30 May 2011

The Thermal Performance of Building Form in the Urban Environment


PAUL HAY Capital Projects

Topic:            Environment & Building Form
Author:          Paul Hay
e-mail:            paul.hay@phcjam.com
profile:           www.linkedin.com/in/phcjam



1.0     INTRODUCTION           

Heat input to buildings originates from internal & external sources.
1.1.1   External sources include (a) Solar Heat-gain, (b) Conduction through the envelope, and (c) Ventilation & Infiltration.
1.1.2   Internal sources include (a) Human occupation, (b) Lighting,  (c) Motors & appliances.
1.2       Heat energy is transferred from one body to another as the result of a temperature difference between them.
1.2.1   Sensible Heat transfers include (a) Solar heat-gain, and (b) Conduction through the envelope.
1.2.2   Latent Heat transfers include (a) 30% of heat from human occupation, (b) Ventilation & (c) processes (such as cooking);
1.3       First century architect, Vitruvius, wrote on the importance of wind to the urban environment and made recommendations for City Planning that are still currently relevant.
1.4       Wind should not be used as a generator of urban forms.
1.4.1.  Solar insolation should be considered
1.4.2.  Socioeconomic factors often prevail in the layout of city streets.


2.0     CLIMATIC PARAMETERS

2.1       Degree Days is the temperature difference above a threshold value that the average outdoor temperature attains on a day
2.1.1   Base temperature is typically 18 deg. C
2.1.2   Parameter is independent of orientation of building surface
2.2       Sol-air temperature is the theoretical temperature equivalent to the sum of outdoor air temperature, absorbed solar insolation, & long-wave heat exchange.

            Tsa = Ta + /ho * I + /ho * (MRT – Ta)                                                                           [2.1]
           
Where,
            Tsa = Sol-air temperature, deg. C                          Ta = Ambient air-temperature, deg. C
                  = Solar absorptance [ 0<<1]                         I   = Solar Insolation, W/m2
            ho    = External surface coefficient, W/m2-K          = Emittance, W/m2-K
            MRT = Mean radiant temperature, deg. C

2.2.1   Long-wave exchange is generally neglected in calculations
2.2.2   Parameter is influenced by surface orientation.


3.0     NATURE OF WIND

3.1       Wind speed increases with altitude above the Earth=s surface.
3.1.1   Friction from the ground reduces wind speeds on the surface.
3.1.2.  The Gradient Height is the altitude above ground where the Earth=s friction has no effect on wind speed.
3.1.3   Gradient height varies from 270 m in open country to 450 m in urban areas.
3.2       Moderate gales, with speeds between 23 – 33 knots, are experienced as an annoyance when walking against the wind.
3.3       Strong gales, of speeds 41 - 47 knots, cause slight structural damage to buildings.
3.4       Whole gales, at 48 - 55 knots, can cause considerable structural damage.


4.0     VEGETATION

4.1       Deflected wind flow results in a zone of reduced pressure on the leeward side along with eddy currents.
4.2       Wind accelerates when blown beneath trees.
4.3       Wind accelerates more when shrubs or low walls are placed on the windward side of the trees, but is deflected upwards when these are placed on the leeward side of the tree.


5.0     LAYOUT OF STREETS

5.1       Orientation of streets have a major influence on velocity control in urban areas.
5.2       Current suggestions for ventilating streets vary according to climate.
5.2.1.  Wind speeds should be minimized in the hot dry tropics;
5.2.2.  Ventilation should be maximized in the humid tropics; and
5.2.3   Ventilation should be minimized in cold climates.
5.3       Wind entering narrow streets increase in speed because of the venturi effect.
5.4       Wind tunnel experiments can be used to verify wind effects.



6.0     BUILDING FORM

6.1       Building height and width also have a major influence on velocity control in urban areas.
6.2       Open structures split air streamlines such that one flows over the structure and the other through the structure.
6.3       Square buildings have the greatest volume of enclosed space relative to the exposed surface  area of their envelopes.
6.4       Victor Olgyay determined that elongating buildings along an east-to-west axis could improve their energy performance
6.4.1   Performance of a square building was used as a reference
6.4.2   Test buildings were insulated frame construction having 40% south glazing & 20% glazing on all other sides.
6.4.3   Tests were undertaken for four (4) different climates.
6.4.4   The influence of thermal mass was not evaluated.
 


Further Reading

Essentials of Physical Geography Today, Theodore M. Oberlander and Robert A. Muller;
Wind in Architectural and Environmental Design, Michele G. Melaragno;
Architectural Handbook, Alfred M. Kemper

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