1.2 Single cell geometry

Passive cooling is found to work well for single-cell geometries, even for flux levels as high as 1000 suns, because of the large area available for heat sinking. Edenburn [5] found a heat sink with longitudinal fins to be a cost-effective cooling arrangement for concentration levels up to 170 suns. He also suggested using a finless housing box as a heat sink for concentration levels below 90 suns, although this would result in very high cell temperatures on extreme days, and a defocusing mechanism might be necessary for "worst-case scenarios". Minano [6] found the cell size to be the determining factor when designing passively cooled single-cell concentrators. His model suggests that a concentration of 1000 suns would be possible for cells of less than 5 mm diameter. Heat sinks for these cells would be similar to those used for power semiconductor devices. Outdoor experiments by Araki et al. [7] on an array of Fresnel lenses which focus the light onto single cells mounted on a heat-spreading aluminum plate show a temperature rise of
cells over ambient of only 18°C, without conventional heat sinks, for a concentration level of about 500 suns. Good thermal contact between the cell and the heat spreading plate is shown to be crucial to keep the cell temperature low. A single cell lens array which employs a heat sink with longitudinal fins is patented by Graven et al. [8].

Edenburn [5] also considered using active cooling on his point focus arrays, but found that the parasitic power losses involved in pumping and in dissipating the waste heat make active cooling more expensive than passive cooling for single cells. The only exemption would be for very large lenses (more than 300 mm in diameter). However, to enable a cost comparison between the different cooling regimes, the possible advantage of using the extracted heat for thermal energy supply purposes is not taken into consideration. Edenburn concludes that if this were done, active cooling would be the most cost-efficient solution.