Model Representation and Result Quantities

Overview

The dynamic simulation represents the heat network as a thermo-hydraulic model: a quasi-steady hydraulics (mass fluxes and pressures) coupled with transient energy balances of the fluid volumes. The time integration is performed with an adaptive step size. This page describes the model representation compactly and lists the result quantities and output files as a reference.

Model Representation

Hydraulics

In each integration step, the nonlinear equation system of mass conservation at the nodes and the pressure-loss relations of all elements is solved with a Newton method. The reference pressure is prescribed at the pressure-maintenance node; from the geodetic heights of the nodes, absolute pressures additionally result (quantities with the suffix Absolute). Controlled valves and pumps enter the equation system directly with their current control variables.

Thermics

Each pipe is subdivided into fluid volumes along its length; the maximum element length $\Delta x$ is the Length of the pipe discretization from the network settings. The number of volumes per pipe is

$n = \max\!\left(1,\ \left\lfloor \tfrac{L}{\Delta x} \right\rfloor\right)$

with the pipe length $L$. For each volume, an energy balance of heat transport with the mass flux and heat exchange with the surroundings (e.g. ground) is solved. A smaller discretization length increases the accuracy of the temperature fronts in the network, but enlarges the equation system and the computation time.

Controllers and Ground Coupling

Controllers (e.g. for valves and pumps) are computed within the network model; their current manipulated and error variables are available as result quantities (see Controllers). Pipes with ground heat exchange and ground heat sources with a coupled ground model are represented via separate ground models generated on export, which are coupled to the network model by co-simulation with the configured coupling time step (see Ground Model).

Time Integration

The default integrator is CVODE, an implicit method with an adaptive step size: the time step is automatically increased or decreased based on a local error estimate (relative tolerance 10⁻⁵ by default, maximum time step 1 h). In numerically demanding situations, the steps can temporarily become very small; the outputs are produced independently of this on the output grid (default: hourly).

Result Quantities

With the option Generate default network-related outputs (see Defining Outputs), hourly outputs are generated for all network elements. Which quantities an element provides depends on the component type:

Quantity	Unit	Meaning
FluidMassFlux	kg/s	Mass flux through the element
FluidVolumeFlow	m³/s	Volume flow through the element
FluidVelocity	m/s	Flow velocity (pipes)
InletNodeTemperature / OutletNodeTemperature	°C	Temperature at the inlet or outlet of the element
TemperatureDifference	K	Outlet minus inlet temperature
InletNodePressure / OutletNodePressure	Pa	Pressure at the inlet or outlet node
InletNodePressureAbsolute / OutletNodePressureAbsolute	Pa	Pressure including the geodetic component
PressureDifference	Pa	Pressure difference across the element
PressureDifferencePerLength	Pa/m	Pressure gradient per meter of pipe length
PressureLossFittingsInlet / PressureLossFittingsOutlet	Pa	Pressure loss due to fittings at the inlet/outlet
FlowElementHeatLoss	W	Heat flux from the element to the surroundings; negative if the element injects heat into the network
HeatingPower	W	Heating power supplied to the fluid (heat exchanger)
PressureHead	Pa	Head of the pump
ElectricalPower	W	Electrical power consumption (pumps, heat pumps)
PumpEfficiency	–	Current pump efficiency
COP	–	Coefficient of performance of the heat pump
CondenserHeatFlux / EvaporatorHeatFlux	W	Heat flux at the condenser or evaporator of the heat pump
RequiredBuildingHeatFlux	kW	Heat power requested by the building (transfer station)
HeatDeficitAbsolute	kW	Uncovered heat demand of the transfer station
HeatDeficitRelative	–	Ratio of heat deficit to requested heat flux
OutletTemperatureSecondary / InletTemperatureSecondary	°C	Secondary-side temperatures of the transfer station
StoredHeatingEnergy	kWh	Currently stored heat quantity of the transfer station
ControllerResultValue / ControllerErrorValue	–	Manipulated variable and control deviation of controllers

Output Files

Alongside the project file, the simulation creates a folder <ProjectName>/:

• results/ – result files
• log/screenlog.txt – solver log

For each result quantity, a file <NetworkName>.<Quantity>.btf (binary format, default) or .tsv (text format) is written, e.g. Network.FluidMassFlux.btf. The format is set by the option Use binary format on the Outputs page. Averaged or integrated outputs receive the suffix -mean or -integral; for an output grid other than hourly, its name is appended.

The TSV files are tab-separated text files: the first column contains the time, followed by one column per network element with the element name, ID and unit in the column header, e.g. SupplyPipe(ID=1612).FluidMassFlux [kg/s].

You evaluate the results directly in the result area of VICUS Districts (false color in the 3D model, line charts) or open them in PostProc 2 – the TSV files can also be further processed in any spreadsheet or scripting tool.

Related Topics

• Simulation Settings and Start
• Defining Outputs
• False-Color Display
• Line Charts