Breakthrough in radiative cooling

New research led by University at Buffalo (UB) engineers has made significant progress in the area of passive cooling.

The study, published in the journal Cell Reports Physical Science, has found a way to turbo-charge an existing passive cooling technique, known as radiative or sky cooling. This eco-friendly technique involves the use of sun-blocking nanomaterials that remove heat away from building rooftops.

A leap forward

While other researchers have typically found it difficult to increase the cooling capabilities of the materials, the UB team has designed an exclusive radiative cooling system that takes a leap forward in this field.

According to test results of the new study, the system:

  • Decreased the temperature of the test box in a laboratory, intended to replicate the night, by over 14°C
  • Decreased the temperature within a test system in an outdoor environment under direct sunlight by over 12°C
  • Concurrently captured sufficient solar power that could be utilised to heat water to around 60°C.

The system consists of what are essentially two mirrors placed in a V-shape. Made of 10 extremely thin layers of silver and silicon dioxide, these mirrors absorb incoming sunlight, converting solar power from visible and near-infrared waves into heat.

They also reflect mid-infrared waves from an “emitter” – a vertical box in between the two mirrors – which bounces the heat they carry into the sky.

“Since the thermal emission from both surfaces of the central thermal emitter is reflected to the sky, the local cooling power density on this emitter is doubled, resulting in a record high temperature reduction,” says the study’s lead author Qiaoqiang Gan.

“Most radiative cooling systems scatter the solar energy, which limits the system’s cooling capabilities,” Gan says. “Even with a perfect spectral selection, the upper limit for the cooling power with an ambient temperature of 25°C is about 160W/m2. In contrast, the solar energy of about 1,000W/m2 on top of those systems was simply wasted.”

Scalable technology

Although the system tested was just 70cm2, the engineers say it could be scaled up to cover rooftops. The study’s goal is to reduce society’s dependence on fossil fuels for heating and cooling purposes, and help communities that have only limited access to electricity.

“There is a great need for heating and cooling in our daily life, especially cooling in the warming world,” says Gan.

“One of the key innovations of our system is the ability to separate and retain the solar heating and radiative cooling at different components in a single system,” says co-first author Lyu Zhou, a PhD candidate in electrical engineering in the School of Engineering and Applied Sciences.

“During the night, radiative cooling is easy because we don’t have solar input, so thermal emissions just go out and we realise radiative 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 area.”

Gan’s lab had been working on creating a cone-shaped system for electricity-free cooling in crowded cities to adapt to climate change. The new radiative cooling system will build on the progress of that project.

“Importantly, our system does not simply waste the solar input energy. Instead, the solar energy is absorbed by the solar spectral selective mirrors, and it can be used for solar water heating, which is widely used as an energy-efficient device in developing countries,” says Gan.

“It can retain both the solar heating and radiative cooling effects in a single system with no need of electricity. It’s really sort of a ‘magic’ system of ice and fire.”

The work was supported by funding from the US National Science Foundation’s Thermal Transport Processes program.

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