Concentrating Solar Thermal Power (CSP): What It Is And How It Works

The largest solar thermal power plant in the world is located in California’s Mojave Desert in San Bernardino County. The Ivanpah Solar Power Facility is a 386-Megawatt facility built and operated by the U.S. Department of Energy. It uses distributed power towers, and large flat mirrors called heliostats.

These massive power plants generate a considerable amount of energy by using heat to turn steam generators, and they operate at very high temperatures. Coal, natural gas, and nuclear power plants also operate using thermal energy, but must burn a fuel source. Despite photovoltaic solar power plants being more popular and widespread, concentrated solar thermal power (CSP) plants have also proven to be a reliable and effective technology for utility-scale solar plants.

However, CSP systems are not very widespread, and further research is needed to advance this technology. Of the 4,116 billion kilowatt-hours (kWh) of electricity generated at utility-scale power plants in the U.S. in 2021, solar thermal power plants generated about 3 billion kilowatt hours. A combination of high costs and technological issues has led to slow deployment, thus CSP projects are relatively rare.

How Does Concentrated Solar Thermal Power (CSP) Work?

CSP technology uses mirrors to focus the sun's heat (solar thermal energy) to rotate steam turbines or engines, generating electricity. Plant operators can also store the energy generated from CSP plants to produce electrical power later. There are several types of CSP energy technologies, including parabolic trough, compact linear fresnel reflector, power tower, and dish-engine.

Like fossil and nuclear power plants, CSP plants commonly use wet-cooling, a process that counts on water to cool the steam turbines. Unfortunately, this is a water-intensive process which is an issue in water-constrained areas. There is another technique called dry cooling, but it uses fans to cool the turbines, and therefore is not as energy efficient.

Some CSP plants are hybrid systems that use fossil fuels, such as natural gas, along with solar energy. However, this does create greenhouse gas emissions. In addition to electricity generation, CSP technology can also be used for desalination and process heat.

Types Of Concentrating Solar Thermal Power

There are several types of CSP technology in use in the U.S., which share some common themes. They all use mirrors to focus sunlight onto a central receiver that circulates a fluid. CSP plants operate at high temperatures and use thermal energy to produce power. Let’s examine the various types of CSP plants.

Parabola/Trough

Parabolic trough systems use long, rectangular U-shaped mirrors to focus sunlight onto receiver tubes that run the length of the mirrors. The fluid that flows through the tubes is heated and sent to a heat exchanger to boil water in a steam generator. Depending on the system design, some plants have mirrors that can track the sun.

Typically, the mirrors are aligned in a north-south layout to maximize the concentration of solar energy because it allows them to track the sun as it moves from east to west across the sky and focus the sunlight continuously on the tubes. There are several such concentrated solar power plants in the U.S. located in Arizona, Nevada, and California.

Dish

Solar dish systems contain a mirrored dish that is a similar shape to a large satellite dish. Numerous small, flat mirrors direct and concentrate sunlight onto the solar receiver, which absorbs it and transfers it to an engine generator.

The solar dish points directly at the sun and concentrates the energy at a focal point, and the fluid temperature can reach temperatures of 1,380°F or greater. There currently are not any utility-scale solar thermal dish projects in commercial operation in the U.S. However, parabolic dish CSP technology can also be utilized on smaller-scale projects with one small dish.

Tower

These power plants contain a solar power tower system with heliostats to concentrate sunlight on a receiver on the top of the tower. Although some plants use water as a heat-transfer fluid, there is experimentation with molten nitrate salt, due to its excellent heat transfer and energy storage abilities. Because solar tower facilities can store heat, they can produce energy as needed.

The Ivanhap Solar Power Facility described above is a solar power tower system, and is one of two solar power tower facilities in operation in the U.S.

CSP Storage

CSP plants use solar radiation for power generation, but the heat can also be stored for later use. There are several thermal energy storage technologies that have been tested and implemented, including the two-tank direct system, single-tank thermocline system, and the two-tank indirect system. CSP storage systems have been tested and in use since 1985.

This enables these plants to be a flexible option for generating power when it is needed most, even if the sun’s energy is unavailable. Therefore, energy storage capabilities allow renewable energy to generate a larger share of the total energy mix while ensuring a sustainable and reliable power supply.

CSP Desalination Plants

Many of the regions with sufficient solar radiation to have CSP plants are also water scarce. These areas often rely on industrial-scale desalination plants, but these facilities require a lot of energy to operate. Therefore, CSP desalination plants can over two of the biggest issues facing humanity: the need for clean energy and a sustainable water supply.

FAQs About CSP Plants

Let’s look at some of the most common questions regarding this solar technology.

How Do CSP and Photovoltaics Compare?

Solar PV panels use light, not heat, to make electricity. Both CSP and photovoltaics are used in utility-scale power plants, but PV technology is much more widespread and doesn’t require water for cooling. Utility-scale PV plants are also easier to construct than CSP facilities, and they do not require the same high level of solar radiation as CSP.

Thus, utility-scale PV plants are more common, even in states with moderate levels of solar radiation. By contrast, CSP plants are commonly situated in the Southwestern U.S., Spain, Morocco, India, South Africa, Israel, and Australia, due to their excellent solar resources.

However, CSP technology has a couple of distinct advantages. It produces alternating current (AC) power, so no inverter is needed. Likewise, CSP plants are often designed to store heat for generating electricity at a later time. This means that CSP plants can be used to meet peak energy demand because the power is dispatchable.

However, utility-scale energy storage technologies for PV systems that use lithium-ion batteries have advanced considerably in recent years. Yet, these battery energy storage systems (BESS) are commonly relatively small by comparison to the energy storage abilities of CSP plants. As battery technology advances and costs decrease, BESS at PV plants will likely have greater capacities and become more widespread.

What is the Largest CSP Plant in the World?

The Noor Complex Solar Power Plant in Morocco has a capacity of 580 Megawatts, enough to meet the electricity needs of 1 million people. The CSP plant uses up to 3 million cubic meters of water and up to 19 tons of diesel fuel daily to keep the molten salt mixture and synthetic oil in the heat transfer fluid that the proper operating temperatures.

Concentrating Solar Thermal Power Could Change Power Plants Forever

Although technological and cost issues are holding back more widespread use of CSP plants, the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories are conducting extensive CSP research to help overcome challenges and develop next-generation approaches. Yet, the rapid decrease in the price of photovoltaics (PV) has made CSP less cost competitive by comparison. Addressing cost issues will be essential for CSP plants to become more widespread and practical.

One big advantage of CSP technology is its ability to store heat to generate clean energy when power demand is highest. This enables CSP to compete with other dispatchable energy sources, like natural gas.

Want to learn more about large-scale solar energy projects? Read our article on thermal runaway.