A few basic facts about water:
Specific heat of water 4.2 kJ/kg/°C [*]
Heat of evaporation of water: 2256 kJ/kg [**]
[*] – energy required to increase temperature of water with 1 °C
[**] – energy required to turn 1 kg of water of 100 °C into vapor of 100 °C
In other words: it takes just as much energy to bring water from 46 °C to 100 °C as it takes to turn boiling water of 100 °C into water vapor of 100 °C. Or in other words: there is a lot of energy associated with phase change. That energy can be won back by condensing vapor back into water. This is what essentially happens in a steam engine: coal is used to heat water and turn it into steam. Next the steam is expanded in a cylinder where it is condensed. Part of the evaporation heat is converted into the desired mechanical energy or motion. Or think of stepping out of the shower dripping wet. You will feel cold because the drops on your body evaporate, which takes a lot of energy which is extracted from your body.
In nature evaporation and condensing of water happens on a gigantic scale, think of rain. Is there a way of capturing some of this energy for human purposes and convert it into useful energy? Scientists of the University of Columbia think there is. The place to generate electricity would by near the surface of lakes (uh-oh). Spores are attached to a surface, absorb water vapor and expand in volume. The useful energy is generated when the spores release the water as vapor which drives a motor. As MIT Technology Review previously reported:
“An eight-centimeter-by-eight-centimeter water surface can produce about two microwatts of electricity (a microwatt is one-millionth of a watt), on average, and can burst up to 60 microwatts.”
That would be 12.5 * 12.5 * 2 microwatt per m2 or 312 microwatt or 0.312 milliwatt/m2. Which is not too impressive to say the least. A solar panel of 1 m2 in contrast can bring you up to 150 Watt. That’s a difference of 48,000 in efficiency.
[nature.com] – Potential for natural evaporation as a reliable renewable energy resource
[theverge.com] – Water evaporation could be a promising source of renewable energy
[technologyreview.com] – Evaporation Engines Could Produce More Power Than Coal, with a Huge Caveat