Passive cooling, like the shade a tree provides, has been around forever.
Recently, researchers have been studying how to apply turbo to passive cooling – known as radiation or sky cooling – with solar-blocking nanomaterials that allow it to heat up. away from roof construction. Although progress has been made, this friendly technology is not common as researchers have struggled to increase their cooling capabilities.
New research led by UB engineers is making great progress in this area.
A study published Feb. 8 in the journal Cell Reports Physical Science describes a specially designed radiation cooling system:
- The temperature inside a test system in an outdoor environment under direct light decreased by more than 12 degrees Celsius (22 degrees Fahrenheit).
- The temperature of the test box in a laboratory decreased, resulting in simulation at night, by more than 14 degrees Celsius (25 degrees Fahrenheit).
- At the same time capturing enough solar power that can be used to heat water to around 60 degrees Celsius (140 degrees Fahrenheit).
Although the tested system was only 70 centimeters (27.5 inches) square, it could eventually be enlarged to cover roofs, engineers say, with the goal of ‘reduce society’s reliance on fossil fuels for cooling and heating. It would also help communities with limited access to electricity.
“Heating and cooling are badly needed in our daily lives, especially cooling in the world of warming,” said the study’s lead author Qiaoqiang Gan, professor of electrical engineering, School of Engineering and Applied Sciences.
The research team includes Zongfu Yu, University of Wisconsin-Madison; Boon Ooi, King Abdullah University of Science and Technology (KAUST) in Saudi Arabia; and members of Gan’s lab at UB and Ooi’s lab at KAUST.
The system consists of two mirrors, made of 10 thin layers of silver and silicon dioxide, which are placed in a V-shape.
These mirrors absorb incoming sunlight, converting solar energy from visible and near-infrared waves to heat. The mirrors also reflect mid-infrared waves from an “emitter” (a box directly between the two mirrors), which then kicks off the heat they carry to the sky.
“Because the thermal dissipation from all surfaces of the central thermal dissipator is exposed to the atmosphere, the local cooling power density of this emitter is doubled, resulting in high temperature reduction,” he says. Gan.
“Most radiation cooling systems dissipate solar energy, which limits the cooling capabilities of the system,” he says. “Even with the perfect spectrum selection, the maximum cooling power with an ambient temperature of 25 degrees Celsius is about 160 watts per square meter. In comparison, solar systems consumed around 1000 watts per square meter in addition to these systems. ”
Gan co-founded spinoff company, Sunny Clean Water LLC, which seeks partners to commercialize this technology.
“One of the key innovations in our system is the ability to separate and maintain solar heating and radiation cooling at different parts in one system,” says co-author Lyu Zhou, PhD candidate in electrical engineering at UB. “During the night, radiation cooling is easy because we have no solar interference, so thermal emissions go out and we understand radiation cooling easily. But daytime cooling is a challenge because the sun is shining. In this situation, you need to find strategies to separate solar heating from the cooling place. ”
The work builds on previous research in Gan ‘s laboratory that involved the creation of a cone – shaped system for electric – free cooling in cities to adapt to climate change.
“The new two-sided architecture realized local cooling power densities in excess of 280 watts per square meter. Under normal atmospheric pressure without vacuum thermal isolation, we realized a temperature reduction of 14.5 degrees Celsius below ambient temperature in a laboratory environment, and over 12 degrees Celsius in an outdoor test using a simple experimental system, ”said the other. co-author, Haomin Song, UB research assistant professor in electrical engineering.
“Importantly, our system is not just wasting solar input energy. Instead, solar energy is absorbed by the spectrum solar mirrors, and can be used for solar water heating, which is widely used as an energy efficient device in developing countries, ”says Gan . “It can sustain both the effects of solar heating and radiation cooling in a single system without the need for electricity. It really is a kind of ice and fire magic system. ”
The research team will continue to explore ways to improve the technology, including a study of how to capture enough solar energy to boil water, making it suitable for drinking.
The work was supported by funding from the U.S. National Science Foundation’s Thermal Transport Processes program.