Port of Dover - Tidal Energy Feasibility€¦ · tidal currents, a second outcome was to provide a...

Port of Dover

Report

October 2015

Port of Dover - Tidal Energy Feasibility

Stage 3 – Detailed Resource Assessment

Report

26800237_doverharbourboard_stage3_report.docx / Initials / yyyy-mm-dd

DHI WATER ENVIRONMENTS (UK) LTD

Ocean Village Innovation Centre • Ocean Way • Southampton • SO14 3JZ • United Kingdom Telephone: +44 2380 381 961 • [email protected]

Port of Dover - Tidal Energy Feasibility

Stage 3 – Detailed Resource Assessment Technical Note

Prepared for Port of Dover

Represented by Ms Vicky Jago Tidal energy resource map

Author Matthew Easton (MCE)

Project manager Nicholas James Elderfield (NJE)

Quality supervisor Ole Svenstrup Petersen (OSP)

Project number 26800237

Approval date 28th October 2015

Revision Draft 1.0

Classification Restricted

26800237_doverharbourboard_stage3_report.docx / mce / 2015-10-28

iii

CONTENTS

1 Introduction ................................................................................................................. 1 1.1 Project history ................................................................................................................................ 1 1.2 Aims and objectives ....................................................................................................................... 1 1.3 Report layout .................................................................................................................................. 2

2 Model basis .................................................................................................................. 3 2.1 Validation of the stage 1 hydrodynamic model .............................................................................. 3 2.2 Stage 3 hydrodynamic model ........................................................................................................ 6 2.2.1 Model setup .................................................................................................................................... 6 2.2.2 Validation of Stage 3 model ......................................................................................................... 10

3 Annual production run .............................................................................................. 13

4 Stage 3 resource assessment .................................................................................. 14 4.1 The available tidal energy resource ............................................................................................. 15 4.1.1 Velocity distributions .................................................................................................................... 15 4.1.2 Maximum current speed .............................................................................................................. 16 4.1.3 Directional alignment and asymmetry .......................................................................................... 17 4.1.4 Tidal Power density ...................................................................................................................... 19 4.2 Technical tidal energy resource ................................................................................................... 21 4.2.1 Turbine power curves .................................................................................................................. 21 4.2.2 Methodology ................................................................................................................................. 23 4.2.3 Results ......................................................................................................................................... 23

5 Array modelling ......................................................................................................... 27 5.1 Model setup .................................................................................................................................. 27 5.1.1 Hydrodynamic model ................................................................................................................... 27 5.1.2 Turbine parameterisation ............................................................................................................. 27 5.1.3 Array layout .................................................................................................................................. 27 5.2 Model runs ................................................................................................................................... 29 5.3 Results ......................................................................................................................................... 29 5.3.1 Energy extraction ......................................................................................................................... 29 5.3.2 Changes to flow patterns ............................................................................................................. 31

6 Summary and conclusions ....................................................................................... 35

References ................................................................................................................................... 36

FIGURES

Figure 2.1 Location of the moored AWAC at Port of Dover. Red square shows the proposed position

based on stage 1 model. Green square shows position achieved during survey. ........................ 4 Figure 2.2 Time series comparison of observed and stage 1model current speeds (depth-averaged)

at the Dover AWAC. ....................................................................................................................... 5 Figure 2.3 Scatter plot of observed current speed against stage 1model current speeds (depth-

averaged) at the Dover AWAC. ..................................................................................................... 5 Figure 2.4 Current speed rose at Dover based on depth-averaged currents and directional bins of

10°. Stage 1 model predictions are shown in grayscale superimposed on observed data


shown in colour-scale. The stems of the rose show the direction from which the current is

flowing. ........................................................................................................................................... 6 Figure 2.5 Domain of the stage 3a hydrodynamic model locations referred to in the text. ............................ 7 Figure 2.6 Stage 3 model mesh and bathymetry around the Port of Dover ................................................... 8 Figure 2.7 Stage 3 hydrodynamic model grid size [m2] for whole domain (upper panel) and area

around the Port of Dover (lower panel). Elements lengths in the area around Dover

Harbour are shown to be below 60 m (Purple). ............................................................................. 9 Figure 2.8 Comparison between filtered and unfiltered current speed data from Dover AWAC. Only

three days during spring tides are shown for clarity. ................................................................... 10 Figure 2.9 Time series comparison of observed and stage 3 model current speeds (depth-averaged)

at the Dover AWAC. Upper panel shows the full period. Lower panel shows a 3 day

subset during spring tides. ........................................................................................................... 11 Figure 2.10 Scatter plot of observed current speed against stage 1model current speeds (depth-

averaged) at the Dover AWAC. ................................................................................................... 11 Figure 2.11 Current speed rose at Dover based on depth-averaged currents and directional bins of

10°. Stage 3 model predictions are shown in grayscale superimposed on observed data

shown in colour-scale. The stems of the rose show the direction from which the current is

flowing. ......................................................................................................................................... 12 Figure 4.1 Stages of tidal energy resource assessment ............................................................................... 14 Figure 4.2 Time-series of modelled depth-averaged current speed at location of DOVER AWAC

during 2014 (Stage 3 model) ....................................................................................................... 15 Figure 4.3 Distribution of depth-averaged current speed at location of DOVER AWAC during 2014

(Stage 3 model) ........................................................................................................................... 16 Figure 4.4 Exceedance curve of depth-averaged current speed at location of DOVER AWAC during

2014 (Stage 3 model) .................................................................................................................. 16 Figure 4.5 Statistical annual maximum current speed for the Port of Dover (Stage 3 model). The

black triangle shows the pre-defined area for the Port of Dover tidal energy project. ................. 17 Figure 4.6 Current speed during a near-spring flood tide (upper panel) and a near-spring ebb tide

(lower panel). Vectors show the current direction and are plotted on a structured grid. ............. 18 Figure 4.7 Dover tidal energy investigation zone (black triangle) showing cross sectional area

chosen for tidal power calculation (red line). ............................................................................... 19 Figure 4.8 Average power density across the DTEZ. Green line shows annual average power

density from stage 3 model. Blue line shows the results from the stage 1 model /1/. The

Orange diamond shows the APD from the field measurement /2/. ............................................. 20 Figure 4.9 Monthly average power density across the DTEZ. Blue line shows the results from the

stage 3 model and the Orange diamond shows the APD from the field measurement /2/. ........ 20 Figure 4.10 Turbine power curves. Clockwise from top-left: Tocardo T2, Sabella D05, Nautricity 8m

rotor, Nautricity 10m rotor. ........................................................................................................... 22 Figure 4.11 Optimal turbine axial orientation for Sabella D05 ........................................................................ 24 Figure 4.12 Technical resource map showing estimated annual average power in kilowatts (upper

panel) and plant capacity factor (lower panel) for a Tocardo T2 turbine within the Port of

Dover Tidal Energy site. Note that areas deemed too shallow for turbine installation are

excluded from the assessment. ................................................................................................... 25 Figure 4.13 Location for maximal resource extraction for different turbine types. Orange marker

