| Valentia Observatory
Solar Radiation
Energy from the sun in the form of ultra-violet, visible and infra-red electromagnetic
radiation is known as solar radiation. It has been measured by Met Éireann
since 1954 when the first pyranometer was installed at Valentia Observatory.
At present, observations of solar radiation are made at seven meteorological
stations as well as by some Universities and other research institutes. Data
are available from Met Éireann's climate database, located in Glasnevin,
Dublin.
Why Solar Radiation is measured
Solar radiation is measured by Met Éireann as one of the many elements
in its observational programme and is being added to its climatological database.
It is used in agrometeorology where plant growth, ripening, drying conditions
and crop yield are related to the solar energy available. Solar radiation has
been supplied to users interested in studying the correlation between solar radiation
and such topics as the growth of plants, algae and planktons; the efficiency
of energy collectors; solar heating, and research into the measurement and analysis
of solar radiation.
Brief description of the nature of Solar Radiation
As mentioned above, solar radiation is a term used to describe visible and near-visible
(ultraviolet and near-infrared) radiation emitted from the sun. Different regions
of the solar spectrum are described by their wavelength range within the broad
band range of 0.20 to 4.0 µm (microns). Infrared radiation emitted from
the atmosphere is referred to as terrestrial radiation. Table 1 below gives
the components of solar and terrestrial radiation together with their approximate
wavelength ranges:
| Table 1 |
|
Ultraviolet
|
0.20 - 0.39µ |
| Visible |
0.39 - 0.78µ |
| Near-Infrared |
0.78 - 4.00µ |
Infrared
|
4.00 - 100.00µ |
Solar radiation with wavelengths greater than 3.0µm (approximately
2% of the total energy reaching the earth) is absorbed by the atmosphere.
Radiation
from other sources with still longer wavelengths (infra-red, micro- and radio
waves) is transmitted through the 'atmospheric window'. Water vapour and CO2
also absorb some radiation in the infra-red region while ozone absorbs much of
the ultra-violet.
Outside the earth's atmosphere, solar radiation has an intensity of
approximately 1370 watts/meter2. This is the value at mean earth-sun
distance at the top of
the atmosphere and is referred to as the Solar Constant. The availability of
energy at the surface is affected by location (including latitude and elevation),
season, and time of day, all of which can be readily determined. However, the
biggest factors affecting the available energy are cloud cover and other meteorological
conditions, which vary with location and time. In Ireland at the surface on a
clear day, at noon, the direct beam radiation will be approximately 940 watts/meter2
for many locations.
Measurement of Solar/Terrestrial Radiation
Table 2 below gives a summary of the solar and terrestrial radiation measurements
made by Met Éireann:
| Table 2 |
|
| Type of measurement |
Description |
| Direct Solar Radiation |
Solar radiation at normal incidence in the direct beam from the
sun |
| Diffuse Solar Radiation |
Scattered radiation on a horizontal surface |
| Global Solar Radiation |
Sum of the direct beam plus the diffuse component on a horizontal
surface |
| Infra-red Radiation |
Terrestrial infra-red radiation emitted by the sky on the Earth's
surface |
| Net Radiation balance |
Combined downward solar radiation and sky infra-red minus upward
reflected solar and terrestrial radiation |
| Turbidity |
Measure of the amount of scattering in the atmosphere |
Pyranometer
This is the most frequently used sensor for measuring Global or diffuse radiation.
It consists of a thermopile composed of a series of thermocouples with alternate
'warm' and 'cold' junctions. 'Warm' junctions are in thermal contact with a
'black' absorbing surface with low thermal capacity. 'Cold' junctions are in
contact with a material with relatively high thermal capacity and remains at
ambient temperature. When solar radiation falls on the 'black' absorbing surface
almost all of the incident radiation is absorbed. This results in a rise in
temperature at the 'warm' junction proportional to the intensity of the radiation.
A voltage is then generated by the thermopile, which is proportional to the
temperature difference between the 'warm' and 'cold' junctions. This output
voltage is converted Watts/meter2 by use of a sensitivity factor (whose units
are µV/(W/m2)) which can be established through sensor calibration with
standard lamps in controlled conditions.
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