WAVE, TIDAL AND OCEAN THERMAL ENERGY

GENERAL

Wave energy can be extracted from the vigorous surface disturbances initially stirred up in open water by large-scale weather systems, which in turn are set up by the Sun’s influence on the atmosphere. Tidal energy is recoverable from the mechanical energy of strong tidal flows created by the Moon’s gravitational effect on the oceans. Ocean thermal energy is obtained by installing systems to exploit the temperature differential between warm surface water and much cooler water from medium depth; once again this energy is ultimately derived from the Sun.

The total potential of such energy sources is hard to estimate, but globally must be very large indeed. The energy density available to water-driven systems of moderate scale is much higher than that of wind power systems of comparable dimensions. One estimate suggests that wave energy alone has the capability to provide 2,000 TWh/year, which should meet about 10% of global electricity requirements. Tidal sources are highly predictable, while wave energy and thermal energy conversion sites are only marginally less so.

The UK and Eire are particularly well placed to develop and introduce many such systems, since they possess many sites with excellent potential for wave or tidal energy although less so for thermal energy. Marine-based renewable energy systems are critically dependent upon choice of suitable locations - coasts exposed to significant wave action for much of the year, straits where tidal streams are especially strong, or areas where surface water subject to strong solar warming overlies much cooler water. Usable sites also need reasonable proximity to reliable markets, hence good connections to national and possibly international grids are essential. Local port facilities to facilitate construction and maintenance are also required or must be set up.

It is obvious that the design and installation requirements for such systems are very demanding. Power units must be robust and designed for high reliability to stand up to the harsh operational environment; thorough testing of pilot installations is essential to build up operating experience before full-scale deployment. Provision must be made for easy maintenance and repair when necessary, and design must allow for safety shutdown when required to protect damaged or failed units. Finally, a careful survey must be made of potential installation sites to check environmental issues such as shipping, local fisheries etc., and to ensure that operation over the likely range of operating conditions will be economic.

UK PROGRESS TO DATE (2008)

It has been estimated that marine-based renewable energy systems could produce at least 20% of the UK’s electricity requirements (measured at the 2006 level) and this estimate is probably conservative. The UK and Eire both have long coastlines exposed to the Atlantic and highly suitable for wave energy installations, such as Devon and Cornwall, Wales, Scotland and the west coast of Eire. Both countries also have many locations producing strong tidal streams with potential for power generation, eg. Severn Estuary, the Pentland Firth, sea lochs in Scotland and N. Ireland, Channel Islands, etc.

Thus far, the progress made by the embryonic UK industry has been encouraging. UK companies are considered among the leaders within Europe, which in turn has a strong position internationally, although competition is increasing rapidly. Government agencies, both in Whitehall and Edinburgh, clearly appreciate the potential offered by marine renewable energy systems. Both they and the EU have sponsored a number of studies, which in the main reported favourably, and some valuable financial support for development programmes has also been given.

Several test stations have been set up to permit full-scale trials in a marine environment, with grid connections. Pilot systems are not yet at the point of demonstration in fully commercial settings, but this stage should be achieved in 2009-10. Inevitably, there is a need for further research and development as testing programmes proceed, and the validity of operating cost estimates is not yet clear. The rate of progress will depend on the success of early commercial installations, but seems likely to accelerate.

WAVE ENERGY PROJECTS

Ocean Power Delivery completed testing a 750KW version of their Pelamis design at the European Marine Energy Centre, Orkney during 2007. Pelamis consists of a set of long cylindrical pontoons linked by short power sections to convert relative movement under wave action into hydraulic power. This power then generates electricity for transmission via submerged cable to the national grid. The company have just installed 3 machines off the coast of Portugal and there are plans to expand this installation to 12 machines. Another scheme using 5 machines is proposed for a site off Orkney.

