Researcher makes adjustments to a Stirling Energy Systems solar dish-engine system

by Gabe Grimes / March 29, 2010

“In honor of Popular Science putting its entire 137-year archive online for free perusal, this column is about the silent Stirling engine, as seen in the May 1960 issue. According to the article, the engine is started by taking a match to the burner. It’s a “hot-air engine, updated by modern research and a gee-whiz con-rod system like none you ever saw.” The article continues to list the engine’s many pluses: It has better efficiency than gas or diesel engines; it has no ignition, spark plugs, carburetor, exhaust, and is vibration free. The article talks about what improvements have been made to Stirling’s original engine throughout the years, including Ericsson’s caloric engine used in the Monitor, the Philips company’s struggles with Nazi occupation and efficiency (they used hydrogen instead of air, since it held more BTUs per pound and, again quoting the article, “flowed like a greased eel”) and finally the modern improvements (like better materials and regenerators with 98 percent efficiency). The article also mentions how great this thing could be if it were used as a solar-powered engine for outer-space satellites. Even when the satellite is in the Earth’s shadow, the engine will still run on heat stored in the regenerators and walls! The article finishes with a mention of this engine being used in outboard motors for boats, which would allow for a silent boating experience. Harvey Welsh, section chief of the Allison Division at General Motors, promises that the silent outboard should be along in three or four years, with a silent lawnmower engine following (once technology can figure out how to make an inexpensive one).”

Solar Patent King Boeing Teams Up With Stirling Energy Systems
by Katie Fehrenbacher / Mar.30, 2010

A little known fact about Boeing: It’s got more solar patents than anyone else in the U.S. (14 solar thermal patents since 2002 as of January, according to cleantech patent tracking law firm Heslin Rothenberg Farley & Mesiti). So sooner or later the defense contractor would want to commercialize ‘em. This morning Boeing says it has teamed up with solar thermal company Stirling Energy Systems to develop Boeing’s high-concentration photovoltaic (HCPV) solar power technology. High-concentration solar PV technology uses mirrors and lenses to concentrate sunlight onto a high-efficiency photovoltaic cell. The technology provides more power than standard photovoltaic solar panels, and tends to be smaller in scale than the massive solar thermal plants (that don’t use PV) that are being built in the world’s deserts these days.

Boeing started developing the concentrating solar PV tech (dubbed XR700) back in 2007 in conjunction with the U.S. Department of Energy’s Solar Energy Technologies Program. Stirling Energy Systems, which has the exclusive worldwide license to develop, deploy and commercialize the tech, will now use solar cells from Boeing’s subsidiary Spectrolab for the technology, and plans to deploy the systems commercially by 2012. Stirling already makes utility-scale solar thermal technology based on a stirling engine called the “SunCatcher” solar dish. Stirling engines were invented centuries ago, and can be more efficient and quieter than internal combustion engines and use a closed system of gases to generate power. Most solar thermal technologies, by contrast, concentrate the sun’s rays onto liquid, which powers a turbine.

Earlier this year Stirling, which was founded in 1996, and is based in Phoenix, Ariz., inaugurated its first project that uses the SunCatcher. Called Maricopa Solar, the 1.5MW solar project in Peoria, Ariz. (Maricopa County) uses 60 SunCatchers and sells clean power to local Arizona utility Salt River Project. Later this year Stirling says it will start construction of two much larger solar plants in California — a potentially 900MW plant in Imperial County, Calif. for San Diego Gas & Electric, and a 850MW solar plant in San Bernardino County, Calif. for Southern California Edison. Stirling Energy has raised $100 million from Dublin, Ireland’s NTR, which in the process took a 52-percent stake in the company. Stirling isn’t the only company turning to stirling engines for solar power. One example is Infinia, which is backed by a gaggle of A-list Silicon Valley-ers, including Bill Gross’ Idealab and Paul Allen’s Vulcan Capital.

The CEO of Stirling Energy Systems, Steve Cowman, said in a release this morning that adding on Boeing’s concentrating solar PV tech will help the company “[E]xtend our reach into the solar market for future technology deployments with a product that shares many of the SunCatcher’s key differentiating features – scalability, low water use and high-efficiency.”



