Cooling of linear geometries present a greater challenge than for single-cell geometries, and both active and passive cooling have been employed in these systems. Florschuetz

[9]

uses his cost-efficiency model to assess both active and passive cooling options for a linear geometry. He suggests that a plane heat sink would be sufficient for very low concentration levels (less than 5 suns) and a finned one would work for higher levels (10 suns). With reliable winds, these systems could work under slightly higher concentrations. The trough-type photovoltaic concentrator EUCLIDES in Spain [10], with a concentration level of about 30 suns, is cooled by a lightweight aluminium-finned heat sink. The fin optimisation resulted in fins to be 1 mm thick, 140 mm long and spaced about 10 mm apart. A costly manufacturing method was needed which means the heat sink is projected to contribute to more than 15% of the total cost of an EUCLIDES-type plant, while photovoltaic modules and the mirrors contribute around 12% and 11%, respectively.

Edenburn [5] suggests using a "finned mast" (Figure 2) to cool a linear trough design where cells are mounted in a V-type geometry with concentration levels up to 40 suns. He found passive cooling of a linear design to be much more expensive than for a single cell

design and the suggested configuration not to be cost-efficient. Filling the cavity of the "mast" with an evaporative fluid that would work as a thermosyphon to transport heat away from the cells at a very low temperature differential is suggested as a possible improvement. This is further explored by Feldman et al. [11] on a concentration ratio of about 24 suns. With benzene as the working fluid, this gives a maximum evaporator surface temperature of about 140 °C. Outdoor testing shows that the operating temperature is a strong function of wind speed, and less of ambient temperature, wind direction and mast tilt angle. A linear, trough-like system which uses heat pipes for cooling is described by Akbarzadeh and Wadowski [12]. Each cell is mounted vertically on the end of a thermosyphon, which is made of a flattened copper pipe with a finned condenser area. The system is designed for a concentration level of 20 suns, and the cell temperature is reported not to rise above 46 oC on a sunny day, as opposed to 84 °C in the same conditions but without fluid in the cooling system. Florschuetz [9] considers cooling a strip of cells actively by either forced air through multiple passages or water flow through a single passage. Forced air cooling results in a substantial temperature gradient along the cells due to the low heat capacity of air. The required pumping power is also quite large compared to the effective cooling. For these reasons, forced air cooling does not seem to be a viable alternative.

Water cooling, on the other hand, permits operation at much higher concentration levels [9]. Edenburn [5] found active cooling was found to be more cost-efficient than passive cooling in his linear design described above. An actively cooled system, where the cooling methods considered consist of various geometries of coolant flow through extruded channels, is described by O’Leary and Clements [14]. An optimal geometry is suggested based on maximum net collector output versus coolant flow. Because the rate of decrease in thermal resistance drops as the mass flow increases, and the required pumping power increases with increased flow rate, an optimum flow rate can be found for a given system. Russell [13] has patented a heat pipe cooling system where linear Fresnel lenses focus the light onto strings of cells mounted along the length of heat pipes of circular cross­section (Figure 3). Several pipes are mounted next to each other to form a panel. The heat pipe has an internal wick that pulls the liquid up to the heated surface. Thermal energy is extracted from the heat pipe by an internal coolant circuit, where inlet and outlet is on the same pipe end, ensuring a uniform temperature along the pipe. The CHAPS system at the Australian National University [15] is a linear trough system with a concentration of 37 suns where the row of cells is cooled by liquid flow through an internally finned aluminum
pipe. Under typical operating conditions, the thermal efficiency is 57% and the electrical efficiency is 11% for the prototype collector.