A Comparison of Performances of Turbines for Wave Power Conversion

A number of self-rectifying air turbines for wave power conversion have been proposed so far. This paper shows the comparison of the performances of all these turbines proposed for use in the near future. As a result, the impulse turbine with self-pitch-controlled guide vanes is found to have the best performance. Economic and environmental factors are creating ever greater pressures for the efficient generation, transmission and use of energy. Materials developments are crucial to progress in all these areas: to innovation in design; to extending lifetime and maintenance intervals; and to successful operation in more demanding environments. Drawing together the broad community with interests in these areas, Energy Materialsaddresses materials needs in future energy generation, transmission, utilisation, conservation and storage. The journal covers thermal generation and gas turbines; renewable power (wind, wave, tidal, hydro, solar and geothermal); fuel cells (low and high temperature); materials issues relevant to biomass and biotechnology; nuclear power generation (fission and fusion); hydrogen generation and storage in the context of the ‘hydrogen economy’; and the transmission and storage of the energy produced. The journal coverage of energy and policy, and broader social issues, since the political and legislative context influence and decisions.


INTRODUCTION
A Wells turbine is a self-rectifying air turbine which is expected to be widely used in wave energy devices with oscillating water-air-column. There are many reports which describe the performance of the Wells turbine both at starting and at running conditions (Inoue et al., 1988;Raghunathan and Tan, 1982).
According to these results, the Wells turbine has inherent disadvantages: lower efficiency, poorer starting and higher noise level in comparison with conventional turbines. In order to overcome these weak points, many kinds of turbines have been proposed (Takao and Setoguchi, 1996;Inoue et al., 1989;Setoguchi et al., 1990;1996). However, the comparison of characteristics of all these has not yet been made so far.
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The objective of this paper is to compare the performances of turbines, those that could be used for wave power conversion in the near future. The types of turbine included in the paper are as follows: (a) Wells turbine (Inoue et al., 1988); (b) Wells turbine with guide vanes (Takao and Setoguchi, 1996); (c) Turbine with self-pitch-controlled blades (Inoue et al., 1989); (d) Biplane Wells turbine with guide vanes (Setoguchi et al., 1990) and (e) Impulse turbine with self-pitch-controlled guide vanes (Setoguchi et al., 1996).
The present status of these turbines is as follows. The Wells turbine with guide vanes will be adopted for the project so-called "Mighty Whale" organized by JMSTEC, Japan (Miyazaki, 1993). The turbine with self-pitch-controlled blades is connected with the "Azores Pilot Plant" supported from the 130 T. SETOGUCHI et al. JOULE II (Falco et al., 1993), where the turbine has pitch-controlled blades. The project using the biplane Wells turbine is making progress in Islay, UK (Falco et al., 1993), where the guide vanes are not used for the turbine. The impulse turbine with self-pitch-controlled guide vanes will be constructed by NIOT, India (Santhakumar, 1996).

EXPERIMENTAL METHOD AND PROCEDURE
The test rig consists of a large piston-cylinder, a settling chamber and a 300-mm-dia. test section with a bellmouthed entry and a diffuser exit ( Fig. 1) (Inoue et al., 1988;1989;Takao and Setoguchi, 1996). The turbine rotor with v 0.7 was placed at the center of the test section and tested at a constant rotational speed under steady flow conditions. The test Reynolds number was about 1.5 105 based on the relative inlet velocity and chord length of the rotor blade at mean radius. The performance was evaluated by T, Q, and Ap. The uncertainties in torque coefficient CT and input coefficient CA are +2%, respectively.
The details of turbines adopted in the experiment are as follows: (a) Wells turbine (Fig. 2)  and A=-7.5. Note here that the geometries considered for these turbines are the ones found to be most promising in the previous studies (Inoue et al., 1988;1989;Takao and Setoguchi, 1996;Setoguchi et al., 1990;1996). Furthermore, all of them can start (Inoue et al., 1986) by themselves.
Hereafter we will call the turbines shown in Figs. 2-6 as turbines (a)-(e), respectively.
2CTO-ra +u Equation (1) can be solved numerically as an initial problem when CT and a wave motion are given.
This gives the starting characteristics of the turbine at the beginning and the running characteristics in an asymptotic condition. When the solution is in the asymptotic condition, the turbine performance can be obtained by mean efficiency defined by: In the calculation, the flow conditions are assumed to be quasi-steady. Furthermore it is assumed, for simplicity, that there is no delay in the behavior of rotor blades (Fig. 4) (Inoue et al., 1989) and guide vanes (Fig. 5) (Setoguchi et al., 1996), that is, they turn instantaneously to the required orientation corresponding to the flow direction. Figure 8 presents the comparison of mean efficiency for the five turbines noted above. The efficiency of the impulse turbine (turbine (e)) is higher over the wider range of in comparison with the Wells-type turbines. The calculated starting characteristics for the five turbines are shown in Fig. 9. The turbine (e) can start in a very short time. Moreover, the rotating speed while in operation is much smaller than those of others. Therefore, it may be possible to design an excellent turbine due to low relative speed, which is desirable from the viewpoints of noise reduction and mechanical advantage.

CONCLUDING REMARKS
The self-pitch-rectifying air turbines for wave power conversion proposed so far were tested from a viewpoint of usefulness. As a result, the impulse Economic and environmental factors are creating ever greater pressures for the efficient generation, transmission and use of energy. Materials developments are crucial to progress in all these areas: to innovation in design; to extending lifetime and maintenance intervals; and to successful operation in more demanding environments. Drawing together the broad community with interests in these areas, Energy Materials addresses materials needs in future energy generation, transmission, utilisation, conservation and storage. The journal covers thermal generation and gas turbines; renewable power (wind, wave, tidal, hydro, solar and geothermal); fuel cells (low and high temperature); materials issues relevant to biomass and biotechnology; nuclear power generation (fission and fusion); hydrogen generation and storage in the context of the 'hydrogen economy'; and the transmission and storage of the energy produced. As well as publishing high-quality peer-reviewed research, Energy Materials promotes discussion of issues common to all sectors, through commissioned reviews and commentaries. The journal includes coverage of energy economics and policy, and broader social issues, since the political and legislative context influence research and investment decisions.