Exergy as a Useful Variable for Quickly Assessing the Theoretical Maximum Power of Salinity Gradient Energy Systems

Abstract

It is known that mechanical work, and in turn electricity, can be produced from a difference in the chemical potential that may result from a salinity gradient. Such a gradient may be found, for instance, in an estuary where a stream of soft water is flooding into a sink of salty water which we may find in an ocean, gulf or salt lake. Various technological approaches are proposed for the production of energy from a salinity gradient between a stream of soft water and a source of salty water. Before considering the implementation of a typical technology, it is of utmost importance to be able to compare various technological approaches, on the same basis, using the appropriate variables and mathematical formulations. In this context, exergy balance can become a very useful tool for an easy and quick evaluation of the maximum thermodynamic work that can be produced from energy systems. In this short paper, we briefly introduce the use of exergy for enabling us to easily and quickly assess the theoretical maximum power or ideal reversible work we may expect from typical salinity gradient energy systems.

Thermodynamic Optimization for an Endoreversible Dual-Miller Cycle (DMC) with Finite Speed of Piston

“…Analyses of the different optimization objective functions are also important work for studying performance characteristics of the ICE cycles. Some new optimization objective functions on the basis of the power output ( ) and thermal efficiency ( ), such as the specific power [ 36 , 37 ], power density [ 38 , 39 ], exergetic performance [ 40 , 41 , 42 , 43 ] and finite time exergoeconomic performance [ 44 , 45 ] had been proposed. Besides, Angulo-Brown [ 46 ] first introduced the ecological function for the heat engine cycle (HEC), where is the temperature of the cold reservoir and is the entropy generation rate of the HEC, means the power dissipation of the HEC, the definition ignored the difference between exergy and energy, Yan [ 47 ] made a modification later, , where is the environment temperature, means the exergy loss of the HEC.…”

Capacitive Mixing for Harvesting the Free Energy of Solutions at Different Concentrations

“…The amount of energy dissipated due to the mixing of two solutions with different salt concentrations has been evaluated several times in the context of the colloquially called "Blue Energy", or more properly referred to as salinity difference energy [2][3][4][5][6], so we only pose here a simplified analysis of this quantity. Consider two volumes V fresh and V salt of two solutions with molar salt concentrations c fresh and c salt , respectively, of a monovalent binary electrolyte (such as NaCl).…”

“…Finally, the efficiency of energy conversion, i.e., the ratio between the produced power and the quantity of water is also an issue, as water itself, particularly the fresh stream, has a cost. Classical approaches are mainly based on the use of membranes, and some recent reviews focus on their advances [4,[7][8][9][10][11] and therefore they are not addressed in the present work.…”