Towards Reproducible Performance of Grid Connected Photovoltaic Battery Storage

Abstract

Several attempts have been made to define and measure performance parameters of photovoltaic home storage systems. Ideally, performance parameters are accepted by the scientific community as well as by manufacturers, test laboratories and customers. The German industry association for energy storage (BVES) filed guidelines for performance evaluation in cooperation with research institutions and manufacturers. However, most storage systems are tested only once and in one test lab, so reproducibility cannot be taken for granted. In addition, the current version of the standardized report on performance parameters does not consider measurement uncertainty, which may lead to misleading interpretations of the relative performance of different storage systems. Performance parameter deviations between different storage systems are not huge, at least for some parameters. [1] In our work, conversion efficiencies of all system paths have been measured in the SOLARWATT Innovation laboratory and for the same generator coupled storage system at an independent research institution. Here, three photovoltaic generator voltage levels between 285 V and 640 V as well as three battery voltage levels between 107 V and 151 V were measured in both laboratories. Resulting charge and discharge efficiencies were also evaluated for a storage capacity of 2.4 kWh as well as a storage capacity of 7.2 kWh. Measurement uncertainty was calculated in reference to absolute conversion pathway efficiency as defined in the guidelines for performance evaluation. For direct AC usage of photovoltaic generation or grid feed in a maximum efficiency of 97.56±1.66% was measured at maximum MPP voltage and nominal power. For photovoltaic battery charge (i.e. DC usage) a maximum efficiency of 95.04±0.28% was found at nominal MPP voltage and half nominal power. The authors recommend including measurement uncertainty evaluations into an updated version of the guideline for performance evaluation of photovoltaic home storage systems. This way, scientific evaluation of storage systems will become more transparent. Keywords— performance, efficiency, measurement uncertainty, reproducibility