Passive Annual Heat Storage

When most people hear “free heat” they think of passive solar systems.  The sun comes in in the winter and heats the home.  If it is too sunny, it can get too hot.  If it is too cloudy, it can get too cold.

But one brilliant guy in Missoula came up with an entirely different approach:   store the heat from the summer to warm your house in the winter.  And he did a lot of experimenting and managed to optimize it to something pretty simple.Montanageodome

Consider for a moment the “earth berm” house or the “underground” house.  The temperature down deep is a pretty steady 54 degrees.   So these designs shoot for homes that get close to the 54 degree mass with the idea that “it’s easier to heat a house starting at 54 degrees, than to start at, say, 10 degrees.”  But what if we can take that idea just a little bit further and change the 54 degrees to 72 degrees? Then the house stays 72 degrees year round and we don’t have to heat or cool at all.

In passive heating/cooling systems, energy storage is based on a difference in temperature between the storage medium and the energy source. John Hait (Rocky Mountain Research Center) uses the earth to store solar heat and has termed this technique Passive Annual Heat Storage (PAHS).   PAHS uses dirt to surround a dwelling as part of the strucuture’s thermal mass, insulating it from the elements, but not from the walls.  (I have come to think of the phrase “Annualized Thermal Inertia” as a more accurate description, however, since passive annual heat storage doesn’t hint to the cooling effect that occurs in the summer).geodome_current_gazebo

According to Hait, the tremendous mass of the building and surrounding soil in the PAHS design (a volume  of about 45,000 cubic feet (1,800 tons) for the 20 feet beside and below a  30-foot-diameter home) allows the interior temperature to vary only a few degrees  throughout the year.   He claims it will typically float between about 76 degrees Fahrenheit in the  summer and about 70 degrees in the winter without any additional form of heating or cooling required.

By the way, the images in this post are of two homes in Missoula that use this technique.  I would love to visit these homes ….  should anybody know how to get in touch with the owners!

On one of the permies.com posts, a member gave a pretty good description of John Hait’s technique, based on his book:  “How it works: Earth is actually a very bad accumulator. Add to that, that it heats and cools very slow. With PAHS John Hait supposedly created a system that makes use of these “bad” qualities. Any temperature difference between mass and air will start a transfer. This transfer will travel inward in the mass like the waves on a pond, when you drop a stone. A short impulse will fade out, a hot summers day will create a heatwave travelling several meters into the mass. When winter comes, the waves will switch and the heat is drawn back. But because the warmth moves slowly in the mass, you cannot “empty” the store quickly. It takes time. Which means, that – when your thermal mass is constructed correctly – it will give you warmth back all winter long. The tubes ensures that no heat is lost with air supply. Insulation ensures that no heat is lost through the walls. Of course there will be a loss. Your thermal mass should compensate only that. And according to John Hait’s house in Rocky Mountains, it works.”

Another technique, known as annualized geo-solar, is similar to passive annual heat storage.  Don Stephens, living in the often cloudy Northwest, realized the need to store the more plentiful summer heat (as did Hait) but his design stores the heat directly in the earth beneath the dwelling, where the heat rises in the winter through floor surfaces.

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