Long term yield predictions (LTYP) are a prerequisite for business decisions on long term invest-ments into photovoltaic (PV) power plants. The preparation of a LTYP report typically relies on numerical modelling and prediction of the expected electrical yield, based on experience with previous PV power plants, laboratory measurements and more or less the whole knowledge gained in the PV community over the past years and decades. However, though PV system model-ling has been performed for decades, not much effort has been spent on a comprehensive inves-tigation of the uncertainties related to this task. This report tries to collect some insights into the field of uncertainties of several technical aspects of PV system yield prediction and assessment.
The first main section lists typical measurements, dealing either with a PV system component’s properties or with PV system performance. It covers the uncertainties related to the most im-portant measurands in PV solar energy:
- The solar resource
- PV module properties
- System output and performance—including long term effects
Uncertainty in irradiance measurements is in part related to the instruments, and in part to the measurement practices. While existing handbooks and guidelines may help to reduce operational issues, the uncertainties related to the instruments themselves are more difficult to minimize or reduce. However, if a certain non-ideal behaviour of an instrument is systematic and known, then a systematic correction can be applied to reduce or remove its influence on the measurement. In some cases manufacturers already supply information about temperature dependency and/or non-linearity. In the current PVSENSOR project, a wide range of instruments were characterized, and many systematic errors were identified and quantified. With such knowledge of instrument operating conditions it will be possible to quantify each systematic source of measurement error.
STC power measurement of PV modules and the estimation of its uncertainties is a topic that gained attention in recent years and saw remarkable improvements. For a full uncertainty as-sessment, it is important that stability issues are considered in addition to pure measurement uncertainty. There are laboratories with a profound knowledge on their uncertainty budget, typi-cally those with the smallest overall uncertainties (down to ±1.6%), laboratories where the refer-ence cell or reference module calibration dominates the uncertainty budget, and laboratories where apparently uncertainties were not analysed in detail.
System testing looks at the performance of the complete conversion chain of a PV system. The determination of the observed performance ratio PR is rather easy, including an assessment of its uncertainty. The determination of the expected PR (as a quality requirement) is the major issue, as the PR depends on the system design and changes with the system’s operating conditions. Despite this potential weakness, a PR test can form a valuable tool during the commissioning of a PV system.