Download ARC 309 GROUP 14

A
WRITE-UP
ON
THE USE OF
BIOCLIMATIC CHART.
SUBMITTED BY
GROUP 14
OGUNRUKU O.A (ARC/05/5626)
OMATIGA D.J (ARC/05/5639)
ADEYEMI I.S (ARC/05/5579)
SUBMITTED TO
PROF. OGUNSOTE.
DEPT OF ARCHITECTURE. FEDERAL
UNIVERSITY OF TECHNOLOGY, AKURE IN
PARTIAL FUFILMENT OF ARC 309.
DATE.
APRIL, 2008
BACKGROUND
Building designers and owners are faced with the
difficult problem of determining whether natural cooling
techniques can be substituted for mechanical cooling in order
to save energy.
One solution, which would be particularly useful in the
early stages of planning and design, is a manual procedure for
quickly determining the potential of various cooling strategies
in different climates. The procedure should be based on
accepted criteria for occupant comfort and should indicate the
success of possible cooling strategies in terms of the percentage
of time that they are able to (or fail to) maintain human
thermal comfort in the building.
In developing this procedure, there were four principal
technical problems to be solved:
.
Modifying the comfort zone boundaries on the building
bioclimatic chart to reflect the most current comfort criteria
and the special requirements of naturally cooled
buildings.
.
Finding summarized climatic data for assessing the
natural cooling potential for many climates worldwide, so
that the designer need not obtain and process hourly weather
data.
.
Extending and amplifying Watson and Labs’ method,
from a graphic analysis of climatic potential, to a stepwise
decision-making process for determining the cooling
requirements of the building design. This process should be
based on quantified design criteria.
.
Including a procedure for sizing windows to meet the
ventilative cooling requirements determined in the climatic
analysis.
INTRODUCTION
Before refrigeration technology first appeared, people
kept cool using natural methods: breezes flowing through
windows, water evaporating from springs and fountains as
well as large amounts of stone and earth absorbing daytime
heat. These ideas were developed over thousands of years as
integral parts of building design. Today they are called
"passive cooling." Ironically, passive cooling is considered an
"alternative" to mechanical cooling that requires complicated
refrigeration systems. By employing passive cooling
techniques into modern buildings, you can eliminate
mechanical cooling or at least reduce the size and cost of the
equipment.
Passive cooling is based on the interaction of the building
and its surroundings. Before adopting a passive cooling
strategy, you must be sure that it matches your local climate.
There are four passive cooling strategies: natural
ventilation, evaporative cooling, high thermal mass and high
thermal mass with night ventilation. All these passive cooling
strategies rely on daily changes in temperature and relative
humidity.
The passive cooling strategies that are appropriate for your
building site can be identified by using a bioclimatic chart.
This bioclimatic chart defines four passive cooling
strategies based on temperature and relative humidity. This
chart can be used to determine which passive cooling
strategies are appropriate for the climate at the building site.
First, the following local weather information for each of the
months of the year is to be found;