Stirling Energy to Kick Off Its First Plant
by Katie Fehrenbacher / Jan. 14, 2010

As solar thermal firms like eSolar have started flipping switches on their first projects in U.S. deserts, I’ve been eagerly waiting the stirling engine solar folks to officially enter the game. Looks like we’re getting one project at the end of this month: Stirling Energy Systems and its developer partner Tessera Solar are planning an invite-only kick-off event for the media on Jan. 22 to inaugurate the first project to use Stirling Energy System’s “SunCatcher” solar dish. Stirling Energy and Tessera have built Maricopa Solar, a 1.5MW solar project in Peoria, Ariz. (Maricopa County) that will use 60 SunCatchers to sell clean power to local Arizona utility Salt River Project. The ribbon-cutting event is supposed to feature Arizona Gov. Jan Brewer and Department of Energy Solar Program Manager John Lushetsky.

This is just the first small project from the Stirling Energy crew, and the company says that later this year it will start construction of its two much larger solar plants in California — a potentially 900MW plant in Imperial County, Calif. for San Diego Gas & Electric, and a 850MW solar plant in San Bernardino County, Calif. for Southern California Edison. Founded in 1996, Phoenix, Ariz.-based Stirling Energy has developed a 25 KW electric solar dish that focuses the sun rays directly onto a stirling engine. Stirling engines, which were invented centuries ago, can be more efficient and quieter than internal combustion engines and use a closed system of gases to generate power. Most solar thermal technologies, by contrast, concentrate the sun’s rays onto liquid, which powers a turbine.

Stirling isn’t the only company turning to stirling engines for solar power. One example is Infinia, which is backed by a gaggle of A-list Silicon Valley-ers, including Bill Gross’ Idealab and Paul Allen’s Vulcan Capital. Infinia’s technology is similar to Stirling’s and uses mirrored concentrator dishes to track the sun and reflect its rays into a highly efficient Stirling heat engine. Stirling Energy has raised $100 million from Dublin, Ireland’s NTR, which in the process took a 52-percent stake in the company, according to the Cleantech Group.

‘We show a Stirling Engine where fuel is a piece of ice.’

Solar powered Hydrogen assisted sterling engine.

Solar Stirling Engine with parabolic mirror

Energy Farms talk by Bill Gross

Are Engines the Future of Solar Power?
by Cynthia Graber / December 30, 2009

Nearly 200 years after their invention, and decades after first being proposed as a method of harnessing solar energy, 60 sun-powered Stirling engines are about to begin generating electricity outside Phoenix, Ariz., for the first time. Such engines, which harness heat to expand a gas and drive pistons, are not used widely today other than in pacemakers and long-distance robotic spacecraft.

The 1.5 megawatt (MW) demonstration site, known as Maricopa Solar, is set to begin operations early January 2010, with units provided by the Arizona-based Stirling Energy Systems (SES). While 1.5 MW is only a fraction of the power that may be generated at sites SES has contracted to develop in California and Texas, spokesperson Janette Coates says this is a necessary first step in the technology’s commercialization. “It’s important for our industry to see—and our partners and investors—that we can take a small-scale plant and get it operational before we break ground on larger ones,” she says.

That’s because Stirling heat engines have a reputation for being a bit impractical. First invented by Robert Stirling in 1816, the engines use a heat source to warm gas, which expands and is pushed into another chamber. When the gas cools and contracts, it flows back. The expansion and contraction pushes a piston, which in turn produces electricity. In 1996, SES bought solar Stirling design and engineering patents from companies such as McDonnell-Douglas and Boeing. SES then partnered with Sandia National Laboratories, and over the next decade tweaked and refined the technology. In the SES SunCatcher, a circle of curved mirrors, resembling an upturned satellite dish, tracks the sun on two axes and reflects the sun’s heat onto a single focus point, the power conversion unit (PCU). The PCU contains four cylinders, in which hydrogen gas expands and contracts to move pistons.

