Green Tech

Among the Internet cognoscenti, Bill Gross is best known as the head of tech incubator Idealab. Now, as the CEO of solar start-up eSolar, he's working in renewable energy, but he's still putting his digital economy chops to work.

Two-year-old eSolar is having an opening ceremony for its pilot solar power plant in Lancaster, Calif., on Wednesday. There's a veritable glut of solar start-ups, but eSolar has already gotten further than most: it's actually producing electricity at below the price consumers pay in California.

The plant is also the first concentrating solar "power tower" in the U.S., capable of producing five megawatts, or enough power to supply about 1,500 homes or up to 4,000 during peak hours. This emerging utility-scale solar technology uses mirrors to concentrate sunlight onto a tower to make steam, which is then pushed through a turbine to make electricity. The company also has signed deals to build much larger plants in California, New Mexico, and India.

One of the tricks to getting this far, says Gross, is replacing steel with software. In one 46-megawatt eSolar plant, there are 200,000 flat mirrors, each individually controlled by a microprocessor for the optimal angle.

In an interview, Gross predicts that the "more software, less steel" trend will continue in solar, which will help get the cost of solar electricity down. And he says that because banks "are acting like mattresses" rather than lending, other concentrating solar companies will struggle to get utility-scale solar projects off the ground.

Q: Why have you been able to build a demonstration plant in a year while so many others haven't?
Gross: A few reasons. A big one is the land. By using these smaller-piece parcels of land, we don't have to wait for government permission (to use Bureau of Land Management land). We can just use private land. Not to mention we don't have to do environmental impact studies because we're not using pristine desert land, which a lot of people worried about impacting.

The second thing is everybody has to wait for build transmission lines. That could be 10 years or never. You could build a power plant and people could block you from building transmission lines to the main distribution point, so buying land specifically adjacent to transmission helps.

The biggest one, though, is the finance. Most people's projects are billion-dollar projects--they have to be billion-dollar scale before the economics work, and nobody is going to raise a billion dollars in this climate. Our projects work economically at $100 million scale. And we can pay for this plant with cash because we raised $170 million. Everybody else will be held up for years and years until banks will lend on the riskiness of a new project. For us, banks can lend against our project (for new projects) because they can see it works.

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Grand opening

At a ceremony in Southern California, eSolar will bring its five-megawatt concentrating solar plant online.

There's been a catch-22 on new solar technology--projects can't go forward because they only have a PowerPoint.

In the scheme of things, this plant is just five megawatts (about enough to power 1,500 homes). How do you scale?
Gross: A 46-megawatt plant takes one quarter mile, so to build a gigawatt in California would take 20 of those--or 5 square miles. We purchased that land for $30 million in cash, all in small quarter-mile plots, all adjacent to transmission, so we've completely eliminated the owning and permitting issue.

(Project development companies like) NRG will buy the project from us (for a planned installation in New Mexico) and own and operate the plant for 20 years.

So it's large scale but done in a distributed way?
Gross: Exactly. The ultimate distributed solar is to put (photovoltaic) solar panels on every rooftop. The ultimate centralized solar is get 2,000 acres of BLM land and then you have a transmission problem because you need to build a gigawatt of transmission. We're in between--we're large-scale utility but we're still distributed--distributed in small enough pieces.

Two years ago we didn't know how prescient it would be, but we looked at the entire transmission grid of California and where there was 46 megawatts of available capacity, we would go and buy a patch of land right next to that. We inverted the problem. Others said, "Let's build where the sun is best because I need to buy 2,000 acres at once to get economies of scale. And then I'll try to lobby for 10 years to get 100 miles of million-dollars-per-mile transmission built."

Now environmental groups of all things are protesting people using BLM land, and they have a point. Solar is great but you don't want to destroy the pristine desert...Our land is already being used for something (such as farming).

On the technology side, how much more productive or efficient is this solar tower than existing solar trough technology?
Gross: It is a little more productive than solar troughs. Solar troughs run at between 27 percent and 30 percent efficiency, and we are at 34 percent efficiency. But the real thing is we're half the cost. It's not the efficiency that we're much better at, it's the price--that's really the breakthrough. The reason we're so much less cost is that we use hundreds of thousands of small flat mirrors, instead of long, long rows of huge curved mirrors. The troughs use a mirror that is 5 meters wide by 100 meters long. They pay the same price as we do for the mirror--it's the same high-quality Belgian or German suppliers--but that's only 10 percent of the cost.

