Stanford University researchers have developed a system that can passively cool water by up to 5°C by beaming its heat into outer space. According to a new study published in Nature Energy, the technology promises to “dramatically” cut the electricity needed to run airconditioners in commercial buildings.

We’ve previously written about Stanford research into nanophotonic materials that allow radiative cooling to occur during the day, and at a frequency that allows infrared heat to be beamed through the atmosphere into space – thereby using outer space as a heat sink, rather than the local environment.

Now the mechanism has been combined with water and pipes to create “fluid cooling panels”. Here water is pumped through pipes below panels containing the radiative cooling material on the roof, with results showing it can reduce water temperature by between 3-5°C.

A fluid-cooling panel designed by Shanhui Fan, professor of electrical engineering at Stanford, and former research associates Aaswath Raman and Eli Goldstein. Image: Aaswath Raman

The researchers labelled the system “a striking corollary” to solar hot water heating, and said it could “dramatically improve the efficiency” of cooling and refrigeration systems.

The study, Sub-ambient non-evaporative fluid cooling with the sky, contained modelling that showed when integrated with a two-storey office building in a hot dry climate, electricity consumption during summer was reduced by 14.3 megawatt-hours, or 21 per cent of total electricity needs.

“This research builds on our previous work with radiative sky cooling but takes it to the next level,” study author Dr Aaswath Raman said.

“It provides for the first time a high-fidelity technology demonstration of how you can use radiative sky cooling to passively cool a fluid and, in doing so, connect it with cooling systems to save electricity.”

The findings are important, as airconditioning accounts for an estimated 15 per cent of electricity generated globally, and 10 per cent of greenhouse gas emissions, with demand for cooling expected to increase tenfold by 2050.

Generally radiative cooling wouldn’t have much of an effect during the day, as the energy gained by the sun would be more than that dissipated through radiative cooling. However the nanophotonic material created reflects about 97 per cent of sunlight while also sending infrared energy at a frequency that can bypass the atmosphere (important to not contribute to the global warming or the urban heat island effect). Without the heat from sunlight, the radiative sky cooling effect allows cooling below the air temperature, even on a sunny day.

“From off-grid scenarios, to power plants and industrial facilities, our results show how the radiative coldness of the sky can be integrated with energy systems here on the surface of the Earth to deliver meaningful energy savings,” the study authors wrote.

Lead author Dr Shanhui Fan said having something very cold – like space – available meant you could dissipate heat into it.

“Then you can do cooling without any electricity or work. The heat just flows.”

The authors are so certain of their technology they’ve even created a company, SkyCool Systems, to test and commercialise the technology, and also to work out how to integrate the panels with standard HVAC systems. Data centres are a key building type being targeted.

There’s even potential for the savings to be increased by using thermal storage.

“While we have highlighted the system’s capability of functioning during the day, thereby addressing cooling system inefficiencies that arise with hot ambient air temperatures, it should be emphasised that the fluid cooling panel functions 24 hours a day,” the authors wrote.

“Thus, with thoughtful system design and the use of thermal storage at night, when cooling demands may be low or non-existent, even larger energy savings can be extracted from the fluid cooling panels.”

This work was funded by the Advanced Research Projects Agency – Energy (ARPA-E) of the US Department of Energy, which has now been defunded by the Trump administration.

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