Simulation Types

Building Performance Simulation (BPS)

BPS dynamically computes a building’s heating and cooling loads for a chosen period. The simulation results are a base for the design of heating and cooling systems. Even advanced renewable energy systems that require demand management can be sized efficiently. A building’s internal and external thermal loads vary throughout the day. The internal temperature changes are caused by the heat exchange with the surroundings and the building’s thermal mass.

To set up a BPS, one must balance the results’ robustness and the workload. The selected level of detail of the simulation must fit the problem.

Reasons to perform a BPS

It is the current state of the art that a high manual modeling effort is required to create a dynamic simulation model of a building. Therefore, thermal simulation is often performed, if at all, only for a specific time in a later planning phase, when only a few changes to the cubature are expected. Alternatively, static calculation methods are used, which cannot represent the dynamic loads in the building in detail.

The bim2sim tool aims to automate the IFC-based creation of thermal simulation models to make BPS accessible for all IFC-based design processes, as the designers can significantly influence the building’s geometry in the early design phases. By linking the building simulation to the IFC model, cubature variants can be simulated and evaluated without modeling effort. The IFC-based setup of the building geometry lowers the barrier for the detailed dynamic thermal analysis. The considerable CO2 savings potential is to be expected through optimal building orientation and optimization of thermal loads in an early planning phase.

Requirements for BPS in bim2sim

The BPS part in bim2sim requires a sufficient representation of the building. This includes a description of:

  • building geometry

  • materials (the building’s thermal mass)

  • HVAC systems (only covered with ideal loads for now. Coupled simulations are in scope of future releases)

  • internal loads (occupancy, equipment, lighting, schedules)

  • external loads (weather)

  • building’s location and surroundings (shadings)

This information is extracted from the IFC file. Missing data can be added by using e.g. a template-based enrichment.

IFC Requirements

The BPS Plugins (PluginEnergyPlus and PluginTEASER) should only be applied, if minimum IFC requirements are fulfilled:

  • IFC-Version: IFC4

  • Validity: The IFC file must be valid (fulfill all rules of its schema)

  • Space Boundaries: IfcRelSpaceBoundary elements should be included of type 2ndLevel (either IfcRelSpaceBoundary2ndLevel (optimum) or IfcRelSpaceBoundary with Description 2ndLevel)

  • Quality of Space Boundaries: The provided space boundary data must be valid in terms of syntax, geometry and consistency (cf. Richter et al.: ‘Validation of IFC-based Geometric Input for Building Energy Performance Simulation’, 2022 Building Performance Analysis Conference and SimBuild co-organized by ASHRAE and IBPSA-USA, https://doi.org/10.26868/25746308.2022.C033)

Other IFC requirements (optional, improve model accuracy):

  • Material definitions

  • Shading Space Boundaries (only applicable for PluginEnergyPlus)

Heating Ventilation and Air Conditioning (HVAC) Simulation

HVAC simulations are used to simulate the behaviour of different system components in the energy system. For now bim2sim focuses on the heating and cooling generation, while ventilation and air conditioning is planned for the future (see issue 245 # TODO github deploy).

The corresponding simulation models for heating and cooling analysis typically include:

  • hydraulic network including pipes, valves and pumps

  • generation devices for heating and cooling

  • consumers

  • control logic

Hydraulic Network

Since it is not convenient to model every pipe, pipe fitting and all components of the hydraulic network, one part of the creation of simulation models for heating and cooling analysis is the abstraction of the hydraulic network. The abstraction can be quite time-consuming and error-prone, so one of the strengths of bim2sim is offering automated tasks for analysis and aggregation of the network. To do so we convert the imported meta structure elements and their connections into a HvacGraph using networkx python package.
The possible aggregations start with quite simply aggregations like PipeStrand to aggregate multiple straight connected pipes, but also include more complex aggregations like Underfloorheating which tries to identify underfloor-heating or concrete core activation as there is no possibility in IFC to represent these directly. You can find an overview to all aggregations in it the corresponding documentation. Generation devices and consumers are also simplified in aggregations which brings us to the next group.

Generation Devices & Consumers

Generation devices are e.g. boilers or chillers and consumers might be radiators or the already mentioned underfloor-heating.

Control Logic

Even if IFC offers the possibility to include controls, it is not very practical and rarely used. But for a running simulation the control logic is indispensable. So we came up with a mix of two solutions:

  1. For elements where common standard control logics exist we include these logics as default into the mapped Modelica models.
    We try to gather the relevant parameters for the control from the IFC via the attribute system. One example would be the flow set temperature of a boiler. If there is no information in IFC we can either request them during export or mark them as non-existing in the exported model, so the user can input them in Modelica.

  2. For custom controls and complex controls which highly depend on the system and the usage we offer the possibility to deactivate the internal controls inside the modelica models and allow the user to model their own controls.