A hybrid energy storage system (HESS) can combine the efficiency and quick response of batteries with the energy density and cost of synthetic (e-)fuel storage. Exergy analysis is a valuable tool in optimising the efficiency of the electrochemical process chains needed to produce and consume e‑fuels. In the paper we present our ongoing efforts on using exergy analysis and exergy gravimetric analysis in battery- fuel cell integrated energy systems. A couple of case studies are presented. A hybrid energy storage system case in detail and others briefly.  An algorithm for optimal sizing of HESS-supported electrical grids in various locations, powered purely by renewable energy is introduced [1]. Efficient process chains have been developed for several fuels such as hydrogen, methane and ammonia. These process chains were combined with storage sizing algorithms to make design choices for large scale energy storage in the Netherlands [2]. 

Exergy analysis combined with HESS-sizing algorithms can also be used for exergy gravimetric analysis, which is an approach that seeks to optimise efficiency simultaneously with energy density. This approach can be useful for several applications, particularly weight-sensitive applications such as long-range sustainable transport. The use of exergy-gravimetric approaches for designing an optimal minimized-mass power plant for aircraft applications is presented [3-5]. Additionally, ongoing efforts on using the same approach in developing road transport systems are also introduced