The idea came to the researchers at the Christmas market in the Bavarian town of Rosenhiem. "We got upset because the mulled wine" -- Glühwein, in German -- "was always either too hot or too cold," say Klaus Sedlbauer, the head of the Fraunhofer Institute for Building Physics (IBP), and his colleague Herbert Sinnesbichler. "We had to find a solution."
And find a solution they did. The two scientists found it in phase change material (PCM), a wax-like substance used in the construction materials industry that is normally used to ensure comfortable room temperatures in the summer without having to use any energy for air conditioners.
The material can be embedded within plaster boards or placed on walls and ceilings -- where it absorbs and stores warmth, such as that emitted by the sun during the day, and releases it once again after the sun has gone down, thereby creating a pleasant room temperature at all hours. "But," Sedlbauer adds," a lot of winter and ski jackets also contain PCM to keep people warm. Most people probably have some somewhere in their closet."
PCMs also have long-term memory capabilities that make them ideal for storing computer data over extended periods of time without any need for an electrical current. For example, researchers at the Berlin-based Paul Drude Institute for Solid State Electronics (PDI) are looking into exactly why the material has these storage capacities.
A Highly Active Internal Life
The researchers at IBP came up with the idea of using PCM in coffee mugs. If it works in large office spaces and in jackets, why couldn't it also keep a cup of coffee warm? To test their theory, they created the first PCM mug. The high-tech mug is made using a porcelain shell whose hollow interior is filled with a honeycomb structure made of ribbons of highly conductive material, such as aluminum. This honeycomb structure is then filled with PCM. "So now, if you are drinking hot coffee in one of these cups," Sinnesbichler explains, "the drink's heat is directed straight into the still solid PCM. This heat, in term, melts the PCM -- kind of like wax -- and turns it into a liquid."
Once the material has become liquid, it retains thermal energy, but without absorbing any more heat. The temperature at which it becomes liquid depends on the specific type of PCM, each of which has slightly different chemical properties and melting temperature. "Warm drinks -- like coffee or tea -- are best enjoyed at 58 degrees Celsius (136.4 degrees Fahrenheit)," Sedlbauer explains. "In order to reach and maintain this temperature, we fill the mug with a type of PCM that becomes a liquid at exactly 58 degrees Celsius."
The material absorbs the warmth of the mug's content like a sponge, stores it and brings it down to the optimal temperature. And then the PCM helps maintain the content's temperature at this optimal level by slowly releasing the stored heat back into the mug's contents. "Under ideal circumstances," Sedlbauer says, "the optimal temperature can be maintained for 20-30 minutes."
In order to even further insulate the mug and permit less heat to be lost, the outside part of the mug's hollowed-out cavity -- that is, the part farthest from the material whose temperature needs to be maintained -- is lined with a razor-thin layer of either plastic or ceramics. This helps further ensure that the contents of the mug only start cooling down once the PCM has released all of its stored thermal energy and returned to a solid state.
Hot & Cold Double Functionality
But PCMs aren't just about keeping things warm. "Cold drinks or ice can also be well regulated in PCM cups or mugs," Sinnesbichler says. As he explains, beer tastes best at 7 degrees Celsius (44.6 degrees Fahrenheit), and ice is best at -12 degrees Celsius (10.4 degrees Fahrenheit). "So you want to make cups or mugs that have a PCM type that melts at exactly these temperatures," he says. For the consumer, this unfortunately means that you need different types of high-tech mugs for different beverages, depending on whether you want them hot, cold or ice-cold.
Researchers at the Bavarian Center for Applied Energy Research (ZAE Bayern) in Garching think that the fact that PCM is so cheap to produce could make it attractive for numerous additional applications. For example, it could be used to keep perishable foods from spoiling for longer or prevent sensitive machine components from overheating. Another idea involves blanketing the interior walls of museums with the non-flammable material in order to protect paintings from heat damage in case of a fire.
In the meantime, it won't be long before the IBP's new table products arrive in stores. Their steep price tags should easily give them away. "We don't know how expensive they'll be yet," Sedlbauer says. "We are already talking with different companies. If we can find a partner to work together with, the first mugs could already be on sale by the end of the year."