flat-plate PV/T collector had been designed using computer simulation (5). A PV/T system consists of flat air solar air heater, solar cell and plane booster reflector was studied (6). The usage of booster has increased the thermal 1
efficiency, but decreases the electrical efficiency of the system. The design system is suitable as solar dryer. A steady state model to analyze the performance of a single pass and double PV/T solar collector system suitable for solar drying had been developed (7). The performance of four models of PV/T collector for single pass and double pass system had been evaluated (8). Heat balance equation had been identified and solved for each model. A double pass PV/T system with compound parabolic concentrator (CPC) and fins had been fabricated (9). Fins attached at the back of the PV absorber plate act as the heat exchanger. Performance of a solar collector with v-groove heat absorber had been studied (10). The efficiency of the system with v-groove is 12 % more efficient compared to plat plate collector. The thermal efficiency of double pass PV/T system with porous media at the lower channel had been studied (11). A single passs PV/T system with aluminum - grooved absorber plate was analyzed (12). The thickness of the aluminum is 0.7 mm, attached at the back of the PV module. A direct coupled PV/T system outdoor in Kerman, Iran was tested (13). The design of the system involves use of a thin aluminum sheet placed in the middle of the air channel as the heat exchanger to cool the PV panels. Another PV/T solar collector with an active cooling system was fabricated to increase the electrical efficiency of a PV module (14). A parallel array of ducts with inlet/outlett manifolds was attached to the back of the PV module. This experiment studies a single pass air base photovoltaic/thermal (PV/T) solar collector combined with hexagonal honeycomb heat exchanger. A commercially available monocrystalline silicon PV module was used to produce the electrical energy. As for the thermal components, five pieces of corrugated aluminum sheet as shown in Figure 1 were joined together to fabricate a hexagonal honeycomb heat exchanger. The honeycomb was installed horizontally as shown in Figure 2a, Figure 2b and Figure 2c into the channel located at the back side of the PV module in order to enhance the thermal efficiency of the system. Sheet 1 Sheet 2 Sheet 3 Sheet 4 Sheet 5 Hexagonal shape Fig 1: Joined pieces of aluminum sheets Honeycomb heat exchanger Aluminum sheet Photovoltaic module 23 mm Heat Insulator Fig 2a: PV module with honeycomb heat exchanger Fig 2b: Photo of honeycomb installed at the back of the PV module The schematic integrated components of the system are illustrated in Figure 3. It consists of a blower, a heater and ducting to provide consistent air flow through the PV/ /T collector. Two units of variable voltage regulators had been used to control the speed of the blower and the temperature of the heater. Type T thermocouples had been used to measure temperature at various locations. By using variable voltage regulator, the heater temperature was adjusted in orderr to ensure that the inlet temperature is equivalent to the ambient temperature. The system was tested indoor with and without the honeycomb heat exchanger using solar simulator with irradiance of 828 W/m 2 and mass flow rate spanning from 0.02 kg/s to 0.13 kg/s. Fig 2c: The honeycomb installed at the back of the PV module 2
2. EXPERIMENTAL SET-UP The purpose of this experiment is to overcome the loss of PV module electrical efficiency operated at high temperature above 25 C and at the same time to collect and utilize the thermal energy from the design system. Therefore a piece of hexagonal honeycomb heat exchanger had been introduced in the experiment. The performance of the PV/T solar collector had been tested indoor using a laboratory fabricated solar simulator. The solar simulator had been tested in terms of its irradiance repeatability and uniformity (15). The test area of the solar simulator is 120 cm x 53 cm suitable for the size of the solar collector that is tested under the simulator. The distance between lamps and collector is 1.6 m. During experiment, the simulator was set to 828 W/m 2 all the time. A schematic view of the PV/T solar collector is illustrated in Figure 3. It is a single pass system with air as the fluid medium for heat transfer. The electrical energy is collected using A mono-crystalline (SW 85) photovoltaic module manufactured by Solarworld, USA. The thermal system consists of a blower attached into a galvanized ducting with length of 2.5 m. A voltage regulator had been used to control the air speed of the blower between 0.4 m/s to 1.5 m/s flow through the system. A heater controlled by a voltage regulator was installed into the ducting in order to ensure that the inlet temperature is equivalent to the ambient temperature. Type T thermocouples were installed at various location of the PV/T system to measure the inlet, outlet and panel temperature. Locally purchased aluminum sheet had been made into corrugated sheet. Five pieces of aluminum corrugated sheets as shown in Figure 1 had been joined together to fabricate a piece of compact honeycomb with hexagonal geometry. The honeycomb was installed horizontally into the channel located at the back side of the PV module. had been used to cover the ducting and the PV module as thermal insulation to minimize heat loss from the system. Three units of fans were used to minimize the infrared effect from the halogen lamps of the solar simulator. 