Solar thermal energy is a way of producing electricity in which the Sun's energy is concentrated by mirrors or lenses to either heat a fluid-filled pipe or drive a Stirling engine. The oil which is hot is circulated into a water storage system where it is used to change water into superheated steam, that then turns a turbine to generate electricity. The heat that is produced through this process powers a Stirling engine. The Stirling engine provides electricity and mechanical energy by having the fluid from within a cylinder expand, which then drives a piston that turns a shaft.