The economics of firm solar power from Li-ion and vanadium flow batteries in California

Abstract

Abstract The cost of providing near 24-7-365 power from solar panels at a commercial facility in South California was modelled to be similar for vanadium flow batteries (VFB) and lithium ion batteries (LIB) at around $0:20/kWh. In hotter locations, LIB economics suffer due to accelerated background cell ageing. Even within South California there was enough variation to affect the economic comparison. Although LIB degradation could be reduced in a hybrid VFB-LIB system, there was negligible benefit to the overall electricity cost. As a result of falling photovoltaic panel costs in the last decade solar power (PV) is now claimed to be the cheapest source of electricity. However, the intermittent nature of supply means that it cannot solve the energy trilemma alone, and a form of backup power is required for reliability. This application is well suited to batteries, but the cost implications of providing high levels of reliability in this way have not been widely studied. In this work, the levelised cost of electricity (LCOE) achievable by optimal combinations of PV and batteries is determined for a large food retailer at a range of self-sufficiency ratios (SSR). Both lithium ion batteries (LIB), vanadium redox flow batteries (VFB) and hybrid systems of the two technologies are modelled. In combination with an over-sized PV array, both systems are capable of providing a SSR of 0.95 for a LCOE of less than $0.22/kWh. The optimal LCOE values overlap across the SSR range for both technologies depending on cost and ambient temperature assumptions. A VFB is more likely to give the lower LCOE at lower SSR, and a LIB is favoured at high SSR as the cycle rate drops as SSR increases. It is also shown that a state of charge (SOC) minimisation strategy has a significant impact on the LIB economics by reducing calendar ageing. Lastly, hybrid systems combining LIB and VFB were modelled, but in no cases showed an improvement over the optimal single choice. The overlap in the LCOE of the two battery types highlights the importance of other considerations, such as sustainability, space requirements and safety. Graphical abstract