Economics of Power Production from Forward Osmosis

Forward osmosis (FO) uses an osmostic pressure gradient to induce a net flow of water from the solution feed (low salt concentration solution) into the draw solution (higher salt concentration solution) through a semi-permeable membrane. The osmotic pressure can then be converted into a hydraulic pressure used to run a Pelton turbine and to produce green electricity. The permeate flow is then always kept at high pressure (15-30 bars) to feed the turbine using a pressure exchanger; this technology is called Pressure Retarded Osmosis (PRO). Oil terminals are rejecting to sea large quantities of salty produced water (above 100 g/l) and there is a potential for the application of PRO using this produced water as the draw solution and the sea water, river or lagoon water as the low-salt feed solution. Forward osmosis between sea water and production water could be used to produce electricity with low additional environmental impact. A technico-economical study was launched to evaluate the potential interest of PRO technology to produce electricity on the Djeno oil terminal site situated in Congo. The calculated technical cost of electricity is from 350 to 650 euros/Mwhr, which is not competitive with other green energies. This high cost is mainly due to required bulk equipments and their related electrical consumption, the membrane cost being only 10% of the technical cost. So though the technology seems promising, a technical breakthrough on membrane permeability and mechanical resistance is needed to promote PRO as a competitive clean energy.