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Natural Ventilation Theory and Design

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					Sustainability in Built Environment




                The Air We Breathe
- Indoor Air Quality and Natural Ventilation
                             Dr Yuguo Li
                         Associate Professor
                 Department of Mechanical Engineering
                     The University of Hong Kong

                                      Sustainable Built
                                       Environment
 IAQ Information

 http://www.iaq.gov.hk/
   IAQ Information Centre;
   IAQ Management Group
 http://www.epa.gov/iaq/




                   Sustainable Built
                    Environment
What is sustainability?
 “Sustainable development is development that meets the
  needs of the present without compromising the ability of
  future generations to meet their own needs”.

          The United Nations World Commission on Environment and Development,
                                                   The Brundtland Report, 1987


 Environmental Impact = Population 
 Affluence per person  Technology

                              Proposed by Paul Ehrlich and John Holden
                              Sustainable Built
                               Environment
Ambient air quality
    City   Population (Million) TSP (mg/m 3)
   Dalian           3               185
  Shanghai         13               246
   Beijing         11               377
 Stockholm         1.5                9
 Melbourne          3                35
 Hong Kong          6                95

 Note: Data from Air Quality in Hong Kong, 1999, EPD, HK
       and 2000 World Development Indicators, World Bank
                       Sustainable Built
                        Environment
 Topics
 Indoor air quality is more important than
  ambient air quality
 Natural ventilation can be an effective and
  energy-efficient ventilation and cooling
  strategy
 Discussion - What can we do?



                    Sustainable Built
                     Environment
 Our basic needs
 Healthy and comfortable - Thermal, visual,
  acoustical, tongue, air quality
 We spend more than 90% of our time
  indoors
 Indoor air quality is defined as the nature of
  indoor air that affects the health and well-
  being of occupants

                    Sustainable Built
                     Environment
 Chemical balance of human body

 The energy supply of life provided by
  chemical reaction of carbohydrates, fats and
  proteins with oxygen and produces CO2 and
  H2O.
 Carbohydrates from food, can be stored in
  out body
 We extract oxygen from breathing air.
  Oxygen cannot be stored in our body.

                   Sustainable Built
                    Environment
 Do you know?
 For normal conditions and a sedentary person:
    Takes about 15-40 breaths per minute or 22 k to 57 k breaths per
     day
    During each air change, about 1 litre of air is replaced
    We breathe about 10 to 45 m3 of air per day. How big is out bed
     room? 3 X 5 X 3 = 45 m3?
    Other way to find out how much oxygen is needed? – We need
     1500 to 4000 kcal per day. As one litre of oxygen is needed to
     produce 4.8 kcal, we need 300-800 litres of oxygen per day, I.e.
     1.5 to 4 m3 of air.
    Not all the oxygen is extracted from the air inhaled with each
     breath and exhaled air contains 16% residual oxygen.

                             Sustainable Built
                              Environment
 Acceptable IAQ
 Simplistic views: Good air quality means freedom from
  unwanted odours, and restriction of air contaminant
  concentrations below the levels at which irritation or toxic
  effects manifest themselves.
 Cost of poor indoor air quality costs Australians A$12
  billion per year – CSIRO
 There is a need for compromise between good IAQ and
  cost of achieving good IAQ
 ASHRAE 62-1999: Acceptable IAQ means 80% or more
  people do not express dissatisfaction.

                           Sustainable Built
                            Environment
 Indoor Air Quality

 Typical indoor spaces
  Homes, offices, public spaces
 Main indoor pollutants
  Indoor generated
  Outdoor sources
 Control


                   Sustainable Built
                    Environment
 Two approaches to control IAQ

 Source control
  Use better materials
  Cleaning and filtration
 Ventilation control
  Local ventilation
  General ventilation


                   Sustainable Built
                    Environment
 Questions so far -
 Is IAQ different from thermal comfort?
 Is ventilation different from air
  conditioning?
 Indoor air and outdoor air, which one is
  better?
 What are the factors affecting indoor air
  quality?