3. EXPERIMENTAL PROCEDURE The experiment had been divided into two parts. For the first part, the performance of the PV/T system had been tested with and without hexagonal honeycomb with irradiance setting of 828 W/m 2 at 5 different mass flow rates. Mass flow rates of the air through the system were set to 0.021 kg/s, 0.042 kg/s, 0.085 kg/s, 0.110 kg/s and 0.128 kg/s. At each mass flow rate setting, time of not less than 70 minutes was allowed for temperature stabilization. Measurements of short circuit current, I sc (A), open circuit voltage, V oc (V), maximum current, I m (A) and maximum voltage, V m (V) were measured using Agilent 34401A Digital Multimeter. The irradiance value was measured using LICOR pyranometer. These data were used to calculate the maximum power and electrical efficiency of the PV/T system. Ten units of Type T thermocouples were placed at various locations of the system. Inlet temperature, panel temperature, outlet temperature and temperature at the back of the thermal insulator were measured and used to calculate the thermal efficiency of the system. As for the second part of the experiment, the PV/T system was tested at 4 different irradiances settings. The irradiance setting values were 591 W/m 2, 705 W/m 2, 828 W/m 2 and 906 W/m 2. The mass flow rate during the measurement was set to 0.110 kg/s at all times. Once again, the performance of PV/T system was evaluated with and without the hexagonal honeycomb heat exchanger. During the experiment, air with mass flow rate spans from 0.02 kg/s to 0.13 kg/s flowed through the honeycomb. Polyethylene 3
4. RESULTS AND DISCUSSION The aim of this experiment was to enhance the thermal efficiency and the electrical efficiency of the PV/T system by introducing aluminum hexagonal honeycomb as heat exchanger into the system. All parameters involved in the calculation of thermal and electrical efficiency were measured. Equation (1) is a common equation used to calculate the electrical efficiency of a PV/T system. Measurements of short circuit current, I sc (A), open circuit voltage, V oc (V), can be done by direct connection between the multimeter and the PV module. x 100% (1) Maximum current, I m (A) and maximum voltage, V m (V) was determined from I/V curve measurement of the PV module using digital multimeter and rheostat. A c is the area of the solar cell and S is the average irradiance value during the experiment. Maximum power contributed by the PV module, P m is given by equation (2). P m = I m x V m (2) Mass flow rate, m of the air flow through the system was calculated using equation (3). m = ρav av (3) Fig 4: Temperature difference, (T o T i ) C The temperature difference decreases with the increase of the mass flow rate. The maximum and minimum temperature difference between two collectors is 6 C at 0.02 kg/s and 2.7 C at 0.13 kg/s respectively. Results show that PV/T collector with honeycomb absorbs and transfers heat more efficiently. Graph in Figure 5 illustrates the electrical efficiency for both collectors. The electrical efficiency, for both collectors increases with the increase of the fluid mass flow rate. PV/T collector with honeycomb shows slightly higher electrical efficiency, compared to the collector without honeycomb. However, it could be concluded that for both collectors, the electrical efficiency remains almost the same. Mass flow rate of fluid is important to calculate the thermal efficiency of the system. For temperature below 100 C, density of air, ρ is constant. Area of input, A is measured at location X as in Figure 3. Speed of air form blower through the ducting was regulated using voltage regulator. Then, air velocity, V av at location X was calculated. Equation (4) was used to calculate the thermal efficiency of the developed PV/T system. x 100% (4) Where C p is specific heat of air, A p is the area of the PV/T collector, T i is the temperature air at input location, T o is the temperature air at output location of the system and S is the average irradiance value during the experiment. Fig 5: Electrical efficiency, of both collectors The new innovation in this work is the aluminum hexagonal honeycomb as heat exchanger located under the PV module. The structure of the hexagonal honeycomb has large surface area touching to the back of the PV module. Therefore it will enhance the heat transfer from the back of the PV module to the air as the moving fluid through radiation, convention and conduction. With this the thermal and electrical efficiencies of the system will be better. Graph in Figure 4 shows the temperature difference (T o T i ) C of the collector with and without honeycomb at irradiance of 828 W/m 2. Fig 6: Thermal efficiency, of both collectors 4
