Basic principles
The basic osmotic power process (Figure 2) pumps saltwater against one side of semipermeable membranes whose other side is exposed to fresh water. The pressure at which the saltwater is applied to the membranes is ideally 60% to 85% of the osmotic pressure. Fresh water flows across the membranes, diluting the saltwater and increasing its flow rate. The resulting dilute saltwater (in green) is discharged through a hydraulic turbine. The dilution of the saltwater and the diffusion of salt into the fresh water reduce the concentration gradient at the membranes, a phenomenon known as concentration polarization.
2. Back to basics. The basic osmotic power process. Source: Energy Recovery Inc.
Sufficient crossflow or agitation must be provided to minimize concentration polarization on both sides of the membranes, thereby maximizing the osmotic driving force. The net energy produced by the process is the amount produced by the turbine less the consumption by the pumps. Mounting one of the pumps on the turbine's shaft can significantly reduce the system's parasitic losses.
One way to reduce the amount of energy consumed by the saltwater booster pump in particular is to add an energy recovery device (ERD) to the circuit (Figure 3). The ERD increases the potential net osmotic power available by using some of the pressure in the high-pressure loop to dilute the saltwater with fresh water as it permeates the membranes. Saltwater is supplied to the ERD at low pressure. A portion of the brackish discharge from the membranes is also fed to the ERD. The ERD transfers the pressure of the dilute saltwater to the saltwater. The small pressure losses incurred in the membranes and in the ERD are made up by the saltwater booster pump.
3. No pressure. A high-efficiency osmotic power process can be designed to produce a net power output. Source: Energy Recovery Inc.
In addition to improving the performance of the osmotic power process, the ERD simplifies control of the saltwater hydraulics. The ERD seals the saltwater/dilute saltwater portion of the process so the only inlet flow is of permeate and the only significant outlet flow goes through the turbine. A small fraction of the dilute saltwater (0.5%) leaks through the seals of the ERD. Therefore, the turbine flow approximately equals the permeate flow. This relationship holds regardless of system pressure, saltwater flow rate, or dilution rate.
The membranes serve as the "power source" of an osmotic power process. The flux of permeate through the membranes and the resulting pressure on the saltwater side can be quantified in units of gallons per square foot of membrane area per day (gal/ft2/day) and pounds per square inch (psi), respectively. Increasing the water temperature increases the flux. Flux and pressure are related as follows: For a given membrane area, increasing the flux by increasing the flow to the turbine lowers the saltwater pressure.
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