(Sabella D05, Nautricity 8m rotor), purple marker (Tocardo T2, Nautricity 10m rotor). .............. 26 Figure 5.1 Location of tidal turbines included in tidal array modelling at the Port of Dover. See Table

5.1 for reference to turbines included in each array scenario. .................................................... 28 Figure 5.2 Time series of estimated power extraction from three array scenarios at the Port of Dover

during one moth (April 2014). Results calculated using the Tocardo T2 and Nautricity

(10m rotor) power curves. ............................................................................................................ 30 Figure 5.3 Changes in the depth-average current speed as a result of tidal array scenario A during

peak flood tide extraction (upper panel) and peak ebb tide extraction (lower panel). ................. 32 Figure 5.4 Changes in the depth-average current speed as a result of tidal array scenario B during

peak flood tide extraction (upper panel) and peak ebb tide extraction (lower panel). ................. 33 Figure 5.5 Changes in the depth-average current speed as a result of tidal array scenario C during

peak flood tide extraction (upper panel) and peak ebb tide extraction (lower panel). ................. 34

v

TABLES

Table 2.1 Validation of water levels at Ports around the Dover Straits and the Dover AWAC. Note

that the BODC quality control identifies water level data for Dover as “improbable”. [Obs.

= observed, Mod. = modelled]. ...................................................................................................... 7 Table 3.1 Summary of hydrodynamic model settings for model production run .......................................... 13 Table 4.1 Characteristics of tidal stream energy converters ........................................................................ 22 Table 4.2 Annual average power and energy generation estimates based on 1 year (2014) stage 3

model for different devices. .......................................................................................................... 26 Table 5.1 Turbine position and array layout for modelling at the Port of Dover ......................................... 28 Table 5.2 Summary of hydrodynamic model settings for model production run .......................................... 29 Table 5.3 Summary of estimated power extraction from three array scenarios at the Port of Dover

during one month (April 2014). Results calculated using the Tocardo T2 and Nautricity

(10m rotor) power curves. ............................................................................................................ 30

APPENDICES

Appendix A Definition of model quality indices

Appendix B Technical resource maps

1

1 Introduction

1.1 Project history

Dover Harbour Board (DHB) is the harbour authority for the Port of Dover (Kent, United

Kingdom). DHB have highlighted a commitment to delivering a sustainable port operation and,

since 2007, have successfully managed reduction the ports carbon emissions by over 30%.

As the port continues to develop with the construction of a new cargo terminal, DHB have

identified on-site renewable energy generation as key to their efforts in achieving further

reductions in carbon emissions. Tidal-stream energy technologies have been identified as a

potentially significant component of a future renewable energy portfolio. DHB are therefore

looking to deliver a commercially viable tidal project to provide renewable electricity for the port.

This project is performed as part of Pro-Tide1, which seeks to advance public-private

collaboration and advance tidal energy knowledge and implementation.

A feasibility study and resource assessment are important first steps towards establishing the

economic viability of a tidal energy project. Since 2013, DHI have been engaged with DHB to

deliver a specific tidal energy resource assessment for a designated area around the Port of

Dover. The overall study seeks to research and model specific tidal energy conversion (TEC)

devices. The three stages of this study are:

1. Perform a rapid resource assessment to identify the overall power available to the site

2. Undertake field data collection study to confirm/refine the rapid resource assessment

3. Perform a detailed resource assessment with a refined numerical model for a series of tidal

energy technologies.

The information contained within this report describes stage 3 of the Dover Tidal Energy

Feasibility Study. The objective of stage 3 was to refine the existing numerical model employed

during the stage 1 rapid resource assessment /1/ based on the data collected during the stage 2

field study /2/.

1.2 Aims and objectives

The model developed during stage 3 will provide a reliable and accurate baseline description of

the tidal currents around the Port of Dover. Based on this improved baseline description of the

tidal currents, a second outcome was to provide a more robust assessment of the tidal energy

resource at the site.

The primary aims of the stage 3 resource assessment are:

1. Validate the hydrodynamic model of the Port of Dover using field measurements

2. Determine and describe the available tidal energy resource at the Port of Dover

3. Estimate the extractable tidal energy resource based on characteristics of real tidal turbines

4. Model arrays of tidal turbines to estimate power generation and changes to flow patterns as

a result of energy extraction.

1 http://www.pro-tide.eu/

http://www.pro-tide.eu/


1.3 Report layout

Section 2 describes the model basis for the tidal energy resource assessment. First, the

hydrodynamic model developed during stage /1/ is assessed against the site measurements

from the stage 2 study /2/. The hydrodynamic model is then refined in order to more accurately

capture the dynamics of the tides around the Port of Dover.

Section 3 details the production run of the hydrodynamic model developed in section 2.

The tidal energy resource at the Port of Dover is described in Section 4. First, the available

energy resource is described. This is a revised analysis of the type presented in the stage 1 and

stage 2 resource assessment, but utilising the stage 3 hydrodynamic model. Second, the

technical (or “extractable”) tidal energy resource is estimated. The technical resource is based

on power curves supplied by 3 device manufacturers.

Section 5 describes the setup and results of a series of hydrodynamic models including a

number of parameterised tidal turbine arrays. Three scenarios of between 1 and 9 turbines are

presented. The results of the array models are assessed in terms of estimated power generation

and the changes to the flow regime around the Port of Dover.

Section 6 includes a summary of the results presented and draws overall conclusions.

3

2 Model basis

Model validation is the process of ensuring that a model can accurately replicate the real world

conditions. This is most commonly achieved by comparing model predictions against good-

quality observed data and calculating descriptive statistics, also known as quality indices

(Appendix A summarises and the quality indices employed by DHI during model validation).

Once a model is validated, one can have greater confidence in the accuracy of the predictions.

If the model is judged to have failed this validation test, then a model calibration exercised is

performed. Calibration entails refining and adjusting the model parameters in order to achieve a

satisfactory model performance.

The rapid resource assessment forming stage 1 of the Port of Dover tidal energy feasibility study

utilised an existing numerical hydrodynamic model of the English Channel /1/. The model was

shown to capture the basic tidal dynamics of the site based on the BODC tide gauge at Dover,

and the published data within a tidal atlas. However, the model was not validated against current

speed observations at the site due to a lack of in situ data.

A stated aim of stage 2 of the project was to undertake field observations at the Port of Dover

and develop relevant calibration data for use in future modelling /2/. A survey company (Titan

Environmental Surveys Ltd.) were therefore commissioned to undertake various tidal current

surveys. Observations from a seabed mounted Acoustic Wave and Current meter (AWAC) were

shown to provide an improved understanding of the tidal currents offshore of the southern

breakwater.

In the following section, data collected during the Stage 2 survey are used to validate/calibrate

the predictions of tidal currents and water levels at the Port of Dover. These hydrodynamic

conditions are modelled using the MIKE 21 HD FM (flexible mesh), a two-dimensional system

that simulates water level variations and flow based on an unstructured grid.

