Saturday, November 21, 2009

A new material may revive an old technology

In the last week or so I learned about a fascinating modern version of a millennia old technology.
I'm talking about thermal mass...this is a very popular technology among the passive solar crowd--stretching back to the dawn of civilization--and it involves building your home (or castle) with "thick" walls that absorb heat during the heat of the day (cooling the home) and release that heat during the cool night (heating the home).

Of course most modern buildings (with the exception of some "fringe" passive solar designs) do not call for walls that are multiple feet thick. Enter the "new material" commonly known as phase change material (PCM), which is formulated to accomplish the same thermal effect as a 12 inch wall in about a 1 inch package.

The PCM is engineered to absorb heat by melting at a predetermined temperature. The melting material absorbs a massive amount of energy (simple thermodynamics) if the ambient/outside temperature rises above the melting point. And when temperatures fall below the same predetermined temperature the PCM reverts to a solid and releases the stored energy. Basically the PCM acts as a "natural" air conditioner to provide cooling above a set temperature and heating below it. Once the PCM completes its phase change in either the up or down temperature direction it offers no "additional" thermal benefit until the surrounding temperature again crosses the set temperature. (I just point this out to be as clear as possible about what the material does.)

Basically the PCMs behave like ice that can "freeze" at room temperature (or whatever temperature you design them to work at). We all know that if you put an ice cube in a glass of room temperature water, the ice cube melts and the water in the glass gets chilled below room temperature. This is because the melting ice--the phase change from solid to liquid--absorbs heat from the surrounding water. Conversely you need to put liquid water into a freezer for quite some time to convert the liquid back to a solid--this is slightly harder to conceptualize but the liquid water gives off heat (to the surrounding freezer) as it solidifies.

There is a company marketing products that contain PCMs to the (home) building industry called BioPCM. There are other companies that produce PCMs for this and different applications, but this one offers an interesting "green" spin by using "bio-based materials" rather than paraffin derived from petroleum. Also BioPCM has specifically packaged its product for use in buildings (and it really looks easy to install--check their products page) which seems sensible, I have no idea about its cost, but the company claims up to 30% energy savings--FYI.

A thermal mass product works in conjunction with standard insulation (not instead of!), where the insulation slows the rate of heat transfer either up or down while the thermal mass stores (and releases) the heat that gets through the insulation to make a certain desired temperature "sticky".

Assuming you pick the right temperature for your climate, using a PCM can reduce both your heating and cooling bills, as well as increase comfort by providing temperature stability with its natural air conditioning properties. If you are lucky enough to experience zero climate variation, a PCM will do you almost no good, but if you experience frequent (i.e. daily) climate variation you will get the most benefit from a PCM.


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At 8:53 AM, Anonymous SA Sustainable Living said...

Biologist John J. Morony has documented that adobe and compressed earth blocks are also phase change materials. The moisture in them vaporizes and condenses. This results in cooling in the hottest parts of the day in the summer and warmth in the winter. His paper can be viewed in html here:

At 12:39 PM, Blogger Daniel said...

Thanks for the comment SA Sustainable Living.

That is an interesting property of those materials, but unless you are building your adobe house in the arctic, or somewhere the temperature regularly exceeds 212F (the condensation/vapor points of water), neither of which is very comfortable for humans, I'm afraid I don't know how usefull that is to the homeowner in terms of temperature control. Of course those materials do provide thermal mass the old fashioned any massive material (but you need rather thick walls).

The primary benefit of the phase change materials mentioned is that they "freeze" at around room temperature (and hence makes these temperatures "sticky") which is comfortable.


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