BP Solar Facility Tour Provides Photovoltaics Primer
by Monica A. Mallini, P.E., Chair, Industry Applications Society
On a beautiful sunny Friday in November, the Washington Chapter of the IEEE Nuclear and Plasma Sciences Society hosted a tour of the BP Solar plant in Frederick, Maryland. The weather provided a fitting backdrop for the visit, which was led by Chapter Chair Harry Sauberman and attended by 28 IEEE members.
BP Solar hosts Bill Poulin and Jean Posbic displayed an impressive knowledge of technical, economic, and policy issues surrounding the solar energy industry. The presentation, titled “Everything you wanted to know about Solar Photovoltaics but were afraid to ask,” was true to its name.
Why is BP, one of the world’s largest oil and petrochemicals companies, interested in solar energy? The reason is that oil companies are looking ahead, anticipating the decline of the finite fossil fuel supply. Solar and other forms of renewable energy are “the next thing.” Moreover, solar energy is attractive because the photovoltaic (PV) technology that enables it is simple, and its products are reliable. After a payback period, the electrical energy delivered by a PV system is free. For these reasons, there is a rapidly growing worldwide demand for solar and hybrid solar systems.
Most of our energy supply in the United States is derived from coal. Is solar energy a viable alternative? Conceptually, it is, with current technology. A PV array field 300 miles square could supply all the electrical energy currently used in the U.S. This hypothetical array demonstrates the capability of solar energy; however, an enormous central plant is not the correct way to satisfy a nation’s energy demand. The real advantage of solar energy is its modularity, which suggests distributed applications.
Mr. Posbic gave an instructive primer on PV. Each individual solar cell produces 5 amps of direct current at 0.475 volts. By combining cells, a large range of current and voltage combinations can be obtained, to power virtually anything that uses electrical energy. In practice, individual cells are combined in series to achieve the voltage required to drive an inverter. A solar module typically consists of 72 cells, producing 120 to 180 watts of power in full sunshine. The manufacture of solar modules requires as much electricity—for all phases from production to disposal—as the module will produce in two years or less.
The plant tour highlighted key stages of the module production process. One may be surprised to learn that solar cells can be manufactured from only “5 nines” grade silicon, much cruder than the “9 nines” grade needed by the semiconductor industry. In the refining process, large silicon ingots are cast. Impurities float to the top and are simply machined away before the ingot is cut into blocks. The silicon blocks are sliced into polycrystalline wafers of p-doped silicon, 220 microns thick, with a 1-micron layer of phosphorous, forming an n-over-p junction. When photons strike the surface of the wafer, electrons travel to the back, developing a potential of 0.475 volts between front and back.
One limitation of silicon as a material for solar cells is its cutoff wavelength of 1100 nm, the maximum useful photon wavelength. Because 30 percent of the solar spectrum is above 1100 nm, only 70 percent of solar radiation striking the wafer is available for electricity production. A further limitation is that each photon produces only a single electron, resulting in a maximum theoretical efficiency of 30 percent. Optical enhancement is obtained by applying a 700-Angstrom silicon nitride coating to the polycrystalline wafer to fill in crystal defects and improve photon capture rate. As a result, polycrystalline cells may perform nearly as efficiently as the more expensive monocrystalline cells, with module-level efficiencies up to 15 percent. The silicon nitride layer gives the gray silicon wafer a distinctive blue color. Incidentally, it is technically possible to manufacture silicon wafers in any color, although cell efficiency may be reduced with alternate colors due to less favorable optical characteristics.
There are ongoing attempts to extend conversion efficiency, by cascading multiple junctions, and by connecting wafer layers in series. To date, these techniques are problematic. Non-silicon cells have achieved higher efficiencies, but their use is limited to specialized applications such as satellites because the cost may be several orders of magnitude higher. A novel approach to improving overall solar system efficiency is cogeneration. This concept is under study, with some researchers attempting to exploit the potential of solar modules to heat water by using energy not available for electricity.
Why is so much attention focused on module efficiency? In the second installment of this article, we will examine this and the bigger issue of solar PV economics, as explained by Mr. Posbic. Meanwhile, if you have an opportunity to visit BP Solar’s facility in Frederick, this fascinating tour is highly recommended by 28 of your fellow IEEE members.
Read Part 2 of this story.
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Updated 6/1/06
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