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The user should also set the thermal loss coefficient of the soil, which is needed to simulate the thermal losses accurately. The thermal loss coefficient is expressed in W/K, which means that if the stored energy has a temperature of for instance 10°C, the natural soil temperature is 11°C and the thermal loss coefficient is 1000 W/K, the overall thermal power lost to the environment is 1 kW. The thermal loss is higher if the temperature difference is larger or the thermal loss coefficient is higher. Load matching the thermal loss coefficient and the capacity is explained in the next chapter.
The simulation mode “Sine wave” means the temperatures in the soil doesn’t change depending on the injected or extracted energy. The temperatures are exactly the same as the set sine waves by the user. If this simulation mode is selected, the thermal imbalance of the soil won’t be visual by the temperatures of the soil. The pop-up window of the simulation mode “Sine wave” is shown in the figure below.
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To set the sine wave for the soil, the user should insert the maximum and minimum temperature. The sine wave is set up in such a way that the minimum value is reached after the peak winter days and the maximum value is reached after the peak summer days (of an average year). The clarification is visualised in the figure below.
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Load matching BTES
The BTES system can be load matched with measured data when it is available. When the building owner is interested in installing a BTES system, sometimes a TRT (Thermal Response Test) is used to check the application of a BTES system. This TRT can also be used to load match the BTES BC for an accurate simulation.
A TRT is used to determine the thermal properties of the ground. There is no direct way to measure ground thermal conductivity and borehole thermal resistance. The TRT is vital for designing ground source heat pumps and seasonal thermal energy storage systems. More information about it can be found in:
https://en.wikipedia.org/wiki/Thermal_response_test
An example of the TRT test is visualised in the images below.
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This principle can be implemented and simulated in the Hysopt software. The template for it is clarified in BTES load matching and can be found in our Hysopt Inspiration Library (Hysopt Inspiration Library). The template is visualised in the image below.
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Typical results of the TRT test are the following:
undisturbed (natural) soil temperature [°C]
average heat conductivity coefficient [W/m.K]
average heat capacity [MJ/m³.K]
borehole resistance [m.K/W]
The undisturbed (natural) soil temperature can directly be inserted in the base circuit in Hysopt. The borehole resistance can be recalculated to the UA-value of the borehole “heat exchanger” as follows:
UA-value borehole [W/K] = depth borehole [m] / borehole resistance [m.K/W]
After the calculation, the UA-value can be inserted into the base circuit.
The other values aren’t directly insertable into the base circuit. They have a direct impact on the thermal loss coefficient and the capacity of the BTES system. For this reason, these 2 remaining parameters have to be load matched. The load matching can be done by using the trend function (also a typical result from a TRT).
The trend function is a logarithmic function and created for the average soil temperature trend. In the graph below, the measured data is visualised in blue and the trend function is visualised in red. The trend function is typically created without the measured data from the first 12h because only stationary behaviour should be accounted for.
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This trend function can be imported in the software using a .csv file. When doing so, the template can be used to change the thermal loss coefficient and capacity to load match the BTES system. The image below shows the simulation data (red) and the trend date imported in the software (green).
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However, the load matching is only done for 1 probe. So when the user has multiple probes, the parameters should be scaled in a correct way. The correct way to scale them is explained in BTES load matching.