The main cost of the solar thermal plant besides the mirror is the steel and the actuator (for controlling mirrors)--that's 90 percent of the cost...The steel (is needed) to hold the mirror in shape without distorting, to stay in a perfect parabola. Because we use a one-square-meter mirror, we use half the steel. Imagine if you take a piece of flat glass and put a tripod behind it, it'll stay flat. But you need far more steel to bend glass against its will.

So why haven't other solar companies broken up their mirrored troughs into smaller bits?
Gross: The problem is historically it's been a software control problem to track hundreds of thousands of small things. The benefit of one big row is you only need 20 motors to turn troughs--all pointing at the sun--and software control is trivial. We have 24,000 individual mirrors, all pointing in slightly different directions to point at one spot. We're basically making a dynamic parabola in software where they are making a static parabola in steel.

In the last decade, there's been a 1,000-fold increase in computational power, so now we can put a $2 microprocessor in every mirror and it costs almost nothing--almost one and half percent of the (material) cost. So every mirror that is tracking the sun during the day has its own computer. And the computational power of a microprocessor today is mind-boggling. It's a 16-bit microprocessor with eight I/O ports. It's like an IBM AT (PC) in every mirror--that was a $5,000 computer in 1985. This completely wouldn't be possible without Moore's Law.

It's interesting that you've come from the computer world into solar. Will there be other stories like eSolar to come?
Gross: I definitely think so. eSolar has been grown right in the same building as other Idealab companies with all the benefits of IT they had--it uses all the servers built for Internet companies with all the experience and hardening capabilities. And it even uses many of software developers from prior years that we've hired back from places like Yahoo.

If anything, we're more a software company than a solar company. Of course we're a solar company, but software is 50 people out of the whole company. There are 135 people--100 are in engineering, 35 are out running power plants, so half the (engineering people) are in the software group, which is an amazing percentage for a solar company.

The reason that this going to happen more is that, of course, every commodity in the world is going up over time. There will be blips like we're having now, but in general, the cost of things that require natural resources will go up. The only thing going down is computation power. Everything else behaves on a different law--one of scarcity.

Computing costs are going down. If you want to crack a problem where cost is the issue, you gotta bet on the thing going down in price and include more of that. Less steel, more software--that is the right trade you want to make. I think that's going to be used more and more.

Are you optimistic on solar and green tech in general?
Gross: I'm wildly optimistic about it, and we only have this momentary setback due to the recession and the banking industry. But this is going to be a 100-year-long build-out to replicate what we built out with coal and natural gas in the last century. The only way it's going to happen is if you actually lower the price--it's not going to happen through altruism. If you can beat natural gas and coal, then you'll have access to huge, huge markets. If you don't, then you'll be limited to the subsidized market. (eSolar's projects benefit from a 30 percent federal tax credit).

How did you start getting into energy after working in the Internet?
Gross: I have been interested in energy all my life, ever since I was a teenager. I worked on energy projects back in 1973 during the first energy crisis, called Solar Devices, a mail-order business I ran as a teenager. I think I got into Caltech because of that business--I wrote about it in my application. But then OPEC came along, formed, and colluded to drive down the price of oil so that at end of the '70s, nobody was interested in renewable energy anymore. So I had a 20-year hiatus in software and Internet companies and had a string of successes that enabled me to have the capital to come back to my true love in 2000. And of course, by 2000 people were talking about energy issues, maybe running out of energy. That's when we did the research that led to this crop of solar companies (covering different solar markets such as rooftop solar and off-grid solar) over the last nine years.

Updated at 6:30 a.m. PT with corrected figures for the number of mirrors.

Abengoa Solar of Spain on Monday reported successful tests of its second solar tower in operation, in which the sun's heat is used to make electricity.

The 531-foot solar tower, located near Seville, Spain, features a number of improvements on the first design and has exceeded the anticipated output. Called PS20, the installation is the largest in the world with a capacity of 20 megawatts, enough electricity to supply 10,000 homes, according to the company.

A solar tower configuration uses a field of heliostats, or mirrors, to concentrate sunlight onto a receiver held in the tower. The heat creates steam which turns a turbine to make electricity. The PS20 project has 1,255 of these heliostats, with each heliostat having a surface area of 1,291 square feet.

