Design of a new concentrated photovoltaic system under UAE conditions Ahmed Amine Hachicha, and Muahammad Tawalbeh Citation: AIP Conference Proceedings 1850, 110004 (2017); View online: https://doi.org/10.1063/1.4984478 View Table of Contents: http://aip.scitation.org/toc/apc/1850/1 Published by the American Institute of Physics Articles you may be interested in Recent advances in the PV-CSP hybrid solar power technology AIP Conference Proceedings 1850, 110006 (2017); 10.1063/1.4984480 PVMirrors: Hybrid PV/CSP collectors that enable lower LCOEs AIP Conference Proceedings 1850, 020004 (2017); 10.1063/1.4984328 Parameter optimization of a hybrid solar concentrating photovoltaic/concentrating solar power (CPV/CSP) system AIP Conference Proceedings 1850, 030024 (2017); 10.1063/1.4984367 Co-generation and innovative heat storage systems in small-medium CSP plants for distributed energy production AIP Conference Proceedings 1850, 110002 (2017); 10.1063/1.4984476 PV integration into a CSP plant AIP Conference Proceedings 1850, 110008 (2017); 10.1063/1.4984482 CSPonD demonstrative project: Start-up process of a 25 kw prototype AIP Conference Proceedings 1850, 110003 (2017); 10.1063/1.4984477
Design of a New Concentrated Photovoltaic System Under UAE Conditions Ahmed Amine Hachicha 1, a) Muahammad Tawalbeh 1 1 Sustainable and Renewable Energy Engineering, University of Sharjah, PO Box 27272, Sharjah, United Arab Emirates a) Corresponding author: ahachicha@sharjah.ac.ae Abstract. Concentrated Photovoltaic Systems (CPVs) are considered one of the innovative designs for concentrated solar power applications. By concentrating the incident radiation, the solar cells will be able to produce much more electricity compared to conventional PV systems. However, the temperature of the solar cells increases significantly with concentration. Therefore, cooling of the solar cells will be needed to maintain high conversion efficiency. In this work, a novel design of CPV system is proposed and implemented under UAE conditions for electricity generation and hot water production. The proposed design integrates a water cooling system and PV system to optimize both the electrical and thermal performances of the CPV system. INTRODUCTION Concentrated photovoltaic systems CPV is a promising technology in regions like UAE due to the high level of Direct Normal Irradiance (DNI) which can reach a daily average value of more than 400W/m 2 throughout the year [1]. Such devices operate by using an optical assembly to concentrate focus the sunlight on solar cells placed on a focal point and increase the electricity yield [2]. In fact, the PV cell s conversion efficiency improves with increasing irradiation levels and will be able to deliver much more electricity compared to conventional PV systems [3]. On the other hand, the higher is the concentration the higher is the cell temperature which may lead to a significant drop of the performance and therefore a cooling system is necessary to overcome this drawback. The main effect of the increase of the cell temperature is the reduction of the open circuit voltage which results in a drop of the maximum electrical power delivered by the cell. This increase of the temperature is due essentially to the high solar flux due to the concentration on the PV cells. In this work, an optical model is developed to predict the heat flux distribution around the PV cell under concentration and a cooling system was designed to optimize both the electrical and thermal performance of the CPV system. SolarPACES 2016 AIP Conf. Proc. 1850, 110004-1 110004-9; doi: 10.1063/1.4984478 Published by AIP Publishing. 978-0-7354-1522-5/$30.00 110004-1
OPTICAL MODEL The modelling of CPV systems is an important step to study the effect of both cooling techniques and maximize the electrical and thermal output of the entire system with the lowest pumping power. The non-uniform temperature distribution around the PV cell is the consequence of the non-uniform heat flux distribution around the PV cells under concentration conditions [4]. The methodology used to simulate the non-uniform distribution of the solar flux around the flat PV cells is based on a new method using finite volume method FVM and ray tracing techniques [5]. This numerical-geometrical method allow to track the finite size of the Sun and determine with high accuracy the distribution of the concentrated solar flux in the front face of the PV cells. The entire domain of the CPV system is discretized in various grid systems while the solar optic cone is divided in symmetrical rays similar to the FVM (see Fig. 1). The optical properties are considered in the calculation of the reflected rays reaching the PV cells. FIGURE 1. Different grid systems used in the optical model The Radiative Transfer Equation (RTE) is first resolved within the PV cells domain which is assumed to be transparent leading to the simplified RTE. di( x, y,s) ˆ ds 0 (1) The PV cells are discretized in different control volumes CVs in the horizontal direction. Each CV intercepts the rays coming from the solar optic cone after specular reflection in the parabolic collector. The integration of all rays intercepted by an absorber CV gives the total absorbed solar flux. q ( ) I (2) abs eff int ercepted 110004-2
