Recently, a user in Rome, architect Marika Prete, shared with us a solar and lighting analysis study she conducted. We thought it provided great insight so decided to share it with you as well. Enjoy!
USE OF SAR AND LAR TOOLS FOR ANALYSIS AND CONCEPT DESIGN
Could an insolation analysis be useful to understand which rooms will have the most or least access to daylighting? Are results from Solar Analysis for Revit and Lighthing Analysis for Revit related?
The answer could be affirmative, since between solar radiation and light there is a direct proportional relationship. But only in some cases is it possible to predict the results of a lighting analysis from a solar analysis. Understanding how a software engine calculates is essential to interpret results from analyses and inform your design.
Solar Analysis for Revit (SAR): Solar Analysis for Revit helps you understand how much solar radiation will strike a given surface during a given time (it allows you to specify date, time, and range). We know that the intensity of the sun varies by the clarity of the atmosphere and the angle at which the sun strikes a surface is the “incident angle.” The more perpendicular the sun’s rays are to a surface, the more heat and light energy. Analysis results, in terms of energy accumulated from a surface (kWh/sqm), include direct radiation (Ib), diffuse radiation (Id), radiation reflected from the ground (Ir), shading from surrounding objects (Fshading), the portion of the sky “visible” by the surface (Fsky), and the angle of incidence between the sun and the face being analyzed (theta). The basic equation behind the values provided by the software is below:
Incident solar radiation = (Ib* Fshading * cos(theta)) + (Id * Fsky)+ Ir
Ib direct beam radiation, measured perpendicular to the sun
Id diffuse sky radiation, measured on horizontal plane
Ir radiation reflected from the ground
Fshading shading factor (1 if a point is not shaded, 0 if a point is shaded, a percentage if measured on a surface)
Fsky visible sky factor (a percentage based on the shading mask)
theta angle of incidence between the sun and the face being analyzed
Since incident solar radiation is just a measure of the amount of sun hitting a surface, it does not depend on material properties. The basic equation does not take reflected radiation into account, as we don’t have spectral information about building exterior surfaces.
Example of urban studies of Incident Solar Radiation through SAR. See more at http://sustainabilityworkshop.autodesk.com/buildings/solar-radiation-metrics
Lighting Analysis for Revit (LAR): LAR includes visible components of solar radiation, not only direct and diffuse, but also reflected. Therefore spectral information about building exterior and interior surfaces are taken into account. The algorithm used can be described in more detail here. Every transparent or opaque material must have an RGB value which is equal to a specific reflectivity coefficient which can vary from 0% (RGB 0-0-0) to 100% (RGB 255-255-255).
Daylighting performance depends on the design in addition to the day, time, and sky conditions. When using the LAR plugin, the analysis will be run considering a clear sky condition on the fall equinox at 9am and 3pm. The simulation engine uses actual weather data and automatically chooses the closest date to 9/21 that has a clear sky measurements at 9am and 3pm, and has the highest global horizontal value.
The weather file considered both by the SAR and LAR tools is based on the user selected weather station. Running a solar and lighting analysis on the same date and time can test and provide insight to the weight of the reflective component.
Assuming that solar radiation is the parameter that influences visual comfort the most (visible light is a fraction of the entire solar radiation spectrum) we can map radiation from the fall equinox and identify the maximum and minimum radiation conditions for the NE exposure of a site in Rome.
Solar radiation average analysis at 9 AM and 3 PM: Radiation values vary between the bottom level and the top.
Next we can identify sample rooms located in the areas with higher and lower values of incident solar radiation and test them using Lighting Analysis for Revit. The rooms tested were on the first and seventh levels of the building and had the same shape, floor area, glazing layout and properties, and RGB values for the interior finish materials. The purpose of using identical settings between the rooms was to determine the sensitivity of the reflectivity coefficient of neighboring buildings.
In conclusion, reflections could influence behavior of daylight levels at certain times of the day.
Diagrams explain the impact that reflected light could have on daylight levels.
Within a range of low-medium reflectivity coefficients, LAR results will show an illuminance level variation from bottom to top directly proportional to the solar radiation variation calculated with SAR tool. On the other hand, within a range of medium-high reflectivity coefficients, illuminance level variation could be inverted. As long as neighboring building don’t have high reflectivity coefficients SAR results could inform us about daylight levels.