OWC wave energy devices with air flow control

Abstract

A theoretical model is developed to simulate the energy conversion, from wave to turbine shaft, of an oscillating-water-column (OWC) plant equipped with a Wells air-turbine and with a valve (in series or in parallel with the turbine) for air-flow control. Numerical simulations show that the use of a control valve, by preventing or reducing the aerodynamic stall losses at the turbine rotor blades, may provide a way of substantially increasing the amount of energy produced by the plant, particularly at the higher incident wave power levels. From the hydrodynamic point of view, a by-pass valve or a throttle valve should be used depending on whether the wave energy absorbing system is over-damped or under-damped by the turbine.

Introduction

The oscillating-water-column (OWC) wave energy device is probably the most extensively studied type of wave power plant and one of the few to have reached the stage of full-sized prototype. The device consists essentially of a floating or (more usually) bottom-fixed structure, whose upper part forms an air chamber and whose immersed part is open to the action of the sea. The reciprocating flow of air displaced by the inside free surface motion drives an air turbine mounted on the top of the structure. A system of non-return valves may be used to rectify the flow in order to allow a conventional turbine to be used, as proposed by Moody and Elliot (1982). Alternatively the plant may be equipped with a self-rectifying turbine, which eliminates the need for the rectifying valve system. Several versions of self-rectifying air turbines for wave energy conversion have been proposed and studied, namely McCormick's counter-rotating turbine (Richards and Weiskopf, 1986; McCormick and Surko, 1989), the impulse turbine with self-pitch-controlled guide vanes (Setoguchi et al., 1991) and the Wells turbine (Raghunathan et al., 1985; Gato and Falcão, 1988; Raghunathan, 1995). The latter was adopted in most OWC wave pilot plants built so far, namely in Norway (Falnes, 1992), Japan (Ohno et al., 1993), India (Koola et al., 1993), UK (Whittaker et al., 1993) and Portugal (Falcão, 1998).

A general feature of turbomachines lies in their efficiency being strongly dependant on flow rate. Large drops in efficiency at far-from-design conditions are known to arise from flow separation when the angle of incidence of the flow approaching the rotating and/or stationary blades or vanes becomes excessive. One way of reducing the sensitivity of the efficiency to flow changes (at the expense of higher mechanical complexity and cost) is to employ variable geometry machines, as is the case of Kaplan water turbines and variable pitch propellers and wind turbines. Wind turbines, as well as turbomachines equipping wave energy devices, are especially subject to large variations in flow conditions. Variations associated with changes in wind or wave average conditions with time scales much larger than the response time of the machine itself can be matched by adjusting adequately the rotational speed as far as this is allowed for by the coupling to the electric generator. The inertia of the turbo-generator rotating elements makes it unfeasible to adjust the machine speed to faster oscillations of flow (those whose time scales are less than, say, 1 min); especially for turbomachines of fixed geometry, such flow oscillations may severely reduce their efficiency.

The efficiency of oscillating water column (OWC) wave energy devices equipped with Wells turbines is particularly affected by flow oscillations basically for two reasons. First, because of the intrinsically unsteady (reciprocating) flow of air displaced by the oscillating water free surface. Second, because increasing the air flow rate, above a limit depending on, and approximately proportional to, the rotational speed of the turbine, is known to give rise to a rapid drop in the aerodynamic efficiency and in the power output of the turbine. A method which has been proposed to partially circumvent this problem consists in controlling the pitch of the turbine rotor blades in order to prevent the instantaneous angle of incidence of the relative flow from exceeding the critical value above which severe stalling occurs at the rotor blades (see Gato and Falcão, 1991). Although considered technically feasible (Salter, 1993) this has never been implemented at full scale owing to mechanical difficulties.

Alternately, the flow rate through the turbine can be prevented from becoming excessive by equipping the device with air valves. Two different schemes can be envisaged. In the first one the valves are mounted between the chamber and the atmosphere in parallel with the turbine (by-pass or relief valves, on or near the roof of the air chamber structure) and are made to open (by active or passive control) in order to prevent the overpressure (or the underpressure) in the chamber to exceed a limit which is defined by the aerodynamic characteristics of the turbine at its instantaneous speed. In the second scheme a valve is mounted in series with the turbine in the duct connecting the chamber and the atmosphere. Excessive flow rate is prevented by partially closing the valve. In both schemes, the air flow through the turbine is controlled at the expense of energy dissipation at the valves. Theoretically the two methods, if properly implemented, are equivalent from the point of view of limiting the flow rate through the turbine. However, the resulting pressure changes in the chamber are different (reduction and increase in pressure oscillations in the first and second cases, respectively). Consequently the hydrodynamic process of energy extraction from the waves is differently modified by valve operation in the two control methods.

The main purpose of this work is to analyze theoretically the performance of an OWC wave energy device when valves are used to limit the flow through the turbine. Both schemes are considered and compared: a valve (or a set of valves) mounted in parallel with the turbine (by-pass or relief valve) or a valve mounted in the turbine duct. The hydrodynamic analysis is done in the time domain for regular as well as for irregular waves. The spring-like effect due to the compressibility of the air is taken into account and is discussed in some detail. Realistic characteristics are assumed for the turbine. Numerical results are presented for a simple two-dimensional chamber geometry for whose hydrodynamic coefficients analytical expressions are known as functions of wave frequency.