Earth-air tube design
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This is the design ideas for the Green garage Earth-Air Tube
- Design Goals
- Output temperatures: Winter = 40F; Summer = 70F
- Dehumidification: Output Summer RH 60%
- Air Volume: 800cfm
- Air Quality the same as ambient air
- Easy to maintain
- Tube Length
- It seems that the temp transfer occurs in about 100ft of tube.
- Tube Depth
- Place the tube at least 8 feet below the surface of the earth.
- Temp of earth in Detroit MI is approx 50F at 30ft.
- At 8' below the surface the soil temp will vary approximate +/- 10F.
- At 8' the earth low temp will be in late March and high temp in late Sept.
- Slope the tube at a 2% grade
- Allow a drain at the bottom...with one-way material.
- Place the tube at least 8 feet below the surface of the earth.
- Tube Material
- General Guidelines
- Some studies have shown that any material is just as good at transferring the temperature. This defies the physics, as thermal conductance varies greatly by material type and directly effects heat transfer rates. See some examples here. The Engineering Toolbox has this data:
- Polypropylene PP : U = 24.5 W/m2K
- Steel : U = 25.0 W/m2K
- Aluminum : U = 25.0 W/m2K
- Avoid materials with ridges that may hold water without drainage approach.
- Select tubing that is rigid so there are not any dips to trap water.
- Select a material that has a long life and has a low chance of fracturing or corroding.
- Avoid water leaks.
- Allow water to pass out of the tubes but not in.
- Some studies have shown that any material is just as good at transferring the temperature. This defies the physics, as thermal conductance varies greatly by material type and directly effects heat transfer rates. See some examples here. The Engineering Toolbox has this data:
- Green Garage
- Two Options
- Black Flexible Ridged tubing
- Positives: simple, cheap, available, no seams, can't break, flexible, won't corrode, creates air turbulence, high surface area, continuous drainage (cut at seam)
- Negatives: low conductivity, high pressure loss
- Mitigating factors: could do a hybrid with smooth insulated pipe rising to manifolds.
- Product Suppliers: Timewell Pipe, ADS Products...there are many more.
- Concrete Pipe
- Positives: medium price, available, higher conductivity
- Negatives: Can fracture, seams could leak (could minimize), need to develop drainage approach, large-heavy pipe
- Black Flexible Ridged tubing
- Two Options
- General Guidelines
- Tube Size
- Use 3 - 10in diameter tubes (235 sq in...approx 1.63 sf results in air velocity = 8.2 ft/sec)
- Tube Location
- Ideal area is shaded, moist and clay soil.
- Place the tube in the backyard of the Green Garage because the
- Locate 3 feet away from any buildings.
- Ideal area is shaded, moist and clay soil.
- Inlets
- 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.
- Construction Recommendations
- Tamp the clay soil firmly around the tube to eliminate all air pockets.
- Drainage Design - to avoid mold / bacteria
- "open to air" catch basins that have their bottom in the gravel drainage level.
- Additionally drill down through the clay layer to provide a drain.
- Slope the tube / drainage
- Continuous drain
- Open Issues
- Water table
- entry in a Civil Engineering Forum
I have seen one and built one at the residential scale of construction. Factors to consider include the soil moisture content as mentioned by others because this significantly affects the thermal conductivity of the soil, heat load, and other similar engineering concerns. The temperature difference between the soil and the surface is a function of the depth, both in the magnitude and the time lag compared to the surface temperature. I suggest you look at some reference materials on earth sheltered construction for more detail on this, but at 30' (9m) depth, the time lag is typically 6-months and the delta-T over the year is close to 1-deg C.
Practical concerns are: Introduction of toxins, unwanted odors, etc. from the soil (so probably a lined earrth tube is better). Control of water condensation during humid warmer weather along with the probability of mold growth if water is standing (continuous slope to appropriate type of drain). Ability to clean if needed. Protection from entry of unwanted animals, insects, etc. And, preservation of the air entrance location from damage or removal by people in the future who may not know its purpose.
My empirical one was with 10" dia (254 cm) PVC pipe with rubber gasket joints so it would remain flexible in the soil (SDR-35 in the USA), encased in compacted clay about 5' (1.6m) below the surface, 80' (26m) long. It started outside with an above-ground vent in the corner of the building, insulated from it, ran out into the yard where it turned and came back under the basement floor, and ended inside by connecting to the return air plenum of the central heating and cooling unit. Thus, whenever the air was being circulated in the building, there was a mild positive pressure equal to the pressure loss due to friction in the return air system. All air leakage points in the building were therefore exfiltration only. All exhaust fans had a generous source of earth tempered fresh air into the building. The low point in the earth tube had a small drain pipe attached, which went to a nearby sump pump pit.
Thre are many methods of passively heating or cooling buildings, but the local climate you have will be the biggest concern in deciding which one(s) to use. Best wishes in your work. Contact me, if you want at jmueller245@yahoo.com.
John Mueller, Kansas City, USA
