If a bio-based product that releases energy as its freezes and thaws to help regulate indoor and thermal energy temperatures and reduce a building’s energy load sounds a little Space-Age and far-fetched, just place an ice cube on the counter and watch it melt.
That ice cube is a low-tech version of a phase-change material, or PCM. Higher-tech versions using a variety of organic materials, such as salt hydrates, paraffins (waxes), and fatty acids, have been around since the 1970s and are establishing a range of building products and systems-related applications in Europe that energy experts hope will soon jump the pond.
First, the science: As a PCM heats up (like an ice cube), it changes to a liquid state that absorbs and stores heat; as it cools and returns to a solid state, it releases that heat. In a building, the net effect helps maintain indoor temperatures naturally instead of relying on the heating and cooling equipment to adjust for cool mornings or hot afternoons, thus reducing the energy demand on that equipment and resulting in greater indoor comfort.
To work effectively in building products and applications, however, PCMs must change from liquid to solid and back again at far higher temperatures than water/ice, ideally within the range of a building’s indoor temperature swings. And, they appear to work best in climates that cool down far enough at night to cause a phase-change back to a solid state, and less so in cooling or mixed climates.
That’s likely why logical PCM-enhanced products including gypsum wallboard, insulated glass units, ceiling tiles, aerated concrete, and supplemental insulation mats have had a difficult time breaking into the domestic market. Simply, it’s not yet cost-effective to manufacture climate-specific products in enough volume, and the science to deliver a happy medium remains unproven.
“Both BASF and DuPont have tried it (in Europe and the U.S.), but cost has killed the idea,” for those applications, says Zafer Ure of Phase Change Material Products Limited in Cambridgeshire, U.K., which develops encapsulated PCMs and PCM-enhanced products and systems. “It’s the biggest barrier to their widespread use.”
Ure estimates a 20% to 50% installed-cost premium for such PCM-enhanced applications, though he anticipates PCM’s pricing will be cost-competitive within five years, at least in Europe; in the U.S, the subsidized (and therefore lower) cost of energy and space-cooling systems further inhibit PCMs from making inroads domestically.
In the meantime, Ure is looking to other, perhaps more suitable and cost-effective applications of the technology now—specifically thermal energy storage systems that effectively warehouses high- or low-temperature energy for later use to stabilize thermal loss and mitigate or avoid peak-time needs and utility rates for that energy. To date, his company has developed and installed encapsulated PCMs for heat pumps, chillers, domestic hot water systems, solar energy, and cogeneration (combined heat and power) equipment.
For athlete housing during the 2006 Commonwealth Games in Melbourne, Australia, for instance, during which per-unit occupancy in each apartment far exceeded the average household size, adding encapsulated PCM balls rated for 58 degrees C (136.4 F) into standard tank water heaters increased their hot water storage capacity by up to four times, all but guaranteeing hot showers for the athletes.
For photovoltaic arrays, meanwhile, Ure uses PCMs to capture the heat from the solar panels in addition to what they can convert into electricity—perhaps 25% more than their electric output capacity.
That hot thermal energy is then stored, perhaps in a water tank, to be used as a primary or supplemental space or water heating source, as needed. Conversely, PCMs can also capture and store the cool night air around the PV array to be used as a cooling energy source in the heat of the day, often during peak-use times and rates.
The best news is that PCM applications are not limited to large, non-residential buildings, as so many new energy technologies tend to be. “Size and scale are irrelevant to its effectiveness in buildings,” says Ure. “Technically, anything is possible.”