Resource analysis of the Pentland Firth A. Owen and I.G. Bryden Abstract Understanding the complexities of tidal current flows is fundamental to successfully harnessing their energy. The usual picture given by proponents of devices (if it is even mentioned) is of a well behaved, unsteady, uniform flow that reverses every 6.2 hours. This assumes that the flow is constrained by a smooth channel with no significant surface defects to propagate vortices and eddies. This is clearly not the case for real flows where the channel is not straight, usually varies in depth, and has a number of bumps and holes that will disturb the flow[1]. In addition, the flow will usually draw in water from all sides at the channel entry, but then jet into the bay at the channel exit. The tidal current resource is often perceived as being potentially large [2] but the magnitude, and the potentially exploitable proportion, of the energy embodied within a tidal current, is only now beginning to be properly understood. The characteristics of tidal currents may vary considerably over the 12.4 hour flood/ebb cycle, and a turbine installation will need to take this variability into account. It is not possible to develop an indicative value of the exploitable energy simply from knowledge of the harmonic constants or local tide tables, and over- optimistic resource forecasting has resulted from attempts to do so. Tidal currents are bounded, finite systems with no capacity for energy replenishment from other sources, whereas the atmospheric energy source is, from a wind turbine standpoint, almost infinite, being easily replenished from the upper atmosphere or neighbouring weather systems. Cross-application of the wind farm resource model led to tidal turbines offering to extract more energy than existed within the stream. The nature of the flow at the proposed site must be appropriate for extraction, and many sites (particularly the Pentland Firth), exhibit sizeable areas of organised and random vortex activity in ebb and/or flood directions. These large areas of coherent vortices will not provide viable power, at least not with current technology, and must therefore be excluded from resource models. Earlier attempts to quantify the exploitable resource used the wind farm model and forecast a 2025 extractable resource figure of 25TWh/yr [3] for the tidal stream 1 in the Pentland Firth, which is now thought to be capable of providing about 1/5 th of that figure. Data sources Tidal data and predictions are readily available for most of the world, particularly areas of regular shipping movements. The UK Hydrographic Office (UKHO) publishes navigational charts and tidal atlases, as does the US equivalent, the National Ocean Survey (NOS). All major UK harbour authorities publish tide tables appropriate to their locality, and many hundreds of smaller ports usually administered (in the UK) by the local authority, will extrapolate their local time- shifted data from these major port sources. The importance of local characteristics in the analysis of tidal currents cannot be overstressed, for example the tide times at Burra Sound in Orkney vary by as much as 40 minutes from the published tables for Kirkwall, only 5 miles away. It may be the case that such specific knowledge is not widely published and that local community knowledge must be consulted. Similarly, for particular features of tidal currents, the knowledge and experience of fishermen, ferry operators and pleasure craft pilots will be invaluable in building an accurate picture of vortices, jetting and debris transport within the stream. The UKHO predicted tidal heights and current velocities are based on mean values with any extreme events removed, and therefore the predictions will not be appropriate for severe weather conditions or anomalistic astrological events. Tidal heights are referenced to LAT (Lowest Astronomical Tide) in the United Kingdom and MLW (Mean Low Water) in the United States. Most tidal current velocities are based on data sets that would be considered inadequate for tidal height prediction [4] and inspection of nautical charts reveals much of the information included thereon can be over 50 years out of date. The availability of data is therefore, often much greater than its continuity or accuracy, and this fact must be borne in mind when modelling the resource. Some readily available on-line sources of tidal data are given in Table 1, below 2 Institution URL Data type National Tidal and Sea UK tidal predictions, British http://www.pol.ac.uk/ntslf/ Level Facility dependencies tidal data British Oceanographic Tide gauge, bathymetry, http://www.bodc.ac.uk/ Data Centre bottom pressure Permanent service for Sea level data and tidal http://www.pol.ac.uk/psmsl/ mean sea level constants Harbour Authorities http://www.portoftyne.co.uk Tide times, levels, predictions (examples) http://www.aberdeen-harbour.co.uk National Oceanic and Historical data and predictions Atmospheric http://tidesandcurrents.noaa.gov/ for a range