Frequently Asked Questions
Why should I have a “Thermal Response Test” done?
A Thermal Response Test is done to measure certain important thermodynamic parameters of the soil and bedrock surrounding the borehole.
The most important of these are the conductivity, also known as lambda-value or simply lambda.
The conductivity is the ability of the rock or soil to transfer heat. The higher value the more heat can be transferred per time and distance. The SI-unit is W/mK, Watt (or Joule per second) per meter and degree Kelvin (or degree Celsius).
Thus the conductivity is very important to know since it is the most important factor when calculating how much heating and cooling a number of boreholes can deliver.
The normal variation of the conductivity within even seemingly homogeneous geological areas can easily result in a 30 % difference of the amount of borehole meters one needs to drill for a given heat load!
Without a Thermal Response Test one risk to either drill to few borehole meters resulting a poor performance, or to many, resulting in unnecessary costs.
A Thermal Response Test is well spent money and in practise necessary to design a good Geothermal Energy solution!
Other parameters measured during a Thermal Response Test, and also important for calculating the amount of borehole meters, are the undisturbed average temperature along the borehole and the so called borehole resistance, explained elsewhere.
What is “borehole resistance”?
Simply speaking “borehole resistance” is the barrier the heat has to cross going between the borehole wall and the fluid in the heat exchanger. It manifests as a temperature difference between the bore hole wall and the fluid in the heat exchanger.
Its unit (expressed in SI-units) is Km/W which is Kelvin degrees (same as Celsius degrees) and meter per Watt.
The borehole resistance decides the temperature difference between the borehole wall and the heat carrying fluid (to be precise the average temperature of ingoing and out going fluid) in the heat exchanger and is depending on the amount of power added or extracted into the borehole.
An example. If the borehole resistance has a typical value of 0,12 Km/W and a typical amount of power extracted is 25 W/m then the temperature difference will be 0,12*25 which is 3 degrees Kelvin (or Celsius).
The borehole resistance is measured with a Thermal Response Test.
What is “active depth” of a borehole?
Active depth is the part of the borehole where you have thermal contact between the heat exchanger and the surrounding, i.e.the part where heat is conducted.
In practise it is the part where the heat exchanger is not surrounded by air. In non-grouted boreholes the active part of the borehole starts where the subterranean water level is, a grouted borehole is usually grouted in its total length and thus the active depth is the same as the entire length.
What is “grouting”? And why would one do it?
Grouting is when you fill the borehole (after having lowered the heat exchanger) with something like bentonite, concrete or sand. It is done fore one or several of following reasons:
1. The subterranean water level is far down and you want more of the borehole to be active.
2. You want to increase the conductivity between the heat exchanger and the surrounding for more efficient heat transfer, i.e. lower the so called borehole resistance. Water has a rather low conductivity compared to what you grout with,
3. For environmental reasons. To protect against things like oil spill and to prevent hydrological contact between different aquifer systems.
4.The authorities is demanding grouting as a prerequisite for obtaining permit to drill for geothermal energy.