OT: Cisterns for Geothermal...any chance it'd work?

Let me preface this by saying that I have very little knowledge of geothermal heating/cooling systems. I'm not a "lieburral" tree-hugger by any means; I'm just trying to come up with an idea that may help Mom on her heating/cooling expenses at the farm. [And she has a Farmall Cub, in case anyone would object that there's no tractor-related content in this post.]

The house is heated by an oil furnace. As you all know by now, oil prices aren't going down...and since her last heating oil purchase was invoiced as "#1 off-road diesel," it probably won't get any cheaper this off-season, with all the political stuff going on.

The farm house has TWO old cisterns located near the house. One was used as recently as 20 years ago, when grandma finally decided to go to 100% "city water." Mom says the other cistern leaks; my uncle, who also has an ownership interest in the farm, says BOTH cisterns leak.

Leaks aside, I'm wondering if we might just dig up the tops of those two cisterns, and install a couple of those modern poly cistern tanks as a "liner"...and then use the cisterns as a geothermal source.

Would it work? Why or why not? I'm thinking that using the 53-54 degree F water in these existing cisterns [re-lined] would make more sense than digging up the yard or the barn lot to run a lot of lines...IF it would work.

So...does anyone who is experienced in dealing with geothermal heating and cooling systems have an idea? I know you could give me a more accurate answer if I knew how many gallons the cisterns are, but I simply don't know.

There is no natural gas line nearby...so the only options are the existing oil furnace, propane [I have propane, and I do NOT recommend switching to it], or wood...which she would have to purchase...and at 82 years young, I don't think she needs the added work of feeding a wood furnace. I just think that a geothermal heat pump "assist" might make her oil usage go down, and cut her heating expense over the long term...as well as enhancing the value of the property, should they ever decide to sell.

Thanks in advance for any informed ideas.

Do you have a depth and diameter, or round about, of them?

The calculation will be how fast heat can come and go from the walls of the cistern to the surrounding earth. The top 5 feet don't count.

The plastic liner will be an insulator.

And that will give an answer of how many btu can transfer in a day.

Rather than the loops of tube spread out, you will have a concentrated point of exchange.

What size furnace does the house have, 50,000btu, 100,000 btu, ?

I don't know much more than this, but it will be the focus of your design - can enough heat move through the plastic and cistern wall and surrounding dirt to keep up. Or will you just chill down the contents of the plastic liner and there you are, a cold hole in the ground.

Paul

They will not work, Cisterns are a storage vessel, you need a well that you can pump from that is recharged at the rate needed, or tubes in the ground.

I don't understand why, if we pump both into and out of the cisterns--and possibly even from one to the other--we wouldn't have the circulation necessary to make it work. If you could explain further, I'd appreciate it.

Heat is exchanged by molecules moving, that is done by water molecules moving in a pipe, and water molecules moving in the earth. You need a large surface area and movement on both sides of that surface.

Moving water from one cistern to another just causes them to become equal, you do not have motion from outside molecules to transfer the energy to.

Perhaps you didn't understand my proposal. When we have the tops of the cisterns open, I don't see why we can't run a series of pipes in the cisterns, using the water IN the cisterns to transfer heat to/from the water in the pipes.

If you thought I was just referring to pumping water into and out of the cisterns, without a set of pipes to transfer the water up and down the depth of the cistern and take advantage of the heat transfer abilities, I'm sorry if you got that impression. I'll try to express myself more clearly from here out.

NOW tell me why it won't work.

Heat transfers from hot to cold. Like it's been mentioned. The poly tank will act as an insulator since the water will not come in direct contact with the earth.
You have to remember. When cooling the house you're taking heat out and absorbing it into the liquid lines then this heat must be taken out some where and absorbed into something else which would be the ground. The more surface are of pipe that comes into contact with the soil or well water the faster the heat will transfer.

The thought is to use "loops of tubes" within the confines of the cisterns, with the water outside the tubes used to transfer heat to/from the water inside the tubes.

I don't have the dimensions of the cisterns at this point; they were dug somewhere around 100 years ago, and were filled mostly with diverted rain water from the downspouts...with an assist from the well, as needed [as I understand what I've been told].

One time I googled geothermal heating. There are several sites where you can do btu calculations. I have a good well to use and was thinking about digging a small pond to hold the well water used.
If you do the btu needed, I do not think you that have 2 cisterns large enough to work.

Thank you. That's the kind of information I'm looking for.

Further explanation...

I know that having a couple of old, unused cisterns on a farm can be a liability; I'm just trying to see if I can possibly turn the potential liability into an asset, while also helping Mom out on her heating bills.

I know not all "good" ideas are practical. Just want to explain that the idea is to run tubes inside both cisterns, pump water through the tubes, and use the water in the cisterns to help the heat transfer...in case I didn't make that clear in the initial post [which I apparently did NOT do].

I don't think your cistern will hold enough loops to get much heat from. The preacher has geothermal and he has several hundred feet of loop in his pond. His is an open system. I toyed with a closed loop for our house but it required 25-30 feet of trench to accommodate all the required loops.

Larry

I know what you are trying to do. A friend in Mississippi did that on a space under a new house. He was using black pipe on his roof to heat the water in the day and heating the house at night with the hot water in the cistern. I know the big HVAC outfits have people that can help with that Here in Lexington Brock-McVey has been very helpful to me.

Actually, I hadn't thought of doing it that way. But it sounds like it might be helpful. I used to run a parts route that went through Lexington twice a week, so I'm slightly familiar with who Brock-McVey is.

Thanks for the ideas.

Buzzman,

Your question was clear at the get-go. The place to start is determining how much heat your mom needs. I don't know much about oil furnaces, but you need to quantify her heat needs, in btus or a modern equivalent. Preferably, total for a normal winter.

