Sunny forecasts for energy-efficient cooling | NOVA | PBS | NOVA

As autumn gives way to winter in the northern hemisphere, it is sometimes easy to forget that the planet is warming up. Air conditioners and electric fans account for about 10% of global electricity consumption. And as global average temperatures continue to rise, the number of air conditioners in the world will more than triple by 2050.

This created a vicious circle. In the United States alone, homes release more than 100 million tonnes of carbon dioxide annually into the atmosphere, increasing warm-up times and encouraging people around the world to search again. more comfort in temperature-controlled buildings.

But researchers are developing tools that allow us to inject unwanted heat into space without using the required air. Today, in the newspaper Joule, a team from Stanford University goes further in innovation with a device that could not only keep us cool in the absence of air conditioning, but also simultaneously exploit solar energy for meet energy demand elsewhere on the network.

The refreshing half of this new technology, developed by electrical engineer Shanhui Fan of Stanford University, takes advantage of the natural ability of all objects and organisms to emit heat by radiation. Unlike conduction and convection – the other methods by which thermal energy is transferred – radiation does not require direct contact or circulation of fluids to move heat from one place to another: it emanates simply any material whose temperature is above absolute zero (or -459.67 degrees Fahrenheit … so, for all practical purposes, call it "all matter"). For example, radiation is responsible for the heat you feel coming out of a hot oven even when its door is closed.

With radiation, heat can be jettisoned to space – and if it removes the obstruction from the atmosphere, the point of no return has passed: space is practically a huge, insatiable heat sink. Unfortunately, the radiation coming out of most wavelengths does not always make it that far. Some of these particles ricochet molecules of water in the atmosphere and boomerangs up to the Earth; another part of it is absorbed and retained in the atmosphere by carbon dioxide and other greenhouse gases.

But not all materials emit radiation at the same wavelength. If a surface emits radiation whose wavelength is between 8 and 13 micrometers, it strikes an ideal point that bypasses many of these atmospheric obstacles. By sliding through this "average infrared window", the heat can finally penetrate into a cold and cold space, thus allowing the radiating object to dive below the temperature of the ambient air. Here it is: unlocked cooling achievement.

Fan and his colleagues first exploited this window and its associated cooling factor in 2014, with the design of a radiative cooler. Designed with silicon compounds and other materials emitting in the mid-infrared, it was able to stay around 5 degrees Celsius (or 9 degrees Fahrenheit) below the temperature of the body. ambient air on the roof of a sunny building in California. Two years later, Fan's lab published a follow-up using a similar device to cool running water – a breakthrough that, if scaled appropriately, could reduce the dependency of air conditioning and heat. refrigeration with electricity.

For Fan, space is the ultimate untapped well of renewable energy. One of the most interesting potential applications of this technology is to spot roof coolers with radiative coolers. Unfortunately, this move could ruffle the feathers of those who try to do the same thing with solar absorbers and solar cells, which usually colonize the same space to harvest energy in the sun.

At first glance, the two strategies seem incompatible: one is hungry for heat, the other is cold: placing conventional radiative coolers and solar panels side by side could even reduce the efficiency of both processes. But as humanity struggles to reduce carbon emissions, it is clear that we need all the help we can. In collaboration with lead author Zhen Chen, a mechanical engineer at Southeast University of China, Fan brought together a team of researchers to create a peaceful coexistence between the two technologies.

"The solar energy that arrives on Earth must end up coming out of the Earth," says Fan. "These technologies are therefore complementary: the recovery of solar energy tries to take advantage of the incoming energy flow, while the radiative cooling tries to take advantage of the outgoing flow."

In fact, the spectrum of solar energy most relevant to a solar panel does not overlap with the average infrared window, which means that even if the two devices were superimposed, they would not be obliged to cancel. . To put this idea to the test, Chen, Fan and their colleagues slid a silicon-based radiative cooler under a germanium solar absorber, which absorbs sunlight for energy-efficient heating (not to be confused with a cell, which adds to the extra step of converting this energy into electricity). Unlike conventional solar absorbers, however, this panel is transparent to radiation in the average infrared range of 8 to 13 micrometers. This allows the chiller to let infrared heat pass through the solar absorber and eject it into the unhindered space.

It is important to note that both layers of this device operate completely independently. The solar absorber accumulates heat, but none of this energy needs to be routed to the radiative cooler located underneath, which by definition works without energy. This means that the solar charge collected above can theoretically be transported elsewhere.

Although the two halves of this device are not interdependent, they can still benefit from each other's company. Many conventional radiative coolers work best at night, out of the heat of the sun; For efficient daytime use, the fixtures need highly reflective surfaces that can deflect light. Although this keeps the temperature low, it also wastes what might otherwise be a big boost to solar energy. With the solar panel that covers the sun, the cooler does not need this reflective boost.

Protected by the solar panel, the team's radiator cooler temperature dropped to 29 degrees Celsius (52 degrees Fahrenheit) below room temperature, while the absorber burned to a pleasant temperature of 24 degrees Celsius (43 degrees Fahrenheit) above ambient air. huge differential. According to Chen, an immediate application has already been revealed: the temperature gradient created by these two extremes could possibly be exploited to power a heat engine.

Even more exciting for Chen and Fan is the idea of ​​replacing half of their solar absorber with a solar cell that generates electricity. If this switch does not work, double action technology could complement our conventional cooling methods, which consume less electricity, while simultaneously providing a renewable source of energy.

"We are trying to change the way people perceive rooftops to harness renewable energy," says Chen. "During the hot summer days, you can continuously collect electricity and simultaneously use the device to cool your home."

However, Chen does not think that radiative cooling will evolve to the point of completely replacing air conditioning or refrigeration. Even high efficiency coolers should probably be expanded to cover entire roofs or more – probably an unsustainable engineering demand, especially considering the cost of materials used by the team in their prototype. The average infrared window also knocks cloudy or rainy days, making this technology the most literal of friends in good weather. But, says Chen, radiative cooling could still be in the sun (at least on a small scale) in arid climates or regions with irregular access to power. Researchers are also studying the possibility of rebuilding their device with more economical materials.

In the meantime, there are still many technological hurdles to overcome. While the theory behind the methods is elegant, says Emily Warmann, a mechanical engineer at the California Institute of Technology and a student in photovoltaic cells, who did not participate in the study, "all practical implications are very, very far apart". the adjustments needed to increase the efficiency of the device, or allow the passage of a solar absorber to a solar cell, could very well compromise the critical infrared transparency of the panel.

In addition, the prototype radiative cooler is sealed in a vacuum chamber to minimize unwanted heat loss due to conduction and convection. To function as an alternative air conditioning or refrigeration, as in her previous work, she would need access to water, but exposure to ambient air would reduce her ability to cooling down to about 5 degrees Celsius (9 degrees Fahrenheit) below room temperature. as opposed to 29 degrees Celsius (52 degrees Fahrenheit) obtained in a vacuum.

Despite these hurdles, Chen, Fan and their colleagues have demonstrated something new and important, said Mark Nurge, a physicist at NASA's Applied Physics Lab, who did not contribute to the new discovery. For the first time, it is clear from this device that these two renewable energy sources do not need to jostle for space – and with radiative cooling on our side, even the sky is no longer the limit.

"It's a creative idea," says Gabriela Schlau-Cohen, a chemist who studies light recovery at the Massachusetts Institute of Technology and who has not participated in the study. "If we really want to meet our growing energy needs, we need to do it in a paradigm-changing way. And that means different ideas from incremental improvements to our current structure. "