The BTES 1.4 Base Circuit is based on the hydraulic configuration of a concept commonly used in Belgium. Since a heat exchanger is used, this is a closed system.

Operating conditions

The BTES 1.4 BC has different operating conditions. There are two suggested operating conditions the BC should operate in: Cold withdraw and Cold deposit.

Cold withdraw

In the operating condition “cold withdraw”, the cold stored in the ground is used to cool down the building, meanwhile heating the fluid and the heat stored in the ground.

In other words, during cold withdraw, cold stored in the ground is delivered to the building to perform cooling. The warm return is pumped back into the ground, therefore heating up the fluid in the ground and increasing the amount of heat stored in the ground (reducing the amount of cold, therefore, soil temperature goes up). This condition is mostly used in summer.

Cold deposit

During cold deposit, the heat pump (or different source) is delivering heat to the building, the rest cold of the heat pump is then stored into the ground, which lowers the soil temperature. This condition is mostly used in winter.

Design

The BTES system has two design conditions, one for summer “Cold withdraw” and one for winter “Cold deposit”. All the components inside the BC and on the primary side of the BC are active in both design conditions. The design of every component is clarified by going through the design flow for both full load conditions.

Design flow rates

The design flow rates are separately calculated for “Cold withdraw” and “Cold deposit”. All the temperatures and thermal powers are needed. To clarify the design conditions, the user can toggle the hotkey “Ctrl+O”, which shows the two full load design conditions. “Cold withdraw” is visualised in the figure on the left and “Cold deposit” in the figure on the right.

Cold withdraw

In the figure below the required input data is visualised to calculate the volume flow rate for “Cold withdraw”.

The supply and return temperature of the heat exchanger can be altered by the user and is needed to calculate the design flow and the UA-value of the heat exchanger.

The borehole temperature regime can’t be altered by the user, because the temperatures are already set for the heat exchanger. It can, however, be visualised by clicking on the pencil icon, which shows a popup window shown in the figure below. Depending on the power spread between cold withdraw and the production side, the temperatures are recalculated. The outgoing cold temperature from the borehole and the ingoing hot temperature in the borehole are visualised with the blue and red dot.

The thermal power spread between cooling power delivered by the BTES system (bottom blue arrow) and by the production units (top left blue arrow) is default 100%, which means the BTES system will be designed giving all the thermal power needed at the end units. The cooling power delivered by the production units is designed not to deliver anything to the end units.

The user can override this by inserting the soil power or soil power percentage.

The user can also insert the design soil temperature, which should be lower than the borehole regime temperatures. With the temperatures and the thermal power known, the design flow rate for “Cold withdraw” can be calculated.

Cold deposit

In the figure below the required input data is visualised to calculate the volume flow rate for “Cold deposit”.

The temperature regime of the borehole can be altered by the user by clicking on the pencil icon. In the popup window shown in the figure below. The ingoing cold temperature in the borehole and the outgoing hot temperature from the borehole are visualised with the blue and red dot.

The user can also insert the design soil temperature, which should be higher than the borehole regime temperatures. After the cold deposit power is entered, the flow rate of “Cold deposit” can be calculated as well.

The flow rate that is being propagated to the production side of the Base Circuit (left side) is the maximum flow rate, which can either be the “Cold withdraw” or “Cold deposit” flow rate. The corresponding temperatures of the maximum flow rate are propagated as well.

UA-value calculation

There are two heat exchanger in the BTES BC, a plate heat exchanger and the borehole heat exchanger. The UA-value of both heat exchangers is calculated when the user does the calculation “Compute design flows” in Hysopt.

In the figure below the parameters of the plate heat exchanger are shown. The KV-values are needed for the pressure drop calculation on the primary and secondary side of the heat exchanger. The UA-value is only calculated for the design condition “Cold withdraw”. The user can change and lock the UA-value if the value of the heat exchanger is known. Furthermore, the heat exchanger flow type can be selected, for instance, counter flow or parallel flow.

In the figure below the KV-value and the UA-value of the borehole heat exchanger are shown. The KV-value is needed for the pressure drop calculation.

The UA value is separately calculated for “Cold withdraw” and “Cold deposit”. The largest UA-value is taken as the actual value, except if the user overrides the UA value by locking it. This is done to get a UA-value that is usable for both “Cold withdraw” and “Cold deposit”.

If for instance the UA value for “Cold withdraw” is larger than “Cold deposit”, the implemented temperatures for “Cold deposit” will no longer be valid, as they are depended on the UA-value. With that in mind, the design temperature of the soil is recalculated. This example is given in the figure below, where the UA-value is calculated for cold withdraw mode, therefore the soil temperature in cold deposit is recalculated.

