ZeoPTES: Zeotropic Pumped Thermal Energy Storage with an Ammonia-Water Mixture as Working Fluid

Abstract

The rapid rise of renewable energy production necessitates the development of largescale electricity storage systems. Pumped thermal energy storage (PTES), where electricity is stored in hot and cold storage units, has recently garnered a lot of interest. Previously proposed PTES systems rely on pure fluids as working fluids in Brayton- or Rankine-based power cycles. Herein, ZeoPTES, a PTES system using a zeotropic mixture of ammonia and water as working fluid, is introduced. The mixture exhibits nonisothermal evaporation and condensation, which allows utilization of industrially available sensible storage units, like water or molten salt, to store the thermal energy from these phase changes. A simulation using the REFPROP 10.0 database is written to analyze the cycle. Eight parameters are identified as having an effect on system round-trip efficiency and 30 000 simulations with random values for these parameters are run to identify the impact of all. Although efficient compressors and expanders are necessary to reach a high power-to-power round-trip efficiency, the largest systemwide losses arise from entropy generation and thermal stream mismatch in the cold and hot side heat exchangers. Using state-of-the-art microchannel heat exchangers with extremely low pinch points allows for round-trip efficiencies in excess of 70%.