Marine Energy Systems – Which Is Best?

Marine Energy Systems – Which Is Best?

Marine energy is derived from the movement of the water and provides three primary types of energy, wave, current and tidal. It does not include offshore wind because the energy is not being derived from the movement of the ocean. There are three principal methods of capturing marine energy, wave, tidal and current.

Wave Systems – Captures the energy imparted into the water by the wind of sufficient strength builds up waves by the friction with the water surface over long distances. The energy can be captured and converted into electrical energy in several ways;

  • Surface systems that ‘ride’ the waves and move or oscillate with the wave period and converts the system’s kinetic energy.
  • Subsea systems that move or oscillate with the submersed energy and converts the system’s kinetic energy.
  • Above water systems that uses the air displaced by wave motion to drive an air turbine or overfill driving a low head submerged turbine.

 

Tidal Systems – Captures the energy from the changing height of tide and converts it into electricity. Tidal energy is often used to refer to the currents produced by tidal movement but these are more properly called current systems. Tidal energy converts the vertical differential in two principle ways;

  • Inflow and outflow to and from a man-made impounded area driving a turbine.
  • Inflow and outflow using the vertical tidal differential to drive a turbine.

 

Current Systems – Captures the energy of the mass movement of water resulting from tidal or other forces and converts that to electrical energy.

  • Current Turbines placed in high velocity current areas.
  • Sail or kite systems placed in low velocity current areas.

 

In the U.K. at present the primary systems being developed are surface wave, impounded waters and current turbines. Collectively there is about 4 MW of systems being tested, however there are no commercial marine energy arrays in operations, though several are planned. It is anticipated that by 2020 there will be about 300 MW of marine energy contributing to the national grid. The concepts being utilised in the marine energy sector are proven, so the developments here are not matters of proof of concept, but are rather developing the technologies to capture the maximum energy potential.

Wave Systems
In all of these cases the technology is in the conversion of wave energy into mechanical motion and then converting the resulting kinetic energy into electricity. The ideal sites for these systems are open ocean coastal waters where the long fetch (distance over which the wind can generate waves) and shelving seabed allows long period waves to build up. These have sufficient vertical difference between trough and peak and a long enough period, distance peak to peak, to give a good range over which motion can be obtained.

All these systems suffer from two significant challenges which must be overcome, the relatively low power generation capacity and the survivability in ocean storms. There is a phenomenal amount of power in ocean waves and unfortunately a force 12 Atlantic storm can generate waves with so much energy that wave power generators must be recovered to the shore or be battered to pieces. This necessity to protect and remove or recover systems depending on the weather places a high costs of maintenance and reduced the profitability of the device. The second issue with wave energy is that it is wind dependent. Just like wind turbines, no wind means no waves and no waves mean no power generation and no profits.

Tidal Systems
In general these system are the most attractive from the point of view of both technology and usability. The natural ebb and flow of the tide is used to fill a reservoir by forcing the flow through a restricted space, usually a pipe with a turbine in the middle. Alternately a tidal barrier across a tidal bay or river achieves the same effect. The inwards flow generates power while the reservoir is filling and as the tide falls the flow is reversed, emptying the reservoir and generating power on the way out. Construction is really a matter of civil engineering commonly used for harbours, moles and breakwaters, so no new inventions here, as the engineering from ports and harbours can be drawn on. The power generation process is not unlike hydro power so the turbine technology is quite well known. Generating capacity is consistent and calculable, making investment assessment more assured and therefore more attractive.

All that is left then is finding locations which are amenable to development from an environmental and social acceptance point of view. Some of the best locations are also areas of great ecological or aesthetic importance. This could be destroyed by such a development, so any proposal in these areas is likely to meet fierce opposition.

Current Systems
These systems have been troubled by problems which can be categorised into two major areas, blade technology and installation methodology. The blades in many of the earlier current turbine prototypes suffered under the strain of the forces imposed upon them, as well as possibly objects carried by the current. Unlike wind, where the most damage is likely to be done to flying birds rather than the blades, the marine environment has very large animals and can carry very large neutrally buoyant debris which can destroy blades. Blades therefore must be manufactured to withstand the torque applied as well as the most likely impact scenarios. Bladed current turbines also suffer from the limitation of physics, where the power capable of being derived from the marine turbine is in relation to the swept area of the blades. The more power desired the greater the swept area and the more robust the blades need to be. It is thought that this will reveal an upper limit beyond which blade technology cannot be developed.

Installation methodology poses an even bigger challenge. To date the turbines have been placed singly or in small groups on selected coastlines, and generally the most benign and accessible areas. However, to actively harness the highest current flow sites there will be installation challenges that will test the viability of placing a turbine in some locations. The seabed topography and geology will test engineering design to find a robust anchoring and framing system that can not only hold the turbine in place, but also not contribute too much drag or produce Vortex Induced Vibrations (VIV).

The tide is either diurnal or semidiurnal, which means it moves in and out once or twice a day, one high and one low water, or two highs and two lows a day. The period of the change of tide, or slack water happens only at full high water or full low water and lasts for a few tens of minutes before flow starts the other way. Including workable current time, this means there may be only an hour or so in each tidal cycle to actually work on the installation. This also applies to maintenance when deployment or recovery can only be done at slack water.

Sail and kite systems that enhance low current velocities are great ideas, but unfortunately that is where they may stay. The amount of power capable of being generated is limited and the system have yet to be proven from a survivability point of view. These may be in deeper water but the effects of wave energy cannot be underestimated. As an hydrographer in the oil and gas industry I have seen 30” steel pipelines in water depths of over 50 metres bodily lifted and moved 10-15 metres sideways by wave energy, even with concrete matting stabilisation. So these rather fragile kite and sail systems may not fare well in open ocean deployment, restricting their usability to benign coastlines.

In General the two systems most likely to be viable in the longer term are the impounded water tidal systems and the current turbine systems. From the investment point of view impounded water is probably the safest, but if the current system developers solve the reliability and installation challenges then they are the most likely to produce large quantities of power.

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Tidal Systems Collage:
Images clockwise from top left.
1. Aquamarine Power ( http://www.aquamarinepower.com )
2. Islay Energy Trust ( http://islayenergytrust.org.uk )
3. Open Hydro ( http://www.openhydro.com/home.html )
4. Scot renewables ( http://www.scotrenewables.com )
5. Wavegen ( http://owcwaveenergy.weebly.com/wavegen.html )
6. Minesto AB ( http://minesto.com/deep-green/ )
7. Swansea lagoon ( http://www.tidallagoonswanseabay.com )

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