Large-scale deployment of concentrating solar power (CSP), a renewable energy technology for generating electricity, has the potential to affect the availability of water resources in the Southwest for other uses. Because the water demand of CSP is highly dependent on the type of CSP facilities constructed (e.g., whether thermal storage is included and whether wet cooling is used) and their locations, and because the data for these evolving technologies are preliminary, there remains much uncertainty about the impacts of CSP on Southwest water use. Water resource constraints are likely to prompt adoption of more freshwater-efficient technologies, or decisions not to site CSP facilities in certain locations. However, water constraints do not necessarily preclude CSP in the Southwest, given the ability to reduce the freshwater use at CSP facilities. This report presents a brief primer on CSP, then discusses the potential water implications of CSP deployment in the Southwest. CSP comprises a set of technologies that convert thermal solar energy into electricity. The quantity of electricity produced at these facilities, the water intensity per unit of electricity generated, and the local and regional constraints on freshwater will shape the cumulative effect of CSP deployment on southwestern water resources, and the long-term sustainability of CSP as a renewable energy technology. As the 111 th Congress considers energy and climate legislation, and as individual states take actions such as adopting renewable energy portfolios and identifying geographic regions suitable for renewable energy development, the implications of large-scale adoption of renewable technologies are receiving greater attention. How large-scale expansion of solar generation in the Southwest may affect the people, economy, land, protected species, and water resources of the region are being analyzed in order to compare the local, regional, and national advantages and disadvantages of CSP compared to other electricity options. Site-specific and cumulative water resource implications are among many factors (e.g., cost, climate and air pollution emissions, land and ocean impacts, wildlife and the environment impacts) to be weighed when judging the tradeoffs between different energy options. CSP—or solar thermal power, as it is also known—typically employs large arrays of ground-based mirrors to concentrate sunlight onto a heat transfer medium (e.g., oil, salt, or water), which is heated above 212°F (100°C) to roughly 662°F (350°C), depending on the medium. The heat is used to generate steam via a heat exchanger to spin a steam turbine. Alternatively, steam can be generated directly by the mirror arrays. Photovoltaic (PV) solar is a separate class of solar technology that uses panels of solar cells to convert sunlight directly into electricity. Operation of CSP and PV facilities to generate electricity does not directly release greenhouse gases. There remains significant uncertainty about the future rate and level of CSP deployment. In some CSP installations, sunlight is concentrated from the mirror arrays onto a single, central location atop a tall solar tower where the heat typically melts salt or boils water. In other CSP installations, solar parabolic-mirror troughs, also known as solar troughs , focus sunlight on miles of piping running through a field of mirrors, heating the heat transfer fluid (usually oil). Pressurized steam, produced by the heat transfer medium, then drives a turbine-generator producing electricity. Other less common CSP technologies exist; this report focuses on solar trough and solar tower CSP technologies. CSP currently is better suited than PV installations for larger facilities, and the steam-cycle used in CSP is more familiar to utility engineers. Electricity from a large CSP plant is estimated to cost $0.10 per kilowatt-hour (kWh) over a facility's operating life; electricity from a large PV facility is estimated at $0.26/kWh given current technologies. In terms of cost, both PV and CSP facilities at the scale now being developed (generally between 50 megawatts (MW) and 1,200 MW) are at an early stage in their commercial adoption and use; these early facilities are anticipated to produce electricity at more than twice the cost of conventional coal plants. CSP generation costs are expected to decrease as more solar plants are installed and technologies and operations improve. As of March 2009, 11 large solar trough facilities were operating in the United States (1 in Arizona, 1 in Nevada, and 9 in California), with a total capacity of 339 MW. An additional 16 large-scale solar trough and solar tower plants are planned. The 15 in the West ─ 1 in Arizona, 2 in Nevada, and 12 in California ─ have planned capacities totaling 4.0 gigawatts (GW). The one in Florida is planned as a 75 MW facility. Adding heat storage, such as molten salt storage, can improve the economics of CSP operation because it allows the heat retained in storage to produce electricity into the night hours. The availability of thermal storage technologies gives CSP an additional advantage over PV. The CSP facility currently operating in Arizona has plans to add thermal storage. The first large-scale CSP plant with thermal storage began