Renewable Energy from Evaporation

Evaporating water can act as an energy source to generate electricity using bacterial spores found in soil.

Researchers at Columbia University have discovered a novel way to harness energy from evaporating water, using bacterial spores as 'middle-men'.

Water molecules evaporating from the surface of a pool of water can be captured by the spores, which extract energy, and then release the water molecules back into the atmosphere.

Ozgur Sahin, lead author of the paper published in this week's Nature Communications, explains, "We came up with this idea whilst we were studying bacterial spores. We were interested in their mechanical properties and scientists had previously shown that spores can take up moisture, expand, and then contract when they give the moisture back. We found that this process comes with a lot of force and this can be used as a source of energy."

The bacterial spores swell and shrink according to changing levels of humidity, creating a force that can be used to push or pull objects. By fixing the spores to a thin double-sided plastic tape, anchored at one or both ends, this force can then be transferred to a generator to produce electricity.

The mechanism works as a new form of 'artificial muscle' which is powered by evaporating water.

"Evaporation is a great force of nature," says Sahin, "It is a process that takes place everywhere that water is present."

Heat energy is absorbed by the top layer of particles at the surface of the water causing them to vibrate and become excited. Eventually they have enough energy to break free from the bonds attaching them to their neighbouring particles and enter the atmosphere as a gas in the form of water vapour.

Since energy cannot be created nor destroyed, the laws of physics tell us that it should theoretically be possible to capture the energy used for evaporation. But the difficult question is how to go about doing this.

Sahin and his team believe they have the answer with two ingenious devices: the 'Evaporation Engine' and the 'Moisture Mill'. The latter has been used successfully to power 'Eva', the world's first evaporation-driven car.

Ultimately, they would like to scale-up the technology to harness the energy from much larger bodies of water, such as lakes and reservoirs.

"Reservoirs evaporate around 5% of freshwater, so by using this technology that actually slows down the rate of evaporation, we can not only generate energy from this process, but also save water. The device can be optimised to reduce the evaporation rate by half," says Sahin.

There is still a long way to go before we see such developments, but evaporation certainly has the potential to provide more energy than current renewable sources.

"The energy of the Sun goes through many stages before it creates wind and waves. Evaporation is at an earlier stage in the cycle and so it is advantageous to capture the energy from evaporation. It is more powerful than wind and waves," Sahin explains.