Reflecting to stay cool
(appeared in Oct 2018)

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Surfaces that throw out heat would reduce the energy it takes to cool things down, says S.Ananthanarayanan.

Air conditioning and refrigeration are among the largest consumers of electricity. One hand, we need to reduce the use of electricity to contain global warming. With progressing climate change, however, the demands for cooling will increase, for comfort, productivity and conserving food.

Dr Fan and his group at Stanford have shown that ‘radiative cooling’ can keep things cooler than surroundings, and they work on ways to convert this effect into positive power savings. In the year 2014, Shanhui Fan, Aswath Raman and others had written in the journal, Nature, about a material that promoted emission of heat at wavelengths that pass out of the atmosphere and into outer space. In a more recent paper, Fan, Raman and Eli A. Goldstein implement a method where radiation from a reflective surface, even while the sun is shining, helps air-conditioning work more efficiently, with a 21% saving of electricity.

The whole problem with the earth, and warming is that the atmosphere captures and conserves heat. It is the rise in this capacity, with CO2 buildup, that leads to rising temperatures on the earth. Because of this property, the radiation from a warm object gets absorbed and does not allow cooling beyond a point. The Stanford group, in 2014, had noted that there was a feature of the atmosphere which could be exploited to get finally rid of some of the heat. This is radiation in the 8-13 micrometer band of wavelength, where the atmosphere is transparent to Infra Red radiation. While the bulk of the heat that an object radiates is absorbed by the atmosphere in its vicinity, the heat that is emitted in this frequency window goes through the atmosphere and out into space – there is positive cooling.

The arrangement that was tried out on the rooftops of Stanford in 2014 used a sheet of the material that radiated in this frequency band, and it was found to get 4-5°C cooler than the surroundings. The applications envisaged were for cooling at remote places, where electricity could not reach, or as the roof material in storage of food or medical supplies. It was an ”unexplored opportunity of using the cold darkness of the Universe as a fundamental renewable thermodynamic resource for improving energy efficiency here on Earth”, the authors of the paper had said.

In the more recent paper, the team examines a technology where radiation by surfaces helps bring down the radiator temperature of air-conditioning equipment, and hence increase their efficiency. While radiative cooling does not have the capacity to take the place of air-conditioning or refrigeration, it can make a sizeable difference to the power needs of cooling systems. The Stanford paper starts by saying that 15% of the electricity produced globally, accounting for 10% of global greenhouse gas emissions, is consumed by cooling systems. The demand for cooling is likely to increase ten-fold by the year 2050, the paper says. Hence, “improving the efficiency of cooling systems is a critical part of the twenty-first-century energy challenge”, the paper says.

The way usual cooling systems work is by using the drop in temperature that takes place when a volatile liquid is allowed to evaporate. Once evaporated, the vapour needs to be condensed for evaporation again. This is done by compressing the vapour, but compression causes warming. And then, before use as an evaporative coolant, the vapour needs to be cooled. This takes place by the condenser acting as a radiator, which loses heat by warming the surrounding air or any water that is allowed to run over it.

Now, the laws of thermodynamics are such that the best efficiency possible for a refrigerating machine depends specifically on the temperature at the condenser, at which the arrangement sends heat out to the environment – the lower this temperature, the higher the efficiency. “As a rule of thumb, the electricity input into a cooling system is reduced by 3 to 5 % for every 1°C reduction of the condenser temperature”, the paper says.

As the temperature rises quite high during compression, the vapour cannot be cooled completely down, but it comes down to between 5 to 10°C above the temperature of surroundings. If the condenser is also bathed in running water, there would be evaporation of water and the vapour would get a little cooler. Doing this, however, can get complex, as the water needs to be treated, so that it does not deposit salts on the condenser, and then, water needs to be cooled before it is used again. Arrangements like cooling towers are not practical for smaller applications. There is also significant water loss, by evaporation, the paper says. This is a serious negative, as water itself, in the regions of the earth where economical cooling is most required, is scarce.

Radiative cooling, with application to cooling the condenser of refrigeration plants, then becomes an attractive possibility. The Stanford group made use of aluminum plates, some ¾ of a cm thick and just 2 x 2 feet in dimensions, with a winding copper tube embedded inside the plate. The radiative cooling surface, which combined high reflectivity with the capacity to radiate at the most advantageous wavelengths, was attached to the metal sheet with an adhesive that conducted heat. The cooling surface was hence closely in contact with the copper tubing, through which a stream of water was passed. Sunshades were also placed so that the casing of the arrangement did not heat directly under the sun.

The stream of water was found, during trials, to cool by 2°C and 3°C, with flow rates, respectively, of 0.29 litres a minute and 0.12 litres a minute, for a 1 square metre cooling plate, at the hottest time of the day. Averaging over 72 hours, the arrangement gave cooling of 3°C to 5°C, at an effective flow rate of 2.67 litres a minute. This corresponds, the paper says, to a rate of discharge of heat of 40W to 70W by each square metre of the cooling plates.

The paper goes on to estimate the effect that this rate of extraction of heat would have on the efficiency of an air-conditioning plant if the cooling arrangement was linked to the condenser of the plant. “Over four summer months (May through August of a Typical Meteorological Year) in Las Vegas, Nevada, USA, we show that by covering 60% of the roof on a two-storey commercial o?ce building, 14.3 MWh of electricity could be saved, corresponding to a 21% reduction in the electricity required for cooling”, the paper says.

It is not that radiative cooling has not been used before. There are arrangements where a water tank connected to a roof-top heat exchanger can generate cool drinking water during the night, in arid areas. The paper cites work by Fernandez, Wang, Alvine and Katipamula where the same surface materials used here were employed with a heat storage system to achieve higher energy savings, 45% in Las Vegas. The system used by the Stanford group, which needs no arrangements for heat storage, however, is more suited for use with building cooling systems, the paper says.

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