- New passive cooling system provides effective cooling in hot environments without using any fuel-produced power.
- In dry environments, it can provide cooling of up to 40°C below the ambient temperature.
- It could supplement existing cooling systems to store vaccines and food in hot, off-grid areas.
The existing cooling mechanisms rely on fluid-cooled and vapor compression systems, which are quite complex and expensive. Passive cooling system, on the other hand, like atmospheric radiative cooling uses Earth’s atmosphere at mid-infrared wavelengths to bring down temperatures at significant levels.
For years, researchers all over the world have tried to develop passive cooling systems, and so far they have been able to build these systems using complex photonic instruments that are expensive to manufacture and can’t be widely used. Most of them are developed to perfectly reflect all wavelengths and emit mid-infrared radiation.
However, to achieve this, you need to add multiple layers of materials and control their thicknesses to nanometer precision.
Now, researchers at MIT have developed a new, inexpensive passive system that provides cooling on a hot sunny day, without requiring fuel-produced power. It could supplement other cooling systems to store/preserve medicines and food in off-grid areas.
Theoretically, the system is capable of providing cooling of up to 20°C below the ambient temperature. However, while demonstrating it, researchers were able to achieve a cooling of 6°C. To further bring down the temperature, one could use traditional thermoelectric cooling or refrigeration systems.
How Does It Work?
The new passive cooling system simply blocks the direct sunlight, using a small strip placed at 90 degrees to cover the path of the Sun across the sky. This eliminates the need of tracking instrument(s).
A simple white instrument, developed by combining polished aluminum, low-cost plastic film, and insulation, enables enough emission of heat via radiation in the mid-infrared range. More specifically, it leverages the spectral overlap of the radiative emission of terrestrial objects near ambient temperature between the wavelength of 8 and 13 micrometers.
By placing an umbrella over the system and supplementing it with insulation, researchers made cooling more effective during the daytime. They performed outdoor experiments and found that the system was effectively blocking the direct sunlight while continuously emitting the mid-infrared wavelengths to the sky.
A few couples of decades ago, scientists had only been developing systems to reduce heating. But now, they are smartly using shades along with supportive light filtering mechanisms to bring the temperature down.
Limitations and Applications
Image credit: Bikram Bhatia / MIT
The systems doesn’t work properly in a humid atmosphere. It reduces the amount of infrared emission through the air, limiting the amount of cooling to 20°C (below the ambient temperature). However, if the environment is dry, it could yield up to 40°C of cooling.
Also, this system can’t be used to cool entire houses or buildings. It could only be used for refrigeration applications, like storing food and medicines in hot, off-grid areas. Although the approach is not powerful enough to bring down temperatures all the way to required levels, it can at least decrease the loads on existing cooling systems, to provide only the last phase of cooling.
It could also be used in concentrated photovoltaic systems, in which special mirrors focus Sun rays onto solar cells to elevate their efficiency. These systems often overheat and demand regular thermal maintenance. The passive cooling system can easily control this heating issue, eliminating the need of active thermal management.
In the coming years, researchers will try to improve the system. The biggest challenge in doing so would be discovering an insulating material capable of preventing the entire system from heating up excessively from the external air.