Validation of a pit thermal energy storage model: Demonstration of a comprehensive approach

Pit thermal energy storage (PTES) systems can be a viable storage option in future energy systems to bridge the gap between supply and demand driven by the increase in variable renewable energy sources. The considerable size makes PTES construction projects capital-intensive ventures that require thorough planning, often supported by modeling and simulation efforts. The assessment of the models’ credibility and accuracy is therefore of fundamental importance. Previous studies have not considered relevant uncertainty contributions in determining the modeling error. The present work aimed to conduct a comprehensive model verification, validation, and uncertainty quantification (VVUQ) process for a newly developed PTES model in the modeling language Modelica. Based on an existing VVUQ standard (ASME V&V 20-2009), we considered the measurement, input parameter, and numerical uncertainty, and used five years of measurement data of the PTES in Dronninglund, Denmark. The reasonable agreement of the most important storage metrics with the measurements proves the model's general credibility, and in line with the common practice in the discipline, the model is considered validated. The estimated error in predicting the annual charge and discharge energy, and the internal energy is within ±5%; and the accuracy in predicting the storage water temperatures is ±2 K. The limitations in the experimental conditions did not allow a reliable estimation regarding the annual thermal losses; however, the mean comparison error was approximately 9%. The demonstrated comprehensive validation framework provides model accuracy insights for PTES projects and serves as foundation for validation studies of related storage technologies.