60185 - Off-Design of a Pumped Thermal Energy Storage Based on Closed Brayton Cycles 

The growth of renewable energy source requires reliable, durable and cheap storage technologies. However, the available storage technologies are either impaired by geographical limitations or cost considerations. A technology, namely the Pumped Thermal Energy Storage (PTES), is drawing much interest as it appears not to be affected by similar limitations. PTES is less efficient than pumped hydro and batteries, but it could achieve good efficiencies, show better economic performance and lower environmental impact. The technology is based on thermal energy storage, rock-solid packed beds, and turbomachines that operate two closed Brayton cycles (for charging and discharging). Although the PTES theoretical behaviour is well understood, the interaction between the components (i.e. the thermal energy storage and turbomachines) is rarely investigated. The present analysis comprehensively models a PTES system, investigating the relationship between packed-bed storage behavior over time and turbomachines operating conditions. In this way, PTES off-design and part-load performance are estimated, and the technology is characterised in a more realistic framework. A regulation technique especially suited for closed Brayton cycles, i.e. the inventory control, is used to control the system load in input and output. As it resulted, PTES is characterised by a good performance when operated in part-load, which could represent a significant advantage compared to other competing storage technologies. Based on the results of the outlined analysis, the PTES technology positive and negative features are discussed, and its future outlook assessed.