In the figure above the flow rate of “Cold deposit” is larger than the flow rate of “Cold withdraw” which means the flow rate of “Cold deposit” is propagated to the production side. However the UA-value of “Cold withdraw” is larger, which means the design temperature of the soil for “Cold deposit” changed.

Notice that the UA-value can also be inserted by the user and locked, which means both design conditions for “Cold deposit” and “Cold withdraw” change.

Pipe selection

The pipes on the right side of the BC are designed on the volume flow rate of “Cold withdraw”, the pipes on the left are designed on the maximum flow rate which is for “Cold withdraw” or “Cold deposit”.

There are also two pipes inside the BC itself, visualised in the figure below. These pipes represent the length between the energy centre with all the pumps, valves and header, and the heat exchanger (borehole tubes).

The information needed to select the pipes is the length and pipe type. With the information, the pipe diameter can be calculated, and if needed overwritten and locked by the user.

The user also has the option to:

Insert thermal losses of the pipes, by selection the insulation class and the environment temperature.
Insert a zeta-value or surplus percentage to represent extra pressure drops

Optimisation system components

The only component inside the BTES BC that needs to be calculated is the pump.

Pump in BC

The pump inside the BC is calculated using the maximum volume flow rate, either for “Cold deposit” or “Cold withdraw”. All the pressure drops of every active component in the operating condition with the maximum flow rate is taken into account to calculate the pump head, including the primary side.

Pump secondary side BC

There should also be a pump on the secondary side of the BC for the operating condition “Cold withdraw”. This pump needs to overcome all the pressure drops of every active component in the operating condition “Cold withdraw” on the secondary side of the BC. The pump doesn’t have to overcome the pressure drop of any component on the primary side of the BC or inside the BC, except for pressure drop on the secondary side of the heat exchanger.

Pump warnings

If a pump is missing on the secondary side, an error is shown when doing a “system check”, because if there is no pump, there won’t be any cold delivered to the cooling end units.

A warning is also shown during a “system check” if a pump is placed on the primary side because two pumps are in series, which is suboptimal in most cases. More info on pumps can be found in Pumps.

Simulation

In a simulation, controls are needed to make sure the BTES BC does what the user wants it to do. To control the BC correctly, different enable signals (green) and measuring signals (red) are implemented in the BC.

Pump modulation signal
If a signal between 0 and 1 is sent to this node, the pump will modulate between 100% rpm and its minimum modulation (in default 10%). This is only the case if the pump is activated.
Pump activation signal
If a signal 1 is sent to this node, the pump is activated. If a signal 0 is send the pump is deactivated and won't generate any flow.
Borehole temperature production side
Soil temperature
Borehole temperature end-unit side

Simulation parameters

Furthermore, there are still a few setting the user can change for simulation. To change these settings, the user should activate the simulation layer on the right side of the library.

After the activation, the user can see a couple more parameters, visualised in the figure below. The minimal pump head percentage and change speed are not discussed in this page because these are specific parameters for the pump itself, which is the same as in other BC’s. More info on pumps can be found in Pumps.

The natural soil temperature is necessary to simulate the thermal losses of the stored heat or coolth compared to the natural soil surrounding the stored energy. The default value of the natural soil temperature is 12°C.

The simulation settings can be changed by clicking on the pencil icon, visualised in the figure below.

A popup window gives the user more options on how the soil should be simulated. There are two simulation modes: “Capacity” and “Sine wave”. If the simulation mode “Capacity” is selected, the popup window is shown in the figure below.

The simulation mode “Capacity” is used to visualise and more accurately simulate the thermal imbalance and the thereafter soil temperatures during the simulation.

The soil capacity is the storage capacity of the wells and is expressed in J/K. If for instance the soil has a capacity of 10 GJ/K, and during a certain amount of time the soil is injected with 10 GJ of thermal energy, the soil temperature will increase by 1°C. To use an accurate capacity, the user can simulate and see how the temperatures behave and change the capacity if the temperatures change too quickly or too slowly. If the temperatures change to quickly the capacity should be increased if the temperatures change to slowly the capacity should be decreased. The temperatures in simulation can be matched with measured data if the capacity and other parameters are changed correctly.

The user also has the option to overwrite the soil start temperature during the simulation. As a default, the start temperature is chosen depending on a certain sine wave and the start time of the simulation. The sine wave can be altered in the simulation mode “Sine wave”. If the user, for instance, starts its simulation in the middle of the winter, the soil is filled with cold energy, therefore, will start with a low temperature.

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.

The thermal loss coefficient is the same as in the “Capacity mode”. The thermal loss, however, doesn’t have any impact on the temperature variation of the soil.

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.

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.

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 (Inspiration Library - Templates ). The template is visualised in the image below.

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 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 12 to 24h because only stationary behaviour should be accounted for.

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 (green) and the trend date imported in the software (purple).

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.