DEMAND CONTROLLED VENTILATION (DCV) SYSTEMS

Transcription

1 DEMAND CONTROLLED VENTILATION (DCV) SYSTEMS Mari-Liis Maripuu, PhD CIT Energy Swegon Air Academy, Portugal, 27 May 211

2 Outline Introduction Concept of a DCV system Adapting DCV for commercial and public buildings Technical aspects of a DCV system Economical aspects

3 INDOOR CLIMATE HVAC systems should provide: Good indoor air quality no health risk perceived as fresh and pleasant Good thermal comfort no draught risk temperature and humidity in the comfort range Good acoustical environment quiet supply and removal of air

4 HVAC TASKS - THERMAL CLIMATE AND AIR COMPOSITION Thermal Climate Air and radiant temperatures, air velocity, humidity Air Composition Gases (i.e. CO 2, H 2 O, O 3, NO 2, etc.); Particles Heating Supply (+) Cooling Removal(-) Ventilation Supply (+) Exhaust (-) Gases Supply (+) Removal (-) Particles (Supply +) Removal (-) CAV Constant Air Volume Ventilation (1 or 2 steps, i.e. on/off) Examples of CAV: - manual operation (e.g. on/off) - time control (e.g. day/night) - supply-air temperature control (temperature COD) - outdoor-air/recirculation-air control (enthalpy COD) VAV Variable Air Volume Ventilation (>2 steps or continuous variation) Examples of VAV: - Manual Operation or predetermined pattern - Open-loop Control (OCV) - Closed-loop Control (CCV) VAV with automatic control-on-demand DCV

5 5 Energy VAV and DCV DCV is a subgroup of VAV VAV with automatic control-on-demand DCV VAV Variable Air Volume Ventilation (>2 steps or continuous variation) VAV with automatic control in relation to demand DCV Manual Operation or predetermined pattern DCV Demand Control - Automatic variation with demand

6 DEMAND CONTROLLED VENTILATION A ventilation system with feed-forward and/or feedback control of the airflow rate according to measured demand indicator The demand is determined by set of values and parameters affecting thermal comfort and/or air quality

7 DCV-SYSTEM vs CAV-SYSTEM DCV for thermal comfort control (removal of heat) DCV for indoor air quality control (removal of pollutants) Heat surplus Heat deficit Heat supply needed in the room AC in operation Need of space heating Time of day Cooling capacity of the supply air - CAV system Airflow rate CAV system Cooling capacity needed = cooling capacity of a DCV system Airflow rate DCV system Occupancy Time of day

8 ADVANTAGES OF A DCV-SYSTEM Minimizing the energy use for ventilation and space heating/cooling Lower electrical energy use for fans Lower energy use for supply air heating Minimizing the heat supply needed in the room Advantages can be taken from freecooling

9 ADVANTAGES OF A DCV-SYSTEM Low supply air temperatures possible all year around Outside temp C Air flow Supply air temp Cooling effect 14 C 1 kw C 18 C 1 kw Airflow rate m 3 /s Electrical 3 Power kw 2 Airflow rate m 3 /s DCV Measured Airflow rate m 3 /s Electrical Power kw 3 Airflow rate m 3 /s Electrical Power kw 2 CAV Estimated 1 Electical Power kw h/year h/year

10 APPLICATION OF DCV SYSTEMS For premises with varying load conditions Conference rooms/ auditoria; Schools; Restaurants; Theatres ; Offices; The more the loads are varying in time the more energy savings can be expected

11 EXAMPLES OF DCV-SYSTEM PERFORMANCE Refurbishment of an office building (CAV DCV) 35 m 2 including 17 cell office rooms Energy use for ventilation befor and after refurbisment 2 15 yr 2 1 / m h k W5 Energy performance improvement Indoor climate improvement Before renovation After renovation Heating energy for supply air Electical energy for fans Cooling energy for supply air (district cooling)

12 EXAMPLES OF DCV-SYSTEM PERFORMANCE Refurbishment of an office building (CAV DCV) 35 m 2 including 17 cell office rooms Electric Airflow Power [kw] rate [m³/s] Design airflow 5.6 m 3 /s Fan power Supply airflow rate Time, [h/year] Airflow Supply rate airflow m3/s Electricity Power kw Average 4.2 m 3 /s Average 2,6 kw

13 EXAMPLES OF DCV-SYSTEM PERFORMANCE Refurbishment of an office building (university administration) 25 m 2 including 76 office rooms Electric Airflow Power [kw] rate [m³/s] Design airflow 3,6 m 3 /s Supply airflow rate m3/s Electricity kw Average airflow 1,3 m 3 /s Time, [h/year]

14 EXAMPLES OF DCV-SYSTEM PERFORMANCE New office building (Bengt Dahlgren AB - HVAC consultancy) 42 m 2 of offices, conference rooms, restaurant Photo: Bengt Dahlgren AB Total energy use of the building 75 kwh/m 2 yr DCV system for thermal comfort control Sequence control of cooling machines Performance evaluation is ongoing

15 APPLICATION OF DCV-SYSTEMS - KEY-POINTS TO CONSIDER 1) Specify the demand: what is the purpose of ventilation? Removal of pollutants (control of indoor air quality) Removal of heat (control of thermal comfort) Safety, etc 2) Specify the control: What is suitable indicator for controlling the airflow rates? 3) What measurement techniques are available (types of sensors)? 4) What are the requirements to be set on the system, its components and control system?

