Research @ the Solar Energy Group

Cooling of Concentrator Photovoltaic Cells

The cells are the most expensive part of a photovoltaic system. A simple way of reducing the system costs is therefore to replace some of the photovoltaic area with less expensive optics such as mirrors or lenses, which focus the sunlight onto a small area of cells. Because fewer cells are needed, one can afford to use cells of better quality, with higher efficiency. However, the photovoltaic cells only convert a small (below 30%) portion of the sunlight to electricity. The remaining photons are dissipated in the cells as heat. If this heat is not dissipated efficiently, it will result in a higher cell temperature, which in turn leads to a significant drop in efficiency. Silicon cells should generally be kept below 60C to avoid any long-term damage. Figure 1 shows how the cell temperature will rise for a range of concentrations if the dissipated heat is removed through the back of the cell with a cooling system characterized by its thermal resistance R [1].

Figure 1: Cell temperature rise as a function of concentration level [1].

There are advantages and disadvantages both in the use of mirrors and lenses for concentrating sunlight. Lenses are associated with a relatively high optical loss, typically about 30%, due to reflection and absorption. The heat load is not a problem in lens systems because the cells are spread out on a plate, with plenty of space for each cell to dissipate the waste heat. With mirrors the typical losses are only 10-15%. However, because mirrors focus all of the light onto one highly illuminated area, the cells must be placed closely together in an array. All of the heat load must therefore be dissipated through the back of the cells, which makes cooling in these systems a challenge [1]. In fact, the efficient removal of this heat load is one of the major obstacles for creating viable mirror-based high concentration systems. There is a need for a cooling device that use a liquid coolant such as water, because air cooling is not efficient enough at higher concentrations, cools efficiently across the entire surface, while not shading the concentrator, and operates at a low pumping power.

At the University of Sydney we are currently investigating the use of impinging jets in cooling devices for concentrator cells [2]. Basically speaking this means the water is forced out through an array of small holes and impinge onto the surface to be cooled. This method is used extensively for the cooling of gas turbines, thermal treatment of metals, cooling of internal combustion engines and thermal control of high power density electronic devices. By using this method one can achieve a very high heat transfer rate at a relatively low pumping power. Figure 2 shows a comparison of different cooling technologies available, where impinging jets are one of the most promising options [1].

[1] A. Royne, C. J. Dey and D. R. Mills, Cooling of photovoltaic cells under concentrated illumination: a critical review, Solar Energy Materials and Solar Cells 86 (4), 451-483 (2005).

[2] A. Royne and C. J. Dey, Design of a jet impingement cooling device for densely packed PV cells under high concentration, Solar Energy (submitted).

[3] A. Royne, C. Dey and D. Mills, Cooling of photovoltaic cells under concentrated illumination: a review, Proceedings of the EuroSun 2004, Freiburg, (2004).