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Studies on tidal power from the estuaries of North-West England
Conference Paper · January 2010
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STUDIES ON TIDAL POWER FROM THE ESTUARIES OF NORTH-WEST ENGLAND
Nicholas Yates1, Richard Burrows1, Ian Walkington2, Terry Hedges1
Department of Engineering, University of Liverpool, United Kingdom, Brodie Tower, Brownlow Street,
Liverpool L69 3GQ
2
Proudman Oceanographic Laboratory, United Kingdom, Joseph Proudman Building,
6 Brownlow Street, Liverpool L3 5DA
1
ABSTRACT
The paper presents findings from a recent two-year Joule Centre (JC) study, ‘Tapping the Tidal Power Potential of the
Eastern Irish Sea’ funded by the Northwest Development Agency (NWDA), as well as subsequent developments. This
evaluated the scope for reliable electricity generation from a combination of estuary barrages/lagoons and tidal-stream
energy devices using 0-D and 2-D computer modeling. The emphasis was towards conjunctive operation incorporating
allowances for other schemes outside the region, including a ‘Severn’ barrage, an integral part of the study being
potential impacts of the energy removal on the overall tidal dynamics of the Irish Sea and environmental issues arising.
Estimates arising suggest that the estuarial waters of the North West stretching from the Dee & Mersey, to Morecambe
Bay and the Solway Firth are capable of meeting about 5% of national electricity demand. This places it on a par with
the ‘Severn’ estuary, as a potential major contributor of renewable energy in the UK.
INTRODUCTION
The geographical location of the United Kingdom and the seas that surround it provide internationally enviable
renewable resources. Technologies for wind power extraction are now mature and an increasing role for the
opportunistic capture of this intermittent energy source for the electricity grid is firmly established. Marine wave energy
offers even greater scope for the future. Even more exclusive, however, is the potential for tidal energy extraction from
around the UK coastline, it being estimated that the UK holds almost half the European resource at 50 TWh/year. The
most attractive locations for harnessing tidal power are estuaries with a high tidal range for barrages, and other areas
with strong tidal currents (e.g. straits and headlands) for free-standing tidal stream devices. Barrage schemes, drawing
on established low-head hydropower technology, are fully proven. The La Rance plant in France has now passed its 40th
year of operation.
In the 1980s, 8 major estuaries were identified where tidal barrages would be capable of procuring over 40TWh/year. In
rank order of scale, they were the Severn, Solway Firth, Morecambe Bay, Wash, Humber, Thames, Mersey and Dee
(see UKAEA, 1980 and 1984, Baker, 1991). Thus, about half of this energy was located in the North West of England.
A barrage delays the flux of water as the tidal level changes: holding back the release of water as the tide level subsides
under ‘ebb generation’ so that the ‘head’ (water level) difference is sufficient for turbine operation; deferring the entry
of rising tidal flow to the inner estuary basin for ‘flood generation’. In ’dual mode’, there is a combination of both. Each
mode has some restricting effect as energy is extracted from the tide, so reducing the range of tidal variation within the
basin: ebb generation generally uplifts low water levels; flood generation reduces levels; and dual mode results in small
mean level changes (see Figure 1). Therefore, a degree of environmental modification is inevitable, but this does not
necessarily imply serious degradation. It is also important to recognise that barrage schemes are unique amongst power
installations, being inherently multi-functional infrastructure, offering flood protection, road and rail crossings and
significant amenity/leisure opportunities, amongst other features.
The recent NWDA/Joule Centre (JC) sponsored study (www.liv.ac.uk/engdept/tidalpower) aimed to re-appraise the
earlier estimates of the potential energy yield from North West barrages and to further this detailed technical scrutiny
with assessment of the various operational mode options (ebb, flood or dual) and the broader effects arising from their
conjunctive action. To this end, two levels of technical evaluation were explored.
MODELLING
Zero-dimensional (0-D) modelling
Zero-dimensional modelling (employing the M2, lunar semi-diurnal, and S2, solar semi-diurnal, tidal constituents) was
used to synthesise the local behaviour of a barrage with turbine and sluicing systems, assuming flat water levels either
side. This approach rigorously treated the hydraulics and energy generation from the double-regulated bulb turbine
systems considered here, but made no allowance for the external tidal hydrodynamics. The bespoke software routines
were developed using MATLAB, with the user interface shown in Figure 2. These are presently being extended to
represent lower-head tidal ‘fence’ or ‘reef’ systems.
