Meteorological measurement campaign design

Overview

A meteorological measurement campaign provides ground-measured solar and meteorological data needed across the lifecycle of a PV power plant. The quality of this data directly determines the accuracy of energy yield estimates, site adaptation of satellite-based solar data, post-construction acceptance tests of a power plant, and long-term plant performance evaluation.

A successful campaign rests on two pillars:

• Procurement of high-accuracy instruments installed according to World Meteorological Organization (WMO) standards.

• Rigorous operation and maintenance (O&M) procedures with systematic quality control.

Both are equally important - even the best instruments will produce unreliable data without consistent, well-documented maintenance.

A good measurement campaign has two stages. The pre-construction stage provides data for site adaptation, solar resource assessment, and energy yield assessment. The operational stage converts the campaign into a perpetual measurement system supporting performance evaluation throughout the plant's lifetime.

Station setup

Primary and secondary stations

A well-designed measurement network typically consists of one primary meteorological station and, depending on the size of the plant, one or more secondary meteorological stations placed at different positions within the project area.

IEC 61724-1:2021 specifies the number of sensors based on project size, but in practice, high-quality measurements can be achieved with fewer instruments than the standard recommends - provided O&M follows strict quality standards.

This setup offers several advantages over deploying many instruments across many stations:

• Fewer stations allow more frequent, focused maintenance per site - directly improving data quality.

• A primary plus secondary station configuration provides redundancy and backup in case of equipment failure.

• Fewer instruments make data quality control more efficient and effective.

• Capital and operational costs are lower.

The station should be designed and installed by a company with experience in both the installation and operation of solar meteorological stations. Professional design ensures that instruments are positioned to avoid mutual shading and reflections, are easy to maintain, and represent the site conditions.

Note: The station design should account for the surrounding terrain, land cover, and any obstacles that could affect measurements. Avoid installation near heat sources, reflective surfaces, or objects that obstruct airflow or cast shade on sensors.

Measurement parameters

GHI (global horizontal irradiance) should be measured at every station. GHI instruments have easily controllable alignment and offer the most options for quality checks. All pyranometers measuring GHI and GTI (global tilted irradiance) should be equipped with ventilation units to prevent dew, frost, and dust deposition on the sensor dome.

Figure 1: Example layout of a primary meteorological station with solar tracker, pyranometers, and meteorological sensors.

The recommended recording interval is 1 minute, with each record timestamped. This granularity captures short-term variability needed for performance evaluation and provides sufficient data for site adaptation of satellite-based solar data.

*Each instrument should include a ventilation unit

Only Class A pyranometers (ISO 9060:2018) should be used for solar radiation measurements. Photodiode sensors and reference cells carry a risk of instability and drift, and their data reliability is limited. Lower-accuracy instruments produce data with higher uncertainty.

Wind speed and wind direction sensors should be installed on a 10 m mast, as specified by WMO standards. Air temperature and relative humidity sensors should be housed in radiation shields with free airflow, installed 1.5 to 2 m above ground.

Source: WMO Guide to Meteorological Instruments and Methods of Observation, 2008

DNI and DIF measurement options

Two instrument options are available for measuring Direct normal irradiance (DNI) and Diffuse horizontal irradiance (DIF):

• Rotating shadowband radiometer (RSR): Measures GHI and DIF directly; DNI is derived. RSR requires moving parts but is less sensitive to soiling, which reduces cleaning frequency. This option has lower CAPEX and OPEX and is typically installed at secondary (Tier 2) stations. Measurement uncertainty is indicatively 5% under ideal conditions.

• Solar tracker with pyrheliometer and shaded pyranometer (Tier 1): Consists of a Class A pyrheliometer (DNI), a ventilated Class A shaded pyranometer (DIF), and a ventilated Class A pyranometer (GHI), all mounted on a solar tracker. This option delivers the highest data quality (DNI uncertainty < 1%) and is recommended for primary stations, but requires more intensive O&M.

Note: The SPN1 sunshine pyranometer is sometimes used in the industry as a third option for DNI measurement. Solargis does not recommend this instrument. Its measurement uncertainty is ±5.0 % daily, and higher in real-world operation. Time series data from SPN1 instruments typically show consistent artifacts caused by the internal shading mask design. These artifacts can resemble soiling events but occur regardless of instrument cleanliness, making quality control unreliable.

