By Jeremy Martin

While solar and wind remain at the forefront of most renewable energy discussions, another form of power generation is emerging that has the potential to be a significant source of energy: osmotic power.

Osmotic power is created through the natural phenomenon of osmosis. When two fluid streams of varying salt content—such as low-salinity river water and high-salinity ocean water—meet on either side of a membrane, osmosis causes the less salty water to be drawn toward the seawater side. The membrane blocks the salt, allowing only fresh water to flow through. This creates a pressure on the seawater side that can be used to drive a turbine to generate electricity.

In November 2009, Norwegian energy company Statkraft opened the world’s first pilot osmotic power plant in Tofte, Norway to demonstrate the viability of this emerging alternative energy source. According to Statkraft’s research, osmotic power has a global potential of 1,600 to 1,700 terawatt hours (TWh) annually. To harness this energy production potential, it is essential to understand precisely how the process works, the challenges of the technology, and the critical elements necessary to overcome these hurdles ensure the net-positive production of energy.

How osmotic power works
In the natural world, osmosis requires three components: a low-salinity water supply, a high-salinity water supply, and a membrane that blocks the salt while allowing fresh water to flow through it. To sustain osmosis in a man-made process and create osmotic power, the same three elements are required. For example, river water can serve as the low-salinity water supply, the ocean can supply the high-salinity water, and membranes capable of separating salt from water, which have been developed for modern desalination applications, can be adapted for osmotic power.

When the river water and seawater merge, osmosis causes water to flow through the membrane from low salinity to high salinity. The membrane selectively blocks and filters out salt, only allowing fresh water to pass through. The force exerted by the water—called osmotic pressure—is highest when the salinity difference across the membrane is the greatest. Therefore, wherever a river flows into the ocean, osmotic energy is potentially available if an efficient osmotic power process is used.

The osmotic power process: Low-salinity water and high-salinity water are supplied to the opposite sides of a membrane. Fresh water is drawn across the membrane by the seawater, building pressure on the seawater side and diluting it. The pressure of the diluted seawater is released through a turbine, which turns a generator to make electricity. The diluted water flows to the sea just as it would have without the process. The result is a clean, continuous source of renewable energy.

Overcoming challenges of the osmotic power process
Maximizing energy output through the osmotic power process is not as simple as mixing river water and seawater. But challenges must be overcome. Seawater is supplied to the membranes with a high-pressure pump. River water is supplied to the membranes at low pressure. Osmosis “pumps” the river water across the membrane where it merges with the seawater, increasing its flow rate. This dilute water is released through a turbine that drives a generator. In this design, 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.

However, one of the challenges is the dilution of the seawater. As the river water mixes with the saltier ocean water, the osmotic pressure reduces. With a lower osmotic pressure, the driving force that moves the water across the membranes is reduced. Therefore, more seawater must be supplied at high pressure to keep the permeate flowing and maintain high pressure in the dilute water stream—a process that consumes more energy and reduces the net power output.

A second challenge 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. A more viable design would eliminate the high-pressure pump so that more of the electricity generated by the turbine could be sent to the electric grid.

The high-efficiency osmotic power process
To overcome the challenges associated with the basic osmotic power process, energy recovery devices can be inserted into the process. As outlined in Figure 3, an energy recovery device eliminates the high-pressure pump and only a portion of the dilute water from the membranes passes through the turbine.

The rest of the high-pressure dilute water flows to the energy recovery device where it is replaced with salt water. This exchange occurs at extremely high efficiency, with no energy consumption and without significant reduction of the pressure of the stream. A circulation pump (which consumes very little energy because it does not pressurize the stream) moves the now pressurized salt water through the membranes to be diluted again in a continuous process. As a result, nearly all of the water that enters the high-pressure loop through the membranes leaves through the turbine.

Incorporation of energy recovery devices solves both the problems identified in Figure 2. Specifically, this technology allows for the replacement of the high-pressure pump with a low-energy circulation pump and limits the flow through the turbine to just that necessary to produce net power, making the process more efficient. Energy recovery devices also effectively keep the salinity in the high-pressure loop high, increasing the driving force for osmotic power production.

Potential power
Osmotic power has tremendous potential as a global source of renewable energy and Statkraft is already demonstrating the net-positive production of power at its pilot facility in Norway. The plant is designed with a capacity of 10 kW and by 2015, Statkraft plans to build a full-scale osmotic power plant with a capacity of 25 MW, which could produce more than 166 gigawatt hours (GWh) of electricity per year—enough to power 30,000 homes in Europe.

The potential for osmotic power is undoubtedly significant, but requires precise engineering and a design that properly addresses the challenges associated with the basic osmotic power production process. Eliminating the high-pressure pump by implementing highly efficient energy recovery devices into the process helps ensure net-positive production of electricity. Significant refinement of the membrane technology is required before this process becomes commercially viable. However, the incorporation of energy recovery devices will help the world to realize the enormous potential of osmotic power as an emerging source of renewable energy.

Jeremy Martin is the director of engineering at Energy Recovery, Inc., a company that designs and develops technologies for reducing energy consumed by desalination, oil and gas processing, osmotic power, and other processes.

Energy Recovery, Inc.
www.energyrecovery.com