How CSP Works: Thermal Solar

Concentrating solar power technologies use different mirror configurations to concentrate the sun’s light energy onto a receiver and convert it into heat. The heat can then be used to create steam to drive a turbine to produce electrical power or used as industrial process heat.

Concentrating solar power plants can integrate thermal energy storage systems to use to generate electricity during cloudy periods or for hours after sunset or before sunrise.These attributes, make concentrating solar power the most attractive renewable energy option in the world’s sunbelt regions.

CSP systems can be also combined with combined cycle power plants resulting in hybrid power plants which provide high-value, dispatchable power. They can also be integrated into existing thermal-fired power plants that use a power block like CSP; such as coal, natural gas, biofuel or geothermal plants.

CSP plants can also use fossil fuel to supplement the solar output during periods of low solar radiation. In that case, a natural gas-fired heat or a gas steam boiler/reheater is used.

There are four types of CSP technologies used, with the earliest in use being trough, and the fastest growing as of 2017 being tower. For each of these, there are various design variations or different configurations, depending on whether thermal energy storage is included, and what methods are used to store solar thermally.

Parabolic Trough Systems:

The sun’s energy is concentrated by parabolically curved, trough-shaped reflectors onto a receiver pipe running along the inside of the curved surface. The temperature of the heat transfer fluid flowing through the pipe, usually thermal oil, is increased from 293ºC to 393ºC, and the heat energy is then used to generate electricity in a conventional steam generator.

A collector field comprises many troughs in parallel rows aligned on a north-south axis. This configuration enables the single-axis troughs to track the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes.

Linear Fresnel Systems:

Another option is the approximation of the parabolic troughs by segmented mirrors according to the principle of Fresnel. Most CSP designs can incorporate thermal storage—setting aside the heat transfer fluid in its hot phase—allowing for electricity generation several hours into the evening or during cloudy days.

Power Tower Systems:

Power tower or central receiver systems utilize sun-tracking mirrors called heliostats to focus sunlight onto a receiver at the top of a tower. A heat transfer fluid heated in the receiver up to around 600ºC is used to generate steam, which, in turn, is used in a conventional turbine-generator to produce electricity.

Early power towers, such as the Solar One plant, utilized steam as the heat transfer fluid; current designs use molten salts because of superior heat transfer and energy storage capabilities. Some other designs use air as heat transfer medium because of its high temperature and its good handability.

Parabolic Dish Systems:

Parabolic dish systems consist of a parabolic-shaped point focus concentrator in the form of a dish that reflects solar radiation onto a receiver mounted at the focal point. These concentrators are mounted on a structure with a two-axis tracking system to follow the sun. The collected heat is typically utilized directly by a heat engine mounted on the receiver moving with the dish structure. Stirling and Brayton cycle engines are currently favored for power conversion.