Five basic forms of ocean energy can be harvested for to generate electricity and fresh water by various means: a) tide energy ; b) wave energy ; c) tidal currents ; d) thermal gradients ; e) salinity gradients. Table 1 illustrates the estimated global resources of each form.
|Form of ocean energy||Estimated global resources (TWh/year)||Percentage of current global electricity production.|
Current production: 17 400 TWh (TWh/year)
|Tides energy||300+||1, 7 %|
|Wave energy||8000 - 80 000||46 % - 460 %|
|Tidal currents||800||4, 6 %|
|Thermal gradients||10 000||57, 5 %|
|Salinity gradients||2000||11, 5 %|
Electricity generation through the use of salinity gradients between salt and fresh water is a relatively new concept. While discovered and discussed in the 1970s, research has been slow and most of it only recently. Two practical methods concerning membrane technology are currently being researched: the reverse electrodialysis (RED) method and pressure retarded osmosis (PRO). Both technologies are dependent on the semi permeable membrane. A semi-permeable membrane is selective in its permeability, i.e. only specific substances can pass through the membrane. Both processes rely on ion-specific membranes. The technology RED as well as PRO is in the research and development phase. The major obstacle is the cost of the membrane. Two countries, Norway and the Netherlands, are especially active in the R&D of osmotic power.In addition, the hydrocratic generator technology also relies on salinity gradients for energy generation.
Salinity gradient power plants are based on the natural mixing of fresh and salt water (IEA, 2009). Collision of fresh and salt water provides large amounts of energy, which this technology aims to capture. Many areas exist where industrial users (such as sewage treatment plants) discharge substantial volumes of fresh or low-salinity water into the ocean; such locations could be ideal for implementing prototype salinity gradient systems.
Pressure Retarded Osmosis (PRO)
Pressure Retarded Osmosis (PRO) uses the selective diffusion of water across a membrane in order to pressurize seawater. Freshwater and seawater are placed on either side of a membrane, and the seawater side is pressurized. As the seawater side increases in pressure and decreases in salinity, part of the water is discharged through a turbine while the rest is put in a pressure exchanger to pressurize the incoming seawater, as illustrated in Figure 1.
The pressure difference across the membrane is the main supplier of energy and can be as much as 200 meters of hydraulic head (IEA, 2009). Membrane performance in the 4 - 6 W/m2 range is currently the target range by the main research institution investigating PRO. The lifetime of the membrane also needs to increase to around 7 to 10 years before the technology can become commercial. Test modules have so far demonstrated energy densities of about 1.7 W/m2 (IEA, 2009). Video 1 illustrates the PRO concept.
Reverse Electrodialysis (RED)
Reverse Electro Dialysis is another membrane-based technology that uses an electrochemical reaction rather than osmotic pressure. The form of the device is a stacked series of membranes, half of which are permeable to sodium and half chloride, with seawater and freshwater flowing alternately between each pair of membranes as illustrated in Figure 2.
The stack controls the diffusion of the sodium and chloride ions in the water, which then cause oxidation and reduction at the iron anode and cathode. Currently, the technology has been tested only at a very small (100 mW) scale. (IEA, 2009). Video 2 illustrates the RED concept and a conceptual idea of a salinity gradient power plant in the Netherlands.
Hydrocratic ocean energy
The hydrocratic generator is a system capable of extracting power from salinity differences without the use of a membrane. The generator consists of a tube mounted on the seabed that is filled with holes to allow the entry of seawater. A turbine is mounted vertically in the tube and connected to a generator underneath the pipe. Fresh water is injected at the bottom of the tube, and the mixing of the freshwater and saltwater results in an upward flow of brackish water larger than the initial fresh water injection. This flow turns the turbine and generates power. Designs that involve the coolant discharge of power plants or the discharge of waste treatments plants as the main source of fresh water have been created. Basic tests of water flows through the device have been conducted at sea (IEA, 2009).
The primary requirement of a salinity gradient power plant is the availability of both a supply of fresh water and a supply of salt water. This makes the technology relatively location specific, although there are still a large number of possible locations.
The majority of components required for an salinity gradient power plant have already reached commercialization. In PRO and RED, the two unique components are the pressure exchanger and the membrane (Sandvik & Skihagen, 2008). To commercialize osmotic power the improvement and scale up of these components is required.
PRO and RED are both dependent on the salinity, volume and cleanliness of the water supply. The higher the salinity gradient between fresh- and saltwater, the more pressure (i.e. energy) will build up in the system. Similarly, the more water that enters the system, the more power can be produced. At the same time, it is important that the freshwater and seawater be as clean as possible. Substances in the water may get captured within the membrane’s support structure or on the membrane surfaces, reducing the flow through the membrane and causing a reduction in power output. This phenomenon, which is called fouling, is linked to the design of the system, to the characteristics of the membrane, and to the membrane element (Sandvik, Hersleth, Seelos, 2009).
Hydrocratic energy still needs to enter the demonstration phase of R&D. As such, it is still unclear what the main operational necessities and areas of improvement are.
The technology is still in its infancy, as illustrated in Figure 2. The salinity gradient technologies are in their part-scale (tank) phase which means that the concepts and prototypes are undergoing tests in the laboratory environment (IEA, 2009). Clearly, the other ocean energy technologies have advanced further in the R&D process. However, the global potential of the technology of using the salinity gradient for energy is estimated to be around 2000 terawatthour on an annual basis (TWh/year) (IEA, 2007, p.80.).
A PRO pilot project capacity has been established in Norway. At a capacity of 4 kW, the pilot plant is limited in scope and production capacity and is intended primarily for testing and development purposes (OES-IA, 2009). The pilot project has two main aims: a) to confirm that the designed system can produce power on a reliable 24hr/day basis; b) to further test the technology achieved from parallel research activities in order to substantially increase the efficiency (Sandvik & Skihagen, 2008). Membrane modules, pressure exchanger equipment and power generation are the main areas of focus (Sandvik & Skihagen, 2008). Figure 3 is an illustration of the coiled membranes in the pilot plant in Norway. At a capacity of 1 kW, a small RED pilot project has been installed in the Netherlands (OES-IA, 2009).
There is limited information available on the local impacts of salinity gradient power plants on the environment. The two pilot projects currently in operation have not yet published their findings. Moreover, the small number of projects, limited deployment times and small scale of these projects means that there is still a reasonable degree of uncertainty about the long term impacts of large scale deployment.
At this moment, it seems that salinity gradient power plants will have minimal environmental impact. The mixing of fresh and salt water occurs naturally at delta's of rivers with no negative effects. In addition, the power plant will require no fuel to run, it just needs a regular supply of both fresh and salt water (Jones & Finley, 2003).
A possible source of impact could be the effect of brackish water which will be discharged by the osmotic power plant into marine environment. This may alter the local marine environment and result in changes for animals and plants living in the discharge area. However, the osmotic plant will only displace the formation of brackish water in space without modifying the water quality so this will not be a significant environmental impact (Sandvik, Hersleth & Seelos, 2009).
Only during the course of further development, through more environmental impact assessment studies and larger constructions will the salinity gradient energy community be able to gain a firmer idea of any potential impacts.
Salinity gradient power plants directly contribute to climate change mitigation by providing a completely renewable energy source free of GHG emissions (beyond the initial GHG gases associated with production and installation that could be expected to be offset in small time frames). However, as discussed in the following section on ocean energy economics, the total installed capacity will very likely remain small for salinity gradient energy technologies meaning that their overall contribution to mitigation with the next decades will be relatively small.
As all ocean energy renewables are still largely at the R&D and demonstration phase, with a corresponding lack of commercial devices, it is very difficult to accurately estimate costs. The IEA (2008) put lifetime delivered energy costs of marine renewables at USD 150/MWh to 300/MWh across the range of technologies (minus tidal barrage), generally well outside the range of current electricity revenues even when current carbon finance incentives (available in certain countries) are considered. They estimate that costs will need to reduce to between a third and a quarter of their current levels to be feasible without significant support.
While the investment cost for an salinity gradient power plant is relatively high per installed unit of power compared to other renewable energy sources such as wind and solar, an important difference is that salinity gradient power plants will be designed for base load operation and are thus qualitatively different from most other renewable energies (SPP, 2004). Due to the large number of operating hours and its reliability, the technology's annual energy cost per kWh might be a better measure (SPP, 2004).
Uncertainties about the costs and technical performance of marine energy technologies must be overcome before significant commercial investment can be attracted. Large-scale prototype/demonstration schemes can help in this respect to inform investors regarding the key issues of reliability, efficiency, reparability.
The issues mentioned above do not make marine renewables undesirable; in fact many countries have been supporting research programmes and private sector development. The incentive for such support is the potential for ocean energy to provide a new set of industries and jobs in early adopting countries as well as providing an alternate source of energy security and a diversification of the energy supply to reduce the intermittency associated with the dominance of a single type of renewable.
IEA, 2009. Ocean Energy: Global Technology Development Status. International Energy Agency Implementing Agreement on Ocean Energy Systems Annex I: Review, Exchange and Dissemination of Information on Ocean Energy Systems. IEA-OES document No.: T0104 available at: http://www.iea-oceans.org/ 
Jones, A.T., Finley, W., (2003). Recent Developments in Salinity Gradient Power. Proceedings of the Marine Technology Society OCEANS, 2003. Available at: http://www.waderllc.com/technical.htm 
SPP, 2004. The Salinity Power Project: Power Production from the Osmotic Pressure Difference between Fresh water and sea water. Available at: http://cordis.europa.eu/home_en.html 
OES-IA, 2009. International Energy Agency Implementing Agreement on Ocean Energy Systems Annual Report 2009. OES-IA document A09. Available at: http://www.iea-oceans.org/ 
Sanvik, S.O., Skihagen, S.E., (2008). Status of technologies for harnessing Salinity Power and the current Osmotic Power activities. Article to the 2008 annual report of the International Energy Agency Implementing Agreement on Ocean Energy Systems Annex I: Review, Exchange and Dissemination of Information on Ocean Energy Systems. Available at: http://www.iea-oceans.org/publications.asp?id=1 
IEA, 2007. Energy Technologies at the Cutting Edge. International Energy Technology Collaboration IEA Implementing Agreements 2007. Available at: http://www.iea-oceans.org/publications.asp?id=11 
IEA 2008. Energy Technology Perspectives 2008. Scenarios & Strategies to 2050. Paris, France
Sandvik, O.S., Hersleth, P., Seelos, K, (2009). Unleashing renewable energies from the ocean: Statkraft’s experience in developing business opportunities in immature technologies and markets - The forces of osmosis and tidal currents. HYDRO2009, October 2009. Available at: www.statkraft.com