Thermal Resistance

The borehole equivalent thermal resistance (Rb*) tab has two major differing layouts: the one where you can enter a Constant borehole equivalent thermal resistance (see also are FAQ: How do you input a TRT?) and the one with a lot of flexibility: the Dynamic page. The view you see, is determined in the Options tab.

Constant

When you have measured (or estimated) a certain constant borehole equivalent thermal resistance, you can enter it here. Typically, this value ranges from 0.05-0.25.

Input for the constant borehole thermal resistance

Warning

Using TRT-data for the aim of determining the required depth is dangerous, since the measured borehole equivalent thermal resistance is only valid for one specific depth (since this parameter is a function of the depth). Since this aim changes the depth of the boreholes the borehole resistance will change as well. This effect can be very significant if you have a fluid regime on the border of laminar/turbulent flow (see also What is the Reynolds number?).

Please use this only as a first estimate and for final depths that are in line with the depth of the measured borehole.

Dynamic

Most of the time, however, you are working with a Rb* that is calculated dynamically. This gives you the flexibility of playing around with certain parameters and really get a feel of how they influence the borehole equivalent resistance. In order to be able to calculate the resistance, you have to enter values w.r.t. the Fluid parameters and values w.r.t. the Borehole internals.

Hint

You can vary the pipe and fluid parameters and immediately see the effect on the equivalent borehole thermal resistance below.

Fluid parameters

The fluid parameters can be set either by using a certain % of glycol to form a water-glycol mixture or by setting the fluid parameters all by yourself.

Caution

Be aware that there may be regulatory rules in your region w.r.t. what fluids you can use for geothermal applications.

Input for the fluid glycol parameters
  • Type of glycol Select the type of glycol you want to use.

  • Percentage of glycol [%] The percentage of glycol in the glycol-water mixture.

  • Reference fluid temperature [°C] The reference temperature for the fluid properties.

  • Mass flow rate [kg/s] Mass flow rate per borehole.

If you have a very specific fluid, you can enter its properties directly into GHEtool if you select Custom in the Fluid properties input.

Input for the custom fluid parameters
  • Thermal conductivity [W/mK] Thermal conductivity of the fluid.

  • Density [kg/m³] Density of the fluid.

  • Thermal capacity [J/kgK] Thermal heat capacity of the fluid.

  • Dynamic viscosity [Pa s] Dynamic viscosity of the fluid.

  • Mass flow rate [kg/s] Mass flow rate per borehole.

Warning

Please make sure you enter the dynamic viscosity and not the kinematic viscosity. You can convert one into another by using: \(\mu = \nu \cdot \rho\), where:

  • \(\mu\) is the dynamic viscosity [Pa s] (= [Ns/m²])

  • \(\nu\) is the kinematic viscosity [m²/s]

  • \(\rho\) is the density [kg/m³]

Note

The Mass flow rate within GHEtool is the mass flow rate per borehole. This means that if you have for example 0.2kg/s of flow rate and a double U-tube, this gives 0.1kg/s of flow through each tube. If you have a single U-tube, this 0.2kg/s will pass through this single U-tube.

Hint

If you want to take into account a higher flow rate when working with a series connection of borehole, you enter the resulting fluid mass flow rate into GHEtool. For example, you have a total mass flow rate of 0.4kg/s for 4 boreholes, with 2x2 in series, you enter 0.2kg/s as a mass flow rate into GHEtool.

Note

Within GHEtool, you can set a reference temperature for your fluid parameters (see Thermal Resistance), which has an influence on the Reynolds number and hence the thermal behaviour of your system. By default, this reference temperature is 10°C, which is more or less the undisturbed ground temperature.

If you want a more conservative approach, you can set this temperature to the maximum or minimum average fluid temperature. In that case you are sure that, whenever the thermal resistance matters the most (i.e. during the peak loads), they are calculated with the worst case fluid parameters.

Borehole internals

For borehole internals, there are two major options: using U-tubes or using a coaxial pipe.

Input for the U-pipes in the borehole
  • Number of pipes [-] The number of U-tubes in the borehole.

  • Grout thermal conductivity [W/mK] Thermal conductivity of the grout.

  • Pipe thermal conductivity [W/mK] Thermal conductivity of the pipe. (This mostly stays unchanged.)

  • Inner pipe radius [m] The inner radius of the pipe in meter.

  • Outer pipe radius [m] The outer radius of the pipe in meter.

  • Borehole radius [m] Radius of the borehole in meter. (This input is duplicated from the Borefield tab.)

  • Distance of pipe until center [m] This is an estimation of the locations of the pipe inside the borehole.

  • Pipe roughness [m] The roughness of the pipe. Most of the times, since we are working with PE, this is very small.

Note

The pipe roughness plays a role in when a fluid goes from laminar to turbulent flow (see also our article on What is the Reynolds number?). Some manufacturers use rough pipe internals so they achieve a turbulent flow at lower mass flow rates. If this is the case, please contact the manufacturer for the correct data to put into GHEtool.

You may also always contact us at info@ghetool.eu if you want a specific brand to be implemented in the software.

Input for the coaxial pipe parameters
  • Grout thermal conductivity [W/mK] Thermal conductivity of the grout.

  • Pipe thermal conductivity [W/mK] Thermal conductivity of the pipe. (This mostly stays unchanged.)

  • Inner pipe inner radius [m] The inner radius of the inner pipe in meter.

  • Inner pipe outer radius [m] The inner radius of the outer pipe in meter.

  • Outer pipe inner radius [m] The outer radius of the inner pipe in meter.

  • Outer pipe outer radius [m] The outer radius of the outer pipe in meter.

  • Borehole radius [m] Radius of the borehole in meter. (This input is duplicated from the Borefield tab.)

  • Pipe roughness [m] The roughness of the pipe. Most of the times, since we are working with PE, this is very small.

Note

The pipe roughness plays a role in when a fluid goes from laminar to turbulent flow (see also our article on What is the Reynolds number?). Some manufacturers use rough pipe internals so they achieve a turbulent flow at lower mass flow rates. If this is the case, please contact the manufacturer for the correct data to put into GHEtool.

You may also always contact us at info@ghetool.eu if you want a specific brand to be implemented in the software.