Daylight Simulation Analysis using Radiance & Rhino

DESC9169 � Daylight in Buildings Daylighting simulation analysis NAVIN KUMAR SRIDHAR School of Architecture, Design and Planning, University of Sydney, NSW, Australia

Abstract This report presents a study of the daylight performance of courtyard café, located in the University of Sydney, using traditional approach evaluated in terms of Daylight Factor (DF), calculated manually on an overcast sky and also using daylight simulation tool, such as Radiance. A digital model of the measured space was made using RHINO and integrated with radiance through grasshopper. An annual climate dataset is used to generate the values of sun and sky conditions on the place, where the measured space is based. Other metrics such as Daylight Autonomy (DA) and Useful Daylight Illuminance (UDI) was also used to evaluate the illuminance predictions from the simulation model.

The outcomes of the simulations after certain calibrations are first presented for understanding the existing scenario and its correlation with the measured data.

Based of the results, specific design interventions has been made and the simulations are used to show the improvements of the daylight performance.

The café had good daylighting , but the lighting was not uniform. There were dark spots in the space, especially at the service counter , which is supposed to be the important space of the café, but as the counter was not close to any of the windows nor the glass door, daylight did not reach the area. lighting strategies, such as use of light shelves and light tubes were introduced to improve the daylight performance of the space . Also the existing skylight was redesigned to cut down sunlight and bring in diffused light without compromising on the illuminance levels. The results from the simulations produced a better understanding of daylight performance, with the help of different metrics, such as Daylight Factor (DF), Daylight Autonomy (DA) and Unified Daylight Illuminance (UDI).

1. Space description Space of study : Courtyard Café Location : Home building, University of Sydney Usage : The cafeteria is used for eating and drinking and also for various other purposes, such as reading, writing, and socializing.

Space Dimensions : Length-16m, Width-14m, ceiling height-3.6m Present Lighting Condition : The space receives its daylight from the eastern, western and southern facades. The center of the space is receives good daylight from the two skylights.

However, artificial lighting is also highly being used for for both aesthetic and functional purpose.

Problems : ❖ The counter being the important part of the cafe, where the food is being displayed and served, does not receive enough daylight, so the space is majorly lit by artificial lighting. The illuminance level has to be increased to 500-600 lux. ❖ The daylight entering through the skylight is just being focused on the central part of the café and the sides (specially east side) are comparatively dark. The lighting is not uniform in the café. ❖ The northern façade which contributes to the large amount of day light entering the space through its huge windows, but has tinted glass that nearly cuts down half of the natural light entering the space. Also, it allows direct sunlight during most times of the year. ❖ The entrance of the café has a covered space, with completely transparent glass covering it, which allows a lot of daylight, which produces glare for occupants sitting under the space or close to the entrance.

Standards : AS1680.1-1990 illuminance standards suggests, for Normal range of tasks and workplaces that are continuously occupied – simple or moderately difficult tasks, recommended Maintained Illuminance is 160-320 lux. According to CIBSE DF of 5% or more is appropriate to completely avoid the use of electrical lighting during the daytime.

Figure 1 Interior View

2. In-Situ measurements from Assignment 1

Figure 2 Sun path Diagram

Figure 3 Section showing Sunrays angle & surroundings

Figure 4 Floor plan showing the illuminance and DF readings taken at the selected points Number of points taken for measuring the indoor illuminance is 16, which is divided into 4 zones with 4 points within each zone. Artificial lights couldn’t be turned off, so readings were taken at night time for all the points, which corresponded to only artificial light. These values were subtracted from the values recorded during the day, at respective points, to obtain only the daylight illumination.

Figure 5 Section A

Figure 6 Section B All units in the measured drawings are measured in Meters Scale : NTS

Daylight Factor calculation and strategies

Table 1 Measured DF for Horizontal Illumination

Figure 7 Formula used for calculating DF Daylight distribution in the café is more widespread. The average daylight factor is 6.64 %, which is a good level to be maintained in a café. This place has a very vibrant and active feel to it, which is due to the material and color choice as well as multiple openings in both planes – horizontal and vertical. Daylight remains mostly constant and high in the seating areas but is lesser around the counter because of the location of skylight. Even with good DF, artificial lighting is being used because of the larger difference in the illuminance of central seating, corner seating and counter space. The Daylight Factor is being calculated using the mathematical formula in Figure 7, where Ein is the horizontal internal daylight illuminance (lux); Eout is the outdoor horizontal illuminance under the overcast sky (lux).

