port of dover - tidal energy feasibility

Port of Dover Technical Note

June 2014

Port of Dover - Tidal Energy Feasibility

Stage Two – Assessment of field data

doverharbourboard_stage2_fieldstudy.docx / nje /November 2013

DHI WATER ENVIRONMENTS (UK) LTD • Davidson House • Forbury Square • GB-RG1 3EU Reading • United Kingdom

Telephone: +44 1189 000745 • Telefax: +44 1189 000746 • [email protected] •

Port of Dover - Tidal Energy Feasibility

Stage Two – Assessment of Field Data

Prepared for Port of Dover Represented by Ms Vicki Jago

Port of Dover tidal streams

Project manager Nick Elderfield

Quality supervisor Ole Svenstrup Petersen Project number 26800237

Approval date June 2014

Revision Final 1.0

Classification Restricted

doverharbourboard_stage2_fieldstudy.docx / mce /June 2013 i

CONTENTS

1 Introduction ........................................................................................................ 1 1.1 Project description ............................................................................................................... 1 1.2 Previous work ....................................................................................................................... 2 1.3 Objectives and nature of assessment .................................................................................. 2

2 Field survey ........................................................................................................ 3 2.1 Survey methods ................................................................................................................... 3 2.2 Static survey ......................................................................................................................... 3 2.3 Quality assurance ................................................................................................................ 4

3 Results of the field survey ................................................................................. 5 3.1 Time series ........................................................................................................................... 5 3.2 Rose plot .............................................................................................................................. 1 3.3 Tidal harmonic analysis ....................................................................................................... 1 3.4 Vertical current speed profile ............................................................................................... 3

4 Resource assessment from field data .............................................................. 4 4.1 Velocity distribution .............................................................................................................. 4 4.2 Maximum velocities .............................................................................................................. 5 4.3 Directional alignment ........................................................................................................... 5 4.4 Power density ....................................................................................................................... 6 4.5 Available and extractable energy ......................................................................................... 8

5 Conclusions and recommendations ................................................................. 9

References .......................................................................................................................... 10

FIGURES Figure 1.1 – Location plan of the proposed tidal site (image supplied by DHB) ...................................... 1 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 3.1 Time series of water depths at the deployment site ............................................................ 1 Figure 3.2 Time series of u-velocity component (depth averaged). Positive values represent

flow from west-to-east (i.e. flood tide) .................................................................................. 1 Figure 3.3 Time series of v-velocity component (depth averaged). Positive values represent

flow from south-to-north (flood tide at Dover) ...................................................................... 2 Figure 3.4 Time series of current speed at Dover (depth averaged). ................................................... 2 Figure 3.5 Current speed rose at Dover. Based on depth averaged values and directional bins

of 15°. ................................................................................................................................... 1 Figure 3.6 Tidal constituents derived from harmonic analysis of observed depth-averaged

currents at Dover.................................................................................................................. 2 Figure 3.7 Tidal constituents derived from harmonic analysis of modelled tidal currents at

Dover (Stage 1 resource assessment model) ..................................................................... 2 Figure 3.8 Vertical current speed profile during conditions of peak flood flow (left panel) and

peak ebb flow (right panel) ................................................................................................... 3 Figure 4.1 Distribution of current speed (depth-averaged) during field survey ..................................... 4

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Figure 4.2 Exceedance curve of current speed (depth-averaged) from field survey ............................ 5 Figure 4.3 Time series of tidal power density at observation point over one month. For

reference the average power density (APD) from the deployment is shown along with the average power density estimated from the rapid resource assessment (light blue) ............................................................................................................................ 7

Figure 4.4 Average tidal power density across the DTEZ for one month, compared to the fixed point assessment from the field data (green) ...................................................................... 7

TABLES Table 3.1 Parameters for harmonic tidal analysis of current speeds ................................................... 2 Table 4.1 Scatter table of observed current speed against current direction at Dover........................ 6

APPENDICES Appendix A – Outputs from tidal harmonic analysis

doverharbourboard_stage2_fieldstudy.docx / mce /Jun 2013

1 Introduction

1.1 Project description

Dover Harbour Board (DHB) is the harbour authority for the Port of Dover had has highlighted its commitment to delivering a sustainable port operation that will minimise its environmental impacts. The port has sought to achieve year on year reductions in its biggest emissions source, purchased electricity.

