Evaporation: the overlooked alternative energy source?
So-called evaporation engines could add another stream of renewable power to a diversified energy strategy, researchers say. But the technology has a way to go – and some questions to answer – before it can be deployed.
| New York
There may be a new renewable energy source on the distant horizon.
Evaporation, a key process in the hydrological cycle, is an overlooked source of alternative energy that researchers say could be more reliable than solar or wind power. The technology has a long way to go to reach deployable scale – and there are environmental concerns to consider – but, scientists say, evaporation energy could one day be a vital component of a diversified energy strategy.
Researchers at Columbia University in New York estimate that energy derived from evaporation off lakes and reservoirs could, in some states, exceed energy demand. That figure assumes that freshwater bodies could be covered entirely in evaporation engines, which is not likely to happen. But the researchers hope that their calculations will inspire engineers to explore this untapped natural resource, that they say has the potential to bolster energy security while saving water.
“No one previously has provided an estimate as to how powerful evaporation could be,” says Ahmet-Hamdi Cavusoglu, the lead author of a study published last month in the journal Nature Communications that attempts to quantify the evaporation technology’s energy potential. “With our work, we hope we provide added motivation for future research and development on this evolving class of materials.”
How it works
The evaporation engine described by Dr. Cavusoglu and colleagues relies on some familiar hydroelectric concepts. A dam generates energy by utilizing the flow of water from high to low places: Gravity moves the water downward into a turbine, which rotates to power an electric generator. But water can also move upward through evaporation.
“If we put an engine between an evaporating surface of water and dry air, we can capture energy similar to how we use dams,” says Cavusoglu. “To capture that energy, we need something – an artificial muscle – that mechanically changes due to being wet or dry.”
The researchers previously developed such a muscle out of spores of Bacillus subtilis, a bacteria commonly found in soils, affixed to plastic strips that sit on the water’s surface. This muscle expands and contracts with changes in humidity. As water evaporates from the surface of a lake or reservoir, the bacterial spores absorb the water and the muscle swells. When the muscle reaches capacity, a shutter opens and allows the water to evaporate back into the air. As the spores shrink back to their original size, they pull at a turbine to generate energy.
The team has successfully produced small amounts of energy with their device – just about enough to power a toy car. But their latest study suggests that natural evaporation in US lakes and reservoirs (not including the Great Lakes) could theoretically generate up to 325 gigawatts or 2.85 billion megawatt hours per year. In 15 of 47 US states studied, the researchers found that the total theoretical capacity exceeds consumption.
That estimation depends on the unrealistic assumption that evaporation energy harvesters would cover bodies of water entirely. More likely, researchers say evaporation energy could be one component of a diversified energy strategy that relies on a suite of energy sources.
Building a more reliable energy grid
“The fact is that diversification helps create a stronger and more reliable energy grid,” says Cavusoglu.
Dependence on a single energy resource or provider is thought to reduce overall energy security – volatile prices and reliance on foreign oil can cause both economic and political problems, not to mention potential outages. But by shifting focus to an energy mix of both renewable and nonrenewable sources, and by emphasizing domestic-scale renewables such as rooftop solar, a country can theoretically mitigate those issues and protect itself from energy disruptions.
“As we move to different types of energy sources, we do end up dealing with the question of renewable intermittency,” says Hisham Zerriffi, head of the University of British Columbia’s Energy Resources, Development and Environment Lab. “The wind might not be blowing right now, but the sun is shining. The more you rely on different types of energy sources, the more potentially resilient and energy secure your system is.”
Evaporation energy offers some advantages over wind and solar. While sunlight and wind speed fluctuate depending on the weather and time of day, an evaporation engine could theoretically maintain continuous power flow by using shutters to adjust energy production as conditions change. Such devices can also be built with bio-materials, unlike solar panels and wind turbines.
“The types of active material that capture energy from evaporation are typically biological – spores, wood, hair, silk,” says Cavusoglu. “That means the engines could be grown, in contrast with the steel frequently used for wind turbines and silicon semiconductors used in solar photovoltaics.”
The study also suggests that an evaporation engine would work best in conditions with low relative humidity and low wind speed. As it happens, those are exactly the kind of conditions projected by current climate trajectories, says Catherine O’Reilly, an assistant professor at Illinois State University who studies nutrient cycles and freshwater biogeochemistry.
“We know that there’s going to be longer dry periods punctuated by intense storms, so that should generally increase the amount of time that we do have relatively low humidity,” says Professor O’Reilly. “We also know that there’s sort of a general decrease in wind speeds called global stilling. That also would provide the kinds of conditions that would support this type of technology.”
Heading off environmental concerns
But before evaporation energy can come into its own, researchers will need to address some potential ecological problems posed by their technology. By slowing the amount of direct evaporation from a body of water, Cavusoglu notes, one could inadvertently change water temperatures by reducing the natural effects of evaporative cooling. Alternatively, by physically covering parts of a lake’s surface, these engines could reduce the amount of solar radiation absorbed by the water.
“Because water is very sensitive to changes in temperature, those changes can have lots of different rippling effects through the ecosystem, [such as] how water mixes in the lake and how fast organisms reproduce,” O’Reilly says.
The technology could also affect freshwater oxygen levels, says O’Reilly. In lakes and reservoirs, oxygen is produced by algae, which need sunlight to photosynthesize.
“Putting things on top of a lake can restrict its ability to get oxygen in two ways: by reducing the gas exchange between the atmosphere and the water, and by reducing sunlight available for algal growth,” says O’Reilly. “Under those circumstances, you could expect the amount of oxygen in your water to possibly decrease.”
Such changes could have substantial impacts on both freshwater and ocean ecosystems. So-called dead zones, like the ones found in the Baltic Sea and the Gulf of Mexico, are characterized by hypoxia, or low oxygen levels. Fish die-offs usually follow hypoxic conditions, which are in turn exacerbated by microbes that consume even more oxygen as they break down dead matter. And stagnation isn’t just a problem for marine organisms.
“If a lake or reservoir is used for drinking water and irrigation, these would be serious considerations,” says Stephanie Hampton, director of the Center for Environmental Research, Education and Outreach at Washington State University. “Many lakes are also important for lakeshore economies, recreation, and wildlife. Much depends on how the tech is designed.”
The incentive to address those logistical concerns is significant, the study authors say. In addition to expanding the field of renewable energies, the technology could help save water during drought conditions by cutting evaporative water loss by nearly half. “By slowing down the loss of water due to evaporation, we effectively save water,” says Cavusoglu.
[Editor's note: This story has been updated to reflect a correction that has since been issued by the study authors. The total potential of evaporation energy from all the freshwater bodies in the United States (excluding the Great Lakes) is 2.85 billion megawatt hours per year.]