Straight Green: Phase Change Materials
A new product now on the market helps improve thermal performance of buildings in a unique way. Phase Change Materials have been studied and used in buildings for over 40 years but have only recently become available in the U.S. as a mass-produced building material. Available in rigid boards or flexible sheets, PCMs help to reduce energy consumption in buildings by acting as an equivalent to thermal mass in a small, lightweight, easily installed assembly.
HOW DO PCMS WORK?
PCMs absorb and release thermal energy during transformation between solid-to-liquid and liquid-to-solid within a predetermined temperature range. Within this temperature range, the material is either absorbing heat or releasing heat. During heat absorption, PCMs are changing from a solid state to a liquid state. Heat is released from material as it changes from a liquid to a solid. During this release and absorption of thermal energy, the material remains at a constant temperature. PCMs in building products never melt beyond the consistency of peanut butter. The material does not “melt” to the point that it would ever leak into (or out of) the building.
SMART THERMAL MASS, WITHOUT SO MUCH MASS
PCMs work similarly to more traditional thermal mass materials like concrete and masonry but work smarter and without the associated weight and bulk. Concrete and masonry materials don’t care what the interior or exterior temperature might be and do the same thing, at the same rate, all the time. They are slow to absorb heat and slow to release it. Heavy thermal mass materials, to be effective in the interior of a building, must be exposed; covering them with gypsum board and other interior finishes makes them largely ineffective. When the thermal mass is exposed, it can work well under certain conditions, but only a small fraction of its total depth is necessary. When it comes to concrete and masonry used as thermal mass in buildings, more is not better. In a typical commercial office building, where heating and cooling takes place over throughout the day, a 4-inch thick concrete floor will provide the maximum amount of thermal energy storage possible. Anything thicker is a waste of material.
PCMs, on the other hand, are manufactured to be active within a stipulated temperature range, depending on performance requirements. It’s a “smart” thermal mass. And it takes only a small fraction of the space, and weighs only a fraction of the amount of a traditional, heavy thermal mass material like concrete or masonry. Unlike heavy thermal mass materials, PCMs do not need to be exposed to be effective. PCMs completely change the rules by allowing designers to add thermal mass to just about anywhere in the building. Need some thermal mass in the computer server room walls and ceiling? No problem. Under the metal roofing panels? It’s a cinch with PCMs. Try that with concrete or CMU!
PCMS IN BUILDINGS
In Europe, BASF and Dupont sell paraffin-based, rigid PCM-infused building panels for use at interior walls and ceilings. These products are not available in the U.S., mostly due to the fact that a paraffin-based interior product would be impossible to use in hourly-rated U.S. construction assemblies. The European panels are also quite expensive, at over $5 per square foot, which for most U.S. buildings would represent an unacceptably lengthy return on investment period.
In the U.S., two manufacturers produce PCMs for use in building construction. Mikrotek Laboratories Inc. makes microencapsulated PCMs that are “very small particles consisting of a core material-the PCM-and an outer shell or capsule wall.” These tiny little spheres (15-25 microns in size) can be combined with building materials such as plaster, coatings, and cellulose insulation. Phase Change Energy Solutions makes flexible plastic PCM panels called bioPCmat which have plastic blisters containing the PCM filled as required for the desired performance. The panels are designed to work with standard U.S. construction module at 16½ inches wide by 48 or 96 inches long. Custom sizes are also available upon request.
In a recent Oakridge National Laboratory report “Field Testing of Cellulose Fiber Insulation Enhanced with Phase Change Material,” PCMs were lab tested and field tested in residential building enclosures. The report concludes that, properly placed, PCMs can have significant impact on reducing building energy consumption. One of the most significant findings made in the report is that use of PCMs at interior surfaces such as gypsum board walls and ceilings is not very effective because of the relatively small temperature fluctuations of these interior surfaces. Instead, the material offers much bigger energy savings potential used in areas of a building that are subject to large temperature fluctuations.
