TWO-PHASE COOLING OF CONCENTRATING PHOTOVOLTAIC CELLS

Abstract

Experimental investigation of a heat sink for cooling of photovoltaic solar cells up to 2000 suns concentration was conducted. Flow boiling of refrigerant HFC-134a in a pin-fin microchannel was investigated in the range of mass flux 220–380 kg/m 2 s, heat flux 30–170 W/cm 2 , and an exit vapor quality, xout, from 0.2 to 0.75. The heat sink was a pin-fin microchannel module installed in an open flow loop. Deviation from the measured area average temperatures was 1.5 ◦ C at q = 30 W/cm 2 , and 2.0 ◦ C at q = 170 W/cm 2 . These results indicate that use of pin-fin microchannel heat sink enables keeping an electronic device near uniform temperature under steady state and transient conditions. The heat transfer coefficient varied significantly with refrigerant quality and showed a peak at an exit vapor quality of 0.55 in all the experiments. At relatively low heat fluxes and vapor qualities, the heat transfer coefficient increased with vapor quality. At high heat fluxes and vapor qualities, the heat transfer coefficient decreased with vapor quality. Solar cells are more expensive than traditional crystalline silicon; the total cell area needed to provide a specified power level can be reduced, due to their inherently higher efficiency and the use of concentration, thus minimizing solar cell material cost. It is expected that concentrating photovoltaics (CPV), in which the large area of expensive semiconductors is replaced with an equivalent area of relatively low-cost optical reflectors, will lead to considerable cost savings. The power density per unit area of the cell is greatly enhanced by collecting and focusing the light into a small intense beam leading to a reduced cell footprint for comparable power generation. Parabolic or circular paraboloid dish concentrators work by reflecting all incoming light incident on its surface to a single focal point, where the receiver containing the cells is located. Parabolic dishes can be scaled up or down in size and have a theoretical concentration limit of 10,000 suns. This factor is lower in practice due to imperfections in the reflecting surface, but 2000 suns or more is attainable. Solar cells, like most semiconductor-based electronic devices, are adversely affected by temperature. When the temperature rises, more electrons are excited into the conduction band and, in a PV cell, this has the effect of reducing power conversion efficiency. Concentrator photovoltaic systems, or CPVs, are used to gather solar energy by using reflective surfaces to concentrate solar light onto a small photovoltaic cell. This reduces the cost of energy production because the materials for the reflective surfaces are cheaper than solar cells. The problem with this method, however, is that the high concentration of sunlight heats up the solar cells to high temperatures very quickly and consequently may decrease the efficiency of the electricity production. Also, nonuniform temperature distribution on the heated surface may result in potentially destructive thermal stresses along the interface between the chip and the substrate or heat sink. This is one of the key justifications for seeking nearly isothermal heat sinks. Furthermore, a large temperature gradient is undesirable for the electronic performance since many electronic parameters are adversely affected by a substantial temperature rise (Workman et al., 1998; Bohr, 1995). Two-phase flow in parallel microchannels, for which the feed is from a common manifold, displays interesting phenomena, as the two phases may split unevenly when entering the parallel piping. A nonuniform distribution of a