'Evaporation engines' could be the key to providing a reliable and renewable source of energy to much of the world.
Water released over lakes and reservoirs by the sun's energy could drive power production, while at the same time conserving the resources they cover, a new study claims. 
Researchers created a miniature 'evaporation engine' machine to prove their theory, and used calculations to scale up the results.
They found that their approach could generate three times the power of a wind farm that covered a similar sized area.
Researchers at Columbia University found that US lakes and reservoirs could generate 325 gigawatts of power.
That is nearly 70 per cent of what the United States produced in 2015.
They made the finding using their Evaporation Engine, a small scale version of what could one day become an industrial sized evaporation power plant. 
In a 2015 paper, Dr Sahin showed how this basic process can be exploited to do work.
The current study was designed to test how much power this process could theoretically produce. 
The team estimates that natural evaporation could yield between 2W and 10W per square metre.
This is around three times as much as conventional wind power, according to the researchers, and more than coal.
It would however require every standing body of water larger than 0.04 square miles (0.1 sq km), excluding the Great Lakes, in 48 US states to be covered with the devices.
Covering Lake Windermere in Cumbria could generate enough power for 65,000 homes in the UK, around least 29.5MW. 
'We have the technology to harness energy from wind, water and the sun, but evaporation is just as powerful,' says the study's senior author Ozgur Sahin, a biophysicist at Columbia.
'We can now put a number on its potential.'
Evaporation is nature's way of cycling water between land and air. 

HOW IT WORKS 

The Evaporation Engine, developed in 2015, controls humidity with a shutter that opens and closes, prompting bacterial spores to expand and contract.
The spores' contractions are transferred to a generator that makes electricity. 
When water on the surface below evaporates, it drives piston-like back and forth motion. 
When connected to a generator, that motion produces electricity. 
The evaporation engine sits on the surface of water (blue) here. When water on the surface below evaporates, it drives piston-like back and forth motion. When connected to a generator, that motion produces electricity.
The evaporation engine sits on the surface of water (blue) here. When water on the surface below evaporates, it drives piston-like back and forth motion. When connected to a generator, that motion produces electricity.
The current study was designed to test how much power this process could theoretically produce.
Researchers envisaged a larger version of the machine that uses a water-responsive plastic muscle to absorb moisture.
With its bottom and top shutters closed, the material expands and swells, turning a turbine.
The southern and western United States have the greatest capacity to produce evaporation-generated power from lakes and reservoirs, a new study in Nature Communications finds.
The southern and western United States have the greatest capacity to produce evaporation-generated power from lakes and reservoirs, a new study in Nature Communications finds.