DHB has identified that marine renewables may be part of a future energy mix for delivering its operations and as such requires an initial investigation of the feasibility of a small scale near shore tidal array of up to 0.99MW capacity within the Port jurisdiction. As a busy commercial port, the number of potential locations has been limited by a series of physical constraints and have led to the location plan is shown in Figure 1.1.

Figure 1.1 – Location plan of the proposed tidal site (image supplied by DHB)

The overall study will seek to research and model specific Tidal Energy Conversion (TEC) devices. Providing the results from testing are viable, the project will lead into a small scale array.

The proposed overall schedule includes a Feasibility Phase, a Design Phase and then an Implementation and Evaluation Phase. This Tidal Energy Resource Assessment study is the first in a three staged approach to define the initial feasibility of a small scale array.

The three stages are summarised as:


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

2. Undertake required field studies to confirm/refine the resource assessment.

3. Provide a detailed resource assessment for a series of devices/options.

1.2 Previous work

DHI have previously completed Stage 1 (rapid resource assessment) for the site at the Port of Dover /1/. This initial investigation was performed using an existing numerical model held and maintained by DHI. The model employed during the rapid resource assessment was not calibrated or validated at the site, and thus the calculated available tidal energy resource was regarded as a preliminary estimate.

1.3 Objectives and nature of assessment

The stated objectives of the Stage 2 work undertaken are twofold:

• Undertake an assessment of the available tidal energy resource based on measured data.

• Develop relevant calibration data based on fixed point and moving vessel surveys for use in future modelling assessment.

Wherever possible the first objective above utilises existing guidelines produced by the European Marine Energy Centre (EMEC) in order to provide a reliable assessment of the available tidal energy resource /2/.


2 Field survey

2.1 Survey methods

The primary aim of the survey was to increase understanding of the tidal energy resource at the port of Dover, and to provide calibration/validation data for future modelling work.

Titan Environmental Surveys Ltd. were commissioned to undertake the deployment, servicing, and recovery of a metocean mooring and vessel-mounted tidal current surveys. Only data from the metocean mooring are presented here as this provides the best temporal coverage and is consistent with the recommendations for a stage 2b resource assessment in /2/ (note that the metocean mooring is referred to as a ‘static survey’ in the language of the guidelines).

Data originating from the vessel-mounted surveys can provide further insight into the spatial variation of currents speeds at the site. However, there are no established methods for calculating energy resource from these vessel-mounted observations. It is expected that data from vessel-mounted data should become useful when calibrating/validating numerical models. In the interim, the plots in Appendix D of the survey report /3/ provide an initial indication of the spatial variation of tidal currents across the site.

2.2 Static survey

2.2.1 Characteristics The survey report in /3/ details all the relevant information regarding the measurement of current speeds as recommended in Section 5.4 of the guidelines /2/. This includes information on type of device, device setup and calibration, deployment location, and sampling rate.

We highlight that the following characteristics of the static field survey that are consistent with the recommendation for a stage 2b resource assessment:

• The period of observations was longer than the minimum recommended period of one month.

• The instrument was deployed at a location anticipated to exhibit high velocities (Figure 2.1). Such a location was inferred from the modelled current speeds from the Stage 1 rapid resource assessment /1/.

• To the best of existing knowledge, the instrument was deployed in a location free from obstructions or rapid gradients in water depth that may adversely affect the observed data.

• The data collection interval was 10 minutes (ensemble averaged)

• Observed velocities were vertically binned at a resolution of 1m, with the centre of the first (lowermost) bin located 2.02 m above the seabed.


