The word pyranometer comes from the Greek words ‘pyr’ for fire, and ‘ano’ for above. A pyranometer measures the total solar insolation, i.e., power density in W/m2 (watt per meter square) incident at a surface. It is an essential instrument in any laboratory dealing with solar energy. Planning of solar energy installations is facilitated by the solar data provided by a pyranometer. Other uses of pyranometers are in building physics, climatology and meteorology.
The pyranometer is different from a Lux meter, in that the latter only measures optical power.
Pyranometer for solar systems (Photo courtesy: Kipp-&-Zonen)
Construction of pyranometer
The heart of the pyranometer is the sensor secured on a flat base, and connected to the amplification and processing circuitry through a cable. A transparent hemispherical dome covers the sensor, giving the pyranometer a shape like half burger. The base and the dome seal the sensor against air movement and humidity. Both air movement and humidity have the potential to cause errors in sensing. Since the pyranometer is an outdoor instrument, and may be left outside for extended periods the dome may be of a weatherproof self-cleaning material. A replaceable desiccator module is also enclosed within the same volume to absorb any vapor that might get through. The base has leveling arrangements, so that measurements can be made even over an inclined surface.
Depending on the model and the cost, the pyranometer may have an amplifier and a digital display.
The pyranometer is required to respond to solar energy –both light and heat. This radiation is also referred to as shortwave radiation. The solar spectrum extends from 280 nanometer to about 3000 nanometer wavelength. In terms of colors it extends from ultraviolet B, ultraviolet A, through visible colors, and infrared. After passing through the atmosphere the band width is restricted from 300 to 2500 nm. However, practically most of the energy is contained within 350 to 1500 nm. Practical pyranometers have upper limits starting 1200 nm to 2800nm. This upper limit does not really matter for photovoltaic work because currently solar panels only respond to light component of solar energy.
Directional Response and Field of View
The pyranometer is designed to give a cosine directional response, which means it will give maximum response when facing the sun directly, and if offset, the response will be proportional to cosine of the offset angle. The same is the behavior of the photovoltaic panels-maximum output when the sun is facing directly and decreasing as cosine of the offset angle. Maximum field of view is 180 degrees.
Standards and Calibration
ISO 9060, also adopted by the World Meteorological Organization (WMO) defines three levels of accuracy relative to the primary World Radiometric Reference (WRR) at Davos. The best accuracy is given by the secondary standard instrument calibrated directly against the WRR standard. The next lower classes in accuracy are called first class and second class instrument, respectively.
ISO standards demand a response time less than 15s, 30s, and 60s, respectively, for the secondary standard, first class, and second class instruments. Response time is not really critical for planning solar energy generation.