A solar tower, also known as a solar power tower, is a way to concentrate solar power to make it a more powerful energy source. Solar towers are sometimes also called heliostat power plants because they use a collection of movable mirrors (heliostats) laid out in a field to gather and focus the sun at the tower.
By concentrating and collecting solar energy, solar towers are considered a type of renewable energy. Solar towers are one kind of solar tech (including parabolic trough or dish-engine systems), all of which can make up a concentrated solar power (CSP) system. According to the Solar Energy Industries Association, CSP plants in the United States have about 1,815 megawatts of energy capacity.
How a solar tower works
As the sun shines down on a solar tower's field of heliostats, each of those computer-controlled mirrors tracks the sun's position on two axes. The heliostats are set up so that over the course of a day, they efficiently focus that light towards a receiver at the top of the tower.
Large, flat, sun-tracking mirrors known as heliostats focus sunlight onto a receiver at the top of a tower. A heat-transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine generator to produce electricity. (Photo: Office of Energy Efficiency and Renewable Energy/U.S. Department of Energy)
In their first iteration, solar towers used the sun's focused rays to heat water, and the resulting steam powered a turbine to create electricity. Newer models now use a combination of liquid salts, including 60% sodium nitrate and 40% potassium nitrate. These salts have a higher heat capacity than water, so some of that heat energy can be stored before using it to boil the water, which drives the turbines.
These higher operating temperatures also allow for greater efficiency and mean that some power can be generated even on cloudy days. Combined with some kind of energy-storage device, this means solar towers can produce reliable energy 24 hours a day.
There are some obvious environmental advantages to solar towers. Compared to fossil-fuel burning plants like coal or natural gas plants, there's no air pollution, water pollution or greenhouse gases typically created in the energy generation process. (There are some emissions created in the building of a solar tower, just as there would be in another type of power plant, since materials have to be moved to the location and built, all of which requires energy, usually in the form of fossil fuels.)
Negative environmental impacts are similar to other power plants: Some toxic materials are used to make the components of the plant (in this case photovoltaic cells). When you clear land for a new plant, the animals and plants that live there are impacted, and their habitat destroyed — though some of this impact can be mitigated by choosing a location that has minimal impact on local plants and animals. Solar towers are often constructed in desert landscapes, which by their very nature are somewhat fragile, so special care must be taken in siting and construction.
Some solar towers are air-cooled, but others use ground water or available surface water for cooling, so while the water isn't polluted with toxic waste as it can be at other power plants, the water is still being used, and that can impact the local ecosystem. Some solar towers might also need water for cleaning the heliostats and other equipment. (Those mirrors work best to concentrate and reflect light when not covered in dust.) According to the US Energy Information Center, "solar thermal systems use potentially hazardous fluids to transfer heat." Ensuring those chemicals don't make their way into the environment in the event of a storm or other unusual circumstance is important.
An environmental issue unique to solar power towers is bird and insect deaths. Due to how the heliostats concentrate light and heat, any animal flying through the beam as it is transmitted to the tower will be burned or killed by the high temperatures (up to 1,000 degrees Fahrenheit). A simple way to minimize bird deaths is to ensure that no more than four mirrors are aimed at the tower at the same time.
History of solar towers
The first solar tower was the National Solar Thermal Test operated by Sandia National Laboratories for the U.S. Department of Energy. Constructed in 1979 as a response to the energy crisis, it still runs today as a test facility that's open to scientists and universities to study.
"The National Solar Thermal Test Facility (NSTTF) is the only test facility of this type in the United States. The NSTTF's primary goal is to provide experimental engineering data for the design, construction, and operation of unique components and systems in proposed solar thermal electrical plants planned for large-scale power generation," according to Sandia's website.
The first commercial solar power tower was Solar One, which ran from 1982 to 1988 in the Mohave Desert. While it was able to store some energy into the evening (enough for start-up in the morning), it wasn't efficient, which is why it was modified to become Solar Two. This second iteration switched over from using oil as a heat-transfer material to molten salt, which is also able to store thermal energy and has the added benefits of being nontoxic and non-flammable.
In 2009, the Sierra Sun Tower was built in California's Mojave Desert, and its 5 megawatt capacity reduced CO2 emissions by 7,000 tons per year when it was running. It was built as a model but was shut down in 2015 because it was deemed to costly to operate.
Outside the United States, solar tower projects include the PS10 solar power plant near Seville, Spain, which produces 11 MW of power and is part of a larger system that aims to produce 300 MW. It was built in 2007. Germany's experimental Jülich solar tower, built in 2008, is the country's only plant using this technology. It was sold to the German Aerospace Center in 2011 and remains in use. Other U.S. and European projects are detailed below.
In 2013, Chile put $1.3 billion into the Cerro Dominador CSP project, Latin America's first solar tower project. It was begun in hopes of phasing out coal-fired power by 2040 and being completely carbon neutral by 2050. But delays due to a bankruptcy by the project's funder, meant that by the time the plant's construction was resumed, it's technology had already been outpaced by cheap solar panels from China, and widespread adoption of renewable technologies. The prices that Cerro Dominador would charge would already be three times higher than what other renewables could provide. The project is now on hold indefinitely.
Solar towers around the world
An ideal location for a solar tower is one that's flat, dry and isn't too windy or stormy. Plant operators will need access to some water supplies (if only for cleaning the heliostats) and areas that receive rain or snow in any significant amount should be avoided. Naturally, a high number of sunny days and as much direct solar radiation is best, so minimal cloud cover is the goal. This is measured by a number called Direct Normal Intensity (DNI) of the sun, and that information is available through the National Renewable Energy Laboratory.
Anywhere that these criteria are met are good locations for solar power towers, including the Middle East, the U.S. Southwest, Chile, southern Spain, India, South Africa and China.
Solar tower challenges
A number of solar tower projects have been canceled or decommissioned. Challenges range from financial issues with investment, to competition with other renewable energies on price, to time needed to build a tower, to environmental concerns.
Cancelled solar tower projects
- Cerra Domidor in Chile was begun but not completed due to the bankruptcy of the financier behind the project.
Closed solar tower projects
- Eurelios was a pilot solar tower plant in Sicily operated from 1981 to 1987.
- Sierra Sun Tower, ran from 2009-2015 in the Mojave Desert.
- Solar One and Solar Two in the Mojave Desert operated from 1982 to 1986, and 1995 to1999, respectively.
- SES-5 operated in the former USSR from 1985 to1989.
- Maricopa Solar in Arizona was built in 2010 but decommissioned in 2011 and sold.