Daylighting analysis
3.2.1 Drawbacks and potentials of daylight metrics
Sensitivity in describing daylight amount of DF compared to Annual Light Exposure Among the group of the CBDM that were calculated in this study, the ALE is the metric which describes the overall daylight availability inside a room, resulting for this reason somewhat comparable to the DF, as they both assess the indoor daylight quantity without referring it to a threshold value. Obviously it is important to stress out how the Daylight Factor is a ‘static’ metric, expressed as the indoor to the outdoor unobstructed illuminance ratio, accounting for diffuse skylight in presence of overcast sky conditions only, while the ALE is a dynamic climate-based indicator which accounts for ‘realistic’ direct sunlight and diffuse skylight conditions.
A first analysis dealt with comparing the Daylight Factor DF and the Annual Light Exposure ALE, in order to assess the sensitivity of both metrics in describing the indoor daylight amount. In Figures 3.2 and 3.3 Daylight Factors and Annual Light Exposures are compared in order to point out their different sensitivity in considering room orientations and site climate. In particular, in Figure 3.2 the mean Daylight Factor calculated for each case-study is plotted versus the corresponding mean Annual Light Exposure for the three considered orientations (South, West and North-facing rooms). In spite of the inherent differences of the two metrics, a close fit can be observed between them if the data are correlated separately for the 3 orientations (a linear fit was found with R2 > 0.95) The different gradient of the three functions confirms how the ALE metric accounts for the orientation of the room.
In Figure 3.3_a, the sensitivity of the two metrics with respect to the room orientation is further analysed: in particular, the relative difference between North-facing and South-facing rooms located in Torino is shown. In Figure 3.3_b, the relative differences of DF and ALE for a same room located in Torino or in Palermo is plotted, so as to analyze how the two metrics account for the specific climate of the site.
In both cases the relative differences (ΔDF and ΔALE) were calculated considering the working plane mean value.
The data shown in both figures confirm how the Daylight Factor is not sensitive to orientation nor to the site latitude: the mean value of the relative DF differences between South and North-facing rooms is ΔDFm= 0.5% and ΔDFm= 0.05% for Palermo-based and Torino-based rooms. In other words, the difference tends to zero (if it is not equal to zero, as expected from DF definition, this is due to the simulations, which are based on Radiance which in turns relies on a Monte-Carlo algorithm to generate rays: this means that repeating the same simulation may result in slightly different results).
Figure 3.2 Mean Daylight Factor vs. mean annual light exposure for all case-studies relative to Turin: South,
West and North-facing rooms are shown separately to highlight the fit between the 2 metrics.
Figure 3.3 Relative difference of DF and ALE values for different room orientations and sites.
On the contrary, the ALE relative differences change in case of both different room orientation and site latitude: the mean relative difference between North oriented and South oriented configurations is ΔALEm = 117%; the mean relative difference between Palermo-based and Torino-based rooms is ΔALEm =19%. In particular, as far as the effect of orientation is concerned, the relative differences between North to South-facing rooms located in Torino are quite high for unobstructed configurations or in case of obstruction angles γ up to 30° (ΔALEm = 179%). As the obstruction angle raises, though, the relative differences decrease (ΔALEm = 85% for obstruction angles of 45° and 60°) and become negative for highest obstructions (ΔALEm = - 8% for γ = 75°). In this latter case, in Torino the sun results shaded throughout the year: as a consequence, the role played by the orientation in achieving different daylight provision is drastically reduced. Negative ΔALE values, which imply a higher daylight availability inside North-facing rather than South-facing rooms, seem to be related to the multiple reflections of direct sunlight on both the obstruction and the target building itself.
On the other hand, the highest values of relative ALE differences between Torino and Palermo are observed for highest obstruction angles of 60° and 75° (ΔALEm = 34%), while they
decrease for lower obstruction angles (ΔALEm = 11% for γ up to 45°). This seems to be related to the different sun position, higher in Palermo than in Torino for the same time-step: the higher the obstruction and the more frequently direct sun rays are shaded in Torino while they are not in Palermo.
