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Project Motivation To increase awareness of pressure retarded osmosis (PRO) as a potential source of renewable energy, an interactive device that demonstrates power generation and osmosis through a semipermeable membrane was developed by a group of students from Humboldt State University’s Section of Society of Women Engineers. The lab-scale device was fabricated using a 3D printer, which constructs three-dimensional solid objects from digital models. This method of construction allows for replication of the device; requiring only the digital plans, a 3D printer, and the thermoplastic used by the printer.

The project objective is to build a community of engineers, scientists and educators which utilizes the PRO device and contributes to the community by providing feedback and sharing their findings. SWE will help build this community by making the device available and promoting the system to aforementioned interest groups.

Pressure retarded osmosis (PRO) is an emerging technology in which power is generated by the interaction between saltwater and freshwater through membrane permeation. During PRO, a low-salinity (feed) solution is drawn through a membrane into a pressurized, high-salinity (draw) solution (Figure 1) Power can then be generated by releasing the pressure through a turbine.

Water flux (Jw) is dependent on the membrane’s water permeability coefficient (A), the osmotic pressure differential (π), and the hydraulic pressure differential (P). The power generated per unit membrane area (W), i.e. the power density, is dependent on flux and the hydraulic pressure difference across the membrane.

π, the osmotic pressure differential, is equal to the magnitude of the hydraulic pressure differential (P) that would prevent net transport of water across the membrane. OLI Stream Analyzer 2.0 software (OLI Systems, Inc., Morris Plains, NJ) gives the following relationship between osmotic pressure (in kPa) and molarity for sodium chloride:

Concentration polarization (CP) is a phenomenon that describes the accumulation or depletion of solutes near the interface of a membrane [1]. Two types of CP occur in a PRO configuration: dilutive external CP and concentrative internal CP. In this investigation, predicted flux and power density values will be compared to experimental values. Due to CP, an increase in osmotic pressure differential is not expected to cause a proportionate increase in flux and power density.


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References

[1] Achilli, A., Cath, T. Y., and Childress, A.E. (2009). “Power Generation With Pressure Retarded Osmosis: An Experimental and Theoretical Investigation.” Journal of Membrane Science, 343, 42-52.

[2] Achilli, A., and Childress, A.E. (2010). “Pressure Retarded Osmosis: From the Vision of Sidney Loeb to the First Prototype Installation.” Desalination, 261, 205-211.

[3] Anastasio, D. and McCutcheon, J. R. (2013). “Using Forward Osmosis to Teach Mass Transfer Fundamentals to Undergraduate Chemical Engineering Students.” Desalination, 312, 10-18.

[4] Cath, T. Y., Childress, A. E., and Elimelech, M. (2006). “Forward Osmosis: Principles, Applications, and Recent Developments.” Journal of Membrane Science, 281, 70-87.

[5] McCutcheon, M. E., and Elimelech, M. (2006). “Influence of Concentrative and Dilutive Internal Concentration Polarization on Flux Behavior in Forward Osmosis.” Journal of Membrane Science, 284, 237-247.



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