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Old 09-24-19, 04:41 PM   #1
menaus2
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Default Earthtube-fed air conditioning?

I have an idea that I haven't been able to find really discussed elsewhere online, so I thought I'd throw it out here.

Basically, instead of using earth tubes to directly condition a space, you would use it to feed cooled air to a 2-hose portable air conditioner.

Potential advantages:
1. Increased efficiency for the air conditioner
2. Eliminates mold concerns associated with earth tubes since heat exchange air and
3. Portable A/Cs are relatively inexpensive ~$400, can easily be upgraded/swapped out.
4. Earth tube design and materials could be simplified since mold isn't so much of a concern.
5. Simplicity/fail-safe nature of air as a fluid. No leaks, special fluids to worry about.

Potential Disadvantages
1. Alternative GSHP, or A/C's could be inherently more efficient with less work.
2. Static pressure in earth tubes, need for booster fans
3. Cost/work involved in excavation for tubes.

Questions:
1. How much of an efficiency gain would the air conditioner see from pre-cooled air?
2. What sort of depth of pipe would be necessary? (a/c only) and in what kind of climates?
3. Thermodynamic effect of condensation in the earth tube?

Cheers!

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Old 10-09-19, 11:24 AM   #2
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Hard to dig up solid information on the impact of outside air temperature and air conditioner efficiency. The closest study I've been able to find is here:

https://aceee.org/files/proceedings/...193-000015.pdf

Figure 3 & 4 show massive power consumption differences (0.2KW at -5 to 0 deg. F difference to inside building vs 2.5-3KW at 30-35 deg. F difference) Basically, 15 times less power used.

The question is teasing out how much of that is simply the difference in building cooling load.
If the air tubes can cool 90 deg. F outside air to 65 deg. F (pretty average summer soil temp for only a few feet underground in Wisconsin) and the air conditioner reduces that to say 50 deg. F then could we say that the earth tubes are doing roughly 63% of the cooling? My totally uneducated, ignorant calculations would suggest the A/C would have to work about 3x less.

So would a 11 SEER $400 portable A/C combined with earth tubes perform similar to a 33 SEER $1,600 minisplit?
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Old 10-09-19, 06:42 PM   #3
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The entering air of an AC condenser being 65 degrees instead of 90 degrees doesn't reduce the power consumption by that much. My compressor uses about 2200 watts with a 90 degree entering temperature and 1900 or so with a 65 degree entering temperature and the output capacity at the evaporator is more but the total likely doesn't end up being more than about 15%.

The best source of information for entering condenser temperature, power usage, and output with a TXV driven unit can be found from HVAC manufacturers information. Goodman has a solid product specification sheet for their units.
https://www.goodmanmfg.com/pdfviewer....pdf?view=true

One more important factor - An AC condenser requires unimpeded airflow, they can't handle much static pressure without losing flow, if you are adding a blower to push air through an earth tube, you MUST add that power back in, which could cause a net loss in efficiency.
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Old 10-13-19, 01:20 PM   #4
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If i remember correctly some did on this blog built one of these a couple yrs ago for his air coupled outside unit. Maybe do a search and iíll also see if i can find my bookmark
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Old 10-23-19, 12:13 PM   #5
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If the 15% reduction turns out to be accurate, it would seem to be the final nail in the coffin for the idea. It looks like a high SEER mini-split would be the far more economical choice, and a D/C solar one could make it completely off-grid.

I like the fail-safe simplicity of earth tubes, but extracting the cooling while isolating a potentially contaminated air stream in an efficient way might not be doable. It would be interesting to see some real world results from that Blog Ron342!

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