Unlike photovoltaic (PV) systems, which use the sun’s light to generate electricity, concentrating solar power systems generate electricity using the sun’s heat.

The United States was a pioneer in the development of CSP, or solar thermal power, and California’s Mojave Desert hosts some of the earliest operating CSP plants in the world, installed in the 1980s. Although most large-scale solar capacity is now PV, CSP was once at the forefront and recent projects have significantly increased the installed U.S. capacity of CSP [1]. By the end of 2014, the United States had 1,700 megawatts (MW [2]) of CSP, 85 percent more than in 2013 [3].

CSP technology

Solar concentrators come in two main designs:

- Parabolic Trough. The most common CSP technology, it consists of long, curved mirrors that concentrate sunlight on a liquid (generally oil) inside a tube that runs parallel to the mirror. The liquid, at about 300 degrees Celsius, runs to a central collector, where it produces steam that drives an electric turbine.

- Central receiver. Central receiver CSP facilities, or "power towers," use fields of mirrors to concentrate sunlight on the top of central towers. The intense heat, carried by molten salts, boils water, and the steam then drives on-site generators.

Another type of CSP, though not yet broadly in use, is the parabolic dish. Similar to trough concentrators, this system focuses the sunlight on a single point. Dishes can produce much higher temperatures and thus have the potential of producing electricity more efficiently.

A variation on dish concentrating technology uses a stirling engine to produce power. Unlike a car or generator’s internal combustion engine, in which fuel exploding inside the engine produces heat that causes the air inside the engine to expand and push out on the pistons, a stirling engine produces heat by way of mirrors that reflect sunlight on the outside of the engine. Dish-stirling generators producing tens of kilowatts of power each could replace diesel generators in remote locations.

U.S. power tower activity began with the Solar One project in Barstow, California, which operated from 1982 to 1988. Reconfigured as Solar Two during the early to mid-1990s, the facility successfully demonstrated until 1999 the ability to collect and store solar energy efficiently [5]. Solar Two's success opened the door for further development of this technology, notably the 392 MW Ivanpah project, which was the largest solar power plant in the world when it came online in 2014 [6].

More commercial-scale solar concentrator projects are under development in the United States, thanks mostly to various state policies and incentives.

One advantage of CSP over PV and many other renewable energy technologies is its ability to store the sun’s energy. Molten salt, for example, can be stored and kept hot for many hours so that when electricity is needed, the heat can be used to make steam to drive a generator.

CSP, in this way, has the ability to make electricity when the sun is no longer shining and at times when it may be most valuable to the grid. This storage lets CSP systems extend the “shoulder hours” of their generation patterns and generate electricity a few hours before the sun rises and a few hours after it sets, making it easier to integrate electricity from such plants into the grid [7].

Even without storage, since CSP systems generate electricity using very high temperatures, momentary cloud cover does not lead to the same minute-by-minute variation in electricity production that PV systems experience [8].

Siting CSP

Because CSP plants require very strong solar resources where clouds and haze do not interfere, their development in the United States has been largely in the desert Southwest and California, although facilities have also appeared in Florida, Colorado, and Hawaii [9].

CSP plants require significant infrastructure for collecting steam and generating electricity, and large areas of land, which limits project design options and locations for CSP plants and generally makes them an option for large-scale generation only [10].

The economics of CSP

CSP’s ability to store energy and, to an extent, provide electricity on demand is an important characteristic that can make the electricity more valuable to the grid and can help utilities avoid the costs of building new power plants to meet projected future demand.

However, CSP projects have not seen the rapid growth that the PV market has experienced in recent years, largely because of their overall less-favorable economics. The main components of CSP projects—steel and mirrors—have not experienced the dramatic cost declines that solar panels have.

Several large solar projects slated to use CSP—including the proposed 1,000 MW Blythe solar project near the California-Arizona border [11]—switched to PV technology due to the relatively rapid decline in the cost of PV panels.

Yet strides are being made to reduce the cost of CSP power generation. Most notable is the DOE’s SunShot Initiative, which continues to work with private industry partners, laboratories, and universities with the goal of making CSP cost-competitive with traditional power generation by 2020 [12].

