Earthtube-fed air conditioning?

menaus2 · 09-24-19, 04:41 PM

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!

menaus2 · 10-09-19, 11:24 AM

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?

MN Renovator · 10-09-19, 06:42 PM

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.

Ron342 · 10-13-19, 01:20 PM

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 i’ll also see if i can find my bookmark

menaus2 · 10-23-19, 12:13 PM

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!

09-24-19, 04:41 PM	#1
menaus2 Apprentice EcoRenovator Join Date: Feb 2011 Location: Driftless Region, North America, Earth, Solar System, Orion Arm, Milky Way Galaxy,Virgo Supercluster Posts: 116 Thanks: 13 Thanked 35 Times in 25 Posts	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!

10-09-19, 11:24 AM	#2
menaus2 Apprentice EcoRenovator Join Date: Feb 2011 Location: Driftless Region, North America, Earth, Solar System, Orion Arm, Milky Way Galaxy,Virgo Supercluster Posts: 116 Thanks: 13 Thanked 35 Times in 25 Posts	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?

10-09-19, 06:42 PM	#3
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 940 Thanks: 41 Thanked 117 Times in 91 Posts	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.

10-13-19, 01:20 PM	#4
Ron342 Apprentice EcoRenovator Join Date: Feb 2010 Location: Maryland Posts: 120 Thanks: 26 Thanked 12 Times in 8 Posts	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 i’ll also see if i can find my bookmark

10-23-19, 12:13 PM	#5
menaus2 Apprentice EcoRenovator Join Date: Feb 2011 Location: Driftless Region, North America, Earth, Solar System, Orion Arm, Milky Way Galaxy,Virgo Supercluster Posts: 116 Thanks: 13 Thanked 35 Times in 25 Posts	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!