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Concentrator Photovoltaic System Basics

August 20, 2013 - 4:12pm

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Concentrator photovoltaic (PV) systems use less solar cell material than other PV systems. 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—the solar cell. One measure of the effectiveness of this approach is the concentration ratio—in other words, how much concentration the cell is receiving.

Concentrator PV systems have several advantages over flat-plate systems. First, concentrator systems reduce the size or number of cells needed and allows certain designs to use more expensive semiconductor materials which would otherwise be cost prohibitive. Second, 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. Third, 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.

However, challenges exist for concentrators. First, 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. Both reflectors and lenses have been used to concentrate light for PV systems.

Illustration of a typical concentrator unit. A trough-shaped metal housing is covered with Fresnel lenses. Inside are secondary optics and cell assemblies. Each cell assembly contains, from top to bottom, a secondary concentrator, an electrical contact, a cell and prismatic cover, solder, a copper heat spreader and electrical contact, conductive adhesive, and the module housing.
A typical concentrator unit consists of a lens to
focus the light, a cell assembly, a housing
element, a secondary concentrator to reflect
off-center light rays onto the cell, a mechanism to
dissipate excess heat produced by concentrated
sunlight, and various contacts and adhesives.

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. 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 90% to 95%, and in practice, most transmit less. Furthermore, concentrators cannot focus diffuse sunlight, which makes up about 30% of the solar radiation available on a clear day.

High concentration ratios also introduce a heat problem. When solar 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, requiring sophisticated heat sync cooling designs.

One of the most important design goals of concentrator systems is to minimize electrical resistance where the electrical contacts on the cell carry off the current generated by the cell. A pattern using wide grid lines, known as fingers, in the contacting grid on top of the cell 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 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.

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