Influence of Heat and Mass Transfer on the Performance of Large-Scale Thermal Energy Storage Systems

The current path towards the reduction of fossil-based energy carriers in District Heating systems involves an increasing use of renewable energy sources. In this perspective, the use of efficient thermal energy storage (TES) systems represents an important solution to allow a wider diversification and integration of renewable energy sources, which would otherwise be strongly limited. The use of large-scale buried TES (in particular pit TES) requires however a proper design, which takes into account both the TES operation characteristics and the envelope material properties. In particular, the TES envelope design requires a detailed study, considering its role in preserving the stored energy and the quality of surrounding groundwater. This study will focus on the thermal performance of the TES cover, being the most challenging envelope element. In order to gain detailed information regarding the behaviour of the insulation material in this specific application, it is necessary to investigate the effective materials properties in non-ideal conditions (i.e. moist conditions and high temperatures). The case study of foam glass gravel (FGG), an insulation material widely used in ground applications, is thereby presented to provide an exemplary overview of the challenges that can present in real operation conditions, such as the presence of residual moisture content from the construction phase and the development of convective heat (and mass) transfer. The analysis is supported by numerical simulations and experimental tests in order to investigate the influence of boundary conditions (i.e. heat flux direction, operation temperature, moisture content) and material configuration (i.e. compaction degree, thickness) on the effective insulation performance. Aim of the study is the definition of indications concerning the application of porous insulation materials in TES envelope, considering the challenges and the possible solutions.