Sometimes the simplest solutions offer the best outcomes — along with a little help from human ingenuity. This is the case for night sky radiant cooling (NSRC), a process that combines the temperature in space with the continual efforts of environmental systems to reach thermal equilibrium. In other words, objects hotter than their surroundings get colder until the system balances out. On clear nights, however, thermal emissions from objects can outpace nearby environments, resulting in lower-than-ambient temperatures.
Bolstered by technology, this process may offer a way to help offset the environmental impacts of heating, ventilating and air conditioning (HVAC) systems that use massive amounts of energy and generate substantive amounts of heat. Here’s how.
It’s Getting Hot in Here
As global temperatures continue to reach new extremes — data from the National Oceanic and Atmospheric Administration ranked 2021 as the sixth-hottest year on record, while 2020 and 2019 both ranked in the top three — demand for cooling systems is on the rise. The need for heating solutions is also heating up; even as the rest of the world sweltered, Antarctica recorded its coldest temperatures since 1976.
The problem? All this heating and cooling is actually making our problem worse. HVAC systems (along with lighting) now account for 28% of total greenhouse emissions on Earth, meaning that even as we try to control the temperature in our immediate vicinity, we’re creating larger environmental issues that in turn drive a greater need for HVAC systems. It’s a classically human problem: While technology affords us ways to circumvent natural processes and allow people to live in extremely warm or cold climates that would otherwise wipe us out, this same technology creates ripple effects in the world around us.
According to NASA, these effects aren’t limited to temperature changes. Many parts of the world have already seen average temperature increases of 1.5 degrees Celsius compared to pre-industrial levels. If these changes reach 2 C, more than 60 million people across urban areas worldwide could be subject to severe drought, while other regions could see substantially increased flooding. A 1.5 C change also leads to a 50% reduction in geographic range for 6% of insects, 8% of plants and 4% of vertebrates surveyed. Even more worrisome? Current predictions put us on track for a 5 C increase by 2100.
NSRC may offer a partial solution.
Keeping It Cool
The underlying concept of night sky cooling is simple: Use the temperature in space to reduce the temperature of objects on Earth. All you need is ground, sky and a clear night.
Here’s how it works: All objects slowly radiate heat. On a clear night, this radiation can travel up into the vast expanse of space and allow objects to significantly cool down, even dipping below the ambient air temperature. This sky-concept cooling has been used for centuries in places where even the night provides little solace against accumulated heat. People in North Africa, India and Iran leveraged earthen pits or plant-insulated ceramic trays to hold water overnight. Heat radiated upward to cool the water, while natural insulation kept the water from warming up again when it fell below the ambient outside temperature. The result? Ice, despite an air temperature well above freezing.
While other heating and cooling solutions rely on energy-intensive technology to either remove or add heat for human comfort, NSRC uses existing system components to improve efficiency and reduce total energy expenditures.
(Night) Sky’s the Limit
How cold is outer space? It depends on where you are. At the edge of Earth’s atmosphere, the average temperature is a not-so-chilly 10 degrees Celsius. Move past the upper atmosphere and temperatures can fall to -100 C; keep going and you’ll see the mercury drop to -270 C. So how cold is outer space? Cold enough.
This falling temperature gradient, however, sets the stage for night sky cooling. On a clear night, there’s nothing to stop heat energy from radiating all the way up into space instead of hanging around here on Earth. But how do we make the best use of NSRC? While turning water into ice overnight offers a historical glimpse of this cool concept in action, how can it be adapted in predominately urban settings to help offset energy costs?
One approach from Aaswath Raman — a materials scientist at the University of California, Los Angeles (UCLA) — looks to expand the impact of night sky cooling by bringing it into the light of day. Along with his colleagues, Raman has created an incredibly thin reflective film that allows objects such as solar panels to radiate substantial amounts of heat while absorbing virtually none. The results speak for themselves: Equipped with this film, the temperature of objects fell by 10 C or more, even in the middle of hot, sunny days. Applied to other HVAC systems such as air conditioners and furnaces, this film could help minimize heat generation while simultaneously improving performance.
Work is also underway to generate rather than radiate energy from night sky cooling. It all started as a proof-of-concept effort from researchers at the UCLA Samueli School of Engineering. The team purchased $30 worth of materials at hardware and electronic supply stores to create an aluminum disk and painted it black on one side. The researchers placed the disk on a rooftop during winter with the black side facing the sky, and it began to radiate heat, which was then captured by a thermoelectric generator and converted into heat. Worth noting? It wasn’t much energy — 25 milliwatts per square meter, or enough to power an LED bulb. With purpose-built technology, however, the team believes they could increase the energy efficiency to 0.5 watts per square meter, enough to power a room full of lights. Expand this to a whole rooftop full of radiating discs, combine it with solar power generated during the day and you’ve got a substantial renewable resource scenario.
Like Night and Day
Night sky cooling looks to flip the script on energy consumption and heat production. By using natural temperature differentials to radiate heat into space — and leverage that heat to generate renewable energy — there’s an opportunity to both improve HVAC efficiency and get something usable in return.
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