Cool homes and hot water are there on the cheap

Xiulin Ruan, a Purdue University professor of mechanical engineering, with his lab’s sample of the whitest paint on record. Image: By Purdue University/Jared Pike

Would you like cool homes and hot water without paying to power them? They’re already working in the laboratory.

LONDON, 27 April, 2021 − It sounds like the stuff that dreams are made of: fit equipment to provide cool homes and hot water, and then pay nothing in running costs.

US scientists have worked out how to install the equivalent of 10 kilowatts of cooling equipment without even switching on the electricity. It’s simple: paint the place white. Not just any old white, but a new ultrawhite pigment that can reflect back into the sky more than 98% of the sunlight that falls on it.

And another US team has devised a passive cooling system that could be turned into a roofing material able to lower room temperatures by 12°C by day and 14°C at night, while capturing enough solar power to heat household water to about 60°C.

Each innovation is still at the demonstration stage; neither is likely to be commercially available soon. But each is a fresh instance of the resourcefulness and ingenuity at work in the world’s laboratories to address what is soon going to be one of the hottest topics of the planet: potentially lethal extremes of summer heat as global average temperatures rise, in response to ever more profligate use of fossil fuels.

The problem could grow to nightmare proportions. Researchers have warned that in the next fifty years, up to 3bn people could face temperatures now experienced only by those who live in the Sahara desert.

Increased energy appetite

By 2100, some half a billion people could face heat extremes of 56°C − about the hottest recorded anywhere so far − and people in the cities may face even higher hazard levels.

Air-conditioning systems driven by electricity might cool the homes of the well-off, but they also heighten the demand for energy, and will raise the temperature in the streets. And once again, the poorest people in the most crowded cities will be most at risk.

So for years researchers have been examining new and sometimes ancient techniques for passive cooling. Researchers in Indiana have already devised a pigment that could reflect more than 95% of the sunlight that hits it. Now, in the American Chemical Society’s journal ACS Applied Materials and Interfaces, they report that their latest paint formulation based on barium sulphate particles can deflect up to 98.1% of the light away, while releasing infrared heat as well.

“If you were to use this paint to cover a roof area of about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts,” said Xiulan Ruan, a mechanical engineer at Purdue University, and one of the authors. “That’s more powerful than the central air conditioners used by most houses.”

And at the University of Buffalo, New York state, electrical engineers have experimented with a passive system that under direct sunlight can not only lower the temperature of the chamber it shields: it can also capture enough solar power to heat water.

“It can retain both the heating and cooling effects in a single system with no electricity. It’s really a sort of a ‘magic’ system of ice and fire”

Right now, they say in the journal Cell Reports Physical Science, their mirror-based system is no more than 70cms squared, but it could be scaled up to cover rooftops.

It could not only reduce the need for fossil fuels to generate heat and power cooling systems; it could also one day help those with little or no access to electricity.

The mirrors, based on silver and silicon dioxide, absorb sunlight, and then convert it to heat which is funnelled into an emitter that sends the warmth back into the sky. In outdoor tests it reduced temperatures by 12°C; in the laboratory, it achieved a cooling of more than 14°C.

“Importantly, our system does not simply waste the solar input energy. Instead, the solar energy is absorbed by the solar spectral selective mirrors and it can be used for solar water heating,” said Qiaoqiang Gan, an electrical engineer at Buffalo.

“It can retain both the solar heating and radiative cooling effects in a single system with no need of electricity. It’s really a sort of a ‘magic’ system of ice and fire.” − Climate News Network