Defining a solar flux

When you model the solar effects of the sun in your model, you can set a value for the solar flux, or compute it taking into account the atmospheric and other planet effects.

In the Solar Flux tab of the Solar Heating dialog box and the Solar Heating Space dialog box, you can:

• Specify the magnitude of the solar flux as either a constant or one that varies with time.

• Let the software to compute the solar flux value taking into account atmospheric and other planet effects.

The Solar Heating simulation object only computes the solar flux based on atmospheric and planetary effects on Earth, but the Solar Heating Space lets you analyze these effects for earth and other celestial bodies.

After choosing the planet and time of year, the software populates the dialog boxes with standard computed values, which you can modify.

By defining the solar flux, you can complete the definition of the diurnal trajectory of the sun based on the specified latitude and the time of year.

Understanding atmospheric attenuation

When you define solar flux characteristics, you can model atmospheric attenuation effects due to:

• The variation in the Earth-Sun distance.

• The length that the sun's rays must travel through the atmosphere.

• Factors, such as cloud cover, vapor content in the atmosphere, and air pollution.

• Presence of the solid and liquid particles.

In the Solar Heating Space dialog box, if you select a planet other than Earth from the Planet list, these coefficients are set to have no effect on the calculations. However, you can still modify their values to model the effect of an atmosphere on the selected planet.

Understanding the diffuse sky radiation calculation

You can use one of the following methods to calculate the diffuse sky radiation:

• Atmospheric extinction coefficient

• Altitude and turbidity

When you choose the atmospheric extinction coefficient method, the software computes the diffuse solar irradiation for all illuminated elements as follows:

Idif=Idn x C x Fss

Where:

• Idif is the diffuse sky irradiation.

• Idn is the direct irradiation of a horizontal surface.

• C is the ratio of the diffuse irradiation to the direct normal irradiation of a horizontal surface.

• Fss is the view factor between the surface and the sky. In case of explicit sky modeling, the software calculates the view factor. Otherwise, Fss = 1+cos(E)/2 where E is the angle between the surface and the ground.

Using this method, the solver calculates the direct irradiation on the horizontal element facing the sky which receives it for the given sun position and solar flux. Then, the solver uses it to calculate the diffuse sky irradiation of each element. An element pointing toward the ground does not receive any diffuse sky irradiation since its E angle equals 180°. (ASHRAE. 1995. HVAC Applications. pp. 30.1-30.5)

When you select the altitude and turbidity method, the software computes the diffuse solar irradiation for all illuminated elements as follows:

Idif=G0 x Tn(TLK) x Fd(h0)

Where:

• G0 is the normal extraterrestrial irradiance.

• Tn(TLK) is the diffuse transmission function depending on the Linke turbidity factor TLK.

• Fd(h0) is the diffuse solar altitude function depending on the solar altitude h0.

To compute sky view factors and shadowing effect, you can model the sky explicitly to model the shape of the sky dome.

Understanding ground surface reflectance calculation

You include the effects of ground reflection in your analysis. For ground reflection calculations, the software does not model the ground. It applies the following equation to all selected illuminated elements without performing any shadowing:

Ir=ItH x Pg x Fsg

Where:

• Ir is the reflected radiation from the foreground.

• ItH is the total horizontal irradiation (including both direct and diffuse irradiation).

• Pg is the ground surface reflectance (albedo).

• Fsg is the view factor between the surface and the ground. In case of explicit planet modeling, the software calculates the view factor. Otherwise, Fsg = 1+cos(E)/2 where E is the angle between the surface and the ground.

The software considers your model as floating over the ground. Therefore, even the bottom of your model receives ground reflection radiation.

• Horizontal elements that face the sky do not receive any reflected radiation from the foreground.

• Horizontal elements that face the ground receive the maximum amount of reflected radiation.

To include the ground in the shadowing process, you can model the ground explicitly to model the shape of the ground.

Computing radiation under overcast conditions

To compute the overcast irradiance/irradiation from clear-sky raster maps, the thermal solver uses a factor parameterizing the attenuation of the cloud cover method (Maxwell, E.L. 1987. Solar Energy Research Institute, US).

This method uses the clearness index Kt, which is the ratio of radiation on a horizontal surface to the extraterrestrial radiation to correct the direct radiation.

How do I

Define Solar Heating

Define Solar Heating Space

Learn more

Defining model orientation for solar heating

Inputs to expressions

Look up more details

Auto-generated expressions

Quick links

Simcenter 3D Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal boundary conditions

Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

Browse Simcenter 3D help by product area

Defining a solar flux, Simcenter 3D 2021.1 Series

© 2020 Siemens

window.mainLanguage="en_US"

window.delivId=""

window.projectId=""

MathJax.Hub.Config({ TeX: { extensions: ["autoload-all.js"] }, tex2jax: { displayMath: [ ] }, "SVG": { scale: 125 } });