16 DCV SYSTEMS IN COMMERCIAL AND PUBLIC BUILDINGS Airflow rates are based on the demands on air quality and/or thermal comfort (air cooling) Air quality is influenced by the pollutants emitted by people and their activities people furniture Oudtoor air Building materials Photo: Chalmers

17 CHOICE OF AN INDICATOR FOR IAQ CONTROL Carbon Dioxide is the most common indicator for pollutants due to human occupancy (easy to measure) Airflow rate control based on occupancy (occupancy sensors) Measurement of volatile organic compounds (VOC) Relations unclear: perceived air quality concentration levels comfort and health Sensors need further development for indoor climate control

18 CO 2 AS AN INDICATOR CO 2 is used as an indicator for perceived air quality CO 2 is an indirect measure of pollutants emitted by people CO2 does not influence health and comfort < 5 ppm Recommended CO 2 levels for good IAQ perception is about 1 ppm Percentage of dissatisfied (PPD) % Note: valid for adult visitors! Fanger, 1988 CO 2 concentration (ppm)

19 CO 2 CONCENTRATION AND AIRFLOW RATE CO2 kontsenratsioon, concentration, ppm 3 Õhuvooluhulk Airflow rate 1 l/s inim. per kohta person 2 l/s inim. per kohta person Ruumis viibimise Time, minutes aeg, minutid 4 l/s inim. per kohta person 6 l/s inim. per kohta person 8 l/s inim. per kohta person 1 l/s inim. per person kohta 12 l/s inim. per kohta person

20 TECHNICAL ASPECTS OF A DCV SYSTEM Holistic approach needed - Room, zone and system level Air-handling unit DCV variation in airflow rate variation in pressure Rooms

21 TECHNICAL ASPECTS OF A DCV SYSTEM Airflow control on a room revel Illustration: Fläktwoods VAV- dampers IAQ sensor Airflow control with VAV-dampers + works within a wide pressure range + manages bigger airflow rates - noise problems may occur - limited minimum airflow rates careful supply air diffuser design and higher supply air temperatures required to avoid risk of draught Temp sensor

22 TECHNICAL ASPECTS OF A DCV SYSTEM Airflow control on a room revel Illustration:Swegon Airflow control with VAV-diffusers Supply duct connection VAV- box VAV- diffuser Traversing motor Exhaust air Air quality IAQ sensor Temperature Temp sensor VAV-diffuser Controlling plate

23 TECHNICAL ASPECTS OF A DCV SYSTEM Airflow control on a room revel Illustration:Swegon Airflow control with VAV-diffusers VAV- box VAV- diffuser Exhaust air Air quality IAQ sensor Temperature Temp sensor CAV-diffuser

24 TECHNICAL ASPECTS OF A DCV SYSTEM Airflow control on a room revel Illustration:Swegon VAV- box VAV- diffuser Exhaust air Air quality IAQ sensor Temperature Temp sensor Airflow control with VAV-diffusers + stable air movement pattern within a wide airflow range + good control of low airflow rates + low supply air temperatures + integrated control functions - can be pressure dependent (active control dampers needed) - limited maximum airflow rates per device

25 REQUIREMENTS ON DCV AIRFLOW CONTROL AND SUPPLY AIR DEVICES Selection criteria Stable supply air pattern in the room in a wide airflow range Airflow control in a wide airflow range (1-1 %) and pressure range Quiet operation within the working range Low supply air temperature (approx.+15 C) without risk of draught AHU PS 2-12 Pa

26 MIXING VENTILATION vs DISPLACEMENT VENTILATION DCV-system is commonly based on the mixing ventilation principle Displacement ventilation is more complicated to adapt to DCV-system approach 24 C Exhaust 26 C Supply 16 C 24 C 24 C 18 C Mixing ventilation Displacement ventilation

27 DISPLACEMENT VENTILATION Ventilation effectiveness C exhaust C breath Ventilation effectiveness when cooling Mixing ventilation.9-1. Displacement ventilation Illustration: Skistad (23) Illustration: Skistad (23) 27

28 DISPLACEMENT VENTILATION Ventilation effectiveness C exhaust The ventilation rate required to maintain the stratification at a certain height C breath Illustration: Skistad (23) h The ventilation rate must match the airflow in the plume Illustration: Skistad (23) 28

