Zbornik - SDR

“ Energy efficiency in INTERIOR Lighting – a Romanian Case Study ” R-3
the windows; obstruction density and reflection; windows transmittance and dimensions; windows
on one or two sides of the room. The results show important drops in average illuminance till 60%,
which is a factor of concern for offices real illuminance.
Influence of the time of the year and obstruction type. Figure 1 shows the OL fluctuations during
a day, from 8 a.m. to 4 p.m. for January, April, July and October 15th. It can be seen from these
charts that OL for overcast sky is constant, but for clear sky there are high fluctuations. It also can be
noticed that no matter the sky type, the OL are lower around 10 and 12 a.m., when sun altitude is
near the maximum. From here a dependence relation between sun altitude and OL for South and
West windows. For North windows there are minor daily fluctuations for clear and partially clear
sky.
The chart shapes are changed for each period of the year. In some periods of the year, at 8 a.m.
there are the highest OL, and in other periods there are the lowest OL The illuminance level reduction
for overcast sky is not dependent of the time of the year, with average values of 32.81% for heavy
obstructions, 27.37% for medium obstructions and 12.3% for light obstructions. In the morning, for
partially overcast or clear sky and for North or West window orientation, illuminance reduction is not
significantly influenced by the time. The highest fluctuation of illuminance reduction for 10 a.m.
appears in the case of South or East windows, with a peak in the winter.
Figure 2: Obstruction Losses as
function of VFR || , VFR ⊥ and VFR (for
overcast sky)
Influence of obstruction orientation in relation to the window. In the studies about obstructions in
the case of electric lighting the Vertical to Floor Ratio – VFR factor was introduced as an expression
of room content [5]. VFR is the ratio between obstructions vertical surface area above the floor area.
We proposed for daylight to decompose VFR in VFR || for parallel surfaces with windows, and VFR ⊥
for perpendicular surfaces with windows. (NB: VFR || + VFR ⊥ = VFR). When the obstructions are
rotated with 90°, the VFR || and VFR ⊥ swap, VFR remains constant, but OL drops in average with
3.6% for overcast sky and heavy obstructions and with 4% for partially overcast and heavy
obstructions. So, the OL for overcast sky can be interpreted as function of VFR || , VFR ⊥ and VFR. The
chart from Figure 2 shows that the trend is different from the linear slope of OL/VFR, which is
characteristic for electric lighting. However there is a relation of proportionality between VFR and
OL, especially for light and medium obstructions, but which is no longer valid for heavy obstructions
(presence of partitions). Despite these problems, the simulations have proved that a reduction of
obstruction density lead to smaller OL, as the orientation of partitions from parallel to perpendicular
to windows wall.
“Lighting engineering 2006” stran 15