Concentration of sunlight onto photovoltaic cells, and the consequent replacement of expensive photovoltaic area with less expensive concentrating mirrors or lenses, is seen as one method to lower the cost of solar electricity. However, only a fraction of the incoming sunlight striking the cell is converted into electrical energy. The remainder of the absorbed energy is converted into thermal energy in the cell and may cause the junction temperature to rise unless the heat is efficiently dissipated to the environment. The major design considerations for cooling of photovoltaic cells are listed below:

— The cell temperature generally needs to be kept below ~60°C and should never exceed ~100°C [1].

— The cells should be kept at a uniform temperature [2, 3].

— The cooling system should be reliable, simple and low-maintenance.

— The total energy output of the collector is increased if the thermal energy can be used, for example as domestic hot water or low temperature process heat [4]. This makes it desirable to have a cooling system that delivers potable water at as high a temperature as possible.

— The power required of any active component of the cooling circuit is a parasitic loss to the system [4], and should thus be kept to a minimum.

— Materials use should be kept down for the sake of cost, weight and embodied energy considerations.

1.1 Concentrator geometries

The requirements for cell cooling differ considerably between the various types of concentrator geometries (Figure 1). In small point-focus concentrators, sunlight is usually focused onto each cell individually, so that each cell has an area roughly equal to that of

the concentrator available for heat sinking. Single cell systems commonly use various types of lenses for concentration. Line focus systems typically use parabolic troughs or linear Fresnel lenses to focus the light onto a row of cells. In this configuration, the cells have less area available for heat sinking because two of the cell sides are in close contact with the neighbouring cells. In larger point-focus systems, such as dishes or heliostat fields, the receiver generally consists of a multitude of densely packed cells. This arrangement presents greater problems for cooling than the two previous configurations, because, except for the edge cells, each of the cells only has its rear side available for heat sinking. This means that, in principle, the entire heat load must be dissipated in a direction normal to the module surface, which generally implies that passive cooling can not be used in these configurations at their typical concentration levels.