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Concentrating Solar

October 28, 2011

There are three types of concentrating solar power (CSP) systems. They are all fundamentally different from photo-voltaic (PV) solar which converts sunlight directly to electricity.

The first of these CSP systems is referred to as a “Power Tower” or “Solar Tower”, where a field of mirrors surrounds a tower and focuses sunlight from the mirrors onto a receiver at the top of the tower. The mirrors are electronically controlled to follow the sun to focus as much sunlight as possible onto the receiver. The receiver contains a fluid that is used to produce steam or vapor, which then drives a turbine generator.


CSA Power Tower
Power Tower





The second CSP system consists of solar troughs with long rows of mirrors that focus the sunlight onto a tube that extends the length of the trough. The tube contains a fluid that is used to produce steam or a vapor that then drives a turbine generator.


CSA Trough

Trough collector






The third type uses a parabolic dish to focus the sunlight onto a receiver at the focal point of the parabolic dish. The receiver contains a fluid that can drive a generator. The original design used a Stirling engine driven by an expanding fluid, such as air.

CSA Parabolic

Parabolic Dish








Some of the projects that were to use concentrating solar (CSP) are being converted to PV arrays where PV panels are arrayed over a wide area. Costs are driving these changes.

For example, Stirling Energy Systems filed for bankruptcy on September 23rd.

Many of the CSP projects are receiving loan guarantees or grants from the Department of Energy.

Concentrating solar requires high levels of insolation (solar intensity) and can only be used in the desert southwest of the United States. The measurement for insolation is watts-per-square meter. Spain, with moderatly high levels of insolation was a major developer of CSP. The dark orange areas of the map are the areas suitable for CSP in the United States.

(Insolation should not be confused with insulation.)


USA Irradiation Map
USA Irradiation Map






 These systems have capacity factors of between 16% and 22%, which means that they produce small amounts of electricity when compared with natural gas, coal or nuclear, which have capacity factors of between 75% and 92%.

Efforts are being made to improve the ability of a CSP system to generate electricity after the sun sets by storing heat in a salt reservoir. The heat from the salt reservoir can last for around four hours, which extends the CSP plant’s ability to generate electricity.

The cost of generating electricity from CSP systems is around 30 cents per kWh without subsidies, though this figure is disputed by CSP advocates. Obviously, if CSP could compete with natural gas and coal, CSP plants would be built without subsidies. These systems also require expensive, dedicated transmission lines to bring the electricity to where it can be used.

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