Design, simulation, and testing of a novel hydraulic power take-off system for the Pelamis wave energy converter

doi:10.1016/j.renene.2005.08.021

Renewable Energy

Volume 31, Issue 2, February 2006, Pages 271-283

https://doi.org/10.1016/j.renene.2005.08.021 Get rights and content

Abstract

The economic viability of a wave energy converter depends largely on its power take-off system. Active control of the power take-off is necessary to maximise power capture across a range of sea-states and can also improve survivability. The high force, low speed regime of wave energy conversion makes it a suitable application for high-pressure hydraulics.

This paper describes the hydraulic power take-off system employed in the Pelamis wave energy converter. The process of the system's development is presented, including simulation and laboratory tests at 1/7th and fullscale. Results of efficiency measurements are also presented.

Introduction

The extraction, by any wave energy converter (WEC), of useful energy from ocean waves requires that the waves apply force to some form of responsive mechanism able to resist the working force that the waves apply, and some form of reference against which that mechanism can react. The mechanism by which energy is transferred between the waves and the WEC, and subsequently or directly into useful form, is generally known as the power take-off (PTO). Control of the response of a WEC can be achieved through active control of the PTO.

Control of the PTO is central to the economic viability of wave energy; a major sustainable resource that has, as yet, remained untapped. It is important to show the potential of wave energy at an early stage by firstly, ensuring that machines can survive under arduous conditions, and secondly, by absorbing and converting a respectable amount of energy. These goals can, in large part, be achieved through control of the PTO.

To extract a maximum amount of power from the incident waves, the PTO should be capable of applying a restraining force varying appropriately with time as the WEC responds. This requires real-time measurement of response, and control of the PTO over the wave cycle. Moreover, to operate effectively across different sea-states, the control and PTO must be able to adapt such that power absorption is maximised in small seas and the risk of damage is minimised in large seas.

For maximum power absorption, a WEC should respond in resonance with the waves such that the exciting force and the response velocity are in phase [1]. While, with appropriate design, a WEC can possess inherent dynamics such that its natural response frequency is in the correct range to match the central excitation frequency of most sea-states, active control is necessary to maximise power capture across a range of sea-states and in irregular (realistic) seas where a range of component frequencies are present. Control can also play a vital role in improving survival characteristics although, whilst crucially important, this aspect is not generally expanded on in the existing literature.

Section snippets

Hydraulic PTO

Wave energy conversion can be considered a very suitable application for hydraulics. Waves apply large forces at slow speeds and hydraulic systems are suited to absorbing energy under this regime. Moreover, it is a simple matter to achieve short-term energy storage, necessary to achieve the smooth electricity production required for a marketable machine, with the use of cheap and available high-pressure gas accumulators.

Hydraulic systems have many favourable characteristics and many WEC

Pelamis PTO

The Pelamis [12], [13] is an offshore, floating, slack-moored wave energy converter consisting of a set of semi-submerged cylinders linked by hinged joints. Ocean waves perform work on the Pelamis by moving adjacent cylindrical sections relative to each other across two degree of freedom joints. The two axes that comprise each joint are inclined to the horizontal to allow a net inclined response to be induced by the PTO, which resists and reacts against the relative angular motion of the

PTO simulation

A suite of numerical simulation software has been developed by Ocean Power Delivery Ltd (OPD) to verify and develop the Pelamis design and allow prediction of annual average power capture at specific sites [14].

Simulations and tank models initially assumed linear mechanical impedances at each axis (i.e. a moment is applied to each axis proportional to angular position and velocity). Control of this type is easily quantified and is straightforward to model both mathematically and experimentally.

1/7th scale model

Early in the Pelamis development programme, a need was identified for an intermediate scale ‘systems’ demonstrator with which to develop and prove the full-scale Pelamis hydraulic, control and data acquisition systems. A scale of 1/7th was chosen as large enough for functionally realistic systems to be tested while remaining small enough to be tested in existing (large) wave tanks and avoid the need for specialist handling equipment.

The 1/7th scale joint systems were designed to be functionally

The 1/7th scale test rig

The 1/7th scale hydraulic system was developed and tested using a laboratory rig. The 1/7th scale PTO test rig, initially actuated by hand, was later adapted for actuation by a ball-screw operating under closed-loop control to perform the role of the waves. Pre-prototype hydraulic circuit and component test assemblies were designed and constructed for ad hoc experimentation (Fig. 4).

This work culminated in a design applied to build seven ‘power pack’ units, six for the joint axes of the 1/7th

Full-scale test rig

After making use of a laboratory test rig at 1/7th scale, it was decided to apply a similar approach to the full-scale PTO prior to deployment on a full-scale Pelamis. A single power module, representing one of the universal joints of the Pelamis, was constructed using the same component assemblies planned for use in the full-scale prototype Pelamis. A 1 MW rated hydraulic actuation system was mounted externally to perform the role of the waves (Fig. 8).

Initially, the control system of the

Simulation verification

The PTO simulation was verified using angle and control data from experiments conducted on both the 1/7th and full-scale test rigs. The simulated PTO system outputs the applied moment, flows, pressures etc. resulting from the time-series of angle and of control output recorded during the test.

Results from the computer simulation were compared with corresponding experimental results from the 1/7th scale and the full-scale test rigs. Fig. 12 shows chamber pressure time-series from the PTO

Conclusions

A novel hydraulic power take-off system (PTO), designed for use in the Pelamis WEC, has been demonstrated. First at 1/7th scale, then at full-scale. In each case, a laboratory test rig was used for experimentation and development. The aim of this staged development was to maximise learning and minimise risk. Each successive demonstration of the technology relied on knowledge gained and technology developed in the previous stage.

1.
The PTO allows an arbitrary moment to be applied at each joint axis

Acknowledgements

The work outlined in this paper forms part of an industrial programme of development carried out by Ocean Power Delivery Ltd. Edinburgh Designs Ltd created hardware and software infrastructure for this programme. Projects were supported by the UK Department of Trade and Industry.

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