Glass blowers have known for hundreds of years that a heated vessel can generate sound. But it is really only in the last 20 years that a few dedicated scientists have begun to realise the potential of thermoacoustics — the relationship between heat and sound and the conversion of one into the other.
Physicist Steven Garrett of Pennsylvania State University is one of the pioneers in this relatively new field, and he believes that efficient, environmentally friendly, thermoacoustic engines could provide a viable alternative to traditional refrigeration systems that rely on ozone-depleting chlorofluorocarbons (CFCs) and their substitutes.
As early as 1974 it was suggested that the widespread use of the highly stable CFCs would lead eventually to the depletion of the ozone layer that protects the earth from the sun’s harmful ultraviolet (UV) radiation. UV radiation can cause skin cancer and cataracts, and it interferes with the human immune system. The animal kingdom is also at risk. UV light has been shown to cause cataracts in birds and kill plankton that live near the ocean’s surface.
Subsequent scientific studies confirmed this danger, identified a hole in the earth’s ozone layer above the Antarctic, and eventually led to the adoption of the Montreal Protocols in 1989. The Montreal Protocols called for an international ban on all CFC production by the year 2000, a ban that was subsequently brought forward in some countries to 1995.
In 1997, the Kyoto Protocols on Global Warming went one step further by calling for a ban on the production of the chemical alternatives to CFCs known as HCFCs and HFCs, although these agreements have yet to be ratified by many countries, including the United States.
"When the link between CFCs and ozone depletion was discovered, everyone scrambled around to find alternative chemicals to use in the same technology," Garrett explains. "Ultimately, of course, the alternatives are also harmful. We need something completely different."
Slow progress in the search for alternative refrigerants and concern for the environment motivated Garrett to examine more closely the discovery in the early 1980s that it was possible to extract heat from a gas by producing a high-amplitude sound wave within the gas. In fact, Garrett was one of the first scientists to demonstrate that if temperature differences could produce sound — as glass blowers had been doing for centuries — then sound could produce temperature differences.
Space and Sea Trials
Using a high-intensity loudspeaker, thermoacoustic engines work by harnessing the oscillating pressure created by an acoustic standing wave to compress and expand gas in a sealed tube. The sound wave forces the gas molecules in the tube into a dense, compressed mass at the far end of the tube, heating them up. As they collide with a plate in the tube — the thermoacoustic stack — the plate absorbs the heat, which is then conducted away through a heat exchanger.
"Over the past two decades, substantial progress has been made in thermoacoustic engine and refrigeration development," says Garrett who began his research in this area while working at the US Naval Postgraduate School (NPS) in Monterey, California.
The outcome of his research was a project that earned him a Rolex Award in 1993. The project aimed to produce commercial refrigerators and air conditioners using high-intensity sound waves — up to 10,000 times louder than a rock-and-roll concert — to pump heat in a very efficient engine with only one moving part.
Garrett had already proven the capability of a thermoacoustic refrigerator when he built the first battery-operated model which was sent into orbit with the space shuttle "Discovery" in January 1992. The unit was designed to demonstrate the feasibility of using thermoacoustic refrigeration to provide highly reliable, low-vibration cooling for satellite opto-electronics, and it produced a temperature span of 80 degrees Celsius.
A second device, built by Tom Hofler, also at NPS, produced a temperature span of 118 degrees Celsius — enough to turn boiling water into ice despite being constructed of basic components such as a commercial loudspeaker, polyester film sheets and nylon fishing line.
Subsequently, Garrett’s team at NPS built a 400-watt device called the Shipboard Electronic ThermoAcoustic Chiller (SETAC), which was used on a US Navy vessel in a successful experiment to measure thermoacoustic cooling of a shipboard radar in April 1995. The SETAC unit demonstrated a cooling capacity 100 times greater than the cooler designed for the space shuttle.
Garrett’s research group moved to Penn State University in 1995. They are now in the third year of a four-year, US$3 million project to develop another shipboard thermoacoustic refrigeration unit with a cooling capacity of 10 kilowatts, which could be used to cool an entire office or a small business.
This project is known as TRITON — since 10 kilowatts is the equivalent to the cooling capacity of three tons of ice per day — and it has 25 times the capacity of SETAC.
Laboratory tests of TRITON’s performance are due to be completed by the end of 2000 and, if successful, will be followed by sea trials the year after. As part of the project, Garrett’s team has developed a new, high-power, high-efficiency loudspeaker that can deliver in excess of one kilowatt of acoustic power with an electroacoustic conversion efficiency of 85 per cent.
