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.
1. Generalities on forward osmosis
Osmosis is a natural phenomenon which occurs when a water-permeable membrane separates two solutions of unequal salinity. The pure water diffuses from the less concentrated to the more concentrated region. The energy of this motion can be captured and converted to green electricity. This process, firstly described by S.Loeb in the seventies [1-3], is usually referred as pressure retarded osmosis (PRO). Considering two fluids referenced as Feed solution (F) and Draw solution (D) of different salinity (and thus osmotic pressure) separated by a FO membrane, the water from the less saline compartment F will transfer through the FO membrane to dilute the more saline compartment D (equation 1 and equation 2). The compartment D flow is then canalized and divided into two streams, one going through a turbine to generate energy (equation 3) and the other going to a pressure exchanger where it increases the pressure of the saline (draw) solution (see Figure 1) . PRO process requires a back pressure on the high salinity draw solution that generates power by depressurizing the solution through a turbine. To maintain a steady operation of the turbine, the permeate flow should always been kept at high pressure (15-35 barg) to feed the turbine. The water permeate flowrate (Jp) against the applied back pressure Pdraw depends on the osmotic pressure difference, which thereby converts the chemical potential energy into mechanical work.