April 1, 2013
This little rock can be found in everything from lightweight aggregate to futuristic insulation.
By Bill Langer
We love living in the Sonoran Desert, but the temperature gets high in the summer, as does our electric bill. To help reduce the cost, my son-in-law, Rob, helped me blow insulation into our attic, doubling the original thickness. My wife, Pam, and I also replaced our halogen lights with LEDs, cutting down both electric usage and the heat generated by the lights.
Apparently, we are not the only people doing this. Our electric company recently added a Lost Fixed Cost Recovery (LFCR) charge to our electric bill because customers have taken so many energy-saving measures that usage has significantly dropped. The electric company can no longer meet its fixed operating costs, so we get to help them out.
Well, a rock called perlite is just waiting to exacerbate the problems of the electric company. Perlite consists of glassy volcanic material which, upon rapid controlled heating, “pops” into frothy, low-density particles. Nearly half a million tons of expanded perlite are produced in the United States every year for use as lightweight aggregate, insulation, soil amendments, and numerous other applications. With a density between 8 and 40 cubic yards per ton, that’s a lot of perlite.
So here is how perlite can help reduce electrical usage. The quantity of heat it takes to increase or decrease the temperature of the thermal mass of a building envelope (the physical separator between the interior and the exterior environments of a building) can vary greatly. For example, an adobe or concrete structure has much more thermal mass than a framed wood wall of equal thickness. Thermal mass reduces temperature variations, resulting in decreased energy consumption in some climates.
Researchers are investigating ways to increase thermal mass by incorporating phase-change materials (PCMs) into the building envelopes. The most recognizable PCM is paraffin wax. Others are chemicals or organic materials, many with unrecognizable names. Most PCMs for building applications change from a solid to a liquid (melt) near room temperature — somewhere between 68 and 86 degrees Fahrenheit.
It takes a large amount of energy (called the latent heat of fusion) to weaken the molecular bonds of a PCM in order to make the jump from an ordered state (solid) to a disordered state (liquid). So, when the temperature surrounding the PCM rises to the melting point, the PCM needs large amounts of energy (heat from the atmosphere) to melt. As it melts, it consumes heat while maintaining a near-constant temperature. This keeps the building cool. When the local temperature falls back to the solidification point, the PCM will discharge heat, maintaining the temperature of the space until the PCM has fully solidified. In short, the PCMs incorporated into the building envelope absorb the higher exterior temperature during the day and dissipate that heat to the interior at night when it is cooler.
The U.S. Department of Energy tested an attic insulation consisting of perlite (a carrier) embedded with a PCM consisting of hydrated calcium chloride. That PCM changes phases from solid to liquid at 82°F. As it melts, it absorbs heat from the hot attic before the heat can penetrate into the home. When attic temperatures cool at night, the PCM solidifies and releases heat back into the attic, moderating the cooler outdoor temperatures.
Some day in the future, perlite infused with PCMs may reduce home heating or cooling loads, thereby producing energy savings for the consumer, a reduction in the need for new utility power plants, and, perhaps, yet an additional LFCR charge.
Bill Langer is a consulting research geologist who spent 41 years with the U.S. Geological Survey before starting his own business. He can be reached at