Concentrating solar power (CSP) is thermal solar power that uses a means of magnifying or concentrating the effective radiation from the sun onto a receiving device that collects the power so that it can be used directly as thermal energy or used to generate electricity. It is generally economical in sizes of 10 MW and larger, from community sized systems up through major grid connected systems. CSP is the most developed of the solar technologies and is on the verge of being competitive with conventional power plants. CSP technologies relatively low cost and ability to deliver power during periods of peak demand mean that it has the potential to be a major contributor to our electrical power needs.
The solar resource for generating power from concentrating solar power systems is plentiful. For instance, enough electric power for the entire country could be generated by covering about 9 percent of Nevada—a plot of land 100 miles on a side—with parabolic trough systems. It is not too much of a stretch to foresee that future power needs for much of the southwest; Arizona, California, Nevada, New Mexico and Texas could be met with concentrating solar power.
Electrical Costs from CSP facilities is coming down at a reasonable price. All of the technologies forecast that their costs could be less than $3.00 per watt by 2015. This corresponds to a cost of $0.10-$0.15 per kWh. Some current incentives reduce the costs to the point that costs are competitive with conventional power plants.
Currently solar toughs are believed to be the lowest cost, with some large projects being initiated. Solar dish costs have not been publicized, but must be on the same order as solar troughs or recent orders for large systems would not have been made. These orders, when completed, will enable solar dishes to overtake solar troughs as having the largest installed capacity of all grid connected solar systems. A 10-15 MW solar tower is under development in Spain. Twenty five MW to 200 MW solar chimney projects are under development in Australia and the US. I am not aware of any actual large orders for concentrating solar photovoltaics, plants of up to 100 kW have been built. Amonix claims that their costs for concentrating solar photovoltaic are approaching competitive values.
CSP is divided into four distinct technologies:
- Dish/Engine Systems - The "dish", sometimes referred to as a concentrator, is a large, mirror covered dish shaped structure which collects the solar energy and focuses it on a receiver/engine suspended above the dish. The structure is rotated on both the horizontal and vertical axis to track the sun continuously. The receiver/engine absorbs the solar energy and heats a gas which is used to drive an engine/generator that produces the electricity. The most common form of heat engine is a Stirling engine.
- Solar Troughs - Sunlight is concentrated by parabolically curved, trough-shaped reflectors onto a receiver pipe running through the focus of the curved surface. This energy heats oil flowing through the pipe, and the hot oil is used to generate electricity using a conventional steam generator. The troughs are rotated, up and down, about their horizontal axis to track the sun for maximum efficiency.
- Solar Towers - A large field of sun-tracking mirrors (heliostats) reflect solar energy and focus it onto a receiver at the top of a tower. A heat transfer fluid is heated in the receiver and used to generate steam which is used in a conventional turbine-generator to produce electricity.
Solar Chimney Systems (or power towers) do not use mirrors or reflectors, but may be confused with solar towers because of their appearance and are usually discussed together with solar towers. In solar chimney systems heat is collected in a large glazed solar collector containing thermal storage media. Ambient air, drawn in from the outer edge of the collector, is heated as it flows over the thermal storage media and then, by convention, it flows up through a very tall chimney. Wind turbines in the chimney, driven by the air flow, drive generators producing electricity.
- Concentrating Solar Photovoltaics - Flat plastic fresnel lens, between the sun and a photovoltaic cell, focus and concentrate the sunlight many times (up to 500 times) onto a small cell area thus reducing the cell area required to an amount approximately equal to the concentration ratio. Small quantities of very efficient, expensive, multijunction cells are use to convert the sunlight into electricity. Two axis tracking is required to maintain maximum efficiency. Two suppliers, Sunball and Sunflower are developing smaller modules of this type for individual dwellings, apartments or small industrial applications.