Concentrating solar power plant. Array of Stirling engine heliostats (mirrors with sun-tracking motion) at the concentrating solar power plant in Maricopa, Arizona, USA. These heliostats reflect sunlight and concentrate it on to a small area. The concentrated solar energy is then used to produce electrical power. Here each mirror is equipped with a Sterling engine driven by a hydrogen gas filled piston. The sun's rays are focused onto the piston, heating the gas and driving the engine to generate electricity.