Global radiation
Under global radiation is defined as the total on the Earth's surface on a horizontal receiving surface incident solar radiation . It is made up of
- solar radiation arriving on the direct path, direct radiation , and
- the short-wave diffuse radiation , which reaches the earth's surface via scattering from clouds , water and dust particles.
The climatologically important net radiation, which indicates the absorbed solar radiation, must be distinguished from global radiation . H. minus the reflected part, the albedo , see radiation budget of the earth . Global radiation is the insolation on the earth's surface.
Measurement and typical values
In order to measure the instantaneous value of global radiation serve pyranometer . This instantaneous value has the unit of irradiance , watts per square meter (W / m²). By adding up the radiated energy over certain periods of time, for example hours, days or years, an energy input (measured in kWh / m²) is obtained which is to be related to the summed up period. Daily entries are given in kWh / (m² · d ), annual entries in kWh / (m² · a) (usually spoken: kWh per m² and year). In climatology, the annual entry is also described by the irradiation power averaged over the whole year (day and night, i.e. 8,760 hours in common years ). An annual entry of 1200 kWh / (m² · a), for example, corresponds to an irradiation power of 137 W / m² averaged over the year.
The instantaneous values of global radiation are subject to strong fluctuations due to weather conditions due to cloudiness and atmospheric cloudiness . Because of the variable angle of incidence of the direct radiation component, global radiation is stronger at noon than in the morning and in the evening, and stronger in summer than in winter.
Due to the steeper angle of incidence, the annual inputs are higher the closer you get to the equator (see ecliptic ). The height above sea level , i.e. the thickness of the earth's atmosphere ( air mass ) that the sun has to penetrate, also systematically influences this value.
The instantaneous value of global radiation in Central Europe on a summer noon with a cloudless sky reaches around 900 W / m². With light cloud cover (but practically unhindered direct radiation), it can briefly rise above 1,000 W / m² due to the diffuse radiation provided by the white clouds. In cloudy, cloudy weather it consists only of the diffuse radiation component and sinks to values around or even below 100 W / m².
The annual total of global radiation in Germany is between 900 and 1,200 kWh per m² per year on a horizontal surface, which corresponds to an average of approx. 100 to 135 W / m². In Spain the global radiation is around 2,000 kWh / (m² · a) (230 W / m²), in the Sahara 2,500 kWh / (m² · a) (285 W / m²).
Global radiation on inclined surfaces
While in meteorology the global radiation on a horizontal receiver surface is usually considered, the radiation on an inclined surface has to be determined for numerous energetic investigations (examples: optimal alignment of solar collectors , heating of house facades or energy input through windows ). The radiation on inclined surfaces contains the solar radiation reflected from the ground as an additional component , which can make a significant contribution in the case of strongly reflective ground surfaces (e.g. snow cover in winter). On the other hand, only part of the field of view of inclined surfaces is filled by the sky, so that the contribution of diffuse radiation on inclined surfaces is less than on horizontal surfaces. The change in the proportion of direct radiation depends on the change in the angle of incidence , which results from the position of the sun and the angle of inclination of the surface.
The determination of typical irradiance levels depending on the orientation and inclination of the receiver surface can be done by measuring on the inclined surface, by converting existing measured values on a horizontal surface or by estimating using suitable solar radiation models.
literature
- VDI guideline 3789 sheet 2: Environmental meteorology - interactions between atmosphere and surfaces - calculation of short-wave and long-wave radiation , Beuth-Verlag, Berlin 1994 (radiation models , including for estimating global radiation for a given location; brief description )
- W. Palz, J. Greif: European Solar Radiation Atlas: Solar Radiation on Horizontal and Inclined Surfaces . Springer-Verlag, Berlin 1996, ISBN 978-3-540-61179-0
Web links
- solarserver.de: Radiation data
- Global radiation. In: Spectrum Lexicon of Optics.
Individual evidence
- ↑ The global radiation model of VDI guideline 3789 sheet 2 provides a global radiation value of 940 W / m² at 46 ° N and 870 W / m² at 54 ° N (sea level, opacity factor 3) for midday on June 21 .