Two Thermodynamic Optima in the Design of Sensible Heat Units for Energy Storage

The paper presents a treatment of sensible heat energy storage units as systems intended to store useful work. An analysis of the thermodynamic irreversibilities associated with storing energy from a hot gas source as sensible heat in huge liquid baths points out two important trade-offs: 1. There exists an optimum, well-defined quantity of hot gas to be used in order to maximize the useful work stored in the liquid bath. Using more than this optimum quantity in the hope of maximizing the amount of thermal energy stored as sensible heat leads to severe thermodynamics losses. 2. There exists an optimum relationship among the gas-liquid heat exchanger design parameters which minimizes the system irreversibility while maximizing its capability of storing useful work. This relationship provides a procedure for estimating the heat exchanger optimum number of transfer units (N_tu). Increasing the N_tu above the optimum in order to upgrade the heat exchanger effectiveness and the thermal energy storage capability leads to prohibitive losses due to fluid friction in the heat exchanger channels. The existence of the two optima demonstrates that designing sensible heat units for maximum thermal energy storage does not necessarily amount to thermodynamically optimizing such systems.