It seems that all operating system with densely packed cells rely on active cooling. Verlinden et al. [16] describe a monolithic silicon concentrator module with a fully integrated water cooled cold plate. The module consists of 10 cells and is supposed to act as a "tile" in a larger array. The design is further described by Tilford et al. [17]. However, details are not given on the way in which the water flows through the cold plate. Lasich

[18]

has patented a water cooling circuit for densely packed solar cells under high concentration. The circuit is said to be able to extract up to 500 kW/m2 from the photovoltaic cells, and to keep the cell temperature at around 40 °C for normal operating conditions. This concept is based on water flow through small, parallel channels in thermal contact with the cells. The cooling circuit also forms part of the supporting structure of the photovoltaic receiver. It is built up in a modular manner for ease of maintenance, and provides good solutions for the problem of different thermal expansion coefficients of the various materials involved. Solar Systems Pty. Ltd. has reported some significant results from their parabolic dish photovoltaic systems located in White Cliffs, Australia [4, 19]. They work with a concentration of about 340 suns. An average cell temperature of 38.5 oC with a corresponding cell efficiency of 24% is maintained. If all of the thermal energy extracted were used, the overall useful energy efficiency in this system would be more than 70%. This demonstrates clearly the benefits of active cooling if one can find uses for the waste heat.

Vincenzi et al. [21, 22] have suggested integrating the cooling function in the cell manufacturing process by using a silicon wafer with microchannels circulating water directly underneath the cells. The system under consideration is run at about 120 suns. Microchannel heat sinks will be presented in more detail later. A system is patented by Horne [20] in which a paraboloidal dish focuses the light onto cells that are mounted vertically on a set of rings, designed to cover all of the solar receiving area without shading (Figure 4). In this system, the water both cools the cells and acts as a filter by absorbing a significant amount of UV radiation that would otherwise have reached the cells. It would also absorb some of the low energy radiation, resulting in higher cell efficiency and a lower amount of power converted to heat in the cells. The Horne patent incorporates a phase — change material in thermal contact with the cells, which works to prevent cell damage at "worst-case scenario" temperatures. Koehler [23] suggests submerging the cells in a
circulating coolant liquid, whereby heat is transferred from two cell surfaces instead of just one. In this way the coolant also acts as a filter by absorbing much of the incoming low — energy radiation before it reaches the cells. The coolant liquid must be dielectric in order to provide electrical insulation of the cells. By choosing the right coolant fluid and pressure, one can achieve local boiling on the cells, which give a uniform temperature across the surface and a much higher heat transfer coefficient.