"Since the generator is flooded, it is cooled naturally by the tidal flow passing over it. The nacelle has buoyancy chambers at the front and rear so that the turbine is neutrally buoyant," he says.
The turbine is held on station via connection to a tensioned mooring line, which is secured to the seabed and held in tension by either a surface or sub surface float. The turbine can be connected to the mooring riser at any point in the water column where the tidal flow has the highest velocity.
"This tensioned mooring approach enables the turbine to be deployed in any depth of water up to 500m deep," says Johnstone.
Funding from the WATERS2 programme will help to fund a commercial demonstration of the CoRMaT technology between now and early 2014, and enable it to be deployed at 'full commercial scale and generate electricity into the electricity supply network.'
"Following this, we would be looking to build this out in Scotland and elsewhere. Markets of interest include North America [and] Asia - and South America is beginning to emerge," says Johnstone.
Oceanflow's 'Evopod' tidal technology is a semi-submerged floating body that is tethered to the seabed by a multi-line spread mooring system. The surface piercing struts support a deeply submerged nacelle that houses the generator and power electronics in a watertight enclosure. As Graeme Mackie, managing director of Oceanflow, explains, the turbine itself is a horizontal axis 'unducted' turbine, similar to that used in wind turbines 'only of a smaller diameter for the same power output due to the much higher density of water compared to air.'
The slow rotating turbine is coupled to a step-up gearbox that drives an induction generator. The variable voltage and frequency AC output from the generator is conditioned to a constant 400V/50Hz output using Siemens power electronics configured by Optima Control Solutions. Power is 'stepped up' to 3,000V for export to shore.
"The unit weathervanes about its mooring position so that it always faces into the tidal stream. The streamline support struts cause minimal wake interference into the downstream turbine," says Mackie.
"Power is exported to the seabed through a Focal slip ring coupled to a flexible umbilical cable, similar to the technology used on oil and gas floating production platforms," he adds.
Oceanflow Energy has already built and tested a 1.5 metre diameter mono-turbine Evopod at a site in Strangford Narrows and, with Scottish Enterprise WATERS1 support, is currently building a larger 4.5m diameter mono-turbine unit, which will be deployed in Sanda Sound near the Mull of Kintyre and connected to the grid later this year. A twin turbine variant has been tested at model scale at Newcastle University and the WATERS2 funded project will involve deploying a version with twin 4.5 metre diameter turbines.
"The project will extend the knowledge gained through operation of the mono-turbine unit and will demonstrate the improved economics of mounting twin turbines off a single support platform," says Mackie.
According to Mackie, Oceanflow has taken a 'stages development' approach - moving up through the scale of the devices it has deployed as it builds up essential in-sea operating experience. In this way, he believes that a small company like Oceanflow can avoid expensive failures and achieve progressive test milestones without expending vast sums of shareholders funds.
The company’s strategy is to build up operating experience with its 35-kW mono-turbine and 70-kW twin turbine Evopod units, deploying multiple units to gain understanding of operating small arrays, before progressing to a full scale 2-MW (2 x 1-MW) twin turbine unit by 2015.
"The Sanda Sound development has demonstrated that such small units have application to community energy projects and provide invaluable knowledge of not just technical performance but also societal and environmental issues," says Mackie.
Scotrenewables Tidal Power
Scotrenewables has developed an innovative floating tidal energy converter, known as the Scotrenewables Tidal Turbine (SRTT). The SRTT is a floating tidal stream turbine featuring two contra-rotating rotors that extract the kinetic energy of the tidal flow, which is converted to electricity though a power take-off system. The main structure of the device is a cylindrical tube to which the horizontal axis rotors are attached via retractable legs. The retractable rotor legs enable the SRTT to operate in two separate configurations — operational mode, with the rotors down to generate power, or transport/survivability mode, with rotors retracted to decrease draught — allowing the device to be towed into harbour for maintenance or reduce loads in heavy seas.
The concept employs a catenary-type mooring system with a single mechanical and electrical connection point. Once the turbine is connected to the mooring system it is free to passively orientate itself into the tide.
"As a floating concept this mooring arrangement allows the turbine to respond to the dynamic changes in current and wave conditions, rather than having to withstand them as fixed structures do," says John McGlynn, Business Development Manager at Scotrenewables Tidal Power.
"A further advantage of the concept is that the floating design of the system allows for the rotors to be placed in the most energetic part of the water column – near the surface and away from seabed induced turbulence," he adds.
The concept has been trialled at increasing scales in both laboratory and open ocean environments since its inception in 2002 and most recently demonstrated at 250 kW ‘full scale’ at the European Marine Energy Centre (EMEC) in Orkney. McGlynn says that results from the 'SR250' test programme to date have been 'extremely encouraging' — with the turbine now undergoing longer term fully grid-connected tests — and have provided the company with the confidence to progress to demonstration of a ‘commercial scale’ turbine rated up to 2 MW.
"The 2-MW will be the first of a number of SRTT units installed in a commercial demonstrator array project of initial 10-MW capacity with eventual expansion to 30 MW," he adds.
The company's WATERS2 project will involve the design, construction and installation at EMEC of a commercial demonstrator SRTT rated up to 2 MW. The SR2000 turbine will be the final stage in the development of the concept prior to commercial rollout. The objective of the project is to build on the success of the SR250 project and to use the considerable learning and experience gained to deliver a commercially viable floating tidal turbine demonstrator suitable for array deployment. Following preliminary EMEC testing the SR2000 will form the basis for a 10MW demonstrator array in Scottish waters.
"We are confident that even in the early stages, the technology can deliver a lower cost of energy than competitors’ long-term projections. This is primarily due to the considerably lower installation and maintenance costs involved in the SRTP concept as well as low capital costs," says McGlynn.
According to McGlynn, the future prospects for the UK and global wave and tidal technologies sectors are very bright.
"The UK is the undisputed world leader in the development of marine energy technologies and will be the site of the first multi-MW commercial array projects. Tidal energy offers the potential to provide a significant proportion of energy demand both in the UK and further afield," he says.
"A generally accepted figure for the UK is from 5% to 6% — excluding tidal barrages. Other countries with real potential include France, Canada, United States, Korea, Japan, Chile, Argentina & New Zealand among many others," he adds.
Crucially, McGlynn stresses that tidal power offers the potential to reduce dependence on imported fossil fuels and thereby contribute to security of energy supply.
"This, combined with the absolute predictability of tidal power are really the key advantages. As well as being predictable, tidal power is also not as intermittent as other forms of renewable generation — a portfolio of sites located around the UK would deliver consistently into the grid," he says.
"The industry has experienced rapid and increasing levels of growth over the past two to three years to the point where we are now seeing the first array projects now take shape — we expect this activity to continue to intensify over the next two years," he adds.