Thermal Equipment Modeling in Modelica
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This document summarizes key aspects of using Modelica for building and equipment modeling. Modelica uses an equation-based object-oriented approach that allows easy reuse and modification of preexisting component models. It offers advantages over traditional simulation programs by allowing engineers to directly write equations as they are formulated and encapsulate component models with standardized interfaces. The document provides examples of Modelica models for HVAC components like heat exchangers and cooling coils. It also discusses validation of a Buildings library component models against ASHRAE Standard 140 and challenges associated with Modelica such as proprietary solvers and limited preexisting models.
Thermal Equipment Modeling in Modelica
- 1. Thermal Equipment and Building Component Modeling in Modelica Kaustubh Phalak
- 2. Scope • Traditional building simulation programs • Modelica • Equation-based language • Features for model development :object-instantiation, object- inheritance • Examples and results • Validation of Buildings Library • Challenges • Modelica: for a Mechanical/Equipment Engineer 2
- 3. Traditional Building Simulation Programs • Written in FORTRAN, C, C++ • Procedural programming • Developer writes a sequence of computer instructions that assigns values to variables in predefined order of execution • Mix of physical model and numerical solution algorithm, example: implementation of pump-system curve • Idealized controllers within HVAC components: hard to implement control algorithms 3 Physical model Fixed program flow logic Own solver Hard to maintain & add new models
- 4. Modelica: Equation-based language • Components/Models described by algebraic and differential equations • Equations encapsulated and represented by an icon • Standardized interface enable modeling across multiple engineering domains: electrical, mechanical, thermal 4 Encapsulation of equations Standardized interface Model reuse and easy model exchange
- 5. • Write equations as they are. • Number of unknowns = Number of equations 5 𝑈𝑈𝑈𝑈 𝑐𝑐𝑝𝑝 = 𝑚𝑚̇ ln(𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏_𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓) 𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏_𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓= ℎ𝑜𝑜 − ℎ𝐴𝐴𝐴𝐴𝐴𝐴 ℎ𝑖𝑖 − ℎ𝐴𝐴𝐴𝐴𝐴𝐴 ℎ𝐴𝐴𝐴𝐴𝐴𝐴 = 𝐸𝐸𝐸𝐸𝐸𝐸𝐸(𝜔𝜔𝐴𝐴𝐴𝐴𝐴𝐴, 𝑇𝑇𝐴𝐴𝐴𝐴𝐴𝐴, 𝑝𝑝) 𝜔𝜔𝐴𝐴𝐴𝐴𝐴𝐴 = 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴(𝜑𝜑𝐴𝐴𝐴𝐴𝐴𝐴, 𝑇𝑇𝐴𝐴𝐴𝐴𝐴𝐴, 𝑝𝑝) 𝑇𝑇𝑜𝑜 − 𝑇𝑇𝑖𝑖 𝑇𝑇𝐴𝐴𝐴𝐴𝐴𝐴 − 𝑇𝑇𝑖𝑖 = 𝜔𝜔𝑜𝑜 − 𝜔𝜔𝑖𝑖 𝜔𝜔𝐴𝐴𝐴𝐴𝐴𝐴 − 𝜔𝜔𝑖𝑖 Modelica: for a Mechanical Engineer
- 6. Modelica Features: Object- instantiation • To use and parameterize an object in a model • Two-port HX: No source-side mass flow rate or heat exchange is independent of source mass flow. e.g. GLHE, DX coil (air cooled condenser), cooling tower • Four-port HX: parallel and counter flow HX, heat pumps 6 Instantaneously mixed volume: no pressure drop, exchange heat through its heatport Flow resistance: fixed flow coefficient Four port heat exchanger: Model transporting two fluid streams between four ports with storing mass or energy
- 7. Modelica Features: Object- inheritance • To reuse existing basic models and refine their implementation • Four port heat exchanger now can be modified as chiller, water to water heat pump • Final model depends on how the physics of heat exchange and control system are designed 7
- 8. Modelica Features: Object- inheritance • Medium = Air • Latent heat: moisture added or condensed 8 Instantaneously mixed volume: with heatPort and latent heat calculation (moisture added/removed) Multi-stage water to air heat pump
- 9. Example: DX Cooling Coil 9
- 10. DX Cooling Coil: Results and Validation • Results of the DX cooling coil model are compared with the model in E+ 10
- 11. Building Components Overhang and side-fins 11
- 12. Buildings Library Validation: ASHRAE Standard 140 • Standard 140 used for testing the accuracy of building simulation models • Standard 140 documents energy performance of a thermal zone using different building energy simulation tools • Validation cases: 600, 610, 620, 630, 600FF, 900, and 900FF (low and high mass building ) • Presented at 9th International Modelica Conference 2012 12
- 13. Buildings Library Validation: ASHRAE Standard 140 • Results are not the same • Each simulation tool use different assumptions, physical models and implementations • The variation of the results is usually in a reasonable range. 13 Source: LBNL-5932E
- 14. Modelica: Challenges • Cost, proprietary solvers • Open source tools do not support all the features: Fluid package • High simulation time for multi-zone building envelopes • Limited number of models 14
- 15. Modelica: Promising future • Expands capability of building simulation tools: interfacing with multiple engineering domains • Flexible environment for modeling • Reuse of models • Less time required for model development 15
- 16. Modelica: for Mechanical/Equipment Engineer • Easy to read and understand • Less lines of code • Similarity with physical systems • Closely represent actual components • Easy to modify or write new model • More focus on model development rather than struggling with implementation issues • Easy to use: drag and drop • Easy to control the boundary conditions 16