Solar Energy

Solar energy is the most abundant renewable energy source on the planet. The energy influx from solar radiation striking land surfaces is sufficient to meet the present primary commercial energy needs of the world thousands of times over. Usable solar influx is affected by diurnal and geographic variation and weather. Worldwide, the yearly average values of effective solar irradiance reaching Earth's land surface vary from a low of 0.06 kW/m2 (kilowatt per square meter; ∼500 kWh m-2 yr.-1 [kilowatt hours per square meter per year]) at the highest latitudes to a high of 0.25 kW/m2 (∼2,200 kWh m-2/yr.-1) in some desert areas of Africa and Australia. Conversion of this potential energy into secondary forms, such as electricity or process heat, depends on technical factors (e.g., efficiency of converters such as photovoltaic cells or thermal collectors), economic constraints (e.g., the absolute and relative costs of technology and fuels), and suitability considerations (e.g., the suitability of land or ocean use for locating the collection infrastructure). Nonetheless, the abundance of solar energy surpasses all other sources of renewable energy. Estimates of the technical potential of solar energy versus wind, biomass, geothermal, and ocean sources of energy show that solar energy represents our largest source of renewable energy supply and a key technology to lower worldwide carbon emissions.

There are several solar energy technologies, which can be arranged along the following continuum: (1) passive and active, (2) thermal and photovoltaic (PV), and (3) concentrating and nonconcentrating. Passive solar energy technology collects the energy without converting the heat or light into other forms. In contrast, active solar energy technology refers to the harnessing of solar energy to store it or convert it for other applications and can be broadly classified into two groups, namely, PV and solar thermal. Solar PV cells are the “high-tech” option among active solar energy strategies, directly converting solar energy to electricity. Solar thermal applications include two distinct technological streams. The first stream, referred to as solar thermal nonelectric, includes applications such as agricultural drying, solar water heaters, solar air heaters, solar cooling systems, and solar cookers. The second stream of solar thermal technology, referred to as solar thermal electric, includes technologies that use the sun's heat to produce steam and generate electricity. The final characteristic in the continuum of solar energy technologies is concentrating versus nonconcentrating technologies. Concentrating solar power (CSP) technologies use mirrors or lenses (concentrators) to focus sunlight and thus increase the intensity of light in the focus area. In addition to CSP, concentrators can be applied in PV technologies as well by using a dish collector or optical lenses to concentrate sunlight on a smaller cell area.

The solar hot water system at the Adams County Detention Facility, northeast of Denver, Colorado, has been operating since 1988; it exemplifies the benefits users can reap from solar heat. This parabolic trough-based system supplies 46% of the hot water used in the jail's kitchen, laundry, and showers, which is about 20,000 gallons per day.

Source: Warren Gretz.

In addition to technological differences, solar technologies can be characterized according to their applications. There are broadly three types of solar applications: (1) centralized utility-scale installations, (2) grid-connected distributed installations, and (3) off-grid installations. Centralized utility installations are typically large-scale PV or CSP systems, operating like other conventional power-producing plants, dispatching electricity over thousands of miles to customers. Distributed grid-connected and off-grid applications, in contrast, are smaller in scale and are located on-site, where the energy will be used. These applications can be electric (e.g., PV) as well as nonelectric (e.g., solar hot water). In recent years, distributed solar applications have experienced rapid growth, representing the fastest-growing new energy generation source in the world, with an astonishing 50% global market growth rate in 2007. Its use is primarily concentrated in the industrialized countries, where policy incentives introduced in recent years have spurred its adoption (e.g., “feed in” laws, renewable portfolio standards, tax incentives, rebates, and net metering). The use of urban rooftops to host the technology makes solar systems adaptive to human settlement patterns. Water-heating solar systems are frequently competitive with conventional energy alternatives. Globally, this technology is being rapidly adopted in both industrialized (Germany, the United States, Japan, Israel) and developing countries (China, India, Brazil, Turkey, and South Africa). China currently has more than half of the existing solar hot water and heating applications in the world. Its Renewable Energy Law encourages installation and use of solar thermal systems, and [Page 2596]parts of China already are moving toward solar water heating obligations.

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