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About PV ----> Modules, Arrays, and Systems ---->Concentrator Collectors

 

Concentrator Collectors

The performance of a 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. A typical basic concentrator unit consists of a lens to focus the light, a cell assembly, a mechanism that houses the lens at one end and the cell at the other, 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 cell may also use a prismatic cover to guide the light around the cell's metallic grid and onto the active cell material. These basic units may be combined in any configuration to produce the desired 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

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-center light rays onto the cell, a 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.

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. Ideally, the concentration ratio is the area of the lens divided by the area of the cell. (We say "ideally" because there are reflection and absorption losses.) Thus, if we have a lens with an area of 200 cm2 focusing light on a cell that has an area of 4 cm2, the concentration ratio is 50. With this ratio the cell receives 50 times the amount of sunlight it would get under unconcentrated sunlight, which means you can cut the cell area by 50 times (relative to flat-plate collectors) to get a desired amount of power.

The concentration ratio, the power at which the sun is concentrated, is equal to the area of the aperture (the opening through which the sunlight enters - in this case, the aperture is a lens) divided by the area of the illuminated cell.

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, up to a point. How much the efficiency increases depends largely on the cell design and the cell material used. Another advantage of concentrators is that they can use small individual cells - an advantage because it is harder to produce large-area, highefficiency 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 simplecovers needed for flatplate modules. Also, most concentrators (those with concentration ratios greater than 10, i.e., those that make the effective sunlight energy striking the cell 10 times greater than that of ordinary sunlight) must track the sun along two axes to be effective. This is because the field of view-the angular window where light must fall to be acted on by the concentrating optics generally gets smaller as the concentration ratio increases. Thus, higher concentration ratios mean using not only expensive tracking mechanisms but also more precise controls than flat-plate systems with stationary structures. Another inherent disadvantage of concentrators compared with flat-plate modules is that they can use only direct sunlight. Concentrators cannot focus diffuse sunlight, which represents about 20% of the sunlight received in a desert region. As a result, concentrators produce little output on cloudy days and are intended for use chiefly in areas with very few such days.

Concentrating systems, especially those with high concentration, require mechanisms that track the sun. This particular system has a reinforced concrete pedestal as a base, a sun sensor (in the middle of the system), and a drive mechanism that allows the array to track the sun east to west during the day and north and south through the seasons.

Besides diffuse sunlight, some direct sunlight is lost as well, because an inherent limitation of any concentrator is that a portion of the sunlight fails to strike effective areas of the lenses. A line-focus concentrator (one that focuses sunlight along a line) can lose as much as 10% of incident direct sunlight, and a point-focus concentrator (one that focuses sunlight at a point) can lose from 10% to 20%.

Concentration can create heat, which can be deleterious to cell efficiency and operation. One way to keep cells cool is to mount the cells on a heat spreader, which spreads the heat and radiates it away from the cell.

High concentration ratios are a particular problem, because the operating temperature of cells increases when excess radiation is concentrated, creating heat. Cell efficiencies decrease as temperatures increase, and higher temperatures also threaten the long-term stability of PV cells (as well as most semiconductor devices). Therefore, PV cells must be kept cool, either by passive cooling (such as metal fins to radiate heat) or by active cooling, such as circulating coolants.

Concentrating Optics

As of the late 1980s, the most promising lens for PV concentrating optics applications was the Fresnel lens, which features a miniature sawtooth design. 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. The combination of Fresnel lenses, high-efficiency silicon PV cells, and two axis tracking is currently considered to be the most promising concentrating approach.

A Fresnel lens, the most common lens in concentrator systems, is a sheet of plastic that is flat on the upper side and that has sawtooth grooves on the bottom (a). If the grooves are etched in parallel ridges (b), they concentrate light along a line. If they are etched in concentric circles (c), they concentrate light at a point.

Special Types of PV Cells

The development of PV cells for concentrators has been evolving in three major directions. Low concentration systems (with a concentration of 10 to 50 times), for expediency and near-term use, generally employ PV cells developed for one sun applications with no modifications. Midconcentration systems (with a concentration of 50 to 300 times) include most of the high-efficiency PV cells being developed today. As to highconcentration systems (with concentrations greater than 300 times), the expense of manufacturing PV cells with such high-efficiency performance may make the entire system too costly.