The WaveGen LIMPET ‘water’s edge’ system is designed for installation in breakwaters and similar structures. Oscillating water levels within a closed chamber compress a trapped air volume, and the derived energy drives a turbine/generator set. One 400KW unit has been commissioned on the coast of Islay, Scotland and other units are planned for deployment in Germany.

Finavera Renewables has developed AquaBuOy technology, currently the most mature wave power system in the US. Vertical movement of submerged buoys drives pumps to produce hydraulic power; electrical generators then produce power for transmission to shore. This system is being used for a project in Makah Bay, Washington State. Further plans are in prospect for a 2MW project off the N. California coast and also a 20MW project in Western Cape, S. Africa.

Renewable Energy Holdings is developing the CETO submerged buoy system, which pumps seawater to shore for power generation and use in desalination. The moorings for CETO wave farm buoys would be set at 25m depths. The first CETO 2 prototype was demonstrated at the company’s Wave Energy Research Facility in Freemantle, Australia in February 2008; full-scale CETO 3 testing is scheduled for 2009.

TIDAL ENERGY PROJECTS

Marine Current Turbines successfully operated their first experimental SeaGen turbine off the N. Dorset coast in 2003-06, and subsequently installed a 1 MW turbine in Strangford Lough, N. Ireland in early 2008. This design uses a pair of 15-20m diameter twin-blade open rotors; variable pitch is employed to allow efficient operation during both flood and ebb tides, and to control system loads and output. The rotors are mounted on wing-like extensions each side of a monopile support structure and can be raised above the surface for maintenance procedures. Studies are in hand for a 10.5MW installation off Anglesey, and cooperative agreements have been signed with Canadian companies for projects employing SeaGen turbines in the Bay of Fundy, Nova Scotia and the Campbell River, off Vancouver. Both these locations are particularly suitable for large tidal power installations. A tidal farm in the Pentland Firth using somewhat similar technology is also under consideration by Tocardo Tidal Energy, a subsidiary of a Dutch company.

OpenHydro have developed their Open-Centre Turbine technology for location on the sea bed in areas which experience substantial tidal flow. The design employs a slow-moving multi-blade rotor in an annular duct, and claims simplicity, robustness and minimal effects on marine life. A 6m test installation is undergoing trials at the European Maritime Energy Centre, Orkney. A small array off Alderney is under development, with a potential for up to 3GW in a large tidal farm, and a demonstration project is also in hand for the Bay of Fundy.

Another design, the RoTech Tidal Turbine, is being developed by Lunar

Energy, initially at a pilot installation off the west coast of UK. This

scheme uses a 16m diameter 5-bladed turbine operating within a venturi

duct to accelerate local flow, reducing overall dimensions; design power

is 1.5MW. Sizeable installations (200-300 MW) are being considered in S.

Korea and New Zealand.

Pulse Tidal are experimenting with a radically different technology, utilising a pair of oscillating hydrofoils to extract power from the tidal flow; their phased motion is coupled to give rotational torque suitable for driving a generator. The system is capable of operating in relatively shallow water and is intended for smaller scale, near-shore installations. A 100KW pulse generator is about to undergo trials in the Humber estuary, the power produced being fed into the supply network of a large chemical works nearby.

OCEAN THERMAL ENERGY CONVERSION

This technique (the acronym OTEC is normally used) was under consideration in a US federal programme in the 1970s and the US Department of Energy then considered building a 40MW test plant off Hawaii. The concept was neglected during the 1980s due to a sharp fall in the price of oil, but rising fuel costs and concerns about energy security have now rekindled interest. The technique utilises warm surface water to gasify a low boiling-point fluid, which then drives a turbine. The exhaust gas is recycled in a closed system and condensed using much colder water pumped up from depth. A pipe some 1000m in length and handling flows up to 100 tons/second may be required; Lockheed Martin was recently given a contract to investigate relevant technology Proponents of OTEC believe that 500MW plants based on floating offshore platforms are entirely feasible. This system may have the potential to provide a large part of global fixed power needs, using a readily available energy source and easily accessible technology.