Capital cost reduction in thermomechanical energy storage: An analysis of similitude-based reversible Brayton systems

The surge in non-programmable renewable production expected for the coming years can only be harnessed with a consequential increase in energy storage capacity. Many technologies are being proposed in the search for durable, affordable, and sustainable storage, and thermomechanical technologies are becoming prominent alternatives. Despite the best efforts, thermomechanical storage technologies still suffer from non-competitive capital costs due to using separate (costly) charging and discharging equipment. This paper proposes merging charging and discharging parts to realize a novel reversible thermomechanical storage, cutting the components’ number and, consequently, significantly reducing the system's capital cost. To do so, a novel technique is proposed and investigated by focusing on Brayton cycles-based Pumped Thermal Energy Storage, a promising technology for large-scale and long-duration energy storage. This technique allows turbomachines to operate under similitude conditions during charge and discharge phases, thus enabling their use in both phases. Of course, operating the turbomachines in similitude imposes several additional constraints on the system design, components’ sizing and operating conditions, the impact of which on the storage cost and performance is discussed in the paper. The analysis encompasses both liquid- and solid-based Brayton Pumped Thermal Energy Storage, assesses the impact of the proposed technique for different storage configurations, and discusses the role of heat exchangers in the realization of reversible storage systems. A techno-economic analysis is performed as a multiobjective optimization problem where performance and cost are pursued as competing objectives. Comparing standard (non-reversible) systems with reversible configurations shows that the latter may achieve an overall plant cost reduction of around 20% in the case of liquid-based systems, whereas reversible solid-based systems show similar costs to their standard counterpart. This occurs because most of the economic gain achieved by reversible configurations comes from reducing the number of heat exchangers, which are absent in solid-based systems.