Jamil Shariff is one of the world’s leading researchers into the ancient technology of Stirling Engines – he reveals why they are about to undergo a revival:
Local governments in the US are starting to develop their own strategies for energy-independence, or micro-grids. One little-known technology, the Stirling Engine, is being revived because it can make use of waste heat from other energy sources.
In Helena, MT, city commissioners are designing a project to cut utility bills by generating electricity at the wastewater treatment plant, the city government’s biggest energy user. Camp, Dresser & McKee Inc. are drafting plans for a Stirling engine and generator, which will use excess methane gas at the plant to generate electricity.
The technology likely will cost several hundred thousand dollars but could pay for itself in as few as five years. So what is a Stirling engine exactly, and why are they not more widely used? If the explanation below is too technical, try Building a Stirling Hot Air Engine (2 Video Set). Click “more” for the rest of the story
The engine has two compartments with a temperature difference between them, and a sealed gas inside. The gas is moved from one compartment to the other cyclically, so that it heats up and cools down and according to the laws of thermodynamics expands and contracts, respectively. As it expands it creates a greater pressure on the piston and is able to push it out, producing what is known as a power stroke. When the gas is cooled the pressure drops, which means less work needs to be done by the piston to recompress the gas on the return stroke, giving a net gain in power.
The hot heat exchanger of the engine can be a chamber in thermal contact with an external heat source, for example, a fuel burner, and the cold heat exchanger a chamber in thermal contact with an external heat sink, like air fins.
The engine boasts no exhaust gas (as the gas is sealed inside), and is most efficient when the temperature difference between the compartments is greatest. Small demonstration engines have been built which will run on a temperature difference of around 15 �C, e.g. between the palm of a hand and the surrounding air, or between room temperature and melting water ice.
The engine boasts a long history, starting from around 1699 when devices known as �air engines� have been recorded to exist, though the English inventor Sir George Cayley is credited with having devised air engines around 1807.
The engines name comes from Robert Stirling, who introduced an important innovation to air engines known as the ‘Economiser’ in 1816. It is now called the regenerator, and it stores heat from the hot portion of the engine as the air passes to the cold side, and releases heat to the cooled air as it returns to the hot side. The Economiser improved the efficiency of Stirling’s engine enough to make it commercially successful in particular applications, and has since been a component of every air engine that is called a Stirling engine.
Stirling engines have even been known to power submarines. Kokum, the Swedish shipbuilder, built at least 10 commercially successful Stirling powered submarines during the 1980s. As of 2005, they have started to carry compressed oxygen with them.
2005 has also seen the Southern California Edison announcing an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantity (20,000 units) sufficient to generate 500 megawatts of electricity. These systems, on a 4,500 acre (19 km) solar farm, will use mirrors to direct and concentrate sunlight onto the engines which will in turn drive generators.
Sunmachine in Germany is planning to commence full scale commercial production of solar-powered Stirling engines this year. A large concave mirror automatically traces the path of the sun, and the concentrated energy gets the Stirling engine up to speed at an output of 2.5 kW. They claim 80,000 hours without any maintenance whatsoever!
WhisperGen, a New Zealand firm with offices in Christchurch, has developed both AC and DC Micro Combined Heat and Power Stirling cycle engine-based systems, gas-fired central heating boilers which sell power back into the electricity grid. A 20 unit trial has started in Germany, and a further 80,000 units are being produced for the residential market in the UK.
A UK supplier of the WhisperGen, is quoting around 10K for a DC system. It’s designed to be used as an electricity generator that also produces heat, and can be used to charge batteries for off-grid locations. The WhisperGen also claims over 90% efficiency.
Of course, you could always try to make your own – check out these links…and good luck!
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