Hybrid energy storage through the passive connection of a Vanadium Redox Flow Battery and a supercapacitor: An experimental, modelling, economic and environmental impact assessment study

Developing a Hybrid Energy Storage System (HESS) involves integrating technologies with complementary attributes. Coupling Redox Flow Batteries (RFBs) with Supercapacitors (SCs) emerges as one of the most promising options. For the first time at our knowledge, here we report about a study on the passive HESS VRFB-SC configuration that was investigated through experimental tests, modelling, life cycle and economic assessments. Indeed, we present a laboratory-sized HESS comprising a Vanadium Redox Flow Cell (VRFC) and a SC, directly connected in parallel without any power converter. Initially, individual tests were conducted on the standalone VRFB and SC using short discharge protocols (5 s). Subsequently, the two systems were interconnected in parallel and subjected to the same discharge protocol. An equivalent electrical model, resembling a parallel R-C circuit, was developed to elucidate the discharge mechanism of the direct parallel system. The tests revealed that the SC mitigates the VRFB's ohmic drop due to the transient behaviour of the R-C circuit. Furthermore, the hybrid system demonstrated enhanced energy delivery at higher currents compared to the standalone VRFB, a phenomenon elucidated by our proposed model. Additionally, the model facilitates the sizing of the SC relative to VRFB performance. In addition to technical findings, this article provides a comprehensive economic analysis and life cycle assessment (LCA) of the proposed system. These assessments highlight the potential cost-effectiveness and reduced global warming potential of the passively connected VRFB-SC HESS, underscoring its viability as a sustainable energy storage solution.