The thermal properties of air make it far less efficient as a coolant medium than water [24]. This implies that more parasitic power will be needed to achieve the same cooling performance. Hence, air is a less favourable option in many cases. Detailed information on the design of forced air heat sinks can be found in [24].

The microchannel heat sink is a concept well suited to many electronic applications because of its ability to remove a large amount of heat from a small area. Tuckerman and Pease [26] were the pioneers who first suggested the microchannel heat sink, based on the fact that the convective heat transfer coefficient scales inversely with the channel width. Two major drawbacks to the microchannel heat sink are a large temperature gradient in the streamwise direction and a significant pressure drop that leads to high pumping power requirements. A numerical optimization that minimises the thermal resistance subject to a specified pumping power is presented by Ryu et al. [27]. Harms et al. [28] conclude that heat transfer performance in microchannels can be increased by decreasing the channel width and increasing the channel depth. Developing laminar flow is
found to perform better than turbulent flow due to the larger pressure drop associated with turbulent flow. Owhaib and Palm [29] show that in the laminar flow regime, the heat transfer coefficient is largely independent of channel diameter, while in the turbulent regime, smaller channels are clearly better. Introducing alternating flow directions can reduce the streamwise temperature gradient in the microchannel heat sink. Missagia and Walpole [30] describe a single layer counter flow technique. Vafai and Zhu [31] suggest using two layers of counter-flow microchannels, which is shown to significantly lower the streamwise temperature gradient compared to a one-layer structure. Chong et al. [32] optimised the counter flow principle for single and double layer channels for both designs for laminar and turbulent flows, and found that laminar flow was to be preferred over turbulent for both cases. The manifold microchannel heat sink (Figure 5), in which the coolant flows through alternating inlet and outlet manifolds in a direction normal to the heat sink, has been modelled and optimised by Ryu et al. [33]. Because the fluid spends a relatively short time in contact with the base, a more uniform temperature distribution across the surface is achieved.

Very low thermal resistances can be achieved through the use of impinging liquid jets. The impinging jets are capable of extracting a large amount of heat because of the very thin thermal boundary layer that is formed in the stagnation zone directly under the impingement, and that extends radially outwards from the jet. However, the heat transfer coefficient decreases rapidly with distance from the jet. To cool larger surfaces, it is therefore desirable to use an array of jets. If measures are taken to ensure the flow from different jets does not interact in such a way that they lower the overall heat transfer, impinging jets are predicted to be a superior alternative to microchannel cooling for target dimensions larger than the order of 0.07 x 0.07 m2 [34]. Webb and Ma [35] give an extensive overview of the literature available on liquid impinging jets.

By allowing the coolant fluid to boil, the latent heat capacity of the fluid can accommodate a significantly larger heat flux and achieve an almost isothermal surface. Although any comprehensive heat transfer textbook such as [36] will give an introduction to forced convection boiling, two-phase flows are complicated to model. The most important parameter in forced convection boiling is the critical heat flux (CHF), defined as the point at
which enough vapour is being formed that the surface is no longer continuously wetted. To achieve maximum cooling, one wants to run the system close to the CHF, but never above. High velocities, large subcoolings, small diameter channels and short heated lengths are known to increase the CHF. Two-phase flows may be a good option for the cooling of photovoltaic cells when the heat fluxes are high. The saturation temperature of water can be brought to 50 °C at a pressure of 0.13 bar [36]. To avoid pressurised systems, other working fluids may be used eg. Vertrel XF [37]. Ghiaasiaan and Abdel — Khalik [38] give an extensive literature review of two-phase flow in microchannels, which includes a thorough description of flow regimes in horizontal and vertical channels, correlations for pressure drops, forced flow subcooled boiling and CHF. Hetsroni et al. [37] describes a two-phase microchannel heat sink that keeps heated surface at a temperature of 50-60 °C, a temperature highly suited for photovoltaic purposes. The working fluid is Vertrel XF, which has the desired saturation temperature and is dielectric, so that it can be brought into contact with the active electronics. The study was performed at relatively low heat fluxes (< 60 kW/m2). Inoue et al. [39] study the use of boiling in confined jets (Figure 6) to cool a very high heat flux (near 30 MW/m2) in a fusion reactor. This system proposes an innovative way of preventing flow interaction between neighbouring jets, and at the same time preventing splash of water from the violent boiling that may occur at the surface under these conditions.

2 Conclusion

Cell cooling is an important factor when designing concentrating photovoltaic systems. The cooling system should be designed to keep the cell temperature low and uniform, be simple and reliable, keep parasitic power consumption to a minimum and, if possible, enable the use of extracted thermal heat.

With single-cell geometries, research shows that passive cooling is feasible and the most cost-efficient solution for concentration values of up to 1000 suns provided the cells and lenses are kept small.

Linear concentrators can also be cooled passively, but the heat sinks tend to get very intricate and therefore expensive for concentration values above 20 suns. A heat pipe based solution is one way to increase the passive cooling performance. Different ways of active cooling by water or other coolants have also been found to work well and should be considered for concentration levels above 20 suns.

For densely packed cells, active cooling is the only feasible solution. At high concentrations, the high heat flux makes a low contact resistance from cell to cooling system extremely important. Recent options such as microchannels or impinging jets generally prove to be good solutions. Microchannels are particularly promising because they have the option of being incorporated in the cell manufacturing process. Forced convection boiling give the possibility of uniform-temperature cooling at extremely high heat fluxes, but a coolant other than water is generally needed in order to keep the cells at the desired low temperature.