OCEAN ENERGY

1.            SCIENTIFIC AND TECHNICAL INFORMATION

1.1        Basic Principles of Marine Energy Technologies

The oceans, covering more than two-thirds of the Earth, represent an enormous energy resource containing vastly more energy than the human race could possibly use. In a four day period they absorb an amount of thermal and kinetic energy that is equivalent to all the world's known oil reserves. The energy is partly stored as kinetic energy from the motion of waves and currents and partly as thermal energy from the Sun. As the ultimate source of energy is the Sun, ocean energy systems are renewable, providing a relatively clean energy when compared to conventional sources such as coal, oil and natural gas. Though they have no fuel costs, they are under utilized mainly for two reasons: their diffuse nature requires a coincidence of favourable conditions for viable exploitation, whereas the high initial costs of plant construction are far too high, which makes it more expensive than conventional alternatives.

Although most marine energy is too diffuse and too far from land to be economically exploited, in special situations it can be effectively captured for practical use. At present the operation of ocean energy systems is mostly restricted to some experimental and demonstration units, but with technological development and a growing concern for global warming, ocean energy might experience a more promising and reassuring outlook for the future.

The main marine energy resources can be summarised, in order of maturity and use, as follows:

·         Tidal barrage energy

·         Wave energy

·         Tidal/marine currents

·         Ocean thermal energy conversion (OTEC)

·         Salinity gradient/osmotic energy

·         Marine biomass fuels.

The most promising technologies – though they are still in the early stages of their development - are ocean thermal energy conversion (OTEC) and wave power plants. Both of them could be deployed to produce electricity from the oceans' vast reserves of energy. Salinity gradients and the cultivation of marine biomass are not discussed herein because their exploitation seems a long way from any practical application, though new research might clarify their potential.

Tidal energy

Tidal energy, which is rooted in the revolution of the Earth and the gravitational forces of the Moon and the Sun, harnesses the potential energy inherent to flowing tidal waters. Tidal energy utilization requests extreme tidal ranges or currents – at least five-metre difference between high and low tides – in order to establish a power plant that is economically viable and which can produce a practical amount of energy. Actually the higher the tides, the more electricity can be generated from a given site. Even though the technology required for tidal power is well developed, ocean tides, which contain extremely large amounts of energy in coastal areas, have not gained a well-established standing. The main barriers to the increased use of tides are the prerequisite of extreme tide ranges, which restricts their use to specific locations, as well as the high construction costs, which makes it rather uncompetitive to conventional energy sources. With regard to these constraints estimates suggest that only 2%, i.e. 60 GW, of the potential 3000 GW of tidal energy is exploitable.

Wave energy

Winds blowing across the surface of oceans create waves that store a vast amount of energy. Ocean wave energy technologies applied to capture this vast amount of energy rely on the up-and-down motion of waves to generate electricity. It is estimated that the total amount of power – approximately 2-3 TW – that is accumulated by waves is twice as much as the world’s electricity production. Wave energy, as demonstrated by certain measurement data, varies substantially from region to region. Eventually there are three common features that are used to describe the flow of water: its height, its period and its length. According to these indicators energy potential in waves differs widely around the world. However, in spite of its variability and diffuse nature, in favourable locations the average energy density of waves comes up to around 65 MW per mile, which enables wave energy to be economically exploited.

Ocean thermal energy

Ocean thermal energy conversion (OTEC) exploits the difference in temperature between warm surface waters heated by the Sun and the colder water stratum of ocean depths. To exploit this thermal energy a temperature difference of 20°C or more between surface waters and depths water of up to 1000 m is required. Such requirements tend to limit the use of the resource to certain areas of coastline offering the coincidence of a suitably intense resource and a potential market for the energy. This makes many published estimates of the enormous global marine energy resources academic. However, when put into practice, OTEC power plants can be located either on-shore or at sea. 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. Shoreline OTEC could be more readily developed and applied economically than devices floating in deep waters. The latest view is that OTEC needs to be applied as a multi-purpose 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.