Concentrating solar thermal technology has been used in desert areas in Spain and the southwest U.S. for decades and is seeing a resurgence as utilities seek out cost-effective solar power.

The traditional solar concentrating power uses rows of mirrored troughs that follow the sun over the course of the day.

Solar tower technology is considered the successor technology to mirrored troughs and is being pursued by a number of solar companies, including renewable energy powerhouse Abengoa and California start-ups BrightSource Energy and eSolar.

Abengoa Solar said that this installation improved on the first solar tower in Spain with better control systems and solar thermal energy storage system.

Updated at 12:05 p.m. PT with additional details from a conference call with reporters, and correction added on the shape of eSolar's mirrors.

Power plant operator NRG Energy on Monday announced a deal with start-up eSolar to build 11 solar power plants in the Southwest U.S.

The agreement will lead to 500 megawatts' worth of solar energy production at peak capacity, enough to power 400,000 homes, according to the companies. The first plant is expected to be operating in 2011. The size of a traditional natural gas or coal power plant is roughly between 500 to 800 megawatts.

As part of the deal, NRG Energy will invest $10 million in eSolar, which counts Google.org as one of its investors. The money will give NRG Energy an equity stake in eSolar and rights to develop three projects that eSolar has already negotiated, including a 245-megawatt power purchase agreement with Southern California Edison.

eSolar is one of several energy technology start-ups pursuing concentrating solar power, where lenses and mirrors concentrate sunlight to make heat. The heat creates steam that turns a turbine.

The company's technology uses an array of computer-controlled flat mirrors, called heliostats, on a supporting structure that concentrates the light onto a tower, where the steam is made. eSolar's plants are designed to be constructed relatively quickly, according to the company.

Earlier this month, BrightSource Energy signed a deal to provide 1,300 megawatts' worth of solar electricity to Southern California Edison over the next several years. BrightSource, too, uses a concentrating solar and tower design.

"eSolar's breakthrough modular power plants use more software and less steel to allow solar energy to be competitive with fossil fuels for the first time ever," said eSolar CEO Bill Gross in a statement. Gross is the founder of tech incubator Idealab.

The projects will be the first solar power-generating facilities for NRG Energy, which operates natural gas, oil, nuclear, and coal power plants, primarily in the U.S.

eSolar's power plants uses prefabricated material to create 46 megawatt facilities, which can be placed on one quarter of a square mile. NRG intends to build projects that use two of these modular components at a time.

Because eSolar's technology is relatively efficient with space, solar power plants can be built by converting existing land, such as farms, Gross said. eSolar has already purchased three locations, which don't require construction of new transmission lines.

"When you look at other types of solar thermal technologies where you have to build in larger chunks--like 300 megawatts as initial size--it requires you to locate a lot greater distance from load centers and there are much higher transmission connection costs," said Michael Liebelson, chief development officer low carbon technology at NRG Energy.

NRG Energy said solar thermal technology offers advantages over solar photovoltaic panels in these desert locations. The company intends to use storage, which will allow the facility to generate electricity even when the sun is not out. And the eSolar's plants use significantly less land per megawatt, Liebelson said.

The company intends to finance construction of the facilities by taking advantage of a 30 percent tax credit for renewable energy investments. It is also exploring loan guarantees from the Department of Energy, NRG's own capital, and outside investors, Liebelson said.

Utility Southern California Edison on Wednesday announced a giant solar energy contract with BrightSource Energy, which could eventually result in powering 845,000 homes with the sun's energy.

Through a series of seven projects, SCE intends to purchase up to 1,300 megawatts of electricity from BrightSource Energy's solar towers that use heat to produce electricity.

Although the companies didn't provide a price tag for the deal, it's one of the biggest solar energy contracts to date in the U.S. and a validation of solar tower technology. The project still needs to be approved by regulators and financed.

The first project from the deal is scheduled to be a 100-megawatt installation in Ivanpah, Calif. which could be operating in early 2013, supplying 286,000 megawatt-hours of electricity per year, or enough for 65,000 homes.

Concentrating solar technology uses reflective troughs or mirrors to concentrate light onto a liquid to make steam. The steam then runs a traditional electricity turbine.

The technology, best suited for desert climates, is being revived in places like California which have a mandate to produce 33 percent of their electricity from renewable sources by 2020.

