In 1998 Queen's University Belfast in partnership with Wavegen Ireland Ltd., Charles Brand Ltd, Kirk McClure Morton and I.S.T. Portugal were commissioned to construct and test a 500kW shoreline wave power plant. The system known as LIMPET (Land Installed Marine Power Energy Transmitter) was installed on the Isle of Islay off the west coast of Scotland and was commissioned in November 2000. The plant has been operating remotely since that time and is supplying energy to the electrical grid in the United Kingdom. The successful unattended operation of the plant since commissioning has demonstrated the potential of shoreline wave energy for contributing towards national energy supplies.
The device comprises three water columns contained within concrete tubes each measuring internally 6m by 6m and inclined at 40° to the horizontal giving a total water surface area of 169m2. The upper part of the tubes are inter-connected and power conversion is via a single turbine generator unit connected to the central column. The water columns with an external width of 21m are located 17m inland from the natural shoreline in a man-made recess with a water depth of 6m at mean water level. The sides of the recess are virtually parallel and vertical.
The power take off system comprises a single 2.6m diameter counter-rotating Wells turbine in which each plane of blades is directly mounted on the shaft of a modified wound rotor induction generator rated at 250kW, giving an installed capacity of 500kW. The output from the generators is rectified and inverted prior to the grid connection and this enables variable speed operation with the range of 700 and 1500 r.p.m. The operational characteristic of the plant is software driven and can be altered. Noise produced by the airflow past the turbine rotors is attenuated in an acoustic chamber prior to discharge to the atmosphere. The turbine generator module also comprises a butterfly and a vane valve between the rotors and the plenum chamber.
The data acquisition system monitors all the main operational parameters throughout the power conversion process. In addition the incident wave energy has been monitored for a limited period using seabed pressure transducers, the wave loads on the front and back walls have been monitored and the water column movements have been measured using both pressure and ultra-sonic transducers.
The project has meet all the objectives originally specified and has been a significant achievement. A considerable amount has been learned about design, construction, power train matching, plant rating and costing. The operational experience gained will be vital to the future development of wave power systems both in the nearshore and offshore locations.
The most significant conclusions and observations are as follows:
- The project has demonstrated the physical practicability of building a shoreline wave energy device in the lee of a natural rock wall cofferdam formed by excavation.
- Subject to normal commissioning maintenance and minor problems, the collector and turbo-generation equipment have proven to be both robust and reliable.
- The “Harbour wall effect” which has been shown in model tests to be beneficial to the performance of near-shore OWC systems is not effective in the LIMPET shallow water gully where the effect of the gully is, contrary to expectation, reducing pneumatic power capture. The reasons for this are not well understood and merit further study.
- The control systems have operated well to allow safe automatic operation of the plant in all weather conditions.
- With the reduced pneumatic power collection the plant operates at around 20% of its installed capacity for a significant part of the year. A most important consideration for future designs is that power train efficiency is maximised at part load. In addition it is probably not cost effective to install M&E plant rated to accommodate the peaks in the pneumatic power delivery. This results in high capital expenditure which is under utilised, and poor efficiency at average power production due to energy overheads related to installed capacity. However, reduced installed capacity necessitates either a bypass or in-line valve to limit the pneumatic power reaching the turbine.
- The contra-rotating Wells turbine does not appear to offer a sufficient improvement in either peak efficiency or bandwidth performance relative to a bi-plane configuration to justify the additional cost of duplication of the mechanical / electrical components.
- The natural rock cofferdam gave less protection against the weather than had been originally anticipated. Whilst this did not prevent completion of the device the construction time and hence the cost of the device was more than planned and to permit commercialisation of the technology we need to drive costs down further. Measurements of structural load on the plant have shown that in future designs this can be accomplished by significantly reducing the concrete and steel in the structure and by making more use of mass-produced elements and novel construction techniques. This will permit a significant reduction of in-situ construction with the subsequent reduction in construction time and cost.
- The commercial exploitation of OWC’s will benefit from the development of a standard range of turbine generator sizes which can be installed either in parallel or even in series and the water columns would be sized to suite the M&E plant as well as the prevailing wave climate. These machines along with their air valves and their electrical control and monitoring systems should be tested in a purpose built facility in order to assure reliability of all components. Wavegen has initiated a design programme for such modular units.
Following on from this work new designs are being developed in combination with different construction methods and new materials. In future a simpler turbine generator module and control system is envisaged which in combination with the reduced structural content of the chambers will result in energy prices competitive with offshore wind systems.
The work completed has formed a vital step in the development of wave power technology and will result in the development of the next generation of oscillating water column systems as well as other types of device. LIMPET continues its grid connected operation on the island of Islay where it has become a major tourist attraction both enhancing the local economy and serving to demonstrate to the public at large the potential of wave energy generation and the role of the EU in supporting the development of renewable energy technologies.