2.1 Validation of the stage 1 hydrodynamic model

The validity of the hydrodynamic model used during the phase 1 study was assessed. The

model setup as described in /1/ was re-run for the month of April 2014, the same period as for

the field data study. The updated model boundary conditions were derived from the same

source used during phase 1 model and no other model parameters were adjusted.

The tidal velocities were extracted from the model at the position moored AWAC and compared

against the data obtained from the moored AWAC deployment. Figure 2.1 shows the position of

the AWAC, located several hundred meters from the southern breakwater of the port.

Information on the survey and the processing of the data described in /2/.


Figure 2.1 Location of the moored AWAC at Port of Dover. Red square shows the proposed position based on stage 1 model. Green square shows position achieved during survey.

A time-series comparison of the observed and modelled tidal current speed shows a

demonstrable under-prediction of current speeds by the phase 1 model (Figure 2.2). This

characteristic was evident during both the flood tide (larger peaks) and the ebb tide (smaller

peaks). Both the mean and the maximal observed current speeds were c. 30% larger than the

modelled current speeds.

A scatter diagram confirms the under-prediction of current speeds by the model. The prediction

bias shows that, on average, model current speeds were 0.24 m/s lower than observed current

speeds (Figure 2.3).

A rose plot provides an overview of how current speeds are distributed through different

directional sectors (Figure 2.4). The rose plot shows that the tidal current directions from the

stage 1 model were skewed towards the north-south axis when compared to the observed

current directions. The RMS error in predicted tidal current directions (for current speeds above

0.2 m/s) was 32°.

UK Foundation for Water Research /3/ have recommended several guidelines for hydrodynamic

model validation, amongst these:

• Speeds to within ±10-20 % of observed speeds; and

• Directions to within ± 20° of observed directions.

The scatter plot in Figure 2.3 shows that the RMS current speed error for the phase 1 model

was 0.31 m/s, or approximately 31% of the observed current speeds. Further, the rose plot

indicted that current directions (binned in 10 degree intervals) show a distinct separation

between the observations and the model.

The validation of the stage 1 model therefore fails with regards to the criteria specified by the

guidelines in /3/.

5

Figure 2.2 Time series comparison of observed and stage 1model current speeds (depth-averaged) at the Dover AWAC.

Figure 2.3 Scatter plot of observed current speed against stage 1model current speeds (depth-averaged) at the Dover AWAC.


Figure 2.4 Current speed rose at Dover based on depth-averaged currents and directional bins of 10°. Stage 1 model predictions are shown in grayscale superimposed on observed data shown in colour-scale. The stems of the rose show the direction from which the current is flowing.

2.2 Stage 3 hydrodynamic model

2.2.1 Model setup

In order to improve the representation of the tidal conditions simulated by the model, a

calibration exercise was performed. The aim of model calibration was to achieve a satisfactory

agreement between the predicted tidal currents and the observed data from /2/. This was

achieved by making adjustments to the existing phase 1 model setup. Initial adjustments were

based on the insight gained during the validation of the Stage 1 model (section 2.1) and also on

previous modelling experience.

The typical parameter that is adjusted to “tune” the model during calibration is the seabed

roughness. The seabed roughness characterises the resistance to the flow, with higher

roughness associated with lower current speeds, and vice versa. In the present work, however,

it was found that model resolution was the most important factor for accurately capturing the

flow around the harbour.

Domain size and boundary conditions The stage 1 model was based on an existing DHI model of the English Channel and Southern

North Sea (see Figure 2.1 in /1/). For the stage 3 it was decided to substantially shorten the

model domain to focus on the local flow within the Dover Straits.

The stage 3 model domain land boundary spanned south-east coastline of Kent and the Nord-

Pas-de-Calais region of France (Figure 2.5). The western sea boundary line joined Dungeness

(UK) to a point just south of Boulogne-Ser-Mer (France), and the eastern open boundary linked

Ramsgate (UK) to the France-Belgium border just past Dunkerque.

7

The stage 3 model was forced by temporally and spatially varying water levels extracted from a

database held and maintained by DHI. However, moving to a smaller domain placed greater

significance on the specification of the model boundary conditions. To ensure that these

boundaries conditions were reliable, modelled and observed water levels were compared at the

ports of Boulogne-Sur-Mer, Dover, and Dunkerque over a 1-month period (April 2014).

Referring to the UK Foundation for Water Research guidelines for hydrodynamic model

validation /3/, the following criteria are recommended for coastal areas:

• Timing of high water to within ±15 minutes

• Water levels to within ±0.1m, or 10% of spring tidal ranges, or 15% of neap tidal ranges.

Table 2.1 show that the modelled tidal elevations were in good agreement with observed data

and satisfied the criteria in the above-mentioned UKFWR guidelines. These results give

confidence that the specified boundaries provide a satisfactory replication of the basic tidal

dynamics through the Dover Straits.

Figure 2.5 Domain of the stage 3a hydrodynamic model locations referred to in the text.

Table 2.1 Validation of water levels at Ports around the Dover Straits and the Dover AWAC. Note that the BODC quality control identifies water level data for Dover as “improbable”. [Obs. = observed, Mod. = modelled].

Port Mean

water

level error

(m)

Spring tidal range Neap tidal range Mean absolute

error in timing of

high water

(minutes) Obs (m) Mod. (m) Error (%) Obs.(m) Mod. (m) Error (%)

Boulougne-Sur-Mer -0.10 8.14 8.25 1.35 3.90 3.93 0.76 10.7

Dover* 0.01 5.86 6.24 6.48 2.98 3.02 1.34 9.0

Dover AWAC -0.16 6.32 6.33 0.16 3.08 3.09 0.32 10.2

Dunkerque -0.11 5.80 5.93 2.24 2.96 3.01 1.69 6.6


Grid size Along with a general under-prediction in tidal current speed, the validation of the stage 1 model

in section 2.1 suggested that observed current directions are more complex than those

predicted. It was previously identified in /1/ that local flow accelerations are due to the interaction

between the tides and the harbour walls, so called “tidal streaming”. These accelerations also

induce strong gradients in the flow and lead the formation of turbulent eddies – complex

phenomena that remove and redistribute energy from the flow.

In the two-dimensional MIKE21 model, the simulation of these energy transfers is, in part, set by

the size of the model grid.

In an attempt to represent these processes in more detail the mesh element length around the

harbour was decreased from 100m in the stage 1 model to around 50 m in the stage 3 model

(Figure 2.6, Figure 2.7). This shift to smaller length scales offers better representation of the flow

around the harbour, but this is offset by greater computational demands.

Figure 2.6 Stage 3 model mesh and bathymetry around the Port of Dover

9

Figure 2.7 Stage 3 hydrodynamic model grid size [m2] for whole domain (upper panel) and area around the Port of Dover (lower panel). Elements lengths in the area around Dover Harbour are shown to be below 60 m (Purple).

Bes resistance The bed resistance characterises the drag on the flow by the presence of the seabed and was

set by the water depth and the Manning Number, M. The Manning number was set as a

constant value (M = 36.8 m1/3s-1) across the model domain, which is within the recommended

values for shelf sea models.

