Solar Thermal Energy

Alex Blandino
December 11, 2014

Submitted as coursework for PH240, Stanford University, Fall 2014


Fig. 1: Global Solar Insolation Chart (Focus on Southwestern U.S.) (Courtesy of NASA. Source: Wikimedia Commons)
Fig. 2: Diagram of Solar Thermal Energy Production Process. [3] (Courtesy of the U.S. Department of Energy)

Solar Thermal Plants are a relatively new technology that are being used in some parts the U.S. as a means to produce clean renewable energy. There are currently four thermal solar plants that operate in the U.S.: three in California, and one in Arizona. [1] The output of these plants ranges from 250 MW to 392 MW and have all been in operation for a little over two years. The deserts in the southwest of the U.S. provide the greatest potential for solar thermal production according to global insolation charts (see Fig 1.) and while solar thermal power accounted for less than 1% of total energy production last year in the U.S., it's potential for clean energy production and its efficiency over solar cell energy production has made it one of the most attractive areas for green energy development. [2]


Solar Thermal Plants use hundreds of thousands of mirrors called heliostats to direct incident sunlight at a focal point located on a receiver. The heat from the sunlight is then used to super heat molten salt held in a receiver tower. The liquid salt is gathered from a "cold" storage tank where it is kept at 290°C (554°F) and heated by the concentrated sunlight to 565°C (1,049°F) and sent to a "hot" storage tank. The molten salt is then used to boil water to create steam. The steam is used under pressure, to rotate a turbine which creates electricity. Once the molten salt has been cooled down in this process, it is sent back to the "cold" storage tank to be used again in the same process. Used water vapor is recollected and cooled down to liquid form and recycled once again in the same process. The four plants currently in use are estimated to prevent around 400,000 tons of carbon-dioxide emissions per year in the U.S. [1,3] (See Fig.2)

Parabolic Trough Design

These thermal solar plants use a parabolic trough design to concentrate sunlight and create heat. The troughs track the movements of the sun throughout the day to focus the sun's energy on a solar tower. [3] (See Fig. 3)

Fig. 3: Illustration of Solar Trough Collector Focusing Sunlight (Source: Wikimedia Commons)
Fig. 4: Sunlight Being Directed at Solar Thermal Receiver Tower in Spain (Source: Wikimedia Commons)

Current Plants in the U.S.

Environmental Concerns and Drawbacks

While Solar Thermal Plants are very attractive because of their potential for renewable clean energy production, there are not insignificant environmental concerns associated with the plants. Over the past two years, large numbers of birds have been killed by the focused sunlight. [1] The birds are attracted by insects which flock to the light. The heat burns beaks and feathers, one bird species put in particular danger in California is the Peregrine Falcon. A report by the California Energy Commission stated that around 28,000 birds are being killed a year by the Ivanpah plant. [2] Another concern is that the plants have no potential for energy storage. While there is enough heat to produce electricity for about 6 hours after the sun has gone down, these plants have no way of storing the power they produce, if the energy is not used it gets wasted. [1]

Future For Solar Thermal Energy

This technology is promising, but it is still proving itself as a viable source of clean, renewable, energy. All of the stations that are currently in use in the U.S. are less than two years old, and research is still being done about the benefits and drawbacks of the plants. Each plant costs between 200 and 300 million dollars to produce, and while there was money to create the existing plants, the ultimate effectiveness of the existing plants will determine if more are built. The Ivanpah Solar Facility, which is the largest of its kind in the world received a 1.6 billion dollar guaranteed federal loan, which it could not have been completed without. If these plants prove to be unsuccessful, that kind of funding will be hard to come by. That being said, California recently has passed legislation that by 2020 33 percent of the state's energy will have to be coming from renewable sources, so there is a lot of room for growth until then. [1,2]

© Alex Blandino. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] D. Cardwell and M. L. Wald, "A Huge Solar Plant Opens, Facing Doubts About Its Future," New York Times, 13 Feb 14.

[2] W. La Jeunesse, "World's Largest Solar Plant Applying for Federal Grant to Pay off Federal Loan," Fox News, 8 Nov 14.

[3] "Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts," U.S. National Renewable Energy Laboratory, NREL/SR-550-34440, October 2003.