Photovoltaics - Concentrator Systems

Background

The performance of a photovoltaic (PV) array can be improved in a number of ways. One option is to employ concentrating optics, which gather sunlight with lenses, thereby increasing the intensity of sunlight striking the PV cell. (This is similar to using a magnifying glass.)

Reasons for Use

A typical basic concentrator unit consists of a lens to focus the light, a cell assembly, a housing element, a secondary concentrator to reflect off-centre light rays onto the cell, a mechanism to dissipate excess heat produced by concentrated sunlight, and various contacts and adhesives. These basic units may be combined in any configuration to produce the desired sized module.

The primary reason for using concentration is to decrease the area of solar cell material being used in a system; solar cells are the most expensive components of a PV system, on a per-area basis. A concentrator uses relatively inexpensive materials (plastic lenses, metal housings, etc.) to capture a large area of solar energy and focus it onto a small area, where the solar cell resides. One measure of the effectiveness of this approach is the concentration ratio (how much concentration the cell is receiving).

Advantages and Drawbacks

Besides increasing the power and reducing the size or number of cells used, concentrators have the additional advantage that cell efficiency increases under concentrated light. How much the efficiency increases depends largely on the cell design and the cell material used. Another advantage of the concentrator is that it can use small individual cells--an advantage because it is harder to produce large-area, high-efficiency cells than it is to produce smaller-area cells.

There are, on the other hand, several drawbacks to using concentrators. The concentrating optics they require, for example, are significantly more expensive than the simple covers needed for flat-plate modules, and most concentrators must track the sun throughout the day and year to be effective. Thus, higher concentration ratios mean using not only expensive tracking mechanisms but also more precise controls than flat-plate systems with stationary structures.

High concentration ratios are a particular problem, because the operating temperature of cells increases when excess radiation is concentrated, and this creates heat. Cell efficiencies decrease as temperatures increase, and higher temperatures also threaten the long-term stability of PV cells. Therefore, PV cells must be kept cool.

Source: U.S. Department of Energy Photovoltaics Program.

For more information on this source please visit National Renewable Energy Laboratory

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