Geothermal Well Drilling—Demystified Part 4: TGCT Test Analysis & Loop Design Considerations

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By David Brown, Vice President of Yadkin Well Co, Inc.

You just received the analysis report from your thermal geothermal conductivity test (TGCT). Now what?

As stated in Part 3, this report is typically only a few pages long, which you may find surprising considering the test takes 48 hours to complete. However, keep in mind that the thrust of this report is contained in the three values we discussed in Part 3: thermal conductivity, thermal diffusivity, and the undisturbed formation temperature. It's now up to the loop designer—preferably a certified GeoExchange Designer (CGD)—to apply this information to the project as a whole.

The CGD will need all of the following to create an efficient and long-lasting loop design:

  • Ground thermal properties (thermal conductivity, thermal diffusivity, and ground temperature)

  • Borehole properties including diameter, planned spacing, grout and piping materials, fluid type, and estimated flow rate.

  • Peak heating and cooling loads on the heat pumps (provided by the mechanical engineer)

  • Performance data on chosen heat pump brand/model(s)

It’s critical that the building design be complete before the loop designer sets to work. Any architectural change that impacts the building's heat gain or loss (e.g., windows, roofing materials, adding or subtracting square footage, etc.) is likely to impact the peak heating and cooling load and, thus, the required loop capacity. No one—especially the owner—wants to learn that the loop field he or she just paid to have installed was based on outdated information.

Short- and Long-term Considerations

The CGDs' efforts and expertise ensure that the loop has the capacity to efficiently meet demand and will continue to perform efficiently over the next 20+ years. Most CGDs design wellfields using specialized design software like LoopLink® PRO. This type of software facilitates the design process, especially the extensive modeling that helps the designer optimize the loop for long-term performance.

One of the most challenging aspects of loop design is making sure that the loop field can consistently meet peak loads without depleting the loop field's heating or heat storage capacity over time. Loop design involves creating a long-lasting balance between the charging and discharging of heat to and from the earth. If, for example, annual cooling loads consistently exceed annual heating loads, the ground may become heat-logged over time. Maximum temperatures may start to exceed design parameters, reducing capacity and efficiency. A similar result can occur on heat-dominant systems when loop temperatures fall below design. All of this can be avoided by right-sizing the loop, accounting for all the factors that impact its performance, including climate and the heat dispersion properties of the formation.

We will explore the nuances of geothermal heating and cooling design in our next series!

David Brown is the Co-owner and Vice President of Yadkin Well Co, Inc. in Hamptonville, NC. He and his technicians drill water and geothermal wells throughout North and South Carolina. The business, purchased by David's father in 1964, is now owned by David, his son, Matthew, and his brother, Vaughn. The Brown family has been drilling wells for geothermal installations since 1985.

David is a Certified Well Driller/Pump Installer (C.W.D./P.I.), a Certified Vertical Closed Loop Driller (Geothermal) (CVCLD), and a Geothermal Accredited Installer (AI) and inspector (CGI). He is also a North Carolina Level A contractor. For more information, click Yadkin Well Co.