VOLTRA

solar load on walls


Home Products VOLTRA

What is the solar energy upon an exterior wall surface, given the geographical location, solar radiation climate data, day of year, orientation, tilt, geometric configuration of possible sun obstacles, solar reflection factor, ground reflection factor ?


Example climate file: European Test Reference Year of Uccle (Brussels, Belgium)
File type: global and diffuse solar radiation on a horizontal surface
Input of climate file names in VOLTRA in Functions window

Consider day number 161 (= 10 June)
Function preview in Function Graph window
The figure below shows the global solar radiation on a horizontal surface

Define north orientation with respect to object coordinate system
Consider south oriented wall (north azimuth = 180 in XY plane)

Define outside environment as solar zone in Colours window (in this example associated with colour number 10)

Define time step (30 minutes), calculation duration (1 day) and day number at start of calculation (161 = 10 June) in Calculation Parameters dialog box

Calculate solar fluxes upon object surfaces (menu command: Calc > Calc Solar Fluxes)
A table report can be made of the average values of these solar fluxes per colour and different orientation in the Text Output window
From this table a graph can be made, e.g. using Excel

In the chart below there are different function curves
Curve 1: global solar irradiation on a horizontal surface (as defined in the climate file)
Curve 2: calculated solar irradiation on a south oriented vertical wall
Curve 3: calculated solar irradiation on a south east 45 tilted wall (i.e. with different values of azimuth and altitude of north orientation in Solar Data dialog box)

VOLTRA allows to calculate the effect of 3D sun obstacles (casting shadows)
Consider a window surface (dimensions: 2.00 m x 2.00 m) that is located 0.10 m deeper than the wall surface (dimensions: 3.00 m x 3.00 m)

         

The figure below shows the shadow image at 9:00 am, made in the Graphic Output window

Corresponding calculated solar fluxes

In the chart above:
Curve 4: (area weighted average) solar irradiation on window surface in south oriented vertical wall (as defined above)
Curve 5: idem after deduction of reflected radiation (suppose constant reflection factor = 0.075)
Curve 6: idem with angular dependent radiation

The reflection factor can be defined as a function of the angle of incidence compared to the surface normal (definition in Functions window)

The reflected solar radiation at exterior wall surfaces is further ignored (as in CAPSOL)
The effect of solar reflection from the environment to exterior object surfaces is included via a ground reflection factor (defined in the Solar Data dialog box)
The incident direct and diffuse solar radiation on an exterior wall surface is increased with this factor

Overshadowing from other structures
Curve 7 (cf. chart above) shows the cooling effect of an overhang (2.80 m x 1.00 m)

Shadow image at 9:00 am

Shadow image at 11:30 am

Absorbed (plus transmitted) solar fluxes at 11:30 am

The direct solar irradiation follows the shadow outlines, while the diffuse solar irradiation provides a background flux that depends on the local sky visibility
The irregular isoflux boundaries are due to the discrete calculation grid mesh (which causes irregular stepwise shadow boundaries) and the randomly generated visibility rays from each surface node to estimate the incident diffuse solar radiation (which cause the noise within the central areas)
The figure below shows the used calculation grid mesh (mesh size = 0.10 m)

Next page: solar load through windows