Marine energy has been dwarfed by the massive development of familiar technologies such as wind and solar – but new advances may see a renewed optimism in the sector as governments and investors seek to avoid financially and environmentally perilous hydrocarbon exploration and extraction.

The ocean is a vast source of potential energy in many different forms, and one that has remained largely untapped. It may be distinguished by three main categories, within which are further subcategories of energy conversion technologies. These include oceanic thermal; whereby energy is extracted by a temperature difference (a giant heat pump, in effect); chemical difference using salinity gradients; and marine hydrokinetic energy (MHK) using various forces to drive turbines or pumps. The last category has been subject to the majority of recent exploration, and is comprised of four main types; tidal range energy (using a barrage or lagoon), tidal stream (where water currents are driven around coastal land features), ocean stream (slower currents driven by tide or temperature differences) and wave energy (where ocean currents and wind exert forces effecting the sea surface layer).

Of these four, tidal range is the most mature, utilising similar technology to a hydroelectric dam. Tidal stream and wave technologies are the latest point of focus within the renewable energy spectrum, leading to the commercialisation of many designs such as Lockheed Martin’s AR1500 turbine, GE/Alstom Oceade buoyant design and the OpenHydro ducted tidal turbine – most of which are either currently or very near to going into continuous production for sale to the global market.

Lockheed Martin AR1500, DCNS France' OpenHydro, Minesto AB Deep Green bouyant tethered turbine

The potential for tidal stream energy is cited by many sources as being up to 1000 GW using current technologies, although new designs may increase this number. The location-specific requirements of tidal stream technologies are due to the fact that conventional turbines can only harness comparatively fast-flowing water – however; new turbine designs may increase this limited potential for tidal currents by utilising slower-moving volumes of water such as more ubiquitous ocean currents driven by other factors such as temperature or salinity.

A newly trialed and enthusiastically regarded technology which may open up this potential at low cost is the buoyant kite turbine design in development by Swedish firm Minesto. Going from extensive testing to the deployment of a full-scale array off the coast of Wales, this design may lead to a revolution in marine energy production. The buoyant kite turbine design works by moving in a continuous figure of eight while tethered to a fixed point, increasing the volume of water passing through the turbine by over 10 times the flow of a stationary object, and thus able to utilise ocean currents of 1-2.5 m/second, where conventional tidal turbines need a flow of above 2.5 m/second. Slower-moving ocean currents are more constant and available than tidal streams, thus posing a much greater overall potential. As well as this, the overall design is far less expensive than a fixed turbine, which needs a heavy base and may weigh up to 1000 tonnes.

Minesto AB bouyant wing turbine in 1/4 scale undergoing trials

As an indicator of the viability and efficacy of the newly patented Minesto design, similar kite designs are in development above the water, using the lateral forces exerted on the tethering mechanism to supply energy to a turbine based on the ground, and utilising the breezier conditions of high-altitude wind, which is more constant and reliable. Minesto should have a 10 MW array in operation in the near future, and we can only hope this is the beginning.

The final technology in development, and one with near limitless application, is wave energy. Two main designs are progressing through trials at the moment, and each is confident that low costs and a broad market for commercialisation will lead to success. The first device, which is currently in its 6th iteration (the CETO6) is being trialled at the Wave Hub test centre in Cornwall, and should begin deployment of a 30 MW array within two years. The 1 MW device is a buoyant structure tethered to a fixed point, which utilises the lateral forces exerted on it to create electricity. Carnegie Wave Resources Ltd. is in talks with manufacturers as whether commercial production can reduce overall costs to approximate that of offshore wind.

CG image of the CETO5 unit. Australian-listed Carnegie Wave Energy company has just launched its $90m (£60m) commercial wave energy project in Cornwall in the United Kingdom, after receiving $15.5m (£9.5m) from the European Regional Development Fund for the first phase.

A further wave energy device being trialled at the test centre near Wales is the WaveSub mounted float, which is touted as utilising the full potential of wave energy by a system of hydraulics that converts both horizontal and vertical wave energy forces. The design has the ability to sink below wave activity during a storm to avoid stresses to the overall structure, and ascend when favorable conditions resume. The design is said to be extremely low cost, requiring no foundations or tethers, and thus easily deployed and repaired.

The UK arguably has some of the most favorable tidal current availability in the world, and has the most advanced marine renewable testing facilities to utilise this resource, with a total of four fully-equipped test centers trialing designs from around the world. A wide range of countries are represented at the test sites, and much hope exists for a cost-effective method of utilising the abundant potential of marine energy.

The first large commercial project to obtain finance is the MeyGen array in Scotland, backed by a range of companies and using a variety of turbine designs. In total, 640 MW of tidal stream energy is slated for production by the early 2020s in the region.

The next site to deploy a tidal array is likely to be near the Island of Aldernay off the French coast, with a projected capacity of 300 MW, utilising 150 OpenHydro turbines at a cost of €600 million.

Two 100 MW arrays are in planning stages for grid connection off the north coast of Ireland, again by a consortium including Irish, Belgian and Scottish companies, and using a combination of turbine designs.

Atlantis has the largest portfolio of tidal power projects in the United Kingdom, representing a combined potential capacity of almost 700MW, in addition to its other development projects throughout China, Canada, India and a number of other parts of the world.

HS1000 Andritz Hydro Hammerfest

They have recently signed a memorandum of understanding in a joint venture to develop a 150 MW tidal stream site in Indonesia.

“The Indonesian archipelago of over 17,000 islands represents an extremely promising tidal stream resource. We are looking forward to working with SBS on this exciting project that offers the potential to provide highly predictable tidal stream power generation to the people of Indonesia.”