Stirling engines are significantly more efficient at converting sunlight into energy than most photovoltaic panels or concentrating solar power plants, whether parabolic trough or tower designs. The test units have reached 31 percent efficiency, compared to 16 percent for parabolic troughs and about 14-18 percent for PV panels in use today (though newer designs not yet on the market range from 24 to as high as 41 percent). The high efficiency numbers alone, however, have not made Stirling an easy sell. The systems have been criticized as being too expensive, unreliable and requiring extensive maintenance thanks to many moving parts. Also, ground has not yet been broken on either California site for which SES signed purchase power agreements in 2005, adding to skepticism that these systems will ever become commercially viable. “At these high temperatures, with this many moving parts, people doubted whether SES could really pull it off,” says Reese Tisdale, research director for solar power at Cambridge, Mass.-based Emerging Energy Research. The relatively small Arizona plant is intended to allay those concerns.

Proponents of the technology point to the advantages it has over other forms of solar power, particularly concentrating solar power (CSP), which also captures the sun’s heat. Most CSP systems require significant amounts of water, which has proven to be a challenge in desert regions of the U.S. where solar power is most attractive, while Stirling engines require none other than small amounts for cleaning the mirrors. In addition, if one engine goes down, it has minimal impact on overall production. SES faced a manufacturing challenge in preparing its SunCatchers for mass production though. “The systems at Sandia were basically hand-built,” says Charles Andraka, a Sandia engineer and Stirling expert who worked with SES on the system’s design. For the Phoenix site, he notes, Sandia and SES engineers built 60 units in three months. “We have to do that many in a day for the larger plants.”

In order to do this, SES turned to the experts in rapid production of engines and related parts: the automotive industry. In partnership with automotive companies such as Tower Automotive and Linamar Corporation, SES managed to reduce the parts in the PCU by 60 percent (to about 650) and slash the weight of the entire system by roughly 2,250 kilograms. Andraka highlights one example of the upgrade: in the original engines, he points out, gas passed over the outside of the engine, with pieces of tubes and fittings at either end, requiring a total of approximately 20 parts. “On the new engine, the gas passage is a part of the block with no external parts. It’s much more reliable, much cheaper to assemble, with fewer parts and fewer places to leak,” Andraka says. The new systems have been running on test sites for more than 100,000 hours.

Maricopa Solar also represents just one scalable module; each multi-megawatt field will be grouped first in 60-engine units that come together to generate 1.5 MW, then those larger units are linked to each other to produce up to 9 MW. Explains Coates, “With the large 750 MW commissions, we won’t have to wait until we have 750 MW of dishes before we start producing power. This means that the utility can get the power prior to the full build-out, which can take years to complete.” This is in comparison to parabolic trough or tower CSP technology, which doesn’t generate electricity until the entire system is complete. Meanwhile, Tessera Solar, SES’s sister company in charge of development, is renegotiating contracts with utilities in California but expects to supply power at or below the cost of other solar technologies, and they plan to break ground on bigger solar Stirling engine power plants in Texas and California in 2010. Tisdale says he remains somewhat skeptical, but also optimistic: “This 1.5 MW site is key to demonstrating that it works.”

prototype Stirling CHP engine

Home-made answer to generating electricity harks back to the past
by James Meek / 2 September 2000

Householders could one day be producing as much electricity as all the country’s nuclear power stations combined, thanks to the revolutionary application of a device developed in the early 19th century. A new version of the device, the Stirling engine, is set to turn ordinary domestic gas boilers into miniature power stations, generating electricity whenever you switch on the central heating or hot water. It won’t make electricity meters run backwards. But for an estimated £500 extra on the price of a new boiler, the machine will generate electricity for the home for nothing, using excess heat that would otherwise escape out the flue. In Britain, a confidential report prepared for electricity companies by energy consultants EA Technology estimates that by 2025, 13m of the country’s 23m households could have their own little power station humming away in the boiler cupboard.

The Stirling engine was patented in 1816 by a Scottish clergyman, Robert Stirling. It consists of two cylinders, filled with gas, each containing a piston. The cylinders are heated and cooled in turn, making the pistons move up and down to turn a drive shaft. In existing domestic gas boilers, about a third of the heat is wasted. With the latest make of Stirling engine fitted, that spare heat is used to drive a small generator. The idea of turning homes into power stations is known as “micro chp” (combined heat and power). EA Technology is championing a Stirling engine made by WhisperTech, a New Zealand company, which can generate a kilowatt of electricity – enough to power three fridges.

