Experimental Studies on Water based Photovoltaic Thermal Collector (PVT)

Transcription

1 Experimental Studies on Water based Photovoltaic Thermal Collector (PVT) ADNAN IBRAHIM*, MOHD YUSOF OTHMAN, MOHD HAFIDZ RUSLAN, SOHIF MAT, AZAMI ZAHARIM AND KAMARUZZAMAN SOPIAN Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor. MALAYSIA and Abstract: - The main reason of combining the photovoltaic/thermal collector systems (PV/T) is to increase the efficiency of the photovoltaic. It is known fact that the efficiency of the photovoltaic panel decreases when the ambient temperature increased. The photovoltaic panel absorbed sunlight from the sun and convert it to electricity. Beside sunlight, it also absorbed heat and this phenominon leads to the decrease of photovoltaic efficiency. An experiement study of a photovoltaic thermal collector involving a specially designed absorber collector known as Spiral flow absorber collector has been for heat transfer enhancement. The results of performance efficiency of the photovoltaic, thermal and also the combination of both, PV/T over range of operating conditions are discussed and analyzed. Results at solar irradiance of 1321 W/m 2 show that the combined PV/T efficiency of 65 %, electrical efficiency of 12% at mass flow rates of kg/s. Key-Words: - Photovoltaic thermal collector, Spiral flow absorber, hot water heating system, thermal and electrical efficiency 1 Introduction Solar energy technology can be broadly classified into two systems; photovoltaic energy system and thermal energy system. It has been shown that these systems can be combined to form hybrid photovoltaic thermal (PVT) system. The term PVT refers to solar thermal collectors that use PV cells as an integral part of the absorber plate. The system generates both thermal and electrical energy simultaneously. As mentioned by Ibrahim et al. [2009] hybrid photovoltaic thermal system inherited all the advantages of PV technology. The advantages such as works on noiseless environment; do not produce any unwanted waste such as radioactive materials etc, highly credibility system with life span expectation is between 20 to 30 years and very low maintenance system are considered an attractive features for PVT system. Numerous researches and development programs have been carried out to improve the applications of solar energy systems. Several design of photovoltaic thermal solar water based collector has been proposed in the past. Among the first was Martin Wolf [1976] who analysed the performance of combining the heating and photovoltaic power systems for residences and conclude that the system was technically feasible and cost effective. Beside Wolf, research on PV/T water based collector bas been conducted by Florschuetz [1979] by extending the Hottel-Whillier model to the analysed the combination of PV/T flat plate collectors with the traditional hot water system and PV panel to minimize the usage of the installation area. It is proven that by combining the system, the installation area produced more energy per unit surface area than a separate system (Florschuetz [1979]). Zondag [2008] examined the various concepts of combined PV-thermal collector technologies and conclude that the design of the channel below the transparent PV with PV-on-sheet and tubes design gives the best efficiency overall. Bergene et al. [1995] perform theoretical examination of a flat plate solar collector model that integrated with solar cells, concludes that the system combination of both produced approximately about 60-80% efficiency. He et al. [2006] studied the hybrid PVT system with natural convection to circulate the water, and discovered that the system produced combined efficiency of 50%, with the daily thermal efficiency contributing approximately 40%. Later, Chow et al. [2003] investigate further the hybrid PVT system of ISSN: X 439 ISBN:

2 He, Chow et al. [2006] and suggested that the system could be improved by placing the PV cells on the lower portion of the collector. Performance simulation of PV/T collectors with seven new design configurations of absorber collectors design has been studied by Ibrahim et al. [2009] and conclude that the best design configuration is the spiral flow design with thermal efficiency of 50.12% and cell efficiency of 11.98%. 2 Experimental setup The design of the specially designed absorber collector known as Spiral flow absorber collector which considered in this paper is shown in Figure 1. system. A standard photovoltaic panel represented as a flat plate single glazing sheet of polycrystalline silicon with single glazing sheet has been used. The Spiral flow absorber collector is designed in the form of continuous coil or tube configured. The spiral coil has at least one inlet and outlet to allow medium (water) to enter and to exit from coil respectively. The inlet and the outlet of the spiral coil are arranged further away to the entre point of the spiral. This will allow the medium (water) to flow in reversed direction and covered the entire photovoltaic panel. The configuration: medium (water) with lower temperature enters the coil and flow in to the centre point and flow out from the centre point leaving the coil as hot water. The hot water can be consumed or stored for later use. In this way solar radiation energy can be fully utilized. In this experiment, standard photovoltaic panel, rating at 80 W power is used. As shown in Fig. 3, the Spiral flow absorber collector is inserted underneath the standard photovoltaic panel and tested outdoor. Fig. 1 The design of Spiral flow absorber collector A specially made Spiral flow absorber collector has been designed and evaluated. The Spiral flow, as shown in Fig. 1, is made of rectangular hollow tubes of stainless steel material with dimension of mm. The tube is connected using a welding method. The absorber collector, as shown in Fig. 2 consist of a single unilateral channel for the water to flow in it with the size of x 30 mm before it is inserted underneath the standard photovoltaic panel with the size of 1 m height, 0.65 m length and 0.3 m thickness. Fig. 2 The assembly view of Spiral flow absorber collector Fig. 3 Complete assemble of PVT system tested outdoor. As shown in Fig. 4, ambient temperature and other temperatures are measured using K-type thermocouple and located at several places. Solar radiations from the sun are measured by Eppley pyranometer for intensity. Mass flow rates for Spiral flow absorber collector are set from 0.034, and kg sec 1 and connected direct to data acquisition system which later link to the computer. Thermal insulator is packed underneath the absorber collector to prevent heat from escaping further and provides more uniform temperatures throughout the ISSN: X 440 ISBN:

