What we generally call daylight is a combination of two components:
the sunlight, which is the direct light originated from the
sun, and the skylight which is the sunlight scattered by air
molecules and dust particles in atmosphere. As daylight levels
vary from one moment to the next due to the motion of sun
and changes in the sky, they are not good measures to base
design strategies.
It is necessary a measure of the penetration of daylight into
a space that is independent of the actual levels prevailing
outside. To get it we calculate the ratio between the illuminance
measured for whole sky and the illuminance measured inside
the building. Although light levels both outside and inside
the building will vary, they vary by the same amount. The
resultant value is called daylight factor and it is constant
at each point of interest in the building.
Target daylight factors are related to the use to which a
space is to be put but as general rule typical average values
for a well-lit space are in order of 5%. Because external
light levels tend to be very high, only relatively low daylight
factors are required to make an internal space serviceable.
The following guidelines in Table 1 were taken from the BRE
Good Practice Guide 245 on Daylighting:
Animations were created to analyse the light levels in the
immediate surrounding area of the windows. The animation (Figure
1) shows the daylight factors in the sunspace and service
area.
Fig. 1: Sunspace daylight levels
As the Daylight Factor is a ratio of the outdoor illuminance
and a particular point within a building, an unobstructed
view of the sky would result in a 100% daylight factor. Therefore,
as expected, the sunspace presents very high factors due to
the large south faced glazing. The animation shows that the
light levels there will generally be too high and so it is
necessary to adopt solar shading control strategies. Points
in a room with only one window such as the bathroom and walk-in
wardrobe would receive much less light, around 2.0 - 5.0%.
Sun Path Animations
In ECOTECT, and many CAD programs, it is possible to generate
shadows at any time of the year, falling either on the ground
or other objects in the model.
The animations below were produced to illustrate how the
design of the Stoneguard C60 research house interacts with
the Sun position throughout the year. In order to comprehend
these animations one needs to first understand its elements.
Stereographic Diagrams
The animations have a 3D chart showing the movement of the
sun. Those are based on stereographic diagrams.
Stereographic diagrams are a projection of the sun position
onto a plane representing the sun's changing position in the
sky. The paths of the sun at different times of the year are
projected on a flattened hemisphere for the current location,
in this case Nottingham. The sun position can be read directly
over the whole year any time of any day. Figure 1 shows the
Stereographic Diagram for Nottingham.
Orthographic Diagrams
An orthographic sun-path diagram (shown in Figure 2 above)
is a 2D graph of the Sun position in Cartesian coordinates.
The azimuth is plotted along the horizontal axis whilst the
altitude is plotted vertically. The sun position can be read
simply reading the two axes.
Date Lines
The stereographic diagram is composed of many lines that
together represent the whole year. Date lines represent the
path of the sun through the sky on one particular day of the
year. They start on the eastern side of the graph and run
to the western side. Figure 3 below shows the date lines highlighted
in yellow.
Hour Lines
Hour lines represent the position of the sun at a specific
hour of the day, throughout the year, always intersecting
the date lines. The intersection points between date and hour
lines give the position of the sun. The hour lines are being
highlighted in yellow in Figure 4 above.
The following two animations were created illustrating the
longest day (Figure 5) and the shortest day (Figure 6) of
the year, when the sun is in its highest and lowest position.
The reason was to demonstrate the two most extreme cases.
Figure 5- (Animation) 21st of June, the longest day of
the year
Figure 6- (Animation) 21st of December, the shortest
day of the year
The next animation (Figure7) was created to demonstrate how
different can be the sun position at midday over the whole
year. It shows the sun at 12pm in each of the months.
Figure 7- (Animation) The sun at midday over the year
Solar & Daylighting animations created by research
student, Lucelia Rodrigues, using Ecotect software (laxltr@nottingham.ac.uk).
