PR as performance guarantee

The Performance Ratio indicator (PR) is widely used as a performance guarantee for PV plants. The PR is calculated from forecast simulations and compared to the PR measured in the field.

PR measurement

The PR calculated by PVsyst is referenced to the transposed irradiance GlobInc, directly from GlobHor without the horizon loss.

The PR evaluated by the measurement should be normalized to a reference irradiance normally measured on-site. This is usually defined as

\(PR_{meas} = EGrid_{meas} / (GlobP_{meas} * PNomPV)\)

However, there are several ways to determine the irradiance GlobP_meas:

• A simple solution is to use hourly or sub-hourly data from a weather data provider. These data should be simultaneous with plant operation. Synchronization should be carefully checked. The available data are global horizontal; the diffuse component must be evaluated and transposed to the collector plane to obtain GlobP_meas.
• A better solution is to measure irradiance in the horizontal plane using a solarimeter (preferably a pyranometer). Transposition to the collector plane must again be performed, which requires detailed calculations in PVsyst. Measuring the diffuse component would improve transposition accuracy.
• The solarimeter is often mounted directly in the plane of array (POA measurement). This avoids using PVsyst for transposition calculations; the measured PR can be directly obtained without transposition. The solarimeter should be carefully positioned on a location without mutual shading and without masking albedo in front of the plant (for example, on top of the first shed).
• With tracking systems, many people place a solarimeter on the tracking axis. In this situation, the solarimeter is affected by mutual shading from neighboring sheds, especially on diffuse and albedo components. Interpretation of GlobP_meas (and PR_meas) requires shading correction as calculated by PVsyst. To avoid this, the solarimeter could be placed on an extension of the axis, outside the tracking array.
• For bifacial systems, the irradiance on the rear side of PV modules is required for PR_Bifi calculation. However, there is no consensus on how to measure it: rear-side irradiance is highly inhomogeneous, depending on position within the shed.

Note: PVsyst simulation also requires measurement of ambient temperature. This should be included in the data along with irradiance. Measured PV module temperature may be used as an alternative.

Accuracy

In any case, the accuracy of the PR_meas is directly related to the accuracy of the GlobP_meas.

External data are typically satellite data with an accuracy of 10 to 20% for hourly values (although long-term data are more accurate). Even if data comes from a weather station, hourly accuracy is not better unless the station is located within 10 km.

When using a solarimeter, excellent calibration is required. Periodic recalibration may be necessary. Positioning should be carefully chosen without shading, and device horizontality is essential. The accuracy of a good solarimeter measurement is approximately 1–2%. The transposition model can be considered sufficiently accurate (less than 1% on monthly periods).

Importing POA measurements (GlobP_meas) in PVsyst reconstructs GlobHor and DiffHor variables, which are necessary for shading calculations. These accurately represent the original GlobP_meas when transposed during simulation, making them independent of potential uncertainties in the transposition model.

Measurement on the tracker axis presents a significant challenge because it is subject to mutual shading, which is evaluated by complex calculations in PVsyst. Additionally, there is no assurance that axis-level measurement fully represents irradiance across the entire tracker.

PR comparison

Therefore, comparing measured and forecast PR is difficult. When specifying a warranty based on PR value, it is very important to specify how this PR will be established on-site.

Many effects should be considered when comparing forecast and measured performance ratios.

Method of comparison

There are two ways to compare forecast and measured data:

• Keep the guaranteed PR value as is and evaluate PR from measured data without using PVsyst (or with limited use for evaluating corrections). In many cases, comparison can be performed on accumulated results, avoiding the need to recalculate the system at detailed time-step level.
• Use available measured Global_meas data for complete resimulation of the system to obtain a new evaluation of reference PR based on measured input data. This should be very close to the forecast result. Then compare production values, which are usually well measured, with E_Grid.

The second solution provides a better flexibility in case of malfunctions of the system, as you can compare the simulated and measured results for a set of situations, cutting the doubtful data.

Effect of the Horizon

The horizon shading loss is typically included in forecast PR by PVsyst because basic irradiance from data providers is usually given for a free horizon (satellite data). In the PVsyst simulation, PR is referenced to GlobInc before horizon loss.

If you measure irradiance on-site, the horizon effect is naturally included in the data. The PR will be different (higher) than the forecast PR. For accurate comparison, use a forecast PR based on the value GlobHrz instead of GlobInc.

Trackers: measurement on the axis

If you measure Global_meas on the tracker axis, your measurement will be underestimated due to diffuse shading loss.

More importantly, on a hill with uneven terrain, the "real" mutual shadings (on beam component) cannot be avoided and cannot be registered by the solarimeter on the axis. This is a significant bias if you want to match Glob_meas to the value calculated by PVsyst. For the correction mentioned below, consider only diffuse and albedo losses, named ShdDLss and ShdALss in the simulation variables.

Therefore, to compare PR to the forecast value, the only approach is to evaluate shading losses (ShdDLss + ShdALss) [kWh/m²] provided by the simulation. Add these shading losses to the measured values to obtain a reference irradiance.

For a global comparison, add these shading losses in monthly or yearly values to evaluate PR_meas.

To resimulate your system based on measured irradiance, first perform a simulation and record shading losses in detailed time-step values (CSV output file). Then add these values to your original Global_meas series and reimport the corrected data into PVsyst for the final simulation.

Effect of the ageing

Be aware that the PV system is subject to degradation due to ageing. Note that degradation already occurs during the first year of operation. By year end, degradation equals one cycle of the degradation rate parameter.

The performance ratio decreases in line with system performance. This is demonstrated in the ageing tool. Account for this when defining the guaranteed PR value, specifically by defining a degressive value.

P50 - P90

The PR is an indicator of overall system losses. It is not dependent on yearly irradiation; therefore, PR is independent of whether the year is favorable or unfavorable.