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.

2.2.2 Output data Amongst the data that were recorded during the deployment was the following required information at intervals of 10-minutes.

• Time (year, month, day, hour, minute, seconds)

• Velocities in three directions at 1 m intervals through the water column

• Standard deviation of velocities in three directions

• Temperature

• Pressure

• Average current speed and direction

2.3 Quality assurance

As detailed in /3/, the instrument performs an internal quality control which excludes “bad” data ensembles. Pitch and roll sensors also enable identification of excessive movement of the device. Following these automated QA procedures a high data return was achieved, with 100% of the data being reported.

A subsequent visual inspection of the data revealed a notable heading change some 48 hours after deployment, consistent with the movement of the ADCP frame by approximately 180°. The survey report suggests that the shift in heading was caused by the rotation of the frame during this event, although the quality of the data should not be compromised. However, the first 48 hours of the deployment were omitted from the analysis, as there was still sufficient data coverage (a whole calendar month) as required for a stage 2b resource assessment.


3 Results of the field survey

3.1 Time series

Time series plots of water depth, u (west-to-east) and v (north-to-south) velocity components and tidal current speed are shown in Figure 3.1 - Figure 3.4.


Figure 3.1 Time series of water depths at the deployment site

Figure 3.2 Time series of u-velocity component (depth averaged). Positive values represent flow from west-to-east (i.e. flood tide)


Figure 3.3 Time series of v-velocity component (depth averaged). Positive values represent flow from south-to-north (flood tide at Dover)

Figure 3.4 Time series of current speed at Dover (depth averaged).


3.2 Rose plot

A rose plot gives a succinct overview of the how current speeds and directions are distributed at a fixed location. Figure 3.5 shows the rose plot for depth averaged currents as observed during the field survey. Note that direction is the direction from which the current is flowing (the inverse to that defined in the Titan report). Further analysis of the directional variation in tidal current direction is included in section 4.3.

Figure 3.5 Current speed rose at Dover. Based on depth averaged values and directional bins of 15°.

3.3 Tidal harmonic analysis

A tidal harmonic analysis of these observed current speed data was conducted (IOS method) to separate the tidal and non-tidal (residual) components. A total of 29 tidal constituents were used for this analysis (Table 3.1).

The dominant contribution to the observed currents at Dover was associated with the principal lunar semidiurnal tidal component, the M2 tide, with secondary contribution from the principal solar, S2 tide (Figure 3.6). A full list of the derived tidal components is included in Appendix A.

It is of interest to compare the tidal constituents extracted from the observations with those from the model used during the Stage 1 assessment. This provides a means with which to assess the model performance without performing a new simulation. Note however, that a more rigorous model validation exercise would be undertaken as part of a Stage 3 assessment.


As for the field data, the harmonic analysis of the model current speeds was based on a full calendar month and utilised 29 harmonic constituents (Table 3.1). For the modelled data, by far the largest constituent was the M2 tide, followed by the S2 tide (Figure 3.7). A full list of the derived tidal consistent is included in Appendix A.

A comparison of the histograms in Figure 3.6 and Figure 3.7 suggests that overall the modelled tidal constituents at Dover are comparable to the observed. On closer inspection, it is noted that the magnitude of the observed M2 tide is around 30% larger in the observed data. The implication of this is that the model used in the Stage 1 assessment under predicts the tidal current speeds, and therefore also the available tidal energy resource.

Table 3.1 Parameters for harmonic tidal analysis of current speeds

Data Source Field data Stage 1 model

Method IOS IOS

Number of constituents 29 29

Location 1.35°E, 51.11°N 1.35°E, 51.11°N

Period of analysis 30/03/2014 12:00 – 30/04/2014 15:00

15/03/2013 00:00 – 15/04/2013 00:00

Figure 3.6 Tidal constituents derived from harmonic analysis of observed depth-averaged currents at Dover.

