Ari Seppälä

Thermodynamic optimizing of pressure-retarded osmosis power generation systems

A transport equation for a solution flow increasing due to osmosis inside a hollow cylindrical fibre is derived. The equation can be applied for either direct, pressure-retarded or reverse osmosis, when the membrane is highly selective. This transport equation is used to study theoretically the net power delivered, and the entropy generated by two different concepts of a pressure-retarded osmosis power production system. As a result, the system can be optimized either by maximizing the net power or maximizing the ratio (Ψ) between the net power and entropy generation. In both cases the optimal values of the initial hydrostatic pressure difference between the inner and the outer sides of the fibre, the initial velocity of the solution and the fibre length could be specified. However, in some cases these two methods of optimization result in remarkably different optimal values. The resulting net power, when Ψ was maximized, was found to drop to less than half the maximum net power. The local entropy generation was found always to result in a minimum value at a certain longitudinal position inside the fibre.

Experimental study of the osmotic behaviour of reverse osmosis membranes for different NaCl solutions and hydrostatic pressure differences

The osmotic behaviour of five different present-day reverse osmosis membranes was studied, using different NaCl solutions (<6 wt.%) and applying different hydrostatic pressure differences (<5.2 bar) between the high and low concentration sides. The osmotic water flux through the membranes was found to increase non-linearly with respect to the increasing concentration difference over the membrane, leading to non-constant values for apparent water permeability. The polyamide composite membranes studied resulted only in very weak osmosis for even relatively small hydrostatic pressure differences and for strong NaCl solutions. Asymmetric cellulose acetate membranes showed somewhat better osmotic behaviour. However, all the osmotic water fluxes measured were still far too small for practical application in the production of energy.

The effect of solute leakage on the thermodynamical performance of an osmotic membrane

Abstract We derive an equation for the Second Law efficiency of an osmotic membrane. This expression of efficiency depends on the ratio between the solvent and solute flux, but remains independent of the actual values of the fluxes and of any transport model. The equation can be used to find the magnitude of solute leakage that can be accepted for optimally performing osmotic membranes. Special emphasis is given to osmotic power generation systems. We additionally compare the fraction of total power destruction that occurs inside the selective layer of the membrane, and the fraction that occurs inside the support material of the membrane.

ENTROPY ANALYSIS OF WET-SURFACE COOLING SYSTEM FOR AIR FLOW

SUMMARY A wet-surface heat-exchanger, where the e§uent air is moistened, is analysed according to a thermodynamic theory and data of experimental tests. An entropy generation function, which takes into account the changes of temperature and humidity of air, is derived. The analyses show that it is thermodynamically possible, without any cooling machine, to achieve extremely large temperature drops. With certain parameters the system results in a maximum rate of entropy generation which is used to analyse di⁄erences between two di⁄erent methods of spraying the moistening water. Approximal limits for the eƒcient working fluid mass flow value are also given. ( 1998 John Wiley & Sons, Ltd.

A STUDY ON THE ZERO FLUX APPROXIMATION IN CONVECTION-DIFFUSION MASS TRANSFER PROBLEMS

A one-dimensional steady state convection-diffusion mass transfer problem (for a binary mixture of components A and B), where the molar flux of component B (J B ) is considerably smaller than the molar flux of component A, is taken under study. A usual approximation for the case, especially if the fluxes are in the opposite direction to each other, is that J B = 0. The error resulting from this assumption, when the naturally-developing Stefan flow or concentration distributions are estimated, is evaluated. A simple analysis shows that if the molar fraction of the almost stagnant component is sufficiently low, the zero flux assumption causes a significant error. Such cases occur, for example, in osmosis and in evaporation near or at boiling point. A corrected Stefan flow equation and a modified mass transport equation for more general than one-dimensional problems, based on the use of a mass transfer coefficient, are presented