Concentrator PV Systems
The primary reason for using concentrators is to be able to use less solar cell material in a PV system. PV cells are the most expensive components of a PV system, on a per-area basis. A concentrator makes use of relatively inexpensive materials such as plastic lenses and metal housings to capture the solar energy shining on a fairly large area and focus that energy onto a smaller area, where the solar cell is. One measure of the effectiveness of this approach is the concentration ratio—in other words, how much concentration the cell is receiving.
The largest system being tested at Arizona Public Service Company's Solar Test and Research (STAR) Center is a high-performance concentrating solar power generator. Although they're not suitable for small projects, concentrator systems could be very effective in large-scale power generation. The STAR Center unit produces 20 kilowatts of electricity—enough to power about five homes. Systems like these may some day supply power to entire communities.
Several advantages of concentrator PV systems, as compared to flat-plate systems, can be enumerated. Concentrator systems increase the power output while reducing the size or number of cells needed. An additional advantage is that a solar cell's efficiency increases under concentrated light. How much that efficiency increases depends largely on the design of the solar cell and the material used to make it. Another advantage is that a concentrator can be made of small individual cells. This is an advantage because it is harder to produce large-area, high-efficiency solar cells than it is to produce small-area cells.
A typical basic concentrator unit consists of a lens to focus the light, cell assembly, housing element, secondary concentrator to reflect off-center light rays onto the cell, mechanism to dissipate excess heat produced by concentrated sunlight, and various contacts and adhesives. Notice that the module depicted uses 12 cell units in a 2x6 matrix. These basic units may be combined in any configuration to produce the desired module.
However, several challenges exist to using concentrators. For example, the required concentrating optics are significantly more expensive than the simple covers needed for flat-plate solar systems, and most concentrators must track the sun throughout the day and year to be effective. Thus, achieving higher concentration ratios means using not only expensive tracking mechanisms, but also, more precise controls than those of flat-plate systems with stationary structures.
Both reflectors and lenses have been used to concentrate light for PV systems. The most promising lens for PV applications is the Fresnel lens, which uses a miniature sawtooth design to focus incoming light. When the teeth run in straight rows, the lenses act as line-focusing concentrators. And when the teeth are arranged in concentric circles, light is focused at a central point. However, no lens can transmit 100% of the incident light. The best that lenses can transmit is only 90% to 95%, and in practice, most transmit less. Furthermore, concentrators cannot focus diffuse sunlight, which makes up about 20% of the solar radiation available on a clear day.
High concentration ratios also introduce a heat problem. When excess radiation is concentrated, so is the amount of heat produced. Cell efficiencies decrease as temperatures increase, and higher temperatures also threaten the long-term stability of solar cells. Therefore, the solar cells must be kept cool in a concentrator system.
One of the most important design considerations is to minimize electrical resistance where the external electrical contacts carry off the current generated by the cell. Wide fingers in the contacting electrical grid are ideal for low resistance, but they block too much light from reaching the cell because of their shadow. One solution to the problems of resistance and shadowing is prismatic covers. These special covers act like a prism and direct incoming light to parts of the cell's surface that are between the metal fingers of the electrical contact grid. Another solution is a the back-contact cell, which differs from conventional cells in that both the positive and negative electrical contacts are on the back. Placing all the electrical contacts on the back of the cell eliminates power losses from shadowing, but it also requires exceptionally good-quality silicon material.