Wave and Tidal Generation Devices: Reliability and availability
There are many wave and tidal devices under development but as yet very few are actually in revenue earning production. However the engineering problems are gradually being solved and there is an appetite to invest in these renewable generation technologies for harsher environments. To some extent the wave and tidal generation industry is following in the wake of the wind industry, particularly learning from the growing experience of offshore wind farm deployment. This book combines wind industry lessons with wave and tidal field knowledge to explore the main reliability and availability issues facing this growing industry. Topics covered include an overview of wave and tidal development; resource; reliability theory relevant to wave and tidal devices; reliability prediction method for wave and tidal devices; practical wave and tidal device reliability; effects of MEC device taxonomy on reliability; availability and its effect on the cost of marine energy; wave and tidal device layout and grid connection; design and testing for wave and tidal devices; operational experience and lessons learnt; monitoring and its effect on operations and maintenance; and overall conclusions. Wave and Tidal Generation Devices: Reliability and availability is essential reading for wave and tidal engineers and researchers and students of renewable energy.
Inspec keywords: tidal power stations; ocean waves; wave power generation; power generation reliability
Other keywords: marine energy; sea wave resource; tidal wave resource; reliability theory
Subjects: Wave power; Conference proceedings; Reliability; General electrical engineering topics; Tidal power stations and plants; Tidal and flow energy
- Book DOI: 10.1049/PBRN018E
- Chapter DOI: 10.1049/PBRN018E
- ISBN: 9781849197342
- e-ISBN: 9781849197359
- Page count: 300
- Format: PDF
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Front Matter
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1 Overview of wave and tidal development
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The challenge for the development of marine energy converters (MECs) is to achieve an investment cost and rate of return that will make them viable, compared to other sources of electrical power. While the energy resource is of reasonable density, it is intermittent and MECs do not operate at fully rated power most of the time, despite being exposed to the resource 24 h a day, 7 days a week. Before we can address, in detail, the challenge that this presents, we must first have an overview of the wave and tidal developments to date.
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2 Resource
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Power generation from the sea can exploit either sea waves or tidal flows. The main attraction of this form of generation is the combination of a substantial resource and relatively predictable output, although less so in the case of waves. Against this is the challenge of constructing reliable systems, which can generate economically in a hostile environment. The field is immature and, unlike wind generation, there is no obvious preferred solution.
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3 Reliability theory relevant to wave and tidal devices
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This paper sets out the reliability theory relevant for marine energy convertor (MEC); however, the amount of reliability data available for such devices is severely limited. The theory below is set out with this in mind so that MEC innovators, developers and operators can collect the appropriate data; see how reliability can influence their machines and based on the data make appropriate design, deployment and maintenance decisions. A 1-2 MW MEC is a complex, robotic, electro-mechanical generating machine mounted in or on a steel and concrete structure, with a steel or concrete foundation or moored to steel anchors or concrete foundations.
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4 Reliability prediction method for wave and tidal devices
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It is proposed that a reliability prediction model for prototype marine energy converters (MECs) could predict incident failures, analyse a system's future performance and compare different architectures, so that lower failure rate devices could be developed. Challenges in establishing such a method include the absence of historical data and the low level of deployment of wave and tidal stream generation technology in comparable locations. In speaking of MEC reliability concepts, it is useful to present the definition of probability-based reliability and reliability modelling. Based on MIL-STD721C, reliability in general is the probability that a part or system will perform its intended function for a specific time interval under stated conditions. It is measured quantitatively and consists of several reliability characteristics, the results being different for non-repairable and repairable systems/sub-systems/assemblies/ sub-assemblies/components. This paper sets out a system assessment method to quantitatively evaluate MEC technology reliability.
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5 Practical wave and tidal device reliability
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This chapter summarises the knowledge available on the failure rates of engineering sub-assemblies used in MECs. It demonstrates that there is a great deal of reliability information already available, some of it from a renewable energy environment similar to that into which MECs will be deployed.
