HVAC Systems Energy Demand vs. Building Energy Demand

1. HVAC Systems Energy Demand vs. Building Energy Demand Ivan Korolija Institute of Energy and Sustainable Development De Montfort University, Leicester, UK email: ikorolija@dmu.ac.uk

2. 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

3. Overview • Building model description • Analysis of building cooling/heating loads • HVAC system models description • Analysis of HVAC system models simulation outputs

4. 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

5. 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

6. Building Model • 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

7. 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

8. Cooling/Heating Demand • Cooling/heating seasons • Higher cooling demand • Equipment electricity demand – constant profile • Light electricity demand – varies by the time of the year

9. HVAC System Models How 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 Variable Air Volume System (VAV) Constant Air Volume System (CAV)

10. HVAC System Models Air-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 Fan-coil System (FC) Chilled Ceiling System (ChCeil)

11. Energy Demand of Systems • Equipment and lights electricity demand, • Heating energy demand, • Cooling energy demand, and • Auxiliary equipment electricity demand.

12. System Heating Demands • System heating demands lower than building heating demand mainly due to: • Decreased ventilation losses • 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.

13. 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.

14. 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.

15. 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).

16. 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.

17. Thank You… ? ikorolija@dmu.ac.uk