Filtering of observed tidal currents During peak flow, the observed currents displayed high-frequency variations in their magnitude.

The source of these variations was not known, but likely reflects local turbulent effects due to

the interaction of the tides and the harbour structure. These complex processes occur at time

and space scales that are not captured by the model, which instead provides a much smoother

representation of current speeds.


To remove the effects of turbulent motion in the data, a filter was employed smooth the

observed currents. A fourth-order Butterworth low-pass filter with a cut-off frequency of 1/3 hrs-1

was applied as recommended in existing guidelines /4/. The depth-averaged u- and v-velocity

components were filtered forward and backwards through time (zero-phase filtering) to prevent

temporal distortion. The depth-averaged current speed was then recalculated from the filtered

components. A comparison between the filtered and unfiltered time-series is shown in Figure

2.8.

Figure 2.8 Comparison between filtered and unfiltered current speed data from Dover AWAC. Only three days during spring tides are shown for clarity.

2.2.2 Validation of Stage 3 model

The modified Dover Straits model as described in section 2.2 was run for the month of April

2014, the same period as for the field data study. Tidal velocities were extracted from the model

at the position moored AWAC and compared against the data obtained from the static field

survey /2/.

A time-series comparison shows that the stage 3 model replicates the (filtered) observed tidal

current speeds with overall decent accuracy (Figure 2.9). The stage 3 model does not attain the

maximal peak currents speeds, particularly during the neap tides (lower peaks) after 11th April.

However, there is clearly a general improvement compared with the stage 1 model (Figure 2.2).

A scatter diagram confirms this improvement (Figure 2.10). The prediction bias shows that, on

average, model current speeds were 0.08 m/s lower than observed current speeds, a decrease

from 0.24 m/s for the stage 1 model. Similarly the RMS error was 0.20 m/s for the stage 3 model

compared with 0.31 m/s for the stage 1 model.

The rose plot shows that the tidal current directions from the stage 3 model were skewed

towards the north-south axis as compared to the observed current directions (Figure 2.11). This

characteristic was also evident in the stage 1 model. However, the RMS error in predicted tidal

current directions (for current speeds above 0.2 m/s) at stage 3a was 18.7°, compared with 32°

for the phase 1 model.

As previously stated, the UKFWR guidelines /3/ for model validation recommends the following:

• Speeds to within ±10-20 % of observed speeds; and

• Directions to within ± 20° of observed directions.

The information on the scatter plot shows that the RMS current speed error for the phase 3

model was 0.2 m/s, or approximately 19% of the observed current speeds. Further, RMS error in

current directions is also within the recommended guidance.

The stage 3 model therefore satisfies the validation criteria specified by the guidelines in /3/.

11

Figure 2.9 Time series comparison of observed and stage 3 model current speeds (depth-averaged) at the Dover AWAC. Upper panel shows the full period. Lower panel shows a 3 day subset during spring tides.

Figure 2.10 Scatter plot of observed current speed against stage 1model current speeds (depth-averaged) at the Dover AWAC.


Figure 2.11 Current speed rose at Dover based on depth-averaged currents and directional bins of 10°. Stage 3 model predictions are shown in grayscale superimposed on observed data shown in colour-scale. The stems of the rose show the direction from which the current is flowing.

13

3 Annual production run

The stage 3 hydrodynamic model was run for a period of 1 calendar year (2014) to provide an

updated baseline model from which to calculate a revised estimate of the tidal stream energy

resource. The model was setup with the specifications as indicted in

The model was run using MIKE by DHI 2014 (service pack 3) on a 6-core computer, using a

GeForce GTX 760 graphic card. The simulation took some 35 hours to complete.

The outputs from the production run were:

• Tidal velocities, current speed, current direction, and water depth at the location of the

Dover AWAC.

• Tidal elevations at the Ports of Dover, Boulogne-Sur-Mer, and Dunkerque.

• Spatial maps of current speeds around the Port of Dover.

Table 3.1 Summary of hydrodynamic model settings for model production run

Setting Value

Mesh resolution Spatially variable. Characteristic element size around

Port of Dover ~50m.

Simulation period 2014-01-01 to 2015-01-01 (1 year)

Model time-step 600 seconds (10 minutes)

Eddy viscosity Smagorinsky formulation with constant = 0.28

Boundary conditions Temporally and spatially varying water levels along open

boundaries

Bed resistance Manning number with constant, M = 36.8m1/3s-1


4 Stage 3 resource assessment

We identify three principal phases of a tidal energy resource assessment as described below

and in Figure 4.1:

• The available resource describes the amount of energy at a site that could potentially be

harvested from tidal currents. The available resource is usually calculated without

consideration of technical, economic or environmental constraints.

• The technical resource is the energy that can be extracted from tidal currents using

envisaged technologies and restrictions (including project economics) without undue impact

on the underlying resource or the environment. The technical resource is a fraction of the

available resource.

• The practical Resource considers criterion that may limit exploitation of development, for

example electricity grid connections or spatial constraints (e.g. shipping, fishing, MOD etc.).

The practical resource is hence a proportion of the technical resource.

During stage 1 and 2 of the Port of Dover Tidal Energy Feasibility study, only the available tidal

energy resource was calculated /1-2/. This analysis was aligned with the steps in Section 7 of

existing tidal energy guidelines /4/. The analysis is repeated here using the results from the

updated stage 3 model. The results are presented in section 4.1.

Estimates of the technical tidal energy resource are presented in section 4.2. The technical

resource was determine based on characteristics of real tidal turbine provided by device

developers following a workshop held at the Port of Dover on 29th April 2015.

Figure 4.1 Stages of tidal energy resource assessment

15

4.1 The available tidal energy resource

The available tidal energy resource is based on calculating the kinetic energy flux incident

through a cross-sectional area. The principal advantage of this “flux method” is that estimates

are independent of TEC device types

The analysis of the observed tidal currents presented in the following section is aligned with the

steps in Section 7 of existing tidal energy guidelines /4/. First, the characteristics of the currents

within the Port of Dover Extraction Zone (DTEZ) are presented. These are vitally important as

the kinetic energy flux scales with the cube of the current speed. Second, the kinetic energy flux

through a cross-section of the extraction zone is calculated.

Unless otherwise stated, the analyses described herein are based on depth averaged tidal

currents.

4.1.1 Velocity distributions

Figure 4.2 shows a time-series of annual modelled depth-averaged current speed at the location

of the Dover AWAC. The mean current over the year 2014 was 0.91 m/s, with a maximal value

of 2.17 m/s.

A histogram analysis shows the percentage of time that the current speed falls within a given

range (or bin). Figure 4.3 shows the resulting velocity distribution for observed current speeds

at Dover. A sampling interval of 10-minutes and a 0.1 m/s bin sizes have been used as

recommended by /4/.

Similarly, the percentage of time that the current speeds exceed a range of values can be taken

from the exceedance curve in Figure 4.4. It can be observed from this plot that current speeds

exceeded 1 m/s during approximately 40% of the year, and exceeded 1.5 m/s for around 10% of

the year.

