Stirling Solar Power

 

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.

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.

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Portland, Ore. -- A project to generate electricity from solar energy using a Stirling engine looks to create farms that will light and cool the households of millions of California customers, at a cost that by 2011 may rival what traditional sources are charging.

The technology originated when Stirling Energy Systems Inc. agreed to supply Sandia National Laboratories with solar dishes in return for Sandia's addition of mechatronics to allow the dishes to track the sun. Together, Sandia (Albuquerque, N.M.) and Stirling Energy Systems (Phoenix) designed a 1-megawatt solar power substation capable of direct connections to the existing U.S. power grid.

"We now have six research dishes online at Sandia National Labs running completely autonomously, turning on and tracking the sun across the sky even on unattended weekends," said Bob Liden, vice president and general manager of Stirling Energy Systems. "We have the first 40-dish 1-megawatt farm started there and plan to have it in operation by 2007."

From 2007 to 2010, the Sandia program will perfect methods of ganging the substations into successively larger groups, operating at increasingly higher voltages.

In California, the state government has mandated that utilities invest in renewable energy sources for at least 20 percent of their power by 2010.

A Stirling engine converts heat into the mechanical motion of the pistons without burning fuel; no combustion takes place. The hydrogen gases inside the engine are sealed and never leave it, making the engine very quiet.

Setting the pistons in motion

The system is closed to the air, with a single connecting pipe between the piston's chambers. The heat of the sun is focused from the system's 82 mirrors onto tubes feeding a piston's chamber. As the first piston is heated, pressure goes up in the chamber, forcing the piston to go down. The second piston rises on its crankshaft, and through the connecting tube, cooler gases enter and cool off the heated chamber. As the first chamber cools, the first piston rises on its crankshaft, driving the cool piston back down. Then the cycle repeats.

Mechatronics enables three control systems to coordinate their behavior for unattended optimal performance even under changing conditions.

By monitoring and adjusting the flow between the system's heating and cooling chambers, the Stirling engine control system keeps the engine running at a constant temperature and power output of 1,800 revolutions per minute directly into a 25-kilowatt 480-volt ac generator.

The farms were perfected in Sandia National Laboratories' New Mexico desert test site under a Department of Energy program.

The DOE predicts that by 2011, Stirling solar dish farms could deliver electricity to the grid at costs comparable with traditional electricity sources. The power would come from more than 70,000 solar dishes in the Imperial Valley and Mojave Desert that would deliver more than 1,750 megawatts to southern California's grid.

Taxes & Politics

Other projects

San Jose, Calif. -- In an oil-addicted society concerned about its environment, entrepreneurs and investors are polishing up clean technologies like solar energy, fuel cells and batteries, looking for new ways to generate electricity and power everything from cars to cell phones. However, contentious government policies and slow- moving technologies dull the shine of this emerging sector.

Front and center in the fight is an initiative on November's ballot in California, where Proposition 87 would levy a fee on petroleum to help fund alternative energy technologies.

"This is probably going to be the most expensive race in the country this year," said Vinod Khosla, one of Silicon Valley's best-known venture capitalists, speaking at the Emerging Ventures conference here last week. He estimated oil companies have already spent $67 million attacking the measure and could spend $80 million to $100 million before the November vote.

A little more than half the Proposition 87 fees would be applied to lowering oil consumption and 30 percent to university R&D. "Clean tech R&D has been declining in this country for 30 years," he said. "We absolutely need to have more R&D in this area."

On a separate front, the government could give a huge push for solar energy if it mandated real-time pricing for electricity, said Barney Rush, chief executive for H2Gen Innovations (Alexandria, Va.), a startup developing hydrogen generation equipment for utilities. Such pricing would show consumers they could save money by using solar instead of utility power at home during afternoon hours when utility demand peaks, Rush said.

Such situations have made legislation a major focus for green tech investors and entrepreneurs, said Bill Joy, former chief scientist of Sun Microsystems turned venture capitalist at Kleiner Perkins Caufield and Byers.

"We hosted a gathering of green-tech innovators to brainstorm, and everyone wanted to go to the policy meeting," said Joy, speaking at the conference. "It's not that the situation is anti-innovation; it's pro-stagnation. It's focusing on the incumbents because it's trying to divide the spoils. We really need to level the field."

Bill Reinert, manager of the advanced technologies group at Toyota's U.S. division and one of the engineers behind the Prius, agreed. He said he butted heads with shifting government mandates earlier in his career while working on Toyota's electric cars, pulled from the market in 1993, and, before that, in the solar industry, which was supported by President Jimmy Carter.

"When [President Ronald] Reagan came in and took out the solar subsidies everything imploded. Within two years, that industry was gone," said Reinert.

"Right now we are using 85 million barrels of oil a day, and by 2020 people expect we will use 125 million. I haven't seen anyone who can show me where this comes from," said Reinert. "We are going to need more than hybrid cars and conservation. We are going to need new fuels and new policies," he said.

Rising interest

"When oil went above $40 a barrel, a host of things became viable," said Khosla, whose Khosla Ventures (Menlo Park, Calif.) has invested in at least seven startups pursuing a variety of alternative fuels.

Khosla is perhaps the most high-profile of many investors increasingly drawn to this small but growing sector. Alternative energy attracted $365 million across 25 deals in the first half of 2006, a record for the sector, said Jessica Canning, a senior research manager with market watcher VentureOne.

