Beaming heat into space could cut airconditioning costs

A cooling system capable of reducing the inside temperature of a two-storey building in a desert climate by more than 20% without significantly boosting electricity consumption or water evaporation has been modeled by scientists at Stanford University in California, US.

The system uses a method called radiative sky cooling, which involves dispersing infrared thermal energy from rooftop panels directly into the atmosphere and outer space, producing internal temperatures lower than those in the surrounding air.

The physics of radiative sky-cooling has been recognised for many years, but the energy exchange produced by it could only be used on clear, dry nights. During the day – when the demand for commercial cooling is at its highest – the necessary combination of high solar reflection and high thermal emission made the mechanism inefficient.

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Now, however, a team led by Stanford’s Eli Goldstein report in the journal Nature Energy on the creation of “fluid cooling panels that harness radiative sky cooling to cool fluids below the air temperature with zero evaporative losses, and use almost no electricity”.

Building prototypes, Goldstein and colleagues found that the panels were able to cool water flowing through them to 5 degrees below the external ambient temperature. Modelling showed that installing panels downstream of the condenser of a large commercial airconditioning system operating in climate conditions typical of Las Vegas resulted in an internal temperature drop of 21%.

The new panel design and construction is remarkably simple, using acrylic walls, polystyrene cover and insulation, and a plate heat exchanger through which water flows.

By radiating and reflecting infrared radiation high into the atmosphere and beyond into outer space, the system effectively uses them as remote heat sinks.

This solves one of the biggest headaches associated with aircon systems: waste heat generated by the condenser warms the air immediately adjacent to it, reducing its efficiency. 

Solving this problem, write researchers Geoff Smith and Angus Gentle from Australia’s University of Technology Sydney (UTS) in an accompanying editorial, would “achieve indoor thermal comfort in mid-summer at reduced cost, with lower risk of blackouts, industry shutdowns and higher energy costs due to power demand spikes.”

In earlier work, Smith and Gentle showed that cooling systems atop large facilities such as shopping centres and airports can warm the surrounding air up to 10 metres above the roof. 

Significant improvements can be made by simply changing the roofing materials so that they reflect more sunlight. Doing this in Australia, the pair report, in some cases saves up to 50% of the energy required to run the cooling system “from the combined effects of reduced heat flow from the roof, a 25% boost in air conditioner performance, and cooler ventilation air”.

This type of efficiency gain is considered urgent and critical. Goldstein and colleagues note that cooling systems currently account for 15% of global electricity consumption and produce 10% of global greenhouse gas emissions.

“With demand for cooling expected to grow tenfold by 2050, improving the efficiency of cooling systems is a critical part of the twenty-first century energy challenge,” they write.

Smith and Gentle praise the Stanford designs, although they have yet to be scaled up in the real world.

Their elegance, they say, show that “further large gains are well worth pursuing, and may be achievable with simple upgrades to existing units”.

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