Energy from Evaporation
July 16, 2015
My first lesson in the thermodynamics of evaporation came from my high school chemistry teacher. As most people know, teacher's salaries aren't all that high. This teacher lived with his family in the rented upstairs apartment of a two-family house, since he couldn't afford his own house. His heating was supplied as part of his rent, but his thermostat temperature adjustment was fixed at a setting too low for his preference.
His trick for getting more heat when he felt too cold was to place an alcohol-soaked rag over the thermostat. The evaporation of the alcohol caused a local cooling that tricked the device into giving more heat. The physical mechanism for this is the latent heat of vaporization (also called the enthalpy of vaporization) combined with the temperature change associated with heat capacity.
The latent heat of vaporization of isopropyl alcohol (IPA) at "standard conditions" of 298.15 K and 1 atm is about 45 kilojoule per mole (kJ/mol). The "rubbing alcohol" used by my chemistry teacher is not 100% isopropyl alcohol; rather, it's a mixture of water with only about 70% alcohol. In that case, one milliliter contains about 0.009 moles of IPA.
The heat capacity of water is about 4.18 J/g-K, and the heat capacity of IPA is about 2.68 J/g-K. As you can see, this is turning into a major calculation, since the concentration of IPA and the volume of solution will change over time. It's time for a rough estimate. Cranking through some numbers reveals a cooling of at least 10 °C when a milliliter of the mixture is evaporated. The evaporation happens slowly, so we can expect our teacher's thermostat can be fooled into thinking a room is a few degrees Fahrenheit cooler than it really is.
This simple evaporation process could be used as a costly and inefficient energy source by spreading the alcohol on a thermopile to convert a temperature difference to an electric current. However, if you're trapped in a bar during a hurricane and happen to have a thermoelectric generator in your pocket, you might power a radio with a bottle of vodka.
Such a thing could be done less expensively, but less efficiently, with water. Water has an enthalpy of vaporization of about 44 kJ/mol, nearly identical to that of IPA. However, the vapor pressure of water at room temperature is just 2.3 kPa, compared with IPA's 6 kPa, so its evaporation rate is slower.
A team of scientists and engineers from Columbia University (New York, New York), with an assist from a microbiologist at Loyola University (Chicago, Illinois), has taken an alternative route to extraction of mechanical energy from the evaporation of water. They use the property of bacterial spores that they shrink and swell with changing humidity. In that way, they can be used to exert a mechanical force to push and pull other objects.[2-]
As anyone can see while looking outside after a warm summer's rainfall, evaporation is an ubiquitous natural phenomenon. It's a major form of energy transfer affecting Earth's climate. Says Ozgur Sahin, an associate professor of biological sciences and physics at Columbia University and the study's lead author,
"Evaporation is a fundamental force of nature... It's everywhere, and it's more powerful than other forces like wind and waves... Our climate is powered by evaporating water from the oceans, and we have no direct way of accessing this energy."[3,6]
There's been previous research on materials that respond mechanically to chemical stimuli, and there have also been biomimetic systems that oscillate, transport fluid, and change shape. These materials have been far less efficient at generating work compared with conventional actuators. Last year, researchers from Columbia University, Harvard University, and Loyola University teamed in a demonstration that Bacillus spores exert a mechanical response of 10 MJ/m3 to water gradients. This is a thousand times more force than human muscles and a hundred times greater than synthetic water-responsive materials.[5-6]
As a survival mechanism, the Bacillus microorganism forms a rigid, dormant spore when starved. When these spores are exposed to humidity, they expand up to 40% in volume, and the effect is reversible. As Sahin explains,
"Changing size this much is highly unusual for a material that is as rigid as wood or plastic... We figured that expanding and contracting spores can act like a muscle, pushing and pulling other objects. We noticed that we could harness the motion of spores and convert it to electrical energy."
In last year's study, the research team identified mutations that almost double the energy density of the spores. They were also able to get the spores to self-assemble into dense, submicrometer-thick monolayers on various substrates, including sheets of elastomer and silicon microcantilevers. In that first study, the research team demonstrated the utility of their material by building an energy-harvesting device in which movement of a flexing membrane creates electricity by moving a magnet inside a coil of wire (see figure).
In the recently published study, the researchers have produced additional devices in which the spores are used for locomotion and electricity generation. The devices, which generate piston and rotary motion, start spontaneously when placed in the humid region above a water surface.[2-3] There's a piston coupled to an electrical generator that produces enough power to flash a light-emitting diode, and a 100 gram miniature car powered by the evaporation of a contained water source (see figure).
While the car's engine concept, called a "moisture mill," could be used as an electrical generator, a much better device architecture is found in their version of an artificial muscle. They constructed such muscles by printing spores in dashed patterns on both sides of a plastic tape. Dry air shrinks the spores, and the tape converts from straight to wavy, while humidity reverses the process to straighten the tape. Many of these tapes are paralleled to produce a greater force, and movement of the tapes also controls shutters to modulate the humidity and cause a piston oscillation.
Potential applications for such evaporation energy-harvesting devices include environmental sensors powered by the natural environment. In theory, such spore devices can generate more power per unit area than a wind farm. This research was funded by the U.S. Department of Energy.
- Data Page for Isopropyl Alcohol, NIST.
- Xi Chen, Davis Goodnight, Zhenghan Gao, Ahmet H. Cavusoglu, Nina Sabharwal, Michael DeLay, Adam Driks, and Ozgur Sahin, "Scaling up nanoscale water-driven energy conversion into evaporation-driven engines and generators," Nature Communications, vol. 6, article no. 7346 (June 16, 2015), doi:10.1038/ncomms8346. This is an open access publication with a PDF file available here.
- Renewable energy from evaporating water, Columbia University Press Release, June 16, 2015.
- Renewable Energy from Evaporating Water, YouTube Video by ExtremeBio, June 16, 2015.
- Xi Chen, L. Mahadevan, Adam Driks, and Ozgur Sahin, "Bacillus spores as building blocks for stimuli-responsive materials and nanogenerators," Nature Nanotechnology, vol. 9, no. 2 (2014), http://dx.doi.org/10.1038/nnano.2013.290.
- Beth Kwon, "Biophysicist Harnesses Power of Evaporation, Discovers Potential New Source of Renewable Energy," Columbia University Press Release, January 28, 2014.
- Bacterial Spores Harness Evaporation Energy, YouTube Video by extremebio.org, January 27, 2014.
- Extremebio.org Web Site.
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