A cascade organic Rankine cycle reversible Carnot battery based on top cycle decoupling: Thermodynamic performance enhancement and economic advantage validation

The reversible Carnot battery, with its relatively low levelized cost of storage over its entire life cycle, has become a focal point in current research. Addressing the challenge of balancing efficiency and cost-effectiveness in traditional reversible Carnot batteries, this paper proposes a cascade reversible Carnot battery system. Through innovative design of the thermodynamic cycle, this system achieves a synergistic optimization of energy storage efficiency and cost control. This system decouples the top cycle of the cascaded organic Rankine cycle (C-ORC). The reverse cycle corresponds to the charging mode, in which the top cycle of the C-ORC operates as a heat pump. The forward cycle corresponds to the discharging mode, where both the top and bottom cycles of the C-ORC perform two-stage power generation. The new system not only ensures high efficiency in energy conversion but also effectively reduces the investment costs of key components. Thermodynamic analysis indicates that, at a waste heat source temperature of 90 °C and a high-temperature thermal storage temperature of 115 °C, with external low-grade waste heat supplied during charging, the system achieves a PTP of 104.07%, calculated as the ratio of electrical output during discharging to electrical input during charging. Economic assessment reveals that, compared to traditional reversible Carnot battery systems, the proposed cascade structure, with its higher energy storage efficiency, significantly reduces the levelized cost of storage (LCOS) and the investment payback period. Under ideal operating conditions, the investment payback period is as short as 7.5 years.