On the present and future economic viability of stand-alone pressure-retarded osmosis

Abstract

Pressure-retarded osmosis is a renewable method of power production from salinity gradients which has generated significant academic and commercial interest but, to date, has not been successfully implemented on a large scale. In this work, we investigate lower bound cost scenarios for power generation with PRO to evaluate its economic viability. We build a comprehensive economic model for PRO with assumptions that minimize the cost of power production, thereby conclusively identifying the operating conditions that are not economically viable. With the current state-of-the art PRO membranes, we estimate the minimum levelized cost of electricity for PRO of US$1.2/kWh for seawater and river water pairing, $0.44/kWh for reverse osmosis brine and wastewater, and $0.066/kWh for nearly saturated water (26% wt) and river water, all for a 2 MW production system. Only a pairing of extremely high salinity (greater than 18%) water and freshwater has the potential to compete with wind power currently at $0.074/kWh. We show two methods for reducing this cost via economies of scale and reducing the membrane structural parameter. We find that the latter method reduces the levelized cost of electricity significantly more than increasing the membrane permeability coefficient.

Economic framework for net power density and levelized cost of electricity in pressure-retarded osmosis

“…In South Korea, PRO was integrated with RO and membrane distillation (MD) systems to process the brine at even higher salinity. These leading pilot plants and recent studies on PRO ( [3,4]) suggest that the most popular salinity pairing of seawater and river water is not feasible and that a more saline draw stream is necessary to make PRO viable. Although these studies identified the need for high salinity, the power density needed for economic viability remains unclear.…”

Thermodynamic analysis of brine management methods: Zero-discharge desalination and salinity-gradient power production

“…This trend suggests that large minimum flux values can be used without significant energetic penalty. Because capital expenditure (CapEx) is the dominant cost factor for PRO [48], increasing the minimum flux (or decreasing the system size) can be an effective way to reduce the overall cost. Ultimately, the savings from operating expenditure (OpEx) in the form of energy reduction should be compared to the additional CapEx of the PRO unit to determine the economic viability of coupling PRO to a desalination system.…”

“…The local reverse salt flux and the water flux at any axial location (x) can be calculated using Eqs. (24) and (25), respectively [48]:…”

“…In specifying the operating conditions, a functional form (dimensional) of the power production equation is useful.Ẇ PRO (the dependent variable) is a function of 6 variables: the minimum water flux (J min ), the hydraulic pressure of the draw stream (P d ), the flow rate ratio (MR) and the velocity (v) of either stream [48]:…”

“…These nonlinear equations were solved using Engineering Equation Solver (EES), which is a simultaneous equation solver based on Newton's method [47]. The modeling approach follows Chung et al [48] and is summarized here.…”