Cost-benefit analysis of reversible reciprocating-piston engines with adjustable volume ratio in pumped thermal electricity storage

Decarbonisation of heating, cooling and/or power services through the utilisation of renewable en-ergy sources relies on the development of efficient and economically-viable energy storage technolo-gies, ideally without geographical constraints. Pumped thermal electricity storage (PTES) is a strongcandidate technology – along with reversible Rankine cycle, (advanced adiabatic) compressed airenergy storage (CAES), and liquid air energy storage (LAES). One of the leading PTES variants isthe reversible Joule-Brayton cycle engine, where energy is stored as sensible heat in hot and coldthermal stores, while the temperature difference is achieved through gas compression and expansionprocesses. For cost reasons, and to achieve high round-trip efficiencies, it is advantageous for thecompression and expansion machines used in PTES plants to be reversible. Positive-displacementdevices offer this possibility. In particular, recent developments in pneumatically or electromagneti-cally actuated intake and exhaust valves could pave the way for high-efficiency reversible reciprocat-ing compression-expansion devices based on variable-valve control in real time. Advanced variablevalve timing (VVT) is a promising feature that allows piston machines not only to be operated bothas reversible compression and expansion devices, but also to maintain high efficiencies over a widerange of operating conditions, thanks to the possibility of adjusting the built-in volume ratio of a par-ticular machine. With enhanced part-load performance, such disruptive piston machines offer greatpotential for round-trip efficiency enhancement and cost minimisation of PTES storage plants. In thiswork, a cost-benefit analysis of innovative VVT-fitted reciprocating-piston technology is performedusing: (i) comprehensive dynamic reduced-order models to predict the compressor-expander perfor-mance for design optimisation, and (ii) Schumann-style one-dimensional models for simulating heatand mass transfer in the packed beds. The aim of this techno-economic assessment is to quantifythe potential of adjustable volume ratio positive-displacement machines for energy storage technolo-gies. Over the range of investigated charging/discharging rates, the round-trip efficiency of a VVT-freePTES system lies between 32% and 52% and its levelised cost of storage (LCOS) between 180 and300 £/MWh, while the same PTES system fitted with advanced adjustable valve actuation exhibitsround-trip efficiencies ranging from 40 to 53% and LCOS from 180 to 250 £/MWh. It is also foundthat the PTES system that features advanced variable valve timing outperforms that with fixed valvetiming, with performance increase of up to 35% and lower LCOS by up to 25% in part-load conditions