The Quest:
The Perfect Heat Storage Medium for Solar Homes
Early solar homes experienced desert-like climates: hot days and cool nights. To reduce the huge temperature swings, designers began to include large amounts of thermal mass: cement blocks, water walls, rock bins, etc. These features tended to moderate the swings, and provided heat during the following night. But during cloudy periods, these houses had to revert to backup heating systems. Solar builders dreamed of ways to "save" heat over a period of days.
Solar advocates thought that they had found the answer with Glauber's Salt, a phase-change material. This salt had a very useful property: like ice, it absorbed a lot of heat when it melted, and gave off heat when it became a solid. However, this material had two unfriendly properties: it was expensive, and it tended to become ineffective in humid conditions.
Researchers, under grant from the
Department of Energy, have identified other phase change materials that are inexpensive (less than a dollar per pound) and are less affected by humidity. These materials are types of paraffins. They have identified paraffins that, as the number of carbon atoms increase, their melting points also increase, with compounds that melt around 60, 70 and 80 degrees Fahrenheit.The interesting characteristic about the paraffins with a range of melting points is that homes can be outfitted with a mix of these materials for use in storing warmth in winter and "coolth" in summer. To store "coolth" using phase change materials, strategically located windows should be opened on cool nights. In addition, electric homes can be heated or cooled during non-peak hours to minimize costs and the homes can be kept within a comfortable range using phase change materials.
How much heat can these paraffins hold? To put this into perspective, let us consider its properties relative to cement, commonly used in solar homes as a heat storage medium. A cement block has a thermal conductivity of about 29 BTUs per cubic foot, while the paraffin is about 5 BTUs per cubic foot. But this refers to the capacity to hold heat, not the heat of the phase change. In addition, the paraffin has a latent heat capacity of about 2500 BTUs per cubic foot. This latent heat capacity is the energy that is absorbed by the molecules changing to liquid or solid.
These phase-change materials can be absorbed into
wallboard (with a fire retardant) without leaking, as long as they comprise less than 25% of the wallboard's weight. These materials can also be used in thermal storage components (concrete blocks, empty walls filled with phase change materials, cement floors, floor tiles). Ivan Salyer, researcher at the University of Dayton drywall project, recommends placing these materials directly into cinder blocks for maximum control of temperature fluctuations. These paraffin materials are also being used to develop an electric water heater that can be heated during off-peak hours, and are being considered for highway bridges (to minimize icing, in winter clothes, and in firefighters uniforms and athletic headbands (to keep firefighters and joggers cool).
Other Links:
Phase Change Material Home Page
Use of Phase Change Material To Control Indoor Thermal Environment
Phase-Change Materials for Specialized Clothing
I still have several 55 gallon drums of the K18 (which melts at 77 degrees Fahrenheit) and K19 (which melts at 83 degrees Fahrenheit). If you are interested in purchasing ($300 each), contact me at