Supporting parameters

The following supplemental measurements can improve performance evaluation quality:

• Ground albedo: Recommended when bifacial PV modules are installed. Accurate albedo measurements improve GTI modelling, particularly for rear-side irradiance. Instruments must be mounted 1.5 to 2 m above the ground to ensure a representative field of view and minimize self-shading.

• Soiling: Used to optimize PV module cleaning schedules and support soiling loss analysis. Several soiling kit and reference cell solutions are available from instrument manufacturers.

• PV module temperature: A temperature probe mounted on the back of a PV module. Required for temperature-corrected Performance Ratio calculations.

Operation and maintenance

Campaign duration

For pre-construction assessment, at least one year of high-quality measurements is needed to cover all seasons. Two or more years provide more robust results, particularly for site adaptation of satellite-based solar data. Shorter measurement periods may not capture all seasonal deviations and will increase uncertainty.

After commissioning, the measurement campaign should transition into a continuous, long-term operation alongside the plant. Ground measurements collected during plant operation are essential for performance evaluation and O&M decision-making.

Measurement errors and their causes

Solar irradiance measurements are sensitive to a range of errors that can cause visible or hidden anomalies in recorded data. A thorough quality check is required before any data use. Error sources fall into three categories:

• Instrument characteristics: Temperature response, cosine and azimuth effects, spectral sensitivity, non-linearity, calibration drift

• Installation and setup: Instrument levelling, sun tracking or shadow ring misalignment, cabling, data logging and transfer

• Environmental and operational issues: Shading and reflections from nearby objects, soiling (dust, snow, bird droppings, dew), mechanical or electrical malfunctions, unplanned shutdowns

Many of these errors can be prevented or corrected through rigorous, systematic maintenance by qualified personnel.

Maintenance procedures

High-quality and consistent maintenance is the single most important factor for obtaining reliable long-term measurements. The following events must be documented with timestamp as they are critical for downstream quality assessment and data analysis:

• Changes in instrumentation, equipment, or station setup

• Calibration of instruments

• Cleaning events

• Changes in instrument geometry or behavior

• Data collection, transfer, or storage errors

• Extreme weather events

Cleaning is essential for thermopile sensors, which have a larger contact surface than photodiode sensors and are therefore more susceptible to soiling. In moderate conditions, cleaning 1 to 2 times per week is generally sufficient. In areas with high soiling risk (high aerosol optical depth, sandstorms, soot, or long dry periods), daily cleaning may be required.

Cleaning events should be recorded using a push-button event logger - a switch connected to the datalogger that is pressed before and after each cleaning. This timestamps cleaning events alongside measured data, making them available for quality control and post-processing. If a fenced station with a gate is used, a gate switch can also automatically record site access.

Daily or near-daily maintenance checks should include:

• Instrument levelling and alignment.

• Cleaning of pyranometer domes and pyrheliometer glass.

• Checking for condensation inside instruments.

• Verifying solar tracker alignment.

• Checking fan and wiring operation.

• Confirming data acquisition continuity.

Calibration certificates for all instruments should be current, with recalibration performed on the manufacturer-recommended schedule or at least every two years.

Data quality control and aggregation

Quality control (QC) should be applied to collected data regularly - ideally daily. QC procedures flag or remove data readings that fail defined criteria. Missing values are marked by flags; data gaps may be filled by qualified experts where appropriate, but non-expert modification of raw data risks irreversible corruption.

Note: Solargis Analyst is built specifically for solar data management, quality control, and analysis. A general-purpose spreadsheet application is not suitable for this purpose.

Care is required when aggregating 1-minute data into longer intervals (10-minute, 15-minute, 30-minute, or hourly). During aggregation, incorrect measurements can be averaged with valid ones, systematic errors such as morning dew can be hidden, and missing values can distort aggregated results. Aggregation should only be performed using appropriate tools and by personnel familiar with the data characteristics.

Further reading

• "Photovoltaic system performance – Part 1: Monitoring" - IEC 61724-1:2021

• "Specification and classification of instruments for measuring hemispherical solar and direct solar radiation" - ISO 9060:2018

• "WMO Guide to Meteorological Instruments and Methods of Observation"

• "NREL Best Practices Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications, 4th Edition"