Engineering the Process

Deriving electricity from osmotic potential is not as simple as mixing river water and seawater. The membranes required for the process must separate the salt and allow the fresh water to flow through on a continuous basis. The membranes used for desalination, unfortunately, are not practically suited for sustaining osmotic flow because salts accumulate in the membrane layers and reduce the effective osmotic pressure. While researchers are actively working on osmotic power membranes, considerable advances have been made to the rest of the process.

To explain the challenges that must be overcome to make the osmotic power process work, consider the diagram of a basic osmotic power process shown in Figure 2. Salt water is supplied to the membranes with a high-pressure pump. Fresh water is supplied to the membranes at low pressure. Osmosis “pumps” the fresh water across the membrane where it merges with the salt water, increasing its flow rate. This dilute water is released through a turbine that drives a generator. The high-pressure pump consumes electricity and the turbine produces electricity. Because the flow rate through the turbine is greater than the flow rate through the high-pressure pump, the process has the potential to produce more energy than it consumes.

Figure 2 – Basic Osmotic Power Process

Two fundamental problems prevent the process illustrated in Figure 2 from being a practical way to produce energy. One problem is dilution of the salt water. As the membrane permeate mixes with the salt water, its osmotic potential reduces. With a lower osmotic pressure, the driving force that moves the permeate across the membranes is reduced. Therefore, more salt water must be supplied at high pressure to keep the permeate flowing and maintain high pressure in the dilute water stream.

A second problem is mechanical losses in the pump, turbine and generator. Even large, efficient modern devices experience some losses when they convert electrical energy to hydraulic energy or vice versa. Overall efficiency could be improved by mounting the pump and the turbine on the same shaft such that only the net mechanical energy produced by the process is converted to electricity. However, since all of the high-pressure water still passes through both the pump and the turbine, the combined losses result in very little net power production, even with very efficient components. A more viable process design would eliminate the high-pressure pump and minimize the flow of water through the turbine to just that necessary to produce net power.

Copyright 2009 Osmotic Power Inc.