"Very big funding deals [in clean tech] are getting done that never make the stats. People are keeping these deals dark because the development cycles are so long," said Erik Straser, general partner at Mohr Davidow Ventures (Menlo Park). Mohr has announced only four of its nine recent deals, he added.

The sheer size of the energy markets is attracting top entrepreneurs, conference panelists said. "I'm not trying to convince people to use electricity or put gas in their cars, we just want to resegment an existing market, and these markets are measured in trillions of dollars," said Straser.

"The most promising thing about clean tech is the entrepreneurs are moving there," Straser said. "Today in the Valley we see a tremendous amount of [career] flexibility." In particular, people in IT say they don't want to be there, he added.

Batteries, cells and solar

Battery technology is one of several investment plays heating up. "The storage of energy is very inefficient, and there are huge opportunities in that area," said Brook Byers, a partner at Kleiner Perkins.

"I have looked at more than 20 battery companies, though we haven't invested in any yet," said Khosla. "If the right battery comes along, the automotive industry will shift dramatically."

Reinert said Toyota is working both with partners and internally on lithium-ion batteries to address tough issues such as large swings in charge voltages and end-of-life disposal.

"All these startups may have better lithium ion than we do, but we are the ones who have to get the lawsuits, provide a 10-year warranty on a car and talk to first responders about how they cut open a car with a lithium-ion battery when it rolls over in a ditch," Reinert said.

In addition, today's fuel cells need longer membrane life, reduced dependency on bulky hydrogen and air compressors, better water management systems and lower-cost materials, he added.

Nevertheless, novel batteries may ride alongside fuel cells in future hybrid vehicles. "We're making good progress on fuel cell stacks in terms of use in cold weather and durability, and we'll get an order of magnitude improvements in cost from new materials and new manufacturing processes," he said.

"I think you will see a combination of batteries and fuel cells in the [cars of the] future but it is quite a ways in the future," Reinert said. "I don't think you will see cars [using fuel cells] by 2010. The real deal is more like 2015-2020," he added.

Franklin Fuel Cells hopes to be one of the providers to tomorrow's hybrid cars. The startup uses copper rather than nickel in its anode and unique catalyst materials that let the cell use as many as 16 different fuels. The company's technology was developed at the University of Pennsylvania. Its fourth-generation prototypes supply energy density of 500 milliwatts/cm² on average, but won't be ready for integration in cars until about 2015, said chief executive John Law.

"This space doesn't move very rapidly, and it is conservative about new technologies. It's a show-me sector," said Law.

Indeed, "the time period [for maturity in the alternative energy sector overall] is probably the next 20 to 30 years," said Dan Nova, a general partner with Highland Capital Partners.

Another fuel cell company, Enerage Inc. (Arcadia, Calif.), is developing components for low-cost disposable fuel cells that could power cell phones. The startup is in pilot production of a high- temperature membrane and is showing prototypes of a single-chamber cell that can be made in a one-step extrusion process.

Here comes the sun

At the opposite end of the spectrum, investors like Khosla see big opportunities for utility plants powered by thermal solar technology.

"I now believe that thermal solar will be cheaper than coal-fired electricity plants. It is far more risky to build a coal-fired plant than a solar thermal one today," said Khosla.

Green Volts (Berkeley, Calif.) is one of many companies trying to address that market using a novel design for solar panels that could generate up to 20 megawatts. The startup's technology uses high-end solar cells with optics that provide a 625x concentration of sunlight on panels that are relatively lightweight and thus inexpensive to install.

Utilities represent an opportunity for solar energy that could amount to hundreds of billions of dollars, said Khosla, who delivered a keynote at a solar power conference in San Jose that attracted an estimated 7,000 attendees last week. Photovoltaic cells that power solar panels have made significant advances with thin film, multijunction technology, he added.

Although many developers are pursuing the low-cost solar cells, Khosla said, "that's exactly the wrong way to go.

"Solar systems would still cost $2 kilowatt/hour if the cell cost went to zero. What we need are higher-efficiency cells. We should be saying we will accept higher costs to get 30 percent efficient cells," he said.

Straser of Mohr Davidow disagreed. "We are trying to move photovoltaic cells from the economies of the semiconductor industry to the printing business," he said. "We want to make it more like printing the New York Times than building the next Intel fab."

With a total capacity of 1.6 megawatts — enough to supply 1,000 average California homes — Google's headquarters will be the largest solar installation on any corporate campus in the United States and one of the largest on any corporate site in the world, according to the search engine specialist.

The project will involve 9,212 solar panels provided by Sharp Electronics. A majority will be placed on the rooftops of some of the buildings in the "Googleplex" and parking lots. The solar energy will be used to power several of Google's Mountain View office facilities.

Google has a strong interest in solar. A startup originally funded by Google in June announced a $100 million financing package and set plans to build what the company claims as the world's largest solar-cell manufacturing facility in California.

Presently in pilot production in its Palo Alto, Calif.-based facility, the solar-cell startup — Nanosolar — has started ordering volume production equipment for use in a factory said to have a total annual cell output of 430-megawatts (MW) once fully built out, or approximately 200 million cells per year.

The company's first volume factory will be located in the San Francisco Bay area. At present, though, Google is apparently using Sharp's solar panels for its campus and not those from Nanosolar.