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G well mixed energy balance equation for each zone

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Unformatted text preview: ep (at average conditions) cont’d… • simple uniform “thermal zone” model “thermal zone” = space in a building under temperature control of a thermostat uniform zone • conditions treated as identical throughout zone • heat gain (or loss) in one part is instantly distributed throughout the zone, e.g. “well mixed” - energy balance equation for each zone determines heating and cooling requirements to maintain temperature we may think of each zone as…. air T thermostat structural mass An individual building may be represented by just 1 zone (e.g. a house) or may have many zones (e.g. large commercial buildings). ASIDE: A commonly used commercial building multiple-zone HVAC system “Variable-Air-Volume (VAV) System” O/A Damper Mixed Air Cooling Heating Variable Volume Supply Fan SAT To Other Zones Economizer R/A Damper VAV box VAV box T ZONE 1 R/F heater T ZONE 2 heater Supply Air Temperature (SAT) is usually cool relative to room temperature. SAT may be held constant (e.g. 13°C/55°F), or increased during periods when cooling load is lower (e.g. based on outside air temperature or zone loads). (End of aside.) DOE-2 / hour-by-hour simulation: • each hour the software calculates an estimate of electricity and natural gas (and other fuels, if applicable) used for key end-uses: • LIGHTING • PLUG-LOADS / RECEPTACLES • HEATING • COOLING • FANS/PUMPS General Calculation Process… Step 1: Calculate Zone Loads for each thermal zone, calculate amount of heating/cooling needed to compensate for net heat loss/gain = − heat gains heat losses (Usually trying to maintain air temperature in range of about 21 to 24°C) Step 2: Calculate HVAC Energy Use energy needed for that hour to operate HVAC equipment and meet the loads Contributions to zone loads? Heat gains from lights zone air (convective) zone structure (radiative) (convective) Heat gains from equipment (radiative) Heat transmission loss/gain through… walls roofs windows doors floors-on-ground etc must describe each • need weather data (hour-by-hour) Heat transmission loss/gain to adjacent thermal zones at different T air-conditioned office non-air-conditioned warehouse partition wall Heat transfer between zone air and zone structure (when ∆ is present between them) air T thermostat structural mass Solar gains through windows, skylights, and other components—including shading effects • amount of sunlight striking each surface each hour (beam, diffuse & reflected; angle of incidence) • solar-optical properties of materials (e.g. SHGC) Infiltration air (envelope air leakage) Heat gains from occupants (and other sources) for every hour – need estimate of how many people in each zone Note: The simulation program doesn’t know anything about the building other than what you tell it (or what the programmer told it) e.g. only knows about the exterior surfaces that you describe losses gains T air thermostat structural mass Load calculation program performs energy balance for each hour to determine amount of heating or cooling needed to maintain space conditions. Energy Balance Example on Room? (Conceptual Example 1) = 0 HVAC System 400 W 300 W = 20 ? T 80 W 40 W 120 W 1000 W 20 W 300 W = !"#$ % Energy Balance Example on Room? (Conceptual Example 2) = 15 100 W 75 W HVAC System = 20 ? 40 W 250 W T 80 W 120 W 700 W () = ##$ % 75 W NEXT STEP: Estimate HVAC energy needed to meet loads for the hour. Example Calculation: Hydronic Heating System with Zone Fan-Coils ZONE 1 T Fan-coil ZONE 2 T Boiler Fan-coil T flue ZONE 3 fuel air T Fan-coil Pump + Motor DOE-2 Default Boiler Performance Curve 100% 80% 60% HIR(PLR) 40% 20% 0% 0% 20% 40% PLR 60% 80% / = 0.082597 *+, -., = / + 1 ∙ -., + 3 ∙ -.,4 1 = 0.996764 3 = −0.079361 100% DOE-2 Default Boiler Performance Curve (Alternate Plot) 100% 80% 60% PLR HIR(PLR) 40% 20% 0% 0% 20% 40% PLR 60% 80% 100% Example Calculation: Hydronic Heating System with Zone Fan-Coils ZONE 1 T Fan-coil ZONE 2 T Boiler Fan-coil T flue ZONE 3 fuel air T Fan-coil Pump + Motor thermostat fan-coil unit T Fan-Coil fluid in fluid out air out coil air in fan + motor air filter finned coil supply and return pipes motor dual fans fluid out T Fan-Coil air out (to room) fluid in fluid out air out coil air in (from room) air in fan + motor air filter fluid in fluid in Air Coil (water-to-air heat exchanger) fluid out fins 1302 W flue boiler 930 W boiler output Zone 1 98 W loss gain fan-coil 1000 W Zone 2 loss gain 70 W electricity boiler input natural gas 1400 W electricity heat distribution loop (boiler losses) fan-coil 930 W pump fan-coil 1000 W 70 W 350 W shaft power motor 500 W electricity motor losses 150 W electricity loss Zone 3 gain Energy Balance on Heating Loop? 1302 W boiler output heat distribution loop 930 W 930 W Zone 1 fan-coil Zone 2 fan-coil Zone 3 fan-coil pump 350 W Heat Out 1302 W + 930 W + 930 W 3162 W pump shaft power Heat In 350 W Net Heat Out 3162 W - 350 W 2812 W boiler output required Bui...
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This note was uploaded on 10/04/2012 for the course ME 760 taught by Professor Davidmather during the Spring '12 term at Waterloo.

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