                     Sustainable Built
                      Environment
Why individual control?
1. To handle personal difference and it may be difficult to
   ensure everyone is thermally comfortable.
2. Most occupants prefer some degree of individual
   control
                               Desired Control Over Office Environment
                                        4%     6%




                                                           23%      No Control
                                                                    Little Control
                                                                    Some Control
                                                                    Complete Control




                              67%




                          Sustainable Built
                           Environment
 Why provide HVAC?
 For IAQ
    Fresh air delivery
    Removal or dilution of pollutants
 For thermal comfort
    Removal or addition of heat loads
    Comfortable living or working conditions
    Certainty of the thermal environment
 Other issues - productivity, cost, flexibility, risk,
  energy efficiency, environment...

                        Sustainable Built
                         Environment
What’s wrong
with the ”old” HVAC and natural ventilation?


   • ”Old” HVAC
       •   Energy consuming
       •   Sick building syndrome
       •   Minimum ventilation rate
       •   Mostly centralised
   • Natural ventilation
       • Do not work at extreme conditions
       • Difficult to control
       • Difficult to predict and design

                              Sustainable Built
                               Environment
Issues
 Air conditioning – use 48% of energy in
                and 30% in residential buildings
  commercial buildings

 30% of offices in HK are sick buildings
 Energy = CO2 + $
 Poor indoor air quality = poor productivity
 Fanger’s study showed 6% improvement in
  productivity

                         Sustainable Built
                          Environment
Three Periods        in Indoor Environment Design




             1900           1973         199?


Natural Design   Mech Design Hybrid/Integrated
                              Design



                     Sustainable Built
                      Environment
Wind Ventilation
             Wind causes a positive pressure on
              the windward and negative on the
              leeward, thus air flows into the
              windward opening and out of the
+       -     leeward one.




                   Especially effective
                   in summer

                  Sustainable Built
                   Environment
Stack Ventilation
              Difference in air density induced
               by temperature or humidity
               differences drives vertical air
               flows




                       Especially
                       effective in
                       winter

              Sustainable Built
               Environment
Combined Ventilation
                           Wind assists (??) stack
                             ventilation
+                -


                        q  q q         2
                                         s
                                                     2
                                                     w
       Flow rate (m3/s)
          qs - flow rate due to stack alone (m3/s)
          qw - flow rate due to wind alone (m3/s)

                     Sustainable Built
                      Environment
 Questions so far -
 With ASHRAE 62-1999, we need 10 litre
  per second outdoor air per person, do we
  need really so high natural ventilation flow
  rate as shown earlier?
 Do you always turn on your air conditioner
  at home?
 Is natural ventilation always good?


                    Sustainable Built
                     Environment
What Natural Ventilation Can
Offer Us?
 Fresh air, dilute or remove pollutants
             Ventilation for IAQ
 Remove heat, provide thermally comfortable air
             Ventilation for Cooling
 Connect us to natural environment
 Remove smoke
 …...


                       Sustainable Built
                        Environment
Opportunities
 Energy saving by 10-50%
 Better indoor air quality in winter, spring
  and autumn
 Adaptive thermal comfort
 Connect users to environment
 Improved productivity if designed properly


                    Sustainable Built
                     Environment
Top-down ventilation
                Better air quality at roof
                 level in urban areas
                Experimentally proved by
                 Hunt and Holford (1998)




     Room


             Sustainable Built
              Environment
Wind towers
                  Features in Arabic
                   architecture
                  Direct winds into building
                  Together with
                   evaporative cooling - cool
                   towers
                  At night, acts as
      Room         chimney?



              Sustainable Built
               Environment
 Questions so far -
 In order to use more natural ventilation, is it
  useful to improve ambient air quality?
 What are the differences between outdoor
  air and fresh air?
 Please list three example natural ventilation
  strategies



                     Sustainable Built
                      Environment
Examples
  Grong School, Norway
  BRE Energy Building, UK
  Manly Lab, Sydney




                       Sustainable Built
                        Environment
 Questions so far -
 Is it possible to use natural ventilation in an
  urban environment?
 What are the roles that architects, engineers,
  users, developers and users play in
  sustainable building design?




                     Sustainable Built
                      Environment
Summary
 Indoor air quality can be more significant than outdoor
    air quality
   Indoor air quality is related to our health, our
    productivity and out living
   Ventilation is different from air-conditioning
   Natural ventilation – a natural choice, but not the only
    one.
   “Air” is just one element of the environment.




                            Sustainable Built
                             Environment

				
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posted:12/3/2011
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