No. 81: Power generation using ocean water and fresh water (September 9, 2011)

Two research activities on power generation using the difference of salinity concentration are going on in Japan. A research team consisting of researchers from Tokyo Institute of Technology, Nagasaki University, and Kyowa Kiden Industry are conducting experiments.

The power generation method they are exploring is called osmotic pressure generation that uses concentrated ocean water, which has two times more salt content than the normal ocean water, coming from the facilities for producing fresh water from ocean water and fresh water produced in the sewage treatment facilities. The ocean water and fresh water runs into eight cylinders, each of which is 1.4 m long and 30 cm in diameter, and permeation membranes are built in the eight cylinders to generate the momentum of the ocean water and increase the flow volume by 50-80%. Used ocean water and fresh water are sent back to the sea and rivers. This process runs the turbine to generate electric power. It is estimated that each of the eight cylinders can have the same generation capacity as a hydraulic power plant with a 300 m drop. The research team successfully generated electricity of 1-2 kW in the experiment.

Nitto Denko is also conducting research on osmotic pressure generation in alliance with Statkraft of Norway, stressing that developing a permeation membrane of 0.1 mm thick is the key technology for the success. In osmotic pressure generation, it is important to maintain the difference of salinity between ocean water and fresh water. That is, the key technology is to infiltrate fresh water into ocean water efficiently while preventing the salinity of ocean water from migrating to fresh water.

It is estimated that the generation cost of osmotic pressure generation is 9-26 yen per kW. It is lower than photovoltaic generation that costs 40 yen per kW and almost the same as wind generation that cost 14-24 yen per kW. In addition, because osmotic pressure generation is not at the mercy of weather, it has a real operating rate of more than 85%. The real operating rates of photovoltaic generation and wind generation are 12% and 20-22%, respectively. Considering the flow volume of rivers in Japan, osmotic pressure generation is estimated to have a potential generation capacity of six million kW, equivalent to the generation by 5-6 nuclear power plants.

The research team led by Tokyo Institute of Technology, which will be joined by Toray and Toyobo, plans to build a commercial plant in three years, while the Nikko Denko group is schedule to build a demonstration plant with a generation capacity of 2,000 kW in 2015.