Tidal Stream Energy

Tidal stream is the periodic horizontal flow of water accompanying the rise and fall of the tide. This horizontal flow of water stores an enormous amount of energy that can be extracted and used for the purpose of power generation. Devices that are used for the purpose of extracting energy from the flow of tidal stream are aptly called ‘Tidal In-Stream Energy Conversion Devices” or TISECs in short.

Although TISECs work pretty much like how windmills work, the predictability of tidal activities and the higher water density as compared to air has made this technology as equally attractive as windmills or even more.

This technology is still in its infancy despite the potential for a reliable and predictable source. Tidal stream technology has the advantage over tidal barrages in environmental and ecological issues. This technology is less intrusive than tidal barrages and the installation and maintenance methods of tidal streams are simpler than barrage. The cost of building tidal streams will be very site specific and will depend on the technology used. Maintenance costs will be the main cost during the life of the project.

 

TIDAL STREAM ENERGY TECHNOLOGY

The devices that are used for the purpose of extracting tidal stream energy come in four common forms: horizontal axis turbines, vertical axis turbines, oscillating hydrofoil devices, and venture devices. While each of the forms has distinct advantages over the others but horizontal axis turbines are in general better developed.

 

MARINE CURRENT TURBINES

The world’s first commercial scale tidal stream turbine was deployed in Northern Ireland’s Strangford Lough in May 2008 and has a rated capacity of 1.2MW in a flow of 2.4m/s. This gigantic tidal energy converter, which is mounted on a tubular steel monopole, is given the name “SeaGen”. It comprises a pair of axial flow rotors with blades that could be pitched through 180o to allow them to operate in both the ebb and the flood tides (bi-directional). Moreover, the blades can be pitched into a neutral position to stop the turbine gently even at full flow, a feature so much better than having a powerful brake as compared to fixed pitch turbines.

 

 

Figure 3.2.1 (a): SeaGen’s rotors rotate at around 10 to 15 rotations per minute, whereas a ship’s propeller by comparison typically runs 10 times as fast.

(Picture obtained from www.marineturbines.com)

 

 

 

The company who owns SeaGen is Bristol- based Marine Current Turbines Ltd (MCT). MCT has long proven its ability in extracting tidal energy ever since year 2003 where a turbine capable of producing 300kW was installed off the north Devon coast and is still thriving in open sea conditions. This turbine is the predecessor of SeaGen and “Seaflow” is the name.

 

 

Figure 3.2.1 (b): Seaflow, a conceptual tidal turbine and the predecessor of SeaGen hit a record-high output of 300kW in year 2004.

(Picture obtained from www.marineturbines.com)

 

 

 

MCT has successfully demonstrated for the first time the commercial potential of tidal energy as a viable alternative source of renewable energy through SeaGen. The power generated by SeaGen is being purchased by an Irish energy company, ESB Independent for its customers in Northern Ireland and the Republic of Ireland.

Very soon MCT is going to plant seven of those SeaGen units off the coast of the Welsh Island of Anglesey in a fast flowing patch of 25 meter deep open sea known as The Skerries

This mammoth sea farm will be able to produce 10.5MW of power and if the planning goes smooth the sea farm can be commissioned as early as year 2011.

 

 

Figure 3.2.1 (c): SeaGen consists of a pair of twin axial flow rotors of 15m to 20m in diameter, each driving a generator via a gearbox.

(Picture obtained from www.marineturbines.com)


 

 

 

 

 

 

Figure 3.2.1 (d): Artist’s impression of a row of turbines such as the ones MCT plans to install off the north Anglesey coast and shows one raised for maintenance from a small boat. This is an important feature because MTC claims that underwater intervention using divers or ROVs (Remotely Operated Vehicles) is virtually impossible in locations with such strong currents as are needed for effective power generation.

(Picture obtained from www.marineturbines.com)

 

 

 

 

CLEAN CURRENT'S TIDAL TURBINE

A demonstration project was carried out at Canada’s Race Rocks. A shrouded turbine designed by Clean Current, given the name Tidal Turbine Generator (TTG) was deployed under the water and was anticipated to provide electricity to replace 2 diesel generators that are used to power up a light house.

