Solid media thermal energy storages with bypass and conventional operation: Development of model-based correlations for designing and evaluation

Abstract

Thermal energy storage systems open up considerable potentials for flexibility due to their time-decoupled operation. This allows a balance between fluctuating energy generation and consumption, thus improvements in efficiency and stability of energy infrastructures. Sensible thermal storage systems are particularly suitable for large-scale applications, whereby solid - regenerators - or liquid salt-based solutions are used depending on the application. In the case of regenerators with packed beds or channel-shaped inventory options, thermal energy is stored by a cyclically moving thermocline, resulting in time-variable outlet temperatures during charging and discharging. These transient characteristics significantly increase the systemic integration requirements: cyclical propagation of thermal inertia at downstream components, restrictions with regard to an optimum operating point, complex simulations and increased control effort. Based on these challenges, a new solid media storage system is presented which achieves - analogous to liquid salt systems - constant outlet temperatures during charging and discharging. The basic idea is to add bypass paths, which guide a part of the incoming mass flows around the storage system and reunite them with the main flows at the respective outlets. This allows constant outlet temperatures by temporal adjusting the mass flow distribution. Investigations into such bypass concepts ideally require analytical models of system-relevant storage characteristics as a function of central dimensioning values. Based on a theoretical derivation, a dimensionless model was developed for this purpose, which was converted into a correlation for solid media storage systems with and without a bypass option. For the first time, it is now possible to perform direct, correlation-based design calculations of system-relevant storage characteristics in the cyclic equilibrium. Comparative calculations with numerical simulation results confirm the good agreement and also show that the mixing-related exergy losses for storage systems with a bypass option are accompanied by a proportionally lower dimensioning requirement. The correlation-based design tool presented here opens up a new path to investigate solid media storages with and without bypass option, which significantly facilitates storage-supported simulations for future techno-economic evaluations of the technology.