the art of natural ventilation balancing heat and air flows? art of natural ventilation... · the...

The Art of Natural Ventilation – Balancing

Heat and Air Flows?

Consulting in the Science of

Buildings, Structures and

Environment

[email protected]

Rowan Williams Davis & Irwin (RWDI) Inc.

Acknowledgements

This presentation is possible because

of the efforts of the many bright and

committed colleagues I have the

pleasure of working with at RWDI.

I’d also like to acknowledge my

appreciation for the opportunity to

work on great projects, with cool

teams for inspired clients.

2

Outline

• What is natural ventilation

• Human requirements

• Driving forces

• Design process

• Examples

• Is it working?

3 WES Seminar – Is Natural Ventilation Working as Advertised? 2012/01/18

Outline

• What is natural ventilation

• Human requirements

• Driving forces

• Design process

• Examples

• Is it working?

• Does wind help or hurt?

4

What do we naturally ventilate?

• We naturally ventilate many spaces

– Hospital patient rooms - parking garages

– Offices - arenas / stadia

– Schools

– Homes

• Sometimes the spaces are about processes and

not humans

– Transformer vaults

– Industrial processes

– Antenna arrays

– Road and rail tunnels

– Emergency (smoke) ventilation

5

Princess Noura University, Riyadh Saudi Arabia

Natural Ventilation of Courtyards With Wind Towers

1 2

0%

10%

20%

30%

40%

50%

60%

Morning Midday Afternoon Evening Night Open

Spring

no tower single direction bi-directional bi-directional with fan

Why do we naturally ventilate?

• Save cooling and fan energy • Only 50% of the net zero capable buildings use natural

ventilation

• Provide acceptable indoor air quality through

fresh air **

• Enhance productivity

• Connect people to outdoors

7

Most people think of this ….

8

For many the reality is different …

9

Photo Courtesy Hanqing Wu, RWDI Photo Courtesy Hanqing Wu, RWDI

Hong Kong

Cairo

Issues to Remember

• Natural ventilation must meet people’s needs

– Temperatures & RH

– Biological airflow requirements

– Contaminants – AQ

• Ideally people should be able to feel it working

• Large consequences can come from (small)

annoyances making a building undesirable:

– Noise

– Odour

– Excessive temperatures

– Dust

– Insects, and

– Allergens 10

11 http://coimbatore.olx.in/

How People Perceive Comfort

• Comfort is a complex phenomena

• It varies from person to person

• A combination of four environment variables

– Wind speed

– Temperature

– RH

– Radiant temperatures (solar impact, hot

surfaces)

• Plus three personal factors

– Clothing levels

– Activity

• Other parameters like gender, height have a

lesser role.

4

3

2

1

0

- 1

- 2

- 3

- 4

Hot

Warm

Slightly

Warm Neutral

Slightly

Cool Cool

Cold

SP

MV

*

Thermal Comfort Indices

• There are lots of different indices promoted to

describe thermal comfort

13

Index Parameters Observations

PMV Wind, sun, temperature, RH,

activity, clothing

Wide acceptance

Only good for indoors

Operative Temp Air temperature, radiation Does not include the

impact of RH or activity.

WBGT RH, air temperature, radiation,

(wind speed)

Used to define heat stress

on a body

PET RH, air temperature, radiation,

wind speed

Defines conditions to an

equivalent indoor T.

Humidex / Heat Index Air temperature and relative

humidity

Described as a perceived

temperature.