Figure 1. Different modes of operation of a tidal barrage.
Figure 2. The 0-D modelling interface: example for Dee scheme.
Two-dimensional (2-D) modelling
Two-dimensional modelling (employing the 5 major tidal constituents for the region: M 2, S2, N2, O1 and K1) was used
to account for energy dissipation by bed friction throughout the modelling domain, and also to allow possible changes
in local circulation patterns caused by tidal stream farms to be explored. Since multiple estuary sites in the Irish Sea
were included, and in order to incorporate any likely impacts from the operation of a potential Severn barrage in the
Bristol Channel, the model domain was extended well beyond the Continental Shelf. The ADCIRC modelling platform
adopted employed almost 750,000 cells forming the unstructured finite element grid, as shown in Figure 3.
Figure 3. ADCIRC tidal modelling domain showing bathymetry and unstructured grid (North West barrages
shown inset).
NWDA/JC STUDY FINDINGS
In the following discussion, a ‘DoEn’ (or 1xDoEn) barrage installation is taken as one with installed turbine capacity
(and complementary sluicing) consistent with the outcomes of the UK Department of Energy’s 1980s studies (UKAEA
1980, 1984), with the characteristics of extracting about half of the available energy during ebb generation. This
arrangement was found in these early studies to yield electricity at minimum cost. Here it is adopted as a baseline
because schemes of multiples of this turbine capacity (i.e. 2xDoEn, 3xDoEn, etc) were considered later as alternatives,
operating in either ebb mode or for dual (two-way) generation.
The 0-D modelling routines consistently under-predicted the DoEn energy generation figures by approximately 15%,
this being attributable to different assumptions in the treatment of sluicing characteristics, unquantifiable departures in
turbine performance characteristics, and the different levels of tide-to-tide control on the selected optimal generation
window. Nevertheless, the work confirmed the scale of resource predicted by DoEn in the 1980s for ebb-only power
generation. These 1xDoEn arrangements were also confirmed as generally leading to the lowest cost of energy
produced (Burrows et al., 2009, (a)/(b), 2010).
In the 0-D studies, different turbine sizes, generator ratings and sluice capacities were investigated; and the ‘best’
options selected in summarising outcomes are shown in Table 1 and Figure 4. Whilst these figures are considered to be
reasonable estimates of potential energy returns, further turbine conditioning (choice of diameter, generator rating and
rated head, etc) and increased sluice capacity might be expected to further enhance energy capture. At the same time,
the detailed hydraulic design of turbine ducts and sluice passageways might give rise to minor (entry/exit) head losses,
so reducing the turbine driving head and hence the energy production. As a result of these contrasting factors, an
uncertainty range in the region of ±10% was suggested.
Table 1. 0-D and 2-D Energy outputs from the NWDA/JC Study
Barrages
Solway Firth
Morecambe Bay
Mersey
Dee
Dee Outer
1 x DoEn Ebb Mode Energy (TWh/year)
0-D Model
2-D Model
8.44
9.66
5.83
5.98
1.07
0.57
1.35
0.89
4.60
1 x DoEn Dual Mode Energy (TWh/year)
0-D Model
2-D Model
7.78
6.82
5.75
3.99
0.98
0.74
1.30
0.80
Figure 4. Illustration of power pulses and basin levels for schemes on the Dee.
The figures in Table 1 suggest that ebb-only generation schemes in the North West with 1xDoEn turbine provision
could meet about 5% of UK demand at the lowest unit cost of electricity produced. Dual-mode operation with 1xDoEn
turbine installations is typically found to yield energy in the range 78-98% of that achieved from the equivalent ebb
mode, and at a cost penalty in the region of 20% on the basis of the turbine having a relative efficiency in the ‘reverse’
mode of about 80%.
Only a limited number of 2-D runs were completed due to time constraints and, for ease of application, unified
operational controls were applied (i.e. constant time delay before generation commences across all barrages in the
system). As a result, no attempt has been made to replicate the best performance achieved in the 0-D modelling of each
individual scheme. Nevertheless, power predictions for the Solway Firth, Morecambe Bay, Mersey, Dee and Severn
have been made under conjunctive operation in a number of operating modes, for which changes in tidal amplitude (see
Table 2) and timings are directly modelled and are included in the power predictions. Changes in regional
hydrodynamics were observed, demonstrating the necessity of modelling all the barrages in conjunction. As a
consequence, location specific effects were revealed which altered the power predictions significantly from the 0-D
modelling.
