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MODELING FUNDAMENTALS

MODELING FUNDAMENTALS. IBPSA - USA. Shell Geometry General Concepts. 2. Shell Geometry Use Of Energy Modeling Wizards. In what cases are energy modeling wizards most useful?. After making edits in main program. 3. Shell geometry Rules of Thumb For Simplification.

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MODELING FUNDAMENTALS

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  1. MODELING FUNDAMENTALS IBPSA - USA

  2. Shell GeometryGeneral Concepts 2

  3. Shell GeometryUse Of Energy Modeling Wizards In what cases are energy modeling wizards most useful? After making edits in main program 3

  4. Shell geometryRules of Thumb For Simplification ASHRAE 90.1-2007 Appendix G • Table G3.1, #5 Building Envelope, Exceptions (a) and (b) • Uninsulated assemblies • Exterior surfaces whose azimuth, orientation, and tilt differ by < 45˚ Simplify REALITY ENERGY MODEL • Thermodynamically, only (3) things matter for modeling heat transfer surfaces • Area • Orientation • Tilt • Total volume matters IF infiltration is specified in ACH 4

  5. Shell geometryRelative Placement Of Surfaces • What Matters • Area • Orientation • Tilt Note: With daylighting the building form is important 5

  6. Shell geometryRelative Placement Of Surfaces Annual Energy by Enduse Annual Energy by Enduse 6

  7. Shell geometryGeometry Interfaces 7

  8. Shell geometryGeometry Interfaces • Building Information Modeling (BIM) • Generating and managing building data • Well developed for architecture, needs improvement on MEP side • Early development phase for energy modeling 8

  9. Shell geometryAshrae 90.1 Applications 9

  10. Effective ZoningGeneral Concepts 10

  11. Effective ZoningCriteria For Zoning An Energy Model 11

  12. Effective ZoningSpaces Versus Thermal Zones Energy Modeling • Typical one zone for each space • Hourly loads are calculated based on an energy balance of the space. • At the thermal zone level, the loads from the spaces are considered in conjunction with the temperature set-point and HVAC operating schedules to determine the zone load. Thermal Zone = area controlled by a single thermostat 12

  13. Effective ZoningZone Types Within An Energy Model 13

  14. ConstructionsOverview Types of Constructions Parallel Path Calculations for a Stud Wall Quickvs.D e l a y e d 14

  15. ConstructionsExterior (Delayed) Constructions - opaque What about construction assemblies with parallel heat transfer paths? 15

  16. ConstructionsParallel Path Calcs for Wood Stud Wall ORNL Online Calculator ASHRAE 90.1 Appendix A Wall Section Typical Stud Wall R-Value of Insulated Section R-Value (Inside Air Film) R-Value (Insulation) R-Value (Sheathing) R-Value (brick) R-Value (Gyp. Board) = + + + + R-Value (Inside Air Film) R-Value (Insulation) R-Value (Sheathing) R-Value (brick) R-Value (Gyp. Board) R-Value of Stud Section = + + + + Overall Weighted R-Value of Wall Assembly () ( ) R-Value of Insulated Section R-Value of Stud Section 0.09 = x + x 0.91 16

  17. ConstructionsSlab Heat Transfer Do you need to perform outside calculations? Slab Heat Transfer Underground Surfaces: How to get a better underground heat transfer calculation in DOE-2.1 by Fred Winkelman 1) Choose F-factor from a series of tables 2) Calculate the exposed perimeter and area of slab. Use equation Reffective = A / (F*Pexposed) 3) Set Ueffective = 1/Reffective. 4) Create a material with Reffective 17

  18. ConstructionsGlazing Constructions • Glazing Properties • Center of Glass U-value • Solar Heat Gain Coefficient (SHGC), OR Shading Coefficient (SC) • Visible Light Transmission (VLT) • Light to Solar Heat Gain Ratio (LSG) Common Pitfall: Outside Air Films 18

