Chapter 1: Introduction
1.3 Device development protocol
The staged development protocol is widely regarded as the ideal roadmap for advancing WEC technologies from an initial concept to commercial installation. Several different guidelines
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exist, but they all share the same core principals; that is, to follow a prescribed sequence of stages, which consider various levels of technical complexity and investment requirements. The recommendation is to advance through this stages sequentially in order to gain the required understanding of the device characteristics while minimising the project’s risk. One such example is Figure 1:2: IEA-OES 5 stage structured development plan (Figure 1:2) to progress a wave energy converter (WEC) from an idea to a marketable product. European and American developers are urged to adhere to this plan while developing marine energy technologies. The structured, phased programme is influenced by similar Technical Readiness approach utilised by the (US) National Aeronautics & Space Administration (NASA) Technical Readiness approach. In the case of ocean energy devices the stages can conveniently be linked to different device scales by following Froude Similitude Laws and geometric similarity (Holmes & Nielsen, 2010).Figure 1:2: IEA-OES 5 stage structured development plan (source Cahill 2014)
In the wave energy protocol each stage includes defined Technology Readiness Levels (TRL) .A description of the different stages according to (Holmes & Nielsen, 2010) is given for clarity.
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The concept validation stage (TRL 1-3) entails testing a small scale (~ 1:50) model in a set of
monochromatic, regular waves (of a single frequency) followed by polychromatic (many frequencies combined), irregular sea states. Regular wave tests enable more control to the researcher to identify and describe physical processes at specific frequencies. Regular waves allow the research to investigate parameters such as the radiated wave height, which as we shall discuss later, is intrinsically linked to power capture, such that the device geometry can be optimised. Irregular sea states provide more realistic conditions in order to estimate potential device performance in conditions more closely resembling real seaways. Hull seaworthiness and mooring suitability can also be established at this early stage.
The design validation stage (TRL 4) involves using a larger, more sophisticated model (~ 1:10)
and consequently often requires a larger testing facility capable of producing the necessary wave input conditions. Testing at this stage usually covers a more extensive number of sea states, including realistic survival conditions. During this phase, engineering is introduced in the form of a preliminary design and an elementary costing of the system components is established. Based on the measured power absorption in a range of sea states, the annual energy production is estimated using a set of generic wave conditions. More sophisticated numerically modelling techniques such as Computational Fluid Dynamics (CFD) & Finite Element Methods (FEM) are also recommend at this stage. Additionally porotype design feasibility and cost estimates should be considered.
The systems validation stage includes the testing of all sub-systems incorporating a fully
operational PTO that enables demonstration of the energy conversion process from wave to wire. If the cost is acceptable, Stage 3 is entered in more detail with the aim to test the complete wave energy converter at a selected sub-prototype size (circa 1:4) that can safely be deployed at sea and produce power. The device is still small enough to facilitate easier handling and operation but large enough to experience deployment, recovery and maintenance techniques at sea. The first involvement with licences, permissions, certification and environmental requirements will be encountered. Also, design teams will experience manufacturing and production and supply chain issues, though the device may not be grid connected. Productivity remains a key stage gate requirement in these tests
The device validation stage is a critical part of the process and covers a solo machine pilot
plant validation at sea in a scale approaching the final full size (circa 1:1). This stage is a proving programme of designs already established rather than actually experimenting with new options. Tests can be initially conducted at a moderate sea state site prior to extended
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proving at an exposed ocean location. This is a very exacting requirement however, since it involves all components from each sub systems conversion process. The device as a whole must be proven fit for purpose before this stage is concluded. The device must also be grid connected before the end of the proving trials. Heavy engineering operations at sea are involved so health and safety requirements become important, as do O&M of the plant under realistic conditions. Since only a single unit is involved environmental impact will be minimal but monitoring of the machines presence in a given location must be undertaken.The economics validation stage involves multiple device testing, initially in small arrays (circa
3-5 machines) which can be expanded as appropriate. By the conclusion of the previous sea trials, the technology and engineering of a device should be well established and proven. The technical risk of Stage 5 should, therefore, be minimized. However, the consequence of failure would be significant and the financial risks are less certain since it is the economic potential of the devices deployed as a generating wave park that are under investigation. Initially the hydrodynamic interactions of the devices will be investigated, together with the combined electricity supply stability possible via the power electronics. Availability and service scenarios will be important issues as more machines are deployed as will onshore and offshore O&M requirements. Environmental aspects, both physical and biological, can now be studied in detail as well as the socioeconomic effect the wave park will have on the local area. Early stakeholder involvement is recommended.
A wide variety of device concepts exist (as we shall see in section 1.6) and a set of procedures to allow fair comparison between them is described in the EquiMar project (Ingram et al (2011)). The project is a vast inter-disciplinary project that lays out protocols that cover site selection, device engineering design, scaling up designs, deployment of arrays, environmental impact on flora, fauna & landforms, as well as economic issues. The project emphasis the need for standardised methods of resource assessment, tank testing and all to way up to full sea trials, so that devices can be compared in an equitable way.
Weber et al (2013) discuss the importance of technology performance level (TPLs) depicted in Figure 1:3. TPLs are an assessment of all cost and performance drivers grouped into high level categories; power conversion efficiency, availability, capital expenditure (CAPEX) and lifecycle operational expenditure (OPEX). In Figure 1:3, the lower x-axis plots the technology readiness level, the upper x axis shows the increasing costs associated with advancing through the TRL levels. The left y axis gives TPLs and right hand y axis cost of energy (COE). A
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device developer would ideally enter the market at the upper right-hand section of the graph, indicating that the full scale device is operating at a relatively low cost of energy.Often device developers will take the same un-optimised device through the early TRL levels (or worse still skip levels entirely); only to be faced with the double dilemma of having a device that is not performing adequately, whilst also being stuck at a larger scale where operating costs are extremely prohibitive. This inefficient path is represented by the red curve in Figure 1:3. The ideal path as represented by the green curve is to maximise the performance of the device as early as possible, as cheaply as possible.
Figure 1:3: Metric for Successful Development of Economic WEC Technology (Weber et al., 2013)
Optimising device performance over the TRL levels between 1 to 4, can be achieved by using the best physical and numerical modelling techniques as well as developing and implementing advanced control algorithms, as early as possible. These sometimes separate but very much interrelated disciplines of research are essential in improving device performance. The main focus of this thesis is look at improved methodologies of physical modelling at early TRL levels. The research work also takes into consideration the other important areas of numerical modelling and control of wave energy devices.