Heating and Cooling Using Artificial Muscles
By using artificial muscles, researchers at Saarland University have developed an environmentally friendly cooling and heating technology that is twice to three times as efficient as conventional devices.
While heat must be dissipated for cooling, heat energy must be supplied for heating. The prototype of a cooling and heating appliance developed by engineers at Saarland University transports heat using a completely new method that does not have the disadvantages of the conventional methods of cooling and heating. Essentially, the process is based on pulling and relieving wires made of a special shape memory alloy, in this case nickel-titanium (NiTi). "The resulting phase transitions that occur in the alloy's crystal lattice release or absorb latent heat," says Professor Stefan Seelecke, who holds the Chair in Intelligent Material Systems at Saarland University.
Nickel-titanium as a shape-memory alloy
This effect is particularly pronounced in wires made of nickel-titanium. "When pre-stressed nitinol wires are unloaded at room temperature, they cool down by as much as 20 degrees," says Felix Welsch, who has been working on the prototype as part of his doctoral research project, along with his team colleague Susanne-Marie Kirsch. This phenomenon makes it possible to remove heat from the system. "When the wires are mechanically loaded they heat up by a similar amount, so that the process can also be used as a heat pump," explains Welsch. The prototype is the first continuously operating machine that cools air using this process. A specially designed registered cam drive ensures that bundles of 200 micrometre thick NiTi wires are continuously pulled and unloaded during rotation. In two separate chambers, air is blown through the bundles, and is heated accordingly in one chamber and cooled in the other. This way the machine can be operated either as a heat pump or as a cooling machine.
Two to three times more efficiency
Depending on the alloy used, the heating or cooling power of the system is up to thirty times greater than the mechanical power required to load and unload the alloy wire bundles. That already makes the new system at least twice as good as a conventional heat pump and three times better than a conventional refrigerator. The new technology is also environmentally friendly and does not harm the climate, as the heat transfer mechanism does not use liquids or vapours. This way the air in an air-conditioning system can be cooled directly without the need for an intermediate heat exchanger or leak-free, high-pressure piping. The team is currently working on further optimising heat transfer within the system in order to boost the efficiency of the new technology even more. "Our objective is to get to a stage where almost all of the energy from the phase transition is being used for heating or cooling," says doctoral student Felix Welsch.