  19. ConstructionsGlazing Constructions Includes Spectral Data: varies SHGC and Tvis with solar angles 3 Options for Modeling Glazing SHGC = solar heat gain coefficient Tvis = visible light transmission 19

  20. ConstructionsWindow Framing 2 Options for Modeling Framing Common Pitfall: Window 6 does not include framing when you export files Common Pitfall: Modeling large bands of glass 20

  21. Lighting Occupancy & Plug LoadsGeneral Concepts • Total watts of all connected power • Peak number of occupants • Can be input with density values Peak Power and Occupancy • Daily/Weekly/Annual Occupancy Schedules • Hourly fractional multiplier for peak values • Daylight Dimming or Occupancy Sensors Fractional Schedules • Assign proportional amounts of heat to space vs. plenum Fraction of Heat Gain to space 21

  22. Lighting Occupancy & Plug Loads Peak Power and Occupancy 22

  23. Lighting Occupancy & Plug Loads - Schedules • Just as important as peak values! • Unregulated by ASHRAE Std 90.1 23

  24. Lighting Occupancy & Plug Loads - Fraction of Heat Gain to Space Radiative (time lag) vs. Convective 24

  25. Lighting Occupancy & Plug Loads Daylighting 25

  26. Lighting Occupancy & Plug Loads Exterior Lighting 26

  27. Lighting, Occupancy & Plug Loads Overestimates of Peak Equip Power 27

  28. Mechanical SystemsOverview Gain and Losses: • Lights • People • Internal equipment (e.g. computers) • Building envelope (sun, outside temps) • Ventilation/infiltration Q=Σ gains + losses + ventilation load Equipment Sizing Q = (1.08)*cfm*(MAT-SAT) air Q = 500 * ΔT * GPM water 28

  29. Mechanical SystemsCooling and Heating Loads Mechanical HVAC systems move energy from one space to another Cooling systems Reject heat to the outdoors via condensers/cooling towers Heating systems Deliver heat to the internal space k 29

  30. Mechanical SystemsPackaged & Central Plant System Diagrams Central Plant supply fan compressor Water Side condenser PackagedSystem Air Side 30

  31. Mechanical systemsPackaged Systems Energy Modeling Tip: Do not double count fan, compressor and condenser power Packaged systems can serve single or multiple zones 31

  32. Mechanical systemsCentral Plant Systems Energy Modeling Tip: Pay attention to pump power and part load curves Central plant systems typically serve multiple zones 32

  33. Mechanical SystemsTerminal Units Standard VAV box with reheat coil Important Inputs • Min. airflow fraction • Fixed or scheduled • Thermostat type • Proportional vs. reverse acting • Terminal unit fan power Variable airflow Series fan-powered VAV box with reheat coil Constant airflow, fan always on Parallel fan-powered VAV box Variable airflow, fan on when reheat needed Reference: Advanced VAV Design Guideline, Appendix 8 How to Model Different VAV Zone Controls in DOE2.2 www.energydesignresources.com 33

  34. Mechanical SystemsFan Curves • Fan power = f(airflow) for VAV systems • “Canned” & custom curves • Fan Curve Issues: • “Canned” VSD fan curves are often optimistic • If creating a custom curve, plot it and check it, set appropriate minimum value • ASHRAE 90.1 Appendix G specifies the curve to be used for VAV systems Source: DOE2.2 Volume 2 Dictionary 34

  35. Mechanical SystemsFan Curves – 90.1 Appendix G Curve 35

  36. Mechanical SystemsFan Curves – Static Pressure Reset Control • Static Pressure (SP) Reset • Continuously adjust pressure to lowest setting that provides adequate zone airflow • Simulate using fan curve Reference: Advanced VAV Design Guideline, Appendix 5 Includes fan curve coefficients www.energydesignresources.com 36