Sensitivity in describing daylight amount of CBDM compared to DF and Annual Light Exposure
Analyzing the database of results from the parametric study, it was observed that metrics inherently referred to threshold values (the groups of Daylight Autonomies and of Useful Daylight Illuminances) show to be more sensitive in describing the daylighting conditions obtained in a room with various architectural features (window area, room depth, orientation and obstruction angle) than the Daylight Factor or the Annual Light Exposure which account for the global daylight availability without referring to threshold values.
The aim of this analysis is to highlight the sensitivity of threshold-based Climate-Based Daylight Metrics compared to Daylight Factor and Annual Light Exposure, since they both describe the overall daylight availability within a space without referring to a threshold value.
For this type of study it was decided to analyse to variation of daylight between a medium deep room (RD = 7.5 m) and a small room (RD = 3 m). A reduction of room depth from 7.5 m to 3 m causes an average 126% and 122% increase of DF and ALE respectively. This increment results almost constant for variation of obstruction angles, window sizes and orientations. This is confirmed by the value of the relative standard deviation (calculated as standard deviation to mean value ratio) that is 10% for the Daylight Factor and 18% for the Annual Light Exposure. Figure 3.4 shows the relative difference of the two metrics for different WWR, orientation and obstruction angles.
It is interesting to stress out how results are rather different if the relative difference between the 7.5 m and 3 m rooms is assessed through threshold-based CBDM: the percentage variation of these metrics changes significantly with the room architectural features, as shown in Figure 3.5 and 3.6 and Table 3.2.
Statistic parameter DAm DAcon,m DAmax,m UDI fell-short,m UDI achieved,m UDI exceeded,m Mean 155.4% 74.6% 88.8% -55.4% 54.2% 200.2% Relative standard deviation [%] 37.6% 91.1% 91.1% -32.5% 132.2% 137.8%
Table 3.2 Mean relative difference of CBDM (variant: room depth from 7.5m to 3m) and relative standard
deviation.
In particular, analysing data shown in Figure 3.5, it can be observed how, if the room depth is reduced from 7.5 m to 3 m, the relative difference of the Daylight Autonomy ΔDA increases as the window area decreases, the obstruction angle increases and, for low obstruction
angles, in presence of North-facing rooms. Highest obstruction angles, which determine the minimum daylight availability within the room, result in a decrease of the influence of the other room architectural features (window area and orientation).
The relative differences of the Continuous Daylight Autonomy ΔDAcon show similar trends to the ones observed for the Daylight Autonomy ΔDA.
The relative differences of the Maximum Daylight Autonomy ΔDAmax equal to zero in presence of high obstruction angles and for North-facing rooms: in these cases, illuminance values remain below the threshold value of 5000 lx independently of the room depth.
Similarly, as far as the group of UDI metrics is concerned, it emerges how the relative differences between room depths of 3 m and 7.5 m are influenced by room architectural features (Figure 3.6). It could be noted that for low obstruction angles, if the room depth is reduced from 7.5 m and 3 m, the relative difference of the UDIexceeded (ΔUDI2000) increases because of a highest amount of daylight above 2000 lx available in the space. This is valid for all orientations which have been considered. On the contrary the relative differences of the UDIfell-short (ΔUDI100) and UDIachieved (ΔUDI100-2000) decrease.
Increasing the obstruction angle (for instance γ=75°) results in an increase of the relative differences of the UDIachieved (ΔUDI100-2000) and in a decrease of the relative differences of the UDIexceeded (ΔUDI2000), in particular for North-facing rooms.
As already shown for DAs values, highest obstruction angles, which determine the minimum daylight availability within the room, result in a decrease of the influence of the other room architectural features (window area and orientation) on UDIs trends.
Figure 3.4 Relative increment of DF and ALE, for different window areas (WWR values), orientation and
obstruction angles (γ), when the room depth is reduced from 7.5 m to 3 m. Case-studies with more meaningful WWR and γ values are shown.
Figure 3.5 Relative differences of DAs metrics, for various window areas (WWR values), orientation and
obstruction angles (γ), when the room depth is reduced from 7.5 m to 3 m. Case-studies with more meaningful WWR and γ values are shown.
Figure 3.6 Relative differences of UDIs metrics, for various window areas (WWR values), orientation and
obstruction angles (γ), when the room depth is reduced from 7.5 m to 3 m. Case-studies with more meaningful WWR and γ values are shown.