CSP and the environment

CSP shares many of the positive environmental aspects of other solar electricity options, and some challenges.

While the manufacturing of CSP components, like all other energy devices, involves some emissions, CSP electricity generation itself:

- generates no carbon dioxide or other heat-trapping gases that contribute to climate change [13]

- produces none of the other harmful emissions or wastes associated with coal power, such as mercury, sulfur dioxide, nitrogen oxides, lead, and arsenic [14]

- creates none of the long-lasting waste or environmental risks associated with nuclear power [15]

- avoids the environmental risks associated with natural gas, including potential water pollution during extraction [16]

Some challenges posed by CSP projects are similar to those faced by large PV projects, while others are unique to CSP technology.

Land use. CSP facilities require large swaths of intensely sunny, relatively level land, which usually implies locating them in desert ecosystems, which can be fragile. Project developers may scrape and grade sites in order to install the structures that support the mirrors, potentially disrupting the habitats of ground-dwelling animals.

Several actual or proposed CSP projects in the Mojave Desert have run into troubles because of the desert tortoise, a species native to the U.S. Southwest and Mexico that is already threatened by development, climate change, and other issues [17].

While modular PV projects can more easily be built around physical constraints (such as tortoise habitat) or be scaled back to minimize impacts, this flexibility is much less available to CSP. Project developers can, however, reduce impacts to plants and animals by building on already disturbed lands or by placing mirrors more efficiently to make optimal use of land [18].

Photo: NASA

Solar flux. Unique to CSP power towers is the issue of solar flux created by the mirrors. The extreme heat created by the concentrated energy can singe and kill passing birds and bats. Incidents of bird deaths were first reported at the Ivanpah solar facility in the Mojave soon after its launch in 2014 [19]. Understanding and minimizing such wildlife impacts is an important issue for the future of power towers.

Water use. CSP’s water use depends largely on choices around cooling systems. CSP plants often use water to cool the steam once it has been used to generate electricity. Ones with conventional “wet cooling” may evaporate even more water per unit of electricity than coal or nuclear power plants [20].

One solution is exemplified in some CSP projects in the United States, including Ivanpah and the Genesis solar project (also in California), which cool steam with air instead of water, cutting water consumption by 90 percent or more [21].

Policies: research and development on CSP storage

CSP investments have been driven mainly by the requirements of states’ renewable electricity standards. CSP projects also benefit from federal incentives like the federal investment tax credit (which covers 30 percent of the initial costs).

Further, since CSP projects are more likely to be built in the western United States on federal lands, they are benefitting from efforts by state and federal agencies to coordinate permitting studies and agree on investments to reduce environmental impacts.

CSP project designs continue to evolve; therefore, policies that also support research and development into engineering innovations, including how to take advantage of CSP’s storage capability to make it easier to integrate larger amounts of solar electricity into the electric grid, will make CSP costs more competitive and allow the projects to demonstrate the value that renewable resources bring to the grid.

References:

[1] GTM Research and Solar Energy Industries Association (SEIA). 2014a. U.S. solar market insight report 2013 year in review. Boston, MA, and Washington, DC.

[2] Unlike photovoltaic panels, which generate direct current that is then converted into alternating current (AC), CSP systems produce AC directly; capacity figures (MW) on this webpage are expressed in MWac.

[3] Solar Energy Industries Association (SEIA). 2015. U.S. Installs 6.2 GW of Solar PV in 2014, Up 30% Over 2013.Solar Energy Industries Association (SEIA). 2013. Solar Market Insight Report 2013 Year in Review.

[4] U.S. Department of Energy (DOE). 2014. 2014: The Year of Concentrating Solar Power.

[5] Sandia National Laboratories. 2002. Final Test and Evaluation Results from Solar Two Project.

[6] U.S. Department of Energy (DOE). 2014. 2014: The Year of Concentrating Solar Power.

[7] Denholm, P., and M. Mehos. 2011. Enabling greater penetration of solar power via the use of CSP with thermal energy storage. Golden, CO: National Renewable Energy Laboratory. Online at http://www.nrel.gov/csp/pdfs/52978.pdf.

