Concentrated solar power (CSP) uses mirrors to focus sunlight into intense heat, which then drives a steam turbine to produce electricity. Unlike rooftop solar panels that convert light directly into electricity through a chemical reaction, CSP works more like a traditional power plant: it boils water, makes steam, and spins a generator. The key difference is that sunlight, not coal or gas, provides the heat.
The Basic Process: Mirrors to Electricity
Every CSP plant follows the same core sequence. Large reflective surfaces, either curved mirrors or flat mirrors on tracking systems, collect sunlight across a wide area and redirect it toward a single focal point or line called a receiver. The receiver absorbs all that concentrated energy and converts it into extreme heat, often reaching 550°C or higher. That heat is transferred to a fluid, which produces steam. The steam spins a turbine connected to a generator, and the generator produces electricity.
The “concentrated” part is what makes this work. Sunlight spread across a large field isn’t hot enough to run a power plant on its own. But when thousands of mirrors aim their reflected light at one small target, the energy density at that point becomes intense enough to rival or exceed the temperatures inside a coal-fired boiler.
Four Main Plant Designs
CSP plants come in four configurations, each using a different mirror arrangement to concentrate sunlight.
- Parabolic trough: Long, curved mirrors shaped like a half-pipe focus sunlight onto a tube running along their center. A heat-transfer fluid (usually synthetic oil) flows through the tube, absorbing heat as it goes. These systems typically operate between 350°C and 550°C and reach about 15% electrical efficiency. They’re the most commercially proven design.
- Power tower: A large field of flat, sun-tracking mirrors (called heliostats) surrounds a central tower. Each mirror independently angles itself to reflect sunlight onto a receiver at the top of the tower. Because thousands of mirrors focus on one point, power towers achieve higher temperatures than troughs, which translates to better efficiency.
- Linear Fresnel: Similar in concept to parabolic troughs but uses rows of flat or slightly curved mirrors at ground level to reflect sunlight onto a fixed overhead tube. The simpler mirror design costs less to build but captures slightly less energy.
- Dish/engine: A large parabolic dish (like a satellite dish) focuses sunlight onto a receiver at its focal point, where a small engine converts the heat directly into electricity. These are the most efficient per unit of mirror area but are typically small-scale.
Heat Transfer Fluids and Operating Temperatures
The fluid that carries heat from the receiver to the steam generator is a critical part of the system. In parabolic trough plants, synthetic thermal oil is the standard choice, working well up to about 400°C. For higher temperatures, molten salt (a mixture of sodium and potassium nitrate) takes over. Molten salt stays liquid at temperatures well above 500°C and can store enormous amounts of heat energy in a relatively small volume.
Newer, third-generation CSP systems push operating temperatures above 700°C by using different power cycles borrowed from gas turbine technology. At these temperatures, annual electricity generation efficiency jumps to 25%–30%, nearly double what earlier trough systems achieve. Researchers are also exploring salts based on carbonates and sulfates that remain stable at even higher temperatures, which would push efficiency further.
Built-In Energy Storage
This is the feature that sets CSP apart from solar panels and wind turbines. Because CSP generates heat before generating electricity, you can store that heat in insulated tanks and use it later. The most common approach pumps hot molten salt into a large tank during the day, then draws from that tank after sunset to keep making steam.
Typical storage systems hold about 8 hours of full-power output, though some newer plants are designed for more. The round-trip efficiency of storing and retrieving heat this way is remarkably high: 95% to 99% of the thermal energy put into storage comes back out. That’s far better than battery storage, which typically loses 10%–15% in each charge-discharge cycle. The catch is that you’re storing heat, not electricity, so the overall conversion still depends on the turbine’s efficiency. But the practical result is that a CSP plant with storage can generate power well into the evening or through cloudy periods without any backup fuel.
Where CSP Plants Can Operate
CSP needs direct sunlight, not just bright skies. Clouds, haze, and humidity scatter light in all directions, and scattered light can’t be focused by mirrors. The metric that matters is called direct normal irradiance (DNI), which measures how much sunlight arrives in a straight beam from the sun. A CSP plant generally needs at least 2,000 kilowatt-hours per square meter per year to be economically viable.
That threshold limits CSP to hot, dry, sunny regions: the American Southwest, North Africa, the Middle East, South Africa, western China, and parts of Australia and Spain. Out of the major solar thermal plants built in Spain and the United States, most sit in locations exceeding that 2,000 kWh threshold. Standard solar panels, by contrast, work in cloudy climates too, because they can use scattered light. This geographic restriction is one reason CSP hasn’t spread as widely as photovoltaic solar.
CSP Paired With Natural Gas
Some plants blend CSP with natural gas turbines in what’s called an integrated solar combined cycle (ISCC). In this setup, a solar field (usually parabolic troughs) generates steam that feeds into the steam turbine of a gas-fired power plant. The solar energy either produces extra power on top of what the gas turbines generate, or it compensates for efficiency losses the gas plant suffers during hot weather.
A typical ISCC might pair two large gas turbines totaling 330 megawatts of electric capacity with a 220-megawatt steam turbine and a 50-megawatt solar field. The solar contribution is modest relative to the gas, but it displaces fossil fuel during sunny hours. When the solar field feeds into the high-pressure section of the steam system, the combined plant can reach net thermal efficiencies above 64%, with the solar portion alone converting sunlight to electricity at roughly 47%. That solar-to-electric figure is much higher than a standalone CSP plant achieves, because the solar heat is leveraging the already-efficient gas turbine infrastructure.
Water Use and Cooling
Because CSP is a thermal power technology, it faces the same cooling challenge as coal and gas plants. The steam that spins the turbine needs to be condensed back into water, and that condensing step requires removing heat. With traditional wet cooling, a parabolic trough plant consumes about 800 gallons of water per megawatt-hour of electricity. About 2% of that goes to washing mirrors; the rest is cooling.
That’s a problem when your best solar sites are in deserts. Dry cooling (using air instead of water) cuts consumption to roughly 80 gallons per megawatt-hour, a 90% reduction. The tradeoff is that dry cooling reduces the plant’s efficiency, especially on the hottest days when cooling is hardest and electricity demand is highest. Dish/engine systems sidestep the issue almost entirely, needing only about 20 gallons per megawatt-hour for mirror washing since they don’t use a steam cycle.
Cost Compared to Other Solar
CSP has gotten dramatically cheaper. Between 2010 and 2024, the global average cost of CSP electricity fell by 77%, landing at $0.092 per kilowatt-hour. That drop came from longer storage durations, lower maintenance costs, and better performance at high-irradiance sites in China and South Africa.
Still, CSP remains more than twice as expensive as standard solar panels, which reached $0.043 per kilowatt-hour globally in 2024. Solar PV is now 41% cheaper than the least expensive fossil fuel option, while CSP still sits above fossil fuel parity in most markets. The value proposition for CSP isn’t about being the cheapest source of daytime electricity. It’s about providing dispatchable power: electricity you can schedule and deliver on demand, thanks to built-in thermal storage. That capability competes less with solar panels and more with gas peaker plants and battery storage systems.