Near the end of last month's column, I mentioned one of the means of providing hydronic heat to the dwelling — a five-stage ground source heat pump. This is not an off-shelf assembly; it will have to be constructed from scratch. I am not a qualified refrigerant technician, so I am depending upon the assistance of a qualified and certified refrigerant technician to handle the construction of this particular assembly. Ideally, if I could find a variable-speed ground source heat pump with the ability to transfer thermal energy to the water, I wouldn't need to construct this five-headed beast, but unfortunately no one has a marketable variable-speed unit that can be used off-shelf for this application. Necessity is the mother of invention, and I am the father…
Rumor has it that a few manufacturers will soon be out with a variable-speed compressor package for this type of application, but I can't depend on rumors to heat my space, hence the five-headed beast. My reasoning behind using five 1-ton compressors is that I can cycle compressors on and off, depending upon demand and availability. My loop field will be five horizontal loops, approximately 5-ft. below grade.
These loops will actually be bored through horizontal sleeves strategically placed in the basement walls of the mechanical room. I will not be using the current industry standard plastic polyethylene pipe for my earth heat exchanger, but instead will be experimenting with a copper tube within a tube heat exchanger. My thought is that copper is significantly more conductive than PE tubing, hence I can get by with one-third to one-half the soil exposure given the soils conductive nature. Also, the actual exposed heat exchanger surface area of a 2-in. copper pipe is significantly greater per foot than, say, a 1-in. PE tube.
For example, a 2-in. copper heat exchanger, with an internal 1-in. dip tube would have to have 3.82 liner feet of exchanger per square foot of surface area exposure. One inch PE tubing would require 9.02 linear feet for one square foot of surface area exposure. Therefore, instead of 100-ft. of horizontal bore for a 1-in. PE heat exchanger, I am going to use 33-ft. of 2-in. copper pipe.
I will bore a 3-in. horizontal hole to accommodate the copper heat exchanger. As I insert the heat exchanger into the borehole, I will attach two additional pipes to the outside of the copper heat exchanger. One tremme pipe will be used for sending thermally conductive grout to the bottom/end of the hole, forcing air bubbles out and insuring intimate contact between the heat exchanger pipe and the earth. The other pipe will be a perforated dribbler pipe. After the grout has completely set, I can then pump water into and along the length of the heat exchanger, guaranteeing a good, wet heat transfer between the copper pipe and the surrounding earth. The copper piping will also be protected against external corrosion through the use of cathodic protection to minimize the chance of failure due to potentially corrosive soil conditions.
One of the biggest fall backs of shallow horizontal loop fields is that the performance of the heat exchange loop is severely compromised due to changes in soil moisture and related conductivity during drought conditions.
I will be circulating an antifreeze solution through the copper heat exchanger to extract the low-grade heat for harvesting into the thermal storage tanks of my system. The heat exchanger will be bidirectional as it pertains to heat flow. If I have a need for air conditioning within the dwelling, which is doubtful unless global warming kicks into high gear, I can use the earth heat exchanger in one of two ways: to circulate water between the radiant ceiling/wall assemblies and the heat exchanger (50°F fluid temperatures) and to activate reversing valves on the compressors, making them cooling units instead of heating units.
As I previously explained, if I have excess solar thermal or woody biomass energy available, I can divert and bank that energy into the horizontal loops for harvesting at a later time. I have no idea how well this banking/harvesting program will work, and I will have a sensor located within the horizontal loops, so I can monitor the performance of the system as a whole. I do know that the groundwater aquifer is 45-ft. below the bottom of my foundation, so the possibility of losing energy to “wicking” from a high water table is not an issue. It is my hope that by raising the soil temperature I can increase the coefficient of performance of the heat pump enough to warrant the energy consumed in putting the excess energy back into the ground. Only time will tell, and I know it has been successfully done at numerous sites throughout the world. Now Hydronicahh can be added to the map!
Tune in next month as we continue our journey at Hydronicahh. Until then, happy ultra out-of-the-box hydronicing thinking!
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 720-479-9313.
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