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Techno-economic WEC system optimisation – Methodology applied to Wavebob system definition

Date: October 04, 2013 at 20:33 GMT

This paper considers the optimisation of WEC systems and focuses on WEC technology development rather than upon site or project development. Therefore, resource, environmental and operational conditions of a site or sites are regarded as a given and the system of WECs is required to satisfy and exploit such conditions in an economic and optimal defined manner. Site selection and development, where WEC technologies are regarded as given, are not within the scope of these considerations on technology optimisation. Notwithstanding the justified focus on technology development, it is recognised that both development processes are not decoupled. This is particularly evident for WEC technologies targeting particular site types with, for example, minimum water depth, or on the other hand, near- or onshore bottom mounting requirements.

Rightly, development protocols such as in [1], call for the WEC development process to be conducted in a well structured and sequential manner commencing with a focus on absorber concept viability and performance improvement. Over the development process, the systems are considered at increasing scale and with increasing detail. Technological development then follows the completion of absorber concept development and optimisation. Subsequently, with WEC technology design variations in hand, economic considerations receive increasing attention to achieve economic viability. These three broad development stages - targeting conceptual, technological and economic viability and performance - and the manner in which they build upon each other is briefly depicted in Fig. 1.

Over the years, a number of WEC technology developments have suffered considerable setbacks or failure, often associated with the omission of the required, important steps in these development stages.

Figure 1: Sequential system development approach.

Notwithstanding the recognised benefits of the sequential development, an undesirable phenomenon - not precluded by a sequential approach - is becoming
increasingly evident in WEC developments. This is associated with the belated consideration of technological and economic system performance criteria, resulting in setbacks with potential failure of achieving technological or economic viability. As concept and absorber developments advance and reach largely defined embodiments, the consideration of technological and economic criteria may be low and their viability thresholds may be difficult to achieve. Even the sequential optimisation of just two power conversion steps in the conversion chain of a WEC system, or the sequential consideration of absorber shape and power take-off (PTO) control of a WEC can lead to considerable penalty on overall system performance; this has been shown by Weber & Thomas, Babarit and Gilloteaux & Ringwood.

The overall techno-economic WEC optimisation needs to overcome such a sequential consideration of key technological subsystems and advance to a simultaneous consideration of all key performance features and an integrated optimisation of the WEC technology in their economic application. The persisting range and variation of WEC species under development is a testament of the challenge of satisfying the economic performance goals and of the requirement for such an integrated approach. 

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