Under Slab Geothermal Cooling System

[bigAlittlee]

I have little experience with geothermal heat pumps, but it is being pushed as an idea for the conceptual design of an upcoming project. I have been recruited to explore the feasibility of such a system and to start I would mostly like to find an example of such a system already in place.

The system being considered is a vertical closed loop system for cooling only. The field would need to be directly underneath the slab of the building because of space constraints. There may be room to extend the field beyond the actual footprint of the building, but I don't know how far.

As far as calculations are concerned I've been trying to figure out where to start all day and still don't feel very confident about the numbers I am getting. It doesn't need to be perfect and all I really would like to see would be a rule of thumb table relating temperatures (entering water, leaving water, ground, etc.), soil conductivity, pipe size/flow rate, well depth, etc. and come up with a ballpark number of how much cooling capacity I could expect.

Things I know:

1) Ground temp is around 55F at 10m depth so I would guess it gets down to 50F or so when it is deeper.
2) Site is within a couple hundred meters of water on 2 sides of so the soil should be moist/conductive (but is too much moisture a problem? What about water tables would that be a problem?)
3) Site area is about 30,000 sqm so that is the absolute max ground area to work with, but in reality it is probably less.

I would think just based on that I should be able to get a ballpark expectation of useable capacity.

Any comments/help or information on existing similar projects would be greatly appreciated. Thank you.

 

[CH5OH]

the idea of a heat pump:
evaporate a liquid at a certain temperature,
condense it at a higher temperature.
to establish a condencing temp higher than the evaporating temp, a compressor and an expansion valve is required.
The smaller the difference in temperatures, the less compressor power is needed (operating costs)
in heating mode, the evaporator is "connected" to the heat sink (geothermal heat)
in cooling mode, the condensor is "connected" to the heat sink.
As your intension is to use mostly cooling mode, and preference seems to be reducing running cost over installation cost (heat pump preferred over roof mounted AC)
the heat sink of choice should be the water near by.
Your temperature estimate in function of depth suggests the earth centre is an ice cube.

 

[bigAlittlee]

Yeah? I guess that means if the ground you're standing on is flat then you should assume that the earth is flat too.

You should note the difference between "guess" and "estimate." I understand that temperature decreases with depth, then stabilizes, and then increases. Upon further research it seems that it stabilizes at about 10m. Congratulations, you're right.

I am an HVAC engineer that doesn't know what a heat pump is? Come on man.

I was asking about a specific configuration of a heat pump that I am not familiar with...

 

[EnergyProfessional]

you should get the actual ground temperature, not just guessing. Most likely you want a sample bore to see what the soil consitst of for conductivity etc. Otherwise sizing of the (expensive) field is guesswork.

If you only do cooling, it is possible to overheat the soil if the field is large enough. You can seasonal changes but when you switch to heating, this will cool the soil. If you only cool, you don't have that cooling of the soil. Consider a linear field to have more room to have heat travel sideways. You can imagine, if you have a 10x10 bore hole square the center holes don't have much room to dissipate heat.

Use some modeling software, ask some seasoned engineers what to recommend. there used to be a free software, but now there also are some commercial ones. the equipment manufacturers might be able to advice (but consider the source, they sell!)

Don't use a horizontal field!

 

[RossABQ]

I had to look at a similar situation, except the bores wouldn't be under the building. What do you do if a loop leaks and the building is on top of it?!

Figure one ton of rejection/absorption per 150 ft depth of bore, 300 ft deep hole typical. 10x10 meter spacing of bores is OK, pretty standard (25 x 25 ft here). You'll likely see it's not practical or cost effective for any kind of large load. Heat saturation is a real possibility. Depending on soil conditions (rock or sand) borehole costs here vary from $50/ft to $100/ft, including grouting. Tapping a lake or river is a great idea if you won't detectably change its temperature (needs to be a good size). Depending where you are, you might want to look at using that water in a HX to eliminate the compressor some times of the year.

 

[bigAlittlee]

Right, I wouldn't know what to do if you have a leak. You'd pretty much have to cap it and never use it again I guess.

I've spent a lot more time checking out different system types since I first posted, and this type doesn't seem feasible.

However, it sounds like because of high water tables the underground parking will be flooding constantly with water, that's probably around 55F. It sounds like an open groundwater system might make the most sense. The well shouldn't need to be very deep.

Thanks for your comments.

 

[EnergyProfessional]

check with your DNR, here in WI the DNR doesn't allow boreholes underneath the building. In the city peole then put boreholes underneath the sidewalk etc.

not sure about the reason, but for longevity if you ever need to access the hole having it accessible might be a good idea. they say life of the system is 50-100 years. Well, many buildings can be older.

 

[walkes]

Usually the piping is pressure tested before being put into the borehole, then grouted in place. The chance of a leak developing are pretty slim.

A couple of examples for vertical borefields under buildings:
Manitoba Hydro Building, Winnipeg, Manitoba, Canada. The bores are directly below the building as it is in a downtown core in Winnipeg. My understanding is that they had some issues when filling the bores with grout in that there were a number of voids under ground that required excessive amounts of grout to fill them.

There was an article in High Performing Buildings where they used geoexchange systems with thermal storage concrete slabs within the building. I believe that was somewhere in Quebec, Canada.


Well to well systems are usually cheaper assuming you have the water capacity required.

 

[EnergyProfessional]

http://canmetenergy-canmetenergie.nrcan-rncan.gc.ca/eng/software_tools/gs2000.html

this is a free design tool. It is a bit dated... and doesn't take the load changes of the building into account. Like whne you only do cooling all year, your field needed to be larger as if you have changes between cooling and heating, or some heating/cooling at the same time in a building.

I don't know if the professional versions do that, though.

 

[mauricestoker]

You might want to check out WestJet headquarters in Calgary.

Just read an article in ENR about that one (November 2009).