Significant research took place from 1975 to 1985 and gave various results regarding the economy of PRO and RED plants.  The total cost varied from 0.02 to far above 1.3 USD per kWh [1-6].  In a recent review on renewable energy [7] the cost of salinity power is said to be prohibitive.  However, a closer look at the sources reveal that many negative conclusions were based on the performance of existing equipment and not new purpose-built equipment.  Measurements with membranes for reverse osmosis and not equipment adapted to osmosis.  A recent prestudy [8] concluded that a total power cost with PRO could be 0.035 - 0.07 USD per kWh.  This prestudy resulted in further development work by Norway's largest producer of hydroelectric power, Statkraft SF, in collaboration with European membrane expertise and backing from the EU commission.  It is important to note that small-scale investigations into salinity power production take place in other countries like Japan, Israel, and the United States. 
Advantages

The advantageous properties of salinity power can be summarized as follows:

No CO2 or other significant effluents or any global environmental effects;

Completely renewable;

Non-periodic (unlike wind or wave power);

Suitable for small or large scale plants (modular layout).

Drawbacks

The primary drawbacks of salinity power are:

Some plant equipment has yet to be developed with the necessary efficiency;

High capital costs for plant construction -mostly buildings and machinery;

Energy cost is very sensitive to membrane cost and efficiency;

Membranes used for plants are vulnerable to fouling.

Conclusions

Salinity power is one of the largest sources of renewable energy that is still not exploited.  The exploitable potential world-wide is estimated to be 2000 TWh annually.  One of the reasons that this renewable source has not drawn more attention is that it is not readily evident to most people.  Another reason is that considerable technological development is necessary to fully utilize this resource.  Along with the the lack of efficient and suitable plant components, some pessimistic cost forecasts have been issued.  The potential cost of energy from this source is higher than most traditional hydropower, but is comparable to other forms of renewable energy that are already produced in full-scale plants.

References

1.  Emren, A. and Bergstrøm, S.  1977.  Salinity Power Station at the Swedish West Coast: Possibility and energy price for a 200 MW plant, In: Proc. Int. Conf. on Alt. Energy Sources, Miami Beach, December.

2.  Gava, P.  1979.  Energy from Salinity Gradients, European pre-study, Eurocean, Association, Europeenne Oceanique, Monaco.

3.  Jellinek, H.H. and Masuda, H.  1981.  Osmo-power: Theory and performance of an osmo-power plant, Ocean Engng., vol. 8, 2, 103.

4.  Lee, K.L., Baker, R.W., and Lonsdale, H.K.  1981.  Membranes for Power Generation by Pressure-retarded Osmosis, J. of Mem. Sci. vol. 8, 141.

5.  Loeb, S.  1998.  Energy Production at the Dead Sea by Pressure-retarded Osmosis: Challenge or chimera?, Desalination, 120, 247-262. 

6.  Metha, G.D.  1982.  Further Results on the Performance of Present-day Osmotic Membranes in Various Osmotic Regions, J. of Mem. Sci. vol. 10, 3.

7.  Burnham, L., Johansson, T.B., Kelly, H., Reddy, A.K.N. and Williams, R.H. (Eds.) 1993.  Renewable Energy: Sources for fuels and electricity, Island Press.

8.  Thorsen, T.  1996.  Salinity Power, SINTEF Report STF66 A96001, SINTEF Applied Chemistry, Trondheim (in Norwegian).