Cool stuff

Air-conditioning is a popular method of creating pleasant indoor temperatures in hot weather. However, the forecast of increasingly warm summers due to global warming means an increased need for air-conditioning, leading to a potential increase in CO2 emissions.

This environmentally unfriendly prognosis is what inspired researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) to find an alternative solution.

The idea came to Professor Volker Wittwer, who believed that the answer to more efficient air-conditioning lay in the power of nature - and particularly the heat of the sun. This began a research experiment, which, after more than ten years, provided an answer to the question: How can the natural night-time cold be trapped and used to keep a building cool at a later point in time?

Together with his colleague Peter Schossig, Wittwer began to work on so-called "latent heat savers," which function with materials like wax, especially paraffin. To prevent unpleasantly warm indoor conditions, these materials are meant to capture heat inside walls and stop it from spreading into living areas.

This function is based on a physical property that can be observed in an ice cube as it melts inside of a glass. When the ice cube is heated, it starts to melt, but the liquid around it only begins to warm up when the very last ice particle has melted.

"This means I have to put in a lot of energy to make the ice melt, but the liquid remains cold," explained Wittwer. "That's the hidden energy that you need for melting, but that doesn't raise the temperature."

Wax balls retain heat

Whether this effect also works inside a wall was tested in experiments over many years. They showed that when the paraffin melts inside the wall at temperatures between 22 and 26 degrees Celsius (72 to 79 degrees Fahrenheit), the temperature inside the room does not rise above 25 degrees, even on hot afternoons.

It took some time before this theory was turned into a usable product. At the start, the plan was to mix the paraffin directly into the cement or stucco, and then to put it into apple-sized balls and enclose these within the wall, but both methods proved ineffective.

Then Wittwer decided to inject the paraffin into microscopic-sized balls and enclose the individual droplets in small capsules made of acrylic glass. They are less than half the thickness of a human hair and can easily be added to cement, stucco and plasterboards.

An evolving invention

The wax capsules need to cool down and harden at night in order to function again on the next hot day. This means that they are not very effective in tropical regions, where temperatures remain very stable, but the scientists are working on developing a system where the capsules can be cooled by water pumped through walls.

According to Wittwer, the capsules can also be used for other cooling purposes, adding that "there are already bed sheets and diving suits that contain this material." The capsules are also a possible method of cooling batteries in the electric cars of the future.

Author: Richard A. Fuchs (ew)

Editor: Kate Bowen