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This week I would like to look at Stirling engines and their use in generating electricity. A Stirling engine is a closed cycle, external combustion, reciprocating engine that was initially developed by the Reverend Dr Robert Stirling with later contributions from his brother James Stirling. Dr Stirling's was inspired to develop the engine as a safer alternative to the high pressure steam engines that were becoming relatively common in the early part of the 19th century. High pressure steam engines require a high pressure boiler and poor maintenance and lack of proper operation had at that time, caused several devastating accidents when the pressure vessel had failed.
 Stirling engines work on a closed system with two separate chambers, one connected to a heat source and the other to a heat sink. Both chambers contain pistons that are connected to a common flywheel so that they are 90° out of phase. This out of phase oscillation causes the combined volume of the two cylinders to cycle between a maximum and minimum limit. By applying a heat source to one cylinder and heat sink to the other you can produce a cyclic motion and convert heat energy into mechanical energy.
The four steps to the basic cycle of a stirling engine are this:
- The cycle starts with the piston in the hot cylinder near the bottom of its stroke and most of the gas in the heating cylinder. The gas now gains energy from the heating
 medium and as it expands if forces the piston in the cooling cylinder down.
- As the cycle continues the piston in the heating cylinder forces the gas into the cooling cylinder.
- The gas in the cooling cylinder is now compressed but as it has radiated energy into the cooling system the energy required to compress the gas is less than the energy that was gained when it expanded..
- Finally the cooled gas is now forced back into the heating cylinder where the cycle starts over.
There have been numerous modifications and additions made to stirling engines but the basic principle and cycle of operation remains the same.
In theory stirling engines can approach 80% efficiencies but they are limited in the amount of power they can produce by the rate that heat can be transferred to and removed from the gas that is used within the closed cycle. Ideally this gas that is used in the closed cycle should have a high specific heat capacity and good thermal conductivity while not liquefying at any temperature or pressure that occurs during the cycle. The best gasses to use in the closed cycle are hydrogen and helium however due to their small molecular size they are prone to leaking and require regular topping up. Hydrogen also poses the problem of explosion and can infiltrate metals making them brittle,
Because stirling engines are external combustion engines they can use any source of heat including combustion, nuclear, solar, geothermal, hydrothermal, or any other source that can provide a sufficient temperature difference. This makes them ideal for use with renewable energy sources and bio-fuels from rubbish dumps and the like.
Stirling engines have been around for nearly two centuries and can achieve efficiencies that other engines can't even come close to matching. However they are severely limited by the rate that energy can be transferred to and removed from the gas within the closed cycle. This has generally limited their use to static installations where they are used to either generate electricity or supply some sort of drive to a mechanical system. Even so there are numerous companies throughout the world producing stirling engine generators systems and whilst the power to weight ration is relatively poor you can purchase systems with outputs in the order of 55 Kw or more.
You can learn more about stirling engines from the following links
- Stirling Engines: Wikipedia
- Stirling Engines FAQ: American Stirling Company
- Stirling & Hot Air Engines: Robert Sier
- Stirling Engines: How Stuff Works
- New Simplified Heat Engine: eMachineShop.com
What are your thoughts on stirling engines. Do they present an as yet untapped potential to harvest renewable energy sources or have they been developed to a point that they can not be further advanced? If they are so efficient and reliable why are w not using them more frequently? Is the rate heat can be exchanged with the gas a limiting factor that is insurmountable and something that will doom the stirling engine to specialist applications?
This article is licensed under the GNU Free Documentation License and uses material from the Wikipedia article "Stirling Engines"
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Re: Future Energy Sources 1.10.1 Stirling Engines
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