HPControlHeatLed

Basic

The heat pump controller requires the various temperatures as input parameters to control the heat pump:

• Setpoint temperature in the flow line on the heat sink side ▶TFlowRef

• Setpoint temperature to which the heat pump is to be controlled ▶TRef

• Current temperature of the component to which ▶TRef is to be controlled ▶TAct

• Temperature in the flow line on the heat sink side ▶TFlow

• Temperature in the return line on the heat sink side ▶TReturn

The Boolean connector ▶Enable, which comes from an external controller or input signal and specifies when the heat pump should be active, must also be connected. ▶Enable allows the implementation of a complex switch-on control for the heat pump in addition to the internal control of the heat pump controller and to control the heat pump autonomously the connector can be defined with a static true argument. The weather data is transferred to the controller via the ▶EnvironmentConditions. The output parameters must then be linked to the respective input parameters of the HeatPump models.

As the HeatPump uses ambient air, as an environmental heat source, the SourceAir parameter has to be set to true. If the parameter COPcontrol is false the heat pump won't be controlled based on their current COP value, which is the default state of the heat pump controller. Also the heat pump controller is in the basic state configurated to have an external set value for the flow temperature on the heat sink side ▶TFlowRef and lastly the heat pump has no auxiliary heating component as the parameter AuxHeat is false.

The user is given the possibility to vary internal operation time constants (e.g. minimum switch-on/off time etc.) of the heat pump controller. These times are the delay in starting the heat pump compared to the circulation pump tDelayHP, the minimum switch-on time tHPminOn and the minimum switch-off time of the heat pump tHPminOff. If the corresponding data files of heat pump controller are not available the pre-defined files should be used.

For the circulation pump, there is also a time delay for switching off the circulation pump compared to the compressor tDelayCPComp and for switching off the circulation pump later compared to the compressor after the de-icing process tDelayCPIce. The heat pump starts operation later than the source pump by the parameter tDelaySP and the source pump stops later than the heat pump by the parameter tDelaySPComp. More information is available in the Model Background.

When the maximum flow temperature TFlowMax or maximum return temperature TReturnMax is reached, the heat pump is switched off by the controller. While the parameters deltaTActRefLow and deltaTActRefUp represent the limit values for switching the heat pump on or off.

To ensure numerical stability and reduce the simulation time, the assumption of a constant volume flow on the source side of the heat pump qvSource is modeled. When parameterizing ambient air heat pumps, focus must be given to the maximum and minimum source pump volume flows. Due to air heat pumps using a ventilator instead a circulation pump, a lot higher volume flows (e.g. 100 m3/h vs. 10 m3/h) occur. It should thus be ensured that these parameters are defined accordingly to avoid incorrect system functionality. For parametrization reference use the heat pump source files in the data directory (GreenCity/Data/ModelData/heat_pump/hp_data, not visible in SimulationX).

The maximum volume qvMax depends on the selected circulation pump (ref. GreenCity/Data/ModelData/heat_pump/hp_data), however the minimum volume flow qvMin can be defined using the heat pump system data sheet. Ensure that the minimum volume flow qvMin is set to a value above zero, otherwise the volume flow control is not activated. Note that the minimum volume flow should be set to a level where heat power output is possible with an adequate temperature spread (TFlow-TReturn). If this value is too high, the volume flow control can cause system state faults. More information is available in the Model Background.

Apposed to heat pumps using other environmental heat sources (e.g. ground, well water), air heat pumps must be de-iced during the heating process to ensure their efficient functionality. During this process the devices are switched-off and the operation mode is reversed resulting in a heat transfer, so that the ice is melted. Defining these time periods requires measurement data of internal system states of the controlled heat pump system (e.g. working medium temperature). Due to inner processes being neglected in the heat pump model these states are not available during simulation. To enable the consideration of influences of a de-icing process on the heat power output and overall system behavior a statistical approach to define the de-icing process was needed. Measurement results for different heat pump systems were utilized and a statistical approach was developed. Thus, de-icing processes are switched-on and -off after statistically determined time periods. These time periods can however be manually defined by the user using the De-Icing parameter tab.

The operation of a de-icing process also depends on the air temperature used as an environmental heat source. At temperatures below the set parameter value TDeIcingBound (should be set at about 7°C - measurement results) a de-icing process needs a complete turn of heat pump operation. Here, the compressor and circulation pump stays operational and the ventilator is switched-off. Above the set temperature level the compressor and circulation pump can be switched-off during the de-icing process. Only the ventilator must stay switched-on to provide heat for de-icing. The other time constants are the duration of the de-icing process tDeIcing, the average time between two de-icing processes tOperation and the switch-on and shutdown time (tDeIcingOn and tDeIcingOff). If no measurement results, of the specific heat pump system, are available the pre-defined parameters should be used.