British Gas is working on a different design with a similar power output, based on an engine from the US firm SunPower. “We have now got to the stage where we’ve overcome the technical barriers, and it’s simply a matter of product development, things like designing the right colour for the casing,” said EA Technology’s Jeremy Harrison. He said the first units would be on sale in 2002. Depending on the size of the home, he said, they would save householders between £150 and £300 a year on their electricity bills. The beauty of micro chp is that it cuts in when demand on the grid from households is greatest. On a summer’s afternoon, with the fridge on and the TV and VCR on standby, a household might be using only a few hundred watts of electricity.

On a winter’s evening, with kettles, electric cookers, irons, washing machines and tumble dryers working, that might rise to 20 kilowatts. But with the central heating and hot water on, the home would be generating its own electricity, lightening the load on the grid, the family budget, and the environment. EA Technology estimates that at peak times, a network of 13m households would generate 15,000 to 23,000 megawatts of electricity, compared with 15,000 megawatts produced by the country’s nuclear power stations. However, if the government wanted to promote micro chp specifically as a way of cutting down on the carbon dioxide emissions held responsible for global warming, it would need to close high-emitting coal-fired power stations, rather than nuclear ones. The government has been keen to promote chp as a means of implementing the 1997 Kyoto protocols, which called for reductions in carbon emissions.


The same day that Ford and General Motors announced catastrophic third-quarter losses, Dean Kamen was showing off his new electric car. The prototype vehicle, a zippy two-seat hatchback designed with more than a passing resemblance to the Volkswagen Beetle, can go about 60 miles on a single charge of its lithium battery and with practically zero emissions. The secret? “It’s the world’s first Stirling hybrid electric car,” its inventor said with a flourish. Installed in the car’s trunk compartment is a Stirling engine invented at DEKA, Kamen’s technology company in the Manchester Millyard. It powers the features that would normally drain huge power from the battery, notably the defroster and heater. That leaves the battery primarily for propulsion. “You’re running a pure electric, which is enormously cheaper to operate and enormously more environmentally friendly,” Kamen explained. And if the battery does run low, the Stirling can recharge it, so you’ll never get stranded, he said. That’s why Kamen calls his Stirling engine “an insurance policy” for the electric car. Kamen showed off his state registration for his new car, listed as a 2008 DEKA Revolt. “I’m a car manufacturer!” he grinned.

Dean Kamen’s eco hybrid that’ll run on anything that burns
by Ben Purvis / June 28, 2009

Dean Kamen – the multimillionaire inventor behind the Segway personal transporter – is well down the road in the development of a new bike that combines electric power and a radical generator which will allow it to burn almost any fuel. Built around a fairly conventional battery and electric motor combination to provide the drive to the wheel, something Kamen’s experience with the much-hyped Segway makes relatively easy, the radical part of the design is the inclusion of a Stirling engine to recharge the bike’s battery pack. Based on technology that pre-dates the internal combustion engine by nearly a century, the Stirling engine is closer in concept to a steam engine, using external combustion, and without the need for a fuel that can be injected and burned incredibly fast inside a normal engine’s combustion chamber, it can run on virtually anything that burns – opening the door to easily renewable fuels rather than relying on dwindling fossil fuel supplies.

Although the prototype bike has yet to be shown in public, unlike Kamen’s Stirling-engined car which has been demonstrated several times, Kamen himself is understood to have been using the prototype to zip around his own estate. As revealed in Kamen’s own patent for the technology, the bike looks like a conventional scooter, with the Stirling engine and its fuel tank mounted under the seat, a rechargeable battery pack in the floor and a radiator in the front fairing. Although the Stirling engine’s low output – one this size is unlikely to make any more than 5bhp – means it can’t give the bike much performance on its own, it’s able to keep the battery topped up by continuing to supply electricity even when you’re not moving. The energy reserves in the battery can be used when more power is needed. And as the Stirling engine could be left running at low speed even when the bike is parked, the battery would never be likely to go flat. Kamen has already sunk more than $50 million into his development of Stirling engine technology, using the idea for everything from bikes to cars and even to water-purifiers to be used in the developing world.

