(486e) Minimizing Energy Consumption in Reverse Osmosis Membrane Desalination Using Optimization-Based Control

Bartman, A., University of California Los Angeles
Zhu, A. (., UCLA
Cohen, Y., UCLA

In recent years, the interest in the use of reverse osmosis membrane desalination has increased due to the energy efficiency and versatility of this process relative to other water desalination technologies. Water shortages in various areas of the world have necessitated further development of the reverse osmosis desalination process in order to provide clean drinking water to the people in these regions. When operating a reverse osmosis desalination process, it is imperative that the system conditions be monitored and maintained at appropriate set-points in order to produce the required production rate of clean, potable water. Furthermore, with the rising cost of energy, it is also desired to deploy operating methods to reduce the energy consumption of reverse osmosis desalination, especially when confronted with variability of feed water quality. This task requires the development and implementation of effective feedback control strategies. Traditionally, classical (i.e., proportional-integral (PI) or proportional-integral-derivative (PID)) control algorithms have been used to regulate process flow rates and adjust the system pressure in order to achieve a desired rate of clean water production. In addition to classical control schemes, nonlinear model-based geometric control strategies have been developed to minimize the effects of varying feed water quality and also to account and correct for various faults that may present themselves during the operation of a reverse osmosis desalination process. Control methods using model-predictive control (MPC) and Lyapunov-based control have also been evaluated using computer simulations. Reverse osmosis system analysis using linear models and data-based models using step-tests to create approximate linear models have also been demonstrated. Other control methods have also been evaluated in the context of RO system integration with renewable energy sources. While the aforementioned control strategies are able to maintain a constant permeate production rate and deal with feed water variability, they do not directly minimize energy usage by the reverse osmosis process. In a typical seawater RO system, the cost of energy can approach 45% of the total permeate production cost due to the fact that the system operation can require high feed pressure (around 1000 psi) in order to achieve a desired permeate production rate. In recent years significant effort has been invested in order to decrease the energy consumption by a reverse osmosis desalination system; these include the development of high permeability membranes leading to lower required transmembrane pressure, optimization of RO module and system configuration, and also the use of energy recovery devices. It has been recently shown that the specific energy consumption, or SEC (energy cost per volume of permeate water produced), is useful as a metric to quantify reverse osmosis desalination system energy usage. Within the SEC framework, unit cost optimization with respect to water recovery, energy recovery, system efficiency, feed/permeate flow rate, membrane module topology, and optimization of the transmembrane pressure subject to feed salinity fluctuations can be evaluated at a fundamental level. Accordingly, the present work focuses on the design, implementation and demonstration of an optimization-based control system in order to facilitate system operation at energy optimal conditions. In this approach, a reverse osmosis desalination system model was developed for use in conjunction with the system energy usage analysis model developed in previous work [1] to design an energy optimization-based controller. This controller uses multiple system variables and a user defined permeate production rate to calculate the optimal operating set-points that minimize the specific energy consumption of the reverse osmosis desalination system and satisfy the process and control system constraints. The optimization-based control system was implemented in a multi-tiered fashion in the UCLA reverse osmosis (RO) pilot plant that is capable of processing up to about 15,000 gallons per day of feed water. Experimental performance data with the current control strategy demonstrated that it is feasible to operate the RO process such that it is driven toward minimization of RO energy consumption. [1] A. Zhu, P. D. Christofides, and Y. Cohen, ?Effect of thermodynamic restriction on energy cost optimization of RO membrane water desalination,? Ind. Eng. Chem. Res., vol. 48, pp. 6010-6021, 2009.