Concentrated Solar Power

Concentrated solar power (CSP), also called solar thermal electricity, makes power from the sun’s heat rather than its light. Fields of mirrors track the sun and focus its beam onto a receiver, heating a fluid to somewhere between 300 °C and 565 °C. That heat raises steam for a turbine, the same power block you would find in a coal or gas plant. What sets CSP apart is storage: heat costs far less to store than electricity, so a plant with a tank of molten salt can sell solar electricity at night.

Globally, CSP is small. IRENA, REN21 and the SolarPACES project database put operating capacity at roughly 7 GW in the mid-2020s, against more than 2,000 GW of solar PV. The site’s electricity statistics (from Ember and Our World in Data) don’t list CSP separately; it sits inside the solar figures, where it supplies well under 1% of output. It matters less for its current size than for what it can do in sunny, arid regions: provide solar power that can be stored and dispatched on demand.

How CSP works

Every CSP plant has four subsystems:

1. Solar field. Mirrors (heliostats or curved reflectors) track the sun and concentrate direct normal irradiance onto a receiver. Line-focus systems reach about 30-100 suns; point-focus towers reach 500-1,000+.
2. Receiver. It absorbs the concentrated light and passes the heat to a fluid: synthetic thermal oil, molten nitrate salt, or water/steam directly.
3. Thermal energy storage. Usually two tanks of molten salt, a “solar salt” mix of about 60% sodium nitrate and 40% potassium nitrate. Salt is pumped from a cold tank (~290 °C) through the receiver or a heat exchanger into a hot tank (~390 °C for trough plants, ~565 °C for towers).
4. Power block. A Rankine steam cycle with turbine, generator and condenser, the same in principle as a fossil plant.

Only direct beam radiation can be concentrated, so CSP gets nothing from the diffuse light that PV still converts on hazy days. For siting, the number that matters is DNI, not global horizontal irradiance (see the solar potential map).

Main CSP technologies

Parabolic trough

Long rows of parabolic mirrors focus sunlight onto an evacuated receiver tube along the focal line, heating synthetic oil to about 390-400 °C. Troughs are the most mature design and account for most installed capacity. Examples include the Spanish fleet of 50 MW plants (Andasol 1-3, each with 7.5 hours of storage), the 280 MW Solana plant in Arizona, and the 600 MW trough section of Dubai’s Mohammed bin Rashid Al Maktoum Solar Park Phase 4. The oil’s temperature limit holds down steam cycle efficiency, and storage needs an oil-to-salt heat exchanger.

Solar power tower (central receiver)

Thousands of flat heliostats, each tracking the sun on its own, reflect light onto a receiver at the top of a tower. Towers run hotter (molten salt at about 565 °C), so the turbine is more efficient and the salt can be stored directly with no heat exchanger, which lowers storage cost. Noor III in Morocco (150 MW), Cerro Dominador in Chile (110 MW, about 17.5 h storage) and the 100 MW, 262-metre tower in Dubai, the tallest solar tower ever built, are examples. Ivanpah in California (about 390 MW gross, three towers, direct steam, no storage) is the best-known tower without storage.

Linear Fresnel

Linear Fresnel plants use long, nearly flat mirror strips near the ground to focus light onto a fixed receiver mounted above them. They cost less to build and use less land per MW than troughs, but their optical efficiency is lower, particularly in winter and at high latitudes. Plants tend to be tens of MW, such as Puerto Errado 2 in Spain (30 MW) and Dhursar in India (125 MW).

Parabolic dish

A dish focuses light onto a receiver at its focal point, usually driving a Stirling engine of about 10-25 kW per unit. Dish-Stirling systems hold the record for solar-to-electric efficiency (above 30% at peak). They can’t easily take storage, though, and never reached commercial scale: the large US dish projects of the early 2010s were cancelled or switched to PV.

Efficiency ranges are approximate design and operating values from IRENA and SolarPACES literature. Real plants vary with DNI, storage size and how they are operated.

Efficiency, storage and capacity factor

CSP efficiency is a chain of conversions. Optical losses (mirror reflectivity, cosine and shading) leave typically 55-75% over a year; the receiver keeps 80-90% after absorption and heat losses; the steam cycle converts about 35-42%. Multiply them and you get annual solar-to-electric efficiencies of about 13-20%, similar to or below PV modules. The Carnot limit sets the thermodynamic ceiling for the power block:

That temperature advantage is why towers dominate new projects. The solar multiple is the main design lever for dispatchability. With SM = 1, the field is just big enough to run the turbine at noon. With SM of 2.5-3.5, the plant collects surplus heat during the day and stores it for the evening and night. Molten-salt tanks lose only a few percent of their heat per day and cost far less per kWh of capacity than batteries, so 6-15 hours of storage is now standard.

The capacity factor of a CSP plant therefore comes down mostly to storage. Without storage, plants run at roughly 20-30%. With 7-10 hours, 40-55% is typical at good sites, and plants like Cerro Dominador are designed for 60-70% or more. As storage durations grew, IRENA’s weighted average for newly commissioned plants rose from around 30% in 2010 to 40-55% in recent years. The capacity factor calculator shows what this means in annual energy.

