Ragone plots and discharge efficiency-power relations of electric and thermal energy storage devices

Ragone plots (energy-power relations) and discharge efficiency-power relations are important for characterizing energy storage (ES) devices, as they contain the information on the maximum power and the available energy. In this theoretical study, these two characteristics and the losses per energy are derived in the framework of endoreversible thermodynamics for ideal electric and thermal ES systems with different dependencies of their intensive thermodynamic variables (potentials, temperatures) of the reservoirs on the extensive state variables (electric charge, heat) that quantify their state of charge. While for battery and latent heat storage device, the normalized Ragone plot is equal to the discharge efficiency-power relation, the two characteristics differ for electric capacitor and sensible heat storage device due to their intrinsically limited depth of discharge at constant power. Due to the decrease of the exergy of heat at decreasing temperature, the discharge efficiency of the ideal sensible heat storage device exhibits a local maximum at a finite power value.