TTG was installed in about 20 metres of water near Race Rocks during the period of July to September 2006. The unit was tested for 2 months and was later connected to a control system that feeds electricity into the battery storage system at Race Rocks. Though the water lubricated bearing system was a disappointment but Clean Current is dedicated to refitting the design with a new bearing system and to come out with a design which will meet commercial scale.

 

 

 

Figure3.2.2 (a): The power generated by TTG was channelled through to power the light house at Race Rocks.

(Picture obtained from www.cleancurrent.com)

 

 

 

 

 

Figure 3.2.2 (b): Final assembly of the Clean Current tidal turbine generator (TTG).

(Picture obtained from www.cleancurrent.com)

 

 

 

 

The turbine generator is entirely below the surface of the water and has no exposed blade tips owing to the novel duct design. The environmental friendly feature of the design is further enhanced by having a large hole in the middle of the unit for fish and sea mammals to pass through in case they enter the inlet duct area. The tidal generator is supported on a single 32” diameter pile grouted into the bedrock. Therefore, the permanent footprint of the unit on the seabed is also minimal. The unit has been designed to rotate only when the current velocities exceed 2 knots (1m/s).

 

 

Figure 3.2.2 (c): TTG has successfully extracted power in flows up to 6.6 knots.

(Picture obtained from www.cleancurrent.com)

 

 

 

 

LUNAR ENERGY'S ROTECH TIDAL TURBINE (RTT)

Instead of having a couple of gigantic turbines working on converting the movement of tidal flow to electricity, it is likewise possible to have small turbines working in a large group while generating an equivalent amount of electricity. This group of turbines that are much smaller in size as compared to the many commercial tidal turbines replicates a power generation plant, or aptly called a “sea farm”.

 

Figure 3.2.3 (a): Rotech Tidal Turbine. (Picture obtained from www.lunarenergy.co.uk)

 

Britain’s Lunar Energy is probably the pioneer in making the idea of a “sea farm” feasible. Lunar Energy has come out with a product which serves just the right purpose. A Rotech Tidal Turbine (RTT) of 1 MW capacity is likely to be the initial standard commercial unit. It is envisaged that the first commercially viable fields could vary in size from 100 to 500 linked 1MW RTT units, though the economic and physical practicalities of a 2MW unit are already being examined.

The first commercial “sea-farm” is planned to be operational in 2011 and will be located off St. Davids Head on the Welsh Coast. The farm will comprise 8 units of 1MW RTT. The idea of a sea farm was again proven to having commercial values when Lunar Energy signed a Memorandum of Understanding with Hyundai Samho Heavy Industries (HSHI) and Korean Midland Power (KOMIPO) to develop the 1MW RTT unit for deployment into Korean coastal waters. If the testing sounds promising there will be a full commercial development in the Wando Hoenggan waterway of up to 300 units of 1MW RTT at a cost of £500 million. This sea farm will be operational by 2015.

What has made the RTT unit so special is the venturi shaped duct at the heart of the unit. The venturi shape duct accelerates tidal flows which pass through the turbine. Therefore, the energy that can be captured by blades of a given diameter is increased. Lunar Energy claims that the output of the ducted turbine is almost exactly twice that of the unducted turbine at the same yaw angle.

 

Figure 3.2.3 (b): The use of a duct ensures that the energy contained within the tidal stream is always straightened to flow perpendicularly through the turbine itself. (Picture obtained from www.lunarenergy.co.uk)

 

 

 

VERDANT POWER'S FREE FLOW KINETIC HYDROPOWER SYSTEM

Also embracing the idea of a “sea-farm” is Verdant Power, a world leader in the design and application of marine renewable energy solutions. The design, called Free Flow Kinetic Hydropower System is an axial-flow three-bladed turbine. Verdant Power is committed to commercializing the design and has already deployed six of its turbines in New York City’s East River under a demonstration project called Roosevelt Island Tidal Energy (RITE) Project. The demonstration project claims to have the world’s first grid-connected array of tidal turbines, pushing ahead of Lunar Energy’s RTT units.