ARRANGEMENTS

14

Two Airflow Issues

• Get the air in

– Related to the flow through the perimeter

• Circulated it well

– Dictates how efficiently we use the air that is available

• Internal quantities (AQ, temperature) scale with

air flow rate

– makes envelope flows very important

15

Natural ventilation is designed leakage

16

Buoyancy Driven Stack Ventilation

Wind Driven Cross Ventilation Single Sided Ventilation

Scale of Natural Ventilation System is Important

17

Etheridge, Natural Ventilation of Buildings – Theory,

Measurement and Design, John Wiley & Sons, 2012

Single Room

Full building

STACK EFFECT

Some Physics

18

Stack Effect – Building Open @ Top

• Summer Winter

Pressure

Heig

ht

Pressure

Heig

ht

Open skylight

Stack Effect – Building Open @ Top

• Summer Winter

Pressure

Heig

ht

Pressure

Heig

ht

ΔP = Pinside – Poutside

Open skylight

Stack Effect – Building Open@Bottom

• Summer Winter

Heig

ht

Pressure

Heig

ht

Open door

Pressure

Stack Effect – Building Open@Bottom

• Summer Winter Open door

Heig

ht

Pressure

Heig

ht


Pressure

Heig

ht

Stack Effect – Distributed Openings

• Summer Winter

Pressure Pressure

Heig

ht

Distributed


• Summer Winter Distributed

Heig

ht

Neutral

Plane

Pressure

Heig

ht


Pressure

Heig

ht

Stack Effect – Pressurisation

• Summer Winter

Pressure Pressure

Heig

ht

Distributed

Stack Effect – Pressurisation


Heig

ht

Pressure

Heig

ht


Pressure


Wind Impact Windward Side


Heig

ht

Neutral

Plane

Pressure


Pressure


Wind Impact Leeward Side


Heig

ht

Neutral

Plane

Pressure


Pressure

Pressure Difference Across Façade

With Wind, Summer Conditions

Upwind

Downwind

Picking the Neutral Plane is Important

30

Etheridge, Natural Ventilation of Buildings – Theory,

Measurement and Design, John Wiley & Sons, 2012

Needs to be above inlet

of highest room to be

ventilated by stack effect

31

CIBSE, AM10 – Natural Ventilation of non-domestic

buildings, 2005

32

CIBSE, AM10 – Natural Ventilation of non-domestic

buildings, 2005

THE DESIGN PROCESS

33

There are two approaches to natural ventilation design…

• Process 1:

– Draw building

– Draw arrows going in and out of building

• Sometimes colour arrows blue in and red out (always sure to

help)

– Install windows

• Process 2:

1. Understand site & climate

2. Understand needs of building, occupants and/or

process

3. Evaluate means to minimise pollutants / heat loads

4. Evaluate mechanisms to achieve sufficient flow

5. Assess success in achieving objective: iterate

34

Very Very Well Behaved Arrows (WBAs)…

35

Very Very Well Behaved Arrows (VWBAs)…

36

Sustainability features built into

roof element (cooling fins, PV,

wind turbines, etc.)

CIBSE AM10

Very Very Well Behaved Arrows (!)…

37

Natural Ventilation Driving Forces

• The Art of balancing driving pressure

differences and restricting pressure losses

• Two driving forces for naturally ventilated

environments

– Buoyancy due to heat gain/load: 0.3 – 3.0 Pa

– Wind driven pressures: 1 – 35 Pa

• One typically draws air from bottom to top

• Natural ventilation tends to be transient

38

The best naturally ventilated buildings are located

in sites that are planned for natural ventilation

The masterplan can contribute to the success of

natural ventilation and prevent it from being

possible.

39

Success Starts at the Masterplan

40

Deep and Complex Urban Cores Lead to Complex Wind Regimes

• Masterplan site to permit natural ventilation

• Some urban environments are very challenging

41

Even within a Complex Urban Fabric we can Give Buildings Wind

Access

42

Masterplan for Natural Ventilation in Dense Urban Fabric

Results of Assessment - Winds at Lower Roof Level

Northwest Winds

Southwest Winds West Winds

• Ventilation at this level is positive for the west

winds due to building alignment with the winds.

• In courtyard areas between the towers, winds

were very stagnant, even at this level.

• Increased spacing of the towers would allow for

better ventilation of areas between the buildings.

EXAMPLE: LAB BUILDING

UNIVERSITY NEAR TORONTO

Guess at wind Cp values

44

Problem Statement

• Energy and network flow modelling tools have

built in wind values

– Varying levels of complexity

– Not always appropriate

• The issue is sometimes how wrong are they?

• Summary of information from Sim Build Paper

45

The Role of Wind in Natural Ventilation Simulations Using Airflow Network Models

J Good, A Frisque, D Phillips

Comparison of Cp values for different wind directions

47

Wind Pressure Coefficient (Cp)

