Friday, January 2, 2009

MIT researchers split water to store solar energy

Nocera and his MIT colleague, Matthew Kanan, on Thursday will publish a technical paper that describes what they claim is a breakthrough in solar energy storage.

The idea is to use the energy from solar photovoltaic panels (or another electricity source) to crack water molecules into hydrogen and oxygen gas. Those gases would be stored and used later in a fuel cell to make electricity when the sun is not shining.

The concept is a closed-loop system: running the hydrogen and water through the fuel cell creates water, which can be captured and used again.

The hope is that within 10 years, a cost-effective system that combines clean energy generation with storage can be engineered and available cheaply to people around the world.

"I'm open-sourcing this to let everybody run with it," he said. "My plan is that when people see it, they'll see it's easy to do and they'll start working it."

Artificial photosynthesis
The core scientific discovery was finding a way to break oxygen out of the water with a relatively inexpensive and benign material, Nocera said. The catalyst--made of a cobalt phosphate--can operate in plain water at atmospheric pressure, giving it more potential than existing methods, he said.

Commercially available electrolyzers already split hydrogen atoms from water. A hydrogen filling station, for example, could use an electric-powered electrolyzer to break off hydrogen from water.

A finished system that MIT researchers envision would separate both hydrogen and oxygen. Once stored, both gases would be fed into a fuel cell using a second catalyst like platinum to make electricity.

John Turner, a research fellow in photoelectric chemistry at the National Renewable Energy Laboratories (NREL), called the work a "significant result."

But he said that a number of improvements still need to be made before realizing the "hydrogen economy." Right now, systems to produce hydrogen and oxygen from water would require huge amounts of land and materials to make catalysts.

"The initial results look promising but it doesn't answer all the things you need in a catalyst," he said. Turner's research focuses on improving ways of harvesting light energy to crack water molecules.

Using the process of photosynthesis as inspiration, Nocera has spent 25 years researching a way to tap the energy in water molecules' bonds.

He envisions a complete break with existing power distribution infrastructure where each home can produce and store enough electricity to be self-sufficient.

Today, when solar panels generate more electricity than a home is using, the excess is simply fed back into the grid, essentially subtracting from the homeowner's utility bill. In an off-grid application, the excess is put into batteries.

But with radically cheaper storage, remote power plants running on polluting fossil fuels are cut out of the picture altogether.

"It's not totally practical, but because it's easy, people are already all over it," Nocera said. "Being a scientist, I can play outside the rule box."

Active field of work

He said colleagues at MIT's electrical and mechanical engineering departments have already committed to working with the research, which MIT has patented. The work came out of a university-wide energy initiative launched in 2006.

Nocera said that an MIT-Abu Dhabi venture, called Masdar City, to create a self-powering "sustainable city" in the middle east could be a proving ground for the storage method.

On-demand, or "dispatchable," energy storage holds back broader adoption of renewable energy sources like solar and wind. But it is an active area of research and development for centralized power plants.

The newest generation of solar thermal power plants will store hot water or molten salt to provide several hours or even days of electricity. Batteries or pressurized air in underground formations are also considered viable, if niche, technologies for power grid storage.

But wide-scale distributed power generation in people's homes, combined with hydrogen fuel cells, is mostly just a vision at this point.

Turner said that many researchers are pursuing the same goal of cracking water to make hydrogen without losing too much energy in the process. Although existing electrolyzers are expensive, the challenge is devising a system that efficient enough to make energy inexpensively.

"If we're successful, then we'll compete with electrolyzers," he said. "If not, we'll have to find another way to get hydrogen."


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