Geothermal piping calculations

[prichmon]

We are looking at replacing a Cooling tower with Geothermal Cooling for a process water loop.

80KW total power usage.
10*F temp differential on cooling tower design
20*F temp design differential on system
operating pressure 30psi

If we move to geothermal cooling our temp differential will be ~35*F+ due to ground water typically not exceeding 60*F. The thought is to simplify the system by elimination of the HX, cooling tower and tanks.

Based on copper pipe of thermal transfer of 13.11 W/M^2*K; PE pipe 7.09 W/m^2*K

1" copper tubing
10*F differential copper equates to ~4595 feet pipe
35*F differential copper equates to ~1315' pipe

Based on data on ground temperature changes the loops need to be ~20' deep to minimize temp swings to the +/-10* range.

I would appreciate if someone with experience in this area could assist me?

Thank you

Rich

 

[racookpe1978]

How deep and at what assumed spacing between ground loops for that continuous ground thermal absorption?

How "reliable" is your ground water? in Texas, for example, the underground aquifers change depth depending on drought conditions topside, and on the specific county the ground water is in. Florida and Mississippi underground water is more "stable" in elevation, but doesn't "flow" very fast though. Upstate GA and AL and NC and SC and VA is rocky with almost no assurance of underground water.

Is there any difference in heat load you have to reject between summer and winter?

 

[prichmon]

How deep and at what assumed spacing between ground loops for that continuous ground thermal absorption? No Idea. I do not have accurate soil data. Based on some examples on the net it seems at 20' down the temps become stable within +/- 10* which would easily meet the consistency we are after. I have read of people burying loops in the 4-8' deep range but do not feel we will see consistent temps.

At this point I am doing a cost analysis for feasibility. I have to perform more research.

How "reliable" is your ground water? in Texas, for example, the underground aquifers change depth depending on drought conditions topside, and on the specific county the ground water is in. Florida and Mississippi underground water is more "stable" in elevation, but doesn't "flow" very fast though. Upstate GA and AL and NC and SC and VA is rocky with almost no assurance of underground water.

The area is next to a river in a low lying area. Clay soil composition.

Is there any difference in heat load you have to reject between summer and winter? No. The heat load is for process equipment. Under ideal conditions the equipment would run at 40KW 24/7 365.

 

[sspeare]

The original post says: 35*F differential equates to 1,315 feet of copper pipe, at "total power usage" of 80 KW.

My question: what BTU per hour are you trying to reject into the ground? Is it (80 KW x 3413) = 273,040 BTU/hour?

 

[prichmon]

Yes 273K BTU/hr

 

[sspeare]

OK, here are some thoughts:

I assume your 273,000 BTUH includes all the Heat of Rejection from the refrigeration equipment.

Tons = 273,000 BTUH) / (12,000 BTUH/Ton) = 23 Tons to be rejected to the ground.

Borehole Depth: I use a 180 foot deep vertical borehole to reject each Ton. By "Ton", I mean the load that INCLUDES all the Heat of Rejection. Some guys miss the Heat of Rejection, and they under-size their system. My rule of 180 feet of borehole per Ton works in most areas of Pennsylvania; I can't comment on other regions.

So, 23 Tons x (180 feet/Ton) = 4,140 total feet of pipe within the boreholes. I use HDPE pipe that is fabricated for "geo" applications.

If I understand properly, you seem to want to do a "shallow" burial of pipe, not "boreholes" as I have just roughly sized above. I can't help you with a shallow system. All I know is how to get it done with boreholes.

Hope this helps.

 

[prichmon]

sspeare:

Thank you very much your calcs help alot.

Please define for me Heat of rejection??? I am using name plate data in which we will never see the amount of heat due to losses.

As per Ohio Dept of Nat Resources we must dig a minimum of ~20-28' for consistent ground water.

A low burial would be ideal to minimize costs as much as possible. I have area but do not have equipment to bore.

thanks guys...

 

[sspeare]

prichmon, you probably should have posted this to the HVAC/R forum. Anyway, be wary of rejecting heat into the ground 24/7. If you have ZERO heating load, you'll never have to extract any heat from the ground. When you reject heat 24/7 into the ground, the "borehole field" gets "saturated." Saturated is not the right word I'm sure, but the idea is that you reach the point where your borehole field (or shallow "slinky" field) can't absorb any more heat. This happened to me. We had to go back in less than 2 years and cut a Cooling Tower into the geo loop. Most embarassing. Early on, I explained to my boss that we'd get into trouble rejecting heat to the ground 24/7, but he didn't listen. He was fixated on the ground-source heat pump concept, and that's all there was to it. So, the bottom line: be careful if you are rejecting heat all the time, and never need to extract heat from the ground.

 

[prichmon]

Our system runs realistically 20 hrs per day ~50 weeks per year 5 days per week.

Wouldn't the flow of ground water transfer the heat? Was it an area which lacks ground water flow?

I will do more research...

 

[LittleInch]

I would be very careful not to use simple steady state calaulations for this purpose.

