Producing electricity at estuaries from salinity gradient: exergy analysis

Abstract

ABSTRACT Salinity gradient technologies generate electricity from the chemical pressure differential created by differences in ionic concentration between freshwater and seawater. Seawater has a higher osmotic pressure than freshwater due to its high concentration of salt. Two main technology types, Reverse Electro-Dialysis (RED) and Pressure-Retarded Osmosis (PRO), make use of semi-permeable membranes which generate an osmotic potential that can be used to generate electricity using turbines in deltas, estuaries, fjords or hyper-saline lakes. In PRO, semi-permeable membranes placed between the two streams of solutions allow the transport of water from low pressure diluted solution to high-pressure concentrated solution. RED requires two alternating semi-permeable membranes that allow the diffusion of the ions but not the flow of H2O. An exergy analysis of an SGE system of estuary-river can be applied to calculate the maximum potential power for electricity generation. Seawater is taken as reference environment for calculating the exergy of water since the seawater is the final reservoir. In this work, an aqueous sodium chloride solution (NaCl) model is used of the selected estuary (i.e. Sebou estuary (Morocco)) to calculate the thermodynamic properties of seawater in the estuary. This model does not consider seawater as an ideal model and provides accurate thermodynamics properties of sodium chloride solution. The chemical exergy analysis considers sodium chloride (NaCl) as main salt in the water of Sebou estuary. The sodium chloride concentration (SCC) is more than, ∼0.3 mol/L in the estuary. The flow exergy and the power potential calculations of the investigated system ranged from ∼3.5 to ∼8.6 kJ/kg and ∼65 to ∼1303 MW respectively, thereby exposing the opportunity for power generation in Sebou estuary.