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Energy and fuels
In outdoor deployed photovoltaics (PV), standard test conditions (STC) of 25 °C PV temperature, 1000 Wm-2 solar radiation intensity and 1.5 air-mass rarely prevail. PV temperature can rise 40-100 °C above STC inducing a power drop in crystalline silicon PV with a coefficient of -0.4 to -0.65 %/K above STC. Increased operating temperature also results in accelerated PV degradation due to cell delamination allowing moisture ingress. vConventional building integrated photovoltaics (BIPV) cooling techniques using passive
or active heat removal by air or water flow are limited by (i) very low heat transfer or (ii) large capital as well as maintenance costs respectively. A PV cooling technique employing phase change materials (PCM) exploits latent heat absorption during solidliquid phase change in a very narrow range of PCM transition temperature was investigated. The current research aims to investigate suitable PCM materials through experimental characterization in terms of melting point, heat of fusion, thermal conductivity, density
and specific heat capacity to determine the suitability of different PCMs for PV cooling in different climatic conditions indoors and outdoors employed at small scale cell size PV systems as well as larger PV panel size system through extensive experimental work supported by the reasonable numerical modeling to determine the associated power improvement of PV through cooling produced by PCM.Indoor experiments were conducted at small scale cell size PV at 500 Wm-2, 750 Wm-2 and 1000 Wm-2 insolation representative PV operating condition that would require PV cooling in most cases. The effect of (i) thermal mass of PCM (ii) melting point of PCM and (iii) thermal conductivities of PCM and PV-PCM system on temperature regulation
performance of PCM was observed. Two out of five PCM, a salt hydrate (CaCl2.6H2O) and a eutectic mixture of capric -palmitic acid (CP), , an aluminium alloy based PVPCM systems were found optimum for PV temperature regulation at most of the solar radiation intensities. To extend experiments on PV panel size systems, A larger scale PVPCM system with dimensions 700 cm x 600 cm with metallic fins was fabricated. PCM
CaCl2.6H2O and CP found optimum through cell size experiments were characterized at 500 Wm-2, 750 Wm-2 and 1000 Wm-2 insolation contained in the large scale PV-PCM system. The experiments on large scale PV-PCM systems showed promise for PV cooling provided by PCM and associated power gain. PV-PCM systems were then characterized outdoors in Dublin, Ireland (53.33 N, 6.25 W) and Vehari, Pakistan (30.03 N, 72.25 E) to observe their performance in real time outdoor condition in different climates. Higher PV cooling and associated power savings were observed in climate of Vehari than that of Dublin. Out of the two PCMs, CaCl2.6H2O achieved higher PV
cooling and power saving than CP. In the best case, peak PV cooling of 21.5 °C with associated measured peak power saving of 13 % and predicted peak power saving of 14 % were recorded in Vehari on 30-10-2009. The results show that PCM are an effective way to cool PV and maintain higher power outputs in higher insolation climates.
Hassan, A.: Phase Change Materials for Thermal Regulation of Building Integrated Photovoltaics. Doctoral Thesis. Dublin Institute of Technology, 2010.