Sun catchers tuned to crank out the juice
Solar Powered Stirling Generators?

Stirling engines have been around since the early 19th Century, but they've never managed to find major commercial uses beyond toys like the Micro-Stirling-powered spinny thing pictured. The engines are a great concept, with the potential to be far more efficient than internal combusion, but the inherent disadvantages (most notably a long warm-up time and sluggish output variance) have kept Stirlings confined to very specialized jobs. Now, EE Times reports, engineers have determined that harvesting solar power with Stirlings (rather than the traditional photovoltaic cells) can bring the cost of harvesting solar energy way down. Now, this could mean clean energy for homes and businesses, which is great, but I'm more interested in portable solar-powered Stirling generators. Like, backpack-sized. You listening, Voltaic? Stirling solar collection dishes could be a hot fashion accessory.


The Stirling engine is a heat engine that is vastly different from the internal-combustion engine in your car. Invented by Robert Stirling in 1816, the Stirling engine has the potential to be much more efficient than a gasoline or diesel engine. But today, Stirling engines are used only in some very specialized applications, like in submarines or auxiliary power generators for yachts, where quiet operation is important. Although there hasn't been a successful mass-market application for the Stirling engine, some very high-power inventors are working on it.

A Stirling engine uses the Stirling cycle, which is unlike the cycles used in internal-combustion engines.

The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place. Because of this, Stirling engines are very quiet.

The Stirling cycle uses an external heat source, which could be anything from gasoline to solar energy to the heat produced by decaying plants. No combustion takes place inside the cylinders of the engine. There are hundreds of ways to put together a Stirling engine. In this article, we'll learn about the Stirling cycle and see how two different configurations of this engine work.

The Stirling Cycle

The key principle of a Stirling engine is that a fixed amount of a gas is sealed inside the engine. The Stirling cycle involves a series of events that change the pressure of the gas inside the engine, causing it to do work.

There are several properties of gasses that are critical to the operation of Stirling engines:

If you have a fixed amount of gas in a fixed volume of space and you raise the temperature of that gas, the pressure will increase.

If you have a fixed amount of gas and you compress it (decrease the volume of its space), the temperature of that gas will increase.

Let's go through each part of the Stirling cycle while looking at a simplified Stirling engine. Our simplified engine uses two cylinders. One cylinder is heated by an external heat source (such as fire), and the other is cooled by an external cooling source (such as ice). The gas chambers of the two cylinders are connected, and the pistons are connected to each other mechanically by a linkage that determines how they will move in relation to one another.

There are four parts to the Stirling cycle. The two pistons in the animation above accomplish all of the parts of the cycle:

Heat is added to the gas inside the heated cylinder (left), causing pressure to build. This forces the piston to move down. This is the part of the Stirling cycle that does the work.

The left piston moves up while the right piston moves down. This pushes the hot gas into the cooled cylinder, which quickly cools the gas to the temperature of the cooling source, lowering its pressure. This makes it easier to compress the gas in the next part of the cycle.

The piston in the cooled cylinder (right) starts to compress the gas. Heat generated by this compression is removed by the cooling source.

The right piston moves up while the left piston moves down. This forces the gas into the heated cylinder, where it quickly heats up, building pressure, at which point the cycle repeats.

The Stirling engine only makes power during the first part of the cycle. There are two main ways to increase the power output of a Stirling cycle:

Increase power output in stage one - In part one of the cycle, the pressure of the heated gas pushing against the piston performs work. Increasing the pressure during this part of the cycle will increase the power output of the engine. One way of increasing the pressure is by increasing the temperature of the gas. When we take a look at a two-piston Stirling engine later in this article, we'll see how a device called a regenerator can improve the power output of the engine by temporarily storing heat.

Decrease power usage in stage three - In part three of the cycle, the pistons perform work on the gas, using some of the power produced in part one. Lowering the pressure during this part of the cycle can decrease the power used during this stage of the cycle (effectively increasing the power output of the engine). One way to decrease the pressure is to cool the gas to a lower temperature.

This section described the ideal Stirling cycle. Actual working engines vary the cycle slightly because of the physical limitations of their design. In the next two sections, we'll take a look at a couple of different kinds of Stirling engines. The displacer-type engine is probably the easiest to understand, so we'll start there.

Displacer-type Stirling Engine Instead of having two pistons, a displacer-type engine has one piston and a displacer. The displacer serves to control when the gas chamber is heated and when it is cooled. This type of Stirling engine is sometimes used in classroom demonstrations. You can even buy a kit to build one yourself!

