Marine energy the power of the sea

The Pelamis WEC

The waves that crash onto a rocky seashore leave an indelible impression on the mind. The sheer power of the waves as they approach and then recede, one after another, is unforgettable and the action endless.

Wind has a large part to play in the formation of waves. The size of the waves depends on the strength of the wind, how long it blows, and the area over which it blows. Tides also play a big part: the gravitational effect of the sun and moon on Earth is the cause of the waters' rise and fall.

Given that 71% of Earth's surface is covered by water, it seems obvious that mankind should seize the opportunity to harness some of that power. It has been estimated that ocean waves could provide anything between 8000 and 80 000 TWh (terawatt hours) of energy per year, while the figure for marine currents is more than 800 TWh per year.

To put those figures into context: estimated global energy consumption in 2008 was 15 TWh (15 trillion watt hours). Given that an electric locomotive (train) engine produces just 5 or 6 MW, that's a lot of power.

Hard to harness

Harnessing the waves and tides as sources of renewable energy is harder than one might suppose. Scientists have been trying to think their way around the problems for the past 30 years or so. Significant research and development into wave energy conversion only began approximately 5 years ago. Investigation into ocean energy conversion is now being undertaken by several European Union countries, as well as Canada, China, India, Japan, Russia, and the USA.

The challenges are multiple. Installation of devices is demanding in lively seas. It is extremely expensive to develop and fund prototypes and first-generation devices. Then, other elements such as the deterioration of devices in seas that are perhaps deep as well as hostile have to be taken into account, as well as the effect of the devices on marine life.

For these, and other reasons, it has been difficult to establish the commercial and economic viability of devices. Little money has been committed by governments for development, though the picture is changing with investment in the United Kingdom (particularly in Scotland) and the USA.

Other sources of marine energy

The ocean can be used to create energy through other technologies too. One makes use of the considerable difference between the temperature of the water on the surface of an ocean compared to that in its depths. This temperature can be as great as 20 degrees Celsius, notably in coastal areas of equatorial regions. The amount of energy available in the temperature gradient between hot and cold seawater can be substantially larger than the energy that is required to pump the cold seawater up from the lower layers of the ocean. This is the area of standardisation work of IEC TC (Technical Committee) 4: Hydraulic turbines, since the electricity is generated using hydroelectric rotating machinery.

There are currently two predominant types of OTEC (Ocean Thermal Energy Conversion) technology. In a closed-cycle OTEC system, fluid with a low boiling point, such as ammonia, is used to rotate a low-pressure turbine. The warm surface water is pumped through a heat exchanger. As the other fluid boils, it expands as it evaporates and turns into steam; this steam turns a turbo-generator. Cold, deep-sea water is then used to condense the steam back into a liquid that is recycled through the system.

India has taken an active interest in OTEC technology and is building a closed-cycle, floating OTEC plant. Projects in the 100 MW (megawatt) range are under way in Hawaii and on a US Navy base in the Indian Ocean.

The other main type of OTEC system is open-cycle. In this type of desalination technology, warm seawater from the ocean's surface is placed in a low-pressure container, where it boils. The expanding steam drives a low-pressure turbine generator. Because all of the water's contaminants, including salt, are left in the low-pressure container, clean (drinking) water results. The water can also be used for air conditioning, chilled-soil agriculture, aquaculture, hydrogen production, and projects that are being developed in mineral extraction.

Another potential source of energy lies in harnessing the osmotic energy associated with the salinity gradient found typically at the mouths of rivers as they reach the sea. The energy can be 'trapped' using a process of pressure-retarded reverse osmosis and then converted. Alternatively, fresh water can be run up through a turbine that is immersed in seawater, or an electrochemical reaction can be exploited.

Technology, a changing picture

Melanie Nadeau, Chair of IEC TC114: Marine energy – Wave, tidal and other water current converters, says, 'Developers want to ensure that the technology is kept in place and that the data collected is safe.' She adds, 'There is not yet a fully-fledged marketplace. It is not commercial and there are not yet economies of scale.'

