Theoretical analysis for heat exchange performance of transcritical nitrogen evaporator used for liquid air energy storage

Abstract

In view of violent changes of thermo-physical properties, the segmental design method is adopted to explore the heat exchange performances of the transcritical nitrogen (T-N2) evaporator used for liquid air energy storage, in which cold N2 is heated up successively by hot propane and methanol in two wide temperature sections. The local heat capacity rate ratio between cold and hot fluids has crucial effects on the local heat exchange performance of evaporator, such as local effectiveness, local entransy dissipation, and local required heat conductance or local heat transfer rate. They have extremums near the positions where the local heat capacity rate ratio equals one, but their optimal values need to be determined by combining the changing trend of the local heat capacity rate ratio. The total heat exchange performance of evaporator is evaluated using total entransy dissipation and total exergy efficiency. When the heat load is fixed, the total performance is improved with the decrease in the mass flow rate of methanol, but at the expense of the required total heat conductance; The total performance can be optimized by precisely tailoring the heat load ratios between the two temperature sections. When the heat conductance is given, the optimum total performance can be obtained by adjusting the mass flow rate of hot fluids at a fixed heat conductance ratio; Increasing the heat conductance ratio of the low temperature section can further elevate the optimum total performance whereas the affordable heat load or the outlet temperature of N2 is notably decreased. Increasing N2 pressure elevates the total performance of evaporator but diminishes the extractable cold amount from the liquid N2 in the same temperature rise. This work is beneficial for selection of key parameters to achieve optimal operation of the T-N2 evaporator.