The demonstration project has successfully delivered power to two businesses on Roosevelt Island; a supermarket and a ‘motorgate’ parking facility. Not to be over contented, Verdant Power is now working on a next-generation design that will be cheaper to mass-produce and is anticipating to install a farm of at least 100 turbines at the East River, hoping to generate enough power for nearly 8000 New York homes in very near future.

Not to be left out by its counterpart in the South, Verdant Power Canada has also launched a demonstration project at St Lawrence River near Cornwall, Ontario. It is anticipated that a farm of a commercial scale and having a capacity of 15MW could be built no later than 2012 at the site. Though having to commercializing the site is the ultimate goal, Phase 1 of the project will simultaneously demonstrate an updated methodology for deploying and anchoring turbines for enhanced operational and cost effectiveness.

Figure 3.2.4 (a): The Free Flow kinetic hydropower system of Verdant Power has demonstrated a Fully bidirectional tidal operation.

(Picture obtained from www.verdantpower.com)

 

 

 

 

 

 

 

Figure 3.2.4 (a): Artist’s impression of a tidal farm with Verdant Power’s Free Flow kinetic hydropower system.

(Picture obtained from www.verdantpower.com)

 

 

This study aims to identify coastal areas in the State of Sarawak that have the potential of supplying a minimum of 150 kilowatts of power by making use of the energy stored in tidal streams (currents).

 

LITERATURE REVIEW

Limitations Pertaining to the Preliminary Study of the Extraction of Tidal Stream Energy in Sarawak
A report prepared by Triton Consultants Ltd. for Canadian Hydraulics Centre is in a way similar to the report presented here. The Canadian report was meant to provide an analysis background to a preliminary tidal stream resources inventory under the Canada Ocean Energy Atlas Project.

Table 3.4 (a): The list of sites under investigation of this report. The speeds of the tidal stream for the first seven were published in year 2005 and 2006 by the Director of Marine, Sarawak. Such information is no longer published since 2006.

 

DESCRIPTIONS

CANADA/TRITON[12]

SARAWAK/ UNIMAS

Sailing Direction

For the entire of Canada, 21 of
them.

None

Nautical Charts

 

489 digital charts from NDI, St. John’s, Newfoundland; 700 charts from CHS;
unknown number of charts from Triton’s own chart database and at the map library
at the University of British Columbia.

44 digital charts from the Director of Marine, Sarawak; 12 digital charts from Sime Darby.

Tide and tidal stream tables

7 volumes, unknown number
of different sites.

2 volumes of tidal stream
tables (similar sites), for year 2005
and 2006 respectively; Sarawak Hourly and High & Low Tides
Tables (Including standard ports of
Sabah), 2008

Tidal stream stations

The report claims to have “probably less than 50 stations”.

None

Tidal modelling software

CEAPack, owned by Triton

None

Number of sites investigated

Not mentioned anywhere in the report but the top 50 sites of highest potential were listed.

8


Table 3.4 (b): Comparison of the resource available for both the Canadian and Sarawak report. Both reports have resorted to using information which was published by third parties (but reliable sources). The Canadian report was prepared in year 2006.

 

GUIDELINES FOR TIDAL ENERGY MAPPING

The Electric Power Research Institute, EPRI is a non-profit organization actively involved in the energy industry. It claims to have an annual budget of about $300 million and provides expertise to solve today’s toughest energy and environment problems. In year 2006, EPRI published a set of guidelines for the use of preliminary estimation of power which is generated by turbines or any devices deployed in a tidal stream [7]. These devices were termed “Tidal In-Stream Energy Conversion” or ‘TISEC’ devices in the EPRI report.

The guidelines were prepared under a demonstration project called “the EPRI North American Tidal In-Stream Power Feasibility Demonstration Project”. Co-producers of the report were prominent institutions including Virginia Tech and University of Washington. The methodology proposed in the EPRI report is adapted and used in this report. The methodology proposed by EPRI helps in finding the extractable power at all eight sites that are under investigation by this report. The methodology is summarized in Table 3.5.

 

 STEPS

DESCRIPTION

 APPLICABILITY

 REMARKS

 

Site screening

Finding sites with
average peak more than
1.5m/s.

Applicable

Nil

 

Acquiring information

Obtaining the speed of
tidal stream at the
surface.

