Thermal Equipment and Building Component Modeling in Modelica

Kaustubh Phalak

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

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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

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Physical model

Fixed program

flow logic

Own solver Hard to

maintain & add new models

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

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Encapsulation of equations

Standardized interface

Model reuse and easy model exchange

• Write equations as they are.

• Number of unknowns = Number of equations

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𝑈𝑈𝑈𝑈𝑐𝑐𝑝𝑝

= �̇�𝑚 ln(𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏_𝑓𝑓𝑏𝑏𝑐𝑐𝑓𝑓)

𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏_𝑓𝑓𝑏𝑏𝑐𝑐𝑓𝑓=ℎ𝑜𝑜 − ℎ𝐴𝐴𝐴𝐴𝐴𝐴ℎ𝑖𝑖 − ℎ𝐴𝐴𝐴𝐴𝐴𝐴

ℎ𝐴𝐴𝐴𝐴𝐴𝐴 = 𝐸𝐸𝐸𝐸𝑓𝑓ℎ(𝜔𝜔𝐴𝐴𝐴𝐴𝐴𝐴,𝑇𝑇𝐴𝐴𝐴𝐴𝐴𝐴, 𝑏𝑏)

𝜔𝜔𝐴𝐴𝐴𝐴𝐴𝐴 = 𝑈𝑈𝑏𝑏𝑏𝑏𝐴𝐴𝐴𝐴𝑚𝑚(𝜑𝜑𝐴𝐴𝐴𝐴𝐴𝐴,𝑇𝑇𝐴𝐴𝐴𝐴𝐴𝐴, 𝑏𝑏)

𝑇𝑇𝑜𝑜 − 𝑇𝑇𝑖𝑖𝑇𝑇𝐴𝐴𝐴𝐴𝐴𝐴 − 𝑇𝑇𝑖𝑖

=𝜔𝜔𝑜𝑜 − 𝜔𝜔𝑖𝑖𝜔𝜔𝐴𝐴𝐴𝐴𝐴𝐴 − 𝜔𝜔𝑖𝑖

Modelica: for a Mechanical Engineer

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

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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

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

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Modelica Features: Object-inheritance

• Medium = Air

• Latent heat: moisture added or condensed

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Instantaneously mixed volume: with heatPort and latent heat calculation (moisture added/removed)

Multi-stage water to air heat pump

Example: DX Cooling Coil

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DX Cooling Coil: Results and Validation • Results of the DX cooling coil model are compared with the

model in E+

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Building Components Overhang and side-fins

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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

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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.

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Source: LBNL-5932E

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

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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

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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

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Questions?

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Thank you!