Common Practices for Pyranometer Recalibration
The initial calibration process and subsequent recalibrations are critical steps for accurate measurement of solar radiation for any Solar PV Plant.Pyranometer recalibration ensures the data is reliable for efficiency & performance ratio calculations.
ISO Calibration Standards- Various international standards related to solar energy, including IEC 61724-1, require that pyranometers are calibrated in accordance with following standards:
ISO 9846:1993: Calibration of a pyranometer using a pyrheliometer
ISO 9847:1992: Calibration of on-field pyranometers by comparison to a reference pyranometer
Why Re-Calibrate?
After a period of a year or 2 in the field, the sensitivity of a radiation sensor is slightly altered due to prolonged exposure to the sun, and that cause deviation in the sensor’s measurement properties.
Regular calibration of the sensors is critical for measuring performance parameters with the least error.
Recalibration Interval
Most of the sensors need a recalibration every 1 year.IEC (International Electrotechnical Commission) suggests to either work with a 1-year interval or to follow the sensor manufacturer’s recommendation.
Generally, utility-scale PV power plants deploy multiple pyranometers and they re-calibrate half of the pyranometers in one year and another half of them in next year.By this alternate schedule they maintain more accurate measurements as well as keep the uptime of sensors high
How ?
Pyranometers are calibrated by comparing the measurement values from the test instrument with those given by one or more reference instruments under controlled conditions. Any of the below methods can be used for purpose of recalibration
Recalibration in a Certified Laboratory by following ISO 9846:1993
1) Equipment needs to be dismantled ,shipped & reassembled post re-calibration.
2) Plant will be without an irradiation sensor, resulting in loss of valuable data.
3) Suggestion: Purchase an extra sensor and keep it in stock so that you can replace the existing one with extra sensor and send the on-site sensor for recalibration which can be then used for another site due to recalibration.
On-site Recalibration by following ISO 9847:1992:
1) ISO recommends to perform field calibration during several days; 2 to 3 days under cloudless conditions, 10 days under cloudy conditions.
2) In case of field comparison; ISO recommends field calibration to a higher class pyranometer.
3) Indoor is not equivalent to outdoor conditions. Calibration equipment can be affected by the surrounding environment.End-to-end calibrations are necessary to limit uncertainties and systematic errors.
