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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]
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
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
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
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
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
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
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
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)
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)
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.
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.
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.
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.
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).
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.
