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Evaporation from the lakes and reservoirs in the US could generate nearly 70 per cent of the power that the country currently produces, a study has found. Though still limited to experiments in the lab, evaporation-harvested power could be made on demand, day or night, overcoming the intermittency problems plaguing solar and wind energy. "We have the technology to harness energy from wind, water and the Sun, but evaporation is just as powerful. We can now put a number on its potential," said Ozgur Sahin, a biophysicist at Columbia University in the US. Researchers have previously shown how evaporation can be exploited to do work. One machine developed, dubbed the Evaporation Engine, 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. The new study, published in the journal Nature Communications, was designed to test how much power this process could produce. One benefit of evaporation is that it can be generated only when needed. Solar and wind power, by contrast, require batteries to supply power when the Sun is not shining or the wind is not blowing.
Batteries are expensive and require toxic materials to manufacture. "Evaporation comes with a natural battery. You can make it your main source of power and draw on solar and wind when they are available," said Ahmet-Hamdi Cavusoglu, a graduate student at Columbia. Evaporation technology can also save water. In the study, researchers estimate that half of the water naturally lost from lakes and reservoirs through evaporation could be saved during the energy-harvesting process. In their model, that came to 25 trillion gallons a year. States with growing populations and sunnier weather can best capitalise on evaporation's capacity to generate power and reduce water waste, in part because evaporation packs more energy in warm and dry conditions, researchers said. Drought-prone California, Nevada and Arizona could benefit most. The researchers simplified their model in several ways to test evaporation's potential. They limited their calculations to the US, where weather station data is easily accessible, and excluded prime locations such as farmland, rivers, the Great Lakes, and coastlines, to limit errors associated with modelling more complex interactions. They also made the assumption that technology to harvest energy from evaporation efficiently is fully developed.