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Phase-change materials that freeze at around room

Thu, 01/05/2012 - 6:23am
New Scientist

THE sun has risen, and a brand new building on the University of Washington's campus in Seattle is about to melt.

It is no design flaw: encapsulated within the walls and ceiling panels is a gel that solidifies at night and melts with the warmth of the day. Known as a phase change material (PCM), the gel will help reduce the amount of energy needed to cool office space in the building - scheduled to house the molecular engineering department when completed this month - by a whopping 98 per cent.

PCMs don't have to be as high-tech as this, of course. We have been using ice, a phase change material that melts at 0 °C, to keep things cool for thousands of years. But advances in materials science and rising energy costs are now driving the development of PCMs that work at different temperatures to help people and goods stay cool or warm, or to store energy.

PCMs are attractive energy-savers because of their ability to absorb or release massive amounts of energy while maintaining a near-constant temperature. "To melt ice takes the same amount of energy as would be required to warm an equal volume of water by 82 °C," says Jan Kosny of the Fraunhofer Center for Sustainable Energy Systems in Cambridge, Massachusetts, who began to explore the potential of PCMs three decades ago by looking at beeswax as a way to store heat from the sun. The reason PCMs are so useful is because energy is needed to break the molecular bonds between atoms when a substance melts, and is released when bonds are formed as it solidifies.

The "bioPCM" gel in the university building, derived from vegetable oils, will be "charged" each night when windows automatically open to flush the building with cold outdoor air. The solid gel then absorbs heat as it melts the next day. The idea is the same as using thick concrete or adobe walls, which reduce indoor temperature fluctuations, but only a fraction of the material is required. "Our bioPCM is 1.25 centimetres thick yet it acts like the thermal mass of 25 centimetres of concrete," says Peter Horwath, founder of Phase Change Energy Solutions, based in Asheboro, North Carolina.

A recent report by technology research firm Lux Research predicts the use of phase change materials in buildings will grow from near zero today to $130 million in annual sales by 2020.

Meanwhile, a number of other applications are emerging. UK-based Star Refrigeration is using carbon dioxide, which changes phase from liquid to gas at a very low temperature, to keep data centres cool. Heat emitted by today's high-performance server farms can overwhelm even the most advanced water cooling systems. By piping CO2 through heat exchangers, the company recently demonstrated an ability to pull nearly twice as much heat from the computers as the systems used at present.

In western China, PCMs derived from yak butter and local plant oils are helping yak herders keep warm. The material is encased in plastic and then woven into traditional clothing. It melts as herders work up a sweat walking to mountain pastures then, when they stop moving, the pent-up heat is slowly released, keeping them warm as they watch their herds. More than 100 families are now using the materials as part of an ongoing pilot project that also includes bed rolls warmed by cooking stoves in the day to keep people warm at night. "Families that use them are starting to see a significant difference in the amount of fuel they need," says Scot Frank of One Earth Designs, also based in Cambridge, which developed the compounds.

Another promising application for PCMs is vaccine delivery in developing countries. Vaccines need to be kept cold during transport, which is a challenge in countries with limited refrigeration. They are typically packaged in ice, but their effectiveness can be severely compromised if they freeze. Using materials that change phase between 4 and 8 °C, US packaging manufacturer Sonoco says it has developed a solution that can keep vaccines cool for up to six days. Sonoco is now testing the Greenbox with a non-profit biotechnology developer called PATH, to meet World Health Organization standards.

Harnessing PCMs for energy storage could also give solar power a boost. Today systems that concentrate solar thermal energy rely on liquid salts to store heat. This allows power plants to produce energy when the sun is not shining, but requires massive amounts of liquid and large, well-insulated storage facilities. By using chemicals that change phase instead, German manufacturer SGL Carbon says it can reduce the volume of storage material required by roughly two-thirds. The company is currently testing a prototype.

For Kosny, all of the recent interest in PCMs is something of a vindication. "Ten years ago, when I argued for the development of phase-change materials, no one was interested," he says. "Now we can't seem to develop these materials fast enough."

Issue 2846 of New Scientist magazine

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