It is people, not the surroundings, that feel the heat, says S.Ananthanarayanan.
In our approach to beating the heat by creating bubbles of coolness, we expend resources in cooling things that do not appreciate the effort. The heat capacity of things in a room that air conditioners cool is many times more than the cooling the people need.
Eric Teitelbaum, Kian Wee Chen, Dorit Aviv Kipp, Bradford, Lea Ruefenacht, Denon Sheppard, Megan Teitelbaum, Forrest Meggers, Jovan Pantelic and Adam Rysanek, from the Singapore campuses of ETH, Zurich and Berkley Education Alliance, Princeton University and the universities of Pennsylvania, British Columbia and California, report in the journal, Proceedings of the National Academy of Sciences (PNAS), an experiment with ‘radiative cooling’ – a method to allow the body to throw out heat, but without spending energy in cooling the surroundings.
Just as we can throw coloured light on an object, but we cannot ‘shine darkness’ on it, only warm bodies radiate heat – cool things do not ‘radiate coolness.’ But cool things absorb more heat than they emit, and their presence helps warm things lose their heat and cool down – the question is only of limiting the cooling to things that need to be cooled.
The common method we use to help people stay cool in warm weather is Air Conditioning, which is to blow refrigerated air into the spaces that people occupy. To help them stay warm in the winter, we do use ‘radiators’, or warm objects that warm the air in the room, but people can also directly sense the warmth of radiators. For cooling, however, the only practical way is to chill air by passing it through a refrigerator, and then blow it into the room (apart from just blowing air, which causes a little cooling by evaporation of moisture on human bodies). And the cool air blown into the room needs to chill the walls, the furniture and the floor, before the people in the room begin to be comfortable.
The human body stays at an almost constant temperature of 37°C or 98.6°F. As the surroundings are usually cooler, there is a balance of heat production within the body and heat loss to the surroundings. The heat loss is mainly by radiation, where a warm body loses more heat than the radiation that it absorbs from its surroundings. And there is some heat lost to the surrounding air, by contact, but this is not much, as the air has little heat capacity. The problem, however, starts when the surroundings get warm, as on a summer day, and the body does not lose enough heat. The Body they needs to sweat, to lose heat by evaporation, but if it is humid, this is not effective and discomfort increases.
The method the PNAS group have used is to position in the room a specially fashioned, substantial ‘cool’ surface, which would absorb heat radiated by human bodies but not return much radiant heat to the surroundings - a kind of one-way window, through which radiation can escape. The reason why just any cold surface in the room cannot do the same thing is that an ordinary surface would rapidly warm by the heat from the surrounding air, which would blow over it in convection currents. And then, the moisture in the air would condense on the cool surface. Condensation releases a great deal of heat. In the case of Air Conditioning, in fact, the power used in drying the air that is circulated is greater than the energy needed for cooling it. The arrangement that the PNAS group has created takes care of these two effects and allows the cool surface to receive heat only by radiation that falls on it.
The arrangement, which was tried out in an outdoor pavilion in Singapore, where the temperature was almost 30°C, was a wall, with a metal surface that was kept cool, at 17°C, by circulating refrigerated water. Normally, currents of air would have warmed the sheet. But this was prevented by an insulating, low density, polyethylene film, a membrane that was placed before the metal sheet. “…we eliminate this unwanted convection as a mechanism of heat transfer,” the paper says. The film, however, was transparent to thermal radiation, and the heat from warm human bodies in the pavilion could pass through, to be absorbed by the cool surface.
As for condensation – the insulating film kept the surface exposed to air from getting much cooler. Trials showed that even with 66.5% humidity, and a ‘dew point’, or the temperature at which condensation starts, of 23.7°C, there was no condensation on the wall surface. The chilled water circulating inside the wall could go down to 12.7°C below the dew point, the paper says, before condensation set in.
And for the human comfort? There was a study of how 55 persons, who participated in a trial, perceived cooling, during the days between January 8 and 27, the summer in Singapore. Of the 55 persons, 37 entered the pavilion when the cooling system was on and the remaining when the system was switched off. The group which entered when the system was on, reported a ‘satisfactory’ ambient temperature in the pavilion 79% of the time, in those hot days, the study says. It was seen that warm objects in the area lost heat to the absorbent wall, and the humans, the warmest objects, were the main losers. The air temperature, however, was hardly affected, falling from 31°C to just 30°C. This last observation shows that very little energy is being spent in cooling the air – which is the sole vehicle of low temperatures in conventional cooling systems.
The technology reported is thus an effective stand-in for Air Conditioning. The reduction of energy consumption can be 50%, using only what it takes for chilled water to circulate in the heat absorbent walls, an author of the paper says. This is significant, as Air Conditioning forms a major part of the world’s energy budget, and by the year 2020, the demand is projected to surpass the demand for heating. Another advantage of the new system is that the cooled spaces can be well ventilated. In Air Conditioning, economy demands that much of the cooled air be recirculated. One person with a cold (or worse) in an enclosed office is hence sure to give it to all the others.
The technology of radiant cooling works because the heat from human bodies is not absorbed by the surrounding air, to be radiated back, but can reach the cool surface. At the scale of the earth, however, the atmosphere does not allow heat to escape, which is why the earth does not cool deep below freezing at night (and why changes in the atmosphere are causing global warming). There is, however, a narrow band of wavelengths, 8-13 micrometres, to which the atmosphere is transparent. Emission of heat in this wavelength window goes straight out into space. This has been made use of by covering objects with films that convert heat into the required wavelength range. The result is that the objects can cool, through radiative cooling, 4-5°C below the ambient, while they are out in the sun. A useful thing, for ‘green’ cold stores, and solar energy panels, which become less efficient when the get warmer.
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