Parabolic Troughs: In parabolic trough systems, curved, trough-like collectors reflect and concentrate sunlight onto a receiver, a pipe running along the inside of the curved surface of the trough. The concentrated solar energy heats a heat transfer fluid (usually oil) flowing through the pipe; this heated fluid is then used to run a conventional steam generator for electricity production.

If we install numerous troughs in parallel rows, we have what's known as a collector field. The field is typically aligned on a north-south axis, which allows the troughs to track the sun from east to west during the day. This ensures that the sunlight is continuously focused on the receiver pipes and that electrical output is highest in the summer months when it is needed most. Trough systems with thermal storage capabilities can also store thermal energy for electricity generation later in the evening. The largest trough systems operating today generate about 80 megawatts of electricity (for comparison, a 5- to 15-kilowatt system can provide most of the power needs of an average U.S. home), however, it may be possible to build plants as large as 400 megawatts which can greatly reduce the cost of delivered energy.

Currently, all parabolic trough plants are hybrids. This means that they include a fossil fuel system to supplement the solar energy at night or when it's cloudy. The fossil fuel part of a hybrid system runs on natural gas.

Power towers: A power tower system is made up of many large, sun-tracking mirrors (heliostats) that focus sunlight on a receiver at the top of a tower. The sunlight heats up a heat transfer fluid in the receiver, which then is used to generate steam. The steam, in turn, is used in a turbine-generator to produce electricity.

In early power towers (such as the Solar One plant), steam was the heat transfer fluid. Current designs (including Solar Two, pictured) made use of molten nitrate salt because of its superior heat transfer and energy storage capabilities. Individual commercial plants can be small or large enough to produce anywhere from 50 to 200 megawatts of electricity.

Dish-engine systems: A solar dish-engine system is an electric generator that "burns" sunlight instead of gas or coal to produce electricity. The major parts of the system are the solar concentrator and the power conversion unit.

The dish, or solar concentrator, is the primary solar component. It collects the sun's direct-beam energy and concentrates it on a receiver located at the focal point of the dish. The reflective surface of the concentrator is made of glass mirrors, which reflect approximately 92% of the sunlight that strikes them.

The power conversion unit includes the thermal receiver and the engine/generator. The thermal receiver the interface between the dish and the engine/generator absorbs the concentrated solar beam, converts it to heat, and transfers the heat to the engine/generator. A thermal receiver can be a bank of tubes with a gas, usually hydrogen or helium, which is the heat transfer medium. Thermal receivers can also be heat pipes in which an intermediate fluid boils and condenses to transfer heat to the engine. The engine/generator uses heat from the thermal receiver to produce electricity. The most common type of heat engine in dish-engine systems is the Stirling engine, which uses heat from an external source (like the sun) to create mechanical power that in turn drives a generator to produce electricity. The Solar Energy Technology Program is investigating concentrating PV receivers that use high-efficiency PV cells to generate electricity-the advantage being the elimination of moving parts and potential for very high efficiencies and low cost.