Tidal Stream Turbines - Are we there yet?

Talk by David Collier, MeyGen
Review of the Presentation of MEYGEN on their Pentland Firth Project

David Collier gave the meeting an overview of the MEYGEN Pentland Firth Project, the engineering challenges involved and the outstanding issues that remain to be resolved.

A basic tidal flow generator consists of a totally submerged turbine/generator installed in an area of high tidal flow. The energy that can be extracted from a tidal flow is proportional to the flow velocity cubed and the area swept by the blades, so effective tidal stream generation requires suitable marine sites with high tidal flows and with a depth of water that allows the use of large diameter turbines.
AK1000 Tidal Turbine
The UK has an estimated Tidal Stream resource of 18TWh/yr. Three quarters of this is around the Scottish coast. Whilst the sea bed is owned by the Crown Estate it has leased out a number of sites, amounting to an estimated 1.7GW, to companies planning to develop tidal flow generation stations.

MEYGEN has obtained a lease on the channel between the tip of Scotland and the Isle of Stroma lying in the Pentland Firth. The tidal flow through this 2 mile wide channel, between the Atlantic and the North Sea, is 4 to 5 meters/sec (10 to 11 knots) at peak spring tide. This is seen as a good site in a strategic location close to the Scottish mainland with an exceptional fast tidal current and with sufficient water depth (32-35 meters) to allow the use of 18 meter diameter turbines with a 8 meter clearance for costal navigation. The channel is free of silt due to the high tidal flow which allows direct access to the bedrock for turbine installation but makes laying cables more problematic.

MYGEN are planning to install multiple generator arrays within this channel over the period 2013 to 2019 cumulating in a total installed 398 MW capacity.This is intended to be phased as follows:
  • 20MW       2013/14
  • 66MW       2015
  • 78MW       2016
  • 78MW       2017
  • 78MW       2018
  • 78MW       2019
Engineering Challenges

Future AK-1000. Turbines

Optimising Generator Installation. It is planned to install the generators in arrays of 86 ( max) permanent magnet generators of 1MW each, however, there are issues with the optimum spacing of the generators in order to maximise the power output. To prevent interaction between the turbines and the tidal flow the distance between turbines needs to be maximised, whilst site constraints require they are spaced as closely as possible to maximise the number of generators possible.

Getting Power in to the Grid. There are a number of options available:
  • Daisy Chain the generators together and have a single or limited number of connections back to the on-shore sub-station as have been used with offshore wind
  • Connect each generator back to the shore via a dedicated cable.
Current thinking is to go for the latter option as:
the distance to the on-shore sub-station is low compared with that of the usual offshore wind farm and unlike off-shore wind generators, where connections can be made in the support structure above water level, tidal flow generator connections need to be made under water. Underwater connections pose reliability issues and if a daisy chain approach is used a failure in an underwater chain connection can in the worst case take down the whole chain.

Power Conditioning. The generator outputs vary widely, in both frequency and voltage, dependent on the flow rate of the tidal current at any particular time. Consequently the output needs to be conditioned before it can be fed into the grid. There are options to provide such conditioning either within the generator housing or back in a shore based sub-station. The current thinking is that it will be more cost effective and provide better reliability to centralise the conditioning on-shore. Lifting a failed generator is high cost as a specialised lifting vessel is required and the operation will be weather and vessel availability limited. The reliability of the generator needs to be maximising by minimising its complexity. Replacement of a shore based conditioner is much simpler and less costly

Cable Routing. It is intended to connect the generators back to the conditioning sub-station using standard sub-marine cable but there are considerations about holding the cables in place under the high tidal flow conditions. The bed rock is a relatively soft multi-strata sand stone which has been eroded into ridges and troughs by the tidal flow. As there is no silt in the channel in which to bury the cable consideration is being given to either drilling cable ducts through the rock from the shore to the generators or alternatively laying cables within the natural troughs in the bed rock. Current thinking is to use a combination of these techniques by drilling out well into the channel and picking up the troughs near the generators.