Figure 4.2 Time-series of modelled depth-averaged current speed at location of DOVER AWAC during 2014 (Stage 3 model)


Figure 4.3 Distribution of depth-averaged current speed at location of DOVER AWAC during 2014 (Stage 3 model)

Figure 4.4 Exceedance curve of depth-averaged current speed at location of DOVER AWAC during 2014 (Stage 3 model)

4.1.2 Maximum current speed

Figure 4.5 shows the statistical maximum tidal current speed during 2014. The highest current

speeds (>2.5 m/s) occurred slightly west of the tidal energy project site. This area is adjacent to

the entrance to the port, and is caused by tidal streaming around the breakwaters. Within the

tidal energy project site, the fastest currents are located to the west (2.3 – 2.4 m/s).

17

Figure 4.5 Statistical annual maximum current speed for the Port of Dover (Stage 3 model). The black triangle shows the pre-defined area for the Port of Dover tidal energy project.

4.1.3 Directional alignment and asymmetry

The largest tidal currents occurred during the flood flow (approximately 1 hour after HW at

Dover). Figure 4.6 (upper panel) shows that flood current speeds are largest through the centre

of the extraction area. Near to the harbour walls, the flood tidal current appear to be deflected to

a more northerly direction.

The ebb tidal currents are approximately 50% weaker than the flood currents through the

extraction area. However, Figure 4.6 (lower panel) shows that ebb current magnitude and

direction are more homogenous through the extraction area.


Figure 4.6 Current speed during a near-spring flood tide (upper panel) and a near-spring ebb tide (lower panel). Vectors show the current direction and are plotted on a structured grid.

19

4.1.4 Tidal Power density

A cross-section through the Port of Dover extraction zone (as selected in /1/) was chosen for the

energy flux calculation (as represented by the red line in Figure 4.7). The cross section extends

from the maximal offshore extent of the extraction zone to within 100 m of the southern

breakwater and was aligned perpendicular to the mean current directions.

The average kinetic tidal power density (APD) available from the flow was calculated from the

depth-averaged currents via equation 4.1:

𝐴𝑃𝐷 = 1

2𝑁∑ 𝑈𝑖

3

𝑁

𝑖=1

4.1

where was the density of seawater (assumed constant of 1025 kgm-3), U is the depth-

averaged current speed at time step i, and N is the total number of model time steps.

Figure 4.8 shows the variation in APD across the DTEZ from the stage 3 hydrodynamic model of

the Port of Dover. The APD increases steadily from c. 240 W/m2 some 100 m from the southern

breakwater to a maximum value of c. 680 W/m2 at around 600m from the breakwater. There is a

slight decrease in APD further offshore, however the APD is consistently above 600 W/m2.

For comparison, the estimates from the stage 1 model and stage 2 AWAC deployment are also

shown in Figure 4.8. This shows that the APD from the stage 3 model are consistently larger

than the stage 1 model, but are lower than those from the observed data at the AWAC from

stage 2.

Figure 4.7 Dover tidal energy investigation zone (black triangle) showing cross sectional area chosen for tidal power calculation (red line).


Figure 4.8 Average power density across the DTEZ. Green line shows annual average power density from stage 3 model. Blue line shows the results from the stage 1 model /1/. The Orange diamond shows the APD from the field measurement /2/.

Figure 4.9 Monthly average power density across the DTEZ. Blue line shows the results from the stage 3 model and the Orange diamond shows the APD from the field measurement /2/.

21

4.2 Technical tidal energy resource

4.2.1 Turbine power curves

The turbine power curve characterises the turbine power output and is fundamental for technical

resource assessments. Important characteristic of a power curve include:

• Cut-in current speed: the threshold current speed above which the turbine starts generating

energy.

• Cut-out current speed: the threshold current speed above which the turbine ceases to

generating energy.

• Rated current speed: the current speed at which turbine power output matches the rated

turbine power

• Rated turbine power: typically this is the limit of the safe operating range of the turbine

generator.

It is important to note that a larger rated power does not necessarily mean that more energy

may be generated by a turbine. Energy is the product of power and time, thus the interval

between cut-in and cut-out current speed is also important. For example, a low power machine

operating with a relatively slow cut-in speed may give a larger energy yield than a relatively high

power machine with fast cut-in current speeds.

DHB and DHI entered into discussions with a number of device developers regarding the Port of

Dover tidal energy project. Following a questionnaire and workshop, three developers were

selected for the stage 3 technical resource assessment. Each company was requested to

provide the following information:

• Details of turbine power curve (the relationship between turbine power and axial flow

speed)

• Information on the relationship between turbine power coefficient and the axial flow speed.

• Turbine diameter and swept area of turbine rotor

• Information on whether the turbine is “fixed-axis” or yawing (either passively or actively)

Table 4.1 summarises the key operating characteristics of the turbines and Figure 4.10 shows

power curves for each of the devices. Note that the power curve for the Tocardo T2 device was

not supplied directly, and has instead been inferred from published characteristics from the

Tocardo website.

The Tocardo and Sabella devices are “fixed axis” turbines. A fixed-axis turbine cannot rotate

about its vertical axis to automatically align with the flow direction. Thus, there is a need to

determine the optimal orientation of the turbine with regard to power generation. It was reported

in the stage 1 report /1/ that flood and ebb tidal currents in the project area are predominantly

rectilinear (i.e. directly opposing), providing favourable conditions for a fixed axis device.

The Nautricity device is neutrally buoyant and connected to a tensioned mooring, meaning that

drag forces ensure that the turbine remains perpendicular to the flow at all states of the tide.

Sabella also offer a technical solution for increasing energy yields using a ducted turbine. The

duct operates by funnelling the flow. The upstream currents are accelerated through the rotor,

thus producing more power during this stage of the tide. This is offset by a deceleration in the

downstream current speeds, resulting in lower power production.


Table 4.1 Characteristics of tidal stream energy converters

Turbine

diameter

[m]

Swept

area [m2]

Rated

speed

[m/s]

Cut-in

speed

[m/s]

Cut-out

speed

[m/s]

Rated

Power

[kW]

Max.

Power

[kW]

Turbine

Axis

Tocardo T2 9.9 76.8 2.0 0.4 2.6 103 103

Fixed

axis

Nautricity

8 50.3 2.0 1.0 2.5 100 108 Free/

Moored 10 78.5 2.3 1.1 2.5 100 107

Sabella 5 16.6 - 0.15 > 3 109 109 Fixed

axis

Figure 4.10 Turbine power curves. Clockwise from top-left: Tocardo T2, Sabella D05, Nautricity 8m rotor, Nautricity 10m rotor.

23

4.2.2 Methodology

For each of these turbines, an estimation of the annual energy yields was determined. The

calculations were based on depth-averaged current speeds from the stage 3 model. The period

was 1 year (2014) and the time resolution was 10 minutes. The methodology was as follows:

1. Within the pre-defined area around the Port of Dover, identify model cells with enough

water depth to accommodate each device. The condition was that the diameter of the

turbine rotor should not exceed 2/3 of the water depth below lowest astronomical tide.

