The ATES 1.2 Base Circuit is based on the hydraulic configuration of a concept suggested in the ISSO publication 39, which is the main guideline for ATES systems in the Netherlands. The Base Circuit is shown in the figure below.

Operating conditions

The ATES 1.2 BC has different operating conditions. There are only three suggested operating conditions the BC should operate in. The other operating conditions are suboptimal, mainly because the heat exchanger works in parallel flow.

Cold withdraw

In the operating condition “cold withdraw”, the cold stored in the cold well (visualised as the left blue bucket) is used to cool down the building, meanwhile heating the fluid and stored in the hot well (visualised as the right red bucket).

Cold deposit

In the operating condition “cold deposit”, the heat stored in the hot well is used to heat the building (commonly done by a heat pump), meanwhile cooling down the fluid and stored in the cold well.

Bypass

In the operating condition “Bypass”, the ATES system isn’t used because the heat exchanger is bypassed.

Design

The ATES system has two design conditions, one for summer “Cold withdraw” and one for winter “Cold deposit”. Some of the components are only active in “Cold withdraw”, some are only active in “Cold deposit” and some are active in both. 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 temperature regime of the soil side can be altered by the user by clicking on the pencil icon. In the pop-up window shown in the figure below, the cold extraction temperature from the cold well and the hot injection in the hot well is visualised with the blue and red dots

The temperature regime of the circuit side, however, can’t be altered by the user, because the temperatures are propagated from the secondary side of the BC to the BC itself. It can, however, be visualised by clicking on the pencil icon. Depending on the power spread between cold withdraw and the production side, the temperatures are recalculated.

The thermal power spread between cooling power delivered by the ATES system (bottom blue arrow) and by the production units (top left blue arrow) is by default 100%, which means the ATES system will be designed to give 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.

With the temperatures on both sides of the heat exchanger 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 soil side can be altered by the user by clicking on the pencil icon. In the popup window shown in the figure below. The cold injection temperature in the cold well and the hot extraction temperature from the hot well are visualised with blue and red dots.

The temperature regime of the circuit can be altered as well.

After the cold deposit power is entered, the flow rate of the “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

The UA-value of the heat exchanger is also calculated when the user does the calculation “Compute design flows” in Hysopt. In the figure below the UA-value is shown, as well as the type of heat exchanger (counter or parallel flow) and the KV-value on the primary and secondary sides. The KV-values are 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.

If for instance, the UA value for “Cold withdraw” is larger than “Cold deposit”, the temperature regimes and flow rates for “Cold deposit” aren’t correct anymore. With that in mind, the temperature and volume flow rate are recalculated. This example is given in the figure below.

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 temperatures 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”, and 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 center with all the pumps, valves, and header, and the heat exchanger which is typically close to the boreholes.

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 selecting the insulation class and the environment temperature.
Insert a zeta-value or surplus percentage to represent extra pressure drops

Optimisation system components

The components inside the ATES BC that need to be calculated are:

Pump on the circuit side of the heat exchanger
Balance valve

The remaining components on the circuit side are on/off valves, which don’t have to be calculated because valve authority is not an issue.

The pressure drops on the soil side of the heat exchanger are not yet taken into account, so the remaining pumps don’t have to be calculated. In the future, Hysopt will also implement a hydraulic solver on the soil side of the heat exchanger, but for now, our main goal is to accurately simulate the system itself, not necessarily accurately simulate the soil. Hysopt does however implement an adequate thermal solver for the soil side.

Pump in BC

The pump inside the BC is calculated using the volume flow rate for “Cold deposit” because the pump is called a “charge pump”. All the pressure drops of every active component in the operating condition “Cold deposit” are taken into account, including the pressure drop on 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”, including the pressure drop on the primary side.

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.

If a pump is placed on the primary side, a warning is shown when doing a “system check”. If there is a primary pump, the primary pump and the charge pump during “Cold deposit” are in series, which is suboptimal in most cases.

