Building Systems Simulation

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Building Systems Simulation

• Fred Buhl

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General Philosophy of the EnergyPlus Interface
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Trade-off Between Simulation Flexibility and
Simplicity

• HVAC Simulation Based on Configurable Input
  Template Systems
• Code Allows Connection Flexibility
• Input Defines Specific System Configurations -
  Connections are in the Input
• Completely Modular Systems and Real-Time Controls
  Handled by Links to SPARK and TRNSYS

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HVAC Simulation High Level Structure
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Defining HVAC Systems as Components on a Fluid
Loop

• Loops Define the Movement of Mass and Energy
  Within an HVAC System
• Air Loop
• Plant Loop
• Condenser Loop
• Electric Loop
• Systems Defined by the Components on the HVAC
  Loops

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Air Loop Simulation

• Connects water side with zone equipment
• Simulates central air distribution, mixing, and
  conditioning
• Equipment supply return fans, central heating
  and cooling coils, heat recovery, mixed air box
• Control supply air temperature, outside air
  economizer

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Zone Equipment Simulation
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Simplified Plant Node and Equipment Diagram
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Typical System
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Demo Building
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Describing a System

• System Configuration
• directed graph arcs and nodes
• Component Instantiation
• create each component with unique data
• Attach Components to System Graph
• associate components with nodes
• Specify System Control

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Eplus Components

• Simple
• input initialize calculate report
• Compound
• assembled from multiple components
• simulated sequentially with overall control
• Complex
• component may be a system

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Eplus System Control

• High Level Set Point Managers
• algorithms establish fluid loop set points
• similar to an EMS
• Control Inividual Components
• sense state at one point in system
• control flow at another point to match setpoint
• Plant component sequencing load management

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Zone Equipment

• Exhaust Fan
• Fan Coil Unit
• Window Air Conditioner
• Unit Ventilator
• Unit Heater
• Hot Water Baseboard

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Terminal Units

• Dual Duct Constant Volume
• Dual Duct Variable Volume
• Single Duct Constant Volume Reheat
• Single Duct Variable Volume Reheat

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Air System Components

• Hot Water Coil
• Chilled Water Coil
• Electric Heating Coil
• Gas Heating Coil
• DX Coil Unit
• Supply Retrun Fan
• Outside Air Mixer Economizer

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Plant Equipment

• Boiler
• Water Heater
• Chiller (performance curve based)
• Pump
• Cooling Tower
• Air Cooled Condenser

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Special Input Objects

• Schedules
• Curves
• Branches and Connectors
• Nodes
• Plant Operation Schemes
• Etc.

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Summary

• Loops are backbone of HVAC simulation
• all equipment attached to fluid loops
• zone connected to fluid loops via zone equipment
• Loop structure permits assembly of any system
  through input--not hardwired code
• Manager software structure allows future use of
  general solvers

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General Philosophy of Input/Output/Weather

• Simple, free-format text files
• SI units only
• Comma-separated
• Object-based
• Somewhat self-documenting
• Two partsdictionary and data or simulation
  results
• Not user-friendly Interfaces needed
• Can become large
• Designed for Flexibility for Developers

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Input Data Dictionary Example

• ZONE EXHAUST FAN,
• A1 , \field Exhaust Fan Name
• A2 , \field Availability Schedule
• \type object-list
• \object-list ScheduleNames
• N1 , \field Fan Total Efficiency
• \type real
• \minimum 0.0
• \maximum 1.0
• N2 , \field Delta Pressure
• \units Pa
• N3 , \field Max Flow Rate
• \units m3/sec
• A3 , \field Fan_Inlet_Node
• A4 \field Fan_Outlet_Node

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Input Example

• ZONE EXHAUST FAN, Zone 2 Exhaust Fan,
• FanAndCoilAvailSched,
• 0.6, ! efficiency
• 125., ! pressure rise
• 0.1, ! flow rate
• Zone 2 Exhaust Node, ! inlet node
• Zone 2 Exhaust Fan Outlet Node ! outlet
  node

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Input Summary

• IDF Objects can be in Any Order
• IDF Objects are retrieved by each module to keep
  data local
• There are No Hard-wired Input Limits in
  EnergyPlus Limits depend on Disk and Memory of
  Users Computer

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User Output Reporting Flexibility

• User can Select Any Variables Available for
  Output
• User can Specify Output at Time Step, Hourly,
  Daily, Monthly, or Environment Intervals
• User can Schedule Each Output Variable
• User can Supply a Meter, i.e. Specify All Chiller
  Plants for Total Energy Consumption