Evaporation, the process that dries washing on the line and supplies clouds with rain water, could one day produce vast stores of clean energy, researchers suggest.
Exploiting this natural process could generate up to 325 gigawatts of power over existing lakes and reservoirs in the United States, equivalent to about 70 percent of the country's electricity generation in 2015, they wrote in the journal Nature Communications.
There is only one problem: the technology to harvest this power does not exist.
"A device that is placed at the air-water interface would be needed to capture this energy," study co-author Ozgur Sahin of Columbia University in New York told AFP.
"Moisture that is coming out of the water body would first go into the device before reaching the atmosphere," he said. "The technology has to be developed."
For their study, Sahin and a team relied on models of the energy-generating potential of evaporation, which is the process by which liquid water turns into gas, or vapour.
They looked at the impacts of humidity, wind speed and temperature on evaporation rates, and assumed the availability of devices to capture the energy produced as a result.
The scientists then calculated the potential of evaporation from water bodies in the United States larger than 100,000 square metres (1 million square feet), other than the Great Lakes.
- Driven by spores? -
Energy harvesters could be based on the natural action of spores -- tiny structures formed by bacteria in response to harsh environmental conditions. Spores can survive drought or freezing to give rise to new bacteria when conditions improve.
"The protective outer layers of spores can absorb and release water when the relative humidity of the surrounding air changes," Sahin said.
"At high humidity, they absorb water and expand, and at low humidity, they release water and contract. In this process they act like a muscle."
Theoretically, spores can be integrated into materials that generate energy when humidity levels change, as during evaporation.
"We have made proof-of-concept devices out of spores that generate electrical energy when they are placed right above water," Sahin said.
"These devices contain spore-coated plastic strips that elongate and shorten with changing humidity.
"We have connected the moving end of the strips to a generator, which produces electricity."
The dam or lake could itself be used to store the sun's heat -- which drives evaporation -- so that power can be generated only when needed, the team said.
Water is an exceptional reservoir of heat, and may provide a solution to the problem of energy "intermittency" -- turbines or solar panels that generate power only when wind or sun is available, the team said.
Distributing evaporation-derived energy would require proximity to the electricity grid, they added. Reservoirs used to generate hydropower are already well placed.
The findings warrant further research into technology to convert energy from evaporation, the researchers said.