Scientists are trying to bottle solar energy and turn it into liquid fuel

Scientists are trying to bottle solar energy and turn it into liquid fuel

- in Science
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By Wayt Gibbs

What if we could bottle solar energy so it could be used to power our homes and factories even when the sun doesn’t shine?

Scientists have spent decades looking for a way do just that, and now researchers in Sweden are reporting significant progress. They’ve developed a specialized fluid that absorbs a bit of sunlight’s energy, holds it for months or even years and then releases it when needed. If this so-called solar thermal fuel can be perfected, it might drive another nail in the coffin of fossil fuels — and help solve our global-warming crisis.

Unlike oil, coal and natural gas, solar thermal fuels are reusable and environmentally friendly. They release energy without spewing carbon dioxide and other greenhouse gases into the atmosphere.

“A solar thermal fuel is like a rechargeable battery, but instead of electricity, you put sunlight in and get heat out, triggered on demand,” says Jeffrey Grossman, who leads a lab at MIT that works on such materials.

A molecular Jekyll and Hyde

On the roof of the physics building at Chalmers University of Technology in the Swedish city of Gothenburg, Kasper Moth-Poulsen has built a prototype system to test the new solar thermal fuels his research group has created.

As a pump cycles the fluid through transparent tubes, ultraviolet light from the sun excites its molecules into an energized state, a bit like Dr. Jekyll transforming into Mr. Hyde. The light rearranges bonds among the carbon, hydrogen and nitrogen atoms in the fuel, converting a compound known as norbornadiene into another called quadricyclane — the energetic Mr. Hyde version. Because the energy is trapped in strong chemical bonds, the quadricyclane retains the captured solar power even when it cools down.

Image: The energy system MOST works in a circular manner
The energy system works in a circular manner. First, the liquid captures energy from sunlight, in a solar thermal collector on the roof of a building. Then it is stored at room temperature. When the energy is needed, it can be drawn through the catalyst so that the liquid heats up.Yen Strandqvist

To extract that stored energy, Moth-Poulsen passes the activated fuel over a cobalt-based catalyst. The Hyde-like quadricyclane molecules then shapeshift back into their Jekyll form, norbornadiene. The transformation releases copious amounts of heat — enough to raise the fuel’s temperature by 63 degrees Celsius (113 degrees Fahrenheit).

If the fuel starts at room temperature (about 21 degrees C, or 70 degrees F), it quickly warms to around 84 degrees C (183 degrees F) — easily hot enough to heat a house or office.

“You could use that thermal energy for your water heater, your dishwasher or your clothes dryer,” Grossman says. “There could be lots of industrial applications as well.” Low-temperature heat used for cooking, sterilization, bleaching, distillation and other commercial operations accounts for 7 percent of all energy consumption in the European Union, Moth-Poulsen says.

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