Turbine Support Structure and Deployment. The Turbine support structure and its fixings to the sea bed must be adequately robust to withstand the high tidal flow and three types of structure have been considered. These are:
    Riding the waves: TGL\'s 500kW prototype turbine is towed to the European Marine Energy Centre, off Orkney, with one of its three blades breaking the surface
  • Monopole cemented into a hole drilled into the sea bed
  • Tripod cemented into holes drilled for each foot in the sea bed
  • Gravitationally weighted tripod structure
The installation of the turbines and support structures is an expensive business requiring specialised offshore vessels capable of heavy lifting and pin point positioning even in adverse weather. Such vessels are used by the oil and gas industry and are normally operating far off shore in deep water during the summer months. The availability of such vessels for inshore use is limited and can cost £80K to 100K a day to hire. In order to minimise costs and make maximise the utilisation of such vessels the latter option is preferred as the vessels do not have to be on station for extended periods carrying out drilling operations.

Environmental Impact. The government needs a demonstration that there will be no deleterious effects on the environment. Initial studies on previous small trial installations are encouraging. An environmental impact study costing £1M plus is currently under way for this specific project.

Project Economics. The cost of installing generation capacity can be assessed in one of two ways: Cost per rated MW installed. Cost of energy generated over the life time of the installation - including installation, running and decommissioning costs. For new technologies this method is problematical as realistic operating and end of life costs are difficult to assess.

The current installation costs for various technologies are assessed as follows:
  • Tidal Flow -------- £5M - £6M/MW
  • On-shore Wind --- £2M/MW
  • Off-shore wind --- £3M-£4M/MW
  • Coal ------------- £1M/MW
For the emerging technologies of Tidal Flow and Wind the Government are providing support during the learning period by allowing electricity from these technologies to be sold at a premium. For Tidal Flow this premium is currently 6 times open market power price with wind at 2 times the open market power price .

The MEYGEN Project estimates that over the period 2015 to 2019 their installation costs will decline from £6M-£7M/MW installed to £4M- £5M/MW as the lessons of each installation phase are learnt.

Are We There Yet?

By way of summary David addressed this question as follows:
  • The theoretical issues have been identified.
  • All issues are believed to be surmountable but the issue of can it be done within an acceptable cost remains to be proved in practice.
  • The Grid in the north of Scotland will need to be upgraded and this is dependent on political will.
  • The Environmental issues are not seen as presenting major problems.
  • The current cost is high, but costs will come down as installation and operational issues are resolve, a growing understanding of how the turbines affect the tides is gained and the economies of scale can be reaped.
Questions and Answers

Following his talk there followed a lively Q&A session. Below are some of the questions that arose:-

Q1 Can the effects of turbine arrays on the tidal flow be modelled?
A1 The effect of turbine arrays is as yet not fully understood and the modelling is not yet proven

Q2 What effect will the unevenness of the sea bed have on the correct alignment of the generator support structures?
A2 It is acceptable for the turbine to be up to 5 degrees out of level. The support structure can be profiled to compensate for the sea bed profile where necessary.

Q3 Is there a problem with fouling or is the flow rate too great to promote growth of weed and marine animals?
A3 Fouling from weed and barnacles is an issue and can reduce the turbine efficiency considerably. It is planned to paint the turbine blades with anti-fouling paint.

Q4 Given then number of challenges involved will this ever be a cost effective technology?
A4 Turbine installation is feasible using high cost oil/gas vessels designed for use in deep water far off shore. In the long-term a custom designed vessel would be more cost effective for installation and maintenance purposes. The key driver to getting the operating costs down is to get the reliability of the offshore items up. The use of on-shore electrical conditioning is seen as major contribution to containing operational costs.

Q5 Could a similar design be used to generate electricity from river flow?
A5 To achieve effective power generation a large diameter turbine is required and a very significant water depth, not normally found in rivers and estuaries, is needed to accommodate this. Vertical axis turbines or run of the river turbines may be a solution to this constraint.

Q6 Power will only be produced around the times when the tide is at full flow. Where will the electricity come from during the periods around slack water/ high water?
A6 Tidal power is predictable unlike the wind and the level of base load can be planned accordingly. The tides progress around the coast and a number of geographically distributed tidal flow stations could provide cover for each other. There is also the possibility of Hydro Electric Generation could provide a base load.

MEYGEN Web Site - For anyone wanting to know more about this project or see some pictures may visit: www.meygen.com

Julian Todman