Graph in Figure 6 shows the thermal efficiency, of both collectors at 828 W/m 2. The experimental result shows that the thermal efficiency increases with the increase of mass flow rate up to 0.11 kg/s. Above mass flow rate 0.11 kg/s, the thermal efficiency remains stable. The maximum thermal efficiency, of the collector with the honeycomb is 87 % and without is 27%. Result shows large difference of ~60% between two collectors. This proves that the structure of the hexagonal honeycomb with large surface area touching to the back of the PV module is capable to enhance the heat transfer from the back of the PV module to the air as the moving fluid. At irradiance of 828 W/m 2 and mass flow rate of 0.11 kg/s, the thermal efficiency of the collector with the honeycomb is 87 % and without is 27%. The thermal efficiency is enhanced by 60 % for system with honeycomb. The electrical efficiency of the PV module remains almost the same for both systems. It can be concluded that this design system is suitable to be further investigated as solar drying system and space heating. This innovation is simple for manufacturing process and compact which enables it to be easily implemented for building integration. ACKNOWLEDGEMENT Fig. 7: Comparison of electrical efficiency, of both collectors at different irradiance As for the second part of the experiment, both PV/T collectors were tested at irradiance setting of 591 W/m 2, 705 W/m 2, 828 W/m 2 and 906 W/m 2. The mass flow rate during the measurement was set to 0.110 kg/s all times. Graph in Figure 7 illustrates the experimental results. Electrical efficiency of collector with honeycomb is slightly better. The electrical efficiency is lower at higher irradiance due to the increasing temperature of the PV module caused by infrared radiation effect from the halogen lamp of the solar simulator. Referring to the experimental results, the aim of this experiment which is to enhance the thermal and the electrical efficiencies of the PV/T system, by introducing aluminum hexagonal honeycomb as heat exchanger into the system had been successfully achieved. Results from this experiment compared to other works before for similar collector system shows an improvement mainly in the thermal efficiency. Therefore this innovation can be further improved in the near future for building applications. CONCLUSION The performance of a single pass air base photovoltaic/thermal (PV/T) solar collector with hexagonal honeycomb heat exchanger is more efficient compared to the PV/T collector without honeycomb. The structure of the honeycomb with large surface area touching to the back of the PV module enhances the heat transfer from the back of the PV module to the flowing air. The experiment described in this paper was financially supported by SIRIM Berhad, Malaysia. The technical supports are received from University Kebangsaan Malaysia under research grants UKM-DLP-2011-053 and PRGS/1/11/TK/UKM/01/12. The author would like to express great appreciation to both organizations for their valuable support. Furthermore, the author would like to thank Mr Abdul Rashid Hj. Zainal Abidin, Senior General Manager of National Metrology Laboratory, SIRIM Berhad, Mrs. Hafidzah Othman and Mrs. Irene Safinaz Hassan for their ideas and advices. REFERENCES (1) T.T.Chow., A review on photovoltaic/thermal hybrid solar collector technology, Applied Energy, 87, 365 379 2010 (2) Prakash.J, Transcients analysis of a photovoltaic/thermal collector for co-generation of electricity and hot air/water. Energy Conversion and Management 35, 967-972, 1994 (3) Winston. R., Principles of solar concentrators of a novel design., Solar Energy 16, 89 95,1974 (4) Florschuetz. L., Extention of the Hottel Whillier model to the analysis of combined photovoltaic/thermal flat plate collectors, Solar Energy, 1978 (5) Raghuraman, Cox., Design considerations for flat-plate photovoltaic/thermal collectors, Solar Energy 35, 227 241, 1985 (6) H. P. Garg, R. K., The effect of plane booster reflectors on the performance of a solar air heater with solar cells suitable for a solar dryer, Energy Convers. Mgmt, 543-554.,1991 (7) K. Sopian, K. S., Performance analysis of photovoltaic thermal air heaters, Energy Convers. Mgmt, 1657-1670, 1996 (8) Hegazy, A. A., Comparative study of the performances of four photovoltaic/thermal solar air collectors, Energy Conversion & Management, 861-881, 2000 5
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