Figure 3.7 Tidal constituents derived from harmonic analysis of modelled tidal currents at Dover (Stage 1 resource assessment model)


3.4 Vertical current speed profile

The survey results as presented above (as well as the resource calculations in section 4, below) are for depth-averaged tidal velocities. In most cases, depth-averaged values are considered to provide a useful and reliable overview of the site conditions. Furthermore, the depth-averaged values can be compared directly to the output from the two-dimensional hydrodynamic model.

In reality, tidal velocities vary with depth below the surface. This variation is called the velocity profile. For example, the influence of friction acts to slow velocities closer to the seabed. On the other hand, meteorological effects (wind and waves) may influence velocities nearer to the sea surface. Vertical velocity profiles are often characterised via well-known empirical relationships.

As the availability of tidal energy scales with the cube of the current speed, the shape of the velocity profile has implications for generation. It is important, therefore, to consider this variation for tidal energy resource assessment. In Figure 3.8 we highlight examples of the variation in current speed around the time of peak flood flow and peak ebb flow. During both events, the current speeds diminish toward the seabed and are generally largest several meters below the surface. These variations are consistent with the expected behaviour described above.

The implication for tidal power is that the available tidal energy is likely to be somewhat greater in the upper part of the water column.

Figure 3.8 Vertical current speed profile during conditions of peak flood flow (left panel) and peak ebb flow (right panel)


4 Resource assessment from field data

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 /2/. Unless otherwise stated, the analyses described herein are based on depth averaged tidal current velocities.

4.1 Velocity distribution

A histogram analysis shows the percentage of time that the current speed falls within a given range (or bin). Figure 4.1 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.

Similarly, the percentage of time that the current speeds exceed a range of values can be inferred from the exceedance curve in Figure 4.2. It can be inferred from this plot that current speeds exceeded 1 m/s during approximately 50% of the survey period, and exceeded 1.4 m/s for around 10% of the survey.

Figure 4.1 Distribution of current speed (depth-averaged) during field survey


Figure 4.2 Exceedance curve of current speed (depth-averaged) from field survey

4.2 Maximum velocities

The maximum velocity is defined as the peak velocity that has been reached for 10-minutes during the whole month. It was previously shown in Figure 3.4 that the peak 10-minute depth-averaged current speed was 2.29 m/s.

4.3 Directional alignment

The current speed rose plot in Figure 3.5 indicates that tidal currents are broadly bi-directional. For certain tidal turbine concepts, notably horizontal fixed-axis designs, flood an ebb currents that are rectilinear (i.e. directly opposing) are more efficient for energy generation. It is useful therefore to consider temporal variations in tidal current direction. This type of analysis is referred to as the “Tidal Ellipse” in the language of the existing guidelines /2/.

Based on the harmonic analysis of observed tidal currents in section 3.3 the major axis of the principal M2 tidal component is c. 25° from the horizontal. This implies a theoretical ebb and flood directions of 65° and 245° relative to north, respectively.

Table 4.1 shows the frequency of occurrence of current speeds binned into directional sectors of 10°. Current directions deviate from the major M2 tidal axis by less than ±10° over 70% of the time. The variation in directions is larger for the flood tidal currents than for the ebb currents, which may reflect a more complex development of flood flow around the western side of the port of Dover.

The existing guidelines recommend that directional offsets on the resource are applied if the flow direction varies by more than 10° for more than 5% of the time. However, this will


not be applied in the subsequent analysis. The is justified on the basis of keeping the resource estimate independent of device type (the directional offset implying a fixed axis device is used).

Table 4.1 Scatter table of observed current speed against current direction at Dover

4.4 Power density

The average kinetic tidal power density (APD) available from the flow was calculated from the depth-averaged currents as:

𝐴𝑃𝐷 = 1

2𝑁ρ�𝑈𝑖3

𝑁

𝑖=1

where ρ was the density of seawater (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.

The APD at Dover was 887 W/m2 when averaged over the one month survey.