2. Within each of the model cells selected from step 1, calculate the available tidal energy

resource, i.e. the average kinetic tidal power density (APD), using equation 4.1.

3. The turbine power output was approximated from the depth-averaged current speeds by:

𝑃 =1

2𝜌𝐴𝑒𝑈3𝐶𝑃 4.2

Where U is the depth-averaged current speed perpendicular to the turbine and Ae was the

swept area of the turbine blade (Table 4.1). The power coefficient CP was determined from

interpolating the relevant turbine power curves in Figure 4.10. For fixed axis devices the

orientation of the turbine that provided the maximum power was also determined.

4. The average annual turbine power was calculated. The energy yield in kilowatt-hours (kWh)

was determined by multiplying the average power (kW) by the number of hours in the year

2014 (8760 hours).

5. The turbine capacity factor was calculated. The capacity factor is a measure that indicates

how much electricity a generator actually produces relative to the maximum it could

produce at continuous full power. This is determined by the ratio of the average power

produced by the turbines rated power.

4.2.3 Results

The results of the assessment are presented as maps in order to identify the areas within the

greatest technical tidal energy resource. Selected maps are shown below, and full results are

provided as part of Appendix B.

Optimal Axis Orientation For the fixed-axis turbines (Tocardo and Sabella) the optimal orientation of the turbine axis was

determined. This is the angle (in degrees relative to North) that the turbine should face, such

that the power production was maximised.

For the Sabella device, Figure 4.11 shows the turbine should be oriented at between 60º in the

west of the site decreasing to 40º in the north-east corner. This reflects the changing direction of

the tidal streams across the site and the influence of the harbour structure on the flow. This

change in direction can also be observed in the current speed vector plot for the flood tide

(upper panel of Figure 4.6).

In the deeper waters and away from the harbour wall, the optimal orientation was either 50º or

55º from North. The same result was also found for the Tocardo Turbine (see Appendix B).


Figure 4.11 Optimal turbine axial orientation for Sabella D05

Annual average power and energy The technical resource map shows the estimated average annual power that would be extracted

via a Tocardo T2 turbine at different locations across the Port of Dover extraction area (upper

panel of Figure 4.12). In this case, areas where the turbine blade diameter (9.9m) exceed 2/3 of

the water depth below LAT have been excluded. The results show that the maximal annual

average resource at a positon just inside of the mid-point of the south-west resource area

boundary. The technical resource decreases from west-to-east. A similar pattern was observed

for other turbine types (see maps in Appendix B).

Table 4.2 provides a summary of the peak annual average power and energy available from

each of the turbines considered.

It is interesting to note that the optimal location for power generation are confined to two

positions, both located along the just inside of the south-west resource area boundary (Figure

4.13). The Sabella/Nautricty (8m rotor) devices produce maximal power at a location

approximately 110 m further inshore than the Tocardo/Nautricity (10m rotor) device. This is

solely due to the imposed constraints set on the water depths. At the Sabella/Nautricity (8m

rotor) location, the water depth was 14.3mLAT was deemed too shallow for the larger

Tocardo/Nautricity (10m) devices.

The Nautricity (10m rotor) turbine produced the largest estimated annual average power and

energy of the turbines tested. This was closely followed by the Tocardo T2 turbine.

Despite having the lowest turbine cut-in current speed, the Sabella D05 turbine produced the

least energy yields. This was due to the relatively shallow slope of the Sabella turbine power

curve (see Figure 4.10). By placing a duct around the device to capture and accelerate the flood

flow, the annual average power and energy was more than doubled. However, a duct placed to

capture more of the ebb flow had the result of decreasing the energy yields that could be

achieved with an un-ducted turbine.

With a capacity factor of 0.17, the Nautricity (10m rotor) and Tocardo T2 were jointly the most

effective turbines for the site. However, it is noted that these values are likely to be considered

as a fairly low utilisation of the available generation capacity.

25

Figure 4.12 Technical resource map showing estimated annual average power in kilowatts (upper panel) and plant capacity factor (lower panel) for a Tocardo T2 turbine within the Port of Dover Tidal Energy site. Note that areas deemed too shallow for turbine installation are excluded from the assessment.


Table 4.2 Annual average power and energy generation estimates based on 1 year (2014) stage 3 model for different devices.

Turbine Variety

Location of peak

generation [m]

(UTM Zone 31)

Estimated

annual average

power [kW]

Turbine

capacity

factor

Estimated annual

energy generation

[kWh]

Tocardo T2 383864 E

5663499 N 17.6 0.17 154,176

Nautricity

8m blade

diameter

383755 E

5663523 N 12.0 0.11 105,212

10m blade

diameter

383864 E

5663499 N 18.7 0.17 163,537

Sabella

Un-ducted 383755 E

5663523 N 6.0 0.08 52,670

Ducted

(flood flow)

383755 E

5663523 N 12.1 0.11 106,104

Ducted

(ebb flow)

383755 E

5663523 N 5.8 0.05 47,982

Figure 4.13 Location for maximal resource extraction for different turbine types. Orange marker (Sabella D05, Nautricity 8m rotor), purple marker (Tocardo T2, Nautricity 10m rotor).

27

5 Array modelling

The hydrodynamic model of the Port of Dover, was elaborated to include the representation of

tidal stream turbines in an indicative array layout. The purpose of the modelling was to predict

the changes to the flow regime around the port due to the extraction of energy by the turbine.

5.1 Model setup

5.1.1 Hydrodynamic model

The stage 3 hydrodynamic model of the port of Dover described in section 2.2 was used as the

basis for the tidal array modelling.

5.1.2 Turbine parameterisation

The effect of tidal turbines is modelled as sub-grid structures using a simple drag-law to capture

the increasing resistance imposed by the turbine blades as the flow speed increases. The drag

force is calculated as:

𝐹𝐷 =1

2𝜌𝐶𝐷𝐴𝑒𝑉0

2 5.1

Where is the density of water, CD is the turbine drag coefficient, Ae is the swept area of the

turbine rotor and V0 is the upstream current speed.

In the following, a single device type was considered. The chosen device represents the

expected behaviour of the larger of the turbines considered for the Port of Dover (e.g. Tocardo,

Nautricity 10m rotor).

The turbine blade diameter was set at 10m, giving a swept area of some 78.5 m2.

The turbine drag coefficient was set at constant value of 0.7. This value was consistent with the

information provided by Tocardo during the Workshop event at the Port of Dover during April

2015. It should be noted that the turbine support structure will impose an additional drag on the

flow. The drag contribution from the turbine support structure is not included in this model setup,

and therefore the predicted effect on the baseline flow is likely to be non-conservative. It is

recommended that the influence of turbines supports should be considered in the future, for

example as part of an Environmental Impact Assessment.

5.1.3 Array layout

Three array scenarios of 1, 3, and 9 turbines were considered as part of this assessment (Table

5.1, Figure 5.1). For a turbine with a rated power of 100 kW, this would provide a total installed

capacity of between 0.1MW and 0.9 MW.