Figure 3 Section showing daylight factor

Material properties

Table 2 Calculation of Reflectance / Transmittance of materials

3. Daylight Modeling for simulation

Figure 8 Outdoor view of the Digital model

Figure 9 View of the cafĂŠ from the open courtyard

Figure 10 Interior views of the digital model

Model calibration With the measured outdoor illuminance, the date and time was identified and calibration of the digital model was made to maintain the Mean bias error of the illuminance values produced by the simulation less that 20%. The calibration was made by adjusting the radiance parameters, such as ambient bounces (ab) and quality. The grid points was placed on an ideal work plane , which is 0.9 m from the floor. The measured outdoor illuminance of 26700 lux on the overcast day at 11:00 AM in the month of September, was considered for calibration. The results of the daylight illuminance and model calibration are presented in Figure 1.

Figure 11 Outdoor point for measuring the illuminance

Table 3 Indoor illuminance measured from the simulation and error percentage

Table 4 DF measured from the simulation and error percentage

The accuracy between the measured and simulation produced illuminance values was assessed using Mean Bias Error (MBE). The error percentage was 19.7%. The Daylight factor was also found to be close to the measured value and the error percentage was 18.6%.

The ambience bounce was set at 7 and the quality was set at 2. The surroundings play and important role on the indoor illuminance values. The surrounding objects including buildings and trees were also modelled in Rhinoceros. The physical dimensions were determined from google map and on-site measurements. The transmittance of the surrounding trees were slightly adjusted to calibrate the model.

Figure 12 Formula for calculating MBE

4.Results for existing scenario Daylight Factor(DF)

Figure 13 DF at ab 7

Figure 14 DF at ab 14

The DF produced by the simulation is 5.4 and the measured is DF is 6.5. The higher the DF, the availability of daylight in the space is more. Spaces with average DF of 2% or more is considered to be daylit, but there will still be the need for electrical lighting to perform some visual tasks. To completely avoid the use of electrical lighting during the day, an DF of 5% or more is necessary (CIBSE, 2002).

The ambient bounce (ab) was set at 7, as there is no major variation in the average daylight factor even after increasing the bounces to 14, as seen in Figure 13 & Figure 14. The uneven distribution of light can been seen from the figures. The glass doors at the south brings in a lot of light. Dark patches can be seen at the service counter space and the centre.

Legend

Figure 15 DF contour mesh

Daylight Autonomy (DA) and Useful Daylight Illuminance (UDI)

Figure 16 DIA

Figure 17 UDI at 100,2000

The Daylight Autonomy (DA and Useful daylight Illuminances (UDI) for 100-2000 lux, for the existing scenario of the space are shows in Figure 1& Figure 2. The average DA for a set threshold of 500 lux is 76.4 and average UDI is 63.6, which shows the good amount of daylighting in the space.

Legend

Daylight Glare Probability (DGP) The Daylight Glare Probability is calculated for two different dates ,which are the summer solstice (22 December ) and winter solstice (22 June). These dates predict the extreme conditions for glare within the space. The results produced by the simulations predicts imperceptible glare in both the set view.

Winter Solstice

Summer Solstice

View 1

DGP = 0.30

DGP = 0.31

View 2

DGP = 0.32

Figure 1 Plan showing the set views

DGP = 0.28

5. Proposal for improvement of Daylight performance The analysis of the current daylight performance of the space shows that, on the whole good daylight is available, as shown by DA and UDI metrics. However, daylight is unevenly distributed within the space, with peak illuminance values being achieved close to the windows and under the skylight. Dark patched can be seen at the service counter and places away from the window. These issues can be corrected by a number of design solutions. The proposals aims at reducing daylight levels close to the windows and under the skylight, while increasing the overall uniformity of daylight.

The first intervention is to add light shelves to the openings in the North and East façade, with internal and external protrusions, which aims to cut down direct sunlight and also reflect daylight further inside, lighting the space between the skylight and the north façade.

Different geometrical shapes have been analysed for the light shelves, by changing the length of the internal and external protrusions. Various combinations of reflectance and shapes were explored. The combination resulted in : • Width of the light shelf : 1m externally + 0.8m internally • Reflectance of the Light shelf : 0.85 The second intervention is to change the design of the skylight to avoid sunlight into the space. The transparent glass has been removed from the top and been added to the south side of the skylight, to capture more diffused light with ought loosing the amount of light currently available under the skylight, because the high level of focused light from the skylight creates a grand and majestic feel to the public space, which I wanted to retain. The present angle of the skylight is kept constant, as the angled shape has supported in spreading the light across the space. The idea of changing the direction of the skylight has been analysed from the BOX MODEL experiment. The transmittance of the glass has been reduced to 0.45 , which is the transmittance of the tinted glass.

The third intervention is to add light tubes to bring in focussed light to the service counter space ,as it is away from all the windows and as well as from the skylight. Light tubes of 0.5 dia has proved well to bring in sufficient daylight. The hemispherical top place at the outdoor has a transmittance of 0.85 , while the glass placed inside has a transmittance of 0.7, which bring in good diffused light in to the space.

Figure 18 Design solutions

Figure 19 Design solutions The final intervention made is to cut down the high illuminance at the entrance on the cafĂŠ , which is covered with the glass sunshade. The idea was to reduce the transmittance of the glass, but still during certain period of the year the space under and close to the glass door, becomes unusable, thus the idea of introducing plants, such as creepers on the sunshade was made. It cutes don the direct light, heat and will also be aesthetically pleasing. The transmittance of the plans was considered to be 0.35.

6. Results for retrofit scenario

Figure 20 DF contour map As seen form the Daylight factor contour map, the distribution of light across the space, while still maintaining the focused light form the skylight as intended. The lighting at the counter space has been increased to the targeted illuminance of 400-600 lux. The light entering from the windows and glass doors has been diffused, with the help of the interventions.

Existing Scenario

DF = 5.4

DA = 76.4

UDI = 63.7

Retrofit Scenario

DF = 5.1

DA = 84.5

UDI = 74.9

Table 5 Daylight metrics of existing and retrofit scenario

Daylight Glare Probability (DGP) Winter Solstice

Summer Solstice

View 1

DGP = 0.29

DGP = 0.30

View 2

DGP = 0.29

Table 6 Comparison of DGP

DGP = 0.29

7. Conclusion The daylight simulation analysis has greatly helped in analysing the daylight performance of a space and also design spaces by predicting the real case setting of sun and sky. The calibration between the measured values and simulated values of illuminance has justified the accuracy of the results of the intervention made. Metrics like DF, DA and UDI has helped to predict the daylight performance of the space more accurately.

8. References Australian/New Zealand Standard Australia/Standards New Zealand.

(AS/NZS

1680.1:2006).

(2006).

Standards

Tandfonline.com. (2010). Empirical Validation of Daylight Simulation Tool with Physical Model Measurement. [online] Available at: http://www.tandfonline.com/doi/full/10.1080/2093761X.2016.1167646 [Accessed 7 Nov. 2018].

CIBSE. (2002). 2- Lighting Levels- by CIBSE.pdf | Lighting | Rendering (Computer Graphics). [online] Available at: https://www.scribd.com/document/250619077/2-LightingLevels-by-CIBSE-pdf [Accessed 7 Nov. 2018].

Li, D., Lou, S., Ghaffarianhoseini, A., Alshaibani, K. and Lam, J. (2018). A review of calculating procedures on daylight factor based metrics under various CIE Standard Skies and obstructed environments.

Kontadakis, A. and Tsangrassoulis, A. (2017). A Review of Light Shelf Designs for Daylit Environments. [online] Available at: https://www.researchgate.net/publication/322142534_A_Review_of_Light_Shelf_Designs_fo r_Daylit_Environments [Accessed 7 Nov. 2018].