[8] Jorgensen, J., P. Denholm, and M. Mehos. 2014. Estimating the value of utility-scale solar technologies in California under a 40% renewable portfolio standard. Golden, CO: National Renewable Energy Laboratory. Online at http://www.nrel.gov/docs/fy14osti/61685.pdf.

[9] National Renewable Energy Laboratory (NREL). 2014b. Concentrating solar power projects in the United States. Online at http://www.nrel.gov/ csp/solarpaces/by_country_detail.cfm/country=US.

[10] International Energy Agency (IEA). 2010. Technology roadmap: Concentrating solar power. Paris, France. Online at https://www.iea.org/media/freepublications/technologyroadmaps/csp_roadmap2010.pdf.

[11] Solar Trust of America. 2011. Solar Trust of America chooses PV technology for world’s largest solar facility. Oakland, CA. Online at http://www.businesswire.com/news/home/20110817006604/en/Solar-Trust-America-Chooses-Photovoltaic-Technology-World%E2%80%99s#.VeiQyvlVhBc.

[12] U.S. Department of Energy (DOE). 2014. 2014: The Year of Concentrating Solar Power. Online athttp://energy.gov/sites/prod/files/2014/10/f18/CSP-report-final-web.pdf

[13] Dell, J., S. Tierney, G. Franco, R.G. Newell, R. Richels, J. Weyant, and T.J. Wilbanks. 2014. Energy supply and use. In Climate change impacts in the United States: The third national climate assessment. Washington, DC: U.S. Global Change Research Program. Online at http://www.globalchange.gov/browse/reports/climate-change-impacts-united-states-third-national-climate-assessment-0.

[14] Freese, B., S. Clemmer, and A. Nogee. 2008. Coal power in a warming world: A sensible transition to cleaner energy options. Cambridge, MA: Union of Concerned Scientists. Online at http://www.ucsusa.org/sites/default/files/legacy/assets/documents/clean_energy/Coal-power-in-a-warming-world.pdf

[15] Gronlund, L., D. Lochbaum, and E. Lyman. 2007. Nuclear power in a warming world: Assessing the risks, addressing the challenges. Cambridge, MA: Union of Concerned Scientists. Online at http://www.ucsusa.org/sites/default/files/legacy/assets/documents/nuclear_power/nuclear-power-in-a-warming-world.pdf.

[16] Union of Concerned Scientists (UCS). 2014a. How natural gas works: Water use and pollution. Online at http://www.ucsusa.org/clean_energy/ our-energy-choices/coal-and-other-fossil-fuels/how-natural-gasworks.html - enviroimpacts.

[17] Defenders of Wildlife. 2014. Basic facts about desert tortoises. Online at http://www.defenders.org/desert-tortoise/basic-facts; United States Fish and Wildlife Service (USFWS). 2014. Mojave desert tortoises: Threats to desert tortoises. Reno, NV: Nevada Fish and Wildlife Office. Online at http://www.fws.gov/nevada/desert_tortoise/dt/ dt_threats.html.

[18] Chu, J. 2012. Here comes the sun: A new sunflower-inspired pattern increases concentrated solar efficiency. MIT News, January 11. Online at http://newsoffice.mit.edu/2012/ sunflower-concentrated-solar-0111.

[19] Kagan, R.A., T.C. Viner, P.W. Trail, and E.O. Espinoza. 2014. Avian mortality at solar energy facilities in Southern California: A preliminary analysis. Ashland, OR: National Fish and Wildlife Forensics Laboratory. Online at http://alternativeenergy.procon.org/sourcefiles/avian-mortality-solar-energy-ivanpah-apr-2014.pdf.

[20] Macknick, J., R. Newmark, G. Heath, and K.C. Hallett. 2012. Operational water consumption and withdrawal factors for electricity generating technologies: A review of existing literature. Environmental Research Letters 7, doi:%10.1088/1748-9326/7/4/045802. Online at http://www.nrel.gov/docs/fy11osti/50900.pdf.

[21] National Renewable Energy Laboratory (NREL). 2014b. Concentrating solar power projects in the United States. Online at http://www.nrel.gov/ csp/solarpaces/by_country_detail.cfm/country=US