Maintaining temperature nature's way

research

Inspired by nature, scientists have come up with a novel way of maintaining temperature in buildings by creating a sheet with channels to carry water like blood vessels found in animal skin, says S Ananthanarayanan

Nature provides ways for animals to adjust their skin temperature so that they can cool down when it is warm or lose less heat when it is cold. Finding a way to incorporate this mechanism while constructing houses and other buildings can save a lot of money spent on air-conditioning in the summer and heating in the winter.

Professor Ben Hatton of the University of Toronto and his colleagues at Harvard University and the Broad Institute, Cambridge, Massachusetts, report in the journal Solar Energy Materials and Solar Cells that they have created a surface to lay on glass windows which has channels to carry water like narrow blood vessels that are found in the skin of animals.

The mechanism is not the same as in animals, but the arrangement helps cool the window during hot weather, at any rate, they say.

Ventilation and lighting of buildings call for large windows. Windows have glass panes and while light streams in through the glass and keeps the interiors well lit, the glass also blocks infrared radiation and the place soon heats up. On a warm day, glass window panes can get as warm as the asphalt outside. The result is that the window panes radiate heat into the building, like large heating coils, much to the discomfort of the occupants and especially the owner, if there is air-conditioning, when she receives the energy bill. The authors of the paper say that of the total building energy costs, for cooling and heating, around 40 per cent is estimated to arise because of windows.

Keeping cool

Natural systems – warm-blooded animals – in the same way as buildings need to control internal temperature no matter how extreme the temperature of the surroundings gets. Thus, the body needs to stay cool even when it is hot and has to stay warm even during harsh winters. Else, the rising temperature would block vital processes like the working of the brain or there would be great energy loss in keeping warm when it is cold outside.

Natural systems have evolved to control temperature in the most efficient way by controlling the surface temperature. Thus, in warm weather, when the body needs to lose heat or receive less heat from the surroundings, the surface temperature, which is the skin temperature, is kept high – almost as much as the temperature inside. But in cold weather, when the body needs to lose the least heat possible, the skin temperature is kept low, well below the internal body temperature.

The way the body manages this is by adjusting the diameter of the blood vessels that are right at the exterior – the peripheral capillaries. When the temperature falls, the nervous system causes constriction of the capillaries and less warm blood flows at the surface.

The surface of the skin is thus cooler and there is less heat loss. On the other hand, if the body needs to lose heat, like after exercise or when it has a fever or when the weather is warm, the surface temperature needs to be higher. The nervous system then widens or dilates the peripheral capillaries and more warm blood flows at the surface and the temperature rises.

People who live in cold climates adapt or generally have constricted outer capillaries, to conserve heat. When such persons visit warmer places, it takes the body some time to adjust and for a few weeks or longer they would find the heat difficult to bear.

The converse is true when people from the plains take a holiday in the hills – they suffer from cold and do not adapt till it is time to come home again!

Alcohol and the capillaries

Alcohol has the effect of dilating the peripheral capillaries. A stiff drink when one comes in from the cold would make the skin grow warm and this would warm up the woollen garment of the person, and the person would be comfortable. But that same drink taken when one is not securely wrapped up would lead to faster loss of heat and may prove harmful.

In this context, a story is generally narrated of soldiers or campers at high altitudes who have a drink before they get into their sleeping bags. The drink makes them uncomfortably hot and being a little drunk, the soldiers unzip the sleeping bags. If they are not careful to pull the zip back up, they may not wake up in the morning!

Window to saving power

Ben Hutton and his colleagues took a cue from nature and experimented with glass windows. They created a plastic sheet made of the material polydmethylsiloxane, a flexible and transparent material, and embedded narrow channels into the sheet. Water was allowed to flow through these channels so that the glass pane on which the plastic sheet was applied could be cooled.

Water at room temperature either began to get warm and flow upwards, due to convection, or pumped through the film. But the result in either case was that the glass pane cooled to almost the temperature within the building and no longer added to the heat load the interior had to handle.

The warm water flowing out of the film can be passed through a cooling tower or used for any warm water application or even with an energy recovery arrangement, they say.

During winter, when windows cause loss of heat to the exterior, the flowing water can stem the loss of heat by carrying it away for recovery and reuse. Ben Hutton and colleagues even suggest using coloured or stained water to control the level of light admitted or for aesthetics.