Hello,

My company is considering using geothermal loops to water cool a chiller, that will in turn have an air-water heat exchanger that cools the air.  We are in west Texas, where the climate is hot and dry.  We also will be using a lot of commercial equipment (lathes, ovens, etc.), so our ratio of cooling to heating out of the year is quite high. We often still run the air conditioner down to 30 degrees F.  We probably only run the heater 10% of the year. We are guessing that we will need about 150 tons of cooling for the building that is about 30,000 sq. ft.

My question is, assuming we have the land, would a horizontal loop or vertical loop be better?

Here are some of my concerns with a horizontal loop:
        If we do not space the trenches out properly, then the grass above will die and we do not want that. Also, over time, the average ground temperature will increase and the loops will become far less effective.  Is there some sort of standard used for various locations that dictates acceptable ground temperatures?

Here are some of my concerns with a vertical loop:
      Cost is a large concern.  We will have to probably drill dozens of bores that are hundreds of feet deep.  Very expensive business.  Plus, I do not know how far down the ground water is, or where the aquifer is.  If we can go down to the ground water or aquifer, that should greatly increase the heat transfer rate, and help to eliminate increasing the ground temperature over time.  However, this is difficult to quantify without an extensive geological analysis, so designing the loop is kind of a shot in the dark.

Any suggestions or maybe just pointing me in the right direction so that I can get started on the design process? Or maybe you know of someone with a similar situation? Any precedent would be helpful.
Thanks

-Mechanical Engineer

As long as you water the grass, I don't think it will die. The sun contributes far more to grass temperature than any 4' down loop does.

You can't use filtered outside air for cooling when it is 50F outside?

You don't have any data as to how far down the water table is?

Historically vertical loops are indicated in your kneck of the woods. You do however have a company on these forums that is currently experimenting with horizontals.
Depending on what your loop design results are you cost of verticals v horizontal may be closer than you think.
Long term studies show a slight raising of ground temps around vertical loops in commercial applications over several years. Is it a problem? I don't know 1 or 2 degress in ten years probably not.

The only thing more expensive than the right loop field for a job is the wrong one.

Thanks for the replies guys.

I personally think vertical loops are the way too go, as they seem more stable. That said, I have exactly zero experience with geothermal loops. I've just been reading about it for a couple days now.

As for filtering the cold air during cold temperature seasons, we are going to do that a little bit with our make-up air. But I don't think that will always be enough cooling considering the large ovens and other equipment we will be using.
I've seen probably close to ten different rules of thumb for vertical loop design, but I feel that those are generally for locations that are unlike Texas weather. During the summer, we are often above 100 F, and the air is extremely dry in west Texas. Is there a reliable rule of thumb to follow for weather conditions in west Texas?

Enough 50F air will cool anything down to 70F. But it might not be practical. A good local HVAC pro can answer most of your questions. Or just to explore options, there is software what will do many of the calculations.

With unbalanced loads your loopfield temp will go up with time. Larger spacing will help, so will any effort to keep out solar gain. Invest into a ground loop design software, or even better, hire someone who does this for a living. There is no rule of thumb with commercial building design. If it saves you a couple boreholes it will pay for itself. Plus, have you done the inside infrastructure layout before? The design, flow rate, pressure drop of the inside equipment will influence the design of the outside heat exchanger.

Many school systems here in the Dallas area have changed over to geothermal HVAC systems.  Big buildings that are not all advanced with respect to energy efficiency, coupled with lots of surrounding parking lots and athletic fields and park aread all combine to make changing to geothermal a 'no brainer.'

Soil geology is studied, test boreholes are drilled, and other practices unique to commercial situations are all employed.

I have yet to see one of these jobs use anything other than verticle boreholes.  I'm sure it could be engineered with a horizontal loop, but, the amount of pipe required, and the pumping required, I would think would actually make total cost of ownership of a horizontal design to be cost prohibitive relative to vertical.

Contact me off line and I can refer you to two excellent commercial geothermal HVAC companies. 

Best regards,

Bill

I would suggest coupling geothermal heat pumps with Calmac thermal storage tanks. Using this technology with off peak hour rates you could increase efficiency and save money, possibly reducing the system size as well.

While I live in Montreal Canada, I have seen Calmac thermal storage tanks used in West Texas, at a petroleum museum in Midland to be exact. Perhaps speaking with the operators of this museum may be useful.

Here’s a link to Calmac:

http://www.calmac.com/products/icebank.asp

Here’s a link to the museum:

http://petroleummuseum.org/index.html

Google this to find a Google satellite map of the museum where you can zoom in and see the tank farm out back with the heat pumps:

1500 Interstate 20 West Midland, TX 79701, Map

The building you’re looking for is in the shape of a right angle like an arrowhead with a circle at its entrance. Note how compact this system is cooling this large facility.

Hiring a professional engineering company with geothermal design experience is a must. So is in-situ bore hole testing of ground thermal conductivity; I’m sure there’s no shortage of drillers in your area!

I hope this is of some interest and help.



SR

Thanks for all the help guys. I'm realizing that it's likely that I don't have the expertise required to design this entire system myself. I haven't contacted a drilling company yet. Are there any suggestions for reputable drilling companies? And about how much does it cost to drill generally? I know it depends on how much you drill, where you drill, etc. But just as a super general ball park, is it like $20/ft? $10000/bore? I don't have even the slightest idea about how it's priced out and about what it might cost.

