ZT: Putting the spotlight on solar power - 英语交流 - 电子工程师俱

2dmin

In some ways the companies that produce solar panels and modules for generating electricity resemble automobile companies of 90 to 100 years ago. Back then, there were hundreds of automobile manufacturers exploiting practically any technology that would propel a wheeled land vehicle—steam, internal combustion and even electricity. Today numerous solar companies use multiple technologies to generate electricity. Like the auto companies, this industry will presumably follow its own evolutionary path. Here we look closely at four diverse companies with their roots in the United States and Europe to see where they are now and where they may be heading.
<strong>First Solar</strong> is best known for having signed a memorandum of agreement (not yet a contract) with the China government for what will eventually be a 2GW solar field near the city of Ordos in Inner Mongolia. Originally strictly a manufacturing company, First Solar grew in the past two years by acquiring two project development firms. Its largest manufacturing plant is in Malaysia; others are in Ohio and in Germany.

<i>First Solar assembly line
[Photo courtesy First Solar.].</i>

First Solar produces thin-film solar cells using cadmium telluride (CdTe). Their technology is thus not silicon-based and is relatively inexpensive to manufacture. The panels are fixed in place (that is, they do not track the sun), and may be on rooftops or in solar arrays planted in the earth. Except in Germany, where the feed-in tariff is high, the company avoids residential installations, although they have a partnership with Solar City in the western United States for residential and small commercial installations.
An important number in any solar technology is its efficiency—the percentage of incoming sunlight that is converted into electricity. CdTe technology has efficiencies that tend to be on the low end of the scale; First Solar currently quotes 11 percent. But the cost per watt of electricity generated is very low. CdTe cells produce reasonable levels of electricity when the sun is low or when there is modest cloud cover or air-borne dust. Currently the company's biggest development outside of the south-western United States is in Sarnia, Ontario, Canada, a chilly and damp location on Lake Huron. One assumes advances in CdTe technology are possible, but the company tends not to discuss such matters
<strong>SunTech Power</strong> makes silicon-based solar modules using both less expensive polycrystalline silicon and its more expensive monocrystalline form. Its markets are residential and commercial applications, including building-integrated photovoltaic (BIPV). Its global headquarters is in Wuxi, China, and the firm has regional headquarters in California and Switzerland. It is building a 30MW plant in Arizona and is expanding its manufacturing plants in China with a goal of 1.4GW (photovoltaic cell and module capacity) by mid-2010.
SunTech's most exciting technology is a silicon-cell variation that it calls Pluto. The company licensed the technology from the University of New South Wales in Australia. Like standard silicon cells, Pluto cells have metal lines to collect electrons, but Pluto's metal lines are both much narrower and closer together than standard metal lines. There are two benefits: the metal lines shade less of the underlying silicon, and the electrons have a shorter distance to travel to find a line (some electrons get lost along the way). SunTech is now producing Pluto cells that achieve efficiencies of about 17 percent for polycrystalline silicon and about 19 percent for monocrystalline silicon, values that have been confirmed by Germany's Fraunhofer Institute.
One very forward-looking SunTech product is a glass-on-glass thin-film product that can be used as the glazing on the side of an office building. It serves as a window (and, like sunglasses, comes in varying degrees of opacity) and has contacts at the sides of the windows. The efficiency is low—about 6 percent—and the technology is currently expensive, but it makes it possible to turn the side of a skyscraper into a giant solar plant.
<strong>SolarWorld</strong> has been producing and marketing silicon solar cells in the United States since 1977; its history is thus longer than the 25- or 30-year field life that solar firms typically claim for their PV installations. Today it manufactures what it likes to the call "the workhorse of the industry," a very reliable monocrystalline silicon cell with an efficiency of 14 percent.
To make its cells, SolarWorld etches the surface of the wafer to create pyramidal structures. The surface area that is able to capture incoming light is thereby increased. At some angles, the light is reflected from the pyramid faces, giving a second chance for it to be captured. This gives slightly higher efficiency, particularly in early morning and late afternoon.
The company pays attention to details, large and small. For example, its products are produced in the United States, and virtually all of the components are purchased within the United States, which reduces transportation costs substantially. Its plants are also highly automated, with the result that labor costs are only 10 percent of their total costs
One of the details it pays attention to is the phenomenon of light-induced degradation. When a solar module is installed, there is an initial drop in power. This drop might be as small as 2 percent or as great as 8 percent. SolarWorld offsets this drop in its module testing; that is, to ensure a 100W module, it will produce a 102W module. It also uses what is called plus-sorting. Across the industry, one of the problems of installation is a mismatched lot of modules, where the wattage from one module to the next might vary by ±3 percent. Such variation degrades power output. SolarWorld keeps the variations minimal to avoid degradation.

<i>SolFocus concentrator PV panels
[Photo courtesy SolFocus]</i>

<strong>SolFocus</strong> uses solar concentrator technology in which a silvered glass parabolic mirror focuses incoming light onto a secondary mirror; from there the light is directed onto the 1cm² cell. The multi-junction cell, long used in space, has a germanium substrate with overlying layers. SolFocus cells achieve a concentration of 650x—that is, a flat, non-concentrated area 650x the size of the cell would receive the same amount of sunlight as the SolFocus cell.
The applications for the SolFocus technology are entirely in small and large power-field applications. It is not suitable for individual rooftop installations, but appropriate for powering towns or cities. The modules (groups of 20 mirror units) move on two axes in order to keep the mirrors aimed directly at the sun.
The technology is suitable for regions with low cloud cover and lots of sun—deserts, semi-deserts and some high-altitude locations. SolFocus has important installations in Portugal, Spain, Hawaii and California. Within its environmental limits, the technology is highly productive: the current cell efficiency is between 38 percent and 40 percent. The theoretical upper limit for multi-junction cells is about 70 percent, compared to about 26 percent for silicon cells. Multi-junction cells also are able to maintain their efficiency at high temperatures, while silicon cells produce about 15 percent less at 30°C and as much as 30 percent less at 45°C.
SolFocus outsources all of its manufacturing, a great deal of which is already done in Asia. What makes outsourcing feasible is that assembly mostly involves glass and aluminium, much like automobile companies, but unlike other solar manufacturers. These qualities also make its manufacturing very scalable. It could double its current 50MW capacity within five months if needed. It is also the only concentrator solar company to have achieved IEC 62108 certification for quality and reliability [at the time of this writing].
- <strong>Tom Adams
<i>Consultant, Sonoscan Inc</i></strong>