Analysis of specific energy consumption in reverse osmosis desalination processes

Specific energy consumption (SEC) – expressed in kWh per m³ of product water – is the most important parameter when it comes to the performance of desalination processes, especially with regards to the entire plant sustainability. SEC comprises contributions from the operation of the main sections of the entire desalination plant; i.e. the feed-water intake facility, the pre-treatment section, the main desalination section (that includes high-pressure pumps, RO membrane trains and energy recovery devices [ERD]), the product water post-treatment section and the brine treatment/disposal facility. The main section, where RO desalination takes place, is the most significant specific energy consumption contributor, contributing by >75% to the total product water unit cost; therefore, this section presents the greatest potential for SEC reduction.

In the RO membrane desalination process, in addition to the thermodynamically imposed minimum energy consumption (SEC_min), the consumed energy is mainly due to resistance to fluid permeation through the membrane, the frictional losses in the retentate and permeate channels of the spiral wound membrane (SWM) modules, and the non-ideal operational losses of pumps and energy recovery systems. The relative significance of these factors should be clarified, to enable SEC minimization through targeted improvements in process and equipment design and operation. For this reason, it is important to evaluate the itemized contributions to SEC and pinpoint those factors with the greatest minimization potential.

A team of researchers under the guidance of Professor A.J. Karabelas from the Center of Research and Technology-Hellas in Greece presented a thorough analysis of contributions to specific energy consumption by dealing with two typical cases of membrane desalination, i.e. brackish and sea water desalination. The analysis was made by employing a comprehensive software, developed in the authors Laboratory (http://nrre.cperi.certh.gr/index.php?lang=en), which allowed detailed performance simulation of the entire pressure vessel of a desalination process. Through this software, the authors were able to predict the spatial distribution of all process parameters, which facilitated various forms of reliable parametric studies. Their research work is published in peer-reviewed journal, Desalination.

The research team analyzed the effect of the key design and operating process parameters on system performance, and identified the membrane resistance to permeation and the inefficiencies of pumps and ERD as the dominant factors contributing to SEC. This implied that priority should be given to these particular issues in R&D works aiming to achieve considerable specific energy reduction. The impact of SWM-module design parameters on SEC was found to be more subtle; i.e. although the direct effects of those parameters on the membrane-channel friction losses were quite small, their indirect effects on spatial flux distribution were more important.  Moreover, the structural SWM module parameters affect the detrimental membrane fouling evolution, that tends to increase friction losses and reduce flux, although this issue requires more R&D work. Under such operating conditions, maintaining constant permeate recovery, leads to feed-pressure increase and concomitant SEC increase due to pump/ERD inefficiencies.

In studying the itemized SEC contributions, a detailed account was presented of the thermodynamic constraints posed by the feed- and spatially evolving retentate-fluid osmotic pressure. As expected, the contribution of the thermodynamically imposed SEC_min to the total SEC was large (>50%) in the case of seawater desalination, but of lesser significance in the case of brackish water due to its smaller salinity.

Prof. Karabelas commented on this study: “This paper is a significant outcome of many years of systematic experimental and theoretical work in this Laboratory, on the key aspects of design and operation of RO membrane desalination plants, that led to the development of an advanced RO process simulator. This unique tool, which is available to industry and academia, allows detailed and accurate predictions on the main RO desalination design and operation issues, including SWM module design (selection of optimal spacer characteristics, membrane sheet number/dimensions), pressure vessel design (optimal arrangement of SWM modules of different permeabilities) as well as optimization of RO process operating parameters”.