A better way to keep cool


We all have our preferred temperature. Me, I like it cool. My poor college roommate can attest to that, since I just about froze him out of our room, aided by the fact I was tall enough to easily reach the air conditioning controls and he wasn’t. But hey, that was in Arkansas, and in Arkansas in late summer, I needed all the air-conditioning I could get.

Humans, despite having originated in hot parts of the world, have long looked for ways to make buildings more comfortable in hot weather. The first attempts in the 19th century involved circulating air over blocks of ice, but modern air conditioning first had to await the invention of mechanical refrigeration.

Liquids absorb heat from their surroundings when they evaporate or boil, and you can control the temperature at which that happens by controlling the pressure: the higher the pressure, the higher the boiling point.

William Cullen first demonstrated refrigeration using this principle in Glasgow in1748, but it was 86 years before Jacob Perkins of London patented the first practical ice-making machine, and it wasn’t until 1911 that Willis Carrier invented a practical air-conditioning system.

In both a refrigerator and an air conditioner, a liquid is boiled in an evaporator. It absorbs heat as it expands, and the warmed vapour is then compressed (which makes it even hotter) and run through pipes that allow it to radiate that heat away (which is why the back of your refrigerator is so hot). In other words, both refrigeration and air-conditioning boil down to (sorry) transferring heat from whatever you want cooled to a place where you don’t mind that heat being released.

The most common refrigerants for the last 80 years have been chlorofluorocarbons. Although later implicated in the erosion of the ozone layer, they were actually developed as a safe alternative to the much nastier refrigerants that preceded them, such as sulfur dioxide. Stable, incombustible and non-toxic, CFCs made air conditioning practical in office buildings, hospitals, apartments, trains and buses, and, by 1950, automobiles.

There are new ozone-friendly refrigerants in use today, but refrigeration-based air conditioning still has its problems. For one thing, it’s energy-intensive, as those with central air conditioning well-know from their sky-high electricity bills in hot weather.

But now comes word that the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) has invented a new kind of air conditioner that could potentially use anywhere from half to a whopping 90-percent less energy than standard systems.

The new system, called DEVap, is based on evaporative cooling rather than refrigeration. In a really dry climate that doesn’t get too hot or too humid (say, Denver), you can cool and humidify air simply by flowing water over a mesh and then blowing air through the mesh.

Unfortunately, evaporative coolers don’t work well enough to cool really hot air to a pleasant temperature, and in a humid climate they actually make things more unpleasant by increasing humidity while hardly cooling the air at all.

DEVap combines an evaporative cooler with desiccants, chemicals that absorb water from the air (you know, like those “Do Not Eat” packages you find in electronics packaging). It uses highly concentrated syrupy solutions of salts such as lithium chloride and calcium chloride that can create very dry air.

One challenge with desiccant-based cooling systems has been their complexity. DEVap has simplified things immensely by using thin membranes that are hydrophobic–water beads on them instead of soaking through them. This allows the membranes to control the flow of liquid within the cooling core, keeping the water and the desiccant separated from the air stream.

What that means in practice: hot, humid air flows into the core and in a fraction of a second becomes cool, dry air that can then be directed into the space to be cooled.

The NREL has patented the DEVap process, and will be refining it over the next couple of years with the goal of eventually licensing it to manufacturers.

It won’t help this season. But in a few years, you may be able to enjoy a cool, comfortable house without any heart-stopping power bills, all summer long.

Permanent link to this article: https://edwardwillett.com/2010/06/a-better-way-to-keep-cool/

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