Abstract
Osmotic power extracted from the mixing of freshwater with seawater is a renewable energy resource that has gained increasing attention during recent years. The estimated energy can significantly contribute to the production of power worldwide. However, this power production will be subject to variation due to both local conditions and seasonal variation. The present paper explores the effect of concentration and temperature on water and salt fluxes in osmosis at zero transmembrane pressure for five different membranes. Further, the measured fluxes have been utilized to model water and salt permeabilities (A and B), and the structure parameter (S). The observed flux variations at different combinations of concentration and temperature have been ascribed to skin properties, i.e., changes in A and B of each membrane, whereas S was assumed constant within the range of concentrations and temperatures that were tested. Simplified equations for the variation in A and B with temperature and concentration have been developed, which enable A and B to be calculated at any concentration and temperature based on permeabilities determined from osmotic experiments at standard test conditions. The equations can be used to predict fluxes and specific power production with respect to geographical and seasonal variations in concentration and temperature for river water/seawater pressure-retarded osmosis. The obtained results are also useful for forward osmosis processes using seawater as draw solution.
Highlights
During recent years, osmotic power from mixing river water and seawater has gained increasing attention in the field of renewable energy research [1,2]
Since a pressure retarded osmosis (PRO) power plant typically will be operated at half the osmotic pressure difference between the two solutions, the maximum mixing energy that can be extracted will be limited to 50% of the reversible mixing energy
The work presented in the current paper has focused on the effects of concentration and temperature on water and salt fluxes in forward osmosis (FO)/PRO
Summary
Osmotic power from mixing river water and seawater has gained increasing attention in the field of renewable energy research [1,2]. Kim and Elimelech [21] studied the effect of temperature on the water flux by using a CTA membrane from HTI They measured the isobaric water flux at 20 ◦ C and 30 ◦ C with a 0.5 M NaCl feed solution and 1 M, 1.5 M, or 2 M NaCl draw solutions, respectively. Touati et al [22,23] studied the effect of temperature and concentration on the water flux using a CTA membrane from HTI and a membrane from the Fraunhofer IGB Institute Both the water and salt permeabilities were fitted to Arrhenius equations, giving good correlation to the observed temperature dependency. Water and salt fluxes, as well as PRO performance, can be calculated for any process condition using an appropriate membrane transport model
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