29 DISPLACEMENT VENTILATION Higher airflow rates are needed Higher supply air temperatures needed (min +18 C) Reduced useable floor area Typical applications are found in the industry (large airflow rates required for removal of heat) Displacement ventilation Illustration: SP

30 REQUIREMENTS ON DCV-SENSORS The sensors must be chosen according to the demands set on indoor climate Uncertainty of measurement must be sufficient for assuring the indoor climate in within the required Response time must be sufficient for controlling the airflow rates Placement of the sensor must consider the activities in the room and physical properties of the room Not in the breading zone Close to the exhaust air duct in bigger rooms

31 TECHNICAL ASPECTS OF A DCV SYSTEM Zone and system level The duct system must manage low supply air temperatures Relative temperature change Ratio = 1 we have lost all the cooling capacity l/s 1 l/s 5 l/s 1 l/s Duct length, m Duct insulation must reduce the duct heat gains (particularly in main ducts)

32 TECHNICAL ASPECTS OF A DCV SYSTEM Zone and system level The duct system must manage low supply air temperatures Pressure control in the system: zone level & system level Pressure sensor VAV Active control damper Pressure sensor 2-5 Pa VAV diffuser

33 TECHNICAL ASPECTS OF A DCV SYSTEM Zone and system level The duct system must manage low supply air temperatures Pressure control in the system: zone level & system level Careful design of central air-handling unit and its components fan system efficiency dependent on airflow rate Optimal design = good control properties and performance of the system Efficiency, % Johan Åström, Chalmers Load, %

34 TECHNICAL ASPECTS OF A DCV SYSTEM Zone and system level The duct system must manage low supply air temperatures Pressure control in the system: zone level & system level Careful design of central air-handling unit and its components For thermal comfort control all supply air devices must manage low supply air temperature Mixing ventilation and deplacement ventilation can not be combined in a DCV system Rooms

35 EVALUATION OF OCCUPANCY- BASIS FOR THE OPTIMAL DESIGN Example 1: office building (university administration) Number of rooms 76 Occupancy factor OF N N occupied _ rooms All _ rooms Occupancy factor OF Measurement every 4,5 minute 5: 6: 7: 8: 9: 1: 11: 12: 13: 14: 15: 16: 17: 18: 19: 2: Time Maximum Average Minimum

36 EVALUATION OF OCCUPANCY Example 2: school building year 7-9 Total number of classrooms 43 Number of classes 16, 35 students Occupancy factor OF N N occupied _ rooms All _ rooms 1, ) F,9 (O,8 r,7 c to,6 fa,5 c y,4 a n p,3 c u,2 O,1, : 8 :3 8 : 9 :3 9 : 1 :3 1 : 1 :3 : Time :3 2 1 : 3 1 :3 3 1 : 4 1 :3 4 1 : 5 1 :3 5 1 : 6 1 :3 6 1 Monday Tuesday Wednesday Thursday Friday

37 EVALUATION OF OCCUPANCY Example 3: Basic school (Tomelilla) 11 classrooms monitored 24 students per classroom Average occupancy in the classrooms

38 OCCUPANCY IN BUILDINGS Depends on the type of activity in the building In office buildings the work tasks define the presence of people In school buildings occupancy dependent on on the type of the school Pre-schools, basic and middle schools use their rooms continuously (few empty rooms) Secondary schools have also rooms for specific courses and group rooms

39 ECONOMICAL ASPECTS OF DCV Evaluate the potential of DCV for the specific building How the loads are varying in time and between the rooms? Occupancy factor for the building? Evaluate the energy use of the system Evaluate the energy savings Evaluate the investments needed

40 EVALUATION OF ENERGY USE OF A DCV-SYSTEM Electrical energy for fans in a DCV-system: Airflow rate and fan power do not follow the traditional cube law Depends on pressure control in the system and placement of the pressure sensor Fan Power, design W t W t Constant static pressure in the system W f ( t V VAV α 2 ) W Airflow rate,v 3 t f ( V VAV design V No pressure control ) Fan power [kw] Example: measured supply fan power at varying airflow rates W W 1.73 t.18.21,73 V VAV f ( ) t V VAV Supply airflow rate [m³/s]

41 INVESTMENTS NEEDED FOR A DCV-SYSTEM Refurbishment projects Dismantling and construction Fan speed control after pressure in the system (frequency inverters+ pressure sensors) New air ducts (when needed) VAV-box and/or VAV-diffuser + sound attenuator Active control dampers (when needed) Sensors + cabling Control systems (+commissioning)

42 INVESTMENTS NEEDED FOR A DCV-SYSTEM New buildings Fan speed control after pressure in the system (frequency inverters+ pressure sensors) VAV-box and/or VAV-diffuser + sound attenuator Active control dampers (when needed) Sensors + cabling Control systems (+commissioning) Duct system can be optimised!

43 THANK YOU FOR YOUR ATTENTION!