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Solar markets: Highlighting PV technology

Posted: 13 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:solar energy? photovoltaics? thermal?

It has been shown that solar energy harvesting will soon go mainstream, but it's not clear which of the many diverse technologies vying for dominance in this emerging market will take precedence. From bulk and thin-film silicon to more exotic compound materials such as III-Vs and new printable organics, a designer may have many choices but, as we explain, each has its own pride of place on the totem pole of conversion efficiency, availability, reliability and cost.

People in the IC industry use the term photovoltaics (PV), interchangeably with solar power. PV refers to a class of power-generation devices that convert light directly into electrical current. Photons, usually coming from the sun with the hope as source for renewable energy, are converted into electron-hole pairs that are subsequently extracted into useful electrical current. The non-semiconductor part of the solar power field is solar thermal-power generation, where radiant energy from the sun produces steam that subsequently drives a turbine similar to the turbines used in conventional coal or nuclear power plants. PV is actually a subset, albeit it is probably the larger portion, of the more general field of solar-power generation.

PV is classified broadly into four generations:

??First-generation solar cells are similar to the earliest practical silicon cells designed in the 1950s on bulk silicon wafers.??
Second-generation devices are the thin-film type. Thin is an advantage because less material means cheaper cost. The smaller volume also means there's less chance of losing an electron because of impurities, so you can use a lower-grade material.??
Third-generation solar cells, which are just starting to get into pilot production, go beyond conventional junction semiconductors to include photoelectrochemical cells, polymers and dye-sensitized cells.??
Fourth-generation devices include future technologies, such as quantum dots and nanowires. Multiquantum well cells are fourth-generation devices, but they're closer to production, thanks to Imperial College spin-off Quantasol Ltd.

Click to view full image.

Complex processes
Solar thermal plants depend on mirrors or lenses to focus sunlight to achieve higher temperatures in a working fluid that will eventually boil water to drive a turbine. Higher temperatures mean higher system efficiencies. Sunlight concentrators are also used for PV systems, forming concentrated PV (CPV). The idea behind CPV is to use higher-efficiency cells and focus a larger area of incident sunlight onto the smaller cells. The exotic materials used in CPV cells are more expensive than their alternatives. Because a very small area of cell is used, CPV systems often stack several different PV materials together into a multijunction cell to capture more of the incident wavelengths and therefore extract more energy from the sunlight.

The focusing optics is inexpensive compared with the cell material and concentrate solar energy from a large surface area. Most CPV systems can follow the sun during the day to keep incident light from striking the PV cell at the steepest possible angle. These systems raise overall system cost because mechanical actuators and associated sun tracking and control electronics are needed to keep the CPV pointed directly at the sun throughout the day. However, these costs are at least partially offset by CPV's higher power-output density.

A subcategory of CPV systems that is starting to gain prominence is based on light-guiding rather than light-focusing optics. MIT startup Covalent Solar and Morgan Solar from Canada are developing systems that channel higher-energy, shorter-wavelength light to the edge of the panel, where an exotic, high-efficiency multijunction cell is placed. The overall efficiency of these systems is relatively high because they extract energy from a wide spectrum of incident sunlight. While the shorter, high-energy wavelengths are led to the high-efficiency cell at the edge, longer wavelengths pass directly through the panel. Solar cells developed from less costly materials cover the larger area directly below the panel plate to extract the unguided long-wavelength radiation. Covalent's technology is based on luminescent dye coatings, while Morgan Solar uses glass or acrylic waveguide plates.

The remaining PV systems are, in one form or another, substantially flat. Flexible cells are seen because they become as flat as the surface to which they are attached. They are known as flat-panel or flat-plate and do not depend on mirrors or lenses to get the sun's rays. Most of the panel area is active for energy conversion, except for wiring or other structures that may block incoming light.

Flat-panel PV systems use more active material, unless the active layers are thinned enough to compensate for the larger area, than CPV systems. Therefore, flat-panel systems tend to be less expensive, less efficient PV materials. They have the edge over CPV in many less sunny climates. Flat panels still work as designed in cloudy conditions, extracting power from diffuse sunlight. Concentrating optics in CPV systems need parallel rays. Otherwise, the small cell at the focal point receives only the light falling on that small surface area.

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