Rebuttal to “Discussion of ‘Second law analysis of reverse osmosis desalination plants: An alternative design using pressure retarded osmosis’ [Energy 2011] 36: 6617–6626]”

Abstract

We have had several conversations with Knutson during recent months, and we certainly agree that the one-dimensional mass exchanger model is more appropriate than the zero-dimensional model which has appeared in much of the past literature and which we also applied. We have been working on revisions of these calculations that incorporate the one-dimensional model, and we report some of our findings in Refs. [2] ; [5]. However, we believe that PRO has clear potential for energy recovery, as we explain in detail below. Further, Knutson's analysis of the power generation is flawed and his conclusions regarding energy recovery are not accurate.

Knutson's method of analysis makes the assumption of infinite membrane water permeability (equivalent to infinite surface area), taking the value of A in Eq. (1) to be infinity. Because the value of A for the membrane is assumed to be infinite, the local water flux is equal to zero at the inlet, outlet, or both ends of the PRO module. In another words, the driving force for the water flux will vanish at the inlet, outlet, or both ends. This is similar to the assumption of zero pinch at the end of a heat exchanger, such as might be obtained with infinite surface area. This is an interesting method, and it should give similar trends as given in Ref. [4] if it is applied correctly.