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Formation Thermal Conductivity | FTC Position

Geothermal Resource Technologies, Inc. (GRTI) understands that the formation thermal conductivity value is very important in the design of the ground-loop heat exchanger for a GHP system. We also fully understand that accurate information leads to accurate conclusions. There has been considerable discussion over the past year concerning test procedures for formation thermal conductivity testing. Some believe that short time duration tests, while not as accurate, are acceptable because they produce results (most often a low formation thermal conductivity value) that tend to lead to conservative loop design lengths. We believe that it is perfectly acceptable to take a conservative approach to the ground loop design if economics allow it. However, we believe that any design decisions belong solely to the design engineer, and our job is to provide a formation thermal conductivity value that is as accurate as realistically as possible. Therefore, GRTI strives to provide information that is as accurate as possible. Our clients hire GRTI due to that fact.

Understanding the Variables

testing unitIn the ideal situation, an extremely accurate conclusion can be made when the analyzer is certain that the data has fully stabilized and that there are no variations in any of the variables. However, the formation thermal conductivity testing process is far from ideal due to the fact that it is done in the field in extremely difficult to control conditions. There are basically two avenues of pursuit for accurate information in the field; one can attempt to fully control all variables, or one can collect enough data that, through an averaging process, the inaccuracies will have minimal effect on the conclusion. Since full control of all the variables in the field is extremely difficult, if not impossible, GRTI has concluded the latter is the best option to achieve accuracy.

Through the years of research that GRTI's principals have been affiliated with, we have concluded that there are certain patterns that the variables tend to follow. As an example, when measuring power, most publicly available power systems have a 24-hour cyclical pattern that reflects system load usage. Similarly, generated power at the test site is usually sensitive to the ambient temperature that will most often vary throughout the test period. Since the stability of the power supplied to the test is the key component to reaching an accurate conclusion, these patterns must be taken into account. Without enough data to be able to recognize an occurring pattern, an inaccurate conclusion can easily be obtained. Also, without sufficient data, one cannot determine if a pattern is occurring. Therefore the duration of the test also becomes extremely important and is, in itself, another variable

Test Duration

The experience of the individuals involved with GRTI that has led to our understanding of the important test variables has also lead to our commitment in making every attempt to collect data for 40 to 48 hours. We also know that, due to the analysis type, data collected in excess of 48 hours is very difficult to use due to the analysis procedure’s increased sensitivity of variations in the power input and data accuracy. We have also found that it typically takes between 10 and 12 hours for most tests to show conclusive signs of stabilization. Therefore, we will typically discard the initial 10 to 12 hours of data prior to analysis, typically leaving 28 to 38 hours of usable, stabilized data. The balance of the data is then used through an averaging process in the analysis.

Example Data Graph

Collecting uninterrupted data for a period of 40 to 48 hours can be extremely difficult at times, and is certainly inconvenient almost all the time. Therefore, we are as interested as anyone in finding a process that might result in reducing the amount of data required in order to achieve a repeatable level of confidence equal to or greater than that which we currently employ. However, until such time that independent, industry recognized research proves that such a process is available, GRTI is committed to continue in taking the position that data needs to be collected for a duration of between 40 and 48 hours.

Conclusion

GRTI is committed to providing its clients with an analysis that is as accurate as realistically possible. Our experience enables us to be in a position to assist our clients in making informed decisions that will lead to increased confidence in both the design process and system performance. In turn, we believe this approach will result in increased acceptance of the GeoExchange technology.