Berkeley Lab, the US national laboratory that conducts scientific research on behalf of the Department of Energy, has developed a roof coating which, depending on the weather, can keep buildings cooler or warmer.
When it’s warm, the material reflects sunlight and heat, but as temperatures fall, the radiative cooling automatically switches off which reduces the amount of energy used for both heating and cooling.
The radiative cooling process withdraws heat (thermal radiation) from the building and into the atmosphere which is pellucid to these wavelengths which means that the heat discharges into space.
Though the process works well in summer, it has the opposite effect in winter – keeping the building cooler.
So, the team at Berkeley Lab have developed a surface coating that instead, automatically shifts to trapping heat when the temperature falls. The new material is called temperature-adaptive radiative coating (TARC).
In 2017, in what appears to be a contradiction in physics, the team found that when vanadium dioxide (VO₂) reaches a temperature of 67˚C, it conducts electricity but not heat.
Using this contradiction, the team developed a material that absorbs and emits thermal-infrared light keeping it away from the building when the seasonal temperature warms up. Conversely, when the temperature cools, the material is transparent to heat so the heat passes through it to the building.
Measurements show that TARC reflects about 75% of sunlight irrespective of the weather, but when the ambient temperature rises beyond 30°C, it emits up to 90% of its heat to the sky. However, when the temperature falls to below 15 °C, TARC emits only 20% of its heat.
From a simulation using the data gathered, and based year-round on 15 different US climate zones, it was estimated that by using TARC an average saving of 10% could be made on a US household electricity bill. By adapting TARC, the researchers say that TARC could also be used as a temperature-regulating material for clothing, tents, electronics, satellites and cars.
The next stage for the researchers is to develop larger TARC prototypes, to test whether it has potential for a practical application as a roof coating.