Imagine coating the roof of your house with a paint that absorbs energy from the sun – and lets you use that energy to power your television, computer or toaster.

Northwestern chemistry professor Mark Ratner hopes that one day you’ll be able to do just that with a can of paint he calls “a battery in a jar.”

The technology would use tiny nanostructures to convert sunlight into energy, similarly to the process of photosynthesis in plants.

It’s just one application of nanotechnology to the energy problem, Ratner said Wednesday night at the monthly Science Café event in Evanston. His talk covered the science behind innovations that could provide clean and efficient energy alternatives.

The problem of scale

With oil prices topping $100 a barrel this week, and recent studies suggesting ethanol and other plant-based fuels may be worse for the environment than conventional fuels, pressure is growing to find a better solution.

“The real issue is that there are a lot of us. There are six billion of us. And there are going to be more. And that means that no little solutions are really very interesting,” Ratner said.

Wind and geothermal power can provide clean energy, but not enough of it. “As wonderful as it would be to have a windmill in everybody’s back yard generating energy for their house, that’s not going to do it for the Earth,” Ratner said. “There isn’t enough energy that way.”

For a solution to be truly effective, it must be scalable. That is, it must produce enough energy to meet the world’s needs – especially considering the rapid growth of countries like India and China.

“They are going to be where we are in a few years,” he said. “And if India and China use energy the way we use energy, then it’s going to get hard to breathe, and the polar bears are going to have a rough time, and the seas are going to get warmer, and the coral reefs are going to die, and it’s going to be a different world.”

A new kind of solar panel

So what is the best scalable energy source? The sun, Ratner said.

“Coal, oil, wind, biomass – all that energy is originally solar energy,” he said. “The energy came here from the sun. And leaves, which are nanostructures, turned it into the kinds of energy that we use today.”

Scientists are now trying to design solar panels using nanostructures that work like leaves, but better. The goal is 30 percent efficiency in converting sunlight into power – much higher than the efficiency of biofuels.

“The corn organism is 3 percent efficient in harvesting the energy of the sun,” Ratner said. “You’ve got to do better than that.” Miscanthus grass, another source of biofuel, is less than 5 percent efficient.

While conventional solar panels made from silicon are about 18 percent efficient, “the cost involved in making them is so high,” he said, “that they’d have to run for several years just to pay back the energy cost in making them.”

Nanostructures, on the other hand, would use inexpensive materials to capture sunlight. That’s where the blue jeans and house paint come in.

In artificial photosynthesis, you need a molecule to absorb the sunlight, but not any molecule will do. (See accompanying video for an explanation of how photosynthesis works.)

“The molecules that we probably want to use are related to the blue jean dye that you’ve got,” Ratner said. “It’s a planar molecule, it has the right shape and it has the right energy properties.”

The dye is called a thalocyanine and is also found in shoe polish.

Once the molecules capture solar energy, that energy must be stored somewhere – otherwise, it will be given off as heat. White house paint contains titanium dioxide, and when mixed with the dye molecules, titanium dioxide holds on to the energy the dye collects.

Turning concept into reality

The next challenge is to develop the right kind of wire to get the energy back out of the paint and dye mixture.

“Right now, that’s a bottleneck,” Ratner said. “Nobody’s found the right wire to be compatible with this whole thing.”

The titanium dioxide in paint has been shown to be up to 12 percent efficient in capturing energy, but there’s still a long way to go.

“When you design a solar energy system, the important point is the word ‘system.’ It’s not like taking an aspirin, which does one thing and, you know, it’s great,” he said. “This has to capture the energy, separate the charges, hold the charges, recombine the charges and do it all efficiently. And do it in a way that’s sustainable and do it in a way that won’t break anything. So you have to be able to use it at least 500 million times in order for it to be practical.”

So will the solar panel paint ever be developed?

“I actually have a little bit of money from the U.S. government to do exactly that,” Ratner said. “They’re interested in, for example, [paint-powered] remote sensors. They would like to power a sensor that’s out in the middle of the desert somewhere trying to count neutrons. Or they would like to [use it to] power a sensor that’s on the highway seeing how fast you’re driving.”