run silent,
run long

The Stirling engine is earning a little more respect these days.

by Harry Hutchinson, Executive Editor

A diesel-powered submarine is a bit of a misnomer. The diesel power supply can run only at the surface, where it has air to breathe. In any event, a diesel would raise too much of a racket to be of use for a sub in hiding. Submerged, the submarine relies on a battery pack with enough charge for about a day's worth of operation.

Nuclear power plants are one way around this limitation, but Swedish submarine builder Kockums AB has found another. It puts Stirling engine power plants into its boats, where they generate electricity for propulsion and for other needs onboard. The company calls its design "air-independent" because, although it is heated by burning diesel fuel, it carries its own oxygen supply. According to Lars Larsson, manager of the Stirling department at Kockums in Malmö, the system allows a boat to stay submerged for weeks.

And, unlike the diesel, the Stirling engine is quiet enough for the submariner's stealthy work.

The Stirling engine is a peculiar thing, probably because we see so few of them. Conceived as a safer alternative to steam, the Stirling engine never quite caught on. Overshadowed these days not by steam but by internal combustion rivals, the Stirling cycle has enjoyed some commercial applications. It drives toys—American Stirling Co. has one that runs on the heat of your hand. Stirling engines are used in cryocoolers and for specialized power generation. But events in the past several months suggest that the Stirling engine may be raising its profile.

Developers of Stirling engines say that their machines make very efficient use of energy and will outlast other types of engines doing the same work. The engine is quiet because, unlike internal combustion, the Stirling cycle needs no explosion of fuel to drive a piston. It needs only steady heat. It makes no difference if that heat comes from a petrochemical fire, a nuclear reaction, or even sunshine.

When, in 1816, the Rev. Dr. Robert Stirling patented the engine now named for him, he was looking out for the welfare of his hardworking parishioners. He intended it as a safe alternative to the steam boiler which, when poorly constructed, can rupture and kill those standing nearby.

Gas, sealed in a cylinder, is alternately heated and cooled to drive a piston. As with the steam engine, the heat source is outside the cylinder, but interior pressures are nowhere near the level needed to keep a steam engine going.

The coming of Bessemer steel did much to improve the performance and safety of steam boilers, so Stirling's idea languished as steam power surged ahead.

An experiment at Sandia aims to get several solar-heated Stirling engines working together, so one day they may be able to cooperate for utility-scale power generation.

It didn't help that Stirling engines require precise machining to build, nor that compared to a steam engine of similar size, they produce less power. And as the world made the transition from boilers to internal combustion, the long warmup time for the Stirling engine was a decided disadvantage.

Once the engine gets going, however, it can, in theory, run reliably for a long time. To find out just how long, ask the folks at Stirling Technology Co. in Kennewick, Wash.

The company has a number of projects in hand to generate electricity on Earth and in space. One is a generator being prepared for NASA. It combines high efficiency and longevity, properties critical for a deep-space probe. A test unit with a 10 W output last August passed an operating landmark, more than 87,600 hours of continuous service, or 10 years' running with no maintenance or decline in performance.

According to Maury White, Stirling Technology's chief technology officer and a founder of the company, the test system is powered by an electrical heat source, but is designed to be heated by a radioisotope in space. It was the subject of an article published in Mechanical Engineering in February 1996 under the headline "Engines That Never Wear Out." Never is a long time, and so is 10 years without a break.

One of the features Stirling Technology stresses about its design is that no moving parts rub together. The piston has about a 25-micrometer clearance in the cylinder. It is supported by flexural bearings, essentially thin metal disks with spiral kerfs cut through them. They are rigid from side to side, to keep the piston centered. They flex with the oscillation of the piston.

The piston drives a linear alternator, so there are no linkages to convert linear movement to rotary, White said. The engine and generator are hermetically sealed in a single case. According to White, the hermetic sealing is one reason that helium is the working gas instead of hydrogen. Helium costs "about a point in efficiency," he said, but suits the design better than hydrogen. According to White, "Hydrogen will inevitably permeate through hot metal and be lost to the system so that is not feasible." He added that helium avoids hydrogen embrittlement of the heater head and allays concerns over the safety and handling of hydrogen.

Stirling Technology says it is currently gearing up to produce 10 W and 55 W generator sets and expects to ship a total of 40 systems to customers in the next three years. It has a $23 million space contract through Lockheed Martin.

A Stirling engine generator set following this design passed a landmark last summer: 87,600 hours, or 10 years, of operation without maintenance or a decline in performance, its manufacturer said.

Another of the company's current projects involves a federal Small Business Innovative Research Program grant to supply a diesel-fueled mobile generator to the Army. According to Stirling Technology, it will deliver a prototype to the U.S. Army Soldier Systems Center in Natick, Mass. Called a micro-cogeneration system, it will provide hot water, which can be used in a field kitchen, and 1 kW of electricity.

