The effect of efficiency and self-discharge parameterization on the sizing of hybrid energy storage systems

A hybrid energy storage system (HESS) is a widely investigated setup as a means to reduce losses and system dimensions by combining storage technologies with complementary characteristics. However, reported benefits are often based on case studies and do not clearly distinguish systemic operational effects from intrinsic parameter advantages. This work investigates under which conditions heterogeneous loss characteristics in a HESS lead to measurable loss savings and sizing gains compared to a single storage system. To eliminate parameterization bias, the concept of loss-equivalent storage units is employed. A linear storage model is formulated as a cascaded and bi-level quadratic programming problem that minimizes system dimensions under defined loss constraints. A parameter study is conducted across 1024 load profiles, varying efficiency, self-discharge rate, and storage dimensions, resulting in more than 800 000 parameter configurations. The dominant predictor of both sizing gains and loss savings is the variation coefficient of the load profile. Strong statistical relationships are observed (Pearson correlation coefficients \(\approx\) 0.9). Nevertheless, practical sizing gains are limited and rarely exceed 3% within realistic parameter ranges. In contrast, relative loss savings are substantial and often exceed 20%, following a saturating functional relationship with sizing gains. The results indicate that load-profile variability primarily governs the benefits of a HESS with regards to loss reduction. While dimensional advantages are modest, loss reductions remain pronounced and relevant for operational cost and thermal management. Data and results are accessible via https://zenodo.org/records/18770498 and https://github.com/s-guenther/hess-study.