Balance valve

The balance valve is set to overcome the pressure difference between the operating condition “Cold withdraw” and “Bypass”. For instance, the operating condition “Cold withdraw” has to account for the pressure drops from the pipes and heat exchanger, although “Bypass” doesn’t have to account for these pressure drops.

Hysopt will automatically set the balance valve correctly, resulting in the same flow in the two operating conditions.

Simulation

In a simulation, controls are needed to make sure the ATES 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.

Charge / Discharge valves
If signal 1 is sent to this node, the charge valve is opened and the discharge valve is closed. If a -1 signal is sent to this node, the discharge valve is opened and the charge valve is closed. If a 0 signal is sent to this node, both the charge and discharge valve is closed.
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 charge valve is open because if it’s closed, there won’t be any volume flow rate through the pump anyway.
Heat exchanger bypass valve
If signal 1 is sent to this node, the bypass valve is opened. If a signal 0 is sent, the valve is closed.
Volume flow rate between wells
The user should send the correct volume flow rate to this node depending on what the user wishes the volume flow rate should be. Because there is no hydraulic solver at the soil side of the heat exchanger, the volume flow rate can be chosen. If it is a positive signal, the volume will flow from the hot well to the cold well (Cold deposit). If it is a negative signal, the volume will flow from the cold well to the hot well (Cold withdraw).
Heat exchanger temperature (cold system side)
Heat exchanger temperature (cold soil side)
Cold well temperature
Hot well temperature
Heat exchanger temperature (hot soil side)
Heat exchanger temperature (hot system side)

Simulation parameters

Furthermore, there are still a few settings 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 on this page because these are specific parameters for the pump itself, which is the same as in other BCs. 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 cold 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 pop-up window gives the user more options on how the soil should be simulated. There are two simulation modes: “Capacity” and “Sinewave”. If the simulation mode “Capacity” is selected, the pop-up window in the figure below is shown.

The simulation mode “Capacity” is used to visualise and more accurately simulate the thermal imbalance and the consequent cold and hot well temperatures during the simulation.

The cold and hot well capacity is the storage capacity of the wells and is expressed in J/K. If for instance the hot well has a capacity of 10 GJ/K, and during a certain amount of time the hot well is injected with 10 GJ of thermal energy, the temperature of the hot well 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 too quickly, the capacity should be increased and if the temperatures change too slowly, the capacity should be decreased. The temperatures in the 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 cold and hot well start temperature during the simulation. As a default, the start temperatures are 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 cold well is filled with cold energy, therefore, will start with a low temperature.

The user should also set the cold and hot well thermal loss coefficient, 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.

The simulation mode “Sine wave” means the temperatures in the wells don’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 cold and hot well.

To set the sine wave for the cold well, the user should insert the cold well 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.

Simulation warnings

During a simulation, a warning or error can occur. The current warnings are the following:

Parallel flow

If the heat exchanger operated more than 5% of its time in parallel flow instead of counterflow, a warning is set because this is suboptimal compared to counterflow operation. When the heat exchanger is operating in parallel flow, the thermal conductivity reduces, which means the temperature difference is lowered. If the temperature difference is lowered on both sides, the return temperature to the system is higher or lower than it should be and the injection temperature inside the cold or hot well is higher or lower than desired. The latter can result in heating up the cold well or cooling down the hot well. The issue is probably in the incorrect control of the BC, so the user should check the controls for any errors.

Heating up cold well

If the temperatures going into the cold well are higher than the temperature of the cold well itself, for more than 5% of the simulation time, the warning is shown saying the cold well was heated up. The user has to analyse the simulation further to correct the mistake. This can be done by doing short simulations with different start temperatures of the cold well and seeing where and why the problem occurs. It is also possible it happens in the reference case, which means an optimisation of the system is needed.

Cooling down hot well

If the temperatures going into the hot well are lower than the temperature of the hot well itself, for more than 5% of the simulation time, the warning is shown saying the hot well was cooled down. The user has to analyse the simulation further to correct the mistake. This can be done by doing short simulations with different start temperatures of the cold well and seeing where and why the problem occurs. It is also possible it happens in the reference case, which means an optimisation of the system is needed.