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6 Effects of MEC device taxonomy on reliability
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The purpose of this chapter is to describe the taxonomy of a number of wave and tidal MEC configurations, so that the reader can understand the principal subassemblies involved and then start to consider how these sub-assemblies can be arranged to generate and transmit electrical energy reliably to shore. The intent will be to develop a standard way of presenting various devices, so that the reader could apply the same methodology to any prototype under consideration. This will then be used to apply some of the reliability modelling techniques.
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7 Availability and its effect on the cost of marine energy
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Very little can yet be known about the availability of MEC devices and farms; however, there are clear indicators from the on-shore and off-shore wind industries as to how availability is likely to develop. It is clear that MEC developers need to pay special attention to strategies to reduce installation costs and O&M costs. The next chapter will show that there are choices for MEC farm developers to do this, and these are possible because, unlike an off-shore WT, a MEC device could be moored on location and removed for maintenance. But these involve radical decisions during the development.
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8 Wave and tidal device layout and grid connection
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This chapter has shown that there is a balance to be made in device layout and grid connection for MECs that are fixed to the sea-bed and MECs that are detachable, floating and moored. In general, arrays with floating and moored devices will either contain fewer devices but be of larger geographical area. There will be an optimisation of the redundancy incorporated into arrays, but experience from off-shore wind farms suggests that the practicable degree of array redundancy will be modest. There is some interest in the development of DC networks, because of device architecture and possible savings in the transmission of energy from arrays far offshore. However, experience from off-shore wind farms suggests that long-distance DC connections will be many years away. There is insufficient installed experience or published information on off-shore MECs to draw specific conclusions about the best way to specify MEC layout and grid connection.
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9 Design and testing for wave and tidal devices
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The testing required to approve a marine energy converters (MEC) device for full service is extensive and could take over a decade, as demonstrated by the Pelamis wave energy converter (WEC) device. However, lessons are being learned and progress to reliable devices is accelerating. It is still important, however, to implement effective testing programmes, and this chapter describes design and testing methods that can be used to drive up the reliability of future MEC devices. Ultimately, it is this that will determine the success of MEC devices in capturing and delivering wave and tidal energy in the long term.
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10 Operational experience and lessons learnt
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The purpose of this paper is to present information on operational experience, of renewable marine energy converters (MECs). In reality, there is relatively little operational experience with wave energy converters (WECs) and tidal stream devices (TSDs), and no published material, since in general only prototypes have been exposed to operational conditions at test or demonstration sites. However, there is good experience of wind devices, both on-shore and off-shore, so this will form the basis of the analysis, with a focus on its applicability to MECs. Where information is available for WECs and TSDs, we will elaborate and consider its significance.
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11 Monitoring and its effect on O&M
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This chapter outlines the current knowledge in the field of CM for renewable generation systems, primarily based on the author and colleague's experience in the electrical machine and wind industry, and on Coronado et al. (2015). Recently published reliability studies are summarised, and the sub-assemblies of most concern for O&M are identified. This is followed by a description of the state-of-theart in CM, looking at both new and emerging techniques being researched, and industry-developed tools, with a particular focus on the economic benefits of CM. Conclusions are then drawn about current systems challenges and limitations.
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12 Overall conclusions
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The main learning from this book is that innovators, designers, developers, original equipment manufacturer (OEMs) and operators of marine energy converter (MEC) farms need to be trained in the predictive reliability estimation of MECs and be competent to judge between different designs, particularly in respect of their maintainability in the field. There must be an increase in training, to improve the testing of devices and prototype sub-assemblies.
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Appendix A: A tidal poem
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Reliability diagrams of MECs
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This book appendix presents reliability diagrams of six marine energy convertors (MECs); it includes wave energy convertors and tidal stream devices.
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MEC reliability data collection based on wind experience
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This section summarises the general principles and guidelines on which the taxonomy will be based and is derived from a deliverable prepared for the EU FP7 ReliaWind Consortium by the author and other Consortium members.
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PSD reliability of key sub-assemblies
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Table D.1 summarises the reliabilities for key MEC systems described earlier taken from the PSD sources set out in tables in Chapter 5, many of the sources above being collated in Delorm (2014), and these values are used in the MEC reliability analyses in the book.
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Back Matter
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