Thermo-Economic Assessment of Pumped Thermal Electricity Storage Systems Employing Reversible Turbomachinery

Abstract

Abstract The objective of this paper is to assess the thermo-economic performance of different cycle configurations for pumped thermal electricity storage (PTES), including the effects of charging electricity cost. Reversible turbomachinery is employed to reduce the capital cost. Brayton cycles with different working fluids and a subcritical Rankine cycle operating with ammonia are compared. Liquid thermal energy storage systems are used. For the Brayton cycle, also packed bed storage systems are considered as an alternative. A new cost correlation for turbomachines, initially developed for organic Rankine cycles, is adapted for compressors and reversible machines. This correlation is based on the number of stages and physical size of the machine, which are estimated considering thermodynamic as well as mechanical limitations on the enthalpy difference that can be attained per stage. The results indicate that for a plant size of 50 MW and a discharge duration of 8 hours, the Brayton system with liquid storage and helium as a working fluid has the lowest levelized cost of storage at 0.138 $/kWh, mainly due to the high thermal conductivity of the fluid. Packed bed systems were found to be more expensive than liquid storage systems due to the large cost of the pressure vessels, with cost parity reached at a discharge duration of 4 hours. However, at this duration lithium ion batteries are likely to be cheaper. The results suggest that the levelized cost of storage for the Rankine cycle is slightly higher, 0.151 $/kWh. In addition, the Rankine cycle has disadvantages related to the higher storage volume and less flexibility for part load operation.