Scientists Have Found A Way To Save Energy And Boil Water More Efficiently

Water boils a lot – whether it’s a cup of tea brewed in the kitchen or a power station that generates electricity. Any increase in the efficiency of this process will have a major impact on the overall amount of energy used for it each day.

One such improvement could come with newly developed treatments for surfaces involved in heating and evaporating water. The treatment improves the two main parameters that determine the boiling process: heat transfer coefficient (HTC) and critical heat flux (CHF).

Most of the time, there is a trade-off between the two – when one improves, the other worsens. After years of investigation, the research term behind this technique has found a way to improve both.

“Both parameters are important, but raising the two parameters together is a bit tricky because they have an intrinsic trade-off,” said bioinformatics scientist Youngsup Song of the Lawrence Berkeley National Laboratory in California.

“If we have a lot of bubbles on the boiling surface, that means very efficient boiling, but if we have too many bubbles on the surface, they can coalesce together, which can form a vapor layer over the boiling surface.”

Any vapor layer between the hot surface and the water creates resistance, lowering the heat transfer efficiency and the CHF value. To solve this problem, the researchers designed three different types of surface modification.

First, a series of micro-scale tubes are added. This arrangement of tubes 10 micrometers wide, spaced about 2 millimeters apart, controls bubble formation and keeps bubbles in the cavity. It prevents the formation of a vapor film.

At the same time, it reduces the concentration of bubbles on the surface, reducing the boiling efficiency. To address that, the researchers introduced a smaller-scale treatment as a second modification, adding bumps and protrusions only nanometer in size inside the surface of the hollow tube. It increases the available surface area and increases the evaporation rate.

Finally, a micro-scale cavity is placed in the center of a series of pillars on the surface of the material. These pillars speed up the fluid withdrawal process by adding more surface area. In combination, the boiling efficiency is significantly increased.

(Songs et al.)

Above: A slowed down video of the researchers’ setup shows boiling water on a specially treated surface causing bubbles to form at specific, separate points.

Since the nanostructure also promotes evaporation below the bubble, and the pillars maintain a steady supply of liquid to the bottom of the bubble, a layer of water between the boiling surface and the bubble can be maintained – increasing the maximum heat flux.

“Showing that we can control the surface in this way to get an increase is the first step,” said mechanical engineer Evelyn Wang of the Massachusetts Institute of Technology. “Then the next step is to think of a more scalable approach.”

“This kind of structure that we created was not meant to be scaled in its current form.”

Taking work from a small-scale laboratory setting into something usable in commercial industry won’t be easy, but the researchers believe it can be done.

One of the challenges was finding a way to create surface textures and the three “levels” of modification. The good news is that there are different approaches that can be explored, and the procedure should also work for different types of fluids.

“Details like that can be changed, and that could be our next step,” Song said.

This research has been published in Advanced Material.

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