Cells designed for concentrators usually have special grid patterns. This cell could handle concentrations up to 200 suns. (Depiction based on a cell used by Applied Solar Energy Corperation.)

At mid-concentration ranges, concentrating modules require some changes in cell features for the best performance. As the intensity of the incident light increases, so does the short-circuit current of the PV circuit. With a higher current, series resistance losses become important. Several resistance-lowering features can be incorporated into high-efficiency cells, and they are particularly important for cells that will be used under high concentration ratios. Usually, cells designed for concentration are thinner than normal cells, have a lower cell resistance, and have special grid patterns devised to carry the high currents without blocking much of the cell's surface area.

The point contact cell, designed by Swanson et al. at Stanford University, is a concentrator cell that incorporates several innovations, including a textured top, a reflective back surface, silicon dioxide layers at the front and back surface, and thousands of small contacts on the back surface to collect charge carriers. (Adapted from the Proceedings of the 18th IEEE Photovoltaic Specialists Conference with permission from IEEE.)

Point-contact PV cells, which are fashioned for use with concentrators, are the most efficient silicon solar cells reported to date and have achieved efficiencies above 28% under concentration. These cells are uniquely designed to capture the maximum amount of light and to generate the greatest amount of current. A conventional solar cell has an electrical grid on the front and a solid electrical contact on the back. Each point-contact cell, in contrast, has tens of thousands of microscopic points of alternating positive and negative contacts that occupy only about 5% of the cell's back surface.

Putting the point contacts on the back removes any shading or recombination that might otherwise have occurred by putting them on the front surface. Making them small significantly reduces the contact surface area, which decreases recombination and contact resistance. And implanting so many contacts enhances efficient collection of lightgenerated charge carriers.

The front surface of the cell is textured with small pyramid shapes; this reduces the reflection of incident sunlight and scatters the light into the cell at angles that allow the light to be trapped inside the cell (reflecting back and forth without escaping) until the light is absorbed. To maximize this "light trapping," the back surface is highly reflective and reflects the great majority of the unabsorbed photons back into the interior of the cell. With these two features the cell can be made very thin (100 microns or less) and still absorb nearly all of the usable light. The high percentage of light absorption produces a large number of charge carriers, and the thinness of the cell helps the carriers reach the terminals for collection before they can recombine. To help reduce surface recombination, the cell also employs thin layers of silicon oxide at the front and back surfaces.

The microgrooved cell designed by Green et al. at the University of South Wales uses very high quality silicon with a thin layer of silicon dioxide on the top surface. Thin grooves are cut into the oxide, over which are laid very thin fingers of a metal grid. (Adapted from the Proceedings of the 19th IEEE Photovoltaic Specialists Conference with permission from IEEE.)

Finally, the cell uses an antireflection coating on the front surface to decrease reflection losses.

Microgrooved passivated emitter solar cells have a more conventional structure than point-contact cells. The base material is very high quality silicon; a thin layer of silicon dioxide is applied to the surface to inhibit recombination.

Several special features enhance the efficiency of this device. Thin slots or grooves in the oxide allow contact between the metal grid and the emitter. The grooves also help to minimize reflection and to trap light inside the cell. The thin metal grid, laid across the grooves at a 45° angle, allows some of the light to be reflected from the metal fingers and to enter a nonmetallized region.

To further enhance efficiency, these cells are also being used with prismatic covers, which help overcome shadowing losses due to top contacts. These special cell covers are etched in such a way that they direct incoming sunlight away from areas of the PV cell covered by metal grid lines. Prismatic covers have been shown in the laboratory to improve efficiency by 20% (relative), and they are beginning to appear in commercial projects.

High-efficiency multijunction PV cells, of which there are many designs, are particularly suited for use under concentrated sunlight. One reason for this is because of the wide selection of material combinations available; you can choose materials that perform well under stressful conditions (heat, in particular) induced by concentrated sunlight. Another reason is that multifunction devices, which can be designed to reach efficiencies of 30% or more under normal sunlight, can reach even higher efficiencies under concentration. (Multifunction devices are discussed more fully in Chapter 3.)

 

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