Radiative sky cooling in low-medium concentration photovoltaic systems

Abstract

Low-medium concentration photovoltaics (LM-CPV) can significantly reduce cost by using cheap optical lenses to reduce the area of the solar cell. Recently, radiative sky cooling (RSC) has been demonstrated to be promising for CPV systems. However, in the reported designs of heat dissipation for CPV systems combining the radiative cooler and a heat sink, a simple flat heat sink was used. Therefore, the effect of the radiative heat dissipation was magnified, but the contribution of convection, which accounts for a significant share in the natural working environment of CPV, was inhibited. To give full play to the role of convection, in this paper, a heat dissipation design combining a radiative cooling layer (RCL) and a finned heat sink was proposed. The RCL was laid on the upper surface of the heat sink to enhance heat dissipation for RSC and realize the effects of thermal radiation. The contributions of the RCL in reducing the temperature of the device, heat dissipation, and obtaining a tolerable concentration ratio under different external environments were examined, and the law whereby the cooling effect of the RCL was influenced by the wind speed and ambient temperature was investigated. The results of numerical simulations shown that the proposed passive cooling device could ensure that the temperature of the solar cell did not exceed 71.5 °C at a concentration ratio of 200, and the maximum difference in temperature inside the device was less than 3.8 °C even in extremely harsh environments (wind speed, 0 m/s; ambient temperature, 50 °C), where this satisfies the requirements of heat dissipation in low-medium concentration silicon solar cells. The radiative heat dissipation power per unit area of the RCL was 201 W/m2, far exceeding the convective heat dissipation. Although the area of the RCL only accounted for 7.9 % of the total area of heat dissipation, its radiative ratio of heat dissipation exceeded 15 % of the total heat dissipation and led to a drop in temperature of 1.76 °C. Moreover, the increase in ambient wind speed significantly improved the cooling effect of the device, which could ensure the rise in the temperature of the solar cell relative to the environment to smaller than 5 °C. However, this also diluted the effect of radiative heat dissipation by the RCL. The increase in the ambient temperature significantly improved the ratio of radiative heat dissipation of the RCL to the overall heat dissipation, reaching 20.2 % when the concentration ratio was 100. In all cases, the cooling power per unit area of the RCL was higher than that of convective and conventional radiative heat dissipation. An even better cooling effect could be achieved by further increasing the relative area of the RCL. Finally, we found that the maximum acceptable concentration ratio of the device was approximately linearly related to the size of the heat sink, and using the RCL could enable the device to withstand an additional 10 ∼ 15 multiples of concentration. The work here provides a new technical option for reducing the cost and improving the efficiency of photovoltaic power generation through the systematic exploration of the influence of the RSC on cooling LM-CPV systems.