CPV systems made of thin-film plastics

Sun & Wind Energy 2/2010
by Reid Smith and Lisa Cohn

Cool Earth Solar, California, a concentrator photovoltaics (CPV) company – formed by a group of scientists and engineers from the California Institute of Technology – is now developing an innovative CPV system design made almost entirely of thin film plastic, an inexpensive and abundant resource.

Cool Earth Solar’s CEO Rob Lamkin and his team started their company with a goal of finding a clean energy solution that could be scaled up to meet the world’s power demands at prices competitive with traditional fossil fuels. In addition, the company wanted to use fewer materials – which in PV systems can be costly and hard to find. “The company set out to find materials that exist in enough abundance on the earth to capture sunlight over millions of acres,” resumes Lamkin.

While many companies focus on increasing efficiency or reducing costs, most are not looking far enough into the future to ensure that their product can be scaled up to meet global demand, Lamkin. Cool Earth Solar is principally concerned with ensuring there are enough materials to meet the expected demand, he adds. “If you look at electricity demand in the US alone, we need 10 million acres of solar panels in order to make a difference on climate change,” says Lamkin. “We didn’t want to provide an energy solution that could not meet the tremendous growth needed for clean energy.”

The limiting factor in the effort to scale up to 10 million acres (1 acre is approx. 4.000 m2) in the next decade is material availability, declares Lamkin. “The land is there, the sun is there, but scaling up and doing it quickly is what is really important, which increasingly becomes a question about materials.” Although the price of silicon and other materials used in solar panels has dropped significantly in the past year, solar companies cannot depend on these prices staying low because the demand will continue to grow.

New technology ...

The Cool Earth CPV design is unique because it uses a fraction of the expensive – and sometimes rare – materials commonly used in PV systems, such as metals, glass, and silicon. To help meet the growing demand, Cool Earth uses thin-film plastic – an appropriate material because global markets produce over 500 billion pounds (225 bil-lion kg) of thin-film plastic every year. The company inflates the thin-film plastic into a balloon-like structure. Cool Earth Solar vacuum deposits aluminium on the plastic, seals it, and inflates it into a balloon shape. “The main structural elements are not steel, aluminum, silicon, or glass; they are air and plastic”, explains Lamkin. The concentrating plastic balloon is ten feet (ca. 3 m) across,
uses two pounds (about 900 g) of plastic, and contains 20 pounds of air. “Air is free and abundant,” he adds.

Cool Earth fills the plastic balloon to the amount needed to allow the sunlight to focus directly onto a nine-inch (22.86 cm) focal point where the solar cell is located. The energy concentrated in the balloon is 400 to 500 times as strong as the sun’s energy. The cells are both crystalline silicon and triple junction solar cells, which can withstand the high concentrated temperatures within the plastic balloon. The small, multi-junction, high-efficiency cells within the balloon allow the CPV system to generate the same amount of electricity as traditional flat-panel PV systems while using up to 300 to 400 times less solar cell material, says Lamkin. “Right now silicon solar cells have the advantage to scale faster and bigger so it seems like a good long-term material.” Triple junction solar cells, on the other hand, have yet to be proven to be scalable but offer the potential to increase efficiency in the future.

The CPV system will be much easier scaled-up in future because the majority of material is thinfilm plastic. In addition to using fewer materials, the plastic is a high-strength, non-toxic polymer that can be recycled and re-purposed into structural and nonstructural building-construction elements, says Lamkin. While using far fewer materials than other companies, Cool Earth Solar’s concentrator offers efficiencies equivalent to other CPV technologies, at about 30 %.

In order to maintain a high efficiency, the plastic balloon and the solar cell are mounted on a groundbased dual axis tracking system that follows the sun across the sky throughout the day in order to maximize efficiency, says Lamkin.

A third-generation prototype will soon be installed near the company’s office in Livermore, California. The first-generation prototype, which Cool Earth also installed near its office, showed the viability of the technology, reports Lamkin. The second generation improved the dual-axis tracking and structural support. The third-generation prototype will increase the useable surface area of the reflective optic, resulting in a 70 % increase in power output.

… and further challenges

Even though Cool Earth Solar offers competitive efficiencies, the company faces several challenges, including maintaining air pressure in the balloon, cooling the solar cells, and developing a new technology for the commercial market. The concentrators must maintain the correct air pressure within the balloon as air temperatures change throughout the day.
Colder air compresses and warmer air expands, so each concentrator has an active air-line connected to it in order to stabilize the air pressure. “We want to maintain the exact focus on the solar cell to maximize power production,” says Lamkin.

In addition to maintaining air pressure, Cool Earth Solar must make sure that the solar cells do not overheat. Instead of using an aluminium cooling system like that used by most CPV technologies, each concentrator has a small water-cooling loop on the backside of the solar cell. “While it is more complex than aluminium, a closed water loop is more efficient,
cheaper, and can scale up easier,” adds Lamkin. In the future, Cool Earth Solar may connect a dozen concentrators to one air supply, one water-pump, and one radiator in order to further reduce costs and simplify installation.

Another challenge for Cool Earth Solar will be proving the viability of the technology at larger scales. Currently, Cool Earth solar is deploying its thirdgeneration prototype that will provide valuable operational data to the company. “This will be the last prototype before we build our first power plant in spring,” announces Lamkin.

The first power plant will be small, consisting of dozens of concentrators with a capacity of between 1 MW and 5 MW. The first power plant will likely be owned and operated by Cool Earth Solar. Lamkin doesn’t want to sell the technology to other users right away. “We want to operate the first power plant because there is still a lot of learning to be gained,” he states. “No one has ever made an inflated solar concentrator to sell electricity.” Cool Earth Solar made this decision despite interest from project developers in the EU and the US in the equipment and design. “We are excited to get to that stage in a few years,” says Lamkin.

As solar becomes more widespread, companies that reduce the need for expensive and rare materials will have a clear advantage, Lamkin is convinced. The goal in five to ten years is to install enough renewable energy to dramatically reduce green house emissions. That means solar firms will increasingly need to consider a broad, long-range view about what technologies can scale-up – and how they can do it quickly, says Lamkin.

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