Applicable

The speed is obtained from the stream tables published by the Director of Marine,
Sarawak and on-site
measurement for Pulau Triso.

 

Extrapolation of current
speed

To be performed if the
site where the turbines
will be installed is
different from the site
where measurements are taken.

Not
applicable.

No cross-sectional
area is assumed.

 

Accounting for depth
variation in speed

Calculating the speed of
tidal stream at below
surface where the
turbines will be deployed.

Applicable.

Nil.

 

Cross-sectional area

Area is needed for the
calculation of mean
annual kinetic tidal
power.

Not
applicable.

No cross-sectional
area is assumed.

 

Calculating available
resource

Obtaining the mean
annual kinetic tidal
power, or simply the
available
theoretical/physical tidal
energy,

Applicable,
but adapted.
See remarks.

Annual kinetic
energy flux density
is calculated instead
because no crosssectional
area is assumed.

Table 3.5: Methodology proposed by EPRI’s report which was meant for preliminary tidal energy assessment. The descriptions of individual steps may not be exactly the same as what is been used in the EPRI report. The column ‘Applicability’ indicates if the individual step is relevant to this report and reasons are given in ‘Remarks’ if the step is not applicable.

 

MINIMUM SPEED FOR POTENTIAL SITE

The UK’s Department of Trade and Industry (DTI) proposed a set of general selection principles in their 2005 report for the purpose of mapping potential high tidal stream energy sites [11]. According to the report, stronger tidal streams are only found around certain features, such as channels or constrictions between islands.

Despite the statements made by DTI, researchers from the Centre for Research in Energy and the Environment of Robert Gordon University claims in a report funded by EPSRC, that there are three typical regions where tidal energy extraction will prove economic. Those regions are:
i. areas with an unusually large tidal range,
ii. enclosed bays and estuaries which are subject to resonance effects, and
iii. channels where the bodies of water at each end of the channel are out of phase with each other, and exhibit at least moderate tidal amplitudes, thereby creating a strong pressure gradient across the channel.

The speed of the tidal stream reveals the available physical energy a site has. It is not worth extracting tidal energy if the speed of the stream reveals less or no potential for energy extraction. Therefore, it is important to determine a desired speed which is the minimum speed for the energy extraction to seem to have a worthy return on investment.

The desired speed varies and relies on the limitation of present technology as well as the economical value of making use of tidal stream to generate power. A few reports which are dedicated to assessing available tidal stream energy made a good reference towards determining the desired speed. One of the desired speeds proposed by the reports will be used in this report.

Blunden and Bahaj of the Sustainable Energy Research Group from University of Southampton made a good summary of tidal energy resource assessment in a manuscript published in year 2007 [5]. According to the manuscript, a European Commission (EC) funded study as part of the JOULE II non-nuclear energy programme produced a database of tidal stream energy resources around Europe, including sites in the UK in year 1996 (hereafter EC96). The criterion for site selection in that report, in most cases was “peak tidal stream speed” greater than 1.5m/s.

Also mentioned in the manuscript, the Energy Technology Support Unit produced an estimation of available tidal stream energy resources in the UK in year 1993 (hereafter ETSU93). ETSU93 took into account sites with “mean spring peak” of greater than approximately 2m/s.

In a report prepared by AEATFES [1], sites having the greatest interest for exploitation are those with a “maximum velocity” of 1.5m/s. The report was entitled “Status and research and development priorities. Wave and Marine Current Energy. 2003.” It was prepared for the UK’s DTI as part of their New and Renewable Energy Programme. The report claimed in the executive summary that the attraction towards ocean energy is clear and the report was dedicated to consider the status of tidal stream and wave power technology and research and development (R&D) requirements.

A report prepared by Black and Veatch which was commissioned by Carbon Trust in year 2005 presented a study on tidal stream resource assessment in the UK [2]. The report took into account sites with a “mean spring peak” of 2.5m/s.