What’s a Stirling engine?
Although the idea for Stirling engines has been around since 1816, they’ve never been mass produced so the concept is still quite unfamiliar. Like a steam engine or internal combustion engine, Stirling engines use pistons to turn a crankshaft, but unlike the alternatives they have no valves as no gas ever enters or leaves the cylinders. Instead, the idea is to use the fact that gas expands when it’s hot and contracts when it’s cool to move the pistons. Although there are several variations on the theme, Kamen’s design uses a design known as a two-piston Stirling engine which has a power piston and a displacer piston.

The cylinder of the power piston is heated from outside, making the gas – normally helium – inside the cylinder expand, moving the power piston and giving a power stroke to the crankshaft. The flywheel weight on the crankshaft keeps the rotation going, moving the power piston back, on what would be the exhaust stroke of a four-stroke internal combustion engine. But rather than sending the expanded gas out into the atmosphere, it’s sent through a transfer port into the second “displacer” cylinder, which has its own piston – at this stage moving down its stroke. Unlike the power cylinder, the displacer cylinder is cooled, so once the gas is moved there it contracts. Again, flywheel momentum keeps the engine going as the displacer piston returns up its bore, forcing the gas – now cool – back through the transfer port into the power cylinder, where it’s heated for the cycle to begin again.

Compared to an internal combustion engine, Stirling engines give out relatively little power and torque compared to their size and weight, they take time to warm up and start working properly and they can’t react quickly to a throttle control – which explains why they never replaced either the steam engine (even slower to warm up, but more powerful) or the internal combustion engine (very powerful, no warm-up time and fast throttle response). But by linking a Stirling engine to a battery and electric motor, the disadvantages start to drop away.

Because they don’t need internal combustion, where you need a highly volatile, liquid fuel to burn at incredibly high speed inside the cylinders, they can run on almost anything and use high-efficiency burners that completely use whatever fuel is being used. With many of the emissions problems from internal combustion engines relating to “unburned hydrocarbons”, Stirling engines can be cleaner.

Think of them like the wood burners that are becoming increasingly popular in homes, which are able to chuck out huge amounts of heat from slow-burning, natural fuel. Just like them, the Stirling engine’s ability to completely burn whatever fuel is being used, making the most of the potential power tucked away inside that fuel.

They also use the heat from burning fuel very efficiently. In an internal combustion engine, heat is a problem and engineers go to huge efforts to get rid of it, Stirling engines use the heat itself to create power. And by running cleanly and at a constant speed, they’re perfect for being linked to a generator to supply electricity.

Has the Stirling engine come of age?
Kamen isn’t the only person to have leapt onto the idea of Stirling engines as the world looks around for cheap, sustainable and clean energy, although with several years’ development under his belt and more than $50 million invested he’s got a head start. Honda has also been looking at the idea, not so much as a way of powering an entire vehicle but as a way of extracting more power from a conventional internal combustion engine. The firm has patented concepts revolving around small Stirling engines bolted to the exhaust system of a normal engine, using the heat from the exhaust as “free” power for the Stirling engine, which could then be used instead of a power-sapping alternator to power a car’s (or bike’s) electrical systems.

Motorsport experts Prodrive have also been helping develop a Stirling engine, not for a vehicle but for your home. The idea is to create a machine roughly the size of a tumble drier that will both heat a house and provide all its electrical needs, all using the Stirling engine concept. Other new applications for the technology include an autonomous robot for the US military, which is being designed around a Stirling engine which allows it to effectively “eat” by feeding itself wood or leaves, creating a machine that can remain active for years on end without needing to be recharged, while on the other side of the world, in Taiwan, tiny Stirling engines are being developed to run off the heat from computer chips, providing power for a cooling fan.

