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

Posted by Paul on June 20, 2014 at 12:00:05 from (76.77.203.247):

In Reply to: Re: OT: Cisterns for Geothermal...any chance it'd work? posted by Buzzman72 on June 20, 2014 at 08:29:03:

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

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