Power generation using pressure-retarded osmosis

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2014

The feasibility of producing electricity from a salinity gradient is under study.

KEY CONCEPTS
• Pressure-retarded osmosis can be used to generate power by taking advantage of a gradient in water salinity.
• A dimensionless PRO model has been developed to determine the potential of this technology as a source of power.
• 95 percent of the PRO system’s maximum power can be generated using 50 percent or less of the maximum semipermeable membrane area.

RESEARCH IS CONTINUING TO FIND ALTERNATIVE METHODS FOR POWER GENERATION that can be done with low-cost raw materials and at high efficiency. Water is such a material and is present at different mineral concentrations (also known as salinity).

This can be exploited as a means to generate power. For example, fresh water from a source such as a river and seawater are present at different salinity levels.

Techniques such as reverse osmosis have been present for preparation of pure water needed in consumer and industrial processing and for treating wastewater. Reverse osmosis enables water to flow through a membrane, while salts and other contaminants are blocked by the membrane. This is accomplished by applying enough pressure to overcome osmotic pressure.

A related setup that uses a membrane and can be used for power generation is known as pressure-retarded osmosis (PRO). This setup involves pressurizing one stream below its osmotic pressure. Leonardo Banchik, a graduate student in the mechanical engineering department at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., says, “PRO is a method for producing power from a gradient in water salinity. By placing sources of river water and seawater on opposite sides of a semipermeable membrane, water can be passed from the less-salty river water to the more salty seawater, which is held under pressure in order to generate power as it is passed through a turbine.”

Another setup known as reverse electrodialysis (RED) uses the ionic difference between fresh water and seawater to produce electricity and was discussed in a previous TLT article (1). In the case of RED, stacks of alternating positive and negative ion-exchange membranes can be used to achieve this goal. However, RED is considered to be inefficient, leading researchers to investigate alternative methods of power production from salinity gradients such as PRO.

Figure 2 shows an image for an elementary PRO system. Banchik says, “Initially, power is required to pump both streams through the system and to pressurize the seawater. After startup, the PRO system can begin to generate net power.”


Figure 2. Pressure-retarded osmosis generates power by having water from a less salty source pass through a membrane to a more salty source under pressure, as shown in this elementary system. (Courtesy of Leonardo Banchik of MIT and Elsevier BV)

While the energetic capability of PRO has been evaluated through modeling by a number of researchers, Banchik says, “Past models have not provided realistic analyses of PRO because they have considered small systems where the osmotic pressure of each stream does not vary throughout the system.”

A new approach is needed to determine the amount of power producible in a large-scale PRO system. Such a model has now been developed.

DIMENSIONLESS PRO MODEL
Banchik, in collaboration with John Lienhard, the Abdul Latif Jameel Professor of Water and Food at MIT, and Mostafa Sharqawy, King Fahd University of Petroleum and Minerals in Saudi Arabia, have developed a dimensionless PRO model that provides insight into the potential for using this technology to generate power.

One of the inspirations for the researchers was to evaluate the operation of heat exchangers. Banchik says, “Consider how a radiator in an automobile exchanges heat between a coolant and the ambient air. If a radiator possesses an infinitely large size, then eventually the temperature of the coolant will reach the ambient temperature as the radiator fully exchanges heat with the environment.”

The researchers looked at the potential for a system which uses an infinitely large membrane to generate power. By looking at five key parameters over a wide range of operating conditions, they found that 95 percent of the PRO system’s maximum power can be generated using 50 percent or less of the maximum semipermeable membrane area.

Banchik says, “The ratio of salinity between the two water streams is an important parameter, which governs the maximum amount of producible power. We looked at a number of different stream combinations and concluded that mixing a highly concentrated desalination brine and wastewater would provide twice as much power as a seawater-river water mix. Such a project is currently being undertaken in Japan to potentially reduce the energy required for desalinating seawater for drinking purposes.”

Banchik notes that the simplified model which the researchers developed for PRO gives an upper bound for the amount of power produced because it does not take into account other energetic losses, which would be incurred in this facility. He says, “Due to the sensitivity of the semipermeable membrane, pretreatment facilities will be needed for both water streams to make sure that any large foreign materials will be removed.” A PRO system would also include a pressure exchanger and pumps for both streams.

Banchik also discussed the possibility of using PRO at the Deer Island wastewater treatment plant, which is located in the Boston Harbor near MIT. He says, “The implementation of PRO at the Deer Island facility to mix less salty treated wastewater with salty seawater could theoretically power the entire facility but would probably require a membrane that is at least 2.5 million square meters. This is the operating area of the largest reverse osmosis plant in operation worldwide.”

Future work will involve an economic study to determine the commercial potential for PRO. Additional information on this work can be found in a recent publication (2) or by contacting professor John Lienhard at lienhard@mit.edu.

REFERENCES
1. Canter, N. (2012), “Synergy from combining two energy-generating technologies,” TLT, 68 (7), pp. 14-15.
2. Banchik, L., Sharqawy, M. and Lienhard, J. (2014), “Limits of power production due to finite membrane area in pressure retarded osmosis,” Journal of Membrane Science, 468, pp. 81-89.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.