SSpearce is correct in that over time, sometimes quite a short period of time, the soil, water, rocks etc surrounding the pipe cease to be at the temperature they started out at and gradually absorb some of the heat and increase in temperature. Therefore your simple steady state calcualtions fall apart. You can do a transient analysis, but need to know the soil heat coefficient when wet and allow a decent amount of heat needing to be transfered through the soil "tube" surrounding your pipe. Don't rely on any signficant ground water "flow". If you're leucky you might get a small amount, but most times it will be insignificant.

If in doubt put double the amount of tube in the ground....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MJCronin]

Its a good thing that the copper system will not corrode in the ground......

 

[sspeare]

To answer prichmon's question: what is the Heat of Rejection?

Suppose you calculate the Total Cooling Load (sensible + latent) of an office space at 10 Tons. You might think that you only have to size the "heat sink" (whether a cooling tower or an array of boreholes) to accept 10 Tons. But, you have to make sure the "heat sink" (cooling tower or borehole array) will accept not only the 10 Tons Total Cooling Load, but also accept the Heat of Rejection. This HEAT OF REJECTION is the heat generated by refrigerant compression, and the heat from operation of the compressor motor. This "extra" or "waste" heat, which is not part of your Total Cooling Load, has to be gotten rid of somehow, right?

You might think of the Heat of Rejection as the price you pay for moving the heat around. Typically, heat pump technical data will list the "THR", or Total Heat Rejected. If you study the catalog data for a ground-source heat pump, you'll see the "THR" is approx 125% of the heat pump rating. In other words, the 10 Ton heat pump rejects 10 Tons building load + 2.5 Tons "waste" heat, which is 12.5 Ton. The THR = Total Heat Rejected = 12.5 Tons.

I'm going into this in detail because I had to learn it the hard way, by mistakes.

 

[sspeare]

In my most recent post, I failed to note a link included to a PDF that contains, among other things, an explanation of "Heat of Rejection."

 

[prichmon]

Thank you for the explanation of THR. The heat of rejection is the load created in an HVAC system for changing the state of the refrigerant. In this case there isn't heat of rejection. The application is a cooling system for brazing. There isn't any additional process.

We currently use a divorced system with heat transfer occurring in the liquid to liquid dual pass heat exchanger. I would prefer to run a single pump and piping directly through the loop system to cool.

We are required to use deionized water as the system cooling fluid. Would anyone know if antifreeze would affect the conductivity of the deionized water?

I found at the local home Depot a set of 1.5" polyethylene pipe rated at 80 psi with the thinnest wall. If the data is correct we would have a K of ~8.77.

Based on the 80 psi pipe I would need ~1965' for a basic system. I will oversize to accommodate heat soak of the surrounding earth.

Does anyone have a link on separation of loops and configuration effects? Depth effects on cooling?

 

[BigInch]

Don't expect a lot of water flowing in clay soils. Above or below, maybe, but water won't flow through it. You're talking 3-6 months just to change its moisture content.

Independent events are seldomly independent.

 

[sspeare]

Anti-freeze?
I have a link below, on Dow-Frost GEO 20. You can get the thermal properties of GEO-20 anti-freeze/water mix from the brochure in the link. Don't use standard automotive anti-freeze, use an anti-freeze formulated for "geo" and/or HVAC applications. Read Dow's literature to learn why.

Loop Separation & Configuration Effects?
This is a field of study in itself. I have used this book: "Ground Source Heat Pumps, Design of Geothermal Systems for Commercial & Institutional Buildings", by Steven Kavanaugh & Kevin Rafferty. It is published by ASHRAE. The ISBN number is 1-883413-52-4. You can buy it at ASHRAE's web site. But, the book only discusses vertical boreholes (wells) and surface water applications (using a pond or lake). It does not discuss the "slinky" tubing buried shallow, which seems to be the direction you're headed.

You still have a problem. If you're rejecting heat all the time, and never removing heat from the ground, your "heat sink" will become less & less effective, until it just won't accept any more heat from your process. That's what happened to me in the job I mentioned in my post of 24 Sep 13 13:08. We had to go back & cut a Cooling Tower into the loop. The geo loop didn't work because we were always rejecting heat.

Hope this helps.

 

[prichmon]

sspeare:

Thank you for the comments. It sounds like this is not a viable option since we never intend to remove heat from the ground. We will end up exactly where we started from with a cooling tower.

If we had access to the river we might be able to make this work.

Thanks all;

Rich

 

[LittleInch]

It might be viable, but you probably need to increase the length / area of your piping by such a significant amount that it starts to become un economic. Some testing on a small scale is probably the only way to figure it out properly...

