Thermoacoustic Stirling Engines
Abstract:
Sound waves in "thermoacoustic" engines and refrigerators can replace the pistons and cranks that are typically built into such machinery. Over the past two decades, physicists and engineers have been working on a class of heat engines and compression-driven refrigerators that use no oscillating pistons, oil seals or lubricants. These so-called thermoacoustic devices take advantage of sound waves reverberating within them to convert a temperature differential into mechanical energy or mechanical energy into a temperature differential. Such machines can thus be used, for example, to generate electricity or to provide refrigeration and air conditioning. Because thermoacoustic devices perform best with inert gases as the working fluid, they do not produce the harmful environmental effects such as global warming or stratospheric ozone depletion that have been associated with the engineered refrigerants such as CFCs and HFC’s. 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.
In a step toward finding alternatives to conventional engines, scientists at the U.S Department of Energy's Los Alamos National Laboratory have developed a remarkably simple, energy-efficient engine with no moving parts, known as the “Thermoacoustic Stirling Engine”. Recent advances have boosted efficiencies to levels that rival what can be obtained from internal combustion engines, suggesting that commercial thermoacoustic devices may soon be commonplace.
Conventional Stirling Engine:
History:
On September 27, 1816, Robert Stirling applied for a patent for his “economiser” at the Chancery in Edinburgh, Scotland. By trade, Robert Stirling was actually a minister in the Church of Scotland and he continued to give services until he was eighty-six years old! But, in his spare time, he built heat engines in his home workshop. Lord Kelvin used one of the working models during some of his university classes. In his spare time, the Reverend Robert Stirling designed, built and demonstrated a rather remarkable type of hot-air engine, one that still bears his name. Unlike steam engines of the era, his invention contained no potentially explosive boiler. In 1850 the simple and elegant dynamics of the engine were first explained by Professor McQuorne Rankine. Approximately one hundred years later, the term "Stirling engines” was coined by Rolf Meijer in order to describe all types of closed cycle regenerative gas engines.
Introduction to Conventional Stirling Engines:
Stirling’s engine depended on the expansion and displacement of air inside of a cylinder that was warmed by external combustion through a heat exchanger. Stirling also conceived the idea of a regenerator (a solid with many holes running through it, which he called the “economiser”) to store thermal energy during part of the cycle and return it later. This component increased thermodynamic efficiency to impressive levels, but mechanical complexity was greater for Stirling’s engine than for the high-pressure steam and internal-combustion varieties (which do not require two heat exchangers), restricting its widespread use.
Stirling engines are unique heat engines because their theoretical efficiency is nearly equal to their theoretical maximum efficiency, known as the Carnot Cycle efficiency.
Working:
Figure 1. Stirling cycle contains four distinct steps—compression, heating, expansion and cooling—which produce a characteristic set of changes in pressure and volume (right). In a simple, two-piston Stirling engine (directly below), the compression step (1) keeps one piston fixed as the other moves inward, the heat of compression being rejected into the adjacent cold reservoir. The next step (2) produces constant-volume regenerative heating, as both pistons move simultaneously, forcing cool gas through the porous regenerator, which was heated during the final step of the last cycle. Next (step 3), heat from the hot reservoir causes thermal expansion of the gas, which forces the adjacent piston to move outward. Finally (step 4), both pistons move together to create a constant-volume regenerative cooling of the heated gas. The changes in pressure and gas velocity within the regenerator of such a Stirling engine mimic the relationship seen in a traveling acoustic wave, where pressure and gas velocity move up and down in phase (bottom pair of panels).
Stirling engines are powered by the expansion of a gas when heated, followed by the compression of the gas when cooled. The Stirling engine contains a fixed amount of gas which is transferred back and forth between a "cold" end (often room temperature) and a "hot" end (often heated by a kerosene or alcohol burner). The "displacer piston" moves the gas between the two ends and the "power piston" changes the internal volume as the gas expands and contracts. Stirling engines are being studied at NASA for use in powering space vehicles with solar energy! Versatile in performance, Stirling engines can be reversed to make refrigerators, cryocoolers, or heat pumps. So efficient is the engine, it can chill to cryogenic temperatures.
What is Thermoacoustics?
Thermoacoustics is the study of the thermoacoustic effect and the attempt to harness the effect as a useful heat engine. A thermoacoustic prime mover uses heat to create sound. A thermoacoustic refrigerator uses the thermoacoustic effect to move heat with sound
What is the thermoacoustic effect?
Simply put, thermoacoustic effect is the conversion of heat energy to sound energy or vice versa. Utilizing the Thermoacoustic effect, engines & refrigerators can be developed that use heat as an energy source and have no moving parts!
To explain the thermoacoustic effect, consider a sound wave generated through a loud speaker in a tube. If we have a stack of plates in the tube and force one end to be hot and the other cold and put that in a tube, we can create a very loud sound. Thus by using waste heat (say from a fire) we could create sound in a tube and use that sound to cool off another part of the tube (say where a beer can is sitting). We have now created a refrigerator that can cool a beer at one end by putting the other end in the campfire! A device that creates sound from heat is called a thermoacoustic heat engine.
Even more spectacular is the fact that it can work in reverse. To explain the thermoacoustic effect, consider a high amplitude sound wave in a tube. As the sound wave travels back and forth in the tube, the gas compresses and expands (that's what a sound wave is). When the gas compresses it heats up and when it expands it cools off. The gas also moves back and forth, stopping to reverse direction at the time when the gas is maximally compressed (hot) or expanded (cool).
Now, put a plate of material in the tube at the same temperature as the gas before the sound wave is started. The sound wave compresses and heats the gas. As the gas slows to turn around and expand, the gas close to the plate gives up heat to the plate. The gas cools slightly and the plate below the hot gas warms slightly. The gas then moves, expands, and cools off,
becoming colder than the plate. As the gas slows to turn around and expand, the cool gas takes heat from the plate, heating slightly and leaving the plate below the gas cooler than it was.
So, what has happened is one part of the plate gets cooler, and one part gets hotter. If we stack up many plates atop each other (making sure to leave space for the sound to go through), place the plates of an optimal length in the optimal area of the tube and attach heat exchangers to get heat in and out of the ends of the plates, we have created a useful refrigerator.