E CONOMIC A NALYSIS AND E CONOMIC D ECISIONS FOR E NERGY RETROFITTINGS.
Institute of Energy and Sustainable Development HVAC S YSTEMS E NERGY D EMAND VS. B UILDING E NERGY...
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Transcript of Institute of Energy and Sustainable Development HVAC S YSTEMS E NERGY D EMAND VS. B UILDING E NERGY...
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Institute of Energy and Sustainable Development
HVAC SYSTEMS ENERGY DEMAND VS. BUILDING
ENERGY DEMAND
IVAN KOROLIJA
INSTITUTE OF ENERGY AND SUSTAINABLE DEVELOPMENTDE MONTFORT UNIVERSITY, LEICESTER, UK
EMAIL: [email protected]
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Institute of Energy and Sustainable Development
INTRODUCTION
• Building size/shape
• Building fabrics
• Glazing percentage / characteristics
• Shading• etc…
• Internal gains• Office space
arrangement• Daylighting• Occupancy• Temperature
setpoints• etc…
Heating sources:• Boilers (gas, coal,
biomass, liquid fuel…)
• District heating
Cooling sources:• Chillers (air-cooled,
water-cooled, thermally driven)
• District cooling
Renewables
Heating,Ventilating and
Air Conditioning(HVAC)System
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Institute of Energy and Sustainable Development
OVERVIEW
• Building model description• Analysis of building cooling/heating loads• HVAC system models description• Analysis of HVAC system models
simulation outputs
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Institute of Energy and Sustainable Development
BUILDING MODEL• Square plan office building• Three-story high• 22.5 x 22.5 m footprint • 3.5 m floor-to-ceiling height• Each floor is divided into four
zones: Zone 1 (open office)
Zone 2 (common spaces)
Zone 3 and 4 (cellular offices)
• Glazing amounts 50% of external wall area
• Building fabrics comply with the latest UK standards
Building Elements
U-value [W/m2K]
External Wall 0.35
Flat Roof 0.25
Ground Floor 0.25
Glazing 2.10
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Institute of Energy and Sustainable Development
BUILDING MODEL
• Indoor thermal condition:
Controlled by dual setpoint thermostat.
Occupied hours (weekdays between 7am and 7pm):- Offices: heated to 22°C or cooled to 24°C;- Common spaces: heated to 20°C or cooled to 26°C.
Setback temperatures: - Heating period: 12°C in the whole building- Cooling period: offices: 28°C; common areas: 30°C
Chilled Ceiling system: Cooling setpoints +2°C
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Institute of Energy and Sustainable Development
BUILDING MODEL
Internal heat gains: Open Cellular
Occupants:(density)
108 W/m2
(9 m2/person)108 W/m2
(14 m2/person)
Office Equipment: 15 W/m2 10 W/m2
Artificial Lighting: 12 W/m2 12 W/m2
• Daylight control is implemented in office zones• Illuminance target: 500 lux
• Fresh air requirements and infiltration rate:• Fresh air requirements: 10 l/s per person• Infiltration rate: 0.3 ach
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Institute of Energy and Sustainable Development
COOLING/HEATING DEMAND
Cooling/heating demands are calculated by taking into consideration standard heat gains/losses which are:
• Transmission heat gains/losses through building envelope elements,
• Solar heat gains through glazed areas,• Internal heat gains/losses from artificial lighting and
office equipments,• Infiltration air heat gains/losses, and• Fresh air ventilation heat gains/losses.
• Simulation Software: EnergyPlus v.4.0• Weather file: London Gatwick
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Institute of Energy and Sustainable Development
COOLING/HEATING DEMAND
• Cooling/heating seasons• Higher cooling demand• Equipment electricity demand – constant profile• Light electricity demand – varies by the time of the year
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Institute of Energy and Sustainable Development
HVAC SYSTEM MODELSHow do typical HVAC systems handle different building loads?
All-Air systems with zone reheating boxes:• Variable Air Volume System (VAV)• Constant Air Volume System (CAV)
VAV System• Main H/C coils are controlled by tsa
• Reheating boxes are controlled by tza (reverse dumper action)
CAV System• Main H/C coils are controlled by
variable tsa
• Reheating coils are controlled by tza
Economizer Box
Constant Air Volume System (CAV)
Variable Air Volume System (VAV)
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Institute of Energy and Sustainable Development
HVAC SYSTEM MODELSAir-Water systems with dedicated air:• Fan-coil System (FC)• Chilled Ceiling System (ChCeil)
Both systems operates with 100% fresh air
FC System• Main H/C coils are controlled by tsa
• Four-pipe fan-coil units controlled by tza
ChCeil System• Main H/C coils are controlled by tsa
• Increased cooling setpoint by 2°C• Embedded chilled water pipes• Radiators for heating
Heat Recovery Unit
Chilled Ceiling System (ChCeil)
Fan-coil System (FC)
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Institute of Energy and Sustainable Development
ENERGY DEMAND OF SYSTEMS
• Equipment and lights electricity demand,• Heating energy demand,• Cooling energy demand, and• Auxiliary equipment electricity demand.
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Institute of Energy and Sustainable Development
SYSTEM HEATING DEMANDS• System heating demands lower than building heating demand
mainly due to:1. Decreased ventilation losses
2. Additional heat gains from fans and pumps• All-air systems - mixing a warm return air stream with a cold
outdoor air stream to maintain desired setpoint• Air-water systems – using a heat recovery units with 75% eff.
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Institute of Energy and Sustainable Development
SYSTEM COOLING DEMANDS• All-air systems performs so well because of the usage of free cooling• Air water systems suffer from limited free cooling which is even more
decreased by supply air temperature setpoint
- The influence of dissipative heat gains cannot be neglected• ChCeil slightly better than FC due to 2°C higher cooling setpoint
which results in a reduction in the building fabric and ventilation cooling loads.
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Institute of Energy and Sustainable Development
SYSTEM AUXILIARY ENERGY DEMANDS
• The auxiliary energy consumption is often overlooked when discussing building energy consumption
• All-air systems have much higher consumption when compared with air-water systems
- Mainly due to higher fan consumption
• The worst system, in terms of auxiliary energy consumption, is the CAV system due to constant operation at maximum air flow rate which results in enormous fan consumption.
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Institute of Energy and Sustainable Development
SYSTEM AUXILIARY ENERGY DEMANDS
• By introducing variable flow rate in the VAV system, fan consumption is more than halved.
• Due to lower cooling demands, all-air systems also have lower pumps consumption.
• ChCeil system requires slightly less energy for auxiliary equipment, mainly due to usage of zone passive heating and cooling equipment (radiators and embedded pipes).
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Institute of Energy and Sustainable Development
CONCLUSIONS
• The presented results clearly indicate that in buildings serviced by HVAC systems, it is inappropriate to evaluate building energy performance based only on its heating and cooling loads.
• For the four investigated HVAC systems (VAV, CAV, FC and ChCeil) the difference between system demand and building demand varied from over -45% to +35% for cooling and between -10% and -70% for heating.
• The auxiliary energy consumption of the HVAC systems should not be overlooked.