UK Tidal Stream Assessment

1. UK Tidal Stream Assessment Stephen Wyatt, Charlie Blair – Carbon Trust Scott Couch, Ian Bryden – University of Edinburgh Andy Baldock – Black and Veatch December 2010

2. The Carbon Trust has worked with leading figures in the industry to reassess the UK’s tidal resource from the bottom up Previous assessments have shown huge variation in the accessible UK tidal resource: • Variations in data are due to the complexity of taking the appropriate approach to energy quantification and the difficultly in understanding the impact of energy extraction on the available energy. Both require fundamental bottom up theoretical studies. • An accurate understanding the accessible tidal resource is important: • To establish the market size and potential for tidal stream technology. • To give project developers and policy makers a clear indication of the most appropriate sites for tidal stream insulations, Hydrodynamic Assessment Methodology The flux methodology, developed by the University of Edinburgh Institute for Energy Systems, uses hydrodynamic modelling to assess the impact of energy extraction from a tidal current. The Carbon Trust has worked with The university of Edinburgh, The Crown Estate and Black and Veatch to deliver a new bottom up estimation of the UK’s tidal stream resource. Developed the underpinning theory though various peer review studies, and this most recent work funded by CT and Npower Juice. The underpinning theory is well established, and has a strong peer review track record. The work builds on pervious studies by Garrett et al’ and the Northwest National Marine Renewable Energy Centre . This approach considers idealised representation of three hydrodynamic mechanisms, which give rise to the tidal current conditions necessary for TEC deployment. All UK sites with tidal current resource have been represented by one of these mechanisms. When farms of TECs are modelled, flow discharge, flow velocities and tidal range are all reduced. These impacts would eventually have an impact on the environment and on project economics that would eventually become unacceptable. Black and Veatch have work with the University of Edinburgh to apply the underpinning theory to real site around the UK. The crown estate have co-funded this work and provided GIS support to assist with site locations and constraints.

3. Hydrodynamic modelling for tidal stream can be represented by 3 specific expressions • Approach: • To accurately model tidal environments it is necessary to consider a number of different mechanisms and scenarios to understand the dominant factors. • Previous approaches cannot be used to generically represent all mechanisms and all scenarios (the MEC 2005 study was steady state and only tested hydraulic current site). • In this current work a new approach is used to formulate a representation of all mechanisms and scenarios for generic application, based on using a percentage of the total (kinetic and potential) energy to formulate extractable energy and corresponding change in tidal range and velocity. • Validation: • Dynamic simulations over a 24 hour tidal cycle; • Various scenarios tested for each mechanism for consistency (change in geometry/bathymetry and tidal velocity generated); These can be represented mathematically as follows: • Three hydrodynamic mechanisms have been identified: • Hydraulic current: If two adjoining bodies of water are out of phase, or have different tidal ranges, a hydraulic current is set-up in response to the pressure gradient created by the difference in water level between the two bodies. • Tidal streaming: the physical response of the tidal system to maintenance of the continuity equation; when a current is forced through a constriction, the flow must accelerate. • Resonant system: Resonant systems occur as a consequence of a standing wave being established. A standing wave arises when the incoming tidal wave and a reflected tidal wave constructively interfere.

