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The total solar energy incident on the surface of the earth averages about 86,000 terawatts (TW), which is more than 5,000 times the 15 TW of energy currently used by humans (of which roughly 12 TW now comes from fossil fuels) and more than 100 times larger than the energy potential of the next largest renewable source, wind energy (Hermann, 2006). Hence, the potential resource of solar energy is essentially limitless, which has led many to conclude that it is the best energy resource to rely on in the long run. Currently, this resource is exploited on a limited scale—total installed worldwide solar energy production totaled 15 gigawatts (GW) in 2008,2 or just 0.1 percent of total energy production, with similar penetration in the United States (EIA, 2009). Solar energy can be used to generate electricity and heat water for domestic use. Passive solar heating can be used in direct heating and cooling of buildings.

There are two main classes of solar energy technology used to generate electricity: concentrating solar power (CSP) and photovoltaics (PVs). CSP technologies use optics (lenses or mirrors) to concentrate beam radiation, which is the portion of the solar radiation not scattered by the atmosphere. The radiation energy is converted to high-temperature heat that can be used to generate electricity or drive chemical reactions to produce fuels (syngas or hydrogen). CSP technologies require high-quality solar resources, and this restricts its application in the United States to the southwest part of the country. However, CSP technologies are commercially available and there are a number of upcoming projects in the United States, particularly in California. The CSP industry estimates 13.4 GW could be deployed for service by 2015 (WGA, 2006). In the short term, incremental design improvements will drive down costs and reduce uncertainty in performance predictions. With more systems installed, there will be increased economies of scale, both for plant sites and for manufacturing. However, new storage technologies, such as molten salt, will be needed in the longer term to make wide

2Energy production is generally reported as the "nameplate capacity" or the maximum amount of energy that could be produced from a given source. For energy sources such as solar or wind, which are intermittent in nature, the actual output is often lower than the nameplate capacity.

spread CSP deployment feasible. The global research community is studying the use of concentrated solar energy to produce fuels through high-temperature chemical processing (Fletcher, 2001; Perkins and Weimer, 2004, 2009; Steinfeld, 2005). At the international scale, the SolarPACES organization is working to further the development and deployment of CSP systems.3 This organization brings experts from member countries together to attempt to address technical issues associated with commercialization of these technologies.

While incremental improvements in CSP performance are anticipated, there is the potential for large improvements in PV electricity generation technologies. Over the past 30 years, the efficiency of PV technologies has steadily improved, though commercial modules achieve, on average, only about 10 to 15 percent efficiency (that is, only 10 to 15 percent of the solar energy incident on the cell is converted into electricity), which is 50 percent or less of the efficiency of the best research cells (NRC, 2009d). Most current PV generation is produced by technologies that rely on silicon wafers to convert photons to electrons (Green, 2003; Lewis, 2007). Recent shortages of polycrystalline silicon have increased prices for PV modules and spurred increases in the use of thin-film solar PV technologies that do not require as much or any silicon. Thin-film solar PV technologies have about a 40 percent market share in the United States (EIA, 2009). In the short term, research is continuing on PV technologies; most of the work on improving these cells has focused on identifying new materials, new device geometries (including thin films), and new manufacturing techniques (Ginley et al., 2008).

The overall costs of a PV system, not just the costs of PV cells, determine its competitiveness with other sources of electricity. For example, approximately 50 percent or more of the total installed cost of a rooftop PV system is not in the module cost but in the costs of installation, and of the inverter, cables, support structures, grid hookups, and other components. These costs must come down through innovative systemintegration approaches, or this aspect of a PV system will set a floor on the price of a fully installed PV system. In the medium term, new technologies are being developed to make conventional solar cells by using nanocrystalline inks as well as semiconducting materials. Thin-film technologies have the potential for substantial cost reduction over current wafer-based crystalline silicon methods because of factors such as lower material use, fewer processing steps, and simpler manufacturing technology for large-area modules. Thin-film technologies have many advantages, such as high throughput and continuous production rate, lower-temperature and nonvacuum processes, and ease of film deposition. Even lower costs are possible with plastic organic solar

3 See

cells, dye-sensitized solar cells, nanotechnology-based solar cells, and other new PV technologies.

If next-generation solar technologies continue to improve and external costs associated with emissions from fossil fuel-based electricity are incorporated into the cost of electricity, it is possible that solar technologies could produce electricity at costs per kilowatt-hour competitive with fossil fuels. This transition could be accelerated through carefully designed subsidies for solar energy, as several other countries have done, or by placing a price on carbon emissions (Crabtree and Lewis, 2007; Green, 2005). Modifications to the energy distribution network along with energy storage would also improve the ability to exploit solar energy resources (see the section Energy Carriers, Transmission, and Distribution in this chapter). However, it should be noted that a bifurcated market for PV systems exists, depending on whether the system is installed on a customer's premises (behind the meter) or as a utility-scale generation resource. Behind-the-meter systems compete by displacing customer-purchased electricity at retail rates, while utility-scale plants must compete against wholesale electricity prices. Thus, behind-the-meter systems can often absorb a higher overall system cost structure. In the United States, much of the development of solar has occurred in this behind-the-meter market (NRC, 2009d).

There are several potential adverse impacts associated with widespread deployment of solar technologies. Utility-scale solar electricity technologies would require considerable land area. When CSP is used with a conventional steam turbine, the water requirements are comparable to fossil fuel-fired plants, making water availability a concern and, in some cases, a limiting factor. For PV technology, there are also concerns associated with the availability of raw materials (particularly a few rare earth elements; NRC, 2008f) and with the potential that some manufacturing processes might produce toxic wastes. Finally, the energy payback time, which is a measure of how much time it takes for an energy technology to generate enough useful energy to offset energy consumed during its lifetime, is fairly long for silicon-based PV.

In addition to electricity generation, nonconcentrating solar thermal technologies can displace fossil fuels at the point of use, particularly in residential and commercial buildings. The most prevalent and well-developed applications are for heating swimming pools and potable water (in homes and laundries). Systems include one or more collectors (which capture the sun's energy and convert it into usable heat), a distribution structure, and a thermal storage unit. The use of nonconcentrating solar thermal systems to provide space heating and cooling in residential and commercial buildings could provide a greater reduction of fossil fuels than do water heaters, but at present it is largely an untapped opportunity. Recently there has been limited deployment of liquid-based solar collectors for radiant floor-heating systems and solar air heaters, but the challenge with these applications is the relatively large collector area required in the absence of storage. Solar cooling can be accomplished via absorption and desic-cant cycles, but commercial systems are not widely available for residential use.

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