by Karl-Lydie Jean-Baptiste, Massachusetts Institute of Technology, Cambridge | The passive, solar-powered system could prevent freezing on airplanes, wind turbines, power lines, and other surfaces. Images of a droplet on a surface show the process of freezing (top row), during which condensation temporarily forms on the outside of the droplet as it freezes. The next two rows show the droplet thawing out on a surface coated with the new layered material. In the middle row, the droplet is heated by the coating immediately upon freezing, and the dashed lines show where the freezing at top is just catching up with the thawing from below. The bottom row shows a slower thawing process. Under identical conditions, the droplet stays frozen without the new coating. (The Varanasi Research Group)
From airplane wings, to overhead power lines, to the giant blades of wind turbines, a buildup of ice can cause problems ranging from impaired performance all the way to catastrophic failure. Preventing that buildup usually requires energy-intensive heating systems or chemical sprays that are environmentally harmful.
A completely passive, solar-powered system was developed that is based on a three-layered material that can be applied or even sprayed onto the surfaces to be treated. It collects solar radiation, converts it to heat, and then spreads that heat around so the melting is not just confined to the areas exposed directly to the sunlight. Once applied, it requires no further action or power source. It can even de-ice at night using artificial lighting.
The usual de-icing sprays for aircraft and other applications use ethylene glycol, a chemical that is environmentally unfriendly. Airlines don’t like to use active heating, both for cost and safety reasons.
As an alternative, the system captures the heat of the Sun and uses it in a passive approach. It is not necessary to produce enough heat to melt the bulk of the ice that forms; all that’s needed is for the boundary layer, right where the ice meets the surface, to melt enough to create a thin layer of water, which will make the surface slippery enough so any ice will slide off.
The top layer of the material is an absorber that traps incoming sunlight and converts it to heat. The material absorbs 95 percent of the incident sunlight and loses only 3 percent to reradiation. In principle, that layer could in itself help to prevent frost formation, but with two limitations: It would only work in the areas directly in sunlight, and much of the heat would be lost back into the substrate material — the airplane wing or power line, for example — and would not help with the de-icing.
To compensate for the localization, a spreader layer was added — a very thin layer of aluminum, just 400 micrometers thick, that is heated by the absorber layer above it and very efficiently spreads that heat out laterally to cover the entire surface. The material was selected to have thermal response that is fast enough so that the heating takes place faster than the freezing.
Finally, the bottom layer is simple foam insulation to keep any of the heat from being wasted downward and keep it where it’s needed at the surface. In addition to passive de-icing, the photothermal trap stays at an elevated temperature, thus preventing ice buildup.
The three layers, all made of inexpensive, commercially available material, are then bonded together and can be bonded to the surface that needs to be protected. For some applications, the materials could instead be sprayed onto a surface, one layer at a time.
Karl-Lydie Jean-Baptiste is a Media Relations Assistant, Freelance Writer and Content Manager at the Massachusetts Institute of Technology (MIT). For more information, contact Karl-Lydie Jean-Baptiste at kjeanbap@mit.edu, 617-253-1682 or Linkedin.
