Wangda Zuo

Modelica Buildings Library

This article describes the Buildings library, a free open-source library that is implemented in Modelica, an equation-based object-oriented modelling language. The library supports rapid prototyping, as well as design and operation of building energy and control systems. First, we describe the scope of the library, which covers heating, ventilation and air-conditioning systems, multi-zone heat transfer and multi-zone airflow and contaminant transport. Next, we describe differentiability requirements and address how we implemented them. We describe the class hierarchy that allows implementing component models by extending partial implementations of base models of heat and mass exchangers, and by instantiating basic models for conservation equations and flow resistances. We also describe associated tools for pre- and post-processing, regression tests, co-simulation and real-time data exchange with building automation systems. The article closes with an example of a chilled water plant, with and without water-side economizer, in which we analysed the system-level efficiency for different control setpoints.

Real-time or faster-than-real-time simulation of airflow in buildings

UNLABELLED Real-time flow simulation is crucial for emergency management in buildings, such as fire and accidental or intentional release of chemical/biological agents (contaminants). The simulation results can then be used to impose proper measures to minimize casualties. Computational fluid dynamics (CFD) is accurate, but too time-consuming. Nodal models are fast, but not informative. To obtain a quick and informative solution, this study proposes an intermediate approach between nodal models and CFD by introducing a fast fluid dynamics (FFD) method. This investigation used the FFD methods with and without turbulence treatments to study systematically four basic flows in buildings, and compared the numerical results with the corresponding CFD results and the data from the literature. The results show that, on one hand, the FFD can offer much richer flow information than nodal models, but less accurate results than CFD. On the other hand, the FFD is 50 times faster than the CFD. The results also show that the FFD with the laminar assumption has the best overall performance as regards both accuracy and speed. It is possible to conduct faster-than-real-time flow simulations with detailed flow information by using the FFD method. PRACTICAL IMPLICATIONS The paper introduces a fast fluid dynamics (FFD) method, which can simulate airflow and contaminant dispersion in buildings with real-time or faster-than-real-time speed and provide informative solutions. As an intermediate approach between nodal models and the computational fluid dynamics (CFD), the FFD can be a very useful tool for emergency management in case of fire and accidental or intentional release of chemical or biological agents in a building or around the buildings. The FFD can also be used as a preliminary test tool for quick assessment of indoor airflows before a detailed CFD analysis.

Functional mock-up unit for co-simulation import in EnergyPlus

This article describes the development and implementation of the functional mock-up unit (FMU) for co-simulation import interface in EnergyPlus. This new capability allows EnergyPlus to conduct co-simulation with various simulation programs that are packaged as FMUs. For example, one can model an innovative Heating, Ventilation, and Air Conditioning (HVAC) system and its controls in Modelica, export the HVAC system and the control algorithm as an FMU, and link it to a model of the building envelope in EnergyPlus for run-time data exchange. The formal of FMUs is specified in the functional mock-up interface (FMI) standard, an open standard designed to enable links between disparate simulation programs. An FMU may contain models, model description, source code, and executable programs for multiple platforms. A master simulator – in this case, EnergyPlus – imports and simulates the FMUs, controlling simulation time and coordinating the exchange of data between the different FMUs. This article describes the mathematical basis of the FMI standard, discusses its application to EnergyPlus, and describes the architecture of the EnergyPlus implementation. It then presents a typical workflow, including pre-processing and co-simulation. The article concludes by presenting two use cases in which models of a ventilation system and a shading controller are imported in EnergyPlus as an FMU.

Simulating Natural Ventilation in and Around Buildings by Fast Fluid Dynamics

Natural ventilation is a sustainable technology that can provide a well-built environment and also save energy. The application of natural ventilation to buildings requires a careful approach in the early design phase, and fast, simple design tools are greatly needed. Fast fluid dynamics (FFD) can provide useful airflow information at a speed much faster than CFD so that it is a potential design tool for natural ventilation. This study thus validated FFD with test cases representing different types of natural ventilation. The results showed that FFD was capable of predicting the main air flow feature and ventilation rate with reasonable accuracy for wind-driven or buoyancy-driven natural ventilation. FFD simulation can reflect the influence of wind direction and surrounding buildings on natural ventilation.

