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The Rock Doc: A new approach to harvesting the sun’s energy

E. Kirsten Peters

What if there were a two-for-one sale on kilowatts? Your power bill would be cut in half – not a bad result for your monthly budget.

Energy drives everything we produce and consume, and global energy consumption continues to grow year after year. The two-for-one image came to mind as I talked with professor Jeanne McHale of Washington State University. McHale is a chemist who researches an alternative approach to making solar cells that produce electricity.

“There’s no question we have a lot of solar energy that strikes the planet each day,” McHale told me. “It’s an often-quoted statistic that just one hour of sunlight all over the planet has enough energy to give us what we need for a year.”

The challenge is capturing that energy at economical rates. Traditional solar cells are made of expensive, high-tech ingredients. They work, but at a relatively high price and with negative environmental impacts. For some time now, scientists have been looking at an alternative version, called dye-sensitized solar cells. Most researchers use synthetic organic dyes or dyes containing an element called ruthenium. The McHale lab is one of the few using plant dyes.

McHale studies pigments like betanin, one of the molecules that makes beets red. Betanin can be used in these alternative solar cells. Recently McHale and her team found a way to have each photon striking the betanin produce two electrons.

“This means we could double the electrical current of dye-sensitized solar cells,” McHale said.

One of the challenges for McHale is that a one-electron reaction occurs in parallel with the desired two-electron reaction, producing what chemists call a “free radical.” Those are highly reactive and damaging molecules. The free radicals in the dye-sensitized solar cells damage the betanin. Currently, McHale is working on what are called co-pigments – molecules that can be attached to betanin to make it more stable under the influence of free radicals.

“The way I think of it is that we have a molecule that’s a model: one that can help us design better molecules that would produce two electrons per photon without the degradation problem,” McHale said.

Calculations show that the maximum possible efficiency of dye-sensitized cells is about 30 percent. What’s been achieved so far is 13 percent. That doesn’t sound too good until you learn that plants – in their process of photosynthesis – have an efficiency of about 1 percent.

“The joke is that if plants went to the government for funding, they would never be awarded a grant,” McHale said.

Although I’m a geologist who spent part of her early career studying the geology of fossil fuels, I think that securing more of our energy needs through solar power would be potentially good for the planet and a triumph of sophisticated science. Here’s wishing McHale’s lab the very best!

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard universities. This column is a service of the College of Agricultural, Human, and Natural Resource Sciences at Washington State University.
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