Professor Qiaoqiang Gan of the University at Buffalo (NY) and his team developed a unique two-in-one system that uses solar energy for simultaneously cooling and heating — without electricity.

Tech Briefs: What got you started on this project?

Professor Qiaoqiang Gan: We have been working on sustainable technologies for quite a long time, focusing on solar energy and water treatment. We worked on using solar energy to purify water. In fact, we have a startup company, Sunny Clean Water, working on evaporating water using solar energy to produce moisture, which can be condensed to get clean water. That brought us to electricity-free cooling technology, which is an emergent research area at this moment.

Prof. Gan

Tech Briefs: Could you describe your system?

Gan: We built a V-shaped structure with two solar absorption mirrors, each at a tilt angle of 45°. The mirrors are made of 10 extremely thin layers of mixed silver and silicon dioxide. In between the mirrors, is a vertically aligned two-sided thermal emitter made from transparent polydimethylsiloxane (PDMS) film.

The special composition of the mirrors allows them to have a double function. Heat from the thermal emitter is radiated from both of its sides to the mirrors, which redirect it to the sky. At the same time, they absorb incident sunlight so that it will not heat the emitter.

Our special double structure has enabled the system to reach over 270 W/m2, as compared to the natural cooling limit of approximately 160 W/m2 for a typical radiative cooling system. We are able to maximize the radiative cooling effect because the energy-absorbing mirrors shield the thermal emitter from sunlight. For example, over the summertime at Buffalo, our roof is black and during the afternoon when we have very strong light, our second floor is very hot because of the solar heating. In radiant cooling, one of the big challenges is how to remove the sun’s heat, but still keep the thermal emission so that we can realize this radiative cooling effect.

Tech Briefs: How did you test your system?

Gan: We actually performed the tests under two different conditions, one indoors and the other outdoors. For the indoor test, the heat from the emitter was radiated to a heat sink to simulate nighttime conditions. For the outdoor test, the sky served as the heat sink.

To perform the outdoor test, we placed the structure in a polystyrene foam box, which was sealed with a polyethylene film.

A mock-up of the radiative cooling system. (Image Credit: University of Buffalo)

Tech Briefs: What were your results?

Gan: Under the outdoor conditions, we were able to reduce the temperature in the test box by about 12°C below ambient. For the indoor test, we were able to do about 14°C.

Tech Briefs: How would the cooling effect get transmitted, say to a room, if this were on a roof?

Gan: We would have to introduce a coolant very similar to that used in standard air conditioning. The cooled liquid would be transported to a room, where it would absorb the ambient heat.

Tech Briefs: You’ve explained how your system would do cooling; how would it be able to simultaneously do heating?

Gan: Previous radiative cooling systems just blocked the sunlight — they didn’t make full use of it. Solar energy is around 1000 W/m2. Our cooling technology is only around 100 to 150 W/m2. Since our special mirrors absorb the incoming sunlight, we can make use of that energy to heat water.

Tech Briefs: How would you scale up the system physically? I'm trying to picture what it would look like scaled up to the size of a full roof.

Gan: Good question. Probably we will design an array of small systems to cover the entire roof. An important factor for scalability is that our system is low-cost.

Tech Briefs: Do you have an approximate idea of how long it might be before you could commercialize your system?

Gan: Currently our startup company is looking for opportunities through Federal or private agencies. So, if someone is interested and puts in resources, I believe we can develop something within one year.

An edited version of this interview appeared in the April 2021 issue of Tech Briefs.