For decades, the solar energy industry has struggled to become cost-competitive with other sources of power generation. Recent technology innovations and creative ways of installing solar generation are beginning to enable solar power to increase its share of the electricity market.
Solar energy is everywhere, yet its use for electricity production remains a faint sliver in the pie that represents total U.S. power generation.
The U.S. Department of Energy’s Energy Information Administration, using data compiled in 2007, shows that 3.1% of the country’s electricity was derived from all forms of renewables. Less than one-third of that amount — less than 1% of the current U.S. generation mix — comes from solar sources, according to the Solar Energy Industries Association (SEIA), a U.S.-based organization dedicated to promoting solar energy technologies.
However, the deployment of solar energy has been increasing rapidly since 2005 in the form of utility-scale photovoltaic (PV) plants and different forms of concentrating solar power (CSP) systems.
To learn what’s hot in solar power, in October, POWER interviewed representatives from two leading solar energy trade associations, two national laboratories, two solar energy technology manufacturers, a software manufacturer, and a national law firm.
The Contenders: Concentrating Solar Power and Photovoltaics
For background, here’s a brief review of the available options for generating power from sunlight. They fall into one of two categories: using solar energy indirectly (CSP) or directly (PV).
Concentrating Solar Power. CSP plants are utility-scale generators that produce electricity by using mirrors or lenses to efficiently concentrate the sun’s energy. CSP, or solar thermal technologies (see our August 2006 issue, p. 84 or http://tinyurl.com/yew4692 for technical descriptions of the different CSP technologies) include the following:
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Parabolic trough systems use parabolic curved, trough-shaped reflectors to focus the sun’s energy onto a receiver pipe running at the focal point of the reflector (Figure 1). Because of their parabolic shape, troughs can focus the sun’s energy at 30 to 60 times its normal intensity on the receiver pipe. This concentrated energy heats a heat transfer fluid in the pipe that is then used to generate steam to power a turbine, which in turn drives a generator.
1. Enjoying its place in the sun. Increasing in popularity, the parabolic trough is a long, trough-shaped reflector that uses pipes containing clear oil that absorbs heat reflected from the trough. Heat from the oil flows through a heat exchanger to heat water to make steam for electricity generation. This parabolic trough is being used at solar power station in Adasol, Spain. Courtesy: Solar Electric Power Association
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Power tower systems use a field of computer-controlled flat mirrors, called heliostats, to focus solar energy on a central collector tower (Figure 2). The energy at this point can be used to heat water to produce steam (and run a central generator), or it can be transferred to a heat transfer material (typically, liquid sodium), which can then store the heat for later use.
2. Shedding light on power towers. Central receiving towers, often referred to as power towers, are tall structures with a boiler on top that contains water. Surrounding the tower are many rows of mirrors called heliostats, which turn to face the sun and focus the reflected sunlight on the boiler throughout the day in order to heat the boiler to create steam. The tower shown was manufactured by Abengoa Solar. Courtesy: Solar Electric Power Association
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Compact linear Fresnel reflectors use flat reflectors moving on a single axis while using a Fresnel lens to concentrate the solar thermal energy onto collectors. The flat mirrors used in this system allow for a greater density of reflectors in the array, increasing the efficiency of land use.
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Dish systems use a large concave dish to track the sun and focus its energy onto a high-efficiency power conversion unit (a heat engine that is sometimes referred to as a Stirling engine), which generates electricity directly. Dish systems typically produce upward of 25 kW.
Photovoltaics. In contrast to CSP, which is only viable financially on a larger scale, PV technology can produce electricity at just about any scale (Figure 3). PV devices generate electricity directly from sunlight via an electronic process that occurs naturally in certain types of semiconductor materials. Electrons in these materials are freed by solar energy and can be induced to travel through an electrical circuit, powering electrical devices or sending electricity to the grid. PV devices can be used to power anything from small electronics to homes and businesses or utility-scale solar power facilities.
3. With solar energy, the sky’s the limit. These photovoltaic arrays are located at a solar power facility in Spain and were manufactured by First Solar. Courtesy: Solar Electric Power Association
Most modern solar cells are made from either crystalline silicon or thin-film semiconductor material. Silicon cells are more efficient at converting sunlight to electricity, but they generally have higher manufacturing costs. Thin-film materials typically have lower efficiencies, but are simpler and less costly to manufacture.
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