How To Make Use Of Passive Geo Thermal Energy For Cooling

From article "Earth Tube Materials"

  In the above picture we see a massive earth tube
construction being built for use by a large building.

  Sometime in the 1970's some engineers decided that they could make use of the geo thermal energy of the earth to precondition air intake from the outside to raise its temperature in the winter time up to 50 some degrees before it is applied to a building.  In turn in the summer time the 50 degree F range of earth temperatures could be used to pre-cool exterior air before it is applied to the interior of a building.  In this way some thermal energy could be added to the building energy calculations to help reduce the use of fossil fuel sources of energy used to power the building.  The method utilizes structures called "earth tubes."  Basically earth tubes act as air to earth heat ex-changers utilizing the massive amount of cold geo thermal energy in the earth surrounding the tubes.  Some things of concern with regards to designing such a system have to be addressed such as water leakage into the system and mold.  What I want to discuss here is a system I have designed using 3D modeling that gets around these concerns by using the interior walls of a basement as the geo thermal mass we will exchange heat to, in order to deal with some heat gain issues lets say in our kitchen.  We should be able to easily deal with thermal energy of about  7000 to 10000 Btu using this method, however lets keep in mind that the longer we run this system the warmer the basement walls will become, and then the system degrades to some degree.  Yet the system should perform useful work for us throughout the day, and renew or lets say recharge it self over night as the home heat gain lowers over night.  As for the amount of degradation that our system will suffer in the course of a day, lets keep in mind that the thermal mass of the basement wall is immense in terms of pounds per cubic foot (~ 145 to 150 lbs) of which the earth behind the basement wall is coupled to this mass, effectively making the thermal mass much large than the basement wall.

As we move on with this discussion please take note of the following pictures of earth tube construction so that you can grasp the concept better.

Here we see the idea behind the typical layout of an earth tube.
The tubes can be made of PVC pipe, clay or metal. 

As mentioned in the previous picture an earth tube can be made of metal.
Here we see corrugate metal tile which  can be laid out merely in a long straight
line as a single pipe, where the corrugated surface causes the air to have
turbulence along it's route and hence is able to exchange more heat to the earth
or acquire more heat than the concepts seen in the previous pictures.

  The earth tube concept is certainly unique in terms of a low tech means to make use of renewable geo thermal energy, yet the concept has some concerns in terms of the system having water infiltration as well as mold growing within the system, and some folk have concerns for such matters as radon gas finding its way into the system, there is a better method of doing the same while reducing the concerns for the matters that some find problematic.  Below are photos of my 3D model you can click a link to and load up and navigate to look at the method of construction used in the concept I have designed for making use of geo thermal energy in an innovative way to deal with some home heat gain issues.  I am sure you will like the idea and some of you might make use of it sometime.

  You can view a 3D model of the above and following picture here at this link:
  This is a model of how a basement can be used as an "earth tube" to cool air that is sent around in a 6" wide cavity created behind an insulated basement wall. As can be seen the concrete wall is cut away near the front corner of the model to reveal a cavity behind the wall.  Behind the interior wall is a heat recovery ventilator with two of its ducts entertaining the wall, and on the cavity side of the wall the two ducts are partitioned with a air stop to divide and separate them from being able to draw air back through the adjacent duct, instead the air must travel the full length of the cavity around the basement's concrete walls before returning to the opposite duct.  The air is made to circulate around the parameter of the basement wall through the cavity that exist between the interior insulated wall and the concrete basement wall, which is cold geo thermal mass. Hence the air is circulate by means of a (HRV) heat recovery ventilator. The ventilator is connected differently than it would be when used to exchange interior air for fresh air, where the air from lets say the kitchen is drawn into the ventilator only to exchange its heat to the basement but not return basement air back to the kitchen. Instead the kitchen air is drawn in and returned. While the basement air is drawn in and returned back into the basement. This allows the heat to be exchanged but keeps the moisture of the kitchen air out of the basement while keeping basement air with any potential radon gas out of the kitchen. Ventilation of this cavity to the outside can be performed from time to time to remove any radon gas that may build up within the cavity.

  Considering the surface area of the basement walls, we have quite a large surface to exchange heat to for some such as a room the size of a typical kitchen, where we have heat being exhaust from the refrigerator and sometimes a freezer as well as from the stove when it is in use.  Of course using the stove will cause the air conditioner to kick on but not as soon and it will not runs as long.  Foremost we should be able to remove most all of the heat gain from the refrigerator and freezer using this method.  The concept of using this system is that it is a low energy system that can use less than 50 watts (in terms of low energy heat recovery ventilator fans) to remove a much larger amount of thermal energy.  A single inexpensive PV solar panel can be used to provide power for the heat recovery ventilator regardless of whether the home owner can afford a PV solar panel system to power their whole house, powering this low energy system on the other hand by renewable energy is quiet inexpensive and hence affordable to invest in, and whats more this system can easily be retrofit into existing homes with basements.  This concept then falls within the category of affordable renewable energy features that can be added to new home designs as well as to existing homes.

  The best way to construct the basement's interior insulated wall is to have at least a wall cavity filled with R 19 which is backed with rigid foam insulation on the cavity side.  The wall frame and its foam insulation can be constructed laying on the basement floor, with a sheet of heavy plastic as a moisture barrier over the foam insulation, and then the wall is raised into place.  Stops in terms of small wood blocks extruding from the frame by 6 inches, should be added along the frame so that it will not be able to reach any closer to the wall than 6 inches. Once all the wall is raised the R 19 can be installed and drywall or paneling can finish of the interior.  Since some folk might want to insulate their basement walls, this then would be an affordable concept to add to such walls.

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Tags: earth_tubes, geo_thermal_energy, innovative_basement_idea


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