Salinity Gradient Energy Propulsion - Will it scale up to tankers?

Sometimes assuming the technology is at our doorstep and determining if it is viable in a given application is a good approach.  So let's look at very large capacity carriers (VLCCs) which can carry between 1.40 - 2.24 million gallons.  How much salt would be required to match the equivalent ocean crossing of 3,000 miles.   We need to work backwards.

VLCC horsepower for a 1.82 million gallon tanker is around 42,500 HP.  If we can assume graphene membranes are in a tight form factor with 100x flux of today's membranes, then we would have approximately 40 m2 per membrane or about 40 Watts x 100 = 4,000 W ~ 5 HP.  Dividing 42,500 by 5 would equal = 8,500 membranes (quite cumbersome).  Therefore, graphene membranes need to be manufactured with large surface areas, two orders of magnitude much larger than current designs.  If this were the case then only 85 membranes would be required to produce 500 HP each and the surface area would have to contain 4000 sq meters.  This would take up a large footprint, but the goal would be to generate electricity and many large ships have externally-mounted electric propulsion motors that pivot, eliminating rudders and also allowing higher maneuverability as well.

If it were possible to propel the ship with PROP, then how much salt would need to be loaded to "fuel" the journey of 3,000 miles?  One would have to determine a concentration based on the water the ship is traversing.  If the ship was in brackish water, it would require less salt than if it were traversing though saltier seas.  More math to follow...

Ubiquitous graphene in the near future?

Ubiquitous graphene seems still a bit out of reach due to the price of $250 per 2" square pieces.  For applications that require graphene for filtration there is the "sticky" situation of graphene sticking to itself which poses a manufacturing problem when compacting sheets of graphene into spiral configurations. 

One wishes to be optimistic and go into the lab and change the future but also to reap some benefits. 

Salinity Gradient Energy: The Future Energy Source for Ocean-going Vessels?

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.

PRO Power? Pipe dream to power generation?

The leader in PRO (pressure retarded osmosis) is Statkraft, a hydro-electric power company in Norway who demonstrated a 4 kw pilot plant in late 2009.  According to Statkraft website:

What is the membrane's efficiency?

The one we are going to test now has a efficiency of less than 1 watt per square metre, but we plan to install membranes that can deliver 2-3 watts after we have run the plant for awhile. The objective is to reach 5 watts.

But with graphene, the flux is theoretically higher and the membrane just 1-atom thick, so mass transfer (internal concentration polarization) should be less of a problem as well as concentration polarization, which plagues the efficiency in current RO membrane architectures.  With orders of magnitude over 100 up to 1,000, the future looks bright for PRO power generation assuming graphene membranes make it to reality.

Graphene Water Desalination: Osmotic pressure of sea water is 24 bar period, so how come the MIT researchers claim that this can be done at a lesser pressure?

After pondering the MIT study previously mentioned, I started thinking that the osmotic pressure of sea water is 24 bar, so this means to reverse the process and obtain fresh water a high pressure pump must exceed 24 bar to produce fresh water regardless of whatever osmotic membrane is employed.  In other words, 24 bar is still required, it is the flux or water flow through the graphene 1-atom thick membrane that increases relative to existing RO membranes.

MIT Graphene Simulation

     David Cohen-Tanugi and Jeffrey C. Grossman of MIT have published a study in Water Desalination about their work on computer simulation of graphene rejecting salt ions.  The report demonstrates at least hypothetically, that water flux or permeability of graphene is orders of magnitude higher than existing reverse osmosis RO membranes.  Take a look at the graph below:

A quick glance would indicate the water permeability ranges from 0.1 to 100 L/cm2/day/MPa, which is almost 1,000 times greater sea water RO.

     Two things come quickly to mind:  1.  (obvious) this will be a game changer if manufacturing of graphene is affordable and 2.  (not so obvious, but definitely a concern) what about the safety of nano-particales in drinking water?  or in other words, what happens as this graphene sheet mechanically breaks apart into nano-particles and gets into the water supply?