Institute of Energy and Sustainable Development HVAC S YSTEMS E NERGY D EMAND VS. B UILDING E NERGY D EMAND I VAN K OROLIJA I NSTITUTE OF E NERGY AND S USTAINABLE D EVELOPMENT D E M ONTFORT U NIVERSITY, L EICESTER, UK DMU. AC. UK

Institute of Energy and Sustainable Development I NTRODUCTION 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 O VERVIEW 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 B UILDING M ODEL 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/m 2 K] External Wall0.35 Flat Roof0.25 Ground Floor0.25 Glazing2.10

Institute of Energy and Sustainable Development B UILDING M ODEL 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 B UILDING M ODEL Internal heat gains:OpenCellular Occupants: (density) 108 W/m 2 (9 m 2 /person) 108 W/m 2 (14 m 2 /person) Office Equipment:15 W/m 2 10 W/m 2 Artificial Lighting:12 W/m 2 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 C OOLING /H EATING D EMAND 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 C OOLING /H EATING D EMAND 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 S YSTEM M ODELS 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 t sa Reheating boxes are controlled by t za (reverse dumper action) CAV System Main H/C coils are controlled by variable t sa Reheating coils are controlled by t za Economizer Box Constant Air Volume System (CAV) Variable Air Volume System (VAV)

Institute of Energy and Sustainable Development HVAC S YSTEM M ODELS 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 t sa Four-pipe fan-coil units controlled by t za ChCeil System Main H/C coils are controlled by t sa 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 E NERGY D EMAND OF S YSTEMS Equipment and lights electricity demand, Heating energy demand, Cooling energy demand, and Auxiliary equipment electricity demand.

Institute of Energy and Sustainable Development S YSTEM H EATING D EMANDS 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 S YSTEM C OOLING D EMANDS 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 S YSTEM A UXILIARY E NERGY D EMANDS 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 S YSTEM A UXILIARY E NERGY D EMANDS 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 C ONCLUSIONS 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.

Institute of Energy and Sustainable Development T HANK Y OU … ? DMU. AC. UK