A powerful proposition13 April 2010
Jack Hardisty is the technical director of Neptune Renewable Energy in the UK. Here he explains why tidal stream power is a serious contender in the renewable energy field
When it comes to discussions on green energy it is all too easy to overlook tidal stream power when set against the high profile rollout of other renewable sources of energy such as wind, wave and solar. The reality, however, is that tidal stream, on a cost benefit analysis, stacks up extremely well against the alternatives and is increasingly being looked upon as a serious contender in the renewable energy field.
The fact that, to date, tidal stream has been largely untapped can be attributed to a number of factors, particularly the need for proof-of-concept devices to be developed and trialled, something which is now happening on the ground. A major advantage with tidal streams is their predictability, which means we can forecast output to a high degree of accuracy and for many years into the future. The capacity factor is therefore fully determinant. The utilisation of sheltered estuarine sites, which nevertheless evidence strong tidal streams, provides ease of maintenance compared to wind or wave with a significant reduction in O&M costs.
Thankfully, things are changing and, after considerable time and effort, tidal stream energy has now reached the stage where it can be offered at a commercially sustainable level. For our own part, at Neptune Renewable Energy Ltd (NREL), we have been developing a tidal stream power capability for the past five years. Building on an intensive research and development process, we have reached a major landmark with the announcement that we are in a position to deploy the first full-scale demonstrator of our vertical-axis Proteus tidal stream power plant in the Humber Estuary at Hull, England.
The Proteus, to be moored in the Humber for an initial three month trial, features a lightweight exoskeletal structure. It has been specifically designed for estuarine sites where there are not the problems, and stresses, associated with wave activity but which nonetheless exhibit powerful currents and have the advantage of lower access, cabling and maintenance costs compared with offshore environments. Looking at the potential to roll-out Proteus, we consider that there are at least 10 estuarine sites in Britain with peak spring currents of more than 2.5m/s-1 which could quite comfortably support this technology. There are, literally, hundreds of suitable sites worldwide.
As highlighted, the major benefit offered by tidal stream power is the delivery of a predictable source of renewable energy compared to more variable, less consistent, options such as wind. Certainly this reliability has to be seen as a key benefit when it comes to the supply of energy to individual sites and also to the national grid. Crucially, once moored in the free stream, the state-of-the-art Proteus is able to work equally well in ebb and flow currents.
Work on the £2M (US$3M) full-scale Proteus NP1000 demonstrator was undertaken over six-months from May 2009, with 25 shipyard workers involved in the construction phase. The completed unit is in the process of being transferred to Hull with commissioning anticipated shortly thereafter.
The patented NP1000 device, which once in place is scheduled to go through in-situ tests on the Humber, is the culmination of five years of intensive effort by the team at Neptune and our partners. Key landmarks in the development process over this time include: the design, construction and testing of 1/10 and 1/40 scale tank models. During the first-half of 2007, computational fluid dynamics (CFD), mathematical and physical modelling resulted in a number of design modifications. By 2008 we were ready to seek investment for the full-scale demonstrator. With sufficient funding in place, March 2009 saw us obtain Lloyds approval for the completed structural designs and the electronic and electrical designs were also finalised. The construction of the Proteus demonstrator itself began in the middle of last year with the finished device currently being moved to the Humber (February 2010).
Anatomy of a tidal power generator
Weighing more than 150 tonnes and stretching to around 20m long, with a beam of 14m, the full-scale demonstrator of the Proteus NP1000 consists of a steel hull, vertically mounted turbine, and buoyancy chambers. The vertical shaft within the Neptune Proteus connects to a 1:200 gearbox and generator in the deck housing. Associated valves and electrical and electronic processing and control for the Proteus are mounted onshore, connected by a floating cable-bridge.
Looking at the Proteus’ turbine in more detail, this is a 6mx6m vertical axis cross-flow design which is mounted within a patented symmetrical Venturi diffuser duct below the steel deck. A cross-flow turbine – such as that adopted in the Proteus – is a radial free stream turbine and due to its specific speed is classified as a slow-speed turbine. Practically, guide vanes impart a rectangular cross-section to the water jet which then flows through the blade ring of the cylindrical rotor, first from the outside inward and then – after passing through the inside of the rotor – from the inside outward.
Controlling the flow
The device’s rotor is maintained at optimal power outputs by sets of computer-controlled shutters within the duct and by the variable electrical load.
Focusing on the patented flow control shutters on the Proteus, these maximise the area of water hitting the turbines to increase torque and power output.