4. The Flux model is used to identify the point when energy extraction becomes compromised by economic or environmental constraints Start Hydrodynamic modelling Methodology: Identify sites applicable for envisaged technology using bulk tidal resource data from the marine energy atlas. (Sites with >1.5kW/m2 and >15m deep). This returns 30 sites key sites. Applying 1 of the 3 hydrodynamic mechanisms allows the ‘Theoretical Resource’ for each site – this is the starting point for the flux method: Calculate effect of energy extraction due to tidal farm on available resource. Iterate new farm design based on new resource. Until one impact (environmental or economic) becomes significant, this gives ‘Technical Resource’. Theoretical resource with initial farm configuration Reduced resource due to energy extraction 1 Farm re-optimised for site 2 1 CoE or ENV’N Impact become dominant 3 2 3 End Iteration continues until one of the impacts hits a prescribed limit (below) Constraint Levels (optimistic, base, pessimistic) to get to Technical Resource: Environmental Constraint (limits): • Tidal range reduction 0.5m (=0.25m tidal amplitude); 0.2m; 0.1 (or 5% of mean spring range) • Velocity reduction <10% (Does not apply in base case – impact is not significant) Economic Constraint: • Cost of Energy increase (site by site modelling) limited to 50%;20%; 10% These constraints are set based on CT and Consultants’ understanding of probable acceptable environmental effects; and of likely project financing. Significant sensitivity analysis was undertaken. As environmental effects become better understood – or a limit is prescribed by the EA or SEPA – these levels could be adjusted. Similarly acceptable CoE increases could be changed to reflect different willingness to pay for tidal electricity. 4 Apply practical constraints (shipping, etc) gives ‘Practical Resource’. Constraints to get to Practical Resource: Practical constraint mapping was undertaken with the Crown Estates Ltd, using their MArS GIS facility. Exclusion zones (with buffer) and restriction zones were mapped. Where these overlapped with Tidal Resource Sites modelled output from the site was reduced correspondingly. Designated areas, wrecks and many potential competing uses for the sea space (other energy, fishing, shipping, dredging etc) were considered.

5. The UK tidal resource has been re-estimated as 29TWh/yr, or 20.6TWh/yr with constraints UK Resource Annual Energy Production Total Resource RESOURCE DEFINITIONS • Total Resource (TWh/y):Total energy that exists within a defined tidal system • Theoretical Resource (TWh/y):Maximum energy that can be harvested from tidal currents in the region of interest without consideration of technical, economic or environmental constraints. • Technical Resource (TWh/y):The energy (which is a proportion of the theoretical resource) that can be harvested from tidal currents using envisaged technology options and restrictions (including project economics) without undue impact on the underlying tidal hydrodynamic environment. • Practical Resource (TWh/y):The energy (which is a proportion of the technical resource) that can be harvested after consideration of external constraints (e.g. grid accessibility, competing uses such as MOD, shipping lanes, etc.). Theoretical Resource Technical Resource Practical Resource Practical constraints (shipping, grid etc) Hydrodynamic (physical) limits Enviro’tal &EconomicImpactFactors Example site map with resource assessment and analysis of tidal mechanism. This is Strangford Lough. A hydraulic current site. ‘Practical’ Resource at 30 sites. Well over half the resource (including Pentland Firth Deep with 31%) is at deep sites. These will require ‘2nd generation’ devices.

6. There is scope for further work that builds on the foundations of this study The Results of this study provide clarity about the potential annual energy production (AEP) from UK waters using today’s technologies and foreseeable 2nd generation variants that will be able to access deep sites. A set of general assumptions have been made and these is scope for applying revised constraints and parameters, but we believe the practical AEP figure provides a good place to work from. New concept devices may become available to access new tidal streams (less than <15 kW/m2 or <15m depth) then there is certainly potential to increase the available resource . Device concepts are being tested (some with support from the MEA) that do hold out promise for accessing new sites - more work would be needed to assess how much more resource these devices could access ifthey prove feasible to build. • Next Steps (for Carbon Trust and other stakeholders): • Work to define environmental impacts. This work has identified sites and made assumption around the macro environmental impacts from TECs at these sites. Environmental stakeholders are now in a position to assess the sites and apply statutory limits to tidal range drops or velocity reductions, either across the UK or on a site by site basis • Work to apply learning about future device costs and energy costs, to better understand Cost of Energy limits under different commercial scenarios. Also to update other parameters used in the financial model • Work to improve the underlying tidal data for UK waters (including ADCP data gathering – what limited ADCP data there is shows some significant discrepancies with Marine Energy Atlas data). • Scope to use a similar methodology to assess low energy (<1.5kW/m2) and shallow (<15m depth) sites, following learning from MEA engagement with new concept device developers. • Carbon Trust intends to publish a report early in 2011 to coincide with publication of the MEA Final Report, and invites peer review of the work. Uncertainties from current study Sensitivities to the various assumptions (particularly the environmental and economic ‘significant impacts’ have been analyzed in some depth. Key uncertainties are as follows: • Inaccuracy of underlying tidal data • Issues with parametric approach – most sites do not perfectly fit one of the idealised mechanisms • Uncertainty about realistic constraint levels and other parameters (although sensitivity analysis undertaken – see below)