average maximum temperature

average minimum temperature

average maximum relative humidity

average minimum relative humidity
NOTE. Information on these kind of weather record are
mostly kept by Local airports.
On the bioclimatic chart, plot two points for each month.
The first point is the minimum temperature and the
maximum relative humidity (RH). The second point is the
maximum temperature and the minimum RH. (Note that the
highest temperature is paired with the lowest RH and vice
versa.) Connect these points with a line. Plot a similar line for
each month. Each line represents the change in temperature
and RH over an average day.
Passive cooling strategies are shown on this version of the
bioclimatic chart as overlapping zones. When your lines cross
zones, it indicates that this strategy may work for your
climate. Some months may lend themselves to several different
strategies. To reduce cost, you would probably choose one or
two strategies that are compatible with each other and the
building design.
The design strategies suggested by this version of the
bioclimatic chart are appropriate only for residences and
other buildings with small internal heat gains. Internal gains
for a residence are assumed to be 20,000 btu per day per
person.
These passive cooling concepts address getting rid of heat
that accumulates in buildings. Of course, you'll also want to
reduce heat gains in the first place with high insulation levels,
heat blocking windows, proper solar orientation and good
shading from building elements and vegetation.
Passive Solar Cooling
Passive solar heating can also be assessed using the
bioclimatic chart. Passive solar heating is usually an
appropriate strategy when the plotted lines fall anywhere
below the comfort zone. More information about applying this
chart to passive solar heating region is published in Sun, Wind
and Light, which can be purchased for $45 plus shipping from
Iris Communications, Inc. (800-346-0104).
Passive Cooling Strategies
Natural ventilation depends solely on air
movement to cool occupants. Window openings on
opposite sides of the building enhance cross
ventilation driven by breezes. Since natural
breezes can't be scheduled, designers often choose
to enhance natural ventilation using tall spaces
within buildings called stacks. With openings near
the top of the stack, warm air can escape, while
cooler air enters the building from openings near
the ground. Ventilation requires the building to be
open during the day to allow air flow.
High thermal mass depends on the ability of
materials in the building to absorb heat during the
day. Each night the mass releases heat, making it
ready to absorb heat again the next day. To be
effective, thermal mass must be exposed to the
living spaces. Residential buildings are considered
to have average mass when the exposed mass area
is equal to the floor area. So, for every square foot
of floor area there is one square foot of exposed
thermal mass. A slab floor would be an easy way
to accomplish this in a design. High mass buildings
would have up to three square feet of exposed mass
for each square foot of floor area. Large masonry
fireplaces and interior brick walls are two ways to
incorporate high mass.
|High thermal mass with night ventilation relies
on the daily heat storage of thermal mass
combined with night ventilation that cools the
mass. The building must be closed during the day
and opened at night to flush the heat away.
Evaporative cooling lowers the indoor air
temperature by evaporating water. In dry
climates, this is commonly done directly in the
space. But indirect methods, such as roof ponds,
allow evaporative cooling to be used in more
temperate climates too.
Ventilation and evaporative cooling are often
supplemented with mechanical means, such as
fans. Even so, they use substantially less energy to
maintain comfort compared to refrigeration
systems. It is also possible to use these strategies in
completely passive systems that require no
additional machinery or energy to operate.
This bioclimatic chart for the summer months in
Medford, Oregon shows that high thermal mass and natural
ventilation could provide adequate cooling. You can use local
weather data to plot a chart for any building site. The table
below shows data from four selected sites around the U.S.
Max. Temp. Min. Temp. Max RH Min. RH
Eugene, Ore.
June
74
50
90
49
July
82
53
88
38
August
82
53
88
39
September
76
49
89
43
June
79
57
80
56
July
82
62
84
56
August
80
60
88
58
September
74
54
87
60
June
76
55
77
55
July
81
59
79
53
Akron, Ohio
Burlington, Vt.
August
78
58
83
57
September
69
49
86
61
June
80
49
70
31
July
87
53
68
26
August
86
52
71
28
September
77
45
77
61
Yakima, Wash.
Conclusion
The basic and most essential use of the bioclimatic chart
in building design is to be able to provide passive cooling
where the conventional techniques are not used.
Passive refreshment is not and will not probably be in the
close future, as efficient as the conventional techniques of
cooling (electric and mechanical systems). However for
persons in hot and uncomfortable climate for whom such an
equipment of refreshment is out of reach, passive cooling
design can be a step towards the comfort with low cost.
References
ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING
AND HOUSING)
(Vol. 8, no. 4 (2007) pages 471-478)
A METHOD FOR DESIGNING NATURALLY COOLED
BUILDINGS USING BIO CLIMATE DATA
(Edward A. Arens_ N. Watanabe†)
SUN, WIND AND LIGHT, (G.Z. Brown)