Abstract
Supercritical carbon dioxide (sCO2)-based cycles have been investigated for pumped heat energy storage (PHES) with the potential for high round-trip efficiencies. For example, PHES-sCO2 cycles with hot-side temperatures of 550°C or higher could achieve round-trip efficiencies greater than 70%. The energy storage cycle and equipment also synergize well with other systems incorporating thermal storage and/or sCO2 power blocks, e.g., concentrating solar power. These sCO2 cycles are closed Brayton cycles whose efficiency and system cost and complexity are sensitive to leakage and makeup/recompression requirements for long-term application. Therefore, incorporating hermetically-sealed machinery is an attractive option for minimizing system leakage and improving system cost and performance. Bearings that enable hermetic machines include sCO2 process-lubricated bearings and magnetic bearings. Ongoing developments in sCO2-lubricated bearings are addressing the well-known limitations that have challenged their use in megawatt-scale machinery (load capacity, damping), yet magnetic bearings have decades of performance in commercial applications at that scale and are worthy of consideration. This paper discusses a proposed sCO2-based PHES system application, and a cycle model establishes nominal conditions that define CO2 environment pressures and temperatures that magnetic bearings would have to operate in. A sensitivity study of the cycle’s round-trip efficiency is presented to see the impact of improved compressor and turbine efficiencies, which would result from expected windage loss and seal leakage reduction from a hermetic machinery configuration compared to one using conventional oil-film bearings. The result is approximately two points of round-trip efficiency for each point of isentropic efficiency from all machines. In the nominal cycle, the highest process temperatures exist for the charge mode compressor and discharge mode turbine, which would require magnetic bearings capable of operating up to 410°C. This exceeds the capabilities of typical commercial magnetic bearings (200°C), though it is within temperature ranges demonstrated for high-temperature magnetic bearings operating in low-pressure air (550°C). However, high-pressure sCO2 presents unique challenges that require further development. The paper discusses how these technical issues can be addressed to advance magnetic bearings for sCO2 applications.
Pumped heat energy storage with liquid media: Thermodynamic assessment by a Brayton-like model
