Pumped heat energy storage with liquid media: Thermodynamic assessment by a Brayton-like model

Abstract

A thermodynamic model for a steady state pumped heat energy storage in liquid media is presented: it comprises a coupled Brayton-like heat pump and heat engine cycles connected to a cryogenic liquid and a hot molten salt by counter-flow heat exchangers. The model considers non-isothermal heat transfers between the working fluid and the liquid media and explicitly includes a set of parameters accounting for the main internal and external losses, heat leak, and pinch point effects for both the heat pump (charge) and heat engine (discharge) modes. Specific expressions for the main magnitudes in the charge (as the input power and coefficient of performance) and discharge (as power output and efficiency) modes and the global round trip efficiency have been analytically derived in terms of isentropic efficiencies of the compressor and turbine, pressure losses in the heat exchange processes, effectivenesses of the external counter-flow heat exchangers, and coupling between the working fluid and the storage and cryogenic liquid media. Round trip efficiencies around of 35 − 40% have been obtained, internal losses being those with main negative influence on the calculated values. The strong constraints imposed by the pinch point effects and liquid media have been analyzed. The model provides a thermodynamic assessment of the main involved processes and their interplay for a selected arrangement (molten salts, cryogenic fluid, and the charge and discharge power blocks) in order to check parametric strategies for thermodynamic optimization and design. These strategies are based on a reduced set of parameters of the overall installation and without the high computational costs of dynamical models.

Thermodynamic Performance of a Brayton Pumped Heat Energy Storage System: Influence of Internal and External Irreversibilities

“…The black point in all curves in Figure 8 corresponds to the maximum round-trip efficiency and the red point corresponds to maximum ( ). As it can be seen, unlike in the endoreversible limit, both maxima are reached at different values, which nonetheless is in agreement with previously analyzed PTES systems [ 12 , 13 ]. It is interesting to note that (see Figure 8 b) the maximum round trip leads to a high value of , not exactly the corresponding maximum, but a numerical value close to it.…”

“…This adds up as a variable parameter. As it has been addressed in the context of energy storage in liquid media [ 12 ] there are physical constraints that define operation regions, that is, the possible values that the operation variables can acquire are restricted and the physical region lastly depends on the values of the irreversibility parameters. The combination of these values might prevent the system from reaching certain operation regimes.…”

“…This process is isobaric in the ideal case. To account for the real pressure losses, , a pressure decay parameter is defined [ 12 ]: where is defined by convenience and is the adiabatic coefficient of the working fluid that is considered approximately constant in the working temperature intervals.…”

“…This process is isobaric in the ideal case. To account for the real pressure losses, ∆p H , a pressure decay parameter is defined [12]:…”

“…The system will be assumed to be at stationary conditions with fixed temperature reservoirs to obtain analytical equations allowing for detailed sensitivity analyses of the influence of all possible losses and different temperature levels in system performance. In previous studies by our group [ 12 , 13 ], PTES Brayton-like systems with liquid storage and their optimization were developed. This work is intended to present a complete and flexible thermodynamic model capable to include all the most relevant peculiarities and thermodynamic details emerging from the combination of Brayton-like heat devices and solid thermal reservoirs.…”