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Abstract

Concentrated photovoltaics (CPV) is advantageous as compared to a conventional non-concentrated photovoltaic (PV) system due to its ability to increase PV cell efficiency and replacing expensive PV material with an abundant and inexpensive reflective or refractive concentrator material. In this work, an overall solar to electrical and thermal conversion efficiencies between a concentrated photovoltaic system thermally regulated by a phase change material (CPVPCM) and a PV system is compared for Qatar climatic conditions. The electrical and thermal behavior of both, CPVPCM and PV, systems are simulated using an already developed and validated coupled electrical-thermal and optical model. A crystalline silicon PV cell is selected which has an electrical conversion efficiency of 14% and temperature dependent maximum power loss coefficient of -0.45% K-1 at standard test conditions. The width of the PV cell is 0.015 m for the CPVPCM system while a 6” ×  6” PV cell is considered for the conventional PV system. The same footprint of 1 m2 is selected as a physical equivalency criterion for both systems to compare their electrical and thermal output. Therefore, 36 PV cells, connected in series, represents footprint of 1 m2 for conventional PV system while the length and configuration of the PV cell for a CPVPCM system depend on the concentration ratio and type of concentrator to achieve equal footprint of 1 m2. The CPVPCM system uses a parabolic trough concentrator to focus sunlight onto a PV cell. The concentration ratios of 10 × , 15 ×  and 20 ×  are considered in this work, which results in lengths of 5.34 m, 3.56 m and 2.67 m to achieve 1 m2 footprint. An increase in incident irradiance density due to concentration instantaneously generates more power from a PV cell in any CPV system. However, it also increases the PV cell temperature and reduces power output because of the temperature dependent negative power coefficient of PV cells. The electrical power output loss due to temperature diminishes the gain in the electrical output due to the concentration of the irradiance. Furthermore, the elevated PV temperature also results in associated degradation mechanisms such as thermal aging, delamination, and mechanical damages. Therefore, cooling of a PV cell in a CPV system is essential to prevent the net power loss and gain the advantages of increased incident irradiance density. Four different commercially available solid-liquid phase change materials (PCM) are investigated for passive cooling of the CPVPCM system. The PCMs absorb and store excess heat during solid to liquid phase change and thereby, regulate the temperature of the PV cell. The heat storage by PCMs also leads to a potential utilization of thermal energy trapped within the PCM. The selection of a PCM depends on its several thermophysical properties such as melting temperature, latent heat of fusion, density, congruency, and stability. In this work, the selected PCMs are Lauric acid, RT42, S-series salt, and ClimSelTM C48 which are reported to have congruency and cyclic thermal stability as per manufacturers' data sheet. The PCMs have melting temperatures ranging from 41 oC to 54 oC and latent heat of fusion in the range of 178 kJkg-1 – 220 kJkg-1. The PCMs are encapsulated inside an aluminum container behind the PV cell. The aluminum container is made of 3 mm thick aluminum sheet having outer dimensions of 0.105 m ×  0.042 m and inner dimensions, to contain PCM, of 0.099 m ×  0.036 m. The lengths of the aluminum container are 2.67 m, 3.56 m and 5.34 m for concentration ratio of 20 × , 15 ×  and 10 ×  respectively. Two consecutive days of each month of a year are selected to simulate the optical, thermal and electrical behavior of both systems. The direct beam irradiance is obtained using the clear sky Bird's model while weather conditions such as ambient temperature and wind conditions are obtained from experimentally measured data from Qatar meteorological department. Since single axis tracking is a pre-requisite for parabolic trough concentrator; therefore, single axis tracking for PV system is considered as well to achieve equivalency regarding input conditions as well. The temperatures of the PV cells, IV curves to calculate maximum power point, the instantaneous power produced over a day using maximum power point tracking, total power produced over a day, and thermal energy storage in the PCMs are predicted by performing simulations of the electrical, thermal and optical behavior of both systems. It is found that the salt performs better than all other selected PCMs due to its high-energy storage density. The electrical output of the CPVPCM for concentration ratio of 10 ×  and 15 ×  is greater than the PV system and comparable at a concentration ratio of 20 × . However, the CPVPCM also store thermal energy, therefore, achieves higher overall solar to the electrical and thermal conversion efficiency as compared to the conventional PV system. Further studies are required, using experimentally measured irradiances instead of using a clear sky model, to investigate and compare the overall conversion efficiencies of the CPVPCM and conventional PV systems.

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/content/papers/10.5339/qfarc.2018.EEPD1017
2018-03-12
2019-11-12
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