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: IKOROLIJA@DMU.AC.UK

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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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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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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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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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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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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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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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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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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ENERGY DEMAND OF SYSTEMS

• Equipment and lights electricity demand,• Heating energy demand,• Cooling energy demand, and• Auxiliary equipment electricity demand.

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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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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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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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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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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.

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THANK YOU…

?IKOROLIJA@DMU.AC.UK