According to the IEA, the topic ‘Ocean energy’ can be divided into the following sub-technologies (IEA, 2012):
• tidal barrage and tidal current energy
• wave energy (largest potential both in Europe and worldwide) • salinity gradient (osmotic) energy and
• temperature gradient (thermal) energy. Tidal barrage and tidal current energy
Tides are a result of the gravitational pull of both sun and moon on the world’s oceans. Depending on the relative positions of the sun, the moon and the earth, the sea level rises and falls twice a day, leading to both horizontal and vertical movements of the seawater, containing a large amount of kinetic and poten- tial energy. Different approaches for the generation of electricity are possible based on these two forms of energy (Frost & Sullivan, 2008).
To use the potential energy of the tidal range (difference in sea level between high and low tide), barrag- es can be built in estuaries or other suitable coastal sections with a large tidal range. With this method, a difference is created in the water level either side of the barrage. The water comes in through sluices with the high tide and is released back to the ocean in times of low tide (Department of Energy and Climate Change (DECC), 2010). As with conventional hydropower, the water drives a turbine to generate electricity (EOEA, 2010). One of the most famous examples for this kind of power generation is the La Rance barrage near Saint-Malo in northern France, which has been in operation since 1964. With a capacity of 240 MW it was the biggest tidal power plant until 2011, when the 254 MW Sihwa plant in South Korea was completed. There are smaller barrages (with capacities of less than 20 MW) in Canada, China and Russia (Daewoo E&C, 2009)
Compared to the construction of tidal barrages, using the kinetic energy of maritime currents is a newer form of power production (Blunden and Bahaj, 2006). The technology is comparable to wind power even though the construction has to be adapted to underwater conditions and the turbines are driven by water instead of air (DECC, 2010). According to Frost & Sullivan (2008), the most promising designs are reciprocating tidal stream devices, Venturi-effect devices and open hydro devices. The first commercial tidal current plant is named SeaGen and started electricity production in 2008 at Strangford Lough in Northern Ireland. It has a capacity of 1.2 MW and is able to supply electricity to 1,500 households (Sauter, 2011).
In general, using maritime currents is a very efficient way to generate electricity because of the specific properties of seawater: its density is more than 830 times higher than that of air, so that the energy of a 5 knot current is sufficient to deliver the same amount of energy as wind blowing with 350 km/h (Frost & Sullivan, 2010b). Moreover, unlike other renewable energy sources, tidal energy is very reliable and predictable far into the future, which has positive implications for the installation and maintenance of facilities. Due to its predictability, there are also fewer problems to integrating tidal-based energy pro- duction into an existing power grid (Denny, 2009; Frost & Sullivan, 2010b). Barriers to the implementa- tion of tidal power plants are the high installation costs and their potential environmental impact: Several billions of dollars are required for the construction of a barrage, and further research is needed into the effect of barrages on the habitats of fish and bird populations (DECC, 2010; The Australian Institute of Energy, 1999).
Wave energy
Waves are created as wind blows over the surface of the oceans due to differential heating of the earth by solar radiation (Thorpe, 1999). The energy content of the waves, which can reach 100 kW per metre of wave front, depends largely on the wind speed and the distance covered (Frost & Sullivan, 2010b). Existing systems to produce energy from waves can be divided into the three categories (Frost & Sullivan, 2012b):
• Onshore devices. For onshore power generation, overtopping devices can be used, which are ‘awashed’ by waves and hold back the water. When the water is released back to the sea, it turns the blades of a turbine (Frost & Sullivan, 2012b). A further approach to extract wave energy onshore is the use of oscillating water column (OWC) devices. Seawater flows into a chamber through a sub-surface opening. The vertical movement of the water column caused by the waves, forces trapped air to stream through an opening where it drives a turbine (Thorpe, 1999). • Near-shore devices. These devices are deployed in water depths of 20-25 metres, up to 500 me- tres away from the shore (EC, 2012c). Point absorbers consist of two connected floating ele- ments moved relative to each other by the waves. The kinetic energy of the relative movement is then converted to electricity (Frost & Sullivan, 2008).
• Offshore devices. These are positioned in depths greater than 25 metres with the advantage that more powerful waves can be used than by onshore or near shore devices (EC, 2012c). Examples for offshore devices are attenuators: long structures consisting of several connected floating el- ements which are arranged parallel to the direction of the waves. The relative movement be- tween the attenuators’ different sections is then transformed into electrical power (Minerals Management Service, 2012; The European Marine Energy Centre Ltd, 2012).
Compared to tidal energy the installed capacity of wave power is still at a low level. In 2000 a 500 kW wave power station commenced working on the Scottish island of Islay. Other wave power plants are located in Spain and Israel with capacities of 300 kW and 40 kW respectively (SDE, 2012; Voith, 2012). Salinity gradient (osmotic) energy
Osmosis is the natural phenomenon of water molecules moving between two solutions with differing salt concentrations through a semi-permeable membrane due to a difference in chemical potential (Post et al., 2007). While water is able to pass the membrane, the salt ions are held back. With water flowing from the solution of low salt concentration to the one with the higher salt concentration, the volume of the latter increases, leading to a corresponding pressure increase if the volume is constrained.19 In an osmot- ic power plant, freshwater and seawater are brought into contact through a semi-permeable membrane. The freshwater migrates through the membrane into the seawater where the pressure rises up to 27 bar (2.7 MPa). This pressurized water can be used for power production in a turbine (Aaberg, 2003; Skilha- gen et al., 2007).
Suitable locations for osmotic energy are places that can provide a sufficient reservoir of seawater and freshwater, e.g. near estuaries (Post et al., 2007). The first (experimental) osmotic power plant was started in Norway in 2009 (Statkraft, 2012).
Temperature gradient (thermal) energy
Using the ocean’s natural thermal gradient for the production of electrical energy is commonly known as ocean thermal energy conversion (OTEC). The thermal gradient in this case is the temperature difference between the warm surface of the ocean and the cold sub-surface water (Frost & Sullivan, 2008).
In a closed-cycle OTEC, a low boiling working fluid, such as ammonia, is evaporated by the warm surface water in a heat exchanger. The ammonia vapour drives a turbine and electricity is generated. The vapour is re-condensed using cold water which is pumped up from the deep in intake pipes (Fujita et al., 2012).
19 The reverse process (reverse osmosis) is used for seawater desalination by applying pressure to the high-salt side of the semi-permeable
An open-cycle OTEC uses the surface water itself as a working fluid. The warm water is evaporated in a vacuum and the steam drives a turbine before it is condensed back to liquid in a heat exchanger by cold deep-ocean water (DiChristina, 1995).
The highest efficiencies of OTECs are reached when the thermal difference between the warm and the cold waters is at least 20°C, which limits their deployment to tropical areas. Moreover, high installation costs - due to the large pipes required – make OTEC technology uncompetitive compared to other re- newable forms of energy production (Frost & Sullivan, 2008).