PROPOSED DESIGN The novel design of the CPV system includes three main elements: the collector to concentrate the sunlight, the receiver where the PV cells are embedded and the cooling system to enhance the electrical and thermal performance of the entire system. The solar collector used in this project is made of a parabolic trough concentrator developed at the University of Sharjah (see Fig. 2). It is made of a steel structure with high reflective aluminum sheet mounted on the mirrors. A new technique was adopted to build the parabola with high precision. Ribs are first made using a laser cutting technique (with a CNC machine) and metallic tubes were used to hold the ribs. The mirror is then fixed to the structure using a sliding technique, i.e. two flat bars are designed as sliders and welded on the ribs with a narrow spaced in between to allow the mirror sheet to take the parabolic shape as shown in figure 3. The sliders consisted of two flat bars tapped together by a screw allowing the mirror to freely slide. This technique is simple to perform and ensure to have a perfect shape of the parabola. FIGURE 2. Existing parabolic trough concentrator at the University of Sharjah FIGURE 3. Mirror slider fixing; middle view (left) and side view (right) The PV receiver and the cooling system are mounted on the focal line of the parabolic trough concentrator. A glass channel with stainless steel channel is manufactured and the PV cells are fixed inside the receiver where it can be cooled with both channels as show in figure 3. 110004-3
FIGURE 4. Receiver design with cooling channels A flexible thin film PV cells are selected for this project (see specifications in table 1) according to the availability and different limitations such as: thickness/conductivity of the PV cell, water proof, small dimensions. TABLE 1. PV specifications 50 ma @4.8V 6.4 V 60 ma P max 0.24W Length 9.4 cm Width 3.6 cm I sc point V open circuit I short circuit With these specifications the CPV system will be able to achieve a concentration factor of 13 suns which is a reasonable value for small scale prototype under Low concentration conditions. As for the cooling system, a water pump is added to circulate the water inside both: the stainless steel and glass channel for the front and back cooling, respectively. The water flow rate is controlled according to the temperature of the PV modules in order to reduce the high temperature of the cells and heat up the water at the same time. EXPERIMENTAL IMPLEMENTATION The water cooling system is designed to enhance the performance of the PV cells without affecting the optical efficiency. In order to determine the optimum thickness of the water layer and therefore select the appropriate design of glass channel, several tests were conducted in the lab using a Potentiostat. I-V curves were measured for different water thickness under the same conditions. Figure 5 shows the variation of the PV cell power with water thickness. The maximum power occurs at a thickness of water of 1 cm where the cooling effect is much more important than the optical losses. The power of the PV cell drops after this optimum and the optical losses become more pronounced. The glass channel was manufactured based on this experiment and the channel depth was fixed to 1 cm. 110004-4
FIGURE 5. Variation of the PV cell power with water thickness The PV receiver should be fixed on the focal line of the parabolic concentrator which has two holders to maintain the weight of the receiver perfectly. The number of PV cells has been determined based on the length of the parabola and avoiding the shading effect of the holders. 18 PV modules are used along the parabolic trough concentrator and connected together in series and parallel to increase the very low current. A thermal tape is used to fix the PV cells on the Stainless Steel duct without affecting the heat transfer from the PV to the duct as shown in figure 6. FIGURE 6. Fixing the PV cells on the receiver using thermal tape Different scenarios have been considered for the experiments with/without concentration and using different cooling techniques (front, back and double cooling). The entire CPV system is set in the roof of the university of Sharjah (see figure 7) on the north-south direction and tracking the sun from east to west throughout the day. FIGURE 7. CPV system installed in the University of Sharjah 110004-5
RESULTS AND DISCUSSION The result of heat flux distribution along the PV cell for a solar radiation of 934 W/m 2 is shown in figure 8. The geometric specification and optical properties were introduced according to the specifications of each element. The entire CPV system was discretised into different control volumes and the solar optic cone into different control angles. The reflected rays are integrated along the PV cell with the respected optical properties. The concentration is high near to the centre of the cell and decrease when displacing towards the edge due to the shape of the receiver. The trend is quite different from a circular receiver and only a small region of the PV cell has high concentration which means the temperature at this region is the highest one. Such increase of temperature