BTES load matching

When simulating a BTES system in Hysopt, the user should first load match it with this template, which is based on a TRT (Thermal Response Test). The correct way to load match is with an already executed TRT. More information about a TRT can be found in BTES 1.0 or BTES 1.4.

A TRT and this template have the following principle. A production (typically electric) is controlled to make sure the temperature difference between the ingoing and outgoing temperature of the probe is constant (in this case 4 K, the user should change the constant value depending on the executed TRT). A constant volume flow is delivered by the pump, which results in a rather constant thermal power injected into the soil.

The following results come from a TRT and should be inserted in the BTES-BC:

Average thermal power injected [W] → Cold deposit power

Undisturbed (natural) soil temperature [°C] → Natural soil temperature

If the pressure drop is also given from the test, the KV value can be calculated by clicking on the pencil icon visualised above. After clicking on the pencil icon, a popup window appears. In this popup window, the user can calculate the KV value by entering the pressure drop and the volume flow rate (which is also given by the TRT).

The remaining parameters needed to correctly simulate the BTES system are found by clicking on the pencil icon of the simulation settings.

After clicking on the pencil icon, a popup window appears with remaining simulation settings. More information on these settings can be found in BTES 1.0 or BTES 1.4.

The soil capacitance, the soil start temperature and the thermal loss coefficient should be load matched with the given trend function from the TRT. In the graph below an example of a trend function (red) is visualised and compared to the measured soil temperature (blue).

The BTES system should be load matched with the trend function using the remaining 3 parameters. To easily load match it, the user should first export the .csv file from the “data file”-BC in the template.

To export the .csv file, the user should click on the pencil icon visualised above. After clicking on it, a popup window appears with the possibility to export or import the .csv file.

The user can use the template file, to implement the measured values. These should then be reuploaded as a custom file into the software.

After the user has successfully imported the data file, the user can compare the simulated soil temperature (green) and the temperature generated from the trend function (purple) in 1 graph by plotting them together. In the graph below the BTES system is successfully load matched with the trend function. The load matching should be done by changing the 3 parameters, spoken of earlier.

However, for a TRT only 1 probe is used to do the test. In a real installation, more probes are typically used which means these parameters should be scaled accordingly. The way to scale them isn’t always linear. The parameter that should be scaled linearly is the cold deposit power.

The KV-value of the borehole heat exchanger and the natural soil temperature stay the same. The soil start temperature should be set the same as the natural soil temperature.

The remaining parameters are the soil capacitance and the thermal loss coefficient which aren’t that easy to scale. In the best case, similar measured data as in a TRT is available of the complete system. In this case the user can simply load match the complete system. If the measured data isn’t available, different methods are possible to scale these parameters. A common method used is based on compactness.

One of the methods using the compactness is from a geometric point of view when scaling the volume and surface area. In this case, the soil capacitance can be seen as the volume and the thermal loss coefficient can be seen as the surface. A calculation sheet is available to calculate these parameters depending on the pattern, spacing, depth, etc.

The space between the different boreholes is typically between 3 to 7 meters. More information on compactness and spacing can be found in the paper written by H. Skarphagen et al. called “Design Considerations for Borehole Thermal Energy Storage (BTES): A Review with Emphasis on Convective Heat Transfer”.

https://www.hindawi.com/journals/geofluids/2019/4961781/

The user should take notice that these calculations are an estimate of what the actual parameters should be. However, the goal of the Hysopt software isn’t necessarily simulating the soil as accurate as possible, but simulating the dynamic behaviour of the complete system. In this point of view, less accuracy of the soil behaviour is allowed because temperature variations in the soil are slower than temperature variations in the system itself. However, some simulation models of large BTES systems in Hysopt were already validated with measured data.

After the installation of the BTES system itself, the thermodynamic behaviour of the complete BTES system should be analysed and if necessary again load matched in the software to ensure an accurate representation of the system.

Furthermore, to quickly use and simulate a complete BTES system in Hysopt, different templates are available in our Inspiration Library and explained in BTES 1.0 templates and BTES 1.4 templates.