After passing through the Earth's atmosphere, solar radiation includes both a direct component from the Sun itself and a diffuse component made up from reflections off clouds, moisture vapour and other particulates within the sky. The diffuse component may also contain reflections off the ground and other elements of the local built environment.
In all Square One software, incident solar radiation is calculated directly from the geometry of the model and using hourly recordings of direct beam and diffuse horizontal solar radiation values taken from the currently loaded weather data file. The following are descriptions of the various solar radiation components you may see used in different calculations, graphs and tables.
Direct Beam Radiation (
)
In the weather data file format, the direct beam component of solar radiation is given as a value in W/m² and is measured on an imaginary surface directly facing the Sun. As the Sun moves through the sky, this measurement surface tracks it so that the direction of incident radiation is always normal (straight on) to it.
Direct Horizontal Radiation (
)
Direct horizontal radiation differs from direct beam in that it is measured incident on a flat horizontal plane. In this case, the direct beam component is modified by the cosine of the angle of incidence (A) at which it strikes the horizontal surface. Thus, it can be calculated as:
Diffuse Horizontal Radiation (
)
The diffuse horizontal component is also given in W/m² and is taken as the energy from the entire sky dome that falls on a horizontal surface, minus the effects of direct beam radiation as it hits the horizontal.
This is important as it means that radiation from low in the sky near the horizon strikes the flat measurement surface at almost grazing incidence - meaning that it contributes much less to the measurement than light from the zenith with strikes the surface at or near normal incidence.
Diffuse horizontal radiation values assume that there are no surrounding obstructions to obscure any part of the sky and, as a result, are typically measured on the top of a tall building or on a pole in a field.
Global Horizontal Radiation (
)
Another value often measured is the global horizontal radiation, being the sum of both the direct and diffuse components as measured incident on a flat horizontal plane. It is therefore the sum of the direct horizontal and diffuse horizontal values. Thus, given as:
or
Global Radiation
You will sometimes see references to global radiation in some Square One graphs or tables. Global differs from global horizontal in that it is a theoretical maximum amount, given as the sum of direct beam and diffuse horizontal values.
Obviously no single surface can always be simultaneously horizontal (collecting diffuse horizontal) and normal to the direct radiation (ie: collecting direct beam). As a result, this value is not meant to represent a real physical quantity, but is actually a better indication of the total available radiation for vertical and other non-horizonal surfaces, especially at low Sun angles.
Units and Measures
Radiation is generally expressed in terms of either irradiance or radiant exposure. Irradiance is a measure of the rate of energy received per unit area, and has units of Watts per square metre (W/m²) - where 1 Watt (W) is equal to 1 Joule (J) per second.
Radiant exposure is a time integral (or sum) of irradiance. Thus a 1 minute radiant exposure is a measure of the energy received per square metre over a period of 1 minute. Therefore a 1-minute radiant exposure = mean irradiance (W/m²) x 60(s), and has units of joule(s) per square metre (J/m²) or Watt hours per square metre (Wh/m²). A half-hour radiant exposure would then be the sum of 30 one-minute (or 1800 one-second) radiant exposures.
Measurement Devices
All the above values are derived from solar radiation measurements available from weather stations equipped with instruments such as pyrheliometers, pyranometers or solar colorimeters - basically sensors that use photo-sensitive material, charge-coupled devices (CCD) or thermocouples to measure the amount of radiation coming from the Sun.
To directly measure the direct beam component, a pyrheliometer is placed at the end of a long tube designed to exclude all but the radiation direct from the Sun (which has an apparent diameter of 0.5°) and its corona (a narrow 2.5° annulus of sky immediately surrounding it). A system of motors and gears is then used to keep the tube pointing directly at the Sun throughout each day. Because of their moving parts, such systems can be expensive and prone to malfunction.
A pyranometer on the other hand is used for measuring solar irradiance from the entire sky falling on a flat planar surface. When mounted and horizontally facing upwards, it measures global horizontal solar irradiance, the sum of the direct beam radiation and the diffuse horizontal. This is the simplest and least expensive value to measure, so many weather stations simply use two horizontal pyranometers - one fully exposed to the sky and the other shaded from just the direct solar component using either a mobile shade or a curved strip of metal that follows the path of the sun.
(Images taken from http://www.eppleylab.com)
Deriving Direct and Diffuse Values
To calculate direct beam radiation, the diffuse horizontal value must first be subtracted from the global horizontal value to obtain the direct horizontal solar radiation. To convert from incidence on a horizontal surface to incidence on a theoretical surface normal to the direct beam, the resulting value must be cosine-corrected to reflect the incidence angle the direct sun makes with the horizontal surface at the exact time of the measurement.
Solar position is usually given as an azimuth (relative to North) and an altitude (relative to the ground plane). For a horizontal surface, whose normal points directly up towards the zenith of the sky, the incidence angle of the sun in degrees can be taken as 90° - SolarAltitude. Thus, to obtain direct beam radiation, the direct horizontal solar radiation must be divided by the cosine of the instantaneous incidence angle.
Depending on the type of pyranometer used, this can sometimes be problematic as values for the direct component of global horizontal radiation are highly dependant on the incidence angle of the sun as it moves through the sky. Recorded values are typically integrated over time to give period-averaged values or sampled instantaneously at regular intervals, so it is critical that solar altitude is calculated at the precise times at which each recording was made.
This is because the results are highly sensitive to even the smallest error, particularly at very low altitudes. This is even more of a problem if the recordings are integrated over relatively long periods, as the same hourly value will pertain to both the time just after sunrise and to that up to 59 minutes later ; during which time the solar altitude will have changed quite considerably.
Such problems can be overcome by using a more expensive cosine corrected pyranometer. As shown in Figure 2, these can use special lenses to focus radiation onto the sensor regardless of direction, and require built-in correction factors to account for focal variation. However, these sensors essentially measure global radiation, not global horizontal as the direct beam radiation is also directed by lens. This makes the derivation of the direct beam component much simpler, becoming a straight-forward subtraction.
However, as hourly weather data for many locations is often NOT readily available from recognised sources, it is usually good to confirm the type of recording equipment used before applying corrections and deriving the various components.
This issue has further implications when calculating the radiation incident on surfaces, however these are explained in detail within the shading design topics.