This engine can run using only the heat from your hand.

In order to run, the engine above requires a temperature difference between the top and the bottom of the large cylinder. In this case, the difference between the temperature of your hand and the air around it is enough to run the engine.


PORTLAND, Ore. - EEs are turning a 19th-century invention into a 21st-century alternative-energy source.

The last leg of a two-decades-long effort by the U.S. Energy Deaprtment to unleash superefficient solar power by 2011 is homing in on the so-called Stirling engine, which is being used to drive solar generators. DOE test site measurements suggest the setup could bring the cost of solar power on a par with traditional fossil fuels and hydroelectric sources - assuming the project engineers can balance the separate power feeds from farms of thousands of simultaneously online 25-kilowatt Stirling solar dishes.

The heart of the design, the engine itself, was invented by the Scottish minister Robert Stirling in 1816.

"The Stirling engine makes solar power so much more efficiently than photovoltaic solar cells can," said Robert Liden, chief administrative officer at Stirling Energy Systems Inc. (Phoenix). "That's because the Stirling solar dish directly converts solar heat into mechanical energy, which turns an ac electrical generator." The bottom line, he said, "is that large farms of Stirling solar dishes - say, 20,000-dish farms - could deliver cheap solar electricity that rivals what we pay for electricity today."

Under a multiyear Energy Department contract that started in 2004, Stirling Energy Systems will supply Sandia National Laboratories with solar dishes for integration into full-fledged power-generation substations capable of direct connections to the existing U.S. power grid. Right now about 20 EEs, including more than a dozen from Stirling Energy Systems, are working full time at Sandia to create the electrical-control systems to manage these sunshine stations.

By the end of 2005, they plan to have six dishes connected into a miniature power station capable of supplying enough 480-volt three-phase electricity to power about 40 homes (150 kW). The next step, in 2006, is a 40-dish power plant that will transform the combined output of the farm from 480 to 13,000 V, for distribution of industrial-level power to an existing substation. From 2007 to 2010, the program proposes mass-producing dishes to create a 20,000-dish farm supplying 230,000 V of long-haul power from its own substation directly connected to the grid.

If the project succeeds, the DOE predicts that by 2011, Stirling solar-dish farms could be delivering electricity to the grid at costs comparable to traditional electricity sources, thereby reducing the U.S. need for foreign sources of fossil fuels.

Eventually, according to DOE estimates, an 11-square-mile farm of Stirling solar dishes could generate as much electricity as the Hoover Dam, and a 100 x 100-mile farm could supply all the daytime needs for electricity in the United States. By storing the energy in hydrogen fuel cells during the day, Stirling solar-dish farms could supply U.S. electrical-energy needs at night too, as well as enough juice for future fuel-cell-powered automobiles, the DOE believes.

Power today costs from about 3 cents to 12 cents per kilowatt-hour, depending upon the customer's location and the time of day. The average is 6.6 cents/kW-hr for the industrial sector in 2004, according to DOE. In contrast, the Stirling solar-powered substations operate only during peak hours (daytime) but at potentially the same or less than the peak rates paid today - or "about 6.5 cents per kilowatt-hour during peak periods," said Liden of Stirling Energy Systems.

Prior DOE tests settled on the Stirling solar dishes by comparing traditional solar power with three kinds of "focused thermal" solar energy - all of which operate in a manner similar to the solar-power generator in the James Bond movie The Man with a Golden Gun. There, Roger Moore narrowly escapes being fried by the concentrated beam from a focused solar mirror that uses the same principle whereby leaves are set on fire with the focused sunlight from a magnifying glass.

The DOE compared the Stirling solar dish, parabolic troughs, power towers and concentrated photovoltaics. The study, conducted at Sandia National Laboratories' Solar Thermal Test Facility, concluded that Stirling dishes outperformed all other sources of solar power.

Today Stirling-powered solar dishes at the Sandia test facility operate at 30 percent efficiency while delivering grid-ready alternating current. In contrast, 30-percent-efficient solar cells are direct current and drop to 16 percent efficiency by the time they generate grid-ready ac. And that's on a hot day. Efficiency can drop as low as 10 percent on a cool day.

"Tests have already shown that the Stirling engine can be made into a very efficient power generator," said Chuck Andraka, project leader at Sandia's Solar Technology Department. "Now what we need to show is that many small Stirling engines can be coordinated in farms that together rival traditional power sources."

