Several times we've been asked to design a GCHP system when the client has already drilled a test borehole (in a couple of cases 2 test boreholes) and had someone conduct a thermal conductivity (TC) test. The test borehole and TC test were conducted before the size and footprint of the building had even been established and there was no idea of what the heating and cooling requirements of the proposed building would be. What a waste of money! Don't get me wrong...
I'm not by any means suggesting that it's a waste of money conducting a TC test, but when during the scope of the project it is done can make a big difference in how deep the test borehole is, where it is located, and what size of pipe is inserted in the borehole.
The cost of a drilling a test borehole typically ranges from $8,000 to $20,000 depending on the depth, geology and location. Conducting the actual thermal conductivity test requires about 3 days to complete and the cost typically ranges from $5,000 to $10,000, making the total cost in the range of $13,000 to $30,000. That's a lot of money, and you want to ensure you get the greatest value when you're spending it.
A TC test is conducted by drilling a test borehole, inserting a U-tube into the borehole, grouting the borehole, filling the U-tube with water, circulating the fluid through the U-tube and a heating device and recording the water temperatures into and out of the U-tube every 1 to 2 minutes for approximately 48 hours. The data is used to measure the average thermal conductivity of the soil and/or rock over the length of the borehole. Note that the test measures the average conductivity of the full length of the borehole. This is important because different soils and rock have different thermal properties. This means that 75' (23 m) of clay overburden conducts heat away from the U-tube at a rate 80% lower than the 400' (122 m) of limestone below it. The average conductivity of 200' (61 m) borehole would be significantly less than a 450' (137 m) borehole.
In determining the feasibility of a GCHP system one of the areas we, as designers, can bring the greatest value to the project by working with the owner and design team to reduce and balance the energy loads of the building. We do this by developing a detailed energy model of the proposed building and (this is important) using the energy model throughout the design phase to recommend changes to the building that will reduce the size of the GHX.
We work with the building because it provides the greatest opportunity to reduce the size and cost of the GHX. Building components and systems can be changed...the geology and the land area available for construction of a GHX can't be changed.
And most important: the energy model provides us with the information needed to calculate how much pipe we have to place in the ground to meet the building loads. Together with information from geologists, local drilling and excavation contractors, it's possible to develop a reasonably accurate budget cost of the GHX needed for the project...and part of the answer the owner needs to build a financial model of a GCHP system for his or her building.
For most projects information is available from geologists, water well records, drilling and excavation contractors to estimate the thermal properties of the soil and rock in a particular location. Drilling contractors can provide good information about the drilling conditions in an area...what depth can they drill to at a reasonable cost. With this information and information from the energy model it is possible to develop a preliminary model of the GHX based on an appropriate drilling depth for the area, the geology and the land area available to determine what is the appropriate depth to drill a test borehole to.
Only after estimating the number of boreholes, borehole depth, U-tube piping size and exact building location (to allow the test borehole to be located where it can be used in the actual system) should the test borehole be drilled and a TC test conducted to confirm the assumptions made during the feasibility assessment.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.
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