The prototype for the Army is an adaptation of one developed for Stirling Technology's licensee, Enatec Micro-Cogen BV in Lichtenvoorde, the Netherlands. Enatec is a joint venture of three Dutch entities—Eneco Holding, a utility; ATAG Verwarming, a manufacturer of heating systems, and Energieonderzoek Centrum Nederland, a research institute. The Enatec system is described primarily as a boiler, which uses a portion of the heat energy to generate electricity. Enatec is offering systems for testing by prospective buyers in Europe and has about 10 in field trials at private homes.

A separate company, Stirling Energy Systems in Phoenix, Ariz., is expanding a program with Sandia National Laboratories that uses a dish collector to concentrate sunlight as the heat source to run a Stirling generator set. The expansion will add five solar thermal systems to one already operating at Kirtland Air Force Base near Albuquerque, N.M. The new ones are expected to begin operating soon.

It sounds strange when you hear for the first time that Stirling Energy Systems licensed the basic technology for the solar design from Kockums, the submarine company. But Kockums developed Stirling engines for underwater use, so it isn't such a stretch that the company should adapt the Stirling idea to other extreme environments, such as the desert.

Bob Liden, president of Stirling Energy Systems, said that each Stirling generator set can turn out 25 kW at peak power, from a dish of about 90 square meters—about the area inside an NBA three-point line. A surface of that size would be catching about 90,000 W of sun power on a bright day in New Mexico, for a conversion rate approaching 30 percent.

For comparison, most commercially available photovoltaic panels, which convert sunlight directly into electricity, work at efficiencies of less than 15 percent. Sarah Kurtz, principal scientist at the National Renewable Energy Laboratory in Golden, Colo., said there have been demonstrations of PV systems of 1 kW or so that have used concentrators for sunlight and have topped 25 percent conversion efficiency.

Stirling Energy's dish concentrates 90 m2 of sunlight to an area about 20 cm in diameter, to heat a four-piston Stirling engine that drives a rotary generator.

According to Sandia, the dish has 82 mirrors from Paneltec of Lafayette, Colo., laminated onto an aluminum honeycomb structure. The honeycomb follows a design invented by Paneltec and two Sandia researchers, Rich Diver and Jim Grossman.

The six generator sets will be able to generate as much as 150 kW, which will be used by the Sandia lab. Chuck Andraka, project leader at Sandia, said the expanded installation will permit "system-level studies and development leading toward large-scale deployment."

Liden said the company is working to bring costs down and to create a final design that will be efficient to manufacture. Prototype systems being installed at Kirtland will cost about $150,000 each. Liden said the company expects a production model to be closer to $50,000. At that level, the price comes out to $2 a watt.

STM Power Inc., a company based in Ann Arbor, Mich., is providing the Stirling-cycle generator sets for a commercial installation to be fueled by waste at a vegetable-oil processing plant in New Jersey.

When a 90m2 dish of mirrors concentrates sunlight to an area 20 cm across, the heat runs a Stirling generator set with a peak power rating of 25 kW. Sandia is testing ways to coordinate six of these systems.

Northern Power Systems of Waitsfield, Vt., is designing the project at a site owned by Aarhus United USA. STM is providing seven 55 kW generator sets that will supply 65 percent of the electricity needs at the Aarhus plant in Port Newark. The Stirling regenerators, where the working gas is cooled inside the engine, are expected to provide 2.7 million Btu of hot water per hour.

Frank Miller, vice president of operations at Aarhus, said the fuel that will heat the engines is the light oil composed of unattached fatty acids that are stripped off during the deodorizing of edible vegetable oils. The distillate is usually stored in tanks and brings a small price when it is sold.

Aarhus hopes to recoup as much as 30 percent of the $1.7 million cost of the project under the New Jersey Clean Energy Program, a state fund set up to promote the installation of renewable energy systems in homes and businesses.

The distillate that Aarhus will use as fuel doesn't lend itself to burning in conventional engines because it is highly corrosive. The Aarhus project is practical because combustion products don't touch moving parts of the Stirling engine.

STM recommends preventive maintenance, including lube and fluid check, every 1,000 hours and a longer procedure every 10,000 hours. Mike Alexandrian, an application engineer at STM, said the major maintenance consists of replacing piston rings, rod seal cartridges, and other components in a procedure that takes about 16 hours. It can be done without removing the engine from the package.

Alexandrian said STM has 25 firm orders, which were due to be shipped by the beginning of this month. All of them are designed to run on landfill or biodigester gas.

The Stirling engine is a technology that has neither come into its own in 190 years nor faded away. It is generally considered the province of hobbyists, niche companies, and howstuffworks.com. There are also companies working to create mass-market products for homes and for grid-quality commercial power generation.

The Stirling Engine Society USA has a slogan that speaks to a world in which increasing numbers of people are becoming concerned over pollution and waste. It calls the Stirling engine "extra green, extra quiet, extraordinary."

That leaves out "peculiar," but then, if the Stirling engine can pick up a little more visibility, it won't seem so peculiar anymore.

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© 2005 by The American Society of Mechanical Engineers