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-   -   DX Geothermal Pond 4 ton Heat Pump (https://ecorenovator.org/forum/showthread.php?t=4477)

randen 07-16-15 09:10 PM

Memphis & AC

I had limited success with the tube & shell That's why I turned to a brazed plate for the price I could not go that route. Memphis is right the refrigerant needs to flow around the outside of the water tube. There are commercial units avalible.

What about your new discovery DX.?? 5/8 or 3/4 copper out to the pond, some 3/8 loops with manifolds and return. No circ. pumps. No heat exchangers, no antifreeze a huge advantage.

One other detail depending how much heat your trying to sequester with a tube and shell you may need antifreeze.

Randen

MEMPHIS91 07-16-15 09:58 PM

Randen I agree that the DX system would probably be the best. But the cons way out weigh the pros right now. Cons are cost, tons of brazing/potential leaks, time consuming install, amount of refrigerant, and heat loss in the ground on the way back to the house. PVC is much easier to insulate.
I am reading your post now about the tube in shell. Why do you say the refrigerant needs to be on the outside? I can make the exchanger very large if need be to gain the efficiency needed. Not disagreeing just SO much data right now.

jeff5may 07-17-15 08:00 AM

The heat exchanger boils down to a time and temperature model, one mass against the other. Water has much higher heat capacity and mass than refrigerant, so less of it (mass) needs to flow than the refrigerant per unit of time. With a coaxial hx, you can make the space between the tubes small, so the gas can flow through the middle. However, this reduces the surface area of water that is exposed to heat transfer.

The main advantage with gas in the center is cost reduction. Since the water is under much less pressure, the outer tube can be made of much less expensive material. PVC or pex is common, as well as cast iron.

With the water flowing through the center, both tubes must be made tough enough to withstand the pressure of the refrigerant. In this type of hx, the refrigerant can leak out to atmosphere or into the water line, so most are made with double walls to prevent an overpressure condition in the water line.

randen 07-17-15 08:46 AM

DX vs Glycol/HDPE loop
 
4 Attachment(s)
AC Memphis

From Wikipedia direct exchange geothermal:The boreholes are drilled to a length of 50, 75 or 100 ft (15, 22 or 30 m) with a diameter of 3 inches (76 mm). A total of 100 feet (30 m) to 140 feet (43 m) of drilling is needed for each ton (3.5 kWth) of system capacity. At 40' from your home a 60' line set is doable.

Its good your weighing all your options.

What are you brazing with?? Silphos brazing is one of the easiest.

What is your plan on the other end? Are you air conditioning and heating with a fan center??

One other thought. If your going to through the loops in the pond both systems will want to float so they will need to be weighted. There is a commercially made system for HDPE made for ponds but I have no idea their cost.

For a day I had a pond with HDPE is this what your thinking??

The shell and tube worked, but I had them freeze up many times even though my flow was good. It was a good thing the outside shell was a flexible plastic but even so it did fail badly. I could imagine inside the ice slowly forming around the tubes reducing the flow and ultimately freezing solid. The proper antifreeze to prevent the freezing isn't cheap either.

Randen

MEMPHIS91 07-17-15 10:05 AM

Jeff that does make sense, so basically as long as it's big enough both ways should work.

Randen, thanks for the pictures. I am not considering a hdpe loop. It's either DX or open loop. Open loop would mean no antifreeze.
The freezing problem does sound bad, that is why I'm now thinking 2 coils with water and refrigerant split between them. And I like the gas on the outside idea because I'm thinking it will limit how much refrigerant flow acts on the water as fast, hopefully helping with freezing as well.
I breeze with 15% sliphos.
The other end is a 4 ton older air handler.
I have a idea I'm going to see if even is possible. I'll post pictures soon.

MEMPHIS91 07-17-15 11:44 AM

Randen this most stable temps in the pond are about 50 feet or from the bank. So the line set would be about 120 feet long. Do you think 3/4 out and 5/8 return would work?? Then a manifold and 5 100 feet loops of 3/8. That really would not be to bad. Hmmmm.... More brain storming to do.

AC_Hacker 07-18-15 01:40 AM

Quote:

Originally Posted by randen (Post 45961)
From Wikipedia direct exchange geothermal:The boreholes are drilled to a length of 50, 75 or 100 ft (15, 22 or 30 m) with a diameter of 3 inches (76 mm). A total of 100 feet (30 m) to 140 feet (43 m) of drilling is needed for each ton (3.5 kWth) of system capacity. At 40' from your home a 60' line set is doable.

Randen, very good info there!

I was just looking on a table of Thermal Conductivity values of some common Materials and Gases. These values are to be used in equations to calculate the quantity of heat flow, when you know delta-T, and other factors. But I think it is useful to know what these values are, to guide intuitive judgment.