BrightSource Energy uses mirrors to reflect sunlight onto a tower to heat water which makes steam. The Google.org-backed company, which has roots with developers who did solar thermal projects in the 1980s, says that its air-cooling process preserves water, an important consideration for desert projects.

Several concentrating solar companies were founded in the past five years, but the credit crisis and economic downturn has made it very difficult to finance these expensive projects. One BrightSource Energy competitor, Ausra, has shifted its business strategy away from giant solar power plants.

California utility Pacific Gas & Electric is expected to announce a large solar deal, which will involve solar photovoltaic panels, some time this quarter.

New Jersey utility Public Service Electric and Gas Company on Tuesday announced that it has applied for a $773 Million Solar Energy program to make 120 megawatts of electricity.

Updated at 10:00 a.m. PT with comments from Ausra providing more detail on its strategy.

Because of a lack of financing, solar-technology company Ausra has dropped plans to make massive solar-thermal power plants in favor of smaller, cheaper units.

Ausra President and CEO Bob Fishman told different news outlets this week that the company has been forced to change its business strategy in response to the credit crisis. The company is also laying off staff as part of the reorganization, according to reports.

The change is a clear sign of how the credit crisis and the current structure of government incentives are hurting the business for large-scale renewable-energy projects.

Backed by top-flight Silicon Valley venture capital firms, Ausra has designed equipment for concentrating solar-power plants. Specially shaped glass plates reflect light to create heat. The heat makes steam, which then drives a turbine to make electricity.

Several new companies have formed with different designs for concentrating solar power, or solar-thermal energy, with an eye toward building power plants in the southwest. California utility Pacific Gas & Electric has contracted with Ausra and others to buy electricity from these planned projects through a contract called a power purchase agreement (PPA).

But Fishman said the financing simply isn't there for solar-power plants that rival the size of coal or natural-gas plants.

"What a lot of people thought when they went out and signed 500- or 900-megawatt power purchase agreements was that it was easy to go from a 1-megawatt demo plant to a 900-megawatt project,'' Fishman told the San Jose Mercury News. "That's simply not reality. The finance market will not support it."

Instead, the company intends to make and run small to midsize power-generating facilities--on the order of 50 megawatts, he said. The company will also emphasize its business of licensing its technology to other project developers and utilities.

Ausra vice president of communications Katherine Potter said that the company still thinks that the business for large-scale solar power is viable. But the company is adjusting to the market conditions by focusing on equipment sales.

The company could, for example, sell a solar field as part of an existing coal or natural gas power generation facility. The solar power delivers electricity at peak times during the day, offsetting a utility's carbon emissions.

Financially, it's easier to do because utilities can deploy Ausra's equipment in six to 18 months, Potter said. By contrast, Ausra's deal to provide 177 megawatts worth of solar power to PG&E will take a few years just to get through the permitting process, she said.

Investors and solar executives say that, even with strong demand, a lack of money to finance projects means that other solar start-ups will either shift strategies, scale back plans, or go out of business.

Renewable-energy investments get a 30 percent federal tax credit. The recession means that fewer corporations expect to have a tax bill to offset, which means that there's a dearth of "tax equity" available to invest, according to financiers.

Last week, PG&E CEO Peter Darbee said the California utility will invest in solar-power generation directly because of the tax equity problem. By building and operating its own solar-power generation facilities, rather than working with a project developer, PG&E can benefit from the tax credit.

Solar power is one of the most heavily invested areas within clean tech, making it a crowded field. Because financiers are being particularly conservative, new technologies will likely have trouble being deployed at a large scale this year.

"I think it's going to be a brutal, brutal year for solar, and a lot of companies will go out of business," said Andrew Beebe, the CEO of Suntech Energy Solutions, which develops solar-power facilities for corporations and utilities. "A lot of mistakes were made. Now is the time of reckoning, and it's going to be ugly."

Update on October 13, 2008 6:30 a.m. PT: Corrections to the SkyFuel's relationship to NREL and it's projected cost of delivered electricity.

Even with the teetering economy, solar companies are bullish that tapping free energy from the sun is a solid financial move.

SkyFue on Friday is hosting an event to unveil its solar power plant system: a parabolic trough made from reflective plastic. Colorado Gov. Bill Rittner will speak at the event, held at SkyFuel's research center in Arvada, Colo.

