Big Picture

What is bim2sim about?

bim2sim is a Python tool that allows to use BIM models (in the form of IFC files) as source to generate simulation models in a semi-automated process. The existing version supports the following domains, where the focus of the development was on the first two:

• Heating, Ventilation Air-conditioning (HVAC)

• Building Performance Simulation (BPS)

• Computational Fluid Dynamics (CFD)

The biggest challenge for this process is the mixed quality of IFC files. IFC as standard theoretically offers the options to hold most of the information needed for a simulation model generation. Nonetheless, even most current IFC files are lacking detailed information about e.g. wall constructions or the relevant characteristics of HVAC equipment.

This makes the process of a semi-automated generation of simulation models quite complex. With bim2sim we provide a tool that offers a lot of functionalities to simplify and unify this process.

How does bim2sim work?

The general structure of bim2sim is shown below:

        flowchart TB

    subgraph PL["`**Plugin**`"]
        default_tasks
        sim_settings
    end
        PL-.->|input| PJ(Project)
    subgraph IP["`**Configuration**`"]
        X[ifc_paths] 
        Y[project directory]

        style X text-align:left
    end 
    IP -.->|input| PJ
    subgraph PJ["`**Project**`"]
    rd("run_default(): \n&nbsp&nbsp for task in plugin.default_tasks:\n&nbsp&nbsp&nbsp&nbsp      playground.run_task()")
    style rd text-align:left
    end 


    subgraph PG["`**Playground**`"]
        C["run_task ()"]
        S[(state)]
    end 
        rd --> C
    C -->|run| D{{Task 1}}
    D <-.-> |data exchange| S
    C -->|run| E{{Task 2}}  
    E <-.-> |data exchange| S
    C -->|run| F{{Task n}}
    F <-.-> |data exchange| S
    D-->E-->F
    S --> |load data|R
    F --> R(Task Export)

    User <---> |decisions| rd
    

Let’s define what each of these elements is and how they work together in short wrap up. For more detailed information please have a look at the detailed linked documentation of the components.

Project: A project is the main object in bim2sim. It takes the selected Plugin and further configuration as input and then performs the tasks configured in the Plugin through the Playground.

Plugin: Plugin are simulation use case and sometimes even simulation tool specific. E.g. one plugin we provide is the PluginTEASER that creates simulation models for building energy performance simulation through the tool TEASER and the modeling language Modelica. A plugin takes two information: the default_tasks which is a list that describes what tasks should be performed and the tasks which allow detailed configuration of the model creation that have default values but can be defined for every project.

Configuration Next to the Plugin the Project needs the object ifc_paths which is a dictionary where the key is a domain and the value is the path to the IFC file. This allows to assign multiple IFCs to a project, e.g. one for HVAC domain and one for architecture domain. Additionally, a project directory needs to assigned. This can either be an empty path where a project should be created or an existing project directory.

Playground: The Playground itself deals as a manager which coordinates the different tasks. * A task is used to fulfill one specific part of the whole process. One task is for example the loading process of the IFC into the bim2sim tool. The results of each task will be stored in the state of the Playground. Every task might have requirements (reads) and outputs (touches). The Playground checks for every task if the requirements are met and then performs the task. Some tasks, like model creation tasks might also export information in form of files. Those will be stored in the export folder of the project directory.

User: To overcome the already mentioned challenges regarding the mixed quality of IFC-files the process might need feedback and additional information from the user. This feedback is given through Decisions.

You find detailed information about each of the concepts in the corresponding documentation.

#TODO this is copied to advanced-user-guide/simulation-types, strip it down here

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.

What are Plugins?

To make bim2sim usable for different simulation domains and tools we use Plugins that are built upon the functionality that bim2sim offers. These plugins put together one or multiple Tasks and a use a set of simulation settings to create a simulation model based on an IFC. Not all plugins are at the same level of development. Following, we give an overview about the current development process and what works and what notin the following table.

Plugin	Domain	Model Generation	Comment	Export	Comment	Simulation	Comment
AixLib	HVAC	Working	improvements aggregations needed	Working		Not working	Modelica models not published yet
EnergyPlus	BPS	Working		Working
TEASER	BPS	Working		Working
LCA	LCA	-	no model	Working	improvements IfcWindows needed	-	no simulation
CFD	CFD	-	no model	Working	documentation missing	-	no simulation

Compatibility

For the Plugins that export a simulation model, following the listed compatible versions and branches are listed, which our Plugins are compatible with at the moment.

Plugin	Repository	version/branch
TEASER	AixLib	development
	TEASER	development
EnergyPlus	EnergyPlus	9.4.0