However, Nadeau is confident that the picture is changing. This is partly due to the security offered by the energy source – unlike fossil fuels, wave, and tidal power will not run out – and partly due to the increased recognition being afforded to renewable energy sources.

Nadeau adds, 'It's exciting to know that there is significant opportunity out there. It's certainly not a stagnant environment and there are a significant number of challenges. There is a desire to collaborate on standards, research and development, deployment and resource characteristics.' Despite her optimism, Nadeau recognises that the present economic situation is exceptionally difficult for investment.

Marine energy – a recent technical committee

Because of the relative immaturity of the marine energy sector, IEC TC114 is one of the IEC's newest technical committees. It was established only in 2007 and held its inaugural meeting just 18 months ago.

Nadeau says: 'The Standards are designed to be not overly prescriptive. They are all technical specifications because they will need to be reviewed, monitored, and changed. We started by identifying the key priorities to work on.'

She continues, 'We have produced the basis for five technical specifications. That's pretty impressive in just over a year. The work programme has ramped up. Now we will let the Standards develop. Meanwhile, we will work on the framework.'

Work in progress on wave, tidal, and other water current converters includes relevant terminology; design requirements for marine energy systems; wave and tidal energy resource characterisation and its assessment; evaluation of performance of wave energy converters in open sea and of tidal energy converters.

To a large extent, the countries that are most active in the development field are driven by circumstance. 'The United Kingdom has invested most in terms of wave and tidal current development,' says Nadeau. This is partly because of its need to diversify, since most of the country's energy is imported. In addition, its land mass is relatively small and it has excellent tide and wave resources.

Note: New Zealand is playing an active part in the work of TC 114 with Dr John Huckerby leading the working group, which is developing two Standards on Wave and tidal energy resource characterisation and assessment. There are six New Zealand members of TC 114 participating actively in these two Standards projects.

Wave energy devices – pumps linked to hydraulic motors

The Scottish government announced funding for the UK's first wave farm in February 2007. Four machines, manufactured by the Scottish company Pelamis, will generate 3 MW of energy. The government's investment is more than £4 million, though this does not represent the full cost of the project. UK energy supplier E.ON has also placed an order for a next generation Pelamis machine, which it expects to be operational in 2010 at the EMEC (European Marine Energy Centre) in Orkney.

The Pelamis machines are long snake-like semi-submerged devices that ride the waves. They consist of cylindrical sections linked by hinged joints. Hydraulic motors within these joints drive electrical generators to produce electricity. Power from all the joints is fed down a single umbilical cable to a junction on the seabed. Several devices can be connected together and linked to shore through a single seabed cable.

According to EMEC, there are six main types of wave energy converter that 'have been developed to extract energy from shoreline out to the deeper waters offshore' and to utilise the power provided by the effect of wind on the surface of the sea.

Tidal current generators – also for use in rivers

Canadian company New Energy Corporation has developed EnCurrent power generation turbine-based units with outputs ranging from 5 kW to 25 kW (kilowatt). These are tidal generators rather than the wave generator units produced by Pelamis. Currently, the New Energy units are primarily intended for use in rivers. The company says that the products are being scaled-up and that larger units, with outputs of 125 kW and 250 kW and more suitable for shore-based application, are expected to be launched next year.

EMEC says that there are currently four main types of tidal energy converter (horizontal axis, ducted horizontal axis, vertical axis, and hydrofoil) in existence and under development by a large number of different companies. The devices, which are 'broadly similar to submerged wind turbines', exploit 'the natural ebb and flow of coastal tidal waters'.

Enticing prospect

While hydroelectric power – which comes under the auspices of IEC TC4: Hydraulic turbines – has played a central part in many countries' power generation plans for the last half century, the 'restless sea' is still out there waiting to be tamed. It offers a tempting and attractive prospect for a number of companies. Melanie Nadeau, for one, believes that the energy is almost within our grasp, with large-scale manufacture of devices just 7-10 years off.

Reproduced from an article by Julia King in International Electrotechnical Commission (IEC) E-Tech November 2009

Published in international.