CondensingBoilerDiscrete

Basic

In the basic configuration the model has to be connected using the turn on/off signals for the boiler and the circulation pump (▶CBon and ▶CPon), the number of currently used burner stages (▶BurnerStage), set point of the volume flow ▶qvRef, the intake and return pipes of the heat sink side (▶FlowCB and ▶ReturnCB) and the dc grid connection for the circulation pump consumption (▶CPGrid1). Most of the connectors should be set with the connectors of the discrete heat regulated controller for the boiler.

The basic configuration of the CondensingBoilerDiscrete does not have an auxiliary heating system (AuxHeat is false) and uses a single-phase connection (dc) for the circulation pump (CPPhase is 1). After the model is configured all connectors should be available for connection to the dc grid (or with a phase tap to an ac grid), boiler controller and heating system (e.g. heat storage or consumers). Make sure that all connectors are characterized to avoid numerical difficulties.

The material properties of the fluid in the heating system (specific heat capacity cpMed, density rhoMed, and volume of fluid in the boiler VCB) are used for dimensioning. Heat losses to the environment are determined using the parameters of the constant ambient temperature TAmbient and the heat loss rate through the boiler insulation QlossRate. The heat loss rate QlossRate is configured for a typical household boiler with good insulation and should be reduced for a boiler with better insulation and increased for poorer insulation. If the heat loss rate cannot be estimated, the default value should be used.

To determine fuel consumption, the volumetric fuel density rhoFuelVolume and the energetic fuel density rhoFuelEnergy must be specified (default values for gasoline).

The efficiency calculation is the main component of this simulation model. The parameter useConstEta determines whether the nominal fuel efficiency of the boiler is constant (useConstEta is true) or is represented by the specific information in the data sheet (useConstEta is false).

For constant nominal fuel efficiency (useConstEta is true), this is specified via the parameter for efficiency relating to net calorific value eta. This efficiency eta is converted to the effective efficiency of the system using the values for relative latent heat LatentHeat, relative exhaust losses ExhaustLoss, and radiation losses RadiationLoss. Typical values for eta for a condensing boiler are between 100% and 115%, for LatentLoss usually 6%, for ExhaustLoss 1% to 10%, and for RadiationLoss 1% to 20%. Further standard values and the calculation of boiler efficiency can be found in the section Fuel Utilization Efficiency.

The other variant uses a temperature-dependent efficiency relating to net calorific value etaTable, which is defined in the parameter file CBFile under Condensing Boiler - Power Data. etaTable is predefined in the ModelData directory for various boiler types and can be selected via the CBFile setting. If a custom condensing boiler system is to be modeled, the boiler data file in the model directory under GreenCity/Data/ModelData/condensing_boiler/CB_data (not visible in SimulationX) can be used as a template. To limit the technically possible temperatures, the maximum temperature limit TMaxEta and the minimum temperature limit TMinEta are also defined. If no exact figures are known for the boiler, then the constant approach should be chosen.

The definition of the heat outputs per burner stage depending on the flow temperature (T of ▶FlowCB) is defined in the file CBFile in the folder GreenCity/Data/ModelData/condensing_boiler/CB_data (not visible in SimulationX), and the heat outputs are specified using the tables QMaxTable and QMinTable. QMaxTable specifies the heat output of the boiler with both burner stages in W depending on the flow temperature in K, and QMinTable specifies the heat output with only the first burner stage. If other condensing boiler systems are to be modeled, use the condensing boiler data file in the model data directory as a template and adjust the values and file name accordingly. To limit the technically possible temperatures, the maximum temperature limit TMaxHeat and the minimum temperature limit TMinHeat are also defined.

For the circulation pump, the power of the circulation pump in W as a function of the volume flow must also be defined in the GreenCity/Data/ModelData/condensing_boiler/circulation_pump folder (not visible in SimulationX) CPFile under the CPTable table. The power factor of the pump must be set using the parameter CosPhiCP, and the technically possible volumes are then limited by the maximum volume flow qvMax and the minimum volume flow qvMin.

To simulate the behavior of the CondensingBoilerDiscrete, the model uses the typical start-up times during the start process for the boiler tThStart and the run-on times for the circulation pump tCP. The start-up time tThStart defines how long the CondensingBoilerDiscrete must be switched on. The run-on times specify the times during which the pumps are still active after the boiler is switched off.

Auxiliary Heating

If the maximum heat generation of the boiler is not sufficient to cover the system heat requirement, the heat generation can be increased via an additional heating component. To do this, the AuxHeat parameter must be set to true and the boiler is then equipped with an additional heating element. After setting AuxHeat to true, the AUXPhase parameter can be used to set the phase type of the newly appeared ▶AUXGrid1 connector (1 = dc, 3 = ac). The connector ▶AUXGrid1 must then also be connected directly to a dc grid or to an ac grid with a phase tap.

Under the tab Condensing Boiler - Power Data must be the nominal heating power of the auxiliary heating system AuxHeatPower defined.

Finally, under the Dynamics tab, the time constant for the auxiliary heating system's response to the boiler starting or shutting down must be defined tAUX. This time constant tAUX works in the same way as tCP (see for more information).

3 Phase Connection

If the individual components of the heat pump have an ac connection, the number of phases for the individual components (CPPhase and optional AUXPhase) can be changed to 3, i.e. ac, in the Model Configuration tab. Now the components electrical connection (e.g. ▶CPGrid3) can be directly linked to the ac grid.

Fuel Utilization Efficiency

The parameters efficiency relating to net calorific value eta, relative latent heat LatentHeat, relative exhaust losses ExhaustLoss, and radiation losses RadiationLoss are used to calculate the system efficiency of the boiler .

If there is no case-specific information available, use the pre-defined parameter sets:

Burner Stages

The burner stage is specified via an external controller. As soon as the value of ▶BurnerStage is greater than 1.5, the second burner stage is activated, and if the last change of burner stage was longer ago than the switch-on time of the burners tThStart, then the burner stage is changed. A model extension is used to enable the utilization of three or more burner stages however model source code modifications are necessary.

Time Dynamics

The times of the pumps and the CombinedHeatAndPowerUnit are modeled in the model via PT1 elements, whereby the entered parameters (e.g. tThStart and tCP) represent the time constant of the PT1 element. When the CombinedHeatAndPowerUnit is switched on, the electrical and thermal powers (e.g. PCP, QHeat and PAux) increase according to the typical behavior of the step response of a PT1 element. This means that after once the time constant (e.g. tCP = 5s) the electrical and heating power rise to 63% of its original value, after twice the time constant (2 * tCP) to a value of 95% and after five times the time constant (5 * tCP) to 99.2%.