operations in Granada, Spain, in November 2008; the facility is a 50 MW plant with seven hours of thermal storage. CSP technologies generate power via the same steam cycle as coal or nuclear power plants; the main difference is the fuel used to turn water into steam. There are two major water processes in a steam turbine system—the steam cycle and the cooling process. Most of the water is consumed during cooling, and the choice of cooling technology largely determines how much water is actually consumed at a facility. Fossil and nuclear power plants use the same wet-cooling technologies as those for CSP. Water is used to produce steam to turn the steam turbines; this water is recycled for the generation of steam in the "closed-loop" steam cycle. Theoretically, water is not lost in the steam production cycle (though real-world imperfections necessitate some "make-up" water to compensate for leaks in the system). Steam is cooled in a condenser and condensed back to a liquid water state to be reused. The condenser itself is then cooled. For wet cooling of the condenser, the most common technology is to use a separate circuit of water to remove the heat from the condenser; this water then flows to an evaporative cooling tower that dissipates the collected heat energy to the environment. Most of this cooling water is lost as clouds of water vapor to the atmosphere as the condenser water contacts the air and the cooling tower. Alternatively, wet cooling can also occur by sending the condensed steam directly to the cooling tower. The 11 large-scale CSP facilities operating in the United States all use wet cooling. In areas where water supplies are constrained, dry cooling technologies may be used, which blow air over extensive networks of steam pipes designed with convective cooling fins to dissipate the heat energy over their surface area. No water is used or consumed in dry cooling. Air, however, has a much lower capacity to carry heat than water; therefore, dry cooling generally is less efficient than wet cooling in removing heat. Often, massive cooling fans are used to remove the heat from the pipe array in dry cooling. These fans consume a portion of the electricity generated by the power plant. Although dry cooling reduces water use, its consumption of energy for cooling fans and reduction of thermal efficiency of the steam turbines, especially on the hottest days of the year, when summer-peaking utilities most need power, is a significant factor impeding its adoption. It may be possible to offset the effect of dry cooling on net electricity generation by using bigger solar collecting fields or perhaps PV systems to run cooling fans. Where efficiency is a concern and water is constrained, a hybrid combination of wet and dry cooling technologies may be used. (See " Reducing the CSP Water Footprint ," below, for more information on hybrid cooling.) CRS was not able to identify any operating large-scale CSP facility in the United States or the world that uses dry cooling, but the technology is being considered as new CSP proposals are being developed in water-constrained areas, such as Southern California counties. Further research on materials and the thermal properties of the heat transfer medium in solar installations may allow the medium to reach higher temperatures, producing hotter steam for the turbines and greater electricity output. However, steam turbine operating characteristics are constrained by the materials used to manufacture the turbines. These materials will then determine how well the turbine is able to accommodate high pressures. Another alternative to increase output would be to add a combustion turbine to the existing solar cycle. If higher-temperature steam can be produced, then combustion turbines operating at lower pressures can be used to augment the solar steam turbines, resulting in more efficient energy output. Ongoing research at the U.S. Department of Energy's National Renewable Energy Lab (NREL) has shown that an integrated solar combined cycle plant would have efficiencies higher than those of a solar-only plant, and potentially higher than those in a fossil-fuel combined-cycle plant. It also has costs 25% to 75% lower than those of a solar-only plant, and offers the lowest cost of solar electric energy among hybrid options. In arid and semi-arid regions like the Southwest, or other areas with intense water demand, water supply is an issue for locating any thermoelectric power plant, not only CSP. The cumulative impact of installing multiple thermoelectric power plants in a region with existing water constraints raises numerous policy questions. As previously noted, there is significant uncertainty about the future rate and location of CSP deployment; this uncertainty significantly restricts the ability to evaluate the extent and location of potential water resource implications of CSP deployment. Additional data and analysis on where CSP may deploy and how it may affect local water availability would benefit federal, state, and local decision-makers when evaluating the potential consequences of general policies and individual permitting and siting decisions. The analysis presented in this report is based on available projections for CSP deployment at the county level.