Energy from waves is derived from concentrated wind energy transferred to the water through friction and there are several techniques for extracting this. Tidal stream energy is a combination of the gravitational forces enacted on the earth by the sun and moon, causing flow of water from one coastline to another; these flows typically drive turbines or hydrofoils.

Various estimates have been made for the global resource that can be practically harnessed for ocean energy and for these two emerging technology areas this can be considered to be in the order of 2,200 – 4,200 terawatt hours per annum (TWh/a) or nearly 40 per cent of the current world electrical demand.

The UK coastal and offshore waters have an excellent capability for wave and tidal stream energy development. In terms of its share in European generation, the UK has about 35 per cent of wave and 50 per cent of the tidal resources of Europe. According to the Carbon Trust Marine Energy Challenge (MEC) estimates, wave and tidal stream technology have a combined potential to contribute about 20 per cent of the current electricity requirement of the UK, equating to 68 TWh/a. The MEC also estimates that 3 gigawatts of wave and tidal capacity could be installed in the UK by 2020, which would generate 8 TWh/a – equivalent to 2.1 per cent of the UK’s annual electricity supply.

There are many ocean energy technologies currently under development. Several of these technology companies are located and active in the UK, where there is growing interest in harnessing the resource. The following gives an overview of a small number of those devices that are closest to being manufactured as full scale commercial products.

Wave power

Leading wave power technology company Ocean Power Delivery (OPD) is the developer of the Pelamis, an attenuating wave energy converter. OPD has undertaken extensive sea trials with its 750 kilowatt (kW) device at the European Marine Energy Centre (EMEC) in the Orkney Islands.

OPD entered into a joint venture with Enersis to install three full scale devices off the coast of Portugal in what will be a 2.25 megawatt(MW) rated commercial wave farm – a world first. A second phase is planned where a further 30 units will be deployed, amounting to an additional installation of 20 MW.

Also leading the way in wave power is Wave Dragon. The Wave Dragon is a slack moored over-topping device that harnesses the energy in the water using conventional hydro turbines. Two large arms focus the waves toward the main body and into a reservoir that is above sea level. This ‘pressure head’ of water is passed through low head water turbines to generate power.

A demonstrator project is planned where a 7 MW device will be deployed off the coast of Wales. This particular device demonstrates very neatly the synergies between the ocean energy sector and the hydropower sector.

Tidal power

In tidal stream there are a number of devices that are at the full scale development stage. Of the developers, Marine Current Turbines (MCT) is leading the way with its ‘Marine Current Turbine’.

MCT deployed a 300 kW pile mounted single rotor system installed off the north coast of Devon in 2003. In May 2008, the company installed a new, full size ‘SeaGen’ 1 MW twin rotor pile mounted grid-connected device off the coast of Northern Ireland. The device first generated at its maximum capacity of 1.2 MW in December 2008 and an MCT spokesperson confirmed that it has since been running at its full rated power. The tidal stream design can utilise both ebb and flow direction and was voted one of the Top 10 Ideas to solve the climate crisis by UK newspaper, The Guardian in July 2009.

Lunar Energy, another UK-based company, is developing the Rotech Tidal Turbine in conjunction with Rotech. This tidal stream turbine is comprises a ducted rotor extracting energy that drives commercially available hydraulic pumps and motors coupled to a generator. All elements are housed within the sub sea unit. As with the MCT, the ducted rotor is bidirectional.

These devices can largely be described as being at the pre-commercial stage. The current cost of energy production from marine energy devices is predicted to be between 9 pence per kW hour (kW/h) and 44 pence per kW/h.

Conclusion

The mid to longer term outlook is positive, provided that reasonable learning rates can be achieved and accompanied by the well known benefits of the economies of scale. The case for this and other environmentally friendly, sustainable energy forms is made ever more robust with the continually rising cost of conventional fossil fuel-derived energy. Dr Drona Upadhyay and Jamie O’Nians are consultant engineers for IT Power, a renewable energy consultancy and technology innovator and incubator. IT Power successfully developed the Marine Current Turbine and is currently assisting the development of five ocean energy technologies.