European Sustainable Electricity (EUSUSTEL) through its report entitled “Energy from the Sea” has published an analysis of future source of energy from the sea [8]. The report outlined four kinds of energy that can be extracted from sea. Tidal stream power or “current power” as been phrased in the report was one of the four kinds of energy mentioned. Sites with tidal stream faster than 1.75m/s are observed as potential sites for exploiting tidal stream power.
The Marine Energy Research Group (MERG) of Swansea University published a report about site selection protocols for tidal stream turbine deployment in the Severn Estuary in year 2008 [13]. Sites with “peak flow” of 2m/s and above were areas interested to them.

EPRI on the other hand, made the desired average peak 1.5m/s [7]. The report presented here follows the methodology proposed by EPRI. As such this report investigates if any sites under investigation have an average peak of at least 1.5m/s.
Table 3.6: The desired speeds as being used by various reports. The desired speed relies on the limitation of present technology as well as the economical value of making use of tidal stream to generate power. The speed in ( ) is the speed which is being used in this report. The column ‘Parameter’ gives the kind of speed or terminology used in each report. Readers should not be too carried away by the different terminologies used. The column ‘Remarks’ gives the definition of the parameter if it is available from each report. Other information is included in the column ‘Remarks’ if no definition is available.

 SOURCE

DESIRED MINIMUM SPEED, m/s

 PARAMETER

 REMARKS

EPRI (2006) [7]

(1.5)

“Average flood or
ebb peak”

Sites were also considered if there was anecdotal evidence for high tidal current speeds, even when the 1.5m/s requirement is not met.

ETSU931 (1993)

2.0

“Mean spring
peak “

Depth greater than 20m is also one of the criteria.

EC 961 (1996)

1.5

‘Peak’

Nil.

AEATFES (2003) [1]

1.5

“Maximum
velocity”

For sites with nonoscillating
currents, the maximum “current velocity” may need to exceed 1.0m/s before the site is considered economic.

Triton2 (2002)

2.0

“Current speed”

 

Nil.

Black and Veatch (2005) [2]

2.5

“Mean spring
peak”

Mean spring peak speed is defined as the speed of tidal stream taken at 5m below the surface.

MERG, Swansea (2008) [12]

2.0

‘Peak’

The report was meant to supply site selection protocols for tidal stream turbine deployment in the
Severn Estuary. The term ‘peak’ was not defined.

EUSUSTEL [8]

1.75

“Current speed”

The phrase “current speed” was not defined.

1. The report was not directly accessed during when this report was prepared. Information pertaining to the reports was extracted from work performed by Blunden and Bahaj, 2007 [5].
2. The report was not directly accessed during when this report was prepared. Information pertaining to the report was extracted from AEATFES, 2003 [1]. The relevant report entitled “Green energy study for British Columbia, phase 2: Mainland, tidal current energy” was prepared in year 2002.

 

CLEARANCES REQUIREMENT FOR VESSELS

If the turbine which will be used to generate power is deployed just below the surface of the sea, using the surface speed to find the extractable energy will make sense. In reality, this will not happen. The turbine will have to be placed deep enough so as not to obstruct and cause harm to sea vessels. The EPRI report proposed a 15-20m clearance for oceangoing vessels [7]. A 5m clearance is enough if the turbine is deployed in places where only shallow-draft vessels will pass through.
The clearance needed at the bottom of the sea is another criterion not to be missed. EPRI claimed that a clearance equivalent to 1/10 of the mean lower low water (MLLW) depth will be required. MLLW is the average height of the lower low waters at each tidal day.


In year 2007, a report entitled “Economic viability of a simple tidal stream energy capture device” was prepared for the UK’s DTI [10]. The report involved various institutions, including WUMTIA of the University of Southampton, ALSTOM Power Ltd and LOG+1. The report compared the overall economics of two different kinds of horizontal axis tidal turbine devices. According to the report, the typical rotor diameter for tidal stream turbine ranges from 15m to 30m based on existing design.

 

HARNESSING TIDAL ENERGY WITHOUT HARMING THE ENVIRONMENT

All energy extraction technologies will have environmental impact. Tidal stream energy extraction is of no exception. In year 2007, a paper entitled “Tidal current resource assessment” was published by a group of researchers representing the University of Edinburgh and the Robert Gordon University. One of the aspects investigated in that paper was the impact of energy extraction on the pattern of tidal flow.

