PROJECT TITLE:

Desalination of Brackish Water with Wind-Powered Reverse Osmosis

PRINCIPAL INVESTIGATOR:

Dr. Clark Liu, Water Resources Research Center/Civil Engineering, University of Hawaii at Manoa

FUNDING AGENCY:

R.M. Towill Corporation

Background

Inadequate supply of fresh water of acceptable quality is one of the critical limiting factors in achieving sustainable development on many remote islands and in coastal regions. Reverse osmosis (RO) has emerged as the most feasible small-scale desalination technology. However, traditional RO desalination is energy intensive and not a viable solution for remote regions where electricity is in short supply. The utilization of alternative energy sources holds promise as a solution to this problem. This approach is especially attractive in areas with supplies of brackish water which requires much lower pressure to desalinate than pure sea water. Under the direction of Drs. Liu and James Moncur, the University of Hawaii at Manoa has conducted research on wind-powered reverse osmosis desalination since 1997, through a joint effort of its College of Engineering, Water Resources Research Center, and Hawaii Institute of Marine Biology. The Industrial Technology Research Institute in Taiwan provided part of the funding in the form of a research grant. Research efforts have been made in terms of mathematical modeling, laboratory experiments, and field-testing. Dr. Jae-woo Park (formerly of UH and now a professor at the Ewha University in Korea), Mr. Reef Migata, and several other students contributed to this research. This research is an offshoot of a previous WRRC investigation conducted at a pilot plant in Ewa, Oahu in the early 1990s that compared several desalination technologies.

A prototype system was constructed on Coconut Island off the windward coast of Oahu, Hawaii. This system consists of a 30-ft tall multivaned windmill/pump (see below), a Filmtec ultra low pressure RO membrane, a flow/pressure stabilizer, and a prefilter. A feedback control mechanism was developed for this project and installed in the prototype system. This control mechanism enhances the system performance by allowing continuous operation under varying ambient wind conditions.

Results of field experiments indicate that the system, when equipped with feedback control, can produce fresh water at a rate of about 0.7 gpm (gallon per minute) from brackish feed water with a TDS (total dissolved solids) concentration of 3,100 mg/l, at an average wind speed of 5 m/s. In future research, this wind-powered RO system will be modified, especially in regard to the flow/pressure stabilizer and pretreatment requirements. The modified system will then be used for desalinating seawater and for treating aquaculture wastewater.

System Operation

A schematic of a wind-powered RO desalination prototype system is shown in Figure 2. Proper operation of the RO module requires that feed water pressure be maintained within a small pre-set range. For the membrane used in this prototype system, the feed water pressure must be maintained in a range of 85 - 105 psi. Feed water pressure equals the water pressure inside the stabilizer, which is continuously monitored by a pressure sensor located on top of the stabilizer. When this pressure is below the minimum value or above the maximum value required for the operation of the RO module, the datalogger sends a signal to a solenoid valve to shut down the operation.

The pressure in the stabilizer depends on the rates of inflow and outflow. Inflow is generated by the wind pump, an input variable that cannot be controlled. The outflow from the stabilizer is the RO feed water, which is separated into flows of brine and permeate.

A manually operated throttle valve on the brine flow was used to provide a simple system control. When the pressure in the stabilizer declined, the throttle valve reduced the flow of brine. This made the water volume in the stabilizer pressure tank increase, as did water pressure. Because the operation of this simple control mechanism was arbitrary, results were less than satisfactory. During a test run conducted on February 23, 1999 with this simple mechanism, the water pressure in the stabilizer went below the minimum value of 75 psi and the system outflow was interrupted. In order to maintain a continuous and efficient operation, a feedback control mechanism was developed and installed on the system (see figure to right).

Feedback Control

The purpose of feedback control is to determine inputs to a system necessary to achieve the desired system response. Control inputs can be manipulated by the system designer as an output of a subsystem called the controller that is to be designed. For a wind-powered RO desalination prototype system, the desirable responses are the flow and quality of the permeate from the RO module. The stabilizer and data acquisition devices (including sensors, data logger, and the control computer) constitute the controller.

Design, Construction, and Testing of the Feedback Control Mechanism

The control of brine flow in the prototype system is accomplished by three parallel sets of solenoid/throttle valves. Signals of water pressure in the stabilizer are sent through sensors and a data logger to a control computer. The computer evaluates these signals and then sends a command to open one set of the solenoid/throttle valves. This control mechanism, which was installed and tested by Jonathan Liu and Clint Shima, allows the system to operate continuously and efficiently.

The figure below shows the time variation of wind speed and water pressure in the stabilizer (feed water pressure) during a field test conducted on July 7, 1999. The figure shows that fairly constant water pressure can be maintained with the feedback control even though wind speed is highly variable.

With feedback control, the rate of flow was relatively constant at about 3 gpm. The system shut down only for a very short time (a few seconds according to the records) when the maximum pressure in the stabilizer exceeded the upper limit.

In conclusion, when equipped with feedback control, the system can be operated automatically with a prevailing wind speed greater than 4 meters per second. A comprehensive mathematical model of feedback control for this prototype system will be developed to achieve optimal operation.