Ground source heat pump

3 gpm is a very low flow water source. Theoritically, you can obtain 72,000 BTU cooling with 60 dF water, but the leaving water temperature would be around 108 dF, and your refrigeration compressor would need be de-rated to accommodate the high leaving water temperature. Your 20,000 BTU number would be a more appropriate capacity for a heat pump using this source.

On the projects I have been involved with, the wells are drill, 200 feet to 400 feet deep, using a 6" hole and a 1 1/2" polyethylene pipe. The wells are spaced about 15' on center and the area is 80% of the footprint of the building. The mechanical people tell me that the rock has twice the heat capacity of the water, but they like the water better than soil. As I understand it the diameter of the well and the flow has nothing to do with the heat tranference. They grout the pipe to for the full length. If there are voids left, where water collects they usually have to redrill the holes.

If the well field is place horizontal, the depth is extermly important and the well field is about 200% of the footprint of the building. The depth is specific to your location.

Thanks for the feedback.I may just use this well for domestic hot water source.It is much cheaper to move heat than to make heat.With approximately 300 square feet of surface area surrounding the water in the well, there should be a lot of available heat in the earth.

It would certainly seem to me that with all that surface area, there would be a lot more heat radiating to/from the earth that is provided by flow alone.

If I knew the thermal conductivity of saturated clay, in BTU/degF/Sq.Ft.I could calculate the full potential of this heat source/sink.

I'm not an expert, but SEDAC had an intro course on ground source heat pumps.

Go to : http://smartenergy.arch.uiuc.edu/html/gshp.html

for information.

Thanks for the link...lots of good info.

ASHREA published an engineering manual "Ground-Source heat pump", detailing about every facit of this system including site selection, load estimating, system selection, etc and including installation. If you are planning on following up on these systems, this manual would be invaluable.

My estimate is that your well could handle 1/3 ton (4,000 btuh) of continuous cooling load. Unfortunately a lot less than you would like it to be. As a rough estimate, you need 200 feet of well for every 1 ton of cooling (12,000 btuh) in a closed loop system.

You are fairly accurate. I use this method to cool and heat my house. My flow rate is 5 GPM, my well is 2500 ft. My house is 3850 sq. ft., We have over 100 degrees F ambient temp most of the summer months (5+). I keep the house at around 74 degrees F day or night, 365 days a year. Cost is about $30.00 per month.

I used a ground source heat pump in a house I built almost 20 years ago, and it used about 10 gpm, with a water temperature change of about 10 degrees. I used two wells, one to supply (the pump and household needs) one to receive. I am not a GSHP expert, by any means, so take this for what it's worth.

KEN,

How many tons were you using?

Thanks

I'm guessing that it was about 55,000 BTU. The house was in Pennsylvania, and was about 2000 square feet.

Well, that correlates pretty close to my inital estimate of 3GPM for 20,000 btu.

I still need to do more studying on this, but it appears that I do not have enough to go whole-house with this well, so maybe I'll just use it to provide hot water.

Thanks everyone for the feedback.

During the perod up to the second world war, a lot of developement took place regarding the use of Heat pumps for commercial and domestic use.

Mc graw hill published a book by Keppler & Ogglesby called "HEAT PUMP APPLICATIONS" describing some of the systems employed for extracting heat from the ground.

One 80,000 BTU system used a cased dry hole 6" dia x 220 ft. deep, a loop of copper tube insulated on its upward journey emmersed in water was used as an evaporator.

This heat sink was a sealed tube, the outsid wall being in contact with the earth,with the inside filled with water as a transfer medium to the copper tube..

So, I guess if you hooked up an evaporator coil, of suitable size, and immersed it just below the surface of the water in your well, you would get heat tranfer from the well walls which are in contact with the water in the well. It may not even be neccesary to pump this water as it would thermally circulate in the presence of the evaporator?condenser you robbed of an old AC unit.

