Abstract
Photovoltaic (PV) systems are frequently covered by performance guarantees, which are often based on attaining a certain performance ratio (PR). Climatic and electrical data are collected on site to verify that these guarantees are met or that the systems are working well. However, in-field data acquisition commonly suffers from data loss, sometimes for prolonged periods of time, making this assessment impossible or at the very best introducing significant uncertainties. This study presents a method to mitigate this issue based on back-filling missing data. Typical cases of data loss are considered and a method to infer this is presented and validated. Synthetic performance data is generated based on interpolated environmental data and a trained empirical electrical model. A case study is subsequently used to validate the method. Accuracy of the approach is examined by creating artificial data loss in two closely monitored PV modules. A missing month of energy readings has been replenished, reproducing PR with an average daily and monthly mean bias error of about −1 and −0.02%, respectively, for a crystalline silicon module. The PR is a key property which is required for the warranty verification, and the proposed method yields reliable results in order to achieve this.
Highlights
The number of photovoltaic (PV) installations in the UK has increased from a few tens of MWp in 2010 to more than 6 GWp in June 2015 [1], indicating that PV is a rapidly growing industry
Utilising average values from dates close to the missing period may give an estimation of performance ratio (PR), but such methods are not adequate to estimate long-term energy yields of the system, especially when missing periods are extended from a few weeks to even months
Ambient temperature is well described with Root-mean-square error (RMSE) and Mean bias error (MBE) of 0.15 and −0.14%, respectively
Summary
The number of photovoltaic (PV) installations in the UK has increased from a few tens of MWp in 2010 to more than 6 GWp in June 2015 [1], indicating that PV is a rapidly growing industry The majority of these systems will operate as financial investments, and in order to manage investment risk associated with system yield shortfalls, many larger scale PV systems are covered by energy performance guarantees. There are no validated strategies that deal with this issue, in maritime climates such as the UK, and any attempts to backfill data using previous dates or days from previous years are at best temporary with very high uncertainty attached to these methods This is because such strategies do not take into account variable weather systems or PV component degradation. An issue remains of how to back-fill lost data values, and to arrive at a valid monthly or annual PR which is required in order to verify the warranties or to predict return on investment
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