Green Tech

Cool Earth Solar has one of those radical green-tech ideas that may actually make a real commercial impact.

In the next two weeks, the company plans to start testing a prototype solar plant built around rows of reflective balloons hung on poles. The solar balloons, which are eight feet in diameter, look something like a tube for sledding or laying around the pool, but each one can generate 1 kilowatt of electricity.

An early version of Cool Earth Solar's solar concentrator without the 'receiver' that holds the solar cell.

(Credit: Cool Earth Solar)

It's a design that combines cheap building materials, notably plastics, with expensive high-efficiency solar cells. Light goes through the side of the balloon facing the sun, is reflected on an aluminum coating on the bottom, and is concentrated onto solar cells in a "receiver."

The method can concentrate light between 300 and 400 times. To keep heat under control, the balloons have an automated water-cooling system.

The test installation for Cool Earth Solar, which was founded in 2006, will be small--on the order of a few dozen suspended balloons, according to a company representative. That's so that the company can make changes to the second version of its concentrating solar balloon.

Following that test system, which will generate about 100 or 200 kilowatts of electricity when done, the company intends to start building a 1.5-megawatt commercial solar power plant this winter, said CEO Rob Lamkin. One and half megawatts is enough power to supply about 400 or 500 U.S. homes. That installation, expected to be constructed in Tracy, Calif., will sell electricity to a utility.

Following that, Cool Earth Solar hopes to ramp up quickly, Lamkin said. It is planning another test facility, sized at 10 megawatts, for next summer, he said.

Unlike the rest
Cool Earth Solar's design is nothing like most concentrating photovoltaic (CPV) systems, which concentrate sunlight onto solar cells to make electricity.

Typically, concentrating solar devices use lenses and mirrors to direct and focus light to squeeze more electricity from high-end cells. The lenses, as well as the mounting and cooling systems, add significantly to the cost.

Cool Earth Solar's device, too, concentrates light onto expensive solar cells. But using materials like aluminum-coated plastic--the same sort of thing you'd find in a Power Bar wrapper--means the company can make electricity at $1 per watt, a few dollars per watt cheaper than other companies, Lamkin said.

Early version of Cool Earth Solar's concentrators mounted on poles.

(Credit: Cool Earth Solar)

He said that Cool Earth Solar will be able to sell electricity to utilities at the low end of what solar power plant providers project, which is in the range of 10 cents to 14 cents per kilowatt-hour.

Equally significant to cost is the flexibility that the balloons allow in terms of location.

Concentrating photovoltaic systems demand the right sun conditions, making them suitable only for certain areas like the southwest United States. That has set off a race among solar technology companies to find and get approvals for appropriate land. But Lamkin said that Cool Earth Solar is looking at a far broader set of possible locations where land is not as much in demand.

"It turns out, land is not an issue for us," he said. "What's exciting is because we are modular, we don't need to find super flat land in the lower Mojave desert. We're happy to be in farm land in rural California that no other solar companies are going after."

Cool Earth Solar raised $21 million in funding earlier this year. Lamkin said the company doesn't intend to raise more money in the coming year.

There are two approaches to magnifying sunlight to make electricity--either concentrating light to make heat or concentrating light to boost solar cells' production.

Cyrium Technologies on Wednesday said that it has raised $15 million to compete with incumbent concentrating photovoltaic cell manufacturers. The series B round was led by the Quercus Trust.

Click on the image to see a photo gallery of concentrating photovoltaic technologies.

(Credit: SolFocus)

The company is one of a handful of firms that makes high-end, efficient solar cells. These high-efficiency cells are used in conjunction with mirrors or lenses to boost output of solar arrays.

Commercial versions of these multijunction cells--essentially three cells stacked on top of each other--have an efficiency of about 37 percent, far higher than silicon cells, which are in the 15 to 20 percent range. But they are substantially more expensive and only used for specialized applications like concentrating photovoltaics or for satellites.

The two leading companies in this area are Emcore Solar Photovoltaics and Boeing company Spectrolab. These companies sell to concentrating photovoltaics firms that design solar arrays around the cells.

At an industry conference earlier this year, Cyrium Technologies President and CEO Stephen Eglash said the company intends to compete with Emcore and Spectrolab by delivering more cost-effective and efficient cells.

The company intends to modify the individual cells by applying quantum dots, a nanotechnology approach which will increase output, he said. Although he didn't specify when products would be available, Eglash said that its first-generation cells will have efficiency of 41 percent, and the second generation will have 45 percent efficiency--higher than most cells created by researchers.

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.

Click on the image to see a photo gallery of utilty-scale concentrating solar power technologies.

(Credit: SolFocus)

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.

Click on the image to see a photo gallery of concentrating photovoltaic arrays.

(Credit: Amonix)

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.

IBM has developed technology that will let solar cells withstand the heat of more than a 1,000 suns.

At a technical conference on Thursday, representatives from IBM Research's photovoltaics research will present a method for cooling concentrating photovoltaics, a solar design where light is magnified onto high-performance solar cells.

A SolFocus 6.2-kilowatt CPV device being tested in Spain. Click on the image to see a photo gallery of other concentrating photovoltaic systems.

(Credit: SolFocus)

Heat is a serious issue when it comes to concentrating photovoltaics, or CPV. The efficiency of cells degrades at high heat and can damage, and conceivably destroy, equipment at extremely high temperatures.

IBM said that its liquid-metal cooling technique, adapted from high-powered computers' chips, can remove roughly three-quarters of the heat generated by a CPV system.

CPV arrays use lenses and mirrors to magnify light onto solar cells that convert light to electricity. By cramming more light onto cells, the panels can generate more electricity.

The technology, which has been around for decades, is being pursued once again by a number of companies, most of which are designing systems for solar power plants.

These plants have rows of CPV systems that track the sun during the day and magnify light hundreds of times.

As part of IBM's Big Green Innovations initiative, researchers looked into applying the company's chip design and manufacturing expertise in solar. It found that CPV companies had not paid enough attention to thermal problems, particularly as they move to higher light concentrations.

"It's clear that everybody wants to go to higher concentrations," said Supratik Guha, lead scientist for photovoltaics at IBM Research. "In the last few years, CPV has sort of been trying to make a comeback and if you look at the numbers, it does have the potential to be really cheap."

IBM looking at CIGS
Because heat dissipation is important for its high-end processors, IBM has developed a cooling system where a thin layer of liquid metals circulates behind a chip to transfer the heat from the chip to a "cooling block."

IBM has built a prototype of a solar concentrator that uses the cooling technology from its high-end chips.

(Credit: IBM)

IBM has built a prototype of the thermal interface layer on a CPV system. In tests, it found that the technique can dramatically lower the heat of high-concentration devices.

The technique only makes sense for very high levels of concentration, which are used on expensive, high-efficiency triple-junction solar cells, said Guha.

IBM doesn't intend to manufacture CPV devices itself but it does hope to license its thermal interface layer to solar manufacturers, he said.

In addition to working on thermal issues, IBM Research is working on "solution process" techniques for manufacturing CIGS cells. The solution process would be an alternative to the slower evaporation process for making CIGS cells.

IBM is also working on manufacturing silicon solar cells on glass. More fundamental research focuses on improving solar cell efficiency through nanowires and nanoparticles, explained Guha.