WWW.Windfair.Net Special Online Editorial - A short look at Ocean Power II

As we move forward in the 21st centuary, nuclear catastrophies such as that in Fukushima are making Mankind look towards other alternative energy sources.

One thing is clear - in order to fill the gapping hole that a stop to nuclear power would leave, Mankind would have to resort to all sources of renewable energy - wind, solar and ocean-based energy resources.

In the longer term, ocean energy could become a much more important part of the world's energy portfolio. The potential worldwide wave energy contribution to the electricity market is estimated to be of the order of 1-10 TW, which is the same order of magnitude as world electrical energy production capacity. Wave energy has the highest density among all renewable energy sources The best resource is found between 40 - 60 degrees of latitude where the available resource is 30 to 70 kW/m with peaks to 100 kW/m. The supply potential is estimated to be 7 TWh/y from ~200000 MW installed wave and tidal energy power by 2050 with a load factor of 0.35 (DTI and Carbon Trust estimates).

To date wave and tidal energy are the most advanced types of ocean energy systems under development. More information on the different types of ocean energy systems and their current status of development will now be discussed.

WAVE ENERGY:
The most important objective for the wave energy sector is to deploy full size prototypes to prove performance at sea and to bring the technology to a point where it becomes comparible with other renewable energy technologies such as wind energy. This step is crucial in order to gain greater confidence in ocean energy as a reliable energy source.

Wave energy systems can be divided into 3 groups:
- Shoreline devices: are fixed to the or embedded in the shoreline, having the advantage of easier installation and maintenance. In addition shoreline devices do not require deep-water moorings or long lengths of underwater electrical cable. The disavantage shoreline devices experience is that they experience a much less powerful wave regime. The most advanced type of shoreline device is the oscillating water column (OWC).
- Near shore devices: are deployed at moderate water depths (~20-25), at distances up to ~500 m from the shore. They have nearly the same advantages as shoreline devices, being at the same time exposed to higher power levels. Several point absorber systems are near shore devices.
- Offshore devices: exploit the more powerful wave regimes available in deep water (> 25 m depth). More recent designs for offshore devices concentrate on small, modular devices, yielding high power output when deployed in arrays.

TIDAL ENERGY:
Tidal energy conversion techniques exploit the natural rise and fall of the level of the oceans and of the seas caused principally by the interaction of the gravitational fields in the Earth-Sun-Moon system. Some coastlines, particularly estuaries, accentuate this effect creating tidal ranges of up to ~17 m. The vertical water movements associated with the rise and fall of the body of water, and horizontal water motions (tidal currents), accompany the tides. These resources therefore have to be distinguished between tidal range energy (the potential energy from the difference in height between high and low tides), and tidal current energy (the horizontal movement, i.e. the kinetic energy of the water in a tidal current).

- Tidal Range Energy: Potential energy associated with tides can be harnessed by building barrages or other forms of engineering constructions across an estuary. Tidal barrages consist of a large, dam-like structure built across the mouth of a bay or an estuary in an area with a large tidal range. As the level of the water changes with the tides, a difference in height develops across the barrage. Water is allowed to flow through the barrage via turbines, which can provide power during the ebb tide (receding), flood tide (allowing water to fill the reservoir via sluice gates), or during both tides. This generation cycle means that, depending on the site, power can be delivered twice or four times per day on a highly predictable basis.

- Tidal Current Energy: Rather than using a dam structure, tidal current devices are placed directly “in-stream” and generate energy from the flow of the tidal current. There are a number of different technologies for extracting energy from tidal currents. Many are similar to those used for wind energy conversion, i.e. turbines of horizontal or vertical axis (“cross flow” turbine, as well as others such as, venturis and oscillating foils). Additionally, there are a variety of methods for fixing tidal current devices in place, including seabed anchoring, via a gravity base or driven piles, as well as floating or semi-floating platforms fixed to the sea-bottom via mooring lines.
In contrast to atmospheric airflows, the availability of tidal currents can be predicted very accurately, as their motion will be tuned with the local tidal conditions. Because the density of water is some 850 times higher than that of air, the power intensity in water currents is significantly higher than in airflows. Consequently, a water current turbine can be built considerably smaller than an equivalent powered wind turbine.

SALINITY GRADIENT:
At the mouth of rivers where fresh water mixes with salt water, energy associated with the salinity gradient can be harnessed using pressure-retarded reverse osmosis process and associated conversion technologies. Another system is based on using freshwater upwelling through a turbine immersed in seawater, and one involving electrochemical reactions is also in development.

Several methods have been proposed to extract this power. Among them are the difference in vapor pressure above freshwater and saline water and the difference in swelling between fresh and saline waters by organic polymers. However, the most promising method is the use of semi-permeable membranes. The energy can then be extracted as pressurized brackish water by pressure retarded osmosis (PRO) or direct electrical current by reverse electrodialysis (RED).

OCEAN (MARINE) CURRENT:
Currents are not generated by tides only, but also by winds, and temperature and salinity differences. The concept for harvesting the kinetic energy from marine, also known as ocean currents, is essentially the same as with tidal currents. Marine current energy converters are based on the same principle of the tidal current ones. At present, marine current energy is at an early stage of development, different pilot plants are in operation or about to be installed. Most devices rely on the horizontal or vertical axis turbine concept. . There are no commercial grid-connected turbines currently operating:

OCEAN THERMAL ENERGY CONVERSION:
The principle of ocean thermal energy conversion (OTEC), consisting in using the heat stored in the oceans to generate electricity, originated with a French physicist, Jacques D'Arsonval, in 1881. Due to solar heating, the top layer of the water is much warmer than deep ocean water. Where the temperature difference between the warmer, top layer of the ocean and the colder, deep ocean water is about 20°C (36°F), the conditions for OTEC are most favourable. These conditions exist mainly in coastal areas located close to the Equator.

There are potentially three basic types of OTEC power plants: closed-cycle, open-cycle, and various blendings of the two. All three types can be built on land, on offshore platforms fixed to the seafloor, on floating platforms anchored to the seafloor, or on ships that move from place to place.

Offshore OTEC is technically difficult because of the need to pipe large volumes of water from the seabed to a floating system, the huge areas of heat exchanger needed, and the difficulty of transmitting power from a device floating in deep water to the shore. The latest thinking is that OTEC needs to be applied as a multipurpose technology: for example, the nutrient-rich cold water drawn from the deep ocean has been found to be valuable for fish farming. In addition, the cold water can be used directly for cooling applications in the tropics such as air conditioning. If OTEC takes off, it is likely to be with energy as a by-product.

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