The array layouts were informed by the results of the technical resource assessment in section

4.2. The turbines were positioned in the area giving high technical resource (see Figure 4.12),

but were displaced 100 m inside of the resource area boundary to avoid any potential conflict

with marine traffic.

For scenario B and scenario C, turbine rows were oriented such that they were aligned across

the flow at the optimal axial orientation for energy extraction. Adjacent turbines were separated

by 5 turbine diameters (i.e. 50m) from hub-to-hub.


For scenario C, the turbines in row B were positioned 7.5 turbine diameters (i.e. 75m)

downstream of the turbines in row A. This separation is necessary so that the wake effects from

one row do not adversely affect the flow speeds at the other row. Laboratory tests have shown

that this these effects are negligible for turbine separations between 5 and 10 times the turbine

rotor diameter /5/.

Table 5.1 Turbine position and array layout for modelling at the Port of Dover

Turbine ID

Position [m] (UTM 31NN) Array layout

Easting Northing Scenario

A

Scenario

B

Scenario

C

A1 383933 5663548

A2 383977 5663523

A3 384020 5663498

A4 384063 5663473

A5 384107 5663448

B1 384020 5663573

B3 384063 5663548

B3 384107 5663523

B4 384150 5663498

Figure 5.1 Location of tidal turbines included in tidal array modelling at the Port of Dover. See Table 5.1 for reference to turbines included in each array scenario.

29

5.2 Model runs

The array model scenarios were run for a period of 1 calendar month (April 2014). The model

was setup with the specifications as indicted in Table 5.2.

The models was run using MIKE by DHI 2016 on an 8-core Intel Xeon 3.20 GHz computer,

using. The simulation time was approximately 4 hours per scenario.

• Baseline (no tidal turbines)

• Scenario A (1 turbine)

• Scenario B (3 turbines)

• Scenario C (9 turbines)

The outputs from the array models were:

• Spatial maps of tidal elevations and current speeds around the Port of Dover.

• Drag force, current speed, and current direction at each tidal turbine

Table 5.2 Summary of hydrodynamic model settings for model production run

Setting Value

Mesh resolution Spatially variable. Characteristic element size around

Port of Dover ~50m.

Simulation period 2014-04-01 to 2015-05-01 (1 month)

Model time-step 600 seconds (10 minutes)

Eddy viscosity Smagorinsky formulation with constant = 0.28

Boundary conditions Temporally and spatially varying water levels along open

boundaries

Bed resistance Manning number with constant, M = 36.8m1/3s-1

5.3 Results

5.3.1 Energy extraction

The average power extracted by the individual turbines in each array was estimated via the

method outlined in section 4.2.2. Only the Tocardo T2 and Nautricity (10m rotor) devices were

considered. The current speed input to calculation was taken from the model output at each

turbine location. The total generation from each array was determined by summing the individual

contributions from each turbine in the array.

Table 5.3 provides a summary of the average power generation by each array for April 2014.

Figure 5.2 shows a time-series of power production over the month. It can be observed that the

Nautricity turbine produces the largest peak power output, however, the Tocardo device

produces the largest average power output. This is due to the lower cut-in speed of the Tocardo

turbine, which means that it generates power during weaker flows (e.g. during neap tides around

the 10 April 2014).


Table 5.3 Summary of estimated power extraction from three array scenarios at the Port of Dover during one month (April 2014). Results calculated using the Tocardo T2 and Nautricity (10m rotor) power curves.

Turbine Variety

Scenario A Scenario B Scenario C

Pave [kW] Capacity

Factor Pave [kW]

Capacity

Factor Pave [kW]

Capacity

Factor

Tocardo T2 10.4 0.101 30.8 0.100 89.6 0.097

Nautricity 10m 8.3 0.077 24.5 0.076 69.2 0.071

Figure 5.2 Time series of estimated power extraction from three array scenarios at the Port of Dover during one moth (April 2014). Results calculated using the Tocardo T2 and Nautricity (10m rotor) power curves.

31

5.3.2 Changes to flow patterns

The removal of kinetic energy from the tidal currents is the principal environmental stress

associated with tidal-stream turbines. The removal of kinetic energy will be manifest by changes

in the flow patterns (tidal current magnitude and direction) around the Port of Dover. Quantifying

and, if necessary, mitigating these changes are a key component of an environmental impact

assessment (EIA). A full EIA is beyond the scope of the current work, however, the predicted

changes to depth-average current speed are briefly discussed below.

Difference plots are used to elucidate the changes in tidal currents after the installation of the

tidal arrays (i.e. hydrodynamic model with parameterised turbines minus the current speeds

form the baseline model). The results below show snapshots of the predicted difference in tidal

currents during peak flood tide extraction [2nd April 2014 13:40] and peak ebb tide extraction [1st

April 2014 08:00].

For array scenario A, Figure 5.3 shows that changes in the depth-average current speed during

peak flood and peak ebb conditions will be negligible (below 0.02 m/s).

For array scenario B, Figure 5.4 shows flood tidal streams are lower than baseline values for

downstream of the turbines (blue shading). This area extend for several hundred meters. There

is also a small area of accelerated (red shading) close to the southern breakwater to the north of

the extraction zone. During ebb flow, the tidal currents are similarly reduced downstream. The

magnitude of these changes (0.02-0.04 m/s) remained very small, however, and are likely

dwarfed by the natural variations in current speeds due to turbulence.

For array scenario C, Figure 5.5 shows a more extensive and more intense area of flow

deceleration/acceleration during flood tide. The current speeds are reduced by up to 0.08m/s

below baseline values through the turbines. This band of decelerated flow expands laterally and

begins to diffuse further downstream. Tidal currents were accelerated to the north of the array

where the flow is constrained between the turbines and the southern breakwater. This results in

the flow in front of the harbour entrance increasing by up to 0.05 m/s. During ebb tide, the

current speeds are reduced for several hundred meters downstream of the array.


Figure 5.3 Changes in the depth-average current speed as a result of tidal array scenario A during peak flood tide extraction (upper panel) and peak ebb tide extraction (lower panel).

33

Figure 5.4 Changes in the depth-average current speed as a result of tidal array scenario B during peak flood tide extraction (upper panel) and peak ebb tide extraction (lower panel).


Figure 5.5 Changes in the depth-average current speed as a result of tidal array scenario C during peak flood tide extraction (upper panel) and peak ebb tide extraction (lower panel).

35

6 Summary and conclusions

A feasibility study and resource assessment are important steps in establishing the viability of a

tidal energy project. This report has detailed stage 3 of the Port of Dover tidal energy feasibility

study – detailed resource assessment. The main conclusions emerging from this project are:

1. A hydrodynamic model of the Port of Dover was constructed and validated against site

observations. It was demonstrated that model resolution is crucial to capturing the complex

flow patterns around the port. The model was found to be slightly negatively biased (under

prediction peak ebb tidal currents), but was shown to provide a useful and reliable

description of the tidal dynamics around the Port.

2. The available tidal energy resource at the Port of Dover

3. Based on characteristics of real tidal stream turbines the technical tidal energy resource

was shown to be largest in the south-west of the Port of Dover tidal energy extraction zone.

This area experiences the fastest tidal streams of up to 2.4m/s during flood tide. It was

estimated that a single turbine would produce an average annual power of between 5.8 kW

and 18.7 kW. The capacity factor for the most effective devices was calculated at 0.17,

which is considered a fairly low utilisation of the available generation capacity.

4. A hydrodynamic model was used to investigate the power generation from an array of tidal

turbines. An indicative array of 9 turbines (with a total rated capacity of 0.9 MW) was

estimated to produce an annual average power of 89.6 kW. This represents a fairly low

plant capacity factor of 0.097.

5. The effects of a single turbine were estimated to have a negligible effect on the depth-

averaged current speeds around the port. Larger arrays were predicted to result in some

changes to the current patterns around the port, including a reduced current speeds behind

the turbines, and accelerated flow close to the harbour walls. The magnitude of these

changes were fairly small, and may be dwarfed by natural variations due to turbulence.

The technical tidal energy resource estimates provided may inform the business and

environmental case for a tidal energy project at the Port of Dover. The energy production by tidal

turbines can be used to determine savings in purchased energy, subsidies available, and carbon

offset by the project.

Any future changes to the exterior layout of the port may impact on the availability of the tidal

energy resource and the best location for exploiting that resource. This could jeopardise both

the economic and environmental case for the tidal stream energy project. The stage3

hydrodynamic model provides an excellent tool for predicting the effects of future developments

on the underlying resource, or informing the EIA of any future port developments.


References

/1/ DHI. 2013. Port of Dover – tidal energy feasibility: stage 1 – rapid resource assessment. Technical

report. November 2013.

/2/ DHI. 2014. Port of Dover – tidal energy feasibility: stage 2 – assessment of field data. Technical

report. June 2013.

/3/ UKFWR. A framework for marine and estuarine model specification in the UK. United Kingdom

Foundation for Water Research. FR0374. March 1993.

/4/ Legrand, C. 2009. Assessment of tidal energy resource. Black and Veatch Ltd. for the

European Marine Energy Centre (EMEC), Stromness, UK

/5/ Maganga, F., Germain, G., King, J., Pinon, G., Rivoalen, E. 2010. Experimental

characterisation of flow effects on marine current turbine behaviour and on its wake

properties. IET Renewable Power Generation, 4, 498-509.

37

APPENDICES

APPENDIX A – Def ini t ion of qual i ty indices

Definition of model quality indices

A-1

A Definition of model quality indices

To obtain an objective and quantitative measure of how well the model data compare to the

observed data, a number of statistical parameters or quality indices (QIs) were calculated. Prior

to the comparisons, the model data were synchronised to the time step of the observations so

that both time series have equal length and overlapping time stamps. For each valid

observation, measured at time t, the corresponding model value was found using linear

interpolation between the model time steps before and after t. The comparisons of the

synchronised observed and modelled data were illustrated in the following figures:

Time series plot incl. statistics

Histogram of occurrence vs. magnitude

Scatter plot incl. sorted data and QIs

Histogram of bias vs. magnitude

Histogram of bias vs. direction

The quality indices are described below and their definitions are listed in Error! Reference

source not found.. Most of the quality indices are based on all data points together, and hence

the quality indices should be considered averaged measures for the entire data set and may not

be representative of the accuracy during rare events.

Table A.1 Definition of quality indices (OBS = Observation, MOD = model)

Abbreviation Description Definition

N Number of valid observations −

MEAN Mean of model data

1

N∑ MODi

N

i=1

BIAS Mean of difference

1

N∑(MOD − OBS)i

N

i=1

AME Mean of absolute difference

1

N∑(|MOD − OBS|)i

N

i=1

RMSE

Root mean square of difference √1

N∑(MOD − OBS)i

2

N

i=1

SI

Scatter index (unbiased) √1

N∑ (MOD − OBS − BIAS)i

2 Ni=1

MEAN

CC

Correlation coefficient

∑ (OBSi − MEAN)(MODi − MOD)Ni=1

√∑ (OBSi − MEAN)2Ni=1 ∑ (MODi − MOD)2N

i=1

PR(Npeak) Peak ratio of Npeak events PR(Npeak) =

∑ MODiNpeak

i=1

∑ OBSiNpeak

i=1

REGS and REGI Regression line slope and intercept Linear least square fit to data

Quantile Scatter plot based on the two data sets sorted after magnitude

The BIAS is the mean difference between the modelled and observed data, and AME is the

mean of the absolute difference. RMSE is the root mean square of the difference.

The scatter index (SI) is a non-dimensional measure of the difference calculated as the

unbiased root-mean-square difference relative to the mean value of the observations. In open

A-2 26800237_doverharbourboard_stage3_report.docx / Initials / yyyy-mm-dd

water, an SI below 0.2 is usually considered a small difference (excellent agreement) for

significant wave heights. In confined areas where mean significant wave heights are generally

lower, a slightly higher SI may be acceptable.

The correlation coefficient (CC) is a non-dimensional measure reflecting the degree to which the

variation of the first variable is reflected in the variation of the second variable. A value close to 0

indicates very limited or no correlation between the two data sets, while a value close to 1

indicates a very high or perfect correlation. Typically, a CC above 0.9 is considered a high

correlation (good agreement) for wave heights.

The peak ratio (PR) is the average of the Npeak highest model values divided by the average of

the Npeak highest observations. The peaks are found through the peak-over-threshold (POT)

method applying an average annual number of exceedance of 4 and an inter-event time of 36

hours. A general under-estimation of the modelled peak events results in PR below 1, while an

over-estimation results in a PR above 1.

The regression line slope and intercept (REGS and REGI) are found from a linear fit to the data

points in a least square sense. A regression line slope different from 1 may indicate a trend in

the difference.

The Quantile plot shows scatter plot based on data sorted after magnitude, i.e. phase

information is neglected. If the data follow the 1:1 line the frequency distribution of the two

datasets are similar. The Q-Q fit line is a linear regression on the Quantile data.

Definition of model quality indices

APPENDIX B – Technical resource maps

Technical resource maps

B-1

B Technical resource maps

B.1 Tocardo

Tocardo T2 – Optimal turbine axis orientation [degrees from N]


Tocardo T2 – Average Annual Turbine Power [kW]

Tocardo T2 – Plant capacity factor


B-3

B.2 Nautricity

Nautricity – 8m rotor – Average Annual Turbine Power [kW]

Nautricity – 8m rotor – Plant capacity factor


Nautricity – 10m rotor – Average Annual Turbine Power [kW]

Nautricity – 10m rotor – Plant capacity factor


B-5

B.3 Sabella

Sabella D05– Optimal turbine axis orientation [degrees from N]


Sabella D05 – unducted - Average Annual Turbine Power [kW]

Sabella D05 – unducted – Plant capacity factor


B-7

Sabella D05 – ducted (flood) - Average Annual Turbine Power [kW]

Sabella D05 – ducted (flood) - Plant capacity factor


Sabella D05 – ducted (ebb) - Average Annual Turbine Power [kW]

Sabella D05 – ducted (ebb) - Plant capacity factor

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