refrigerant pressure in DX ground source heat pump

[fholler]

Hi there, I am interested in installing a direct exchange (DX) GSHP in a vertical borehole in my place. I heard that installers braze the U-bend at the end of the copper tubes before they insert it in the borehole. Should I be worried about leaks, especially at high pressures? Are the seals reliable for conventional GSHP refrigerants? Are they reliable for CO2 refrigerant which requires higher pressures??
Linked to that, I have a theoretical question (sorry I am not an expert): let's imagine that the ground loop of copper tube in the ground is somehow seamless and perfectly sealed. Would that mean that we could apply higher pressures to the refrigerant and hence get a higher COP fro the pump, or is this inexact?
Thanks for your help
Fabien

[Contactor]

Hi there, I am interested in installing a direct exchange (DX) GSHP in a vertical borehole in my place. I heard that installers braze the U-bend at the end of the copper tubes before they insert it in the borehole. Should I be worried about leaks, especially at high pressures? Are the seals reliable for conventional GSHP refrigerants? Are they reliable for CO2 refrigerant which requires higher pressures??
Linked to that, I have a theoretical question (sorry I am not an expert): let's imagine that the ground loop of copper tube in the ground is somehow seamless and perfectly sealed. Would that mean that we could apply higher pressures to the refrigerant and hence get a higher COP fro the pump, or is this inexact?
Thanks for your help
Fabien

You need to pressure leak test at high presure to ensure the joints are not leaking. Assuming you make the joints correctly they will hold pressure.

If you can avoid joints altogether obviously this is a better method but as long as the work is done by someone who knows what they are doing it will be fine.

It's the thickness of the pipe wall that gives it it's strength, this may give you a choice of refrigerants, but you won't have any control over running pressures or the COP which should be considered at the design stage.

[frank]

Sorry for my ignorance.....but the only ground loops i have seen are MDPE pipe.

Do copper pipes get buried directly into the earth?

[One Man]

Yes sir, in a direct exchange system the copper is in direct contact with the earth.

[Peter_1]

Sorry for my ignorance.....but the only ground loops i have seen are MDPE pipe.

Do copper pipes get buried directly into the earth?

Sure they do Frank, we have some running. There ae even systems with the condenser pipes burried as a radiant heating in the house. A friend of mine has these running since +/- 1992 in his house.
Much better COP and SPF then glycol systems. No pumps needed, no antifreeze, no heat exchanger...Therefore, you can evaporate +/- 6K higher because you don't need a heat exchanger.
Thermodynamically seen, this is the best machine.

We achieve COP's of 5.1 while it's outside freezing -10°C. Daikin or Mitsubishi only can dream of these numbers.

Problem for DX systems, you need to be a refrigeration mechanic and many don't see the link with a cooling technician to heat a home. Plumbers and heating technicians choose for the glycol systems because they don't need to get into the refrigeration system. It's like installing a common heating system for them.
They also don't need to be certified (F-gas) with a glycol system.

@fholler, the higher the LP, the better your COP will be but as Contactor said, this is something for the designer.
Anyhow, if you want to use it also to cool your house, it must be able to withstand the HP of the system.

If they're soldered well an pressurized afterwards, then I don't see any problem.

Look once on the sites of Nordic (US) or www.gshp.org.uk or www.gshp.org or Waterkotte Germany or Masser Belgium ECR Technologies Florida, Heliotherm, Oschner (Austrich), ...

[desA]

Excellent post, Peter.

[Peter_1]

Thanks DesA, I can add to this, drill you holes vertical holes not deeper than 25 m (75 ft) , otherwise, I see problems with oil return. A 3/8 and 1/2 will do the job and you can count on a average of +/- 2 kW/ hole.
A common house in Belgium needs +/- 14 to 18 kW, so 7 to 9 holes.
To use it also as a condensor, you will need 50 to 60% from this holes to condense.

Set a 7/8 pipe over the 3/8 -1/2 junction at the bottom and this will work as an oil collector/siphon.

A good technique for drilling: set the auger drillmachine and drill a hole under an angle of +/- 30° to 45°. Then rotate the drill around its axis +/- 50° and drill a second hole and so on till you have your desired capacity.

All your tubes are then concentrated on 1 central point (where the drill stood) where you can connect them all together very easily at this point.

A nice page from Masser http://www.masser.be/nl/warmtepomp.htm where you can see both indoor and outdoor copper coils.

[desA]

Set a 7/8 pipe over the 3/8 -1/2 junction at the bottom and this will work as an oil collector/siphon.


Could you explain this a little further, please, Peter. Perhaps a simple sketch of your thoughts?

[Aik]

Peter_1, very useful information, thanks.
In the nearest future I'll plan work up with theory of calculation evaporator's capacity (tubes in ground). But tube's disposition will be horizontal below the ground frost penetration point. Now I collect informations...
Very interesting to compare theory and practical results...

[Bigfreeze]

We install CO2 probe systems but they are not strictly DX. The probe is filled with CO2 at High pressure. At the top of the bore there is a heat exchanger which exchanges the heat collected by the evaporating/condensing CO2 and tranfers to a R410 DX system. Its a patented technology and returns a COP of about 5.5.

You will have huge problems with oil return if you plan to use purely DX with the refrigerant being pumped from the compressor and actually down into the bore.

[Bigfreeze]

Peter_1, very useful information, thanks.
In the nearest future I'll plan work up with theory of calculation evaporator's capacity (tubes in ground). But tube's disposition will be horizontal below the ground frost penetration point. Now I collect informations...
Very interesting to compare theory and practical results...

If you're using a horizontal Dx system you should be looking to retieve between 12 and 15W per linear meter of collect pipe. Much more than that and you'll create problems with the ground freezing up.

[Peter_1]

If you're using a horizontal Dx system you should be looking to retieve between 12 and 15W per linear meter of collect pipe. Much more than that and you'll create problems with the ground freezing up.

Freezing up depends also very much how many holes you make.

[Bigfreeze]

Freezing up depends also very much how many holes you make.

I was speaking of the horizontal system which would be one big hole :). Pipes usually spaced 500-600mm apart

[Aik]

If you're using a horizontal Dx system you should be looking to retieve between 12 and 15W per linear meter of collect pipe. Much more than that and you'll create problems with the ground freezing up.
This problem I want to solve... I want calculate the distance between two neighboring pipes in wich ground wouldn't freezing up at one piece. Of course, ground around the pipe will freeze up but it will local freezing and on summer it will defreeze, because refrigerant cycle will reverse...

[Peter_1]

I was speaking of the horizontal system which would be one big hole :). Pipes usually spaced 500-600mm apart

Well even the, the more circuits you install, the less prone the system will be for freezing up because you increase the evaporating temperature.

[Peter_1]

....
You will have huge problems with oil return if you plan to use purely DX with the refrigerant being pumped from the compressor and actually down into the bore.
I have several vertical DX's running, the oldest one more than 10 years old without any oil return problem. Proper speed for oil return is important. And not too long tubes of course . We use max 20 m depth.

Regarding a COP of 5.5: I know the patented technology with the CO2 (Ochsner) but a COP of more than 5 is more correct and even this difficult to achieve during the whole season and in all types of soil but this COP of 5 is not due to the CO2 used. It's only related to the temperature lift of the refrigerant in the second circuit.
Don't forget, you have one additional heat exchanger in your evaporator stage which decreases COP.
You can't change nature nor thermodynamic laws.
And finally, a good SPF is much more correct then a COP and in the very near future, all heat-pumps will have to give their SPF instead of COP.