Sotenäs wave energy plant reaches grid connection milestone
Last December, the innovative Sotenäs Wave Energy Plant was successfully connected to the Swedish National Grid via a subsea cable. So, how exactly does the Sotenäs Plant work? And what plans are there for the future development of the device?
The Sotenäs Wave Energy Project is an ambitious initiative consisting of a total of 420 novel energy capturing buoys. When finally completed, the giant €25 million park will provide a total output of about 10 MW, installed over an area of some 0.8 km2 - making it the largest full-scale demonstration project of its kind anywhere in the world, as well as the first ever multiple unit wave power plant. In December last year, the project achieved a major milestone in the development of the first 1 MW of the Plant, when 120 tons of subsea generator switchgear was deployed in cooperation with Norwegian company Cecon Contracting and connected to the Swedish National Grid along a 10 km subsea cable. This was closely followed by the connection, in late January, of several six metre diameter buoys to corresponding linear generator Wave Energy Converters (WECs), signalling the final stage of bringing each unit on line.
The Sotenäs Wave Power Plant uses the motion of waves to directly drive a wave energy converter (WEC). It is made up of a unique design consisting of a direct coupled linear wave energy generator that is attached to the sea bed using a concrete gravity mounting plate and connected via a line to a buoy positioned on the surface of the sea. The theory is that the motion of the buoy perfectly matches the up and down movements of the waves - in the process, also enabling the WEC to generate electricity and eliminating the need for a complicated mechanical transmission system. The WECs themselves are connected to a series of marine substations, which are capable of transmitting an alternating current directly to the onshore grid (see box-out).
The Sotenäs project is jointly financed by the Swedish Energy Authority, alongside Swedish-based wave energy outfit Seabased and Finnish energy company Fortum, with research and development work carried out in close cooperation with academics at the Centre for Electric Renewable Energy Conversion at the Ångström Laboratory, Uppsala University.
As Sebastian Johansen, Senior Technology Expert at Fortum Corporation, explains, the idea for the Sotenäs wave energy device first arose when Fortum provided financing to the team at Uppsala University in Eastern Sweden, where the technology was originally developed. This exercise culminated in 2009, with the creation of the spin-off company Seabased - and the establishment of the ambitious Sotenäs project off the West coast of Sweden.
"A target was set to construct something very new and unique by building a demonstration project using Seabased's wave technology," says Johansen.
"The tailor made direct coupled linear generator stands on the seabed [and] the direct connection results in far less energy loss and avoids difficulties related to dynamic sea cables. Only a simple buoy is floating on the harsh sea surface capturing the energy in the waves," he adds.
In addition to the large scale project at Sotenäs, Johansen reveals that the device has also been deployed at a number of other locations. In particular, there have been 'multiple' pilot tests performed along the Swedish West coast by the Uppsala University in collaboration with Seabased - as well as one site located near to the Åland islands, a region of Finland at the entrance to the Gulf of Bothnia in the Baltic Sea.
Although the project is now well on its way to completion, Johansen admits that the team initially faced a number of technological and regulatory challenges, mostly relating to the mechanical and electrical design of the necessary subsea components - which he points out did not exist on the market when the scheme was first conceived - as well as the lack of an appropriate legislative and regulatory regime related to site permitting of such a 'completely new technology.'
"Most of these issues are overcome by now. It is still too early to make final conclusions because we need more operational experience. However, this technology has great potential and we hope the test period to be successful," he adds.
Looking ahead, Johansen says there are not yet any firm plans for the future commercial development of the Sotenäs plant. At present, the intention is to test the newly built plant for 'some time to come' in an effort to 'gain experience of issues related to operation and maintenance, including energy yield.'
"After the operational experience is gained, the next moves will be decided," he adds.
According to Seabased, although direct drive systems like these benefit from the fact that they possess a relatively low level of mechanical complexity, they also demand more resilient and efficient electrical components. In particular, although the electric current supplied by each Sotenäs device varies in its frequency and amplitude, it must still be converted to a range falling exactly between 50 and 60 Hz of alternating current (AC) before it can be transmitted to and used by the onshore grid.
To facilitate this conversion, the company has developed a proprietary marine electrical system made up of two different types of marine substations. The first is a Low Voltage Marine Substation (LVMS), which 'rectifies, inverts and transforms' the variable alternating current it receives from the Sotenäs units into a grid-compatible AC. The second is a Medium Voltage Marine Substation (MVMS), which is connected to groups of LVMS units in larger wave energy parks - and which further transforms the voltage so that the electricity can be transmitted over long distances to the onshore electrical grid.
By Andrew Williams
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