Multidisciplinary design for performance: Ocean wave energy conversion

Abstract

The multidisciplinary conversion of ocean wave power for a site off the Aust ra l i an East Coast is considered. An Osci l la t ing Water Column (OWC) capture device with a parabol ic collector wall converts incident wave power to pneumatic power while subsequent conversion to electricity is facili tated by a new air turbine-generator unit. A simple methodology is presented for estimating the air t u r b i n e ' s output and longer-term productivity, according to its sizing. Consequently, a methodology is defined which can be used in determining the optimal turbine diameter for a given wave climate and capture device performance. It is predicted that a 0.73m turbine on the 10m wide Australian plant will have a rated capacity of 300kW and will export 770MWh annually at an average rate of 86kW. KEYWORDSi multidisciplinary design; wave energy conversion; air turbine; OWC capture device; renewable energy I N T R O D U C T I O N The beginning to the new millennium has seen a sharp increase in oil prices due to renewed OPEC activity; nuclear power fall from favor due to cost and safety issues; and coal decline further through international commitment to tightening global climate policies. Consequently, the cultural and economic shift towards renewable power continues unabated. The world's ocean waves contain enormous amounts of renewable energy that is dissipated along certain lengths of coastline, although capture devices can now be used to harness this for electricity generation. The Australian wave energy conversion plant presented herein uses a collector wall to concentrate the energy into a semi-submerged air chamber that vents to atmosphere via an air turbine-generator unit (Count, 1980; Ambli et al, 1982; Salter, 1988; French, 1994, Falcao et al, 1994; Whittaker et al, 1997). Initially, the chamber produces an oscillating column of water (OWC) that forces a reciprocating air flow through the turbine-generator unit. The axialflow air turbine is used to create a driving torque from the aerodynamic lift force generated on the blades, pitching and working efficiently in 434 either flow regime. The turbine design is new and is being developed at the Aeronautical Department of Sydney University. Finally, the resultant shaft torque is delivered to a 3-phase induction machine that can act as a generator or motor, and which utilises Programmable Logic Controllers and power converters to provide smooth electrical power at a suitable voltage and frequency. A simple methodology is presented for estimating the Australian turbine's productivity according to its sizing; namely its diameter (Brito-e-Melo et al 1995; Koola et al, 1995; Curran et al, 1998a). Primarily, wave statistics were used in conjunction with the OWC capture device's hydraulic performance characteristics in order to estimate the range of pneumatic power delivered annually to the turbine. The device's energy collection is significantly enhanced by the innovative focusing wall that describes a parabolic shape around the OWC, in plan view, with the 40m wide open mouth being on the seaward side. The OWC is mounted vertically at the locus of the parabola so that it can readily absorb the reflected waves. Initial test results were correlated with previous OWC performance results in order to predict the characteristic variations in hydraulic efficiency due to the variation in wave height and zero crossing period (Mcllwaine, 1992). Subsequently, the pneumatic energy conversion and output was predicted with the inclusion of the new air turbine that supplies rotational power to the electrical generator. The non-dimensional turbine performance data was supplied by a numerical model that was then coupled to further code that allowed the geometry to be chosen to model the effect of turbine damping on the overall efficiency and output of the plant. In summery, the authors wish to describe the Australian wave energy plant and to present the analysis used to calculate its estimated performance. However, the paper also intends to present simple methodology that can be used practically in the sizing of air turbines for given wave climates and OWC output. Improved models for the conversion performance to pneumatic power can be incorporated whilst different turbine prediction models or turbine types may also be accommodated. Furthermore, it is noted that a statistical approach is limited and provides generalised results that do not fully address the complexities of such a multidisciplinary system. However, it constitutes a design framework for the initial sizing, of the turbine and can readily accommodate upgrading when more detailed models are developed and