Radiation arriving at the Earth’s surface from the sun and the sky is split into short-wave radiation (ultraviolet, visible and near infrared) in the wavelength range 300 to 4000 nm (4 μm) and long-wave radiation (far infrared) from 4.5 to beyond 40 μm. PV materials have most of their sensitivity from approximately 400 to 1100 nm, with a peak just beyond the visible range.  There is no response to long-wave radiation, and little to ultraviolet.

Measurements of solar radiation are usually made using thermopile type radiometers with a flat spectral response. The types of instruments are defined by the World Meteorological Organisation (WMO) and the International Standards Organisation (ISO). In a solar monitoring station the short-wave radiation is measured in three ways:

• Global Solar Radiation is measured by a ‘Pyranometer’, which is a radiometer with a glass dome that has a hemispherical view of the whole sky.
• Direct Solar Irradiance is measured by a ‘Pyrheliometer’. This is a radiometer with a 5º view that is pointed accurately at the centre of the sun by an automatic Sun Tracker. It only sees the sun and its aureole.
• Diffuse Solar Radiation is scattered by aerosols in the atmosphere and reflected by clouds. It is measured by a Pyranometer mounted on a sun tracker with a shading mechanism to block the direct solar irradiance.

The output signals are normally acquired by a high accuracy multi-channel data logger that is programmed with the sensitivity of each radiometer, so that data can be stored in units of W/m².

However, for applications such as monitoring the efficiency of a PV installation, it is sufficient to use a single pyranometer measuring the total solar energy available.  This could be permanently installed (horizontally or at the same angle as the PV panels) or used with a hand-held display for field checks.

Kipp & Zonen is the leading specialist for solar radiation instruments. They offer a wide range, from the highest accuracy to cost-effective solutions. They are the main supplier of pyranometers and pyrheliometers to the  PV Industry and the demanding fields of meteorology and climatology at measurement sites around the world.

More information on www.kippzonen.com.