Stirling engines are known to mankind for about 200 years but they are not so widely used today, except maybe for the pacemakers and long-distance robotic spacecraft. This will soon change as 60 Stirling engines will be used in Phoenix, Arizona, to harvest solar power which will be converted into electricity.
A conventional stirling engine operates by cyclic compression and expansion of a gas at different temperature levels, converting the heat into mechanical energy (a piston is pushed into another chamber while the gas is heated, expanded, and flows back when the gas cools down, contracting itself). The movement of the piston is able to generate mechanical work, and thus electricity via an alternator.
The demonstration site, called Maricopa Solar, will begin its activity this month and will be able to produce 1.5 MW of energy. The stirling engine units will be provided by the Arizona-based company called Stirling Energy Systems (SES). Since 1996, SES refined the technology together with their partner Sandia National Laboratories.
After almost 14 years of developments, they came up with an ingenious system: a circle of curved mirrors, resembling to a satellite dish, tracks the sun on two axes and reflects the sun’s heat onto a single focus point, the power conversion unit (PCU). The PCU makes use of 4 pistons that are able to move into their own cylinder. Inside the cylinder, the gas that expands and contracts is hydrogen.
Stirling engines are one of the best options on the market to harvest solar power as they can reach a 31% efficiency compared to just 16% for parabolic trough technology or 14-18% achieved by photovoltaic panels. Because of the high price and high maintenance costs, stirling engines do not stand a chance compared to the photovoltaic systems. Even though the future of the stirling engine may sound crazy today, we never know what tomorrow may bring.