Solar

The earth is constantly receiving vast amounts of energy from the sun in the form of solar radiation. To help meet the nation's energy demand, scientists are trying to convert this energy efficiently into usable energy. However, solar energy is perhaps the least concentrated of renewable energy sources. As radiant solar energy occurs evenly per square foot, the only way to get more is to increase the number of square feet of solar radiance collectors and/or the efficiency at which it is collected. The principal values of solar energy are its ubiquitousness and quietness. The principal disadvantages are cost when used to generate electricity, low efficiency of extraction, and high space requirements.1

There are currently a number of technologies that exist for converting the sun's energy into a usable form (that is, solar thermal; solar heat; solar biofuels, and so forth), but photovoltaics is the topic of discussion for this section. Photovoltaics is where sunlight is converted directly into electricity. There are two main types of photovoltaic solar radiation collectors, also known as solar cells: flat-plate systems and concentrator systems. The more common flat-plate system is discussed here.

A flat-plate is basically a metal box with a glass or plastic cover on top and an absorber plate on the bottom. The absorber plate is a semiconductor, which absorbs solar radiation and converts it to heat. The heat or power is produced when sunlight strikes the semiconductor material, creating an electrical current. Solar flat-plates can be tilted at different degrees to increase the amount of solar radiation reaching the plates. In general, using a tilt angle approximately equal to a site's latitude produces the greatest increase in the amount of radiation reaching the plate. In addition to tilt, there are also flat-plates that can track the sun. One-axis and two-axis tracking flat-plates are available, which are able to pivot and track the sun as it moves from east to west throughout the day. Two-axis trackers are able to capture more radiation than one-axis, and one-axis trackers capture more radiation than no axis flat-plate collectors (See Figure 1).

Economically, the success of photovoltaics depends largely on two things: the amount of solar radiation available and the efficiency with which the solar radiation can be converted into the desired energy product. To determine the amount of solar radiation available for a given area, solar data is available. Solar radiation data is useful because it provides information on how much of the sun's energy strikes a surface at a given location on earth over a particular time period. Solar radiation data is often expressed as kilowatt-hours per square meter (kWh/m2).

Figure 2. Based on 30-Year Average of Yearly Solar Radiation Data (1961-1990) from the National Solar Radiation Data Base (NSRDB).

A number of factors can influence the amount of solar radiation reaching the earth's surface (i.e., time of day; the season; air pollution), but cloud cover is the predominant factor. Solar radiation reaching the earth's surface decreases with increasing cloud cover. Thus, cloudy regions like Seattle, Washington receive far less solar radiation than cloud-free desert climates such as Phoenix, Arizona.

Solar radiation data from the National Solar Radiation Data Base (NSRDB) shows that Washington State has one of the lowest incidences of solar radiation in the United States. For example, the average annual solar radiation in Seattle, Washington for a horizontal flat plate collector is 3.3 kWh/m2/day. In contrast, the average annual solar radiation in Phoenix, Arizona for a horizontal flat plate collector is much higher at 5.7 kWh/m2/day. The only region with a lower incidence of solar radiation is, predictably, Alaska2 (See Figure 2.) An average monthly comparison of solar radiation data for Seattle, WA, Phoenix, AZ, and Anchorage, AK is available in Figure 3.

Although an average annual solar radiation of 3.3 kWh/m2/day for Seattle, Washington may sound like a lot of energy, this is total radiation powering a solar cell that is typically only 12 percent efficient and it is in square meters. There are 10.76 square feet in a square meter, so the equation becomes 306 watts per sqaure foot per day. At an overall system efficiency of 10 percent (12 percent unit efficiency, 90 percent inverter efficiency), this becomes approximately 30 watts per sqaure foot per day. Therefore, it will take 33 square feet to generate one kilowatt and would cost approximately $9,000 to install.3 One megawatt (1,000,000 watts) would require 33,333 square feet and cost $5-10 million to install.1

Solar radiation may be increased by approximately 15 percent through proper orientation (tilt angle) or increased by approximately 33 percent through use of a 2-axis tracking collector.1 In fact, National Renewable Energy Laboratory (NREL) data shows that by using a two-axis tracking flat-plate, Seattle could achieve higher solar radiation in the month of July (30-yr avg = 8.3 kWh/m2/day: See Figure 1) than could Phoenix, Arizona, which has much higher annual solar radiation, using a "non-tracking" flat-plate (30-yr avg = 7.6 kWh/m2/day: See Figure 3) .2 The use of tracking flat-plates and proper orientiation increases the effieciency with which solar power is created and thus makes solar power less expensive than using a typical "non-tracking" flat-plate.

References Cited

1. Northwest Indian College National Indian Center for Marine and Environmental Research and Education. Feasibility Studies for Potential Application of Renewable Energy Technologies at Tribal Colleges and Universities Supplemental Announcement 07. Volume III - Draft Phase 1 Final Report. Submitted: June 30,2001. <http://nwic-research.org/DOE%20PhaseII%20Vol3.htm>

2. National Renewable Energy Laboratory. National Solar Radiation Data Base (NSRDB): 30-Year Average of Monthly Solar Radiation, 1961-1990. <http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/sum2/state.html>

3. Alternative Energy Solutions LLP. 211 Hawthorne St, Bellingham, WA

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