Costs

CSP costs have fallen steeply too, though from a higher starting point and over far fewer projects than PV.

• IRENA (2024): the global weighted-average LCOE of newly commissioned CSP fell from about USD 0.39/kWh in 2010 to roughly USD 0.12/kWh in 2023, a drop of around 70%. Only a handful of plants come online each year (lately mostly in China), so the annual average jumps around.
• Tender prices: Dubai’s 950 MW Noor Energy 1 (Phase 4, 700 MW CSP plus 250 MW PV, 15 hours of storage) was contracted at about USD 0.073/kWh in 2017. Chilean and Chinese projects have bid USD 0.06-0.11/kWh, depending on financing and support schemes.
• Lazard LCOE+ analyses have put US solar thermal towers with storage at roughly USD 120-160/MWh unsubsidised, well above utility PV.
• NREL ATB models substantial further declines for molten-salt towers if next-generation designs (hotter receivers, supercritical CO2 cycles) reach the market.

Installed costs are high, typically several thousand USD per kW depending on storage hours, and the LCOE swings with WACC, plant scale and DNI. The fair comparison isn’t CSP against bare PV. It is CSP against PV plus battery storage delivering the same evening and night energy. For short durations (2-4 h), PV with lithium-ion batteries now wins almost everywhere; CSP looks better at 8+ hours and for industrial heat. You can test these trade-offs in the LCOE calculator.

Flagship projects

• Noor Ouarzazate, Morocco. A 580 MW complex: Noor I (160 MW trough, 3 h storage), Noor II (200 MW trough, 7 h), Noor III (150 MW tower, about 7.5 h) and Noor IV (72 MW PV). Built with multilateral concessional loans, it became the model for large CSP in emerging markets.
• Cerro Dominador, Chile. Latin America’s first CSP plant (2021), a 110 MW molten-salt tower with about 17.5 h of storage, paired with 100 MW of PV in the Atacama Desert, where DNI is among the highest on Earth.
• Mohammed bin Rashid Al Maktoum Solar Park Phase 4, UAE. The largest single-site CSP investment anywhere: 600 MW of trough, a 100 MW tower and 250 MW of PV, with 15 hours of storage.
• Ivanpah, United States. The world’s largest CSP plant when it opened in 2014, it uses direct steam towers with no storage and burns natural gas for start-up. It missed early production targets and drew criticism over bird deaths. In 2025 its offtakers agreed to wind down contracts ahead of a planned partial closure.
• Crescent Dunes, United States. A 110 MW tower with 10 h of storage that had a salt tank leak and long outages. Its troubles cooled investor interest in CSP in the late 2010s.

Land, water and materials

Land. CSP needs roughly 2-4 hectares per MW, about the same as or a bit more than utility PV once the oversized field for storage is counted. Sites must be flat, with slopes generally below 1-3%.

Water. In deserts, water is CSP’s biggest environmental problem. Wet-cooled trough plants consume roughly 3,000-3,500 litres per MWh, similar to coal plants. Dry (air) cooling cuts that by around 90%, to a few hundred litres per MWh for mirror washing and steam make-up, but costs a few percent of output and more capital. Most new plants in water-stressed regions are dry-cooled.

Materials. CSP uses a lot of material: steel and concrete for structures and foundations, glass and silver for mirrors, and tens of thousands of tonnes of nitrate salts per plant. None of these is scarce, but the mass per MW is higher than for PV.

Lifecycle CO2 emissions

• IPCC AR5 WGIII Annex III (2014): median 27 gCO2e/kWh, range 8.8-63 g.
• UNECE (2021): CSP comes out higher on average than PV in this assessment. The reasons are the large quantities of steel, concrete, glass and nitrate salts and, in some designs, natural gas burned for start-up and freeze protection. The figures are still an order of magnitude below fossil generation.

Gas co-firing makes the biggest difference. Plants that burn a lot of gas for start-up or to keep salt from freezing have measurably higher operating emissions than solar-only designs. Molten salt solidifies at around 220-240 °C, so it has to be kept warm through cloudy spells.

Safety and environmental impacts

The industrial hazards are those of any thermal plant: high-pressure steam, hot fluids, and fire risk from synthetic thermal oil in trough plants. Concentrated light near tower receivers can burn birds and insects that fly through it; studies disagree widely on how many birds die at Ivanpah each year. Glare can bother pilots and drivers. Nitrate salts are non-toxic, fertiliser-grade chemicals, although leaks can damage equipment.

Outlook

CSP growth stalled in the 2010s as PV costs collapsed, but storage has brought interest back. China has a large pipeline of tower and Fresnel projects built next to PV and wind to supply firm power for remote “clean energy bases”. Chile, Morocco, Saudi Arabia, the UAE and South Africa are still active markets. CSP’s likely niche is covering the evening and night as a dispatchable partner to cheap PV, and supplying high-temperature industrial heat, since a CSP field can deliver steam directly. Research is focused on receivers above 700 °C, supercritical CO2 power cycles and particle-based storage. For the PV side of solar see solar power; for cost and emissions against other options see the technology comparison.