Earth Tubes

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Contents

What is It?


Earth tubes is a passive heating and cooling technique that uses the constant 50 degree temperature of the earth to pre-heat incoming air in the winter and pre-cool incoming air in the summer by passing the air through tubes buried under the earth before bringing it into the building envelope. This is actually an ancient technique used for thousands of years. It is best suited for climates that have extreme weather conditions...extreme heat and cold. This is when the moderate temperatures of the earth can create comfort value.

Also known as: earth-air tubes; ground-couple heat exchangers; earth air heating; earth air cooling.

Why is it Important?


  • Strengthens our connectedness to the earth and it's natural laws. We are working with the earth's natural laws to contribute to the human comfort zone instead of constantly fighting them with human and fossil energy.
  • Reduces the energy demands for cooling and heating buildings and ultimately the carbon footprint.
  • Creates financial savings through reduced energy costs.
  • Created for a low cost by communities around the world.
  • Creates better and healthier indoor air environments than conventional systems (see resources below.)

When to Use It?


  • Best in climates with extreme heat and cold. The high difference between the ambient temperatures and the required indoor temperatures create the best opportunity for the earth tubes to produce valuable results.
  • Need available land to accommodate the length of tubes.
  • Avoid areas with high water tables.
  • Great opportunity to place them under the building floor when constructing a new building.

Green Garage Use of Earth Tubes


Sustainability Goals

The sustainability goals for the Green Garage Earth Tubes are to:

  • Promote a stronger connection with the earth. "The earth warms our air in the winter and cools it in the summer...naturally."
  • Develop output temperatures: Winter = 40F; Summer = 70F.
  • Provide 67% of the energy (BTUs) required to heat and cool the required (i.e. 800cfm)exchanged air.
  • Develop dehumidification output Summer RH 60%.
  • Develop air volume at 800cfm. Maintain commercial building code requirement for air exchange for 40 people.
  • Provide air quality that is the same as ambient air.
  • Design it easy to maintain with a 100 year life.
Strategy and Conceptual Design

The strategy and conceptual design for the Earth Tubes for the Green Garage are:

  • Tube Length
    • 80 ft is our design length. Experience suggests that the majority of the heat transfer occurs in first 100ft of tube. The length of our backyard is 120ft in length. So we feel we can fit a minimum tube length of 80ft in this area, leaving room for the manifolds (intake and building supply opening) to fit against the building.
    • Supporting science:
      • The longer the tube the greater the surface area and, therefore, the greater the heat transfer.
      • Need calculations from Laurie
  • Tube Depth
    • Place the tube at 6 ft below the surface of the earth. We'd like to do 8 ft below, but we're concerned about ground water.
    • Slope the tube at a 2% grade
    • Allow a drain at the bottom with one-way material.
    • Supporting science:
      • Temp of earth in Detroit MI is approx 50F at 30ft.
      • At 8 ft below the surface the soil temp will vary approximately +/- 10F.
      • At 8 ft the earth low temp will be in late March and high temp in late Sept.
      • More details on earth temperatures
  • Tube Material
    • Corrugated, high density, single wall drainage tubing.
      • Positives: simple, cheap, available, no seams, can't break, flexible, won't corrode, creates air turbulence, higher surface area (with ridges), continuous drainage (cut at seam)
      • Negatives: high pressure loss, potential flooding from ground water
    • Other options considered:
      • Concrete Pipe
        • Positives: medium price, available, higher conductivity
        • Negatives: can fracture, seams could leak (could minimize), need to develop drainage approach, large-heavy pipe
    • Supporting science:
      • Surprisingly the material of tubes matters little, as long as they are not insulated. This is because the thickness of the material absorbing the heat is actually the tube plus the ground around the tube. When this is put in the thermal equations, the thickness of the tube is such a small part of the whole area of absorption that it's conductance doesn't amount to much.
    • Things to remember:
      • Avoid materials with ridges that may hold water without drainage approach. If it has ridges...it will need drains.
      • Select tubing that is rigid so there are not any dips to trap water. If it is flexible it will need drains.
      • Select a material that has a long life and has a low chance of fracturing or corroding.
      • Allow water to only pass out of the tubes, but not in.
  • Tube Size and Number
    • Use 3 - 12in diameter tubes (235 sq in...approx 1.63 sf results in air velocity = 8.2 ft/sec)
    • Supporting science:
      • This is a science balancing act. You're trying to make the ratio of surface area-to-air volume ratio as high as possible and still move the required amount of air at a reasonable speed given the number of tubes. Whew! And our requirement is 800 cfm and maximum air speed around 7 ft/sec. Here's our Earth Tube google spreadsheet that shows our analysis.
      • Another consideration is the cost and availability of the tube you select.
  • Tube Location
    • Ideal area is shaded, with moist, clay soil.
      • Place the tube in the backyard of the Green Garage because the it is in the shade and clay.
    • Supporting science:
      • Ideal area is shaded, with moist, clay soil. This provides the best heat transfer. Sandy soils have air pockets and therefore don't transfer heat well. Shade helps in the summer, as the ground stays cooler.
      • Locate 3 feet away from any buildings to reduce any negative effects either way.
  • Inlet Manifold
    • Provide an inlet with a screen/filter (.3 - .5 microns) to keep bugs and small animals out of the tube.
    • Provide a secondary inlet near the building to bypass the earth tube when needed or the ambient air is preferred (e.g. air temp is 65 degrees and the ground is 50 degrees in the spring.)
    • Develop a manifold on each end. Insulate the upper 4ft of the tube and the manifold.
  • Fans / Pressurize Tubes
    • Provide a fan at the inlet manifold to push the air through the tubes with a damper on the entry to the building. This will create a positive pressure in the tube that will force 1) humidity out of the air, 2) moisture out of the tube, and 3) potentially, radon out of the tube.
    • Supporting science:
      • Increased air pressures will lower the dew points and force more water out of the air (dehumidify.) Water will move from high pressure to low pressure.
  • Construction Recommendations
    • Tamp the clay soil firmly around the tube to eliminate all air pockets.
    • Supporting science:
      • The air acts as an insulator that slows the heat transfer from the air to the ground.
  • Drainage Design - to avoid mold / bacteria
    • The tube drainage is enabled by
      • Slope the tube / drainage
      • Cut a continuous opening at the bottom of each tube for it's entire length.
      • Place the tubes on one-way drainage fabric allowing water to flow out of the tubes.
      • Place the fabric on top of 4 inches of fine aggregate, then on 8 inches of course aggregate.
      • Use "open to air" catch basins that have their bottom in the gravel drainage level. (This needs to be further designed.)
      • If needed, drill down through the clay layer to provide a drain.
      • Avoid draining any roofs or ground areas into the area of the earth tubes.
    • Supporting science:
Proposed Materials / Suppliers
Supporting Science

From Laurie Catey's analysis notes supporting decisions made in the design process for Earth Tubes.

Development Story

The Earth Tube - Development Story page contains many images and video documenting the process used at the Green Garage to design, build and operate our earth tube system.

Related Internal Links

Resources


Valuable links to help make it easier for others to pursue earth tubes. These resources provided learning that influenced our design.


Music video

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