Noting that you're in West Texas, contact Bret Delisle, co-owner, of Earth Tech.  Get contact information from their web site:  http://www.earthtechusa.com/index.html .  These guys are strictly commercial - mostly school building structures.  They're going to know what it will take to properly do your project, noting the West Texas summer heat and noting your cooling load.

I have a personal relationship with Mr. Delisle.  Tell him I refered you to him as part of your introduction.  I would initiate contact with him via telephone versus email.  Use phone number on web site.

Good luck.

Best regards,

Bill

Posted By tgray on 14 Aug 2012 11:52 AM
  But just as a super general ball park, is it like $20/ft? $10000/bore? I don't have even the slightest idea about how it's priced out and about what it might cost.

My super general ball park number is between $10 and $20 a foot.

Thanks for all of the continued input. I called Bret but he was in a meeting so I left him a message.

I know that the chances are high that since we have a very unbalanced heating/cooling load that the ground temperature will increase over time. But, if the building was only operating 8-9 hours a day, then would the 15-16 hour cool down phase not be enough to dissipate the heat in the ground regularly?

8-9 hours/day (vs 24) will effect loop design. Your long term average is much lower but your peak loads are the same (unless you use something like the Calmac storage mentioned above). Open loop is not an option?

This is speculation, but localized venting might be well worth the effort. For example, better to vent the heat from an oven instead of having the AC remove it.

A good HVAC engineer should cover all of these things.

Posted By tgray on 15 Aug 2012 11:32 AM
... the chances are high that since we have a very unbalanced heating/cooling load that the ground temperature will increase over time. But, if the building was only operating 8-9 hours a day, then would the 15-16 hour cool down phase not be enough to dissipate the heat in the ground regularly?

This just isn't correct.  If the loop is properly designed, then the design will include avoiding long term borehole field heating.

Here's an example for a residential system, showing for an almost 4 year studay period, showing how disproportionate the cooling vs heating load can be (red lines) and yet how well mannered the EWT remains:
http://www.welserver.com/cgi-bin/pl...vgHEHR.png

And if Degree Days were further incorporated into the analysis, to provide a pure apples-to-apples comparison year to year, it clearly looks like, so far, this residential design (5.3 tons of cooling equipment connected to 8 boreholes at 300' deep each, spaced at 20' apart, using 1" HDPE pipe and standard grout, in the Dallas climate) does not appear to be suffering from long term borehole field heating.

I think this is a good example where it's the installer (commercial in your case) who really makes the difference, installing off-the-shelf geo equipment, who has the expertise to design a 'system' that works (and 'works' includes no long term borehole field heating in a hot Texas climate).

Best regards,

Bill

Posted By tgray on 15 Aug 2012 11:32 AM
... if the building was only operating 8-9 hours a day, then would the 15-16 hour cool down phase not be enough to dissipate the heat in the ground regularly?

I meant to include further comment in the above posting, on how a loop functions on an instanteous basis, but forgot to until now.  I apologize for the second posting here.

If the loop is designed properly, then yes, the 15-16 hr cool down phase will be enough to dissipate the heat in the ground.

Here's a chart that illustrates the process, minute by minute (actually 3 min. samples), over a 50 hour period:
http://www.welserver.com/perl/plot/...mpsEWT.png 

Here we see that as long as the amount of heat being injected into the earth is less than a certain threshold (20 kBTU/hr - red line) then EWT is dropping due to more heat moving through the earth, away from the loop pipe and towards cooler earth, than what's coming in from the pipe.  And then at times when the injected heat is 75 - 80 kBTU/hr, EWT is rising.  In just a 24 hour period, it's not unreasonable for the EWT high to low swing be 7°F.

This is a residential example.  On a commercial basis, you're still going to need to design for morning to night EWT swings of a reasonable magnitude, but you'll be designing with 10 - 100X kBTU/hr cooling loads and perhaps 10 - 100X boreholes, properly spaced.  I was at a Ft Worth museum facility recently, that required a lot of paved parking area.  This unusable-for-any-structure area was ideal for the borehole field, and they had plenty of space to work with.

Commercial guys can't afford to guess, or use rules of thumbs, to decide on how many holes, how far apart, and what size HDPE pipe to use.  Instead, they'll drill one or more test holes, accurately measure the heat transfer capability of the test holes, and then use the measured data to get the entire borehole field designed correctly.  They don't guess on geology, grout transfer characteristics, or anything else - they spend a bunch of money up front before the borehole field is ever designed.

In fact, the first hole drilled of the entire borehole field is thus hugely expensive.  The rest don't cost nearly as much.

Also in the commercial world, you'll sign a contract for the job before the design in ever done.  Many times, there will be two contracts actually: one to do the testing to determine the numbers in order to cost estimate the project and ultimately design it.  In fact, it's not out of the ordinary for contracts to sometimes stop after the test phase, when geology doesn't yield the kind of heat transfer performance to make geo cost effective versus other cooling technologies.

Best regards,

Bill

Posted By tgray on 15 Aug 2012 11:32 AM
... I called Bret but he was in a meeting so I left him a message.    

I spoke with Bret Delisle this weekend.  He said that his office made an error and thus the message didn't get to him.

His email adress is [email protected] , and his direct line is 214-535-6159.  His name is pronounced 'da lie all.'  I described to him a summary of what you've written here as your needs.  He would more than happy to help with your questions.

Best regards,

Bill

Bill, keep in mind that your runtime is very low due to the oversizing of your system, also your loop is way too big. Also 4 years is not very long to show the effect on the loopfield of an unbalanced loop. It will be very different for a commercial system. Installing a system. Double in size would be cost prohibitive.