Parabolic troughs have been around for decades and are considered the incumbent technology in concentrating solar thermal power plants, which are suited for hot desert climates like in Spain and the U.S. southwest.

Reflective troughs concentrate sunlight onto a liquid which is converted to steam, which turns a power generator.

SkyFuel's enhancement on the basic shape is a trough that uses silver encased in plastic, rather than mirrored glass. Although silver is certainly an expensive material, using plastic cuts down on the overall cost, according to Chris Huntington, vice president of business development at SkyFuel. The company's founder tested the designs extensively with the National Renewable Energy Lab in Colorado.

He said that the cost of an installed solar power plant using its equipment is about 25 percent less than existing parabolic troughs.

The company estimates that it can profitably deliver electricity below the concentrating solar power benchmark of 15 cents per kilowatt-hour. That's the price of other solar thermal technologies, but still more than a natural gas or coal power plant.

Using a solar trough installation at an existing power plant, where there would not be the need to purchase a steam turbine would lower the cost significantly, Huntington said.

Several solar thermal companies have sprouted up in the past five years, with different designs such as Ausra's Fresnel lens glass reflectors or eSolar solar power tower.

Demand is driven by state-level renewable portfolio standards. Utilities in California, for example, need to purchase 20 percent of their electricity from renewable sources by 2010.

Huntington said the company expects to have a small installation of its SkyFuel system--on the order of 2 to 10 megawatts in size--in the next year and then larger installations after those pilots.

Having a less cutting-edge design than other firms is an advantage in a tight funding market, Huntington said.

"The cost of borrowing is going up everywhere and there will be a tighter credit market. But if any money is going to be spent on CSP (concentrating solar power) plants in the near future, I think it's going to be on tried-and-true systems like the parabolic trough," he said.

The company is already working on the second generation product that will include storage through a Department of Energy grant. It probably won't be commercially available for at least three years, Huntington said.

Rather than heat up oil or hot water, the parabolic troughs will heat up tubes of molten salt. That salt can be stored to make electricity even after the sun goes down.

If giant solar thermal power plants spread across the desert are like a mainframe, Sopogy is making the equivalent of a personal computer.

The Hawaii-based company on Tuesday at the Intersolar 2008 conference will show off the latest version of its MicroCSP--essentially a shrunk-down version of concentrating solar power (CSP) equipment used in power plants.

It's a trough with a reflective coating that focuses sunlight onto a pipe that carries an oil. That heated liquid goes through an organic Rankine cycle engine to convert it into electricity.

The conventional thinking in solar these days is to think big. Proposals for concentrating solar power plants call for hundreds of rows of troughs or mirrors to make steam to drive an electricity turbine. The output of these proposed plants will be hundreds of megawatts, approaching the size of traditional power plants.

Sopogy's product, called SopaNova 4.0, is aimed at utilities as well, but for smaller-scale projects, in the range of 250 kilowatts to 25 megawatts. The latest edition is longer--between 12 feet and 18 feet long--than previous editions because of a new manufacturing process.

"On cost per watt, we're cheaper than PV (photovoltaics)," said CEO Darren Kimura. "But that's not what really matters. We can do more production. We actually get more sun energy every day."

With a higher output, the payback on an initial investment comes quicker, he argued. The troughs can be used by corporate customers as well for on-site power generation.

In terms of the efficiency of converting sunlight to electricity, the SopaNova is between 20 percent and 30 percent, he said. That's lower than its larger CSP cousins, which operate at higher temperatures, but better than most solar photovoltaic cells.

Unlike flat solar photovoltaic panels, solar thermal systems have storage today. In practice, Sopogy's trough systems can store a few hours worth of electricity, which can be used when electricity is more expensive or when there isn't light.

Sopogy is thinking relatively small when it comes to its own capital needs.

The company raised $9 million in venture funding earlier this year and got a $35 million special-purpose bond from the state of Hawaii.

Later this year, Sopogy will look to raise another round of equity, which will be more than its past round but far less than the huge deals--some topping $100 million--announced by traditional CSP companies.

"We're trying to demonstrate that you can do solar technology but still be capex (capital expenditure)-light," Kimura said.

Ultimately, the company intends to go public. "The goal in solar is to become a really big company and the market space allows for that. If you don't, you'll get acquired," Kimura said.

