Solar radiation
The prediction of the maximum clear sky irradiance values is primarily of interest for computing maximum cooling loads, or maximum internal temperatures in buildings without cooling, or maximum heating benefit that can be expected from solar radiation during the heating season. For the prediction of mean total irradiation, for any day, month, season or year, needed for energy consumption for heating or cooling, and also for estimating the useful energy which can be collected from a solar device, different estimation methods are available.
Under clear sky conditions there are three sources of radiation:
- Direct irradiance from the sun ID.
- Diffuse irradiance from the clear sky Id
- Reflected direct and diffuse irradiance from the ground Ig.
- Direct normal incidence solar irradiance IDN: Outside the earth"s atmosphere the normal incidence direct irradiance, known as the solar constant, Io is about 1.4 kW/m², the precise figure used in standard calculations is 1353 W/m². This, in fact, varies by a few percent between the two solstices and the two equinoxes due to the variation in sun earth distance caused by the elliptical orbit of the earth around the sun.
- Direct solar irradiance on surfaces other than normal, IDS: Irradiance IDS, on any surface can be obtained from
IDS = IDN cos i
On a horizontal surface this intensity is equivalent to:
IDS = ID¯ IDN sin b
