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Thoughts on earth tubes for Passive Annualized Solar Heating?  RSS feed

 
Peter Ellis
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I'm putting some thought into our passive solar system and pondering whether it's better to have the airtubes in the ground be solid, so nothing gets into them from the surrounding earth, or perforated so condensation can get out.
Does anyone have experience on this subject? My inclination is to keep it sealed and deal with condensation by other means (fan for active air flow to pull it out, a mop on a wire cable) to avoid "stuff" getting into the tubes.

Looking at using 4"dia. "drain tile pipe", either perforated or solid.

Thoughts?
 
Roberto pokachinni
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I don't have the book (yet... might be a Christmas/birthday gift for myself), but have looked online about Earth tubes a bit.  It seems that most of the people I remember seeing are using solid tubes and they use a rope with a sponge/mop attached to pull through and keep moisture/mold/mildew whatnot out of the system.  The rope would have to be long enough to still be at the starting end when the sponge is at the finish end... thus the sponge be in the middle of the length of rope, and the rope be twice as long as necessary to clean the pipe.  My thought on why I might choose to do it this way (with a solid pipe), as opposed to the perforated drain pipe, is because you want to maximize the dryness of your earth/heat storage battery bank.  The continuous, but very gradual drain of liquids might be enough to take away from your thermal battery potential of the system.  That's my only thought on it so far.     
 
Kyrt Ryder
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Roberto pokachinni wrote:you want to maximize the dryness of your earth/heat storage battery bank.  The continuous, but very gradual drain of liquids might be enough to take away from your thermal battery potential of the system.  That's my only thought on it so far.     

On the flip side... isn't damp earth a FAR better heat storage than dry [which is more insulative than absorptive]?

Is there a reason such a system couldn't trap the moisture in the soil by some means [such as perhaps a water-impermiable layer on the bottom] and if so, what is it?
 
Roberto pokachinni
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isn't damp earth a FAR better heat storage than dry [which is more insulative than absorptive]? 
From what I understand, damp earth will conduct heat better and hold heat to a greater degree, but the conductivity will also work against you.  The system as I understand it with the PAHS umbrella, is to have the area dry out as much as possible to reduce the loss of heat.  The umbrella is insulated from above, and is allowed to dry as much as possible.  This maximizes both the insulating qualities of earth, while also getting a great deal of thermal mass out of the earth battery.  I might be wrong.  That's just how I understand it at this time.  Like I wrote earlier: I don't have the book. 
Is there a reason such a system couldn't trap the moisture in the soil by some means [such as perhaps a water-impermiable layer on the bottom] and if so, what is it?
  I'm not sure I understand what you are asking with this second part.  I personally wouldn't want to trap moisture in the system for the reasons stated above, unless it was in contained systems, like closed plastic buckets... which is what I have in mind potentially for other therma mass uses, and may use in the Earth tube house design as well (if I go that route).
 
Peter Ellis
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Kyrt Ryder wrote:
Roberto pokachinni wrote:you want to maximize the dryness of your earth/heat storage battery bank.  The continuous, but very gradual drain of liquids might be enough to take away from your thermal battery potential of the system.  That's my only thought on it so far.     

On the flip side... isn't damp earth a FAR better heat storage than dry [which is more insulative than absorptive]?

Is there a reason such a system couldn't trap the moisture in the soil by some means [such as perhaps a water-impermeable layer on the bottom] and if so, what is it?


The problem as Hait expressed it is that the water carries heat with it as it flows through the ground.  I expect that trying to create a stagnant body of underground water is both more complicated and hence expensive than just doing an umbrella to create a dry (dry-er?) volume of earth to hold heat and that it carries with it a number of potential unintended consequences.

The idea here is not so much to optimize the heat storage capacity of the system, but to very inexpensively and simply use what is very nearly a free storage material - the earth beneath our feet.

Our plans at the moment involve the bulk of the storage area being sheltered under the house itself, but extending beyond both ends to some distance and with intakes for outside air roughly twenty feet out from either end of the house and at a depth of four feet. Areas not sheltered under the house proper will be covered by used billboard tarps. I'm intending to use a solar chimney in the house for draft to pull air through the tubes in the ground, but at this point I've got no clue how fast air should move, or what volume of air needs to be moving through the system, and I don't know how much energy will transfer during the course of travel through the tubes, nor do I know how much energy we will need for our 1,200 to 1,500 square foot house.

So, there's some more work to be done, in terms of how big the system needs to be and things like that   But, I'm pretty sure dry earth is one piece of keeping the energy in the system where we want it.
 
Roberto pokachinni
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So, there's some more work to be done, in terms of how big the system needs to be and things like that
Yes.  I've always wondered how much volume/length of pipe would be optimum for a given volume of house space.  I'm assuming that this is calculated (at least roughly) somewhere in Hait's book?
The idea here is not so much to optimize the heat storage capacity of the system, but to very inexpensively and simply use what is very nearly a free storage material - the earth beneath our feet. 
  From my grasp of it, the idea is to charge that mass/area with heat from the summer, more than what would be there naturally, and thus get that to come back into the house in the winter.  The system uses the Earth for it's natural geothermal potential, but also boosts that potential (through passive solar heat gain and the tubes transferring that heat) and uses the Earth's natural slow geothermal release pattern (with the help of the heat tubes and place with low heat loss) to release summer heat in the winter into the living space.  I probably should get the book to confirm that... but that's how I understand it from all that I've read or watched online.
 
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