Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 39

In order to measure direct solar radiation (i.e. the proportion of direct radiation
related to the global radiation) the pyrheliometer is used. One of two equal, black-
ened thin Manganin surfaces is exposed to direct solar radiation which, in turn,
heats it up. The other surface, without incident solar radiation, is heated with elec-
trical energy to the same temperature as the exposed surface. The production of
heat is proportional to the square of the electric current. Thus the electric current
is equivalent to the absorbed radiation energy. Pyrheliometers are normally ad-
justed to the incident radiation, and are built in a way that enables only direct ra-
diation to incident on the receiving surface, for example by positioning the plane
surface within a tube.
In order to measure global radiation, pyranometers are used (e.g. the Moll-
Gorcynsky pyranometer). This instrument has a radiation thermopile as receiving
surface. Its counter junctions are thermally connected with the casing. The tem-
perature difference caused by heating of the receiving surface due to the solar
radiation generates pressure, which serves as a measure for global radiation. In
order to avoid atmospheric influences on the measuring process, the receiving
surface is protected with spherical calotte made of different materials, in accor-
dance with the spectral range to be measured. In order to measure for example
short-wave radiation fluxes, hemispheres made of silica glass are used. To meas-
ure long and short-wave radiation fluxes, spherical caps made of polyethylene,
and for long-wave radiation fluxes silicon hemispheres are used. Pyranometers are
mainly aligned horizontally. If the direct radiation proportion of the global radia-
tion is cut out, e.g. by shielding from direct solar radiation by a circular disk or a
fixed shade ring (shadow band), these instruments can also be used to measure
diffuse radiation.
In order to calculate the radiation balance, one pyranometer is required for the
upper and the lower hemisphere. Depending on the type of cover, the balance can
be drawn for different spectral ranges.
Often, only the duration of sunshine is measured. It is mostly captured by a
sunshine recorder named after Campbell-Stokes, employing a glass sphere to fo-
cus the sun's rays to an intense spot or a focal point, which will burn a mark on a
curved card mounted concentrically with the sphere.
Distribution of radiation. Worldwide, global radiation is measured at numerous
sites. If these measured radiation values available as hourly, daily or monthly
mean values are added up over one year and the long-term mean values are calcu-
lated, the average expected radiation for this particular site is obtained. Fig. 2.11
shows the global solar radiation distribution on earth.
On a first glance the graphic reveals that the highest global radiation occurs
north and south of the equator. Especially within the deserts and on mountains
located here, the yearly global radiation reaches its maximum. North and south of
these zones the global radiation irradiated throughout the year decreases.