PCMs used in attics, roofs, and exterior wall panels were shown to have a much greater energy savings impact over using them only at interior surfaces. To illustrate this point, in 2007 ORNL field tested the use of PCMs in a metal roof assembly and reported the results in a paper called Field Testing of Second-Generation Residential Attic Using Inorganic PCM Thermal Storage. The test involved placement of PCM mats below metal roofing panels, over a wood framed and sheathed roofing assembly. The test showed that PCMs at this plane of construction during August drastically reduced the temperature of the sheathing beneath, which translates into a lower temperature of the attic space below, and ultimately to a less energy consumed to cool the inhabited space using the HVAC system.
In a similar field test experiment conducted in 2009-2010, ORNL teamed with the Metal Construction Association, CertainTeed, Unisolar, and Phase Change Energy Solutions in building several roofing test panel configurations to show how combinations of metal roofing, insulation, PCMs, and solar photovoltaic panels could work together to increase energy efficiency. Perhaps one of the most significant finding in this experiment is that PCMs combined with PV panels help solve one of the more difficult problems associated with PV panels; while great at producing electricity, dark colored PV panels facing the sun get very hot. The hotter they get, the less electricity they produce. Incorporating PCMs within a PV panels array serves to reduce the heat gain of the assembly and also increase the electricity generated by the PVs.
Sustainable building product and systems like solar PVs, ground source heat pumps, and spectrally selective glazing, can be very expensive and have long pay back periods. As fantastic as they are, because of this they are not used in a very widespread way in today’s buildings. PCMs, however, are not budget busters. At $2.00 a square foot for material cost, payback periods are estimated at between 1.5 to 7 years. That’s right in the sweet spot in a standard developer’s pro forma. Phase Change Energy Solutions estimates between 16 to 30 percent energy savings realized for every building it which its products have been installed. In some cases, building owners have realized more than 50 percent in energy savings.
Independent tests done by PCM manufacturers show that the material does not degrade over time. BASF states on its Web site that a “16-month cyclic test involving 24 temperature cycles per day has attested to a minimum life of 30 years …” Phase Change Energy Solutions states, “We have tested our BioPCM phase change materials to 13,000 cycles which is equivalent to 48 years worth of cycles with no observable breakdown in performance.”
Paraffin-based PCMs installed within building assemblies do increase the flammability of building. Microtek Laboratories uses both paraffin and fatty-acid esters in the production of its PCMs. Although Microtek manufactures spherical PCMs fir use in building products, there are currently no mass produced products available that contain Microtek PCMs. According to ORNL, “Improved microcapsule-skin materials with a higher melting point and with added fire retarders are currently being tested in an attempt to improve flame resistance.”
Phase Change Energy Solutions uses a bio-based PCM made from soy and palm oil and other proprietary ingredients. According to its website, the material has been tested in accordance with ASTM E84 (meeting Class A and Class C requirements), UL 723, NFPA 255, and UBC 8-1.
Microencapsulated PCMs dispersed within building materials such as plaster, paint, and insulations should have little to no impact on the vapor profile of those materials, since the spheres are not tightly compacted and will allow water vapor to easily move around them. The plastic sheeting used to produce Phase Change Energy Solutions’ bioPCM is available in three perm ratings; 0.1, 5.0, and 10.0, selected depending on the Class of perm rating required.
It’s hard to ignore the potential of PCM use in buildings to drastically reduce energy consumption of new and existing buildings now that commercially available products are being mass produced at a reasonable cost. Simply adding the material into a building’s energy model as thermal mass shows where the material is best utilized for greatest energy savings. Phase Change Energy Solutions offers this service for free to any team interested in knowing how the material might be used to reduce energy consumption. Engineers immediately recognize the potential of this material when introduced to it, and are also very skillful at identifying the best uses. I think that use of PCMs in buildings thus far has really only scratched the surface, both in number of buildings and innovative ways to successfully incorporate into buildings.