A team of researchers from the University of California Santa Barbara has developed an adaptive tile that, when deployed in arrays on roofs, can lower heating bills in winter and cooling bills in summer, without the need for electronics.
‘It switches between a heating state and a cooling state depending on the temperature of the tile,’ said Charlie Xiao. ‘The target temperature is about 65° F – about 18°C.’
At about ten centimetres square, the passive thermoregulating device is a blend of Liao’s expertise in thermal science and Elliot Hawkes’s work in mechanism design – a movable surface that can change its thermal properties in response to a range of temperatures. The idea for the project came to them during long drives between Santa Barbara and northern California a few years ago.
‘Both our spouses were in Stanford at the time, so we were taking trips and wondering what we could potentially do together,’ said Liao, who, like Hawkes, is a professor in UCSB’s Department of Mechanical Engineering. They then received seed funding from the California NanoSystems Institute on campus to design mechanically tuneable thermal devices.
It wasn’t until Xiao’s idea of using a wax motor that the idea of adaptive roof tiles took its final shape. Based on the change in the volume of wax in response to temperatures to which it’s exposed, a wax motor creates pressure that moves mechanical parts, translating thermal energy into mechanical energy. Wax motors are commonly found in various appliances such as dishwashers and washing machines, as well in more specialised applications, such as in the aerospace industry.
In the case of the tile, the wax motor, depending on its state, can push or retract pistons that close or open louvers on the tile’s surface. So, in cooler temperatures, while the wax is solid, the louvers are closed and lay flat, exposing a surface that absorbs sunlight and minimises heat dissipation through radiation.
However, as soon as the temperature reaches around 18°C, the wax begins to melt and expand, pushing the louvers open and exposing a surface that reflects sunlight and emits heat. In addition, during the melting or freezing process, the wax also absorbs or releases a large amount of heat, further stabilising the temperature of the tile and the building.
‘So we have a very predictable switching behaviour that works within a very tight band,’ Xiao explained. Testing of the tile has demonstrated a reduction in energy consumption for cooling by 3.1 times and heating by 2.6 times compared with non-switching devices covered with conventional reflective or absorptive coatings. Because of the wax motor, no electronics, batteries or external power sources are required to operate the device, and unlike other similar technologies, it’s responsive within a few degrees of its target range. Additionally, the simplicity of its design lends itself to customisation – different thermal coatings and various types of wax can be used to allow the device to operate at desired temperature ranges – while also lending itself toward mass manufacture.
‘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.
The research has been published in Device.