Marine power

marine po er

The emerging potential of tidal wave energy

Tidal waves generate enormous amounts of power, but this potential is rarely harnessed. New research and technological innovations gives an optimistic outlook for this way of producing renewable energy. Although the absolute production capacity for Norway is limited, Norwegian offshore expertise can be utilized to play an important role in this emerging field.

When the wind energy over the sea is transformed to wave energy, the energy density is increased. Just below the ocean’s water surface, the wave energy flow, in timeaverage, is typically five times denser than the wind energy flow 20m above the sea surface, and 10 to 30 times denser than the solar energy flow.

Various technologies are utilised to convert the waves’ potential energy or forward kinetic energy into electricity. Devices include attenuators, pitching devices, oscillating wave surge converters, overtopping devices, point absorbers, submerged pressure differential devices, and oscillating water columns (OWC). The most common technology used is the oscillating water column, in which the wave, while cresting and toughing in a fixed housing chamber, displaces air at a regular pace. This air flow is directed to a turbine to generate electricity, in a manner quite similar to how wind turbines extract energy. A number of OWC devices have been constructed in countries including China, India, Japan, Norway, and the UK.

Some commercial actors estimate that wave power could produce between 200 and 300 GW, using the most promising sites worldwide. If more areas were exploited, this estimated potential could be even higher. And since the waves producing the highest levels of energy are found close to densely populated regions, including Western Europe, Canada and the USA, wave energy could be a key investigation area for development globally.

Along the Norwegian coast the level of wave energy transport is at 20-40 kW/m. The theoretical potential can be calculated to approximately 400 TWh/year. However, only a part of this potential energy will be feasible for harvesting and a realistic estimate is close to 8 TWh/ year.

Technology status

The technology for wave power plants is still in early stages of development, and further research initiatives are necessary. Pioneer work was carried out in the 80s, especially in Japan, Great Britain, Sweden and Norway. An overview of the Norwegian work from this time can be found in White Paper nr. 65 (1981-82) from the Norwegian Storting, and in the review paper; ‘Wave energy utilization: A review of the technologies’ from 2010 written by António and Falcão. Many different concepts for wave energy harvesting have been suggested. Over the last fifteen years there has been more commercial focus, and some of the largest energy companies in Europe have invested in wave energy projects. Many countries offer stimulating economic conditions, and more test facilities for wave energy have been established, for example at the Orkneys, in Great Britain and in Portugal. Wave farms with a total installed capacity of 170MW are actively under development in Scottish waters.

The British work has resulted in wave power technology like Pelamis which in 2004 was the world’s first

prototype of commercial scale to generate electricity to a national grid from offshore waves. The Pelamis device, being developed in Edinburgh and tested off Orkney, is like a sea-snake, using hydraulics, a large scale device optimised for off-shore installation where there’s more turbulence than near-shore, and waves come from different directions. In 2009 the second-generation Pelamis machine, P2, was launched. The P2 design machine has been sold to utility customers E.ON and Scottish Power Renewables and are currently tested for a number of commercial scale projects.

Following model tests in 2010, AWS Ocean Energy is working towards a full-scale 2.5 MW prototype of its Wave Energy Converter named AWS III. The device is based on air-turbines using floating platforms that convert wave motion into pneumatic power. A full scale demonstrator is planned to be tested in 2014, with full commercialisation to follow.

Aquamarine Power’s Oyster (part of SSE Ventures) wave power technology captures energy in nearshore waves and converts it into clean sustainable electricity. Essentially Oyster is a wave-powered pump which pushes high pressure water to drive an onshore hydroelectric turbine.

In Norway the Storm Buoy from the company Ocean Energy is a concept that addresses the challenges of extreme stress caused by offshore conditions that the machinery must handle by automatic submersion of the buoy floating on the surface. With collaboration with Seabased AB in Sweden a full scale pilot is to be tested on the Centre for Ocean Energy at Runde in the in Norway. Further testing in Gran Canaria is planned in collaboration with the Spanish research institute Plocan.

Challenges

Off the coast of Orkney the wave conditions can be extreme and the waves carry five times the energy density than wind. And add to this the corrosive salt water. It’s a stressful environment. Putting your wave power device in harm’s way carries a reasonably high chance of having it torn apart by the very waves it’s trying to exploit. The necessary capital investments are huge and the time to get to a commercial product is often longer than anticipated. The technology development faces significant struggles to find financial and political support with patience and long-term perspectives to see wave power devices through to commercialisation.

Some estimate that only 155 TWh/year of tidal energy is extractable worldwide. Limiting factors include financial considerations, the infeasibility of power stations in certain areas, electricity transmission issues and efficiency concerns.

From 1973 Kjell Budal established a group for wave energy research in the Department of Physics at NTNU, and SINTEF also got involved. The wave laboratories of NTNU came to good use in the testing of various models. Among the models tested were the concept from Kværner Brug, installed in Øygarden in 1985. Another test plant was built in the same place, based on the tapered channel concept, developed in cooperation with at predecessor of SINTEF.After the oil crisis in the 1970s, the energy prices fell and the funding for wave energy research were reduced. The Norwegian development stalled, but some activity has now been reestablished. After 2003 we have wave energy research at the NTNU-connected institutions CeSOS as well as in various departments connected to construction, power engineering, energy engineering and industrial economy.