3 The overall efficiencies, which is known as combined PV/T efficiency η pvt is used to evaluate the overall performance of the system: η pvt = η th +η e Fig. 4 PVT system tested outdoor with K-type thermocouple and data actuation system. 4 Results and discussion The Spiral flow absorber collector experiment has been performed on 30 December As mentioned, data were collected from 08:00 to 17:00 respectively. As shown in Fig. 5 shows that the peak of solar irradiance on that particular day was at 14:00 with 1321 W/m 2. 3 System efficiency The efficiency performance of the PV/T unit is evaluated for its thermal and photovoltaic solar cell efficiency based on the Hottel and Whillier Equations (Hottel H. C et al. [1958]). For Spiral flow absorber collectors efficiency, the mass flow rates can be calculating using the equation below:. m = ρav av where: ṁ = The mass flow rate, ρ = The density of the medium drain input area and V av = The water velocity. Fig. 5 Solar irradiance versus time Fig. 6 shows the temperature distribution of Spiral flow absorber collector taken at mass flow rate of kg/s. The thermal efficiency of the collector is expressed as: (Vokas et al. [2006]) Ti Ta ηth = FR( τα) PV FRU L GT where: F R = heat removal efficiency factor, τα PV = average transmittance-absorptance of the collector, U L = overall collector heat loss coefficient (W/m 2 ºC), T i = fluid inlet temperature (ºC), T a = ambient temperature (ºC) and G T = solar radiation at NOCT. For temperature-dependent electrical efficiency of the PV module, η ) Tiwari et al. [2006], the ( e expression is given as below: η = η 1 β e r [ ( T T )] where: η e = electrical efficiency, η r = reference efficiency of PV panel (η r = 0.12), β = temperature coefficient (ºC ºC -1 ), T c = temperature of the solar cells (ºC), and Tr = the reference temperature. c r Fig. 6 Temperature of ambient, water inlet and plate versus time Fig. 7 shows the efficiency of the collector versus time. The result shows that the system is time dependence based on solar irradiance. ISSN: X 441 ISBN:

4 Fig. 7 Efficiency of thermal, electrical and overall versus time Fig. 8 show the dependence of electrical, thermal and combined PV/T efficiency on the mass flow rate of the Spiral flow absorber collector respectively. The efficiencies approach steady state values as the mass flow rate increases. The result shows that when mass flow rate increases, the surface temperature decrease and at the same time the efficiencies for electrical and thermal increase. It is clearly shown that as the temperature decrease with the increasing of mass flow rate, the collector thermal efficiency increasing due decrease in the average temperature of the absorber plate. By increasing the flow rate will increase the heat transfer coefficient resulting in a lower mean photovoltaic cells temperature. Fig. 8 Efficiency of photovoltaic (PV), thermal (T) and combined PV/T versus time 5 Conclusions Results indicates that the electrical and thermal production of a PV/T hybrid system increases with decreasing temperature of ambient. The system is considered to be a closed loop system, it is worthwhile to deliver the hot water out of the system for other purposes and cold water should be kept as low as possible. A trade-off between increasing of electricity production and producing hot water is thus necessary. The experiment proved that the hybrid PV/T has a potential as an alternative method of power production. Acknowledgment The authors would like to express their gratitude to Universiti Kebangsaan Malaysia and the Ministry of Science, Technology and Innovation Malaysia for sponsoring the work under project Sciencefund SF0039. References: [1] Bergene, T. and O. M. Lovvik (1995). "Model calculations on a flat-plate solar heat collector with integrated solar cells." Solar Energy 55(6): [2] Chow, T. T., J. W. Hand and P. A. Strachan (2003). "Building-integrated photovoltaic and thermal applications in a subtropical hotel building." Applied Thermal Engineering 23(16): [3] Florschuetz, L. W. (1979). "Extension of the Hottel-Whillier model to the analysis of combined photovoltaic/thermal flat plate collectors." Solar Energy 22(4): [4] He, W., T.-T. Chow, J. Ji, J. Lu, G. Pei and L.- s. Chan (2006). "Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water." Applied Energy 83(3): [5] Hottel H. C and A. Whillier (1958). "Evaluation of Flat-Plate Solar Collector Performance." Trans. of the Conference on Use of Solar Energy 2: 74. [6] Ibrahim, A., G. L. Jin, R. Daghigh, M. H. M. Salleh, M. Y. Othman, M. H. Ruslan, S. Mat and K. Sopian (2009). "Hybrid photovoltaic thermal (PV/T) air and water based solar collectors suitable for building integrated applications." American Journal of Environmental Sciences 5(5): [7] Ibrahim, A., M. Y. Othman, M. H. Ruslan, M. A. Alghoul, M. Yahya, A. Zaharim and K. Sopian (2009). "Performance of photovoltaic ISSN: X 442 ISBN:

5 thermal collector (PVT) with different absorbers design." WSEAS Transactions on Environment and Development 5(3): [8] Tiwari, A. and M. S. Sodha (2006). "Performance evaluation of hybrid PV/thermal water/air heating system: A parametric study." Renewable Energy 31(15): [9] Vokas, G., N. Christandonis and F. Skittides (2006). "Hybrid photovoltaic-thermal systems for domestic heating and cooling--a theoretical approach." Solar Energy 80(5): [10] Wolf, M. (1976). "Performance analyses of combined heating and photovoltaic power systems for residences." Energy Conversion 16(1-2): [11] Zondag, H. A. (2008). "Flat-plate PV-Thermal collectors and systems: A review." Renewable and Sustainable Energy Reviews 12(4): ISSN: X 443 ISBN: