Process and Technology Status – Salinity gradient power is the energy created from the difference in salt concentration between two fluids, commonly fresh and salt water, e.g., when a river flows into the sea. There are two technologies for which demonstration projects are running and both use membranes. Pressure Retarded Osmosis (PRO) uses a membrane to separate a concentrated salt solution (like sea water) from freshwater. The freshwater flows through a semipermeable membrane towards the sea water, which increases the pressure within the seawater chamber. A turbine is spun as the pressure is compensated and electricity is generated. Reversed Electro Dialysis (RED) uses the transport of (salt) ions through membranes. RED consists of a stack of alternating cathode and anode exchanging permselective membranes. The compartments between the membranes are alternately filled with sea water and freshwater. The salinity gradient difference is the driving force in transporting ions that results in an electric potential, which is then converted to electricity. Two main applications exist: as standalone plants in estuaries where freshwater rivers run into the sea; and as hybrid energy generation processes recovering energy from high salinity waste streams. This could be for example, brine from desalination or salt mining, as well as waste water treatment plants. A possible third application is salinity gradient technologies applied to land based saltwater lakes or other types of salt water reserves. In 2013 construction began on a 50 kilowatt (kW) RED plant, based on an existing 5 kW pilot plant. At the same time, the longest running 10 kW pilot project by Statkraft has been stopped, although interested parties are expected to continue. Large-scale production of cheap membranes is one of the key factors for the cost-effective scale-up, and a number of newly created companies are entering this market.
Performance and Costs – An important factor for both the performance and costs of PRO and RED are the membranes. Current net power density (which considers the membrane potential, resistance and the power required to pump the water) of membranes is maximum 2.7 watt per square metre (W/ m2 ), but the latest laboratory experiments have achieved a net power density of 14.4 W/m2 (for a PRO process). Higher power densities could be obtained by changing the cell design, in particular the membrane resistance, the cell length and the use of nanotubes. Changing cell designs could increase the calculated net power density close to 20 W/ m2 . The power density of membranes is, however, not the only indication of performance. Eventually, it is the design of the entire facility, including the circulation of large quantities of water within the plant, which determines the performance of the plant. With the exception of the cost estimates for existing projects (which reflect experimental set-ups), there is little hard evidence for the costs of both PRO and RED technologies. Based on feasibility studies, future costs for 2020 are estimated to be around USD 65-125 per Megawatt (/MW) for PRO, and USD 90/MW for RED. Levelised costs of electricity are estimated to be around USD 0.15-0.30 per kilowatt-hour (/kWh) for PRO, and USD 0.11-0.20/ kWh for RED in 2020. The lower ranges in cost are for hybrid applications that make use of existing infrastructure. The cost ranges are highly speculative, and are relatively positive compared to cost projections for other ocean energy technologies with a more advanced technological status.
Potential and Barriers – The total technical potential for salinity gradient power is estimated to be around 647 gigawatts (GW) globally, (compared to a global power capacity in 2011 of 5 456 GW), which is equivalent to 5 177 terawatt-hours (TWh), or 23% of electricity consumption in 2011. The technical potential does not consider any ecological and legal constraints to salinity gradient deployment, so the actual potential might be less. There are very few detailed studies that consider the ecological and legal responsibility of water extraction; these are found in Canada, Colombia, Germany, the Netherlands and Norway. Current estimates exclude the potential of salinity gradient power used in hybrid applications. The technical and economic potential for these kinds of applications could be high, since waste streams from waste water and desalination plants generally have higher salt concentrations than the surrounding sea water. Consequently, the energy produced per cubic metre (m3 ) of brine would also be higher and the overall costs lower. More research is required to map the potential for hybrid solutions, as well as land based saltwater lakes or other types of salt water reserves. The main economic barrier are the membrane costs, which account for 50%-80% of total capital costs. Their estimated price ranges from EUR 10-30/ m2 and needs to be reduced to EUR 2–5/m2 to be competitive with other renewables. Furthermore, improvements in power density, durability and pressure properties are desirable. On a positive note, an increasing number of companies are entering the market to produce these dedicated membranes and other parts of the installations e.g., stacks or modules. Large quantities of dedicated supplies will be necessary for upscaling, as will sharing documented experiences from pilot projects to help the technology expand across the different regions of the world. The relatively small experience base with salinity gradient technology also has consequences for policy makers, as technology developers look for support and stability to continue to demonstrate this technology. Unfortunately, due to the lack of reliable financial support mechanisms, the company Statkraft, which is one of Europe’s largest generators of renewable energy and a the leading power company in Norway, is going to discontinue its osmotic power efforts this year (2014) and is looking for investors to continue with the research and results attained over the 2006-2014 period.