Pressure-retarded Osmosis （PRO） is a promising system for renewable energy. The system utilizes a spontaneous hydraulic water stream driven by the osmotic pressure difference between brine with high concentration of salts and freshwater, and then the hydraulic water stream is subsequently converted into electricity using a turbine and generator. Because a spontaneous hydraulic water stream is produced by a semi-permeable membrane, the development of high-performance PRO membranes with long-term stability is quite important for making the PRO system commercially feasible. Since this system uses membrane technology, the core item is a semi-permeable membrane. Based on well-established membrane technology for more than 40 years, Toyobo has developed and commercialized hollow-fiber PRO modules consisting of cellulose triacetate.

SaltPower, a start-up company in Denmark, uses high salinity solutions （more than 10 wt％） as draw solutions （DS） that can be used to achieve a commercial PRO plant. Toyoboʼs PRO modules have been officially adopted by SaltPower, and two pilot-scale PRO plants have been demonstrated using high salinity solutions provided by （1） the salt industry and （2） geothermal water. In the case of （1）, we successfully achieved high gross power of 7000 W for 700 h, and this result accelerates SaltPowerʼs first commercialized PRO plant, which will start up within this year.

Pressure retarded osmosis （PRO） is an alternative source of renewable energy. PRO is a powerful generating technology that uses salinity differential as a driving force. The completed PRO system, uses brine and treated sewage water to reduce the power consumed by the seawater desalination plant by 10 ％ on a Mega-ton scale. The power generation cost is estimated to be 10.6 JPY / kWh because general-purpose parts and equipment can be obtained at low cost.

In order to achieve a PRO system that uses seawater and treated waste water, it is necessary to develop a novel PRO membrane module. The PRO membrane module parameters to be developed are A value: 2.05×10^－6 m/s/MPa and B value: 5.5×10^－9 m/s. The power generation cost of a PRO system using seawater and treated sewage water is estimated to be 22 JPY / kWh using the developed novel PRO membrane module. The development of superior PRO membrane modules will revolutionize the worldʼs energy supply.