Here are some of the values:
  • Carbon Steel - 43
  • Aluminum - 205
  • Copper - 401
  • Concrete, stone - 1.7
  • HDPE - 0.42 - 0.51
  • Water - 0.58

It's not surprising that aluminum would be a much better conductor than carbon steel. And that copper is much better than aluminum.

I have listed concrete & stone, because I have seen many tables that give the heat transfer of dry earth, damp earth, wet earth, etc. And the value of concrete and stone are very close to the cluster of values found in bore hole heat transfer conditions.

I was just about to make this entry, to suggest that the loops in water could be made much shorter than the loops in earth, but the thermal conductivity value for water really surprised me.

I would imagine that this value would be more favorable when the system is actually in use, because there would be at least some modest flow of water past the tubing due to temperature differences, whether they are copper or HDPE, but it would certainly not be anything like water that is driven through a HX by a pump.

Also, of interest is that when heat flow is calculated, the various factors that would inhibit thermal flow are all added together, for instance the resistance of flow from the refrigerant itself, the resistance of the boundary layer of refrigerant, the resistance of the transfer tubing (copper or HDPE), the resistance of the boundary between the tubing and water, and the resistance of water itself, there could be other terms added in like resistance due to moss, etc.

So the equation for the overall resistance would look something like:

TOTAL RESISTANCE = Rref + Rbdy1 + Rtube +Rbdy2 + Rwtr

So, although the heat transfer index for copper is a big number, and the heat transfer index of HDPE is a very small number, and the ratio of one to the other is something like 800 to 1, when the terms all get summed up and put into the equation, the difference in overall thermal transfer efficiency is nothing like 800 to 1, it is far more modest.

For example...

Gary, over at Build It Solar did has done a huge amount of testing on many aspects of solar heating, but there was one test that he did that I thought could relate to MEMPHIS91's current project...

HERE is the page that detailed the construction and test setup.

HERE is the page that detailed the test itself, and the results. The conclusion odf the test are quoted below:

Quote:

Commercial Copper Collectors
Commercial copper collectors offer known high performance, long life, and good resistance to high temperature stagnation temperatures for all tilt angles. But, they are expensive (about $25 per sqft), and shipping is costly and can be very frustrating (it took three tries to get an undamaged set of commercial absorber plates for my Solar Shed collectors).

PEX Tubing -- Aluminum Fin Collector
From a cost effectiveness point of view, the PEX collector does very well. If you are willing to put the labor in, you can build the PEX collector for about 1/6 th the cost of a good commercial collector, and only suffer a 15% loss in performance. This makes the PEX collector 5 times as cost effective as a commercial collector on a BTU per dollar basis. In most cases, the loss in performance can be made up for just by making the collector a bit larger. You can literally build the PEX collector for about what it costs to ship a commercial collector to your house!
Make of it what you will...

Best,

-AC

MEMPHIS91 07-18-15 06:58 AM

Awesome info AC. I had seen that table of thermal conductivity some where. And you are totally right, flowing water is much much better at giving off its heat. That is why coaxial coils can be so short, and why you have to pay a arm and a leg for all the DX copper. But water being so low also is what makes a pond a awesome thermal storage area.
I have no idea how much flow is at the bottom of the pond. I know that the great exchange will make it move some plus the pond is full of large fish, all that swimming has to move lots of water.
Great articles on the solar stuff to. That is a project for next year.
I got a quote on 150' of 3/4, 500' of 3/8 and 150 of 5/8. It's was right at $1,000. But that was from only one place. After the building of the coax coils and the extra pump cost doing pump and dump is not going to save to much money. And if I'm going to have to dig up my yard to the pond anyway it's not going to save me much work.

I did make sure that DX could handle the overall 30 foot head and it for sure can. Should move the oil around nicely.

jeff5may 07-18-15 07:18 AM

I am eager to see how much energy can be saved by going dx with a pond loop. The increased heat transfer and lack of a circulation pump could be substantial. The added ground transfer will also come into play if you bury and do not insulate the lines. I would almost want yo run them together as in a refrigerator.

MEMPHIS91 07-18-15 08:06 AM

Yes I will be excited to. Gonna start saving up to buy some copper. You think I should not insulate? I'm thinking the ground will be more effected by seasonal temps. Maybe only insulate the return line?
Should 3/4 to and 5/8 be good enough? Also looking at doing more loops with 1/4. Per AC's post the more surface area I can get the better.

Running calculations on the copper pipe surface area. 500' of 3/8 =662 square inches.
1,000' of 1/4 = 590 square inches.


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