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-   -   DX geotherm explanation please? (https://ecorenovator.org/forum/showthread.php?t=7394)

solarhotairpanels 12-30-20 08:13 AM

DX geotherm explanation please?
 
I'm curious...

Could you folks that are up on DX explain to me how the DX system creates heat in the pipe that is inserted into the ground and how deep... what size pipe? What are the down falls of a DX system? maintainance issues if any?

Are you putting freon inside the copper pipe?
If so how are you transferring that heat to your heat pump?
Side by side heat exchanger piping?

Is the main objective on a DX system to assist your existing heat pump during heating or cooling times?

How perfect does everything have to be for it to run without issues?

Thanks in advance!

jeff5may 12-30-20 11:24 AM

Ok so a little background on the phase change heating and cooling system first.

Any air conditioning or heating system that relies on the phase change system moves heat via mass flow of refrigerant. More mass flow equals more heat flow. Every other variable in the system that affects performance performance affects mass flow.

The phase change of refrigerant (liquid to gas absorbs heat, gas to liquid releases heat) is where most people encounter difficulty when trying to understand the system. This is the "magic" of the system. It's pretty well explained in this thread:

https://ecorenovator.org/forum/showthread.php?t=3160

The main difference between a DX loop and indirect exchange is the efficiency gain of not having a pump that robs power from the overall efficiency of the system. Other potential advantages are:
- heat exchanger maintenance near zero
- no intermediate heat transfer media to bottleneck heat flow

jeff5may 12-30-20 01:55 PM

So with an understanding of the overall heat pump refrigeration cycle, I will focus on the geothermal/ground source end of the system.

With a ground loop based system, the two main things that contribute to heat flow are surface area and temperature difference. With a conventional (glycol, alcohol, water, etc.) based system, the refrigerant exchanges heat through a refrigerant to water hx. The liquid is then pumped through the ground loop. This leads to a reduction in system efficiency due to the reduction of delta temperature between both the refrigerant to fluid and the fluid to ground loop. Also, a circulation pump must be employed to move the heat transfer fluid.

With a DX system, the refrigerant flows through the plumbing directly through the ground loop. Much like a passive solar system, the heat is radiated directly through the plumbing and into or out of the ground. In this heat transfer method, the main limit to heat flow is the immediate temperature of the ground. Since dirt or rock has a relatively low ability to absorb or release heat compared to the pipe, we have localized gradients forming in the ground and this heat travels slowly.

Historically, in general, the HVAC industry seeks to minimize the system that contains pressurized refrigerant. Copper, bronze, aluminium and other metals cost more than a circulation pump, so the vast majority of manufacturers build a refrigeration loop that's "inside the box". This also leads to a smaller mass of refrigerant in the loop, which both simplifies troubleshooting and reduces cost. With a water to water system, the pump controls for both sides, and sometimes one or both pumps are in the same box as the heat exchangers and compressor.

jeff5may 01-02-21 03:25 PM

So, ignoring the residential HVAC industry's reluctance towards anything with "freon" outside "the magic box", a DX ground loop is no more difficult to construct than a glycol or water based ground loop. The ground volume that serves as the "heat well" is going to act the same regardless of the heat transfer plumbing and/or fluid. It's a balancing act: excess well capacity to ensure super efficiency versus material and construction cost. Naturally if you feel like a larger size field or larger diameter piping is necessary, it's going to cost more.

I saw that you had something to say in Jake's (Memphis91) ground source water heater thread lately. That's another one to read all the way through. He ran a single short borehole from some window ac unit guts, built a custom DX coil to retrofit his electric water heater, and tweaked and tuned for a couple of rounds until the numbers added up. Ran what he made, been going strong for years.

The only maintenance I know about is Jake fiddling with the system. The original DX heat exchanger is cap tube metered, custom optimised for his borehole. He caught the tinker bug and decided to try out a super efficient dehumidifier idea. Repurposed what looks like a 2 ton air handler HX and a computer case fan, and metered it with a thermostatic expansion valve. Chop, cut, rebuild, voila! Summer/Winter selectable humidity control!


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