A european wave energy pilot plant on Islay (U.K.)

Bangladesh has one of the highest population densities and the lowest per capita energy consumption of the world. Energy resources are very limited, the main natural resource being gas. The coastline is open to the main wave source from the Bay of Bengal and the generation of electricity from such waves could be an important source for Bangladesh and in addition could ease the problems of wave action on the coastline. Wave heights have been recorded at Patenga (Chittagong) by a wave rider buoy and related to wind date. Maximum wave heights of over 2 m, with a maximum of 2.4 m were recorded. The Ministry of Shipping in Bangladesh has recently started recording wave heights, wave periods, wave direction and so on at the mouths of 2 of the main river systems. Automatic hydromagnetic current meters and wave recorders, type-S4 have been installed at 2 stations to take the readings of current and waves. Possible power output at the Pussur river has been estimated. It is observed that the output power varies from 26 kW/m to 98 kW/m. There are many possible wave power plant sites at the south coastal belt of Bangladesh with a good wave climate. Among them the south west of Cox's Bazar, Moheskhali island, Kutubdia island, Sandwhip island and the Hiron point are very prominent sites. It would be necessary to operate a wave power station in a stand alone mode. While not offering a continuous supply of electricity they could provide a vast improvement on what exists at present. Wave power plants at the suggested sites, would reduce damage on the coastlines and could create better employment opportunities through the installation of new industries and the production of more crops.

The Islay wave power system consists of a 3-stage energy conversion cycle. The stages are: oscillating water column (hydraulic to pneumatic), Wells turbine (pneumatic to mechanical), induction generator (mechanical to electrical). Each medium containing energy can be modelled in general terms as a source generating effort, e(t) which causes a flow, f(t) into a connected system. In this notation e(t).f(t) gives the instantaneous power transfer from the source to the system. For each medium variables can be assigned to effort and flow. This common approach to the modelling of each medium allows analogous models to be drawn up. Transformation of a network of interconnected systems consisting of different media to a single equivalent model simplifies analysis and enhances understanding of the behaviour of the whole network. This approach is particularly useful for the analysis of system dynamic behaviour. The concept is applied to an onshore wave energy extraction device. The model as presented possesses many drawbacks, particularly in the incorporation of several nonlinear elements which defy analytical solution. These nonlinear representations are unfortunately unavoidable in any treatment of this system. Thus the concept is, in this regard, at no greater disadvantage than other methods. The method does, however, possess the important characteristic that it allows the various energy conversion elements to be represented according to a small set of parameters all sharing a common base. This allows the interactions between the various stages to be analysis systematically while contributing to the understanding of the performance of the system as a whole.

The prototype wave power station located on the Isle of Islay was commissioned in May 1991. It is of the oscillating water column type driving a 1.2 m diameter Wells turbine direct coupled to a 75 kW wound rotor induction generator. The plant is connected to the main electrical grid on the island which is in turn connected to the Scottish grid. Although it is the fourth of its generic type to be built in the World to date, there has been very little cross fertilization of technological detail between the teams. This has resulted in a relatively slow rate of development with similar mistakes being made by each group. One of the most common problems has been the over estimation of the rating of the generation plant and a poor appreciation of the air flow regime which the turbine has to accommodate. This is particularly important in the design of the Wells turbine as the incidence angle of the air hitting the blades must be kept within a 15 degree bandwidth to avoid stall and maintain performance. Most of the turbines were designed on the basis of sinusoidal air flow but in practice the situation in the Islay prototype is very different. As a consequence, the turbine on the Islay plant returns an average aerodynamic efficiency of power conversion of around 50% instead of the predicted figure of 70%. The shortfall in performance is not considered to be the result of any deficiency in the fundamental principle of operation of the turbine but can be attributed to insufficient attention to engineering detail. As the primary function of the plant is as a marine test bed, these problems are being addressed and a better understanding of the operation of the turbine is being gained continually.

The principles of operation and factors controlling the performance of an impulse turbine with self pitch controlled guide vanes for wave energy conversion have been investigated. As a result, set of design factors for the impulse turbine have been chosen. Experimental data from small scale tests indicate that this turbine is superior to Wells turbine in overall characteristics. The recommendable geometrical parameters for the rotor blade are: elliptic profile with a blade space to chord length of rotor ratio of 0.5; width of flow path to blade space ratio of 0.4; blade inlet angle of 60 degrees and sweep angle of -7.5 degrees. For the guide vane (monovane type) the parameters are: a guide vane space to chord length of guide vane ratio of less than 0.65; setting angle of upstream guide vane of 15 degrees and setting angle of downstream guide vane of 72.5 degrees. For the splitter type the parameters are: a guide vane space to guide vane length ratio of about 0.8; setting angle of upstream guide vane of 15 degrees and downstream angle of 55 degrees; a slot width to chord length of guide vane ratio of 0.1.

Exploitable results

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