Solar Radiation Metrics Part-1

Autodesk Support

Apr 27, 2018

Solar radiation is an important consideration in any building that strives for energy efficiency. Solar radiation equates to heat if it is allowed into a building or electricity if it is captured by a PV array.

Understanding the significance of solar radiation will help you to mass, orient and program your building to capitalize on the solar radiation characteristics of your site and climate. Knowing the metrics for solar radiation can help with your analysis.

The intensity of the sun varies by the clarity of the atmosphere and the angle at which the sun strikes a surface, called the "incident angle." The more perpendicular the sun's rays are to a surface, the more heat and light energy.

Incident Solar Radiation

Incident solar radiation is the amount of solar radiation energy received on a given surface during a given time. Values are given in units of energy per area (W/m2or BTU/hr/ft2) and are usually the single most valuable metric for early design studies. This is also sometimes called insolation (Incident Solar radiation) and is sometimes quoted in terms of energy accumulated per day or per year (kWh/m2/day or kWh/m2/yr).

Incident solar radiation, visualized in Autodesk Revit

Incident solar radiation values are based on two primary components:

  1. Direct radiation from the sun (direct beam radiation = Ib) which is always measured perpendicular to the sun’s rays
  2. Diffuse radiation that is both scattered by the clouds and atmosphere (diffuse sky radiation = Id) and the ground in front of the surface (Ir). This is always measured on a horizontal surface.

Sky conditions affect the intensity and distribution of solar radiation. A cloudy sky reduces the amount of direct beam radiation and increases the amount of diffuse sky radiation. For example a clear sky will allow for direct light to travel from the sun directly to your site/building, whereas a cloudy sky will filter the sunlight and scatter diffuse light around your location.

From the total energy of the sun, up to one third can be lost (reflected into space), about 20% reaches the surface as diffuse radiation, and the rest reaches the surface as direct radiation (source).

In addition to sunlight directly heating buildings, solar radiation also creates hotter weather and affects humidity. This is one reason it’s included in diurnal weather charts.

Diurnal weather average charts show both direct and diffuse solar radiation. When direct solar radiation varies a lot over the course of a year, it’s cloudy. You can see that Copenhagen is cloudy in the winter because the absolute value of direct radiation is much lower, and the proportion of diffuse to direct radiation is higher.

Absorbed, Transmitted, and Reflected Radiation

While incident solar radiation is just the amount of energy striking a given surface. It does not necessarily tell you how much radiation is being absorbed into the façade of the building, transmitted through a building’s windows, or reflected back. That depends on the material properties and is governed by the following equation:

100% incident – reflected = Absorbed + Transmitted

Analyzing Solar Radiation

Data for direct and diffuse solar radiation are included in the weather files that analysis software uses.
The incident solar radiation values actually calculated and visualized within Revit are based on your specific building geometry. They take the hourly direct and diffuse radiation data from your weather data, your building geometry, and the time period of the analysis into account. The results of the analysis are always over a given time period (often a single hour) and are presented in Wh/m2(or BTU/ft2). You can multiply by 317.15 to convert from kWh/m2to BTU/ft2.

The calculation used by the software includes 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). Since incident solar radiation is just a measure of the amount of sun hitting a surface, it does not depend on material properties.

The amount of sky visible by a surface (Fsky) is determined by a shading mask placed over the sky dome (see more info)

The basic equation1 behind the values given in the software is below:

Incident solar radiation = (Ib* Fshading* cos(theta)) + (Id* Fsky)+ Ir

Where:

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

For more information, see:

Using Revit, you cannot calculate absorbed, transmitted, and reflected radiation values directly. However, based on the incident solar radiation values you can use design judgment and manual calculations to help you design features like apertures, shading, and thermal mass.

1. The actual solar radiation algorithm computed by the software is based on the anisotropic diffuse radiation model developed by Richard Perez, which is a statistical regression formula that takes direct and diffuse radiation into account.

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