July 29th 2020

RadianceIES – Five Daylighting Strategies for the Tropics

RadianceIES – Five Daylighting Strategies for the Tropics

In the previous article, the significance of the integrative design process was elaborated. This process is particularly true for daylighting design, as an immense amount of coordination work from the conceptual massing stage to the commissioning of daylight sensors is needed to ensure the design intention is successfully translated into implementation. If you have tried scoring for the Daylight credit within LEED or other holistic assessment tools, which require at least 55% of regularly occupied space to meet the Spatial Daylight Autonomy (sDA) requirements, you know that this performance-based credit needs an intended and purposeful design.

Daylighting – No Simple Rulebook

Unlike HVAC design optimisation, the conversation of daylighting strategy is dependant on both sky condition and geometry orientation. For a start, we know that daylighting strategy is essentially the optimisation of harvesting daylight to create a comfortable visual comfort environment for the space users. However, what is visual comfort? 

Is it defined by sDA, which refers to the illuminance level occurrence of a horizontal plane? What about the vertical illuminance level of your viewing background? Is Daylight Glare Probability DGP the most accurate glare metric? The point here is, while it is one approach to use certification metric such as sDA to score for LEED credit, it is more holistically to audit the operational daylight levels and run a visual comfort survey. After all, the integrative design process aims to close the feedback loop.

One example is a recent encounter I had in a Green Mark Platinum-certified office with a daylight sensor. Despite its lighting energy-saving feature, the lack of setback with its daylight sensor causes the electrical light to turn on and off every few minutes. I am sure no one loves sitting under an ever-changing luminous environment throughout the day! This encounter is one example where daylighting design is different in the Tropics, where the cloud cover is significantly more than the Temperate climate and hence the constant fluctuation of direct sunlight that affets the indoor daylight level.

Cloud Cover in Singapore is consistently higher than that of London
 

Diffuse Radiation of Singapore’s weather is consistently higher (in fact cumulative double) than that of London’s weather

Daylight for the Tropics

The differences in solar azimuth, cloud cover and solar irradiance level translate our approach in harvesting daylight. In the Tropics, the luminous efficacy of diffuse daylight (~130lm/W) is much higher than that of the direct sunlight (70-100lm/W). Thus, our aim of harvesting only the diffuse daylight ensures that we are not oversizing our HVAC system by compensating the additional external solar heat gain. Unlike the daylighting strategy in Temperate climate which aims a seasonal performance of shading device that responds to low sun angle during the winter solstice, façade design in the Tropics shall be effectively shading against direct sunlight all year round.

Comparison of seasonal sun path between Singapore and London

5 Daylighting Design Strategies for the Tropics

One of the few daylighting bibles I relied on during my Masters Thesis is the International Energy Agency IEA’s Daylight in Buildings sourcebook which has an impressive in-depth evaluation of 28 different daylighting strategies with criteria for the choice of elements. This article compares only five relevant daylighting strategies against a reference scenario of a typical office façade. While this article does not attempt to conclude any form of performance result, it demonstrates how you can use the RadianceIES application within the VE to communicate with your audience through the various illustrative results available.

Defining View Out Windows and Daylight Harvesting Opening

One of the rule books for an effective daylight design in the Tropics is to separate these two elements. In most cases, your view out window has to comply with stringent façade thermal performance metric. Furthermore, incoming glare from the bright sky on the horizon in the Tropics is also a reason why occupants draw down their window blinds and usually, that stays for some time. Hence, the usage of high-performance double glazing with low visible light transmittance (VLT 30-50%), and the additional layer of blind for glare prevention, mean that the incoming daylight level is not useful for the intention of daylight design. Hence, a dedicated daylight harvesting opening, that is shaded against direct sunlight, yet with a high VLT value of 70-80%, serves the intention of maximising useful incoming daylight.

Modelling Daylighting Feature in RadianceIES

The approach of modelling these daylighting feature is a combination of Google Sketchup and VE Components. While it is possible to draw the daylighting feature directly within VE Components, you can draw unique shapes quickly within Google Sketchup and import into VE ModelIT using our VE Sketchup Plugin. Then, export the unique geometry as a GEM file. As each component within the room has its RGB reflectivity and specularity properties, you can assign the similar to the daylighting features after importing the said GEM files.


Comparison of illuminance contours of various daylighting scenarios

 


Comparison of luminance level and Daylight Glare Probability computation of various daylighting scenarios

IEN Consultants have utilised the RadianceIES application to optimise an office space using the Scenario 4 – Daylight Trough. This daylighting strategy, in the Tropics, allows sufficient daylight to penetrate up to six meters into the office space without visual discomfort issue. An site measurement of the internal daylight levels also confirms the accuracy of RadianceIES. Please read here for more information about the proven case study.


Comparison of illuminance level at working plan level of various daylighting scenarios

Using RadianceIES as Your Storyboard

We know that every consultancy work requires strategic communication to deliver an objective presentation. Hence, RadianceIES has an array of simulation options with customisable result parameters to help you. For example, I always prefer to show results holistically with the fisheye perspective angle instead of a working plan level as a 3-dimensional image shows result across the vertical walls and ceiling better. We also have auto computed Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE) with adjustable results bandwidth to help your certification project needs. You can also import IESNA photometric file for electric lighting and see how they interact with the natural daylight level. There are many other features that we believe you can explore with a free VE trial today!

If you have any feedback on improving the RadianceIES application, please contact feedback@iesve.com. If you are interested in enquiring about our consultancy service, please contact us here. If you want more information on our Lunch-N-Learn videos, or any advanced training session, please contact training@iesve.com for more details.

Further reading and training links:

•    On-Demand – Daylight Simulation, Glare modelling and Lighting Energy Calculations
    On-Demand - Using the IESVE to model Daylight Linked Dimming for Thermal Simulations
•    On-Demand – Using the IESVE to conduct Climate Based Daylight Modelling (CBDM) Studies
•    Article - Ten Key Daylight and Electric Light Metrics