Full-Scale Multistage Pressure-Retarded Osmosis for Osmotic Energy Generation: Optimization of Specific and Net Energy Production.

Salinity gradient energy, also known as blue energy, is a clean and renewable option for electricity generation with no direct CO2 emissions. Among the available technologies, pressure-retarded osmosis (PRO) stands out, although its large-scale implementation is not yet economically viable, so predictive models are essential to assess its real potential. In the present work, a multistage PRO system of up to three stages, with one-three hollow-fiber membrane modules (HFMMs) arranged in series per stage, was simulated and the operating conditions were optimized. The model accounts for axial pressure drops, feed concentration and draw dilution along each module, considering two salinity gradients of 29.5 and 59.5 g L-1. The maximum net specific energy generation reached 421.48 Wh m-3 with three stages under the 59.5 g L-1 gradient. Adding HFMMs in series benefited the two-stage system but slightly penalized the three-stage one, and the net specific energy decreased monotonically as the ratio of pressure vessels between the first and the second stage increased. At least three stages are therefore required for a full-scale PRO system to become a net energy producer at moderate salinity gradients, and further energy should be sought through additional staging rather than through longer series.