Introduction of Stirling Cycle
Has the Era of External Combustion Cycle ended?
The growing popularity of micro combined heat and power, which in turn was fueled by the increasing price of fossil fuels, made engineers look for a more efficient and flexible engine. But instead of looking forward, they looked back and finally found the 200-year-old Stirling engine as the solution.
The Stirling engine is an external combustion engine which differs from steam and internal combustion engine because it recycles the same gas through a regenerator. As it produces no emission this engine is suitable for use in alternative energy systems. Furthermore, it can reach an efficiency of up to 50% — highest among all thermal engines.
The Stirling Cycle engine sounds promising, but why aren’t seeing more of it given how old it is? The number one reason is cost: a practical Stirling engine needs materials that can withstand the corrosion effects of the heat it produces. Second is the Stirling engine’s low power-to-weight ratio, which means the engine must be of a great size to produce usable power compared to the other engines.
Because Stirling engines run on heat, it can operate without the use of fossil fuels. Thus, it is found very useful in renewable energy provisions. Cleanergy has developed a CHP Stirling engine that uses biogas with very low methane content and can produce 9 kW of power per unit.
Free piston Stirling engines use linear alternators instead of the crank mechanism in kinematic Stirling engines. As a result, such engine is much reliable and has a higher lifespan. Microgen’s FPSE can run for 50,000 hours and produce 1050 watts of electrical power.
Stirling Cycle engines are also found to be much more efficient in converting solar energy to electricity compared to photovoltaic cells. The Maricopa Solar Plant in Phoenix, AZ contains 60 Stirling-engine powered “SunCatchers” that can each produce 25 kW.
Challenges and Outlooks
Scientists and engineers are still trying to use Stirling Cycle engines as a common automotive prime mover. For years, this has been hindered by the large size of radiators needed to keep the coolant temperature as low as possible. NASA’s Mod II project was the most successful among these ventures but lack of automaker companies’ interest ended further development that could lead to commercialization.
NASA’s Advanced Stirling Radioisotope Generator (ASRG) is a successor to their original SRG which heats helium to drive a piston using a radioisotope heat source. This generator could be used in space missions, particularly probes, and could last for decades of use. Space travel using these machines is expected in 2028.