Concentrating Solar Power

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Concentrating solar power (CSP, or also known as concentrated solar power) uses mirrors or lenses to reflect sunlight onto a surface that absorbs the solar radiation. The heat from the sun – thermal energy – is used to power a conventional generator (often a steam turbine), producing electricity on a utility-scale.

Today, many power plants use nuclear or coal power to boil water to spin the steam turbines. Concentrating solar power systems provide a clean alternative power source. “Hybrid” systems, which use both natural gas and CSP, are also popular.

The technology is known as ‘concentrating’ because sunlight is focused on to a relatively small space.

There are three main types of concentrator systems, which produce varying amounts of energy. Larger CSP systems (up to 100 MW) are used by utilities, whereas smaller systems (say, 10 KW) can be used to provide power for remote villages.

Types of Concentrating Solar Power Systems

Linear Concentrator Systems
Long, rectangular U-shaped mirrors are tilted toward the sun. The incoming light focuses on linear receiver tubes that run the length of the collectors. Fluid inside the tubes is heated by the sun’s rays, and this heat energy is used to power a conventional generator.

A parabolic trough linear concentrator system. Credit: U.S. Department of Energy

This system is known a parabolic trough system (based on the shape of a parabola, i.e. a curve), and a collector field has many troughs in parallel rows. A sun-tracking system enables the mirrors to follow the sun from east to west, maximizing sunlight absorbed.

The largest trough systems generate 80 MW of electricity, although there are systems under development capable of generating 250 MW. Linear Fresnel reflector systems operate on the same principles, but the receiver tube is situated above the mirrors.

Dish/Engine

A dish/engine system. Credit: U.S. Department of Energy

A dish/engine system consists of a solar concentrator and a power conversion unit. A dish-shaped collector, similar in shape to a satellite dish, concentrates sunlight onto a receiver in the center of the dish. A fluid in the receiver is heated, and the thermal energy is transferred to an engine generator.

The most common type is a Stirling engine, hence the name dish/Stirling. A Stirling engine uses the heat to move pistons, creating mechanical power to run the generator. This type of system produces the least amount of electricity out of all concentrating solar power plants, usually around 3 to 25 KW.

Solar Power Towers
Large, flat mirrors (heliostats) on top of a tower track the sun, and focus the sunlight on a receiver atop the tower. A fluid inside the receiver generates steam which powers a conventional generator to produce electricity. Solar power tower technology is less advanced than trough systems, but these systems are very efficient. Though there are few power tower facilities in the U.S., more projects are underway.

The Solar Two solar power tower near Barstow, California. Credit: U.S. Department of Energy

The drawbacks of CSP and the road ahead

Proponents highlight the benefits of using a clean, renewable energy – rather than the traditional fossil fuels or nuclear power – to power utility-scale steam turbines.

As with photovoltaics, one concern with CSP is the ability to produce energy under low sunlight conditions. But while some use water as the heat transfer medium, most new systems us oil or molten salt. These fluids allow the heat energy to be stored for use during cloudy periods or at night.

Another concern is cost. Currently, CSP is one of the most expensive ways to produce energy. According to the Energy Information Administration, the cost of electricity from CSP plants will be much more expensive than natural gas in the next few years.

But some analysts predict that the cost of electricity at CSP plants currently under development will be around the same as natural gas powered plants. While current cost estimates vary, the future cost will depend on continued investment in CSP technology, as well as a global movement away from fossil fuels.

There are currently 514 MW of CSP projects operating in the U.S., though another 1,349 MW are under construction and 4,100 MW are in the development phase. Experts predict an expansion of CSP, with one study claiming that by 2050 CSP could provide up to 25 percent of the world’s energy demand. As of now, Spain has the most CSP plants in the world, but analysts foresee strong future growth in Africa, Mexico and the U.S.

The CSP industry formed a coalition, the Concentrating Solar Power Alliance (CSPA), in March 2012. The group aims to educate U.S. regulators, utilities and grid operators about the benefits of CSP.

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