The basic principle of using solar mirrors is to reflect sunlight onto a point to heat the point up. The more mirror area reflecting sunlight onto the point, the more heat generated at the point. Once the point is sufficiently heated, it can be transformed into electricity.
There are several solar mirror systems currently in use:
- Solar power tower
- Stirling engine
- Parabolic trough
Solar Power Tower
A solar power tower is a system that uses many, usually large, mirrors to reflect sunlight and concentrate it onto a single point high up at the top of a tower. The heat on the tower boils water to create steam that drives a turbine generator. Because this system only works when there is sunlight, a variation has been developed that uses liquid sodium, which is heated first and then heats the water. Because liquid sodium can store heat much longer than water, it allows for the heating process to be run day and night.
Like a solar power tower, electricity generation using a Stirling engine involves using mirrors to focus sunlight onto a point that heats up. The difference is that instead of heating water or a liquid to drive a turbine, the heat is used to drive a Stirling engine.
The Stirling engine was invented in 1816 by Robert Stirling as a way to rival the steam engine. Unlike an internal combustion engine, where heat is generated from the inside, the Stirling engine, like a steam engine, is considered an external combustion engine, because it gets its heat from the outside, in this case from concentrated sunlight. The Stirling engine is a closed system that uses pistons and contains a gas or liquid that in the process of being heated and cooled drives the pistons (see above article for a more complete description). The pistons turn a flywheel that connects to a generator that creates electricity. A great advantage to the Stirling engine is that it can run from any heat source, allowing for the use of fuels that would otherwise not work for internal combustion engines.
Generating electricity from a parabolic trough involves a parabolic trough shaped mirror that focuses sunlight onto a pipe that runs the length of the trough. The pipe contains a liquid, usually oil, which becomes heated. The heated oil then boils water to create steam that drives a turbine generator (see above article for a complete description).
A solar mirror can be substituted by a Fresnel lens, which acts like a magnifying glass to intensify the focus of light.