Stuck on the Limpet

The Scottish Hebridean island of Islay, historically famous for its malt whiskies, is fast gaining a reputation as a centre for alternative energy. This derives largely from the pioneering work of a team from Queen’s University of Belfast, led by Professor Trevor Whittaker, who built a 75kW prototype oscillating water column (OWC) near Portnahaven on the Rhinns of Islay. From its completion in 1991 until its decommissioning earlier this year, the prototype produced a wealth of valuable information and has proved an ideal test bed for developing OWC technology.

While Queen’s University was installing and operating the Islay prototype Wavegen, a private company, was developing commercial OWCs designed to be freestanding in water depths of around 15m. As part of this work the company developed wave tank facilities to measure wave forces on the wave energy collector structure. It developed and manufactured the large Wells turbines invented by company founder Professor Alan Wells and won a contract to supply a turbine unit for a prototype wave energy converter being built on the island of Pico in the Azores (see above).

Wavegen and Queen’s University decided to combine their expertise to promote the construction of a large-scale demonstration unit called Limpet 500 (Land Installed Marine Powered Energy Transformer). Wavegen and Queen’s University also enlisted the help of civil engineering consultant Kirk, McClure & Morton and contractor Charles Brand, both of whom had worked on the Islay prototype. The project champions made a successful application for funding from the European Union’s Joule programme, which resulted in the EU contributing a little over 36% of the total costs. Construction began in early 1999 and the unit will be completed towards the end of the year.

Construction

The basic principle of the Limpet 500 is straightforward. The concrete structure of the wave energy collector is built to form an inclined tube with its opening approximately 2.5m below sea level. In section the collector comprises three 6m2 tubes with free passage between the tubes for the air at the upper end of the column prior to entry into the turbine duct. Changes in the external water level (1, diagram below), as a result of wave action, then cause variations in the internal water level (2) within the collector chamber (3). The changing water level inside the collector chamber alternately compresses and rarefies the air within the chamber causing an oscillating flow through the turbo/generation system. Air exiting the chamber passes through an isolation gate valve (4), which may be closed in emergency, and a radial vane valve (11) which may be used for either control or isolation.

The air flows into an annular passage formed between the exterior ducting (8) and the motor flywheel casing (5) before passing over a pair of monoplane Wells turbines (7). These turbines are self-rectifying, in that for an airflow substantially parallel to the axis of rotation of the turbine they are driven in the same direction irrespective of the direction of the air flow. Each of the two turbines is mounted on a common shaft of an induction generator (6) and flywheel (10). The flywheels provide short-term energy storage to maintain a substantially constant power output from the generator during individual wave cycles. On exiting the turbine, the process air is discharged to atmosphere via an expansion chamber with louvered exits (9). The flow is reversed when the internal water column motion causes rarefaction of the internal atmosphere. The control system monitors instantaneous power generation and energy storage and provides for an acceptable flow of power to the grid. To maintain turbine efficiency during periods of high wave activity and to limit peak internal pressure a series of ‘blow off’ valves (13) are provided in the roof of the collector.

The construction of the prototype reinforced the difficulties of building on an exposed coastline (and of course it is likely that the most favourable sites for wave energy will be exposed). To mitigate this exposure, a construction procedure has been developed whereby the site is excavated a little way back from the cliff edge, leaving a wall of rock between the site and the sea to protect the working area from all but the worst storms. The structure is then built in shelter before the wall, or bund, is removed to allow ingress of the sea. This protection reduces weather downtime and means that the Limpet can be built in a single season.

Flexible operation

In contrast to the Islay prototype, which was connected to the electricity supply grid directly on-line, the Limpet uses inverter drives to change mean turbine speed, so that the energy stored in the system inertia can be used for power smoothing, and to give power factor correction.

The use of inverter drives offers other advantages. In common with other turbine systems the power conversion efficiency of the Wells turbine varies depending on the rotational speed relative to the speed of the air passing through it. Since the air in the OWC is continuously cycling, the choice of operating speed is a compromise dependent upon the mean velocity of the oscillating air and for particular sea conditions there is an optimum operational speed. Sea conditions vary throughout the year and this affects power output. The average air velocity impinging on the turbine varies so that the desired operating speed also changes. The application of inverter drives to the generator controls gives this flexibility to vary speed with significant performance benefits.

There are a number of significant advantages to the Islay site, which make it attractive for wave power development. Most importantly there is a good wave climate. The power in a sea wave varies in proportion to the period of the wave and to the square of the wave height. As such it is found that there is far more power in the long ocean swells than in the locally driven wind seas.

On the UK mainland there are surprisingly few places where the Atlantic swell comes into contact with the shoreline. Ireland protects most of England and Wales while much of the western Scottish coast is protected by the Hebridean island chain. Cornwall and North Devon are directly exposed but the large tidal range as one moves towards the River Severn estuary creates a separate set of problems. Ocean swells can only really be accessed on one of the outer Scottish islands.There are many islands from which to choose. Islay was selected because it is easily accessible, has a significant electricity demand (currently imported from the mainland) and has a population which is both receptive to new ideas and enthusiastic about renewable energy in general and wave power in particular.

The particular site for Limpet 500 at Claddach Farm near Portnahaven faces southwest so that it looks directly into the predominant direction for incoming waves and receives an annual average wave power of nearly 20kW/m of wave front. There are few residents within a kilometre of the site and, as the land slopes down to towards the coast, they will be unable to see it. As such there is negligible visual impact. There is some noise generated by the turbo-generation equipment but again the contours of the land help to keep this to a minimum, and, as noise generation and background noise both increase with wave activity, site noise is well masked by the background.

Prime site

Experience with the prototype device suggests that at the Islay site in particular, noise will not be a problem. This may not be the case elsewhere, so noise control measures have been incorporated into the design and these will be tested as part of ongoing development. There are many other suitable sites for similar devices on the West Coast of Islay. The construction of the prototype and the Limpet 500 have provided significant local employment and all the signs are that further developments of wave energy technology on Islay would be welcomed.

One limitation to such further development is the current ability of the local electricity grid to accept power. The normal supply of electricity to Islay is from the mainland via the Island of Jura. The grid on Islay is generally weak, particularly at Portnahaven which is at the end of the line. This means that before significant generation can be connected to the system the grid must be stiffened by replacing all the current distribution cables with larger conductors. The cost of this is comparable to the cost of installing the wave energy generators. The problem is such that while the Limpet 500 project has been awarded a Scottish Renewables Order (a government initiative to promote economically attractive renewable power) to supply 500kW of power into the grid, the project has had to be phased in two stages.

The first phase, from commissioning later this year, will limit the output of the plant to 110kW, which is the maximum the unreinforced grid can accept. The second phase, whereby the full output of the plant will be delivered to the grid, will commence after grid stiffening. Wavegen is working with Scottish and Southern Energy to determine a resolution to this issue.

Commercial opportunities

While the current project focuses on electricity generation for grid supply, it is feasible that replacing the electricity generator with a water pump will produce high pressure water for direct use in reverse osmosis desalination plant. As in many parts of the world water is in greater demand than power, this may prove a larger market in the longer term.

Wavegen also sees a role for combined technology where the Limpet structures are built-in to harbour walls or coastal defences. The Islay project could demonstrate the viability of wave power generation and open the way for commercial development.