Geologic Thermal Performance in Geothermal Systems

Published On: June 20, 2024By Categories: Drilling, Features, Geothermal Technology

A look at cost implications that come with drilling and installing geothermal heat exchange wells in specific formations.

By David Henrich, CWD/PI, CVCLD

Designing geothermal heat exchange systems is a never-ending evaluation of feasibility.

Whether it be financial feasibility, site suitability, resource availability, or any other of the wide range of factors that influence the prospect of installing a geothermal system.

There are many factors that ultimately need to be considered before installing a geothermal heat exchanger (GHX). This article will focus on one aspect of site suitability: geologic thermal performance.

We’re going to look at different types of geology, analyze soil and rock’s ability to transmit heat, and consider the cost implications that come with drilling and installing geothermal heat exchange wells in these formations.

To start, let’s look at Table 1 that lists some of the thermal properties of soil and rocks.

To a trained driller’s eye, you’ll immediately notice that the majority of rocks with the highest thermal capacity also happen to be some of the more difficult types of formations to construct in. Limestone, for example, can be hard to drill through and its karst forms can consume large amounts of excess grout. Sand and gravel, on the other hand, can be fairly straightforward to construct in, but offer only middling thermal
conductivity and thermal diffusivity values.

Soil thermal performance is generally broken down into two primary components when used in the calculations for a geothermal design: thermal conductivity and thermal diffusivity. From the National Ground Water Association’s Lexicon of Groundwater and Water Well System Terms:

Thermal conductivity: “The rate of heat flow per unit area for a unit thermal gradient normal to that area. Analogous to hydraulic conductivity.”

Diffusivity: “The ratio of conductivity to storage capacity. Examples include thermal diffusivity (thermal conductivity/specific heat) and hydraulic diffusivity (hydraulic conductivity/specific storage).”

In layman’s terms, conductivity is how much energy can be moved through soil and diffusivity is how fast soil can transmit heat. Although these two components are certainly two halves of a whole, in terms of design impact, the thermal conductivity has an outsized influence on loop field sizing compared to thermal diffusivity.

These are two critical inputs when designing a geothermal system. Thermal conductivity (TC) can be easily tested through several different methods, and thermal diffusivity can be tested but is regularly calculated or estimated when doing a geothermal design.

In-situ thermal conductivity testing of geothermal borings has been in practice for several decades. The most common method for testing this property for geothermal borings is the transient line source method. As it pertains to geothermal heat exchangers, this method entails injecting a constant rate of heat into a fluid that is circulated through the loop tube. Thermal data and power input is recorded and analyzed for thermal performance.

Figure 1. Thermal conductivity test report results.

Standard TC tests can measure or calculate a few properties of the GHX. The first and most important is thermal conductivity. The average ground temperature can be measured, provided the system is left to circulate for a period of time without an addition of heat.

Another value that can be calculated is the borehole thermal resistance. This value is useful as a check to ensure the GHX that was specified is what was actually installed. You can see some of these values in the chart shown in a Ground Loop Design TC report in Figure 1.

Thermal diffusivity is a little more challenging. Measuring this value in-situ through conventional means is difficult. There is some research into this area using heat regression analysis from collected data, but largely this value is calculated by the thermal conductivity test professional. In almost every method used to determine this value, some approximation will need to be made. That can make this input a little less reliable but using experienced professionals can ensure reliable results.

When you think about these values and how they affect the design, we can begin to analyze the actual GHX selection. Let’s take a look at the section shown in a drilling log in Figure 2.

There is quite a variety of soils and bedrock to select from. Referring back to Table 1, we can see that the best thermally performing area is a limestone layer.

However, the limestone starting at 248 feet is known to have lost circulation issues. The cost benefit of drilling and then installing a GHX into this formation simply does not exist. Although the thermal conductivity may have increased by as much as 30%, the drilling costs could nearly double.

Figure 2. Drilling log measurements in various formations.

A situation like this highlights the ever-present economic analysis that exists through the entire design process. Working within the bounds of geology on the site and evaluating the proficiencies of the drilling companies in the area is one sure way to deliver maximum value for the building owner.

I’m often asked what formations work best and what depths should we target? Of course, the answer is “It depends.” Yes, it depends on several factors.

The first is local or regional drilling assets. Which types of rigs and drilling disciplines are going to be available to perform this work? Forcing drilling companies out of their comfort zone is a sure way to increase risk and thereby increase costs.

The second is to make the best use of the formations available. When drilling in highly conductive formations, make sure to drill the entire length of that formation.

For example, the need to install surface casing certainly adds cost to any conventional closed loop project. But once the surface casing is installed, how deep can you take the borings? Which formations provide the best thermal performance, and can those formations be drilled effectively? Balancing the answers to these two questions will most assuredly lead to a cost-effective solution.

A third item to consider is selecting a depth and bore location layout that the site can accommodate. In some instances, space limitations on a site may simply require you to select depths that may be more difficult to achieve. If site limitations are playing a major role in selecting the appropriate bore depths, refer again to the previous paragraph to be sure to maximize the GHX’s ability to transfer heat with the ground.

______________________________________________

Soil thermal properties need to be considered when selecting the final configuration of a GHX. Like many other aspects of geothermal system design, the selection will need to be balanced against other factors. Ensuring the thermal performance of the soil is properly weighed versus the cost of installation will yield a system that will operate efficiently and effectively for decades.


David Henrich, CWD/PI, CVCLD, is president of Bergerson-Caswell Inc. in Maple Plain, Minnesota. He served as president of the National Ground Water Association in 2018 and chose the mantra “Better Together” as the theme of his presidency. Henrich received NGWA’s most prestigious award, the Ross L. Oliver Award for outstanding contributions to the groundwater industry, in 2022. He can be reached at david.henrich@bergersoncaswell.com.

Read the Current Issue

you might also like