Improved efficiency boosts PV panel prospects

Cost per watt (CPW) is probably the most important metric in the solar industry. It defines an installation's cost and the point at which the site breaks even. It provides a basis for comparison among photovoltaic technologies and between installations, as well as between solar energy and other sources of electricity. Though other factors contribute to sales, the system with the lowest cost per watt will receive careful consideration for any project.

To improve CPW, manufacturers can either reduce cost -- by improving yield, increasing throughput, and pulling all the other levers available to a manufacturing process -- or they can increase the wattage available from a given panel area. In turn, increasing output power requires improved conversion efficiency: the panel must capture more incident photons, convert more of them to free carriers, and deliver more of those carriers to the panel's terminals. Each of these steps -- capture, conversion, and transport -- shaves points from the ultimate efficiency and offers opportunities for performance improvements.

Concentrating photovoltaic panel.

The search for improved efficiency begins at the front, or sunward, surface of the cell, where the average incident solar radiation is about 1000 W/m2. Though this number is a convenient rule of thumb, it is only an average. At high altitudes or in space, less atmosphere blocks the sun's light. In winter and in the morning and evening, the sun is lower in the sky and less energy reaches the panel. Differences in atmospheric absorption, whether due to location or to weather and pollution, change both the intensity and the spectrum of incident light hitting the panel. These differences affect the economic model for solar energy in fairly intuitive ways: Arizona and New Mexico have access to a much larger solar resource than Washington and Oregon. More subtly, however, atmospheric effects change the performance of particular cell designs.

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