PRESSURE RETARDED OSMOSIS PROPULSION (PROP)
Just to ponder and think outside the box a bit: Introducing Pressure Retarded Osmosis Propulsion (PROP), the future of ocean-transport propulsion. Consider that half or more of the operating expenses (OPEX) in ship transport are fuel, this idea may have some merit.
If the osmostic pressure between sea water & fresh water can create 24 bar and lift water 240 meters, then the forward osmosis process of PROP may scale up and push some screws instead of burning bunker C in a diesel. The new mechanics will be membrane specialists who work with turbines and Gen Sets that run electric motors and charge super capacitors and/or super bacitors (battery hybrid with capacitor - also made with graphene).
1 gram of graphene has the surface area of 3,000 m2 or about the area of a soccer field. If you take the MIT study and be conservative of an increase in water flux of 100x compared to existing RO membranes and you take Statkraft's research of 1 W/m2 (conservative), then it doesn't take too much imagination to realize that 1 g of graphene may produce between 3 kW (at existing Statkraft 1 W/m2) to 300 kW (100W/m2 x 3,000 m2) of osmotic energy. The theoretical may be 1,000x or approaching 3 MW per gram of graphene that is manufactured in a 3,000 m2 sheet either spirally configured or hollow-tube to allow the greatest mass transfer of water to occur without too much internal resistance. If you compare the MIT 1,000x greater flux and correlate directly to power output in watts, then the theoretical salinity gradient energy utilizing graphene membranes would be ~ 3,000 Watts/m2 or about 3x that of solar insolence with the added benefit of operating baseload 24/7 instead of intermittently and about 12 hours or less per day in the case of solar. Of course this thinking is contigent on the MIT computer simulation and other research teams need to see if they can model similar predictions.
OK, let's extrapolate and brainstorm...
If mass transfer can be optimized, then the limiting factor is the volume of membrane pressure vessel assemblies that are constructed. It may be that the majority of ocean-going ship power will be generated by osmotic energy in the next 100 years or less. The power would be generated using the salinity difference between sea water (0.5 M solution) and a much more concentrated salt solution (i.e. 5M solution). Of course collecting rain water from decks would be beneficial as well but not necessary. This energy is completely renewable energy with no pollution. PROP Ships would be limited to ocean ports and traversing the Panama canal may present some problems as well as brackish and fresh water ports.
Personally, I would love to circumnavigate the globe in a ship running on PROP. It would run on fresh water and sea water with maybe a salt "osmo-turbo" concentrator to increase power output at needed times when there was no rain. Incidentally, scientists have thought of using forward osmosis to propel ships for some time (since the 1960s) so this idea is not new. With the advent of potentially inexpensive graphene membranes, the possibility of this idea taking shape is starting to look good. I hope before I die I will see this envisioned and I hope to be part of the making of the future. How ironic would it be to have a PROP-ship transporting oil - that would be like rubbing salt into the wound of the oil & diesel industry.
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