Comparison of Predictions at 180º angle of attack

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Zone1

Zone2

Zone3

Zone4

Zone5

Zone6

Zone7

Zone8

Zone9

Zone1

0

Zone1

1

Zone1

2

Zone1

3

Zone1

4

Zone1

5

Zone1

6

Zone1

7

Zone1

8

Zone1

9

Zone2

0

Zone2

1

Zone2

2

Zone2

3

Zone2

4

Wind Tunnel

IES VE Model



-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Zone1

Zone2

Zone3

Zone4

Zone5

Zone6

Zone7

Zone8

Zone9

Zone1

0

Zone1

1

Zone1

2

Zone1

3

Zone1

4

Zone1

5

Zone1

6

Zone1

7

Zone1

8

Zone1

9

Zone2

0

Zone2

1

Zone2

2

Zone2

3

Zone2

4

Wind Tunnel

IES VE Model

Avg diff = 28%

Avg diff = 29%

Wind tunnel

Commercial Code Estimate

Winds from South

Winds from East

Wind tunnel




-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Zone1

Zone2

Zone3

Zone4

Zone5

Zone6

Zone7

Zone8

Zone9

Zone1

0

Zone1

1

Zone1

2

Zone1

3

Zone1

4

Zone1

5

Zone1

6

Zone1

7

Zone1

8

Zone1

9

Zone2

0

Zone2

1

Zone2

2

Zone2

3

Zone2

4

Wind Tunnel

IES VE Model



-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Zone1

Zone2

Zone3

Zone4

Zone5

Zone6

Zone7

Zone8

Zone9

Zone1

0

Zone1

1

Zone1

2

Zone1

3

Zone1

4

Zone1

5

Zone1

6

Zone1

7

Zone1

8

Zone1

9

Zone2

0

Zone2

1

Zone2

2

Zone2

3

Zone2

4

Wind Tunnel

IES VE Model

Comparison of Cp values for different wind directions

48

Avg diff = 46%

Avg diff = 38%

Winds from West

Winds from North

Wind tunnel


Wind tunnel


EXAMPLE: HIGH SCHOOL

NEAR SEATTLE, WA

49

Courtesy Basseti Archicats

50

www.youtube.com

Courtesy of Bassetti Architects

classrooms

Site Plan & Orientation with Winds

51

Classroom wings

All Seasons (SEATAC)

• Targets:

– 500 cfm = 850 m3/hr

– Temperature at or below ASHRAE Adaptive Target

53

ASHRAE 55-2004, Figure 5.3

Courtesy Basseti Archicats

54

Building Configuration

55

North

Stacked Arrangement of Rooms

59

Anticipated using one

side of chimney as outlet

Inlets need

to heat air

in Winter

mode

60

102030405060708090100110120130140150160170180190200210220230240250260270280290300310320330340350360

4 t

o 6

8 t

o 1

0

12 t

o 1

4

16 t

o 1

8

20 t

o 2

2

24 t

o 2

6

28 t

o 3

0

32 t

o 3

4

36 t

o 3

8

40 t

o 4

2

44 t

o 4

6

48 t

o 5

0

52 t

o 5

4

56 t

o 5

8

60 t

o 6

2

0.0%

0.2%

0.4%

0.6%

0.8%

1.0%

1.2%

1.4%

1.6%

1.8%

Wind Direction

Wind Speed

0

50

100

150

200

250

300

350

-9° to -8°

-5° to -4°

-1° to 0°

3° to 4°

7° to 8°

11° to 12°

15° to 16°

19° to 20°

23° to 24°

27° to 28°

31° to 32°

0.00%

0.20%

0.40%

0.60%

0.80%

1.00%

1.20%

1.40%

1.60%

Wind Direction

Temperature [ºC]

Median T=12ºC

Heat Loads – Must be Distributed

• Occupants – 31 – 2325 W

• Laptops – 31 – 930 W

• LCD Projector – 150 W

• Printer – 100 W

• Lighting – 1080

• Solar

– depends on month & time of day

– 300 – 2700 W <- ironically both in December

61

Modelling Results – January

62

Tue Wed Thu Fri Sat Sun Mon Tue

2400

2200

2000

1800

1600

1400

1200

1000

800

600

400

200

0

Vo

lum

e flo

w (

cfm

)

100

90

80

70

60

50

40

30

20

10

Te

mp

era

ture

(°F)

Date: Tue 01/Jan to Mon 07/Jan

MacroFlo external vent: Lower class (1zone 2004 nowind louversonly.aps) MacroFlo external vent: Upper class (1zone 2004 nowind louversonly.aps) Dry-bulb temperature: (1zone 2004 nowind louversonly.aps)

Air temperature: Lower class (1zone 2004 nowind louversonly.aps) Air temperature: Upper class (1zone 2004 nowind louversonly.aps)

Lower class airflow Upper and Lower Temperature

Outdoor Temperature

Upper class airflow

Modelling Results – March

63

Tue Wed Thu Fri Sat Sun Mon Tue

2400

2200

2000

1800

1600

1400

1200

1000

800

600

400

200

0

Vo

lum

e flo

w (

cfm

)