  37. Mechanical SystemsChiller Curves • Chiller performance model • Capacity = f(temp) • Efficiency = f(temp, part-load ratio) • Represent chiller types • Centrifugal, rotary, reciprocating… • Variable speed, multi-compressor… • Default vs. custom coefficients 1.0 at full load and rated temp. Reference CoolTools Chilled Water Design Guide. Chiller Bid and Performance Tool, (Excel spreadsheet). www.energydesignresources.com Issues Part load efficiency curve typically includes PLR: (EIR = energy input ratio = 1/COP) 37

  38. Mechanical SystemsOutside Air Requirements • Significant implications for annual energy consumption • Energy Models: cfm/person OR cfm/sf OR cfm • PRM: same OA in Proposed and Baseline • Exception: demand control ventilation • Healthcare ventilation: Standard 170 • Exhaust requirements mandatory (section 6.5) ASHRAE 62.1 38

  39. Mechanical SystemsASHRAE 62.1: Ventilation Rate Procedure Vbz = Rp*Pz + Ra*Az Vbz= cfm of outside air required in breathing zones Rp = outdoor airflow rate per person from Table 6-1 [cfm/person] Pz = the largest number of people expected to occupy the zone during typical usage [people] Ra = outdoor airflow rate per unit area from ASHRAE 62.1 Table 6-1 [cfm/sf] Az = occupied floor area of zone [sf] • Used to determine design OA for energy models • Calculating OA for multi-zone VAVs: huge energy implications • At part-load/occupancy, the minimum OA intake flow ≥ Ra*Az. 39

  40. Mechanical SystemsASHRAE 62.1: Indoor Air Quality (IAQ) Procedure Design approach: Allows OA rates to vary if contaminant levels are below recommended levels 40

  41. Mechanical SystemsASHRAE 62.1: Natural Ventilation Procedure • Prescriptive requirements • Ceiling height • Openable passages ≥ 4% of floor area • OR • Engineered system with CFD modeling • 62.1-2010 requires mechanical ventilation UNLESS • OA passages are permanently open, OR • NO heating or cooling system is installed • Controls required for coordination with mechanical ventilation systems 41

  42. Mechanical SystemsDemand Control Ventilation (DCV) • Ventilation airflow resetsbased on occupancy using • CO2 sensors, timers, occupancy sensors or schedules • Higher energy savings for buildings with large occupancy swings • Movie theaters, conference rooms • 10%-30% load reduction and 2-3 yr payback 42

  43. Mechanical SystemsASHRAE Standard 55 Possible to assess within energy models that accurately simulate radiative heat transfer 43

  44. Mechanical systemsSpecific Energy Modeling Notes • EER: break out fan power and compressor power • Part load curves • Altitude effects • Auto-sizing • Rated vs design conditions Common Energy Modeling Mistakes 44

  45. Utility ratesTypes of Charges and Rate Structures $35 per month $0.06 per kWh $7.53 per kW $0.40 per KVAR 45

  46. Utility ratesTypes of Charges and Rate Structures Energy Charge Demand Charge Block 3 Block Charge Block 2 Time of Use Rate Block 1 46

  47. ASHRAE 90.1-2007 Appendix G Applications Utility RatesEnergy Modeling Implications • Same energy rates must be used for Proposed and Baseline • Use either actual utility rates or EIA state averages, except: • Actual utility rates must be used for purchased hot water, steam and chilled water • On-site renewables and site-recovered energy are NOT included with purchased energy 47

  48. Utility RatesEnergy Modeling Implications Case Study: Adam Joseph Lewis Center at Oberlin College • Project Goals • Set an example for energy efficiency and sustainable design • Net-zero energy building • The project achieved significant reductions in total energy use • However, no efforts were made to lower the peak demand, which resulted in a much lower energy cost savings 48

  49. Weather Data 49

  50. Weather DataAnnual Weather Files • Necessary for annual energy and economic analysis • Useful for developing HVAC design strategies • Must include 8760 hours • Generally from sets of averaged data 50

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