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LAWRENCE -- Two tiny cottages at the University of Kansas are helping define a new way of building homes that could make them more energy efficient.
The diminutive abodes are collecting data that will help determine whether an innovative construction technique involving a substance known as phase-change material (PCM) can help decrease the demand on a home's heating and cooling system. Preliminary information collected by Mario Medina, assistant professor of architectural engineering in the KU School of Engineering, and his team of graduate students suggests PCM will.
Phase-change materials are widely used in a number of industries, but not in construction, Medina said. PCMs absorb heat while melting, thus cooling their environment, and they release heat upon freezing, heating the surroundings. The PCMs used in Medina's research constitute a series of linear crystalline alkyl hydrocarbons that are commercially available from by-products of petroleum refining.
"If we are successful, one application could be buildings," Medina said.
The success of the project rests upon the PCMs' ability to change states from solid to liquid as heat moves through the walls of the structure. Medina's research project -- funded by the Center for Energy Research, the KU School of Engineering and a general research fund from the university -- encapsulates the PCM inside 1-inch copper pipes and suspends the pipes horizontally between the structure's studs. The PCM, donated by the German firm Schumann-Sasol, melts in the range of 24 to 28 degrees Celsius (about 75-82 degrees Fahrenheit).
The conductive nature of copper allows heat to transfer through the pipes to the PCM, which melts and retains the heat.
"This kind of material, when it melts, absorbs a lot of heat," said Meng Zhang, a graduate student in architectural engineering from Beijing. A key factor in the project is that the PCM melts and solidifies within the comfort zone in which people operate most buildings.
During the summer months, the PCM would work to retain the heat until temperatures drop in the evening. In winter, the PCM would help retain heat from inside the home and transfer the heat back to the home as the material solidifies.
The research team is collecting a variety of data in each of the structures, which are 6 feet wide, 6 feet tall and 6 feet deep. The buildings are identical except that only one of the homes is outfitted with the PCM-infused copper pipes. The structures even share a single cooling system, which helps maintain the quality of the data being collected.
"They are fully instrumented," Medina said. "They have sensors that monitor the amount of energy for heating and cooling."
In fact, data on interior conditions and external weather conditions are collected every 10 seconds. Data include surface and air temperatures, wall and ceiling heat fluxes, relative humidity, solar radiation, wind speed, and space cooling and heating loads.
While some variables remain to be explored, Medina said the team already has identified that the house outfitted with the pipes with PCM has very constant temperatures. "We may see a transfer in (energy) demand," he said. The material wouldn't negate the need for heating and cooling, but it would decrease the demand during peak times, he suggests.
Data already have been collected for one winter season, and the team continues to collect data from the structures this summer. Jennifer King, a senior in architectural engineering from Boca Raton, Fla., who also began graduate study this fall, soon will develop computer modeling that will explore how this construction technique will fare in a larger, more complex structure. The modeling also will take into account geographic environmental differences.
Medina expects the results from the computer modeling will produce similar results because the geometry and construction techniques of larger structures are similar to those employed in the cottages. The researchers may find there are areas of the country where homes built with this technique will let residents live comfortably without an air conditioner, he said.
If findings are positive, Medina thinks the PCM technique eventually could be economically incorporated into new structures.
"I don't think it would be that expensive, when compared with another layer of insulation," Medina said.
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