A transport equation for a solution flow increasing due to osmosis inside a
hollow cylindrical fibre is derived. The equation can be applied for eithe
r 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 p
ressure-retarded osmosis power production system. As a result, the system c
an be optimized either by maximizing the net power or maximizing the ratio
(Psi) between the net power and entropy generation. In both cases the optim
al 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 metho
ds of optimization result in remarkably different optimal values. The resul
ting net power, when Psi was maximized, was found to drop to less than half
the maximum net power. The local entropy generation was found always to re
sult in a minimum value at a certain longitudinal position inside the fibre
. (C) 1999 Elsevier Science B.V. All rights reserved.