The mixing of freshwater and seawater where rivers flows into the salty ocean releases large amounts of energy. This energy can be harvested and made into electricity using pressure retarded osmosis (PRO). This is the concept of osmotic power, a new and yet unexploited source of renewable energy. The idea of exploiting the energy from mixing fresh water and sea water with PRO was first developed by Prof. Sidney Loeb in the early seventies. After some years in the eighties with limited progress in the field, the Norwegian power company Statkraft engaged in technology development in the mid nineties aiming at cost-effective osmotic power production. Today Statkraft is the world leader in development of osmotic power, and have made state of the art achievements during the last few years. The work has been focused on the design and production of a semi-permeable membrane optimized for osmotic power. During these years the power density of the membrane has increased from less than 0,1 W/m2 up to today's membra...