There are a number of classification schemes for ocean energy conversion technologies. A primary classification can be made based upon the basic energy resource being harnessed:
1. Potential and kinetic energy in waves and currents
2. Chemical potential of seawater (salinity gradients)
3. Heat potential of seawater (ocean heat and geothermal heat)
4. Biological potential of seawater
126.96.36.199 Wave, Tidal and Ocean Current Technologies
These technologies effectively utilize the potential energy (derived from wave height or tidal height differences) and kinetic energy (derived from water movement). Device technologies have four key features:
1. A stable platform or surface
2. A mobile working surface for the wave or current to work against
3. The mobile working surface must, at least partially, resist the wave or current action
4. The mobile working surface must be connected to some power take-off.
Classification of wave energy devices can be made by consideration of the following characteristics: principle of operation, device location and mode of operation (Fig. 26.7; Falcao 2009). Names in bold refer to specific examples of devices in each class.
Oscillating water column
(with air turbine)
Isolated: _Pico, LIMPET
In breakwater: Sakata, Mutriku
Floating: MightyWhale, Ocean Energy, Sperboy, Oceanlinx
(with hydraulic motor, hydraulic turbine, linear electrical generator)
Essentially translation (heave): AquaBuoy, IPS Buoy, F03, Wavebob, PowerBuoy
Essentially rotation: Pelamis, PS Frog, SEAREV
Essentially translation (heave): AWS
Rotation (bottom-hinged): WaveRoller, Oyster
(with low-head hydraulic turbine)
Shoreline (with concentration): TAPCHAN
In breakwater (without concentration): SSG
Floating structure (with concentration): Wave Dragon
Fig. 26.7 Classification of wave energy devices. (Falcao 2009)
There are abundant publications with pictures of wave, tidal and other water current devices, almost all of which are conceptual and a few undergoing full-scale open-ocean deployments. Rather than duplicate these papers with pictures of devices, the reader is directed to the publications of the Executive Committee of the Ocean Energy Systems Implementing Agreement (OES-IA) and particularly the 2008 Annual Report (Brito-Melo and Bhuyan 2009). Further information on the range of wave, tidal and water current technologies can be found in the "Marine and Hydrokinetic Technology Database" of the United States Department of Energy (US DoE 2008).
Seawater has a higher salinity than all river water debouching into oceans. The opportunity to use this chemical potential to generate electricity was recognized in the nineteenth century but commercial technologies are still some way off. Nonetheless any major river entering the sea offers the potential for future deployment of salinity gradient technologies. There are two ways to extract energy from the salinity differences between river water and seawater:
1. Osmosis—the process is called Pressure Retarded Osmosis (PRO)
2. Reversed Electro-Dialysis (RED)
PRO, sometimes called "osmotic power" exploits the chemical potential (i.e., salt concentration) between fresh water and seawater as pressure. Loeb developed
the concept in the 1970s (Loeb and Norman 1975). Seawater and fresh water are brought together across semi-permeable membranes. The resultant pressure is in the range 24-26 bar, depending on the salt concentration in the seawater (Fig. 26.8). Filtering of both the seawater and fresh water are critical, as impurities easily reduce the efficiency of membranes.
The world's first pilot plant for PRO became operational at Tofte, Oslo Fjord, in SW Norway in October 2009. The plant, built and operated by Statkraft, combines river water and water from the fjord to produce up to 4 kW of electricity.
Reverse electro-dialysis is a process, which utilizes chemical potential differences between two solutions, in this case seawater and fresh water brought into contact through an alternative series of anion and cation exchange membranes. The chemical potential generates a voltage over each membrane. This concept is being developed in a first prototype by Dutch researchers (Groeman and van den Ende 2007).
188.8.131.52 Heat Potential of Seawater
The heat potential of seawater was recognized in the 1970s and is available in two forms:
1. Ocean Thermal Energy Conversion (OTEC)
OTEC technologies were first developed in the United States in the 1970s but languished after the oil price rises in the 1980s. OTEC takes deep ocean water, which tends to be at a steady temperature of c. 4°C and combines it—in a heat exchange process—with shallow surface water. The key component of the technology is the 'cold water pipe', usually a large-diameter (>1 m) plastic pipe, extending down for 1 km up which the deep cold ocean water is brought to the surface. Once at the surface, an open- or closed-cycle heat exchange process extracts heat energy, using a secondary fluid, such as ammonia (with a low boiling point) as the exchange fluid and converts it into mechanical energy (Fig. 26.9).
Submarine geothermal energy could potentially be harnessed at those mid-ocean ridges, which are close to the surface and close to shore. Proposed technologies would be submarine heat exchange devices, which generate electricity on the seabed (Fig. 26.10; Hiriart 2008, personal communication). There are proposals to generate potentially drinking water on site to utilize its buoyancy relative to seawater to deliver the drinking water to a surface location.
Various attempts have been made to develop technologies to harvest biomass from the sea for the production of biogas and biofuels (Brehany 1983). In the 1970s research in the United States focused on the harvesting of kelp but this languished in the 1980s as oil prices declined. More recently interest has shifted to the potential for open-ocean harvesting of marine algae for biofuels. The marine algae would essentially be 'farmed' by the chemical fertilization to enhance marine algae growth and concentration. At present there are no technologies capable of concentrating dispersed marine algae from their very low natural levels in the open ocean.
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