January 12, 2012

Phase change materials result in cost savings

A building material that absorbs heat during the day and releases it at night promises to save building owners a bundle of cash when it’s time to pay the monthly electricity bill

They’re called phase-change materials, or PCMs, and scientists have been experimenting with them for decades. And although there have been some efforts to introduce them to construction markets in North America and Europe, they remain largely unknown or ignored.

Now though, PCMs are getting a place in the spotlight.

A new building for the molecular engineering department of the University of Washington, in Seattle, has PCMs encapsulated in its wall and ceiling panels, and the expectation is that cooling costs will be as much as 98 per cent lower than one would expect for a 77,000-square-foot building.

The most common PCM is probably ice. Drop a couple of cubes in your drink and they cool it by slowly absorbing heat as they melt. It’s such an obvious property that one might wonder why it’s important.

But what about other PCMs?

Those used in the Seattle building take the form of a gel derived from vegetable oils and the phase change occurs, not at the freezing point of water, but at room temperature. It remains a gel when the air in the building is cool, but gradually absorbs heat and liquefies as the building’s heat load increases during the day. The result is an indoor environment in which the temperature fluctuations are small, just as they are in buildings with a significant solar mass.

The PCM panels, called BioCPM, were supplied by Phase Change Energy Solutions, of North Carolina, and the company’s founder, Peter Horwath. He says the idea is the same as using thick concrete walls. His panels are 1.25 centimetres thick but, he says, “act like the thermal mass of 25 centimetres of concrete.”

The PCMs are set during manufacture to maintain a temperature of about 23 C. As the building’s heat load increases during the day, the PCMs gradually melt, storing heat and cooling the building’s interior. Then, at night, as the heat load diminishes, the PCMs change back to gel.

In Seattle’s climate, the overnight cooling will be accomplished by having automatic windows open to flood the interior with cool night air.

In other climates, the air conditioning can be run at night, when power rates are low, to cool the indoor air enough to trigger the phase change back to a gel.

The same PCMs used in Seattle are also being introduced into the roofing market, as interest in the concept grows.

Scientists have been experimenting with other PCMs, as well. There is a system that uses tiny capsules of paraffin waxes embedded in drywall. There have been lab experiments trying to use the phase-change properties of beeswax to store solar energy. And a British firm, Star Refrigeration, is experimenting with carbon dioxide, which changes phase (from gas to solid) at extremely low temperatures.

The building on the University of Washington campus is almost finished, with occupancy expected within weeks. It’s a 77,000-square-foot structure costing $77.7 million, and is the first of two phases.

And since it will house the molecular engineering department, it’s nice to see that the building will not be only a working building, but a demonstration project, as well.

Buildings are energy hogs. In the United States, it’s estimated that buildings consume more than 70 per cent of the electricity generated in the country.

There has been a push to make buildings lighter, making them quicker and cheaper to build. The trade-off, though, is indoor temperatures that fluctuate more, which means higher heating and cooling costs.

PCMs seem to offer one way to deal with that trade-off.

Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@dailycommercialnews.com

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