Dynamic Modelling of a Brayton PTES System

The need for storage capacity is continuously increasing due to the progressive penetration of renewable energy sources in the power grids. New storage technologies for large-scale applications are emerging in this context, among which Pumped Thermal Electricity Storage (PTES) is a promising alternative. PTES is based on closed Brayton cycles: a Brayton heat pump is operated in the charging phase, using the off-peak electricity to transfer heat between two reservoirs. A Brayton heat engine is operated between the same reservoirs in the discharging phase, producing electric energy. The purpose of grid-scale storage facilities is to provide flexibility to the grid. Therefore, the proposed technologies must effectively operate in part-load conditions and quickly react when the load must be adjusted. In this study, the transient response of the Brayton PTES technology is studied, with particular emphasis on its off-design operation and control strategy. To this end, a detailed thermodynamic model of the system is developed. Particular attention is given to the sizing of the components relevant to the control. Furthermore, a control strategy suited for the power regulation of closed Brayton cycles, the Inventory Control, is investigated, and implementation is proposed. As it resulted, the system can operate in part-load with negligible performance degradation and is characterized by a rapid transient response. Therefore, Brayton PTES resulted as a promising storage technology for grid-scale applications.