Phase change drywall is an exciting type of building-integrated heat storage material. Currently, it is only produced for research. This type of gypsum drywall, or wallboard, incorporates phase change materials (PCMs) within its structure to moderate the thermal environment within the building.
Phase change materials absorb, store, and release heat when they change state, such as from a solid to a liquid. When the air temperature in a room rises above a PCM's transition temperature (the point at which the material changes phase), the PCM absorbs heat and melts. As the air temperature decreases, the PCM releases the stored heat and returns to a solid state.
PCM drywall is potentially an effective, less costly, and less bulky replacement of the standard thermal mass (for example, masonry or water) used to store solar heat. This could reduce some cost and design limitations of passive solar homes, making them more accessible to lower- and middle-income families. PCM wallboard also improves the feasibility of mass-produced solar heated homes.
Initial findings suggest that PCM drywall could also save energy in some conventional heating and cooling applications. Researchers believe phase change drywall can shift much of the summer air conditioning load to later time periods, allowing customers to take advantage of cool night air and off-peak utility rates. The household temperatures remain relatively stable until all the PCM melts. In the winter, warming the phase change materials from a conventional furnace could reduce furnace cycling and increase efficiency. Computer simulations show that PCM-treated wallboard can eliminate the need for air conditioners in mild climate zones, such as portions of California. This means that such residences will cost less to cool, and builders will not have to incur the cost of installing air conditioners.
Physical Properties of Dependable PCMs
There are several important considerations necessary for combining PCM into drywall. First, the transition temperature, or melting temperature, of the PCM must be near standard or suggested room temperatures (for example, 65-72 ° F [18.3-22.2 ° C] for heating-dominated climates or 72-79 ° F [22.2-26.1 ° C] for cooling-dominated climates). Because the PCM uses the exchange of heat energy from its environment to drive the phase change, this change of state from solid to liquid, or liquid to solid, characteristically occurs within a temperature range of only a few degrees. Second, the PCM product must be effective, offering good heat transfer. Otherwise, it is no better than common gypsum drywall. If manufacturers can mass produce a low-cost, competitive PCM product, PCM-drywall for instance, it will be the result of available, inexpensive PCMs and the cost-controlling manufacture of the product.
Salt Hydrates, Paraffins, and Fatty Acids
The three principal PCMs investigated for use in phase change drywall are salt hydrates, paraffins, and fatty acids. Other PCM applications commonly use salt hydrates. Since they absorb moisture, which decreases their effectiveness, salt hydrates require costly and impractical encapsulation, with a semi-impermeable coating for improved performance.
Paraffins are waxes. They are readily available, inexpensive, and melt at different temperatures relating to their carbon-chain length. Paraffin can be incorporated into drywall in two ways, by direct immersion and by adding permeated plastic pellets to the drywall mixture during the manufacturing process.
Since drywall is a porous material, it can absorb melted paraffin when immersed in it. Extra paraffin runs off, leaving no buildup of wax. Immersion times vary depending on the amount of PCM uptake desired, however, they rarely exceed ten minutes. PCM content ranges up to 30% of the composite weight of 1/2 inch drywall. Drywall dipped in paraffin becomes water resistant. While common gypsum drywall is fire-resistant, PCM-drywall is quite flammable unless treated with fire-retardant chemicals. Immersion is the simplest, lowest cost method for making PCM drywall. To produce large quantities of PCM-immersed drywall, manufacturers would need to make major modifications to equipment and processes. This process is not recommended for do-it-yourselfers.
Polyethylene pellets, saturated with melted paraffin, then mixed with wet gypsum and compressed in sheet form, also yield production quality drywall. Relative to immersed drywall, this material is more fire-resistant, less water-resistant, and conforms to the current gypsum drywall manufacturing process. Both versions work well for heat transfer and storage, and the paraffin remains permanently in the drywall.
The third type of PCM under study, fatty acids, come from meat by-products and vegetables. They are cheap, renewable, and readily available. Like paraffins, different types of fatty acids have different melting points. To tailor the drywall for specific climates, the manufacturer varies the PCM mixing ratio. Fatty acids are also incorporated into the drywall by immersion or encapsulation. They yield similar heat and stability characteristics as paraffin-based PCM wallboard.
Advantages and Disadvantages of PCM Drywall
PCM drywall has several advantages over conventional thermal mass in solar heating applications. Because the exposed surface is so large and the PCM absorbs heat over a narrow temperature range, the drywall need not receive direct sunlight. PCM drywall has a much greater heat storage capacity than conventional thermal mass, and provides excellent heat transfer. It demands no extra structural support and any added installation cost is minimal.
PCM drywall also has some disadvantages. The correct transition temperature for one climatic region will not be appropriate for another. Getting the right temperature becomes doubly difficult in regions that require both heating and cooling. Drywall manufacturers are reluctant to complicate their manufacturing processes to take these regional variations into account. On-site dipping of the drywall may suffer from poor quality control. Other considerations include deposits of surface volatile impurities ("blooming"), fire retardancy, metal corrosion, odor, and traditional application issues, such as the ability of paint to adhere.
There is great potential for phase change drywall. There are significant issues and techniques to address, however, before it is ready for wide market acceptance. PCM drywall is strictly a manufactured product; do-it-yourself applications of phase change materials are strongly discouraged.
Content Provided By the DOE