Due to the cubic relationship with the current speed, a large variation in magnitude of power density exists over the semi-diurnal and spring-neap tidal cycles. The time series plot in Figure 4.3 shows the instantaneous values of the power densities relative to the APD.


Figure 4.3 Time series of tidal power density at observation point over one month. For reference the average power density (APD) from the deployment is shown along with the average power density estimated from the rapid resource assessment (light blue)

As noted in the rapid resource assessment in /1/, that Figure 3.6 showed the variation in average tidal power density along a transect running south from the breakwater. For comparison the latest estimate based on the static ADCP is shown for comparison on an updated version of that figure, now below.

Figure 4.4 Average tidal power density across the DTEZ for one month, compared to the fixed point assessment from the field data (green)


4.5 Available and extractable energy

The final step of the EMEC guidelines (section 9) entails calculation of the energy expected from a tidal turbine. One of the proposed methodologies, commonly known as the “flux” method, is based on the average power density calculated above.

𝑃𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 = 𝐴𝑃𝐷 ∙ 𝐴 ∙ 𝑆𝐼𝐹

DHI refrain from providing such an estimate as this depends on two crucial unknown factors, A and SIF:

• A is the cross sectional area: the observational data is for a fixed position only and thus applying this value over a wide cross-sectional area is likely to yield misleading resource estimate. For example, the stage 1 assessment previously showed that the power density varies across the Dover tidal energy zone, increasing with distance from the southern breakwater. It is suggested that the Stage 3 numerical model is a more appropriate tool for this type of analysis.

• SIF is the Significant Impact Factor: the SIF represents the proportion of the available kinetic energy flux that may be extracted without causing detrimental impacts on the environment or to the underlying resource. Commonly the SIF is taken to be in the range of 10-20%. However, there is no rigorous scientific basis for such an assumption. It is recommended that an assessment of the extractable resource be based on known characteristics of tidal turbines rather than arbitrary fractions of the available resource.


5 Conclusions and recommendations

Based on the results and analysis of the field data at the Port of Dover and the rapid resource assessment in /1/, the following conclusions and recommendations have been drawn.

• Observations from a bed mounted AWAC provide an improved understanding of the tidal currents at the port of Dover, with implications for the assessment of the local available tidal energy resource.

• The observed current speeds from the field study have shown that tidal current speeds at the Port of Dover are larger than predicted by the model used for the rapid resource assessment during stage 1. A tidal analysis has suggests that the major amplitude of the principal semidiurnal tidal constituent is around 30% higher than the modelled tide.

• Based on this new information, we believe it necessary to refine the existing DHI hydrodynamic model at the Port of Dover in order to provide a more accurate replication of observed current speeds. Thus, a detailed calibration-validation exercise will be an integral component of the stage 3 work.

• Based on the site observations, the available tidal energy resource in immediate vicinity of the measurement location is approximately double the value during the rapid resource assessment during Stage 1. This reinforces a key characteristic of tidal kinetic energy flux: a small increase in current speed has a comparatively large effect on the available resource.

• The vertical velocity profile shows that current speed vary with height above the seabed. This may have implications for detailed resource assessment, depending on the specifics of any tidal turbine design. For example, a floating device is likely to experience larger current speeds (and this more kinetic energy) than a device moored on the seabed. In the interim, however, we believe that the assessment from depth-averaged values is both a convenient and reliable basis for informing decision makers.

• Despite the upward revision of the available tidal resource, the ability to capture this “additional” energy remains dependent on identifying suitable technologies for these specific site conditions.


