Scientists have developed an all-season smart-roof coating that keeps homes warm during the winter
and cool during the summer – without consuming natural gas or electricity.
Research findings reported in the journal Science point to a groundbreaking technology
that outperforms commercial cool-roof systems in energy savings.
The researcher says that their all-season roof coating automatically switches from keeping
you cool to warm, depending on outdoor air temperature.
This is energy-free, emission-free air conditioning and heating, all in one device.
Today’s cool roof systems, such as reflective coatings, membranes, shingles, or tiles, have
light-colored or darker “cool-colored” surfaces that cool homes by reflecting sunlight.
These systems also emit some of the absorbed solar heat as thermal-infrared radiation;
in this natural process known as radiative cooling, thermal-infrared light is radiated
away from the surface.
The problem with many cool-roof systems is that they continue to radiate heat in the
winter, which drives up heating costs.
The researcher says that their new material – called a temperature-adaptive radiative
coating or TARC – can enable energy savings by automatically turning off the radiative
cooling in the winter, overcoming the problem of overcooling.
TARC is the first roof coating that automatically switches between cooling in hot weather to
warming in cold weather by regulating its rate of radiative cooling.
Metals are typically good conductors of electricity and heat.
In 2017, the research team discovered that electrons in vanadium dioxide behave like
a metal to electricity but an insulator to heat – in other words, they conduct electricity
well without conducting much heat.
This behavior contrasts with most other metals where electrons conduct heat and electricity
proportionally.
Vanadium dioxide below about 67 degrees Celsius is also transparent to (and hence not absorptive
of) thermal-infrared light.
But once vanadium dioxide reaches 67 degrees Celsius, it switches to a metal state, becoming
absorptive of thermal-infrared light.
This ability to switch from one phase to another – in this case, from an insulator to a metal
– is characteristic of what’s known as a phase-change material.
The researcher's 2017 study revealed that replacing only 1.5% of the vanadium in vanadium
dioxide with tungsten, a technique called “doping,” lowers the material’s phase-change
threshold to 25 degrees Celsius – an ideal temperature for real-world applications.
To see how vanadium dioxide would perform in a roof system, the research team engineered
a 2-centimeter-by-2-centimeter TARC thin-film device from three layers: a reflective bottom
layer made from silver, a transparent middle layer composed of barium fluoride, and a top
layer containing orderly blocks of vanadium dioxide “islands.”
TARC looks like Scotch tape, and can be affixed to a solid surface like a rooftop.
The current study demonstrates vanadium dioxide’s amazing all-weather versatility in a TARC
thin film, and compares its performance with a commercial dark roof coating, and a commercial
white roof coating.
The researchers plan to develop TARC prototypes on a larger scale to further test its performance
as a practical roof coating.
TARC may also have potential as a thermally protective coating to prolong battery life
in smartphones and laptops, and shield satellites and cars from extremely high or low temperatures.
It could also be used to make temperature-regulating fabric for tents, greenhouse coverings, and
even hats and jackets.