100

90

80

70

60

50

40

30

20

10

Te

mp

era

ture

(°F)

Date: Tue 26/Mar to Mon 01/Apr

MacroFlo external vent: Lower class (1zone 2004 nowind louversonly.aps) MacroFlo external vent: Upper class (1zone 2004 nowind louversonly.aps) Dry-bulb temperature: (1zone 2004 nowind louversonly.aps)

Air temperature: Lower class (1zone 2004 nowind louversonly.aps) Air temperature: Upper class (1zone 2004 nowind louversonly.aps)

Lower class airflow

Upper class airflow

Upper and Lower Temperature

Outdoor Temperature

Temperature exceedance….

Modelling Results – Annual Daytime Hours

64

Pressure Losses & CFD Modeling of Inlet Box

• Inlet (K=2.5):

– Louver

– Damper

– Grille

• Outlet (K=1.5)

– Flow turn x 2

– Louver

• Driving pressure =

– 0.005 – 0.010 in H2O (1.25 – 2.5 Pa)

65

Motivation for Wind Tunnel Measurements

• Some chimneys in

recirculation zone

• Some locations of

classroom intakes in

recirculation zone

66

Windtunnel Measurements

68

Chimneys

Conclusions from Wind Tunnel Testing

• Took measurements on three surfaces of

chimney

• Adverse pressure differences could lead to flow

reversal

• Frequency of low flow rates at acceptable levels

– All calculations done assuming window closed.

• depends on orientation of classroom


NATURAL VENTILATION OF

TALL TOWERS

Hypothetical


Natural ventilation in tall towers

• Is difficult

– Wind impacts

– Stack effect

• The benefit is not always apparent


• Internal Flows – Level 31 – North Wind – Δt = 0

Energy Model Using Meteorological Modelling for Upper Elevations

• Interested in seeing impact

• Built a small model and ran

it at two elevations

• Office typology

– Occupancy of 0.04 p/m2,

– plug loads of 8 W/m2,

– lighting of 11 W/m2,

– fresh air of 2.5 L/s-person and

0.3 L/s-m2

79

Energy Model at 600 m

Energy Model at Grade

Comparison of Data

Lighting Occupants Plug Loads Cooling Heating DHW Fans + Pumps Total

(kWh) (kWh) (kWh) (kWh) (kWh) (kWh) (kWh) (kWh)

Ground - orig EPW 34.3 6.3 29.4 123.4 0.8 11.2 22.3 227.6

Ground - WRF EPW 34.3 6.2 29.4 130.4 0.7 11.6 23.1 235.7

Ground - WRF EPW no INF or NV 34.3 6.3 29.4 145.2 0.0 12.4 24.7 252.2

600m - WRF EPW 34.3 6.4 29.4 129.5 3.8 11.7 23.4 238.4

600m - WRF EPW no INF or NV 34.3 6.3 29.4 156.2 0.0 13.0 26.0 265.2

80

• Comparing Line 1 with Line 2 shows the WRF data is close

• Comparing Line 2 with Line 4 shows there is little difference between h=0 and h=600

• Comparing Line 2 with 3 and 4 with 5 shows that the consequence of natural

ventilation is an increase in energy demand

• Further analysis shows that the increase in infiltration offsets the benefit of natural

ventilation.

Natural Ventilation Is Helpful

82

Natural Ventilation

Seasonal Airflow & Cooling

February Infiltration Rates

83

• In Winter and shoulder seasons, when outdoor temperatures are

cooler, natural ventilation is beneficial

• During these cooler months, infiltration rates are typically greater at

lower levels


February Cooling Load w/ & w/o Nat Vent

84

• Without natural ventilation (LEFT), cooling loads are very

similar at low and high elevations

• With natural ventilation (RIGHT), significant reductions in

cooling loads are realized during Winter and shoulder seasons

Ground

600 m


February Cooling Load w/ & w/o Nat Vent

85

• Only modest reductions in cooling load are seen at ground level, even

though infiltration rates are higher

• Significant cooling load savings are seen at higher elevations

• Likely attributed to lower air temperatures at higher elevations

Without Natural Ventilation

With Natural Ventilation

Closing Thoughts

• Lots of opportunity for natural ventilation design

in buildings

– Stack effect control

• We haven’t discussed thermal mass, turbulence

benefits, designing openings, etc.

• This requires close coordination at beginning of

project between architect, MEP, SE, climate

consultant, QS, Construction, FM

89

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