The authors made use of a simple and a complex model to illustrate the impact of tidal stream extraction against the flow pattern of tidal stream as well as the reduction in flow speed. Both models showed that tidal energy extraction slows down the flow. The simple one dimensional flow model used in that paper proved that the flow will be slowed down by no more than 7% if 25% of the tidal stream energy is to be extracted. The question is will such are reduction cause unacceptable impact to the natural environment and to marine life?

No consideration was given on the above-mentioned issue in reports that were prepared in early and mid 90’s. Later reports, having realizing the significance of tidal energy extraction towards affecting the environment had come out with a way to obtain extractable energy without causing disturbance to the environment.

A “ceiling figure” is utilized to represent the maximum allowable extraction of energy from tidal stream. This figure is expressed in percentage and limits the amount of energy to be extracted so as not to cause any harm to the environment and the creatures living in it.

Early modeling was conservative and a 10% figure was suggested by Bryden and Melville in their 2004 report. The proposed figure was based on simple modeling by using open channel flow theory to simulate a tidal channel between two unconstrained water bodies.

Black and Veatch proposed a 20% figure a year after [3]. The report prepared by Black and Veatch was meant to assess tidal stream resource in the UK. Black and Veatch claimed that the 20% figure was ‘indicative’ and varies as according to individual sites.

Garrett and Cummins suggested a 38% high figure the same year Black and Veatch had their report prepared [4]. The proposed figure was again based on numerical modeling and on predetermined conditions.

Bryden and Couch chose to use 25% as the ceiling for extractable energy in their attempt to study the influence of energy extraction against the environment [6]. Again, no justification was given for choosing the number. Although the paper was not meant to propose a figure for extractable energy but as both authors are experienced participants in the industry it is deemed appropriate for this report to present the assumption made by them as a reference.

In the guidelines published by EPRI in year 2006, a 15% figure was proposed for extractable energy [7]. EPRI claimed that researchers who are based in the UK had estimated the mean annual power extracted to be no more than 10% to 20% of the naturally available physical energy flux. A midpoint was therefore picked and was used as the baseline for extractable energy throughout the EPRI report. The 15% figure proposed by EPRI will be used in the report presented here as an attempt to follow the guidelines published by EPRI.

 

 SOURCE

 EXTRACTABLE ENERGY, %

 JUSTIFICATION FOR THE % CHOSEN

 REMARKS

EPRI (2006) [7]

(15)

Mid-point of the estimates made by UK researchers and
based on a paper published by Bryden and Melville
in year 2004 entitled “Choosing and evaluating sites for tidal current development.”

 

It was claimed by the EPRI
report that UK researchers have “variously estimated” the mean annual power extracted to be no more than 10% to 20% of the naturally available physical energy flux. However, the source of such a claim was not further elaborated.

Black and Veatch (2005) [3]

20

No justification was
given in the report.

 

The phrase “Significant ImpactFactor” (SIF) is used to denotethe percentage of extractable power. Black and Veatch claimed that the 20% figure used was ‘indicative’ and should be determined for each site individually. It was also mentioned in the report that the 20% figure is the upper limit which might well require downwards revision.

Bryden and Melville1 (2004)

10

Based on numerical
modeling.

 

Based on the application of
open-channel flow theory to
simulate a tidal channel
connecting two unconstrained
bodies of water (for example,
between two islands)

Bryden and Couch (2007) [6]

25

 

No justification
required. See
remarks.

 

The 25% figure was introduced
to see the influence of energy
extraction on tidal stream in a
simple channel model.

 

Garrett and Cummins2 (2005)

38

 

Based on numerical
modeling.

 

The model takes into consideration bottom drag, flow
acceleration, and exit separation
effects. The 38% value is only
valid when the bottom drag is
much less important than separation effect and it must be
evaluated at the exit of crosssectional area.

ETSU 933

100

Not applicable. See remarks.

See No consideration was given on the effect of extracting energy on flow conditions.

Table 3.6: Maximum allowable energy extraction proposed by various reports and is expressed in percentage of extractable energy (%). The column ‘Justification’ gives the reason behind each selected figure whereas the column ‘Remarks’ elaborates further. The figure in ( ) is the figure chosen for this report.