Researchers found that sections of metal can be made to either promote bubbling (the two rectangles at the edges) or to inhibit bubbling (center rectangles), by switching the polarity of voltages applied to the metal. (Image courtesy of MIT/Jeremy Cho.)
Engineers working with industrial heating and cooling, electricity generation and water desalination all rely on boiling water to drive their operations.
However, their control over this boiling is typically limited to one variable: the temperature.
This means most boiling systems operate with a substantial safety margin in order to avoid the possibility of hotspots that damage equipment.
These hotspots are typically caused by bubbles acting as an insulator when they stick to the sides of equipment for too long.
Therefore, localized control over bubbles in boiling water, and control over the rate of boiling independent of the liquid’s overall temperature would be beneficial. This control may be possible thanks to research by a team of mechanical engineers at MIT.
The team was able to control the boiling process by adding surfactants to the water. The surfactant molecules carry an electric charge, and can be attracted to or repelled by a metal surface based on the voltage polarity applied to the metal.
The change in electric charge causes the metal surface to become either hydrophilic or hydrophobic affecting the rate of bubble development.
Specifically, their method controls the process of nucleation that is necessary for the formation of bubbles in the boiling process. Miniscule irregularities on a metal surface can provide these nucleation points, but the amount and intensity of boiling will vary depending on the material and texture of the metal surface.
By adding the charged surfactant, the metal surface becomes hydrophobic at a charge of -1.0 V, increasing the rate of bubble nucleation. Reversing the charge on the metal surface makes it hydrophilic, inhibiting bubble formation.
Improved Efficiency Through Control of Heat Transfer
Other approaches to modifying the wettability of a metal surface usually rely on the creation of precise nanoscale surface textures. However, the MIT team’s system makes use of the naturally existing irregularities on a metal surface and does not require any special processing.
The researchers found they could achieve up to a ten-fold change in the bubble formation rate simply by switching the charge.
Actively controlling the rate of bubble formation allows greater control over the rate of heat transfer between the metal and the liquid. As a result, wide operational safety margins may be replaced with systems able to precisely control the rate and location of boiling to avoid the production of damaging hotspots. This may enable improved efficiency in power plant boilers which would be able to work hotter.
Though most power plants operate at a steady rate, dynamic control of heat transfer rates could improve efficiency when ramping up or down from full power. This would also enable real-time adjustment of output levels without losing efficiency.
According to Jeremy Cho of MIT’s department of mechanical engineering, this system could provide “the ability to pick the best heat-transfer profile on an as-needed basis,” depending on the process and system being used.
Plant operators will not be limited to choosing a single type of nucleation behavior that allows margins for the most extreme heating expected for a given piece of equipment.
Instead, the MIT team’s system “allows you to pick the optimum rate of heat transfer moment by moment. Having a boiler that can respond to quick changes” in real-time could provide extra flexibility to the electric grid, Cho said.
Potential for Future Widespread Adoption
Cho acknowledges that power plant operators are understandably conservative about making large-scale changes. This is because people rely on dependable, uninterrupted power plant output.
However, existing systems would require relatively minor changes to accommodate this new technology. An operational-scale demonstration plant will need to prove the concept and show that the benefits can outweigh the installation costs before any widespread adoption is likely to happen.
The full paper describing the control of bubbles in boiling water using an electric charge is published in the journal Nature Communications and can be read here.