A surface area 350 sq,ft of well depth under water being all the heat source you would require. for 80,000 BTU

At 60deg you might obtain a COP of about 4.5. 1KW in. 4.5 out as heat.

GF

Good points.

Another common solution is a ground loop, circulating either water or or water with a little antifreeze. This loop needs to be pretty large if it is simply buried in the back yard 3,4,5" deep), and I think in my case I decided the two well system would be cheaper, partly because I had to drill one well anyway, and the second was done at a small incremental cost.

Yet another alternative is to run a pipe loop in and out of the well, and circulate water in that loop and through the heat exchanger in the heat pump. This is a little less (thermally) efficient than pumping water out of the well, but means that it is a little easier to deal with the water outflow, you otherwise have -- which usually means a second well, or a small river in your yard.

With ground loops, the efficiency can drop over the course of a season, as (in summer) the ground around the loop warms up. Drawing water from a well eliminates this effect (but, of course the well pump energy has to be accounted for, because you are actually doing the work of lifting the water, as opposed to simply overcoming pipe friction in a loop).

But when all is said and done, these systems can be very efficient, and cost-competitive, if you were already planning on whole house air conditioning.

Ken,

Did you drill the second well? and if so how far apart were they?

I wondered if 'thermal syphoning" would work, if all the air was removed from within the loop of pipe between the wells? failing that a small circulation pump of about 30 watts input, of the type used for moving water arround hyrdonic central heating systems might help?.

Either way, keeping the the system free of well water crud can be a problem.

What method do you recommend for drilling or driving a well?

GF

Current thinking is unless you have a sandy or very porous geology that allows good water circulation, drill a single well and put in a sufficient length of closed loop pipe to keep ground water solids out of the system. Larger systems would require multiple wells to get sufficient developed length to get enough heat exchange.

Check the link that I previously attached to this thread, SEDAC has gathered shat I think is a good package on current practices.

Did you drill the second well? and if so how far apart were they?

I didn't do the actual drilling; I just had it done by a contractor. They were about 75' apart. The amount of water you can put back into a well is about equal to the amount of water you can take from the well, so both wells have to be pretty good volume.

Gut feeling is that thermo syphon would not provide enough flow, but with a closed loop, a small pump like the one you suggest would probably work, although you'd probably need more than 30 watts. If the back pressure is 30 psi at 10 gpm, that's about 150 watts mechanical which could be 200 watts or so, electrical, I'd guess. The 30 psi is purely a guess, because I'd need another cup of coffee to to actually calculate it for a given run of piping.

I don't have any recommendation re well drilling -- I only know enough about it to open the yellow pages.

ken

If the air was removed from the loop above the water level, you need only overcome the friction of transfering water from one well to the other.

As you say, the water level would drop in one well and get higher in the other, but I dont believe the differance would equal 30 ft,head.

GF

I may be overestimating the friction component, depending upon how much piping you are envisioning. I wasn't sure if when you said "loop" you meant a true closed loop (which typically entails a several hundred feet of pipe) or an open loop in which ground water transfer provides part of the circulation. If the loop is open, then the suction side intake can only be about 30' down, because that's about all the water column atmospheric pressure will support. If instead, you put the pump at the bottom of the well, to allow pumping from greater depth, then you end up with a conventional well, and you return the water to the ground in whatever way you want that complies with local regulations.

So, if instead, the loop is truly closed, running down into and out of one or more wells, then the frictional losses, given a flow of 10 gpm can be moderately high. Friction losses increase roughly with the square of flow rate for a given pipe size. From this chart you can see that at 5gpm 100' of 3/4 inch pipe causes a loss of about 3 psi. At 10 gpm you'd expect a little less than 4 times that, or roughly 12 psi per hundred feet. So with 300 feet of pipe, you'd expect to require at least 36 psi to move the water through a straight section of pipe. Add turns and fittings, and the pressure goes much higher. Going to 1" pipe would help, but the pipe would cost more. But in any case, I think 30 psi would represent a good guess re the expected pressure range.