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Their solar illuminator will help professor Raymond Kostuk test systems that combine optical concentrators with conventional solar cells.

While renewable energy resources are environmentally responsible, they're not necessarily cheaper when compared with fossil fuels.

When their price is competitive – and can even save us money – they catch on fast. That's why you find a hybrid car at nearly every stoplight these days, but few subdivisions with solar-powered homes.

Electricity from photovoltaics still costs about four to five times as much as that produced by fossil fuels, said Daniel Bauer, a senior in optical sciences and engineering, or OSE, at The University of Arizona.

Bauer and three other students demonstrated their “Solar Illuminator and Solar Cell Evaluation” senior design project last week during Engineering Design Day, an annual event on the UA campus in which senior design teams display their projects and compete for cash prizes.

Part of a Larger Project

The project Bauer and the other team members designed is part of professor Raymond Kostuk’s ongoing research effort to produce more efficient photovoltaic systems.

Kostuk, who holds a joint appointment in electrical and computer engineering, known as ECE, and optical sciences, plans to use the students’ solar illuminator to evaluate the performance of commercial and experimental solar cells at moderate levels of sunlight concentration – power levels equivalent to that produced by two to four suns.

As part of the project, the students evaluated three types of photovoltaic cells – crystalline, multicrystalline and amorphous silicon. All of the cells produced more electricity as the solar energy increased. But the cells were less efficient as the energy increased.

They didn't produce twice as much electricity when the solar power was doubled, for instance. That means it would be more cost effective to use two solar cells, rather than one with a device to concentrate sunlight to two suns, unless the concentrator was quite a bit less expensive than the second solar cell.

The students also found that you get what you pay for. The more expensive cells were more efficient. The trick is to decide how much power you need and to then decide if buying fewer expensive cells would be more cost effective than buying more of the less efficient ones, said OSE senior Stephanie Barnes.

The illuminator includes a Fresnel lens to concentrate sunlight and a collimation lens to create a uniform beam. The lenses are mounted on an optical rail and sunlight is focused onto a 3-inch-square area of each solar cell.

Finding the Optimal Load

The students varied the voltage and current being drawn from the solar cell to produce a current-versus-voltage curve for load resistances ranging from almost zero to almost infinity.

“This allowed us to look at a whole range of currents and voltages and find the point on the curve where the cell was most efficient,” said ECE senior Kyle Yeager.

“Knowing the most efficient point on the curve is important when designing solar arrays for homes and other applications,” added team member Ian Tilford, who also is an OSE senior.

Kostuk will now add the students' solar illuminator to his lab gear. “I plan to use this device to evaluate passive concentrator systems that do not require tracking the sun,” Kostuk said. “This is important for developing low-cost concentrator systems that can take advantage of conventional photovoltaic cells.”

Kostuk plans to fund another senior design project this fall that will monitor the passive tracking capability and the effects of temperature variations on the performance of photovoltaic systems that use different kinds of concentrators.

Kostuk is funding the student projects with an honorarium he receives as the Kenneth VonBehren Chaired Professor in ECE.

He also is teaching a new course this semester on solar photovoltaics in which students learn about photovoltaic devices and system design.