Auxiliary Heater

After the AUXheat parameter is turned on, it allows for the modeling of an an auxiliary, internal, electric heating system of the HP. Such a system increases the heat power output when the heat demand exceeds the maximum heat power output of HP system.

To ensure that the auxiliary heating system is only switched-on when the heat demand of the consumers can not be supplied by the heat pump, it only is activated if the actual heating system temperature is below the minimum level, for a pre-defined time period (tBivalence).

In the tab I/O - Control the switch on and switch off temperature differences (deltaTAuxRefLow and deltaTAuxRefUp) must be defined. The switch-off temperature level of the auxiliary heating system deltaTAuxRefUp should be set below the correspondent temperature level, for the overall system switch-off deltaTActRefUp, to avoid extensive usage of the auxiliary heating system (i.e. the auxiliary heating system should work as a slave system only when needed). More information is available in the Model Background.

Ground Heat Source

If the floor is used as a heat source (SourceAir is false), then no de-icing of the heat pump is necessary due to the higher floor temperatures and the de-icing parameters in the De-icing tab are not required.

COP Control

Heat pump operation can additionally be parametrized by setting COPcontrol to true and a minimum Coefficient of Performance (COPmin). This is done as using additional heat systems (e.g. condensing boiler) can be more efficient than running the heat pump at inefficient system states. The new connector ▶COP has to be connected to the ▶COP connector of the heat pump model.

Flow Control

The flow temperature control mode must also be characterized. Concretely, if the FlowControl is true, the reference flow temperature of the HP must be defined dependent on the ambient temperature using the connector ▶EnvironmentConditions.

This is done using the flow temperature characteristics specified in the model data directory FlowFile with the table name FlowTable. For the interpolation of the FlowTable data the maximum and minimum temperature (TAmbientMax and TAmbientMin) must be defined to avoid numerical instabilities. To avoid high dynamic oscillations during the simulation the average ambient temperature is used for flow temperature control. To calculate the average ambient temperature a number of support points SupportPointsAmbientTemperature and a time step size TimeStepAmbientTemperature must be defined in the flow control parameter tab. The time difference between each data point characterizes the time step. Note that for the calculation the initial ambient temperature is used as a data point for every support point.

Pre- and Post-run Time of Components

The logic of the time delay can be seen in the following figure. The heat pump (▶HPon) is only switched on after the circulation pump is switched on and the time tDelayHP (3 seconds in the figure) has elapsed. The logic for tDelayCP is analogue. This results in the ▶HPon signal being switched on at 4 s after the circulation pump is switched on at time step 1 s. The circulation pump is then switched off 4 s (tDelayCP) after the unit is switched off.

Prevention of Continuous Switching

The following illustration shows how the tBivalence parameter works. In this case, it is assumed that the switch-on limit (deltaTAuxRefLow) for the auxiliary heating is 3 Kelvin and the switch-off limit (deltaTAuxRefUp) is 1 Kelvin. This temperature difference is calculated by TRef-TAct. The image shows that the switch-on limit must be reached for at least the duration tBivalence (here 2 seconds) . If the temperature difference decreases within this period (as in the first case at time 3 s), no switch-on signal is issued. The auxiliary heating is only switched on when the temperature difference is reached for a full 2 seconds period (in the image at time 6 s). ▶AUXon is then switched off when the temperature falls below the lower temperature limit at time 10 s.

Volume Flow Control

The volume flow of the circulation pump is defined by a linear approach as shown in the following figure. The decisive factor is the temperature difference between ▶TFlow and ▶TFlowRef. At the value deltaTFlowRefMax (in the image 3 K) the maximum volume flow qvMax is assumed to reduce the temperature stroke inside the generator. At the temperature difference deltaTFlowRefMin (in the image -10 K) the minimum volume flow qvMin is used to limit the temperature stroke. Linear interpolation takes place between the extreme values in order to achieve the desired set flow temperature ▶TFlowRef.

Switch on/off

The heat pump controller uses the maximum flow and return temperature and a temperature hysteresis around the reference temperature of the connected heating system (heat storage, hydraulic shunt, building zone) as a control parameter to switch the system on/off:

If an auxiliary heating system is to be controlled a corresponding temperature hysteresis using the reference temperature of the connected heating system must be defined: