In this article, we will be taking a look at some daylight simulations using the daylight metrics mentioned in this article. Do check that article out to get a brief understanding on the daylight metrics that will be reviewed here.
Simulation Parameters and Set-Up
We will be analysing 4 different daylight metrics, namely Daylight Factor, Useful Daylight Illuminance, Spatial Daylight Autonomy and Annual Sunlight Exposure. All the simulations will be conducted using RadianceIES on IES-VE.
Test Model
A “test box” model will be used for simplicity sake to compare the results between the daylight metrics. The model was designed using ModelIT, a built-in component of IES-VE. All simulations will be done on Room 1 for ease of comparison.
Figure 1: Plan View of the Test Model. Blue lines represent walls and green lines represent windows.
Figure 2: Model View of the Test Model. There is only one window (shown above) in the entire room.
Figure 3: Table of Test Model space properties for Room 1
Surface Properties
The walls are typically made of concrete while the window is a clear glass with Visible Light Transmittance (VLT) of 40%. Below is the glazing properties of the glass window.
Dynamic Daylight Occupancy Hours
For all 3 dynamic daylighting metrics, the simulation was done for all weekdays (Monday to Friday) in a year from 8am to 6pm, which is typical office working hours.
Weather and Location Set-Up
The location of the Test Model is set in Kuala Lumpur Subang, Malaysia using Simulation Weather Data “KualaLumpurIWEC.fwt” provided by IES.
Results
Daylight Factor
Daylight factor simulation was conducted with 10,000 Lux CIE Overcast Sky condition. Images below show static simulation results.
Figure 4: Lux levels throughout Room 1 in Daylight Factor Simulation
Figure 5: Daylight Factor > 1 highlighted in green
Figure 6: Daylight Factor > 3.5 highlighted in green
Based on the results shown, the floor percentage that has a daylight factor between 1 and 3.5 was calculated. According to the Green Building Index Non-Residential New Construction rating tool, at least 30% of the floor area is required to have a DF of 1 – 3.5 ¹. However in this test, only 28.9% of the floor area has a DF of 1 – 3.5, failing to meet the minimum threshold of the requirement.
Useful Daylight Illuminance (UDI)
The colour scheme represents the respective thresholds used for the percentage of time it is within the usable daylight range of 100-2000 lux.
Figure 7: Percentage of hours receiving 100-500 lux a year
Figure 8: Percentage of hours receiving 500-2000 lux a year
The statistics results show that there is a total of 987 grid points in Room 1. It is required that at least 50% of the time, a minimum of 80% of the space shall receive daylight within the usable lux range ².
Based on the results, 71.2% of the space is receives daylight between 100 – 2000 lux at least 50% of the time, however it does not achieve the minimum requirement of 80% which is the recommended compliance level.
Spatial Daylight Autonomy (sDA)
The colour scheme represents the respective thresholds used for the percentage of time it is above 300 lux.
Figure 9: Percentage of hours receiving at least 300 lux a year
The statistics results show that there is a total of 987 grid points in the space. Based on the LEED v4 rating tool, it is required that at least 50% of the time, a minimum of 55% of the space is recommended to receive daylight above 300 lux ³.
70.6% of the space receives 300lux and above at least 50% of the total hours in a year. The minimum requirement for the daylight credit in LEED v4 is met.
Annual Sunlight Exposure (ASE)
Based on the results, 7.70% of the space receives more than 1000 lux for at least 250 hours in a year. This is in compliance with the LEED v4 rating tool, which requires no more than 10% ASE ³.
Comparison of Results
This comparison will show the percentage of useable space according to the requirements of each individual metric. This will help give an idea of the accuracy and differences these daylight metrics have.
It is important to note that the variation in the percentage of useable space is due to the fairly large difference in lux level requirements. For example, UDI defines useable daylight as illuminances that range from 100-2000 lux whereas sDA & ASE only allows daylight within the range of 300-1000lux. This would contribute to the 8.3% difference in percentage of useable space between the two metrics.
The results of the dynamic daylight metrics seem to have a relatively close percentage of useable space as compared to daylight factor. This can be attributed to the dynamic sky conditions that are used in dynamic daylight metrics as compared to a constant 10k lux CIE overcast sky that is used in DF. Due to this discrepancy, overdesigning artificial lightings might occur when using DF as a metric due to the unrealistic assumption of a constant overcast sky condition which disregards actual climate data of the location where the space is.
Summary
As the industry moves towards a more comprehensive and in-depth look into sustainable engineering design, it is important that the methodology is apt to simulate conditions that are as real as possible. With that, the longstanding dominant Daylight Factor may seem to lose its footing due to the demand of a more realistic account of actual daylighting conditions. Due to the static and simplistic nature of how Daylight Factor works, simulations may work well in tropical countries where sky conditions are fairly constant throughout the year but as for countries with highly varied sky conditions, results may be far from actual conditions. This is where dynamic daylighting metrics like UDI and sDA & ASE provide far more realistic conditions during simulation due to the incorporation of actual weather data. These metrics also provide great detail down to the hour so lighting designers can take measures to ensure proper daylighting throughout the year. We hope to see a shift in the daylighting industry as more rating tools start to see the benefits and importance of dynamic daylighting metrics to ensure efficient daylight design.
Additionally, do check out our previous article on Daylight Metrics.
References:
1) Non-Residential New Construction (NRNC) Design Reference Guide & Submission Format, 1st ed. Kuala Lumpur: Greenbuildingindex Sdn Bhd, 2011.
2) C. Patherns, “Education Funding Agency Daylight Design Guide”, Newcastle, 2014.
3) Leadership in Energy and Environmental Design (LEED), 4th ed. United States Green Building Council, 2018.
