Achieving Self-Balancing by Design in Photovoltaic Energy Storage Systems

This paper examines the problem of designing integrated systems of photovoltaic (PV) arrays and battery cells in a manner that achieves self-balancing by design. This paper focuses on two topologies for integrating PV and battery cells, both of which connect PV generation to each battery cell directly, either with or without dc-dc conversion. This paper proves, using Lyapunov stability methods, that both the topologies are globally, asymptotically self-balancing. This means that initial differences among battery cells in either the state of charge or other internal state variables are guaranteed to diminish asymptotically with time. This self-balancing behavior reduces the amount and hence the cost of the power electronics needed for energy storage integration into PV farms significantly compared to conventional integration topologies-an argument substantiated using a financial assessment in this paper. This paper extends previous work by the authors on self-balancing PV energy storage systems in four significant ways. First, the proof of self-balancing is provided using equivalent-circuit battery models of arbitrary order, as opposed to first-order models. Second, this proof is provided for multiple integration topologies, instead of just one. Third, we demonstrate this self-balancing behavior experimentally, using a hardware-in-the-loop setup, for the first time. Finally, we perform a cost benefit analysis of our hybrid topologies in comparison to a benchmark system in order to quantify the integration cost reduction due to self-balancing.