Earth

The earth tab contains information w.r.t. the ground parameters and also the temperature restrictions. Both are explained below.

Earth properties

All borefield calculations start with a correct estimation of the ground parameters. Within GHEtool, there are different ways of setting these ground parameters.

Note

Each country or region has typically its own database with ground data. Please consult these local databases before using the GHEtool default values for more accurate results for your project.

Earth properties
  • Conductivity of the soil [W/mK] The thermal conductivity of the soil (also called the \(\lambda\)-value). The larger this parameter, the better the earth can handle large heating or cooling demands.

  • Ground volumetric heat capacity [kJ/m³K] The volumetric heat capacity of the soil. The higher this value, the more the ground will act like a ‘thermal battery’.

Next, you have different options for choosing the ground temperature data: Measured, Database (default) or Custom.

Measured

When you do a thermal response test or TRT (see also our FAQ: How do you input a TRT?) on a specific location, the undisturbed ground temperature is measured. You can fill it in when you click on Measured.

Database

Most often, especially in early design stages, ground temperatures are not (yet) know. If you don’t have prior knowledge of the location, you can use the default temperature database, which is publicly available here. Below, you can find what the ground temperature and the geothermal heat flux is in that specific location.

Custom

When you have the needed ground data available, you can select Custom and fill in your own ground surface temperature and ground thermal gradient (or flux).

Caution

The undisturbed ground temperature model, as is used the database and in the Custom ground data, is far from accurate. This model assumes a certain ground temperature and a linear temperature increase in function of depth, due to the inherent geothermal gradient.

gradient

Nowadays, especially in city centers, one can notice a big increase in ground temperatures near the surface, do to all the buildings. Here, if you go deeper, the temperature decreases instead of increases (Radioti et al., 2017) 1. This cannot be captured within a simple model and it is therefore suggested to do TRT-tests (see also How do you input a TRT?) whenever you are doing a project nearby a densely built-up area.

Temperature constraints and simulation period

Here you enter parameters w.r.t. the temperature boundaries. Since it is important that over time the ground temperature stays between certain limits, this information is asked on this page.

Temperature constraints and simulation period
  • Minimum average fluid temperature [°C] The minimum average fluid temperature (i.e. the temperature in heating)

  • Maximum average fluid temperature [°C] The maximum average fluid temperature (i.e. the temperature in cooling)

Note

Within GHEtool, the fluid temperatures are the average fluid temperatures, meaning the average between the intlet and outlet fluid temperature. The minimum average fluid temperature is therefore not the lowest temperature you inject into your borefield during the heating peak, but the average between the inlet and outlet temperatures of your borefield.

This is the case since in the background, the only temperature that matters for the heat transfer is the average fluid temperature, and since the inlet and outlet temperatures are depending on the mass flow rate, it is easier to work with an average temperature.

If you typically work with, e.g. a minimum inlet temperature of 0, with a \(\Delta T\) across your borefield of 4°C, you enter 2°C as a minimum average fluid temperature.

  • Simulation period [years] The simulation period in years, oftentimes between 20-60 years.

  • Peak duration heating [hours] The duration of a single peak in heating. For slower emission systems, this is longer.

  • Peak duration cooling [hours] The duration of a single peak in cooling. For slower emission systems, this is longer and typically lower than the duration for the heating peak.

Note

Please note that the peak duration heating and peak duration cooling option is not always visible. It can be hidden when you use calculate using hourly data.

References

1
  1. Radioti, K. Sartor, R. Charlier, P. Dewallef, F. Nguyen, Effect of undisturbed ground temperature on the design of closed-loop geothermal systems: A case study in a semi-urban environment, Applied Energy, Volume 200, 2017, Pages 89-105, ISSN 0306-2619, https://doi.org/10.1016/j.apenergy.2017.05.070.