Geothermal Well Field Calculations 2

[flyeng4]

All,
I have been given the task from my boss to learn about geothermal design. I have spent some time researching on the internet and have not been very successful. Sizing the heat pump unit is done as it is very similar to conventional spit system design which we have experience with. The portion of the project that we are having trouble understanding is the well field design including depth loop pipe sizing and header pipe sizing. I have talked to a couple product reps and well field installers and I keep getting industry 'rules of thumb'. I understand that these are probably safe to go by but probably will not hold up in court. Do you know of any literature such as a hand book or something for understanding well field header, loop, and depth sizing? Are there a set of tables that are industry standard that I could use? Any help will be appreciated.
-flyeng4

 

[flyeng4]

I suppose that I should mention that this is light commercial/residential construction (72,000 btu load on the building).

 

[j79guy]

The information you seek is also dependant upon several factors:
- Which in-ground heat exchange media do you wish to use. (Water/glycol, with exchanger to refrigerant within the heat pump, or direct exchange by using refrigerant directly in the ground loops.
- Horizontal loops, or vertical wells.(You do mention wells, but perhaps horizontal loops, have not been considered?)
- Average ground temperature, and at what depth.
- Soil composition.
- Refrigerant planned to use. (Freon or hydrocarbon based.)

I recently put a 65,000 btu direct exchange geothermal unit in my house, using horizontal loops. (1/2" copper line in the ground, with freon directly in contact with the ground heat source. Located in Northern British Columbia)
I suggest contacting a geothermal specialist company for residential use, to get recommendations from them and perhaps flesh out the info you require. Nordic Geothermal of New Brunswick seems to be fairly knowledgeable.

j79guy

 

[flyeng4]

j79guy, thanks for replying. I have been in touch with a geothermal dealer who has been helping me size the equipment and loops and has been answering some questions. We have decided on vertical loops with a water/methynol medium and R410A refrigerant. The only thing that makes me a little uneasy is everything was sized by some industry rules of thumb. I am confident that the system will work sufficiently, but I as an engineer need to confirm these calculations before the drawings are signed and sealed. It would be nice to see some literature on well field/loop sizing if there are any out there.
-flyeng4

 

[j79guy]

Unfortunately, until you actually flow your water/meth mix through the vertical loops, a guess as to the heat flux rate into the loops is all you will have. Thus the variables that I tossed out there in the earlier response.
What are the chances of hitting ground water in the vertical wells? If there is water, the maximum flux rate goes up. If the soil is dry/sandy the full depth of the drill, the maximum flux rate will be significantly lower, ect. Your local geothermal dealer is probably the best source of seat-of-the-pants knowledge, but as far as concrete engineering numbers, well, you need to know exactly what the soil composition is, the whole depth of the wells.

j79guy

 

[Clymber]

Your first step in an actual geothermal project should be to have a geothermal test bore drilled at the site and then have a formation thermal conductivity test performed for approximately 48 hours This will provide:
1. the formation thermal conductivity,
2. the formation thermal diffusivity, and
3. the undisturbed soil temperature.

With this data, a geothermal dealer could estimate a well depth and flow rate for your required load. If you ask, he may show the calculations to you as well.

HTH,
Clymber

 

[Clymber]

There is actually much information about these systems online. One source I would suggest is the McQuary Geothermal Heat Pump Design manual. It includes a section on boreholes and overall system design.