Adaptive roof tile harnesses wax motor to heat and cool

Based on the change in the volume of wax in response to temperature, a wax motor creates pressure that moves mechanical parts, translating thermal energy into mechanical energy. Wax motors are commonly found in appliances like dishwashers and washing machines, as well as in more specialised applications such as the aerospace industry.

In the case of the tile, the wax motor can push or retract pistons that change the nature of its surface and alter its function. In cooler temperatures when the wax is solid, the louvres are closed and lay flat, exposing a surface that absorbs sunlight and minimises heat dissipation through radiation. When temperatures reach around 18° C, the wax begins to melt and expand, pushing the louvres open and exposing a surface that reflects sunlight and emits heat. The tile is described in the journal Device.

“It switches between a heating state and a cooling state, depending on the temperature of the tile,” explained lead author Charlie Xiao, from the University of California Santa Barbara (UCSB). “The target temperature is about 65° F — about 18° C.”

At about four inches square, the passive thermoregulating device is a blend of Liao’s expertise in thermal science and fellow researcher Elliot Hawkes’ work in mechanism design. According to the paper, testing of the tile demonstrated a reduction in energy consumption for cooling by 3.1x and heating by 2.6x compared with non-switching devices covered with conventional reflective or absorbing coatings. The wax motor means no electronics, batteries or external power sources are required to operate the device, and unlike other similar technologies, it is responsive within a few degrees of its target range.

“We have a very predictable switching behaviour that works within a very tight band,” said Xiao.

Additionally, the researchers say the simplicity of the design lend itself to customisation, with different thermal coatings and various types of wax making the tile tuneable to a range of temperatures.

“The device is still a proof-of-concept, but we hope it will lead to new technologies that one day could have a positive impact on energy expenditure in buildings,” said Hawkes.