Crucially, the efficiency of all turbines depends upon the tip speed ratio (TSR), specifically the ratio of the tip speed to the fluid speed. In the case of the vertical axis, cross-flow rotors on Proteus, maximum efficiencies are achieved at TSR =0.35. The computer controlled shutters direct the flow at variable angles onto the blades in order to maintain this optimum TSR regardless of flow speed and thus maximise the electrical output.
Within the Proteus, the axle torque is taken by a right angle gearing to 2 x 250kW permanent magnet DC generators and then, via industrial regenerative drives and control systems, cabled ashore and grid connected.
Theoretical work and laboratory experiments, suggest that we will be able to realise an overall efficiency for the Proteus in situ of greater than 25% and, potentially, generate 30% more electricity compared to traditional hydro designs.
Extensive model tests at the University of Hull’s Total Environmental Simulator Research Facility have allowed the design to be continuously refined and optimised in a number of crucial ways to reach the stage of the Mark III design of the Proteus NP1000. The Mark I version was constructed at 1/10 scale and tested in the tidal River Hull. Detailed CFD analysis was then conducted to refine the rotor and duct design with the result that, for a Mark II version, the rotor was redesigned and duct angle reduced by seven degrees. Also, a single deflector plate was replaced by two sets of three vertical shutters to direct the flow close to the optimum angle of 16 degrees onto the rotor. Further enhancements introduced for the Mark III (patented) design of the Proteus NP1000 focused on the rotor, ducts and buoyancy chambers.
Cost engineered for estuarine energy production the Proteus offers considerable practical benefits including:
• Generating electricity at both capital and operating costs that are significantly less than other tidal stream power devices and comparable to onshore wind.
• Greater proximity of the generating capacity to the grid or distribution supply points.
• Ability to be moored in relatively sheltered locations meaning that waves are not impacting on the structure, with the potential for damage.
• Being close to land means simpler installation (including cabling) and maintenance.
Why the Humber?
The Humber Estuary for the first deployment of Proteus as its depth and tidal flow is considered one of the best locations in the British Isles for tidal stream power, whilst also being relatively close to our base in North Ferriby, East Yorkshire. The Humber has also been identified consistently as one of the key areas of concentrated tidal stream power potential in British waters, a case in point being the UK Atlas of Offshore Renewable Energy Resources (2004) published on behalf of the DTI (Department of Trade and Industry).
Another benefit of the Humber area was that we were able to call upon the expertise of local naval architects IMT Marine, engineers Water Hydraulics, Ormston Technology in Beverley and Dane Electrical Engineering in Scarborough.
Once deployed we anticipate that the advanced Neptune Proteus NP1000 will generate at least 1000MWh/yr - this is based on a peak spring tidal stream site of about 3m s-1.
Throughout the process, at a municipal level, Hull City Council has been extremely supportive of our efforts and this type of backing is a key consideration for any project of this type. The Council can clearly see the economic and practical benefits of Proteus and have already agreed that the output from the demonstrator will help to power The Deep Submarium (an aquarium tourist attraction in Hull).
Upon the completion of the demonstrator trials, the aim is to have the world’s first tidal stream power array, consisting of advanced Proteus designs, up and running close to The Deep in the Humber during 2011-12. When it comes to the environment, as the Proteus is a moored system, so the demonstrator will have a minimal impact when located in the Humber. Also, the bulk of its steel and composite construction can be recycled in the future.
Operational and cost benefits
At this stage we are not aware of any similar device which is designed to capture the shallow water resource at significantly lower capital and O&M costs. Certainly, a major innovation is the cross-sectional shape that has been adopted by our patented design. In the Proteus’ square turbine cross-section 30% more electricity per unit channel width is generated compared to circular turbines. Additionally the CFD tested and patented flow control shutters adopted, with their computer control, are able to increase the impacted length of the flow on the turbine to almost half of the circumference, thus increasing shaft torque and power outputs.
When it comes to operation and maintenance, all Neptune turbine components have been designed to be serviceable on site except for the lower bearing which can be serviced and replaced with local dry-docking. Crucially, the device can be towed to site using a small tug. Overall, we believe that our Proteus device delivers reduced capital and operation and maintenance costs and, as a moored system, its environmental footprint is low – making it an attractive option in particularly sensitive locations.
Facing the future
The future for Neptune Renewable Energy and Proteus is extremely bright, especially in light of the growing focus on renewable power generation in the wake of the Copenhagen summit. This technology can be successfully expanded and applied to other sites in the UK. It can also be looked at as a practical power proposition for locations worldwide.
For more information on Neptune Renewable Energy Ltd (NREL) please visit www.neptunerenewableenergy.com
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