References

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

/2/ Legrand, C. 2009. Assessment of tidal energy resource. Black and Veatch Ltd. for the European Marine Energy Centre (EMEC), Stromness, UK

/3/ Titan Environmental Surveys Limited. 2014. Dover Tidal Assessment: AWAC and VMADCP survey data report. Titan Report Ref. CS0429.

doverharbourboard_stage2_fieldstudy.docx / mce / June 2013

APPENDICES

doverharbourboard_stage2_fieldstudy.docx / mce / June 2013

APPENDIX A

Output from harmonic tidal analysis

doverharbourboard_stage2_fieldstudy.docx / mce /June 2013 B-1

Table A.1 Output of harmonic tidal analysis of observed currents at Dover

Dover Harmonic Analysis of CS (2014-03-30 - 2014-04-30 ; 10min) Dover Tidal Assessment N = 4483, Method = IOS, Type = Semidiurnal Const. Major Minor Incl. Phase MSF 0.04 0.01 127.71 228.02 2Q1 0.01 -0.00 43.63 46.91 Q1 0.02 -0.00 42.57 325.08 O1 0.10 0.00 24.05 13.24 NO1 0.02 -0.00 16.82 223.06 K1 0.07 0.00 23.81 132.51 J1 0.01 -0.00 26.04 266.45 OO1 0.02 -0.01 19.28 51.54 UPS1 0.02 -0.01 176.74 243.69 N2 0.17 -0.02 21.37 319.64 M2 1.33 -0.13 24.77 339.80 S2 0.46 -0.05 24.03 26.04 ETA2 0.07 -0.01 27.49 250.18 MO3 0.04 -0.01 26.71 351.62 M3 0.00 0.00 24.85 55.15 MK3 0.02 -0.00 37.02 98.18 SK3 0.01 0.00 12.85 127.80 MN4 0.04 -0.01 22.15 287.88 M4 0.21 -0.05 32.52 296.42 MS4 0.15 -0.04 29.64 343.68 S4 0.02 0.00 29.20 87.70 2MK5 0.01 -0.01 125.57 131.60 2SK5 0.00 -0.00 38.74 12.93 2MN6 0.03 0.00 41.53 97.71 M6 0.07 0.00 33.64 125.58 2MS6 0.08 0.01 32.84 173.00 2SM6 0.01 0.00 32.81 229.35 3MK7 0.01 -0.01 67.24 278.92 M8 0.03 0.01 40.99 116.62

doverharbourboard_stage2_fieldstudy.docx / mce /June 2013 B-2

Table A.2 Output of harmonic tidal analysis of modelled currents speeds at Dover (stage 1 rapid resource assessment)

Dover - stage 1 model Harmonic Analysis of CS (2013-03-15 - 2013-04-15 ; 10min) Stage 1 model N = 2977, Method = IOS, Type = Semidiurnal Const. Major Minor Incl. Phase MSF 0.04 -0.00 10.09 44.60 2Q1 0.01 0.00 54.25 305.01 Q1 0.02 -0.00 34.04 326.60 O1 0.08 0.00 38.65 25.86 NO1 0.01 -0.00 33.79 329.12 K1 0.06 0.00 39.85 151.58 J1 0.01 -0.00 34.18 348.13 OO1 0.02 -0.00 31.66 40.93 UPS1 0.00 0.00 33.85 206.24 N2 0.17 -0.01 36.77 319.31 M2 1.05 -0.05 37.24 345.00 S2 0.42 -0.02 37.01 24.35 ETA2 0.04 -0.00 38.73 289.95 MO3 0.01 -0.00 37.55 2.48 M3 0.00 -0.00 33.10 253.14 MK3 0.01 -0.00 49.76 100.36 SK3 0.01 -0.00 33.31 124.71 MN4 0.04 -0.01 43.82 266.84 M4 0.14 -0.02 44.36 292.36 MS4 0.10 -0.02 45.00 339.20 S4 0.01 -0.00 33.84 20.82 2MK5 0.01 0.00 28.35 352.94 2SK5 0.00 -0.00 23.02 268.32 2MN6 0.02 -0.00 38.12 105.80 M6 0.05 -0.00 33.49 135.53 2MS6 0.08 -0.00 34.76 180.76 2SM6 0.02 -0.00 38.72 198.65 3MK7 0.00 -0.00 40.60 45.15 M8 0.01 -0.00 48.78 78.06

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