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Waste not want not, an engine for the future

DOE/Los Alamos National Laboratory : 27 May, 2001  (Technical Article)
In a step toward finding alternatives to conventional engines, scientists at the Department of Energy's Los Alamos National Laboratory have developed a remarkably simple, energy-efficient engine with no moving parts.
Pollution concerns, global warming and shrinking fossil fuel reserves have focused world attention on how engines generate electrical and mechanical power. Engines with higher efficiency help conserve fossil fuels and reduce emissions by consuming less fuel to generate an equivalent amount of power. Today most engines are internal combustion or turbines.

In the Nature paper, Los Alamos researchers Scott Backhaus and Greg Swift describe a thermoacoustic Stirling heat engine consisting of a long, baseball-bat-shaped resonator with an oval 'handle' on the lower end. Filled with compressed helium and constructed of inexpensive steel pipe, the device is highly reliable and decidedly low-tech.

By applying heat to the compressed helium contained within the system through a heat exchanger located on the 'handle,' the engine creates acoustic energy in the form of sound waves. This intense acoustic energy can be used directly in acoustically powered refrigerators or to generate electricity. The power production process is environmentally friendly and up to 30 percent efficient while typical internal combustion engines are 25 to 40 percent efficient.

According to Backhaus, 'the efficiency of conventional heat engines is limited both by the laws of thermodynamics and practical concerns over the cost of building and operating complex engines. Typically, the highest efficiencies can only be obtained from expensive engines like the large turbines used by electrical utilities. Our engine is neither mechanically complex nor expensive.'

The idea behind the engine comes, in part, from the Stirling cycle where a confined volume of gas expands at high pressure and contracts at low pressure, thereby doing work on the surrounding environment. The expansion and contraction of the gas is driven by the absorption and rejection of heat at the engine's hot and cold heat exchangers. The discovery of this principle by Robert Stirling in 19th century Scotland laid the groundwork for the conventional Stirling engine in which a fixed amount of helium is compressed in a cool chamber and then transferred to a chamber heated by an external burner. As the gas expands it drives a piston that delivers energy. As it cools it returns to the cool chamber and the cycle begins again.

According to Swift, there are many possible applications for his engine. 'For instance, small low-cost engines like this could be used in homes for cogeneration. That is, they could be used to generate electricity while at the same time producing heat to warm the home or for hot-water heating.'

Because the thermoacoustic Stirling heat engine contains no moving parts and is constructed of common materials, it requires little or no maintenance and can be manufactured inexpensively.

The future seems to hold great promise for the thermoacoustic Stirling engine. Backhaus and Swift are working on ways to use solar power to heat the engine and, in turn, generate electricity. There may even be uses for the exhaust heat from internal combustion engines to power a car's air conditioning.

The Los Alamos group is also collaborating with Cryenco of Denver on a combustion-driven thermoacoustic refrigerator that liquefies natural gas. 'Associated' natural gas that is currently flared (burned off) at remote oil wells worldwide creates pollution and greenhouse gases without producing any useful energy. Liquefying the natural gas makes it economically feasible to transport the gas to locations with existing pipelines.
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