NEW SOLAR ARRAY SHOWS PROMISE

posted by Paul Ingram

On the University of Arizona campus, researchers with the school’s optical science department are developing a new solar array that could make solar energy cheaper than its competition.

Led by Dr. Roger Angel, in a partnership with REhnu (renew) energy the team is designing a solar collector array using techniques developed by the school’s optical science department. Their goal: getting below a cost of $1 per watt, a target that makes solar energy competitive with nonrenewable energy sources like coal, oil, and gas.

There are several ways to do this, but the team has settled on a design that uses a hemisphere of concentrated photovoltaic cells that sits at the focus of light reflected by a large mirror that’s nearly ten feet across.

The system is fixed to a structure that can follow the sun throughout the day using tracking systems developed by the optical science department for telescopes, including sensors to seek the sun, a GPS system to help the tracker orient to the sun, and motors to shift and tilt the assembly as the sun crosses the sky.

Unlike rooftop systems, which remain in fixed angle to the sun, and therefore lose efficiency in the early morning and late afternoon, following the sun allows the system to collect energy from dawn to dusk.

The system uses a lightweight steel structure, assembled in open cube frames, allowing the system to hold up to eight collectors in each. The structure has the advantage of being relatively stiff, but uses less steel, fulfilling one of the overall goals of the project: shrinking costs by reducing the number of materials.

Blake Coughenour is a research associate and graduate student with the optical sciences department and helped develop the collector.

“I think it’s important because solar is a great resource that we’re not ever taking advantage of, especially in the American southwest we’re protecting ourselves from so much light that we’re wasting,” he said. “If we can find a way to efficiently use that light, we can really solve the world’s energy problems.”

Whittling down the cost and complexity of the device has been a major goal. In the lab at Steward Observatory, the team went through several prototypes for testing, learning as they went even while keeping on eye on an commonly-used efficiency goal of 30 percent. Older solar systems are about 10 percent efficient, meaning that the system captures 10 percent of the energy that's available. Tripling that number makes solar economically feasible.

The energy coming from the mirror is intense, creating several design challenges that the team had to overcome.

“This is a quarter inch piece of steel,” he said. “If we put this at the focus of our mirror, we can burn a hole through it in about five seconds.”

They solved this in two ways, first at the focus on the mirror is a glass sphere, like a fortune teller’s crystal ball. However, the glass ball is made of specialized and expensive fuzed glass.

Properties of the glass make it transparent to nearly all light wavelengths, which means it doesn’t block light from the mirror and it won’t heat up like other glasses. As Coughenour notes, it’s one of the few glasses that crack or “blow up” from the mirror’s energy.

As Coughenour said, “It’s safe, but you’re putting a lot of light into one little place.”

The glass ball also creates a lensing effect, allowing it to keep light on the cPV cells even as the structure moves. This allows the collector to keep pumping out energy under windy conditions. For a solar array sitting in the desert, this is a nice advantage.

They also added a reflective coating to the glass mount, allowing the metal to survive moments when the system is “off sun” and the ray of reflected light hits it rather than the glass sphere. In earlier tests, cables and struts that were in the glare sometimes melted or smoldered.

Behind the glass, a copper bowl holds the cPV cells, allowing each one to get as much light as possible through small funnels that are focused on each cell. This means more energy, but it also means more heat, which requires a closed-loop cooling system similar to the one in a car and a pattern of metal fins at the back they copied from high-end personal computers.

“If we didn’t have a cooing system, the cells would heat up to a couple hundred degrees and they would melt,” said Coughenour. While a cooling system adds complexity, he said, it also allows for a denser system of solar cells, dropping the cost and making it easier to pack many more arrays on a chunk of land.

High-efficiency fans cool the water in an assembly above the collector.

The team is still learning. The current system will move from its home behind Bear Down gym to the University of Arizona’s tech park in the next few months, adding the the UA’s “solar zone.” There, the team will work to make future versions more efficient, using easily to manufacture parts, while producing even more energy.

Their ultimate goal is to deploy these systems at large scale. According to the REhnu website, the corporate partner of the work, a system deployed over six acres could produce 2.5 billion kilowatts a year, enough to power 220,000 homes.

“If we show that we can build this thing, we can take it to manufacturing scale and start rolling out huge megawatt-sized plants,” said Coughenour.