Comparison of Two Alternative Hydraulic PTO Concepts for Wave Energy Conversion

Abstract

Hydraulic Power Take Off (PTO) systems for wave energy usually fall into two broad categories. These are, firstly, variable pressure systems where control of the primary force/torque is achieved by pressure modulation, and secondly, constant pressure systems where control of the primary force/torque is achieved by valve transitions that select between discrete effort levels determined by the approximately constant accumulator pressure and alternative piston areas. Energy storage is integral to the constant pressure category while, in the purest form of the variable pressure category, it is not provided. Hybrid systems which combine elements of both categories are also possible. This paper reports an analysis of the most elementary of systems from each of these categories. The analysis uses a coupled hydrodynamic-hydraulic time domain model. The model is used to assess the effectiveness of the hydrodynamic power absorption and the efficiency of the hydraulic power transmission. The results show that, in each case, the hydraulic motor performance is a critical consideration and the optimal configuration of any one system is dependent on motor selection. In the best instances of both categories of PTO, the indicated performance is sufficiently high to facilitate commercial viability of such systems.