Then you get to determining how many btus you can realistically retrieve from your cisterns. One way to bump that up would be to increase your storage temperature. Several methods that would work. Make it very high, and you really need to insulate your cisterns. Heat storage in dirt around 70º works well, a lot higher does not.

Don't worry about a thin plastic tank reducing your heat transfer from the earth. The formula for conduction is q = A (k/t) ∆T, where A is area, k is thermal conductivity, t is thickness, and ∆T is the temperature differential. A very thin t means a low thermal conductivity isn't very important. The major factors are A and ∆T. Get them both large enough and you've got something interesting.

I design earth heat storage for Passive Annual Heat Storage (PAHS) where we need to determine how much heat is put into the storage, and how fast it can be retrieved. And no, it is not a system that readily retrofits an old house.

Your primary problem will be finding a ground source heat pump company able and willing to do the calculations you need.

The short answer: yes it can work, but the devil's in the details.

It may work until the water in the cistern gets too cold to get any more heat from it.

Thank you. All good information.

You might be far enough south for this system to work fairly well without the wood gasification system. Just the solar and storage tanks.
Nibe heating and cooling systems

I still use a couple cisterns, and I understand the heat transfer principle of a heat pump.

What you are planning to do is pull heat out of a cistern (or 2) and transfer that heat into the house.

As you do so, the water in the cistern will get colder.

Some heat will flow into the cistern from the ground around it.

The question will be:

Does enough heat move into the cistern fast enough to replenish the heat your plastic tubes will be carrying away?

What you probably will end up doing is cooling down the cistern water faster than heat comes into the cistern from the surrounding ground.

The many loops of tube they burry in the ground for a heat exchanger are buried over a wide area, rather deep in the ground. This is so that enough heat can flow through the ground to keep those tubes at 50-55 degrees.

Your cistern is all concentrated in one spot, and you will create a cold tube of water in the ground. It will work great for a few days or weeks, but then as winter gets colder, you will be using heat out of the water faster than it comes to the cistern, and the water will end up at 40 degrees coming from it, not 50-55. And then your heat exchanger will not be efficient.

The things going against your cistern is that you will put a plastic liner in it - that is a thin insulator plus maybe an air gap so that your cistern will not transfer heat from the ground to the water very well.

Plus a cistern is not insulated on top, so will lose heat to the frost line.

The problem is you are trying to gather a small amount of heat from a really big area with a heat pump.

Your cistern will be a small spot that you will be trying to pump all the heat from - and you might succeed in removing more heat than what can get back to the water in the cistern.

So, as I said....

You need to know how much heat the house will need.

And the. You need to figure out how much heat is available from the small area of that cistern.

I will guess the house needs more heat that the small area of the cistern can supply, but I don't really know.



Some people are confusing a well heat pump with the closed loop deal you are trying to work with.

Some heat pumps use water you pump up from a well for the heat source.

The closed loop you are considering is just a whole bunch of loops of plastic tube buried deep and over a wide area to gather the heat from below the frost line. But you want to concentrate all those loops into a relatively small spot, the inside of a cistern, typically 2000 gallos or so.

I don't think heat will flow to that 2000 gallons fast enough to keep up with the amount of heat you will be pulling out of the cistern.

Paul

(quoted from post at 11:31:03 06/20/14) Thank you. That's the kind of information I'm looking for.

Click to expand...

How many gallons of water? how much temperature delta T will the geothermal efficiently work under? Do you know what a btu is ? When heating and both Cisterns.have went change of state from liquid to solid. There is no way to efficiently pull more heat from the cisterns.
How many btu's over the winter to heat the house now ?
Often reducing the hearing and cooling load by improving insulation and plugging air gaps. That is more economical than finding another way to add or remove heat energy.
Limiting the amount of air leakage is probably more important than insulation.

Paul, down below, gives good advice. We put in a geo-thermal unit five years ago. My idea was to lay loops in the bottom of our farm pond that is not very far from the house. The dug pond is about 100 feet wide and about 250 feet long and varies from 3 feet to 20 feet deep on the far end. I had it dug for fill dirt and had big ideas of raising fish, because I like to eat them! Well, that didn't work, so use it for geo-thermal heating and cooling of the house.

Yah right! I don't know where you live, but we're in SE SD. My contractor calculated the cubic feet of water in that pond and determined that in winter, it would turn my pond into solid ice considering that ice on the top would probably be a foot thick.
My pond isn't NEAR large enough to work, so we put down four vertical loops 180' deep. That works.
Depending on your location, your idea probably just won't work. Maybe for a few days like Paul says. Sorry! I'm speaking from my experience. Electricity to run the compressor in January (considered the fuel) was $73.14 for February it was $80.56. Last July for air conditioning the compressor took $6.15.

First off,we(I)don't know where you are at and how many BTUs are needed to heat or cool.The answer is 99% NO it will not work as you have no area to transfer your hot water or cold to get replenished(temp wise).You will be moving 3 to 10 or more gallons per minute thru the heat pump. I had a 6 Ton unit in for 15 yrs.and we changed something with the water system almost every yr.to improve the transfer of heat.When we quit we had 3 4000G cisterns(tanks) staged 15 ft.under ground plus 500 ft.of lines 10ft.down in a open-closed system,plus a well backup if needed.Every gallon of water was monitored for temp.and gallonage per minute and all the Freon pressures were recorded. More then you maybe wanted to know,but the answer is still NO it won't work in your case.

Thank you.

It is a neat idea, good thinking on how to try to use something save energy.

It just seems, without knowing or doing any of the math, that you probably will create 2 giant underground ice cubes that won't be able to transfer enough heat fast enough to a big enough surrounding area to keep working.

Paul