can decrease the efficiency of the PV cell and therefore cooling of the PV cells is required to avoid this drop of performance. FIGURE 8. Heat flux distribution around the PV cell using the new optical model The electrical efficiency of the water-cooled CPV system is the ratio of the output electricity to the incident solar radiation and it is given by (3) The thermal efficiency of the water-cooled CPV module is defined as the ratio of the output heat to the incident solar radiation and it is given by (4) The total efficiency of the CPV/T system is + (5) Several tests have been conducted at the University of Sharjah on 1 st of May and the following results were obtained. Test Type P max (W,Wp) I pp,max (A) TABLE 2. Experimental results for different scenarios V pp,max (V) I sc (A) V oc (V) FF (%) G (W/m 2 ) T PV ( C) Flow Rate (L/min) Back channel PV no 0.9 0.04 25.5 0.06 36 45.9 875 51.3 - - concentration Back Cooling 3.4 0.14 23.86 0.24 40.42 35.9 844 45.5 4.8 - Front Cooling 3.9 0.17 23.43 0.26 37.92 38.4 792 32.6-5.4 Double Cooling 4.22 0.17 24.5 0.27 40 38.6 840 33.1 3.4 1.3 Flow Rate (L/min) Front channel 110004-6
Figures 9and 10show the I-V and power curves of the PV cell under different conditions: no concentration, back cooling, front cooling and double cooling. It is clear that double cooling is the best scenario which give the maximum power and best performance of the CPV system. Figure 9 shows that the short circuit current of the PV cells is much lower for non-concentration compared to the concentration scenario as the light intensity increase by concentration. It is worth to mention that cooling of the CPV is more effective from the front side than the back side and may improve the optical efficiency by removing dust and impurities from the PV cells. Current (A) 0,35 0,3 0,25 0,2 0,15 0,1 0,05 0 0 20 40 60 Voltage (V) FIGURE 9. Experiment results of I-V curve for different scenarios Back Cooling Front Cooling Double Cooling No concentration Power (W) 5 4,5 4 3,5 3 2,5 2 1,5 1 0,5 0 0 10 20 30 40 50 Voltage (V) Back Front Cooling Double cooling No Concentration FIGURE 10. Experiment results of power curve for different scenarios The electrical performance of the CPV system is sensitive to its operating temperature and become significant when the temperature of the cell start to rise. Figure 11 shows the evolution of the electrical efficiency with the temperature for both scenario with cooling and without cooling. The electrical efficiency of the CPV system decreases with temperature and increase significantly with cooling. 110004-7
Electrical Efficiency (%) 9 8 7 6 5 4 3 2 1 0 35 40 45 50 55 PV Temperature (C) With cooling Without Cooling FIGURE 11. Electrical efficiency Vs Temperature of PV cells Although the main objective of the CPV system is to cool down the PV cells and therefore increase the electrical efficiency, the heat gained by the water is also contributing to increase the thermal performance of the system. Table 3 shows the evolution of the absorbed heat and the thermal performance for different scenarios. Following the electrical performance, the front cooling is more efficient than back cooling from a thermal point of view. TABLE 3. Thermal performance of the CPV system Test η thermal (%) Q[kW] No Concentration (No cooling) 0 0 Back Cooling 13.5 0.341 Front Cooling 24.2 0.576 Double Cooling 26.5 0.668 CONCLUSION In this study, a novel design of a concentrated photovoltaic system is proposed and tested at the University of Sharjah. A new technique for manufacturing a parabolic trough solar concentrator has been implemented. Due to the increase of the PV cells temperature with concentration different cooling techniques were adopted and compared. The cooling technique is also used to produce hot water and improve the thermal efficiency of the entire system. It was proven that front cooling is more effective than back cooling and gives better electrical and thermal performance. A novel numerical model is developed to predict the optical performance and the solar flux distribution along the CPV system. It is shown that the solar flux distribution is highly concentrated on the center region of the PV cells where temperature is expected to be higher. 110004-8
ACKNOWLEDGMENTS The authors wish to thank to the University of Sharjah for funding this work under grant: V.C/G.R.C/S.R 83/2015. REFERENCES 1. Islam, M. D., Alili, A. A., Kubo, I., & Ohadi, M. (2010). Measurement of solar-energy (direct beam radiation) in Abu Dhabi, UAE. Renewable Energy, 35(2), 515-519. 2. Tyagi, V. V., Kaushik, S. C., & Tyagi, S. K. (2012). Advancement in solar photovoltaic/thermal (PV/T) hybrid collector technology. Renewable and Sustainable Energy Reviews, 16(3), 1383-1398. 3. Pérez-Higueras, P., Muñoz, E., Almonacid, G., & Vidal, P. G. (2011). High Concentrator PhotoVoltaics efficiencies: Present status and forecast. Renewable and Sustainable Energy Reviews, 15(4), 1810-1815. 4. Coventry, J. S. (2005). Performance of a concentrating photovoltaic/thermal solar collector. Solar Energy, 78(2), 211-222. 5. A.A. Hachicha, I. Rodríguez, R. Capdevila, A. Oliva. Heat transfer analysis and numerical simulation of a parabolic trough solar collector, Applied energy, 111 (2013) 581-592. 110004-9