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What makes good BPS?

Possible Solutions

There are basically two ways to approach the complex and multifaceted problem.

The classical way

One would be to co-simulate separate models for A, B, C, D and E. With the possible exception of mould growth, several models are available that treat A, B, D or E. Some of these models, e.g. EnergyPlus (A), COMIS (B) and Simulink (D and E) have in the scope of research projects been co-simulated using, e.g., BCVTB.

There are two fundamental problems with a co-simulation approach. Firstly, co-simulation is notoriously unstable. The general measure to avoid such instabilities is to decrease the global simulation time-step. This, in turn, may lead to extremely slow simulations. Secondly, a co-simulation environment involving three separate programs (as well as a co-simulation master algorithm), is very far from user friendly. It is unlikely, that the client will be satisfied with such a tool for continued independent simulations.

Multi-physics modelling

The other possible approach to the complex and multifaceted problem is to use a multi-physics modelling environment, i.e. to describe A-E in the same formal framework and thereby avoid the need for co-simulation. Matlab/Simulink/Simscape (from MathWorks inc.) and Modelica (Dassault, Maple, Simulation X, and others) are two potential candidates for such a model.

Although several building fabric simulation (model A) toolkits exist for Simulink, e.g. Carnot blockset and Simbad, no examples have been reported of large models that have been simulated with acceptable numerical performance. It is the possibility of coupling the building model with a control system (model E) that seems to motivate the application of Simulink on buildings. However, the key missing link is a credible coupling with air-flow networks (model B). Small examples have been reported that are based on Simscape, but the lack of more realistic models in the literature is obvious.

Air-flow networks in buildings that are formed by a combination of flow though ductwork, large openings (doors), and small leaks through the building fabric are notoriously difficult to solve numerically. The basic methods for solving non-linear systems of equations perform poorly on these problems due to the severe difference in flow resistance between a small leak and a large doorway. Special purpose tools like Airnet, CONTAM, and COMIS are justified because they are based on special numerical methods that are efficient on this type of problem.

Unfortunately, also the mainstream Modelica environments are based on basic numerical methods (mainly undamped Newton-Raphson) for solving non-linear algebraic equations. Similarly, as for Matlab/Simulink/Simscape, the solving of the air-flow network part of the problem becomes the weakest link. Although several Modelica model libraries have been developed for building simulation, few examples of full-scale building models have been published. Both numerical performance and lack of scalability seem to prevent wider industrial application.

EQUAs solution

The basic research behind EQUAs IDA Simulation Environment (IDA SE) was carried out in the early nineties at the Swedish Institute of Applied Mathematics. The Differential-Algebraic modelling language Neutral Model Format (NMF) was developed in parallel. (NMF later became one of the DAE languages that formed the basis of Modelica.) The development of IDA SE was funded to a large extent by grants from building industry research bodies and, consequentially, building simulation problems have always been considered. In fact, one of the early IDA SE scientific papers was titled “Modeling Air Flows and Buildings with NMF and IDA” (Proc. of the IBPSA Building Simulation '93 conference). The focus already at that stage, was to be able to couple equation-based models for A-E on industrial-scale problems. Indeed, the very first industrial IDA SE application was to develop a clean room design tool.

Today, IDA Indoor Climate and Energy (IDA ICE) with its main market in Europe is one of the internationally leading BPS environments. Its ability to fully couple natural ventilation, controls, and complex HVAC systems is unique on the market. IDA ICE is also the only commercial building simulation environment that is based on equations. IDA component models today are written in Modelica or NMF and, consequentially, the functionality of the tool can easily be extended in the scope of specific projects.

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