Solar Power Tower
Introduction
A solar power tower is a device that uses a large-scale solar installation to transform solar energy into usable electricity. Heliostats, a collection of large, sun-tracking mirrors, are used in the arrangement to direct sunlight toward a receiver perched atop a tower. A fluid is heated and used to produce steam in this receiver. A typical turbine generator is then driven by this steam to produce electricity.
The use of towers like this has the potential drawback of requiring substantial upfront investment in massive facilities. The vast field of mirrors and tower, which may be 50 to more than 100 metres high, can also be considered an eyesore and have an influence on the surrounding scenery.
Design
- Some concentrating solar power towers are air-cooled instead of water-cooled, to avoid using limited desert water.
- Flat glass is used instead of the more expensive curved glass.
- Thermal storage to store the heat in molten salt containers to continue producing electricity while the sun is not shining
- Steam is heated to 500 °C to drive turbines that are coupled to generators which produce electricity
- Control systems to supervise and control all the plant activity including the heliostat array positions, alarms, other data acquisition and communication.
Generally, installations use from 150 hectares (1,500,000 m2) to 320 hectares (3,200,000 m2).
Operation
One of the key elements of a solar power plant is a solar power tower. This tower is situated in the middle of a significant collection of mirrors. These mirrors can be curved or flat, but because they are less expensive than curved mirrors, flat mirrors that follow the Sun are typically employed. The incident sunlight is “caught” by these mirrors as they follow the Sun and is then reflected back to the solar tower.
Many of these mirrors concentrate a significant quantity of solar energy onto the receiver, which is a tiny area of the tower where some fluid is heated. The heat from the sun is transferred to the water via this fluid. Modern designs employ molten salts instead of steam because of their improved heat transmission and energy storage capabilities. Older towers used steam as their heat transfer fluid. Water turns to steam when heat is applied to it. After that, this steam is brought to a typical turbine to create electricity.
As they use the energy from being warm in relation to their surroundings and convert that heat into motion, these solar power towers may be thought of as heat engines. More specifically, because the heat source (the Sun) is distinct from the fluid that flows and performs work, these solar power towers are external heat engines. It uses steam to turn a turbine and is an example of external combustion where heat from the Sun warms a fluid.
Environmental Concerns
Solar power towers are a sort of energy producing technology that is cleaner than generating electricity using fossil fuels, as is the case with other solar power technologies. As a result, one of the greenest methods for producing electricity is through solar power towers.
However, these towers still have related environmental problems. First, a life cycle analysis reveals that greenhouse gas emissions are still linked to the fabrication and construction of the tower and mirrors, as well as to the dismantling and recycling of the plant after it has served its purpose. These emissions are considerably less than those from burning fossil fuels, though.
Since solar towers can consume significant amounts of water to function, the water demand of these plants can also be seen as a problem. Alternative cooling technologies, such as the use of air cooling, are being looked into to reduce the amount of water used.
The detrimental influence that these plants have on birds is a potential end outcome of the deployment of these towers. Birds can be burned if they fly directly into the Sun’s concentrated beams. According to some reports, a bird dies at a power station like this one every two minutes on average.
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 too 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, its 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.
Commercial Applications
Several companies have been involved in planning, designing, and building utility size power plants. There are numerous examples of case studies of applying innovative solutions to solar power. Beam-down (a variation of central receiver plants with Cassegrainian optics) tower application is also feasible with heliostats to heat the working fluid.
Novel Applications
The Pit Power Tower combines a solar power tower and an aero-electric power tower in a decommissioned open pit mine. Traditional solar power towers are constrained in size by the height of the tower and closer heliostats blocking the line of sight of outer heliostats to the receiver. The use of the pit mine’s “stadium seating” helps overcome the blocking constraint.
As solar power towers commonly use steam to drive the turbines, and water tends to be scarce in regions with high solar energy, another advantage of open pits is that they tend to collect water, having been dug below the water table. The Pit Power Tower uses low heat steam to drive the pneumatic tubes in a cogeneration system. A third benefit of repurposing a pit mine for this kind of project is the possibility of reusing mine infrastructure such as roads, buildings and electricity.
Challenges
A number of solar tower projects have been cancelled 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.