of locations. Administration Principally US. University of South Tide predictions for many http://tbone.biol.sc.edu/tide/ Carolina global sites UK Hydrographic Office http://easytide.ukho.gov.uk/EasyTide/ Tides predictor Channel Coastal Tidal and meteorological data http://www.channelcoast.org/ Observatory Canadian Hydrographic http://www.waterlevels- Tides, currents and water Service niveauxdeau.gc.ca levels for hundreds of Canadian sites. Table 1: Some readily accessible world sources of tidal data. The essential point to be made here is that any desk study using data extrapolated from harmonic constituents, or aged historical data, is at best a general approximation. The only accurate method of predicting the energy available for exploitation at a given point in a tidal stream is to measure the tidal current behaviour at that point, and to account for external factors such as prevailing meteorological conditions. Assessment methodologies The acceptability of pre 21 st century tidal stream resource assessments has diminished over recent years. A report for the Carbon Trust [5] concentrates on post 1990 work on the grounds that earlier works had been superseded or rewritten. It is evident however, that the two principal works [6,7] used as the basis for most work prior to 2003-4, utilised the farm methodology previously employed for wind turbine applications. The farm methodology assumes that an infinite supply of kinetic energy is available and that the power output of an installation is governed by the number of devices deployed and their spatial distribution. This model is acceptable for wind energy since its volume of operation is essentially boundless with the exception of the earth‘s surface on 3 which it stands. For tidal currents, improvements in the understanding of the effects of energy extraction on the flow dynamics of a tidal currents [8], and inclusion of the limitations of a hydraulic channel, indicate that the farm methodology can result in predictions of hyper-extraction, i.e. forecasting a greater energy output than is physically possible from the flow available. The earlier works also placed a minimum flow velocity constraint of 2m/s [6] and 1.5m/s [7] before considering a site to be appropriate, whereas velocities of 1m/s may now be more representative of the lower limit. More recent work has sought to quantify the environmentally acceptable level of extractable energy flux, which was then placed tentatively at 10% [9], on the basis that in many cases, the flow speed reductions would be less than the turbulent fluctuations. Further work [10], developed the flux methodology and examined the feasibility of defining a variable Significant Impact Factor, (SIF) that could be applied to a site and applied the mothod to the resource in the Channel Islands. This work was interesting in that it was applied to an open water environment, which did not lend itself to a more analytic approach, without substantial numerical analysis. The SIF can be viewed as a limiting factor, almost certainly site specific, that takes into account the characteristics of, and constraints applicable to, a tidal stream, and can thereby indicate the maximum acceptable level of energy exploitation. Recent work has been directed towards the derivation of a general formula for any channel [11] or fundamental numerical analysis of the nature of the sensitivity of generic tidal sites [14]. This latter approach offers the enticing prospect of generic parameterisation of any candidate site allowing subsequent detailed assessment without costly numerical analysis. It is interesting to note that this recent work supports the hypothesis that knowledge of the undisturbed flow velocities is a necessary but not sufficient criterion for resource assessment and that further understanding of the sensitivity of a site requires knowledge of the bathymetry, topography and boundary conditions. Under some conditions, it can be demonstrated that it might even be possible to extract more energy than appears to be present in the kinetic flux [15][16], which would indicate that the kinetic flux alone does not represent the total available energy and that it might be more appropriate to define a —total flux“, which includes consideration of potential energy and energy dissipated by boundary friction. In this work it has been assumed that 22% of the apparent kinetic flux can be extracted. This is compatible with published work, referred to previously, or work presently in press. 4 The EPRI approach The Electric Power Research Institute methodology was applied to the North American east coast sites in Massachusetts, Maine, New Brunswick and Nova Scotia[17] and it references much of the early work for the flux methodology, outlined above. The site survey begins with an initial screening process to identify sites that may be worthy of closer examination and the criteria for this are, • Peak flow speeds (ebb and flood) of the tidal stream, EPRI places a minimum value of 1.5m/s on this factor.