Cooling problems are found in other areas than photovoltaics. Extensive research has been performed on the issue of cooling of electronic devices. Large cooling loads and strict temperature limitations are also found in the nuclear energy and gas turbine industries. These applications generally deal with larger areas and different geometries from the electronics industry. Research from these three fields should provide a broad base for finding better options for cooling of photovoltaics.

1.3 Passive systems

There is a wide variety of passive cooling options available. The simplest ones involve solids of high thermal conductivity, like aluminum or copper, and an array of fins or other extruded surface to suit the application. An good overview of these systems is give in [24]. More complex systems involve phase changes and various methods for natural circulation. The use of heat pipes, which is a very efficient way of passively transporting heat, is thoroughly described by Dunn and Reay [25]. The use of heat pipes is not feasible for high concentrations because heat pipe performance is limited by the working fluid saturation temperature and the point at which all liquid evaporates (burnout). It should be noted that passive cooling is just a way of transporting heat from where it is generated (in the PV cells) to where it can be dissipated (the ambient). Complex passive systems reduce the temperature difference between the cells and the ambient, or they can allow a greater distance between the cells and the dissipation area. However, if the area available for heat spreading is small and shading is an issue, no complex solutions will help avoid the use of active cooling. Heat dissipation is still limited by the contact point between the terminal heat sink and the ambient, where the convective heat transfer coefficient, and less so the radiative heat transfer (except at very high temperatures), are the limiting factors.