Time for a change

Historically, the Stirling engine could never compete with the bigger bang per cubic inch of a gas-guzzling internal-combustion engine. However, dependence on foreign oil, increasing pollution and America's seemingly unquenchable thirst for more energy hint that it might be time for a turnaround.

The key to Stirling engine solar-dish farms is three control systems being engineered by EEs. "The first is the motor control system that tracks the sun, plus provides safety features such as returning to maintenance position at night or turning to avoid the wind if it gets too high," said Andraka.

The second is a system for engine control and power conversion - making sure the engine runs at a constant 1,800 revolutions per minute and at a constant temperature, by monitoring and adjusting the flow between the system's heating and cooling chambers. When the engine is achieving its target of 30 percent efficiency, the temperature of the hydrogen gases inside varies from 200 to 1,300. But without constant closed-loop monitoring, the system could stall out on a cool day or keep ratcheting temperatures upward, on a hot one, until the engine melts.

The final puzzle piece on which the EE team is working is the plant control system. Andraka called this "the most critical [of the three control systems], because it actually runs a whole field full of dishes on a farm and manages problems like staggering startup so that all the dishes don't go online at exactly the same time."

The dishes behave like sunflowers, following the sun all day and returning to a face-down position facing north at night. Since each dish draws about 10 amps from the power grid for a few milliseconds when it starts up in the morning, startup must be staggered if a large dish farm is to avoid causing a blackout.

"If you have to start up 20,000 dishes, you can't do it all at once or you'll bring down the grid," said Andraka. "But you can't stagger them 5 seconds apart either, or your last one won't even come on by the end of the day. We estimate that staggered startups will need to be limited to 5 or 10 milliseconds if we want all the dishes to go online in a reasonably short period."

Besides control systems, the EEs are pioneering new power-conditioning designs that enable all these small generators to simultaneously operate as if they were one large generator. By conditioning the outputs from multiple dishes with banks of both active and passive capacitors, the engineers hope to get a unity power factor out of their solar substations.

The 25-kW Stirling solar-powered dish utilizes 82 back-silvered mirrors measuring 3 x 4 feet. Manufactured by Paneltec Corp. (Lafayette, Colo.), the mirrors are just 1 mil thick and can easily bend into a slightly concave shape when laminated onto a honeycombed aluminum structure patented by Sandia National Laboratories.

The $150,000 dishes, which have by now logged more than 25,000 hours of "sun-tracking" test time, are being assembled by Stirling Energy Systems from a steel framework made by Schuff Steel Co. (Phoenix) and from engine parts built by various U.S. manufacturers. If produced in mass, their cost is predicted to fall to $50,000 by 2010. The Stirling solar dishes are also easy to maintain, since "the engine only has a single part - a seal - that needs to be periodically replaced," said Liden.

Higher efficiency

Because of the simplicity of its design, the Stirling engine can operate at efficiencies higher than rival technologies. Only cheap fossil fuels have kept the Stirling engine from being commercialized beyond industrial applications as auxiliary power generators and as silent submarine engines.

Unlike internal-combustion engines, the Stirling does not burn and exhaust fuels. Rather, the hydrogen gases inside the engine are sealed and never leave it. The Stirling engine does have a moving piston in its chamber, but no combustion takes place there, making the engine very quiet.

The source of heat for a Stirling engine can come from anything hot - from burning wood to the palm of your hand. (Physics labs often have handheld Stirling engines that are powered by the heat of the human body.) Stirling engine submarines use a giant Bunsen burner as a heat source, thus making them silent compared with diesel- or nuclear-powered subs. In the Sandia project, the Stirling solar dish harnesses the heat from focusing its 82 mirrors onto tubes feeding the engine.

The easiest way to understand the Stirling cycle is by looking at a two-piston engine. The chamber for one piston is heated from the outside (with burning wood, in Robert Stirling's original design) while the other is being cooled from the outside - say, with ice. Since the system is closed to the air with but a single connecting pipe between the piston's chambers, heating the hydrogen gases in the first piston will cause them to expand, raising the pressure and pushing that piston down.

As the heated piston goes down, the pressure in the second piston - positioned lower because of the cold - allows it to rise on its crankshaft. The connecting pipe then feeds the cooler gases from the second chamber back into the heated chamber, where they cool off that piston, enabling it to rise on its crankshaft as the cool piston descends again. Then the gases are heated anew in the first piston and the Stirling cycle continues.


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