Dean Kamen’s Stirling Solution / August 02, 2007

In a world of Stepford executives who never deviate from the corporate party line, there’s something refreshing about an entrepreneur willing to take a tumble – literally – for his latest innovation. In uber-inventor Dean Kamen’s case that meant crashing his Stirling electric hybrid scooter in front of Green Wombat and a photographer. In June, Green Wombat visited Westwind, Kamen’s estate outside Manchester, New Hampshire, to talk to the Segway inventor about his plans to install a Stirling heat engine in an electric car made by Norway’s Think. (See “Have You Driven a Fjord Lately?” in the August issue of Business 2.0.) But first, Kamen wanted to demo the scooter (photo above) to show how a virtually greenhouse gas-free Stirling engine could extend the range of an electric vehicle by trickle-charging the battery. As he zooms down the driveway, the scooter goes sideways – its weight distribution needs some tweaking – sending the inventor flying into the grass. “Say you’re in Bangladesh or anywhere in the world where people don’t have electricity,” says Kamen, dusting himself off and not missing a beat. “You get home and you plug your house into it.” He shows off power plugs behind the scooter’s seat. “It’s your power system, it’s your heating system, it gives everybody electricity. When you leave in the morning, you drive away with your local power plant.”

Over the past decade, Kamen, who made a fortune as inventor of the insulin pump and other medical devices, has spent some $40 million developing Stirling engines. They can use virtually any fuel source to heat a sealed container containing a gas – hydrogen or helium, for instance – that expands and contracts to drive a piston and produce electricity. (The scooter uses a small can of propane as the fuel source.) “We run two villages in Bangladesh on Stirlings that run on freakin’ cow dung,” says Kamen, who envisions Stirling engines powering the world’s off-the-grid villages and using the waste heat produced by the engine to purify water.

But Kamen needs to get to mass production to realize that dream and that’s where Think comes in. Kamen met Think CEO Jan-Olaf Willums last year at MIT. “I took him up to New Hampshire and we spent half the night speculating about how cool the world could be if you put the right technologies in the right place at the right time,” says Kamen. “I need some killer app to put this thing into production. And one way to do that would be to create the world’s first hybrid Stirling electric car.” So Willums shipped a Think City to Kamen, who is now modifying the two-seater coupe to carry a Stirling engine (photo at right) powered by veggie oil, for instance. (“You could drive across the country, stopping a McDonald’s to fill up,” says Kamen.) That would not only extend the Thinks range by hundreds of miles but turn the car into a mobile generator. When electricity demand peaks during the day, thousands of Thinks plugged in at office parks could feed power back to the grid so utilities like PG&E (PCG) and Edison (EIX) could avoid having to fire up planet-warming power plants. The Stirling engine would then recharge the car’s battery for the commute home. When we last spoke in July, Kamen had the autmotive version of the Stirling engine up and running. The next step is to hook it up to the City and see if it’ll work as planned. You probably won’t see a Stirling in a Toyota (TM) or Ford (F) but the device gives Think another power plant to offer its customers.

“If you have enough Thinks out there you would literally change the architecture of the grid,” says Kamen, taking Green Wombat for a drive around Westwind and past his wind turbine before parking the blue Think City near his pair of Enstrom helicopters. (He keeps the Think in a garage that also houses his 1898 steam-driven car and a 1913 Model T.) Kamen heads to the control room of his 33,000-square foot house. An Internet-enabled blue box called a Teletrol controls the home’s power systems, including a Stirling engine about the size of an air conditioner that can act as a backup generator or a mini power plant that kicks on when electricity demand soars. Kamen invented the Teletrol and his company of the same name remotely operates the heating and air-conditioning systems of buildings like the Sydney Opera House. Kamen, of course, would like to see a Teletrol in every house, acting as the interface between your Web-enabled Think and the grid (and, ideally, the Stirling engine that sits in your basement or utility room.) “The big advantage is once we’re in production with that engine, where it will really be uniquely valuable is to the 1.6 billion people on this plant who’ve never used electricity,” says Kamen. “We will become the Con Edison of every village in Asia, Africa and Central America.”

