Focusing on Solar’s Cost

A Hollywood-based solar startup says that it will soon be able to produce electricity from the sun at costs that are competitive with fossil-fuel generation. The key is the company’s dramatic improvement in the performance of concentrated photovoltaic technology.

Sunrgi, which emerged out of stealth mode last week, has created a concentrated photovoltaic system that uses a lens to focus sunlight up to 2,000 times onto tiny solar cells that can convert 37.5 percent of the sun’s energy into electricity. Stronger concentrations of sunlight allow engineers to use much smaller solar cells, making it more economical to use higher-efficiency–but higher-cost–cells. Sunrgi, for example, will use cells based on gallium arsenside and germanium substrates.

Paul Sidlo, one of seven founding partners of Sunrgi, says that the system uses four times less photovoltaic material than other approaches, which typically aim for 500 times sun concentration. This includes systems being developed by California rivals SolFocus and Soliant Energy.

“We’ve miniaturized everything,” Sidlo says. “What this leads to is reduced cost, and the big breakthrough here is all about lower cost.” The company has also designed its system to be produced on slightly modified computer assembly lines, enabling further savings through high-volume production. The higher efficiency also means that a solar park built with Sunrgi’s modules could use one-sixteenth of the space needed with conventional thin-film solar cells, adds Sidlo. The result is lower real-estate costs for developers.

Sunrgi estimates that its system will be capable of producing electricity at a wholesale cost of five cents per kilowatt-hour. Prototypes have been built and tested both in the laboratory and in the field, and the company expects to start commercial production in 12 to 15 months. “It’s quite an aggressive claim,” says Daniel Friedman, a solar-energy researcher at the U.S. National Renewable Energy Laboratory (NREL). He says that most others in the space are still working toward seven or eight cents per kilowatt-hour. “I can’t say Sunrgi won’t achieve what it’s claiming, but right now, it’s just on paper, and costs like that are only going to be a reality at the large manufacturing level,” he says. “Even then, the five-cent figure sounds really optimistic.”

Arguably the biggest breakthrough for Sunrgi is in the area of heat management, which is essential to any concentrated photovoltaic system. The intense heat created by concentrating the sun so much can reduce both the efficiency and the life of the solar cell. At 2,000 times sun concentration, temperatures can exceed 1800 °C–similar to the heat from an acetylene torch, and hot enough to melt the solar cell.

Cells in such systems are usually cooled through a combination of heat conduction, air or liquid convection, and radiation; the goal is to remove as much of the heat as quickly as possible, says Sunrgi partner KRS Murthy, who has been labeled the “thermal wizard” by his colleagues. “At each stage of conduction, convection, and radiation, we’ve made an improvement over what others have done,” he says.

For example, connected to the bottom of each cell is a small fluid-filled chamber that acts as a heat sink. Murthy says that the fluid contains high-temperature composites and nanomaterials that rapidly remove the heat from the cells. This “super cooling” allows the cells to stay cool enough to work, about 10 to 20 °C above ambient temperatures. Murthy won’t say what materials are in the fluid. “It’s our secret.”

Electronics engineer Thomas Forrester, another founding partner at Sunrgi, says that the chamber isn’t filled with much: “We’re talking as little as drops of liquid.” But it’s enough, he says, to absorb the heat and move it to another part of the cell so that it can dissipate rapidly into the environment. Future versions will attempt to capture that waste heat as useful energy. “We have patents pending on other designs that do this,” he says.

Simon Fafard, founder and chief technology officer at Ottawa-based Cyrium Technologies, a maker of high-end cells for the concentrated photovoltaic market, says that the heavy-duty heat sinks that Sunrgi relies on leave little room for error during manufacturing. “It also makes testing the cells a bit more of a challenge,” he adds.

Forrester says that’s why most of the founders of Sunrgi have an expertise in manufacturing. “The question people ask us is, why hasn’t any other solar company done this?” he says. “Well, we’re taking a different approach that directly applies principles from chip manufacturing. That’s one of the keys to our technology.”

But other challenges remain. Concentrated photovoltaic systems need direct sunlight to work, meaning that they must be designed to track the sun through the day. Fafard says that Sunrgi’s system, at 2,000 times concentration, will need to use tracking with pinpoint accuracy to keep the light focused on the tiny solar cells. He compares it to looking at a star through a telescope: the higher the magnification, the more accuracy is required to keep the star within view of the lens. This makes Sunrgi’s system potentially more vulnerable to the elements. “Wind would definitely be bad,” says NREL’s Friedman. “If the thing is shaking even a little bit, the light will go off the cell.”

The need for direct sunlight also means that concentrated photovoltaic systems don’t work on cloudy or hazy days when conventional solar systems can at least capture some of the sun’s energy. “So it makes the most sense for places like Phoenix, Spain, Australia,” says Fafard.

Sidlo says that Sunrgi will initially be targeting utility-scale developments and is in talks with strategic partners, including manufacturers. The company is currently self-funded but says that it is talking with top venture capitalists.