Garrett received the Rolex Award to help fund his efforts to demonstrate that this technology could be successfully applied to domestic refrigerator design and to begin research on a commercially viable unit incorporating efficiency improvements while continuing to use low-cost components. How much progress has he made towards this objective?
"I am still working hard to move the technology out of the laboratory and into commercial acceptance," says Garrett. "We are making quite a bit of progress in gaining support and hopefully acceptance within the US Navy. If things continue to go well, I hope that commercial manufacturers will start to show some real effort instead of just interest. I suspect that interest will be tied to the impact of the Kyoto Protocols on Global Warming gases."
Indeed, although commercial interest in the potential of thermoacoustics has been slow to develop, Garrett says Penn State is currently in negotiations with a large commercial manufacturer of air conditioners, based upon the TRITON project. He expects the first commercial thermoacoustic refrigerators to begin to appear in shops within the next five or ten years.
"My optimism about commercialization has increased, but I am always amazed by how long it takes," he says. Nevertheless, his enthusiasm was boosted again recently by improvements in the efficiency of the technology that have been achieved by another group of scientists under the direction of his colleague G.W. Swift, who is based at the Los Alamos National Laboratory in New Mexico.
Swift’s team has developed a highly efficient, helium-based thermoacoustic engine with a thermal efficiency (the ratio of net work produced to fuel energy used) of 30 per cent, comparable with that of a car engine.
"Efficiency is one of the major obstacles to the widespread adoption of this thermoacoustic technology," says Garrett. "Our efficiency is still about 30 to 40 per cent less than that of a mature, fully engineered vapour compression unit of comparable cooling capacity, but we are gnawing away at that gap. Of course, the conventional technology is a moving target — they are also trying to improve their efficiency."
Part of the inefficiency of current thermoacoustic refrigerators is due to technical immaturity. With time, improvements in heat exchangers and other parts of the system should help thermoacoustic devices become more commercially feasible. It is also likely that the ability to use proportional controls will play a role in the efficiency area, and could prove to be a key advantage.
Conventional refrigerators or air conditioners use binary controls and are, necessarily, either “on”’ or “off”. Proportional control is the ability to "lower the volume" and, like a dimmer on a lamp switch, means that the output can be varied gradually depending on conditions. Since the on-off cycle is limited, the system over-cools or over-heats, wasting energy. Proportional control also avoids energy losses due to the large start-up power surges in conventional compressor systems.
An Emerging Field
Other barriers to commercial adoption identified by Garrett include money, a non-existent supplier base, the inertia of industry and what he describes as the "talent bottleneck". By that he means that there are still relatively few scientists working on thermoacoustics. At the time that he applied for a Rolex Award in the early 1990s, Garrett was one of only three scientists working in this field.
Garrett says the growth in the study of thermoacoustics has been "slow but steady". At a recent seminar held at the National Center for Physical Acoustics at the University of Mississippi, he noted that in 1985 Tom Hofler was the only graduate student working on thermoacoustics.
"Now there are dozens of faculty, graduate students and about 10 postdoctoral fellows working on thermoacoustics at several universities in the United States. There are also groups of students and faculty of varying sizes in Japan, the Netherlands, France and Switzerland and possibly others of whom I am not aware." In 1998, the first textbook on thermoacoustics was published by Prof. A. Tominaga, a Japanese scientist.
The talent bottleneck means that advances in the technology take longer than they might in other fields and that there are not enough knowledgeable engineers to design thermoacoustic devices for the large number of potential applications. Still, the growing interest is encouraging and will surely lead to important new developments.
Reh-Lin Chen, one of the graduate students working under Garrett’s supervision, is developing a solar-powered icemaker as part of his Ph.D thesis. Chen’s calculations suggest that his device can produce ice from concentrated sunlight with four times the efficiency of solar cells and thermoelectric (solid state) cooling units.
Because the one-metre diameter Fresnel lens used to concentrate the sun’s rays is far less expensive than solar cells, and the sound-generating thermoacoustic stack is much cheaper than equivalent thermoelectric units, the new device is expected to be affordable for use in applications in developing countries. Such a unit could, for example, preserve medical supplies in African or South American equatorial areas lacking electricity.
The system operates at a pressure low enough to be produced by a bicycle pump and, because it is heat driven, it has no moving parts, no loudspeaker and therefore should require no maintenance.
"We have high hopes," Garrett says of Chen’s work, but he could easily be speaking in general terms about the future of thermoacoustics. "Industry may be slow to change," he says, "and it certainly takes time to train experts in this area, but the potential for sound to provide solutions is enormous."
Published in 2000