Stirling engine makes a solar-powered comeback
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.
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.