To cut back on radioisotope fuel, NASA goes back 200 years
by Mark Wolverton / February 18, 2008

For more than 30 years now, NASA’s deep-space probes have relied on radioisotope thermoelectric generators (RTGs), devices that use decaying plutonium 238 to warm thermocouples and generate electricity. Now the space agency is poised to replace those heavy, expensive and inefficient RTGs with a system that provides more power with much less radioactive fuel—technology based on a 19th-century invention.

Patented in 1816 by an intellectually restless Scottish minister named Robert Stirling, the Stirling engine is simplicity itself: two chambers or cylinders, one cold and one hot, containing a “working fluid” (commonly air, helium or hydrogen) with a regenerator or heat exchanger between the two. Differences in temperature and pressure between the two cylinders cause the working fluid to expand and contract, passing back and forth through the exchanger and moving a piston. The process thereby converts thermal energy (in NASA’s case, supplied by radioactive decay) into mechanical energy. “Stirling is actually something we’ve been investing in for almost the past three decades at some level, and it’s now reached the point where we’re ready to make the next step forward,” says Dave Lavery, one of the directors of the Solar System Exploration program at NASA headquarters in Washington, D.C.

Lockheed Martin is putting the finishing touches on an engineering test unit that should be ready this spring: the advanced Stirling radioisotope generator. Two Stirling converters inside the generator drive pistons within a linear alternator, generating about 100 watts of electrical power. The unit will be less than a yard long and a foot wide, small enough to fit in the backseat of a subcompact car and weighing just over 40 pounds—less than half the weight of a typical RTG. It will boast a conversion efficiency of 20 to 30 percent, compared with the measly 6 to 7 percent of RTGs—while requiring only one-fourth the amount of radioisotope fuel.

Those characteristics translate into important advantages for spaceflight. Because the Stirling unit will be less massive and thus cheaper to launch, it will allow a spacecraft to carry a larger payload. The fourfold reduction in radioactive fuel—from 20 pounds in an RTG to five in a Stirling—also saves money while considerably reducing safety concerns involved in a worst-case scenario of a launch vehicle exploding in midair. NASA is keenly aware of public concerns about radiological safety, and as Lavery puts it, “for any nuclear-based system, we go through the entire National Environmental Policy Act process,” which requires NASA to collect public comments before any final launch decision.

Richard Shaltens, chief of the Thermal Energy Conversion Branch at the NASA Glenn Research Center, explains that once Lockheed completes initial testing, NASA Glenn will put the device through extended evaluations to begin its transition to flight status. “We’re planning to go forward with the potential use of this technology on future missions in probably the 2012–2013 time frame,” Shaltens says. He also points out that in more than 100,000 hours of lab testing in various environments, the Stirling converters “have demonstrated that they perform as predicted and have the potential for long life” comparable to RTGs.

NASA is so confident in the Stirling radioisotope generator that the agency has already invited the space science community to submit Stirling-based planetary mission concepts. Lavery emphasizes that the generator’s inaugural mission will not be decided until at least 2009, but possible jobs include flights to the outer planets and manned missions to the moon or Mars. “Their overall design right now is to be compatible with either a deep-space interplanetary environment or planetary-surface environment with either atmosphere or vacuum,” Lavery says.

Eventually Stirling technology may phase out RTGs completely. Lavery expects that it “would be the beginning of a new family of [radioisotope power systems] that are significantly more efficient and significantly less costly than the solutions we had available so far.” Reverend Stirling could hardly have imagined that his ingenious invention might well become the prime mover that powers the next great era of solar system exploration.


“Okay, try not to let your mind get blown by the possible time-space paradox we’re about to illustrate, but MSI’s supposedly introducing a new ECOlution motherboard at CeBIT with an “air powered cooler” that operates on the Stirling Engine Theory to transform the thermal output of its chipset into the kinetic energy necessary to power that same chipset’s fan. Of course, as the fan cools the heatsink it deprives itself of energy, supposedly the piston affixed to the crankshaft pulls back down, giving it another potential surge when its heat rebuilds. Supposedly it works at 70% efficiency, so we’ll just let the thermodynamics geeks in the audience mull over the possibility and audacity of it all.”

‘low-temperature-difference Stirling Engine shown here running on heat from a warm hand’

tutorial by William Gurstelle