The company has about 20 customers now. The Natural Energy Laboratory of Hawaii recently broke ground on a project to install thousands of the troughs to ultimately make one megawatt of electricity.

The troughs can also be used to generate process heat, which can be used in a variety of applications, Kimura said.

Hewlett-Packard is licensing flat-panel display technology to a start-up that could lead to dramatically more productive--and aesthetically pleasing--solar panels.

The deal, announced Wednesday, allows Livermore, Calif.-based Xtreme Energetics to use HP-developed transparent transistors to bend light in concentrating photovoltaic, or CPV, solar arrays. CPV systems squeeze more electricity from panels by maximizing the light that hits solar cells.

The company is in the process of raising an "imminent" $5 million series A round of venture funding, and it anticipates a series B $35 million round, CEO Colin Williams said.

It intends to have a first-generation solar array aimed at utilities available in 12 months, he said. A product for corporate rooftops is also in the works.

The transistor materials--made of environmentally benign zinc and tin--and related manufacturing techniques could still be used for very large flat displays, said Dan Croft, director of intellectual-property licensing at HP.

Xtreme Energetics will use the technology to create an electronic "tracker" that directs sunlight to hit solar cells straight-on to maximize exposure.

Typically, these trackers are mechanical devices such as ground-mounting systems that position cells to follow the light during the course of the day.

Xtreme says HP's electronics can do the same task of pointing light. But because it's not a motor-driven steel mount, the company will be able reduce the costs of CPV, Williams said.

"The fact that we are using an electronic mechanism to do tracking means the cost scaling in volume manufacturing will go much more like the cost scaling in the electronics industry, rather than (the) mechanical-manufacturing industry," he said.

The full design calls for a multilayered solar panel with the transparent electronic tracker, a plastic "internal reflection" concentrator, and a high-efficiency solar cell.

Because the tracker and concentrator are transparent, an artistic pattern could be put onto the panel, making it possible to use it on a building facade, Williams said.

Niche buster?
The HP-licensing deal is another sign of the active crossover of technologies and of people between clean tech and information technology. IBM, which has a Big Green Innovations initiative, is adapting chip fabrication techniques to solar power, including concentrators.

Xtreme Energetics has yet to build a product or prototype. Yet its electronic-tracker design could give the budding CPV marketplace a boost.

At a seminar put on by Greentech Media last week, solar expert and Prometheus Institute President Travis Bradford forecast that concentrating solar power--both concentrating solar thermal and CPV--will account for tens of gigawatts of electricity in the next decade, primarily in .

CPV, specifically, remains relatively expensive and, unlike solar-thermal technology, cannot store electricity, Bradford noted. Also, concentrating solar technology works in areas of the globe with the best irradiance, or solar radiation, including the southwest United States, southern Spain, and North Africa.

As a result, he said the extent of the role solar-concentrating power will play in the future is uncertain.

"I firmly believe (CPV) is a market that will be very large, but it doesn't have the ability to work in every market," Bradford said.

Xtreme Energetics' Williams said the electronic tracker tackles one of the biggest concerns with concentrating photovoltaics: the high costs associated with lenses and mounting equipment.

"So long as concentrating PV uses mechanical trackers, it's going to be niche," he said.

Three solar-thermal companies have raised money in the past week in a sector that's showing life, despite a choppy investment environment.

Infinia on Tuesday said it has taken $7 million from Asian contract manufacturer Foxconn Technology Group, part of a total of $57 million in a Series B round first announced in February.

The company uses a dish to concentrate sunlight onto a Stirling engine, which makes electricity. It intends to sell its 3-kilowatt devices to small-scale utility plants.

On Monday, eSolar said it has raised $130 million from Idealab and Google.org. Its solar-thermal systems, designed for utility-scale power plants, use mirrors to reflect light onto a tower that turns a turbine.

And last Thursday, Stirling Energy Systems announced a $100 million investment from renewable-energy developer and waste management company NTR.

Stirling Energy Systems makes a huge concentrator dish that generates heat to turn a Stirling engine that makes electricity. It's under contract for two power plants in the Mojave Desert that would initially generate 800 megawatts of power.

Solar-thermal technology has been around for decades, and Stirling engines date back to the 19th century. But the high cost of silicon-solar cells has made solar-concentrating systems more attractive in desert areas like the Southwest United States and Spain.