Either that or have two or three sets of pipes and switch from one to the other once the return temperature starts to climb beyond your limit. Start with two or three and then just add more as you need - you did say space wasn't a problem....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

If you are always rejecting heat, well then it just doesn't make sense to bury it in earth. Dry earth is a very good insulator and has a rather low heat capacity. As you have already found out, once the heat capacity of the soil immediately surrounding the pipes becomes saturated, it becomes less and less useful. The soil inside the control volume will continue to exchange with the soil outlying the perimeter, but the surface area across which the heat flux will pass is relatively much smaller for the pipe network group. At that point, the soil temperature will start to rise, until you start rejecting heat to the air above, which probably isn't going to be very effective since it will be convective across a flat horizontal surface. Rejecting heat in that situation is probably best accomplished by fin fans, which I have used for natural gas cooling where water cooling is not practical. If you have a relatively cool air temperature in relation to the temperature of your medium, fin fans (entirely above ground) can be effective, however in the Mid East, or South Texas, New Mexico, Arizona... it doesn't work very well when summer air temperatures rise to nearly the same (or possibly higher) as the natural gas compressor discharge temperatures.

Independent events are seldomly independent.

 

[NoHtPp]

OP Prichmon

Please check my opinion:

I have published in NEOHIO 186ft per vertical borehole loaded ton of HEATING off compressor tons, no hot water features absorbing more energy, and
230 to 240 ft per loaded net-block continuous cooling tons. The numbers can be correlated.
the ratio is a simple 4/3 when looking at compressors doing heat production over cooling refrigeration uses. "rejected-heat" and "absorption" to GeoThermal-Loop designers.

FOR my work:
This was because we found at offices operating 22"tons" rejected heat / to your 23-24t the load you refer to, net cooling tons ((our rejecting 262,000 btuh in observation)).
22t is off the hot side of a loaded down a 20-ton compressor not even absorbing 18 net cooling tons at 42-40f fluid temps for fancoils...on the "chiller" side.

YOURS: That 20t compressor but put out 264,000 btuh and on a 1.1/2 hp circulator the loop hit 105+ GOING OUT to the ground heat-sink, and supplied the equipment with 96f fluid for cooling the 264,000 btuh of heat rejected by the electrical energy + the transferred heat recovered from the 17-18 net cooling tons, "cooling" the hot side of the compressor doing 'chilling'.

18-20% prop-glycol...
there (still) is the heat exchange with 12 boreholes, 15 ft apart x 245 ft deep , barely stuffed with 3/4" ID - COMMON PE3408 Poly pipe x dr-11 160 psig rated,
and over 60 ft from the building on 2" dr-11 PE headers, 2 ft apart in that 60ft run.

standing water in the drill holes was nearly 40ft from grade (a little deeper); standard grout
(see KAVANAUGH GROUT SOURCE HYBRIDS GEOTHERMAL ) although with plastic, still PROVED the 15ft spacing was not enough as well as overloading the under 200ft per ton of HEAT REJECTED to the ground-heat-sink. ENHANCED GROUT THERE IN THE OVERLOADING MAY HAVE BARELY BEEN 4 or 5% improved heat transfer, as the entire field was maxing out anyway.

I received calls and spoke with engineers in 3 states. Before I was told I said COOLING x 4/3 for total COMPRESSOR rejected heat- then re "THEY (trainers) TOLD ME 175 FT PER COOLING TON"

I said that is IF your LOAD is on the ground loop as a process cooler. AND if you want over 90f supplied to the load back from an Earth-Coupled Loop in wet clay-sand-gravel overburdened well boreholes... We had to account for actually seeing 240 vertical, wet ft in clays and sand and gravel barely work 12000 btuh input at mean loops of say 93f (+/- 4 or 5f)

IT IS IN FACT THAT YOUR 60F WOULD PROBABLY POINT TO ~ 260-TO -280 VERTICAL BOREHOLE FT OF 300FT DRILLING IF , IF IT WERE DAMP, AND IF YOU NEED CLOSER TO 90F OR 88F RETURNING TO YOUR LOADING.

How so?
We directly cooled only 8 tons at 81 out to the loop of about 3100 ft, and 77f back IN 52f wet soil. (same pump = 3.1/2f temp difference) OHIO clay and sand and gravel. We plotted the 1/3 work done at lower fluid temp results to the overheated ~ 9+ temp differential at 262,000 btuh rejected.
You could graph from info given. LOOK at my footage numbers, and even see if you must use COPPER at all.
In to the overloaded ECL (gle, Earth-Coupled-Loop) for over 3 weeks of "on" 24/7 high rejected 262,000 the ground loop, later "OFF"-line for 3 weeks was at 57f in recovery, here, in 52f soil.

HORIZONTAL boring of 12f to 14f will find you in 60f soil that that soil is probably over 66f to 67f in open sun in August-Sep and you could call folks in TX who had a (good or bad?) Ralph Cadwaller loop install , commercially, from the 90's, just for data... I think then called Loop-Tec.

It may be well thinking of using dry-coolers , less ground loop, and keeping the tower around for a while considering some Hybrid operation with common differential controllers for best "heat-sink" moments. And the dry-cooler and tower can get the ground loop down through the evenings to lower temperatures - if that is applicable, field fitting the above numbers is required.