Improvements in FFD Modeling by Using Different Numerical Schemes

Indoor environment design and air management in buildings requires fast simulation of air distribution. A fast fluid dynamics (FFD) model seems very promising. This work was to develop the FFD by improving its speed and accuracy. Enhancement of computing speed can be realized by modifying the time-splitting method. Improvements in accuracy were achieved by replacing the finite-difference scheme by the finite-volume method and by proposing a correction function for mass conservation. Using the new FFD model for several indoor air flows, the results show significant reduction in computing time and great improvements on accuracy.

Modeling of Heat Transfer in Rooms in the Modelica "Buildings" Library

This paper describes the implementation of the room heat transfer model in the free open-source Modelica “Buildings” library. The model can be used as a single room or to compose a multizone building model. We discuss how the model is decomposed into submodels for the individual heat transfer phenomena. We also discuss the main physical assumptions. The room model can be parameterized to use di↵erent modeling assumptions, leading to linear or non-linear di↵erential algebraic systems of equations. We present numerical experiments that show how these assumptions a↵ect computing time and accuracy for selected cases of the ANSI/ASHRAE Standard 1402007 envelop validation tests.

Simulations of Air Distributions in Buildings by FFD on GPU

Building design and operation often requires real-time or faster-than-real-time simulations for detailed information on air distributions. By solving the Navier-Stokes equations and transportation equations for energy and species, Fast Fluid Dynamics (FFD) model can provide detailed information as a Computational Fluid Dynamics (CFD) model. Compared to the CFD, the FFD is 50 times faster with some compromise in accuracy. But the accuracy and speed of the FFD model can be further enhanced by improving its numerical schemes. In addition, it was found that the computing time of the FFD program can be reduced up to 30 times by executing on a Graphics Processing Unit (GPU) instead of a Central Processing Unit (CPU). Furthermore, the FFD simulation can be accelerated by optimizing the GPU code and by using multiple GPUs. As a whole, it is possible to perform real-time simulation for a moderate size building with 107 grids and Δt = 0.1s using the FFD on GPUs

Improvements of Fast Fluid Dynamics for Simulating Air Flow in Buildings

Fast fluid dynamics (FFD) can potentially be used for real-time indoor air-flow simulations. This study developed two-dimensional fast fluid dynamics (2-D FFD) into three-dimensional fast fluid dynamics (3-D FFD). The implementation of boundary conditions at the outlet was improved with a local mass conservation method. A near-wall treatment for the semi-Lagrangian scheme was also proposed. This study validated the 3-D FFD with five flows that have features of indoor air flow. The results show that the 3-D FFD can successfully capture the three dimensionality of air-flow and provide reliable and reasonably accurate simulations for indoor air flows with a speed of about 15 times faster than current computational fluid dynamics (CFD) tools.

Reduction of Numerical Diffusion in FFD Model

Abstract Fast flow simulations are needed for some applications in building industry, such as the conceptual design of indoor environment or teaching of Heating Ventilation and Air Conditioning (HVAC) system design in classroom. Instead of pursuing high accuracy, those applications require only conceptual distributions of the flow but within a short computing time. To meet these special needs, a Fast Fluid Dynamics (FFD) method was proposed to provide fast airflow simulation with some compromise in accuracy. This study is to further improve the FFD method by reducing the numerical viscosity that is caused by a linear interpolation in its semi-Lagrangian solver. We propose a hybrid scheme of a linear and a third-order interpolation to reduce the numerical diffusion in low order scheme and the numerical dispersion in high order scheme. The FFD model with both linear and hybrid interpolations are evaluated by simulating four different indoor flows. The results show that the hybrid interpolation can significantly improve the accuracy of the FFD model with a small amount of extra computing time.

Coupling indoor airflow, HVAC, control and building envelope heat transfer in the Modelica Buildings library

This paper describes a coupled dynamic simulation of an indoor environment with heating, ventilation, and air conditioning (HVAC) systems, controls and building envelope heat transfer. The coupled simulation can be used for the design and control of ventilation systems with stratified air distributions. Those systems are commonly used to reduce building energy consumption while improving the indoor environment quality. The indoor environment was simulated using the fast fluid dynamics (FFD) simulation programme. The building fabric heat transfer, HVAC and control system were modelled using the Modelica Buildings library. After presenting the concept, the mathematical algorithm and the implementation of the coupled simulation were introduced. The coupled FFD–Modelica simulation was then evaluated using three examples of room ventilation with complex flow distributions with and without feedback control. Further research and development needs were also discussed.