Because sterling engines operate directly of of a temperature difference they can run using a variety of different energy sources. Virtually any heat source can be used to power a sterling engine. Sources of cold temperatures can even be used to drive sterling engines (liquid nitrogen perhaps).
With the recent push toward alternative energy Sterling engines are finding more uses. They can be made to run off of solar, geothermal or other biomass fuels. Solar powered sterling engines can archive better efficiencies then photo-voltaic cells. Sun light is focused, using mirrors, onto the hot side of the engine and the output shaft is connected to an electric generator. Because the combustion is external to the engine the seals of the engine are not exposed to the fuel so corrosive fuels can be used in sterling engines with better success then with internal combustion engines.
Their have been efforts to use sterling engines to power automobiles. This may be impractical due to the difficulty of varying the power output of sterling engines. Due to the fact that the engine it self does not need any oxygen they may be desirable in space or under watter.
Sterling engines produce less vibration then internal combustion engines and can be more fuel efficient. When weight is a large concern like in an airplane then reducing vibration can reduce the flywheel size and engine mount their by reducing weight. The difficulty in varying the output power of sterling engines is not a huge concern for airplanes because for most of their flight they operate at a constant power. Power output can be varied in several ways two of which are wasting power and changing temperature. Changing the engine temperature is the most fuel efficient way to control the power output but it takes a large amount of time to take effect. Wasting power can be accomplished relatively easily and takes effect quickly. In an airplane the airplane could be throttled by wasting power for takeoff and landings where quick power transitions are necessary and during the rest of the flight could be set by setting engine temperature when fast transitions are less critical and fuel efficiency is important.
NASA has developed but not yet flight tested a sterling engine used to generate electric power that uses heat generated by radioactive decay. It would provide electric power to unmanned rovers that would go to mars or into deep space. The sterling engine design is more efficient by a factor of four.
When the shaft of a Sterling engine is rotated the result is heat moving from the cold side to the hot side which is what a refrigerator does. Sterling engines have been used to produce liquid nitrogen in this manor. If the output shafts of two sterling engines are connected together then one can drive the other and the result is a heat driven heat pump. The end result is a device which can use a heat source for air conditioning. A twice double acting sterling Engine is described here it ueses several Sterling engines together to use heat to cool.
