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Pyranometer

A pyranometer is a device used to measure solar irradiance, or insolation, both direct and diffuse on a planar surface. Pyranometers are typically passive devices, requiring no power to operate. Pyranometers use a black-coated thermopile that will absorb all solar radiation across a wide wavelength range. A glass dome limits the radiation to the short wave range only. The thermopile generates a voltage signal that is proportional to the incident solar radiation. Alternatively, a silicon photovoltaic detector can be used to measure the incoming radiation for wavelengths, which generates a current signal that is converted to a voltage signal using a potentiometer. The variable resistance of the potentiometer is used to calibrate the sensitivity for the photovoltaic sensors.

Photo of a thermopile-type pyranometer shown sitting on a pedestal with a field of solar panels in the background. The device is cylindrically-shaped, with a small glass dome on the top in the middle, surrounded by a white round surface that slopes slightly downward and a plug wire coming out of the back.Photo of a Photovoltaic detector-type pyranometer shown attached to a round surface. The sensor in the middle is black with a small round white area in the middle and have a cord protruding from the back.

Thermopile-type pyranometer
PIX #15537

Photovoltaic detector-type pyranometer.
Lieko Earle

Pyranometers are a common sensor for field tests, especially when photovoltaic (PV) panels are installed at the house, or cooling loads are a main subject of the field test. Typically, two pyranometers will be installed: one mounted on a horizontal surface and a second affixed to one of the PV panels (if applicable) in order to measure the available short wave radiation in the plane of the collector. The horizontally mounted pyranometer should be leveled using adjustable feet and the level sensor (shown in the photo above, right). Pyranometers can be used to determine the installed efficiency of PV systems, since the available solar radiation in the plane of the PV panels will be measured. Additionally, insolation can have a large effect on the cooling and heating loads in the house and capturing the on-site insolation, in addition to outdoor temperature and humidity, will help complete the picture of the environmental effects on the home's HVAC system.

For a detailed description of a test procedure for measuring the performance of a residential photovoltaic system, see:

Barker, G. and Norton, P. Building America System Performance Test Practices: Part 1 – Photovoltaic Systems. National Renewable Energy Laboratory, May 2003. NREL/TP-550-30301