Low cost geothermal heat pump: First shots, design and ground loop calculation.

Discussion in 'General Discussions' started by John_geo, Jan 12, 2018.

  1. John_geo

    John_geo New Member

    Dear forum members:

    My name is John, living in Chile, Industrial engineer as profession. I have passion and lot of interest in thermodynamics and heat process. I have a small house “bungalow” in the country (60m2 / 650ft2) that has a 3000W (0,85 Ton) full electric heating system (no natural gas available). Electric bill is too expensive and would like to explore new possible solutions. This heating system is OK and in cold winters days (-10°C / 14°F) has passed all tests maintaining an internal temperature of 20°C / 68°F. I have invested in insulation and that helps a lot. During cold winter months, system has a load of almost 80% - 90% so I guess 3000W (0,85ton) is near the real thermal need for the house. If I have to guess, the system will run approx. 4000 hours/year.

    I am planning to develop a low cost geothermic system using a standard 3300W split heat/cold Freon R-410A air condition with inverter technology. This type of equipment claim to have A class electric efficiency so I guess the COP is near 3,5 using the standard installation in standard temperatures. I will take this as a personal project and try to learn as much as I can in the way. I know this won’t be easy! Furthermore here we have no companies that make this job so I guess it a good opportunity to try.

    Some important points:

    1. Adaptation of the system: My idea is to “disassemble” the external air condition unit and immerse the “Freon-air” heat exchanger in a punt / small tank (fan is removed previously). Some copper tubes adaptations of the Freon exchanger will be needed. All the other parts of the external unit remain the same. The punt / tank will have 1 output to the pump (1/2 HP) that will feed the horizontal ground loop and an input that will receive the heated liquid from the horizontal ground loop. The idea is to create a turbulent ambient inside the tank to improve the heat exchange between the ground loop refrigerant and the Freon heat exchanger. Maybe a mixer is needed inside the tank. House internal heat exchanger is the standard “Freon to air and a fan” of the equipment.

    2. Another possibility instead of 1) is to “remove” the standard external exchanger and buy and install a “counter current” or “co-current” Freon-water heat exchanger. This will be easier as no punk / tank is needed. The bad thing about this solution is that I will need an expansion tank to absorb volume changes of the ground loop. I think this solution will have better results.

    3. Horizontal ground loop setup. Soil has low water content, 50% sand - 50% silt. A good guess is that thermal conductivity is near 1 or 1,2 W/(m.K). Sun hits all day (no trees). Annual rain fall is 800mm, mainly during winter. The horizontal loop I have in mind has 5 trenches of 2,5m / 8,2ft depth, 1m / 3,5 ft wide and 10m / 33ft long each. The garden sector that will have the 5 trenches is near 120m2 / 1300 ft2, dimensions are 11x11m / 36x36ft (sorry but I have more no free space, this is my limitation). Each trench will have 100m / 328 ft of ¾” black polyethylene K4 tube, total tube of 5 trenches is 500m / 1640 ft long. In summary: x5 2,5m / 8,2ft depth trenches with a total length of 50m / 163ft and a total tube of 500m / 1640 ft. Do you find these values somehow correct?

    4. Flow rate: I have done some math and at a flow of 6 LPM = 1,6 GPM I will have a Reynolds of 8400 (turbulent). At this flow, this means that I need a minimum delta temperature of 7°C between the input and the output of the horizontal loop to get 3000W at the heat exchanger in the tank. Pressure drop at this flow is 17,3m / 57 ft =1,73 BAR so a standard 1/2HP pump is big enough and I will probably get a real flow of 9 LPM / 2,5 GPM. Do you find this numbers ok?

    5. Refrigerant. The idea is to add some type of refrigerant to get near -15°C / 5°F freezing point. Main reason is that Freon heat exchanger could freeze in the tank. What type of refrigerant would you recommend? Is standard car refrigerant OK? For further information, my idea is a total refrigerant volume of 200 liters/ 52 Gal in the tank (that will have sunken the Freon exchanger) and approx. 80 liters / 21 Gal in the ground loop.

    6. Local weather has a mean annual temperature of 8.5 °C / 47°F. From what I have read it might mean that ground temperature near or below 3,7m / 12ft will tend constant to this value. Obviously the nearer the surface level, the more it will fluctuate with seasonal changes. Can I assume that the refrigerant fluid during winter (worst scenario) will output the horizontal ground loop near 6 °C / 43°F? See point 2) for horizontal loop setup.

    To many questions! I will really appreciate if you can help me find a way in what you can!

    Best regard, have a have a nice day!

    Last edited: Jan 12, 2018

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