The impact of cooling on cell temperature and the practical solar concentration limits for photovoltaics

The solar concentration limit for densely packed, high-concentrated photovoltaic (HCPV) cells was analyzed for a novel two-phase cooling design. Eight working fluids were examined in the two-phase cooling analysis: R134a, R11, R113, R114, R123, R141b, water, and ammonia. In addition, the study investigated the concentration limit for mass flow rates ranging from 10^-3 to 1kgs^-1. Results from this analysis showed that the limits neared 2000 suns for the six organic fluids examined, whereas for water and ammonia, the practical concentration limit reached about 4000 and 6000 suns, respectively. It was concluded that water and ammonia exhibited greater limits of concentration because they possess greater values of sensible and latent heats compared with the organic fluids examined. The results using this two-phase cooling design were then compared with computational and experimental reference data from other HCPV studies conducted that used cooling mechanisms, such as impinging jets, liquid immersion, and microchannel cooling. Together, the data was compiled and compared with a simplified, one-dimensional, theoretical model using a generic, hypothetical cooling mechanism for densely packed HCPV cells. The general, practical solar concentration limit was predicted to be approximately 10000 effective suns for a cooling device with a heat transfer coefficient on the order of 10⁶Wm^-2K^-1. At this limit, it was determined that the cells' conductive resistance, rather than the external cooling mechanism, becomes the controlling factor for heat removal.