Saving summer warmth into winter so that heating and hot water cost less or nothing at all: this is a dream that can soon come true.
Researchers at the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology (FEP) have developed a process with which thermal energy can be stored in a cold state.
It can be stored for as long as you like without losing any of its energy content over time.
Heidrun Klostermann, project manager at FEP, and her team use zeolite beads with a diameter of five millimeters that are coated with a wafer-thin layer of aluminum. This layer ensures that the solar heat as well as industrial waste heat reaches the deepest layers of the bed. So far, only those beads that come into direct contact with the energy source can be heated, because zeolites are extremely poor conductors of heat. The aluminum counteracts this shortcoming.
In the first attempt, the team was able to double the thermal conductivity. But that is far from enough for Klostermann. "We are aiming for an increase by a factor of five to ten," says the team leader. Zeolites are not only attractive for long-term heat storage because they lose practically no energy. They also store up to five times as much heat per unit volume as hot water, even if it is kept in a well-insulated tank, where it still loses heat to its surroundings.
Zeolites are highly porous ceramics with a tremendously large inner surface. They attract water and store it inside. When charging, the beads are heated so that the water evaporates. This takes place in a heat exchanger, into which no moisture is initially allowed to penetrate after it has cooled down. This only changes at the beginning of the heating season. Then water is let in, which the globules absorb like sponges. During this process, they become hot and in turn transfer this heat to a water cycle that heats the house or office and can provide hot water.
In order to evenly coat hundreds of thousands of spheres with aluminum, the FEP researchers developed a special system.
In it, aluminum wire evaporates in a vacuum.
This steam is then deposited evenly on the beads, which rotate in a drum until the coating has reached the desired thickness.