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By Ed Stiles, College of Engineering
May 23, 2008

Anyone who says there's no free lunch has never used a solar cooker.

Not only is the fuel free, but a solar stove doesn't generate greenhouse gases from burning fossil fuels.

Still, not even solar cookers come without a price. First, you need sunlight. Fortunately that's no problem in the American Southwest and other arid regions of the world, where cloudy days are the exception. So people living in those areas are good to go.

Second, however, you have to cook outside, and you usually have to use a parabolic dish to concentrate the sunlight. This can be very inconvenient and a potential safety hazard – both of which are deal breakers for a lot of people who might otherwise benefit from solar cooking.

Peiwen Li, a mechanical engineering professor at The University of Arizona, hopes to change that by bringing solar cooking indoors.

A Step Closer to the Kitchen

Under Li's direction, a team of engineering students built an indoor, solar-powered stovetop for their senior design project.

What they needed first was a way to concentrate the sunlight outdoors. Then they needed a way to transport the heat generated, or the sunlight itself, indoors.

"We first thought about using a glass dome to concentrate the sunlight, but when we talked with optics professors, we found that was going to be really expensive," said Amy Sopko, a mechanical engineering senior. So the students went online and found a 3-foot-by-2-foot Fresnel lens for only $100. Fresnel lenses, which were first designed for lighthouses, offer a lightweight, inexpensive alternative to conventional glass lenses, Sopko explained.

They bought the lens and started testing it. When they found that it could concentrate sunlight to melt copper, they knew they had a winner.

On to the second step: Early on they decided to transport sunlight indoors because that would avoid significant heat losses and the danger of having very hot pipes running through walls.

Fiber Optics Wins Out

First they thought about tubes and reflectors, but that was cumbersome. "So fiber optics cables turned out to be the best way because you can easily maneuver them through your house," Sopko said.

The students found silica glass cable that was ideal because it could withstand the heat generated by the lens. There was just one problem: A 1-inch bundle of cables made after stripping the cladding from each tiny cable cost $11,000.

"So we ended up going with plastic fiber optic cables instead, which turned out to be pretty limiting for us because the Fresnel lens concentrates the light at a focal point and gets so hot that we started to melt the cable," Sopko said. Even worse, once one tiny fiber melts, it no longer transmits light and its load has to be shared by the other cables, which get hotter, start to melt, and very soon a domino effect renders the whole bundle useless.

By covering most of the lens with cardboard, the students were able to keep the heat down to about 170 degrees Farenheit, which preserved the plastic fiber optic cables – at least until one windy day when they sat down to talk about test results and the cardboard blew off. "Within about two seconds, the wood that we had holding the cable ignited," Sopko remembered.

After that, they built a covering for the lens with a number of circular holes that could be opened individually to control the temperature.

At the cooktop end of the cable, the students built a burner shaped like an inverted pot. They painted it black inside and positioned the end of the fiber optic cable about an inch from the surface. "The heat is generated in that little air space between the fiber optic cables and the surface," Sopko said.

With only 170 degrees at the lens, the burner temperature rose to only 150 degrees. Not enough to boil water, but enough to warm food.

"This is definitely a practical concept," Sopko said. "You just have to find the right cable that can handle the heat and is cheap enough."

That's the assignment for the senior design team that will take over this project next year, Li said. "The data we obtained from this project will allow us to evaluate the light-transfer capacity of our optical cable."

"I want to pique the interest of industry to invest or collaborate to develop a solar light transfer optical cable particularly for the product we are developing. Given the necessity for greener technologies, I believe there is a huge interest in using concentrated solar light for indoor use for both cooking and heating."

The senior design team included Sopko, Kelly Stewart, Derek Downey, and Stacy Darris. All are seniors in mechanical engineering.