Outlook

Renewables are expected to produce 30% of Europe’s electricity by 2020 according to EU targets. At the world’s scale, in 2050, 77% of the world’s energy supply could be covered by the renewable energies. In this context, the development of wave energy has a bright future. Over the last fifteen years there has been more commercial focus, and some of the largest energy companies in Europe have invested in wave energy projects. Many countries offer stimulating economic conditions, and more test facilities for wave energy have been established, for example at the Orkneys, in Great Britain and in Portugal.

Wave farms with a total installed capacity of 170MW are actively under development in Scottish waters.

Tidal energy The revolving of the earth, moon and sun creates tidal differences in the sea level. Tidal energy takes advantage of the energy that is created from the variation of the water level from ebb to flood, or from utilizing the fastflowing tidal streams that exist at some locations. The tidal barrages are dams that rely on the twice daily cycle of the tides. They impound water at high tide, and let the water ebb out through turbines at low tide. This type of ebb generation is generally reliable, but cannot occur continuously as power generation is limited for to less than 12 hours a day. In addition to harvesting energy from tidal barrages, we can also derive energy from tidal and marine currents. The tidal stream is driven by changes of the tides while the marine currents by the global oceanic circulation and seawater density variations. Current technologies to extract tidal stream power include horizontal and vertical axis turbines, venture devices, and oscillating hydrofoil devices.

Tidal power has an energy concentration of 500 to 1000 W/m2 on the Norwegian coast, and the Norwegian coastline has many narrow and deep fjords where the tidal streams are very strong and with higher energy density compared to i.e. wind streams. The technical viable energy potential for production of electricity from tidal streams in Norway is estimated to 2 TWh/ year, according to the Norwegian Directorate for Water and Energy. Thus, tidal power is not a substantial energy resource in Norway, but it can be viable in certain locations, generally in the northern parts of the country. However, the Norwegian competence in offshore installations is considered to be of great value in the development of new concepts for generation of energy from tidal currents.

Challenges

Commercialization of technology for tidal energy is still quite new, and the use of tidal energy for electricity generation is still small in a global perspective. The tidal streams are, however, more reliable and predictable than the wind, and the energy density is higher. The main technological challenges are connected to the harsh environment that sea water represents, e.g. installation of the turbines in tidal streams and fatigue load from waves .

Similar to the deployment of wave energy, tidal power projects are in need of large capital investments and renewable incentive mechanisms in order to be economically feasible. The technologies are immature and largely under development in relation to scale, LCOE, operation and lifetime.

A prototype of a tidal water turbine has been installed in Hammerfest in northern Norway by Hammerfest Strøm in 2003. This was the first tidal turbine utilizing the tidal current that connected to the electrical grid. The experience gained from this prototype lead to the the development of a new prototype which is 1 MW, This turbine has been tested in 2012 at the European marine energy center, EMEC located on the Orkney Islands.

NTNU and SINTEF were involved in the development of a tidal turbine in cooperation with Hammerfest Strøm from 1998 to 2008. The turbine blades was designed and model tests were carried out at NTNU in Trondheim. The prototype has been installed in Kvalsund outside Hammerfest, and has produced electricity since 2003.

Outlook

Renewed interest in tidal energy is being driven by the global interest for renewable energy. Tidal energy is an especially attractive option given its predictability, and impoundments are proven technology. Whereas freestanding underwater turbines designed to capture tidal currents are still being tested at the megawatt scale.

Capturing tidal energy using impoundments is facing increased interest in later years. In 2011 South Korean officials turned on a 254-MW ‘barrage’ style plant akin to La Rance in France, surpassing it by 14 MW. Further tidal installations are in development in South Korea and elsewhere, including the United Kingdom and

China.In Norway the difference between ebb and flood is not so large, but instead there is potential to utilize the kinetic energy in some tidal currents. The energy in such currents can be extracted in floating or bottomfixed turbines, which have much in common with wind

turbines.

Except for South Korea, no other government in the world is really pushing tidal energy which needs massive government support at this stage of development. Small scale pilot plants are being built here and there but they will take a lot of time to become commercial in scale. (The Massive Tidal Barrages being planned in Russia and Philippines will most probably never see the light of the day just like the Severn Tidal Barrage Project in England.)

Tidal wave energy is still a very nice technology with tidal barrages generating most of the electricity in a few power stations. Most of the tidal power plants using the modern tidal turbine technology are still in the pilot phase and generate relatively small amounts of energy. However, tidal power stations have the potential to generate large amounts of energy in a non-polluting way. Though tidal technology is still in the infant phase of development, a number of companies are engaged in research in tidal technology and a large number of tidal stations are being built in Europe and USA.

R&D recommendations:

• New concepts for increasing efficiencies and life expectancy of wavefarms under severe conditions.

• Development of new turbine designs that utilise the kinetic energy in tidal currents.