Comparing the energy generation from the 2-D model with those from the 0-D modelling for the 1xDoEn schemes, it is
seen that predictions were in fair agreement for the Solway Firth and Morecambe Bay, but were significantly lower for
the Dee and Mersey. The Dee and Mersey, together with the Severn, suffer a proportionally larger reduction in M 2 and
S2 tidal amplitudes under the conjunctive actions simulated (see Table 2) and this partially explains the inconsistency.
For the estuaries with constrained outlets (the Mersey by way of the down river ‘Narrows’ and the Dee by its
constrained deep channel and associated skewed turbine/sluice arrangement), the hydraulic flow capacity is expected to
be compromised under enhanced local releases and further exploratory study was called for. For the Mersey, it is likely
that the under prediction is also partially due to the high flux through a small area at the barrage causing Courant
numerical stability issues. For the Dee, the barrage line selected in the 1980s is suboptimal as it was based on the
existing bathymetry, and this needs reviewing in the light of where turbines and sluices could be placed if extensive
dredging is assumed. These issues are the subject of ongoing investigation.
Table 2. Change in tidal amplitude at barrages with 1xDoEn ebb-mode operation.
Barrages
Solway Firth
Morecambe Bay
Mersey
Dee
Severn
M2 component (m)
Amplitude
2.50
2.84
2.82
2.32
3.10
Difference
-0.14
-0.15
-0.36
-0.56
-0.80
S2 component (m)
Amplitude
0.80
0.92
0.86
0.68
1.18
Difference
-0.02
-0.03
-0.13
-0.21
-0.19
Figure 5 summarises energy outputs from the 2-D analysis for 1xDoEn ebb-mode generation (with conditions during
spring tides shown as an inset). The 2-D modelling demonstrates that the Eastern Irish Sea resources are highly
synchronised in tidal phase. They are, however, out of phase with the resource of roughly equivalent scale on the
Severn estuary, making possible extended input to the grid from ebb-mode operation.
Figure 5 1xDoEn schemes in ebb-mode operation (spring output shown inset).
With a reported 64GW mean tidal energy input and installed turbine energy extraction capacity in the region of 20GW
for 1xDoEn ebb-mode barrage operations in the North West and Severn (giving rise to a mean extraction rate in the
region of 5-6GW), some change to tidal circulation might be envisaged. This was one of the main questions to be
investigated in the study. The 2-D modelling outcomes for conjunctive operation of the major estuary barrages do
manifest discernible changes (Wolf et al, 2009). At all the barrage locations, the mean tidal amplitudes are reduced (see
Table 2) and a small increase is experienced on the east coast of Ireland. Furthermore, small changes are observed to
tidal residual currents, bottom stress and stratification parameter within the Irish Sea. The likely influence of these
factors as sediment drift, benthic conditions and eutrophication risk, with the associated potential influences on
biodiversity and fisheries, etc, and these require further consideration.
Tidal-stream devices, in the form of single turbines or multiple units in arrays, or cross-river fences installed in estuaries
are unlikely to exploit a significant proportion of the tidal range resource extractable by barrages. Exploratory 2-D
modelling has demonstrated the tendency for water flows to divert around a tidal-stream farm, as a consequence of the
blockage, which is likely to reduce the fraction of incident energy which is extractable, a factor that has not been
properly accounted for in a number of the earlier assessments of tidal stream resources. (Walkington and Burrows,
2010).
ONGOING INVESTIGATIONS
A number of promising areas and computational issues are currently being explored further –
1. Parameter Study building on the work of Prandle (1984) and MacKay (2009)
This study aims to substantiate the theoretical limits on extractable energy with different barrage operating modes,
turbine and sluice capacities, and to explore the energy gain realisable from pumping. Early findings arePracticable ebb operation is capable of realising at best 30% of the potential energy in the tide.
Practicable dual mode operation is capable of realising 45%, though this implies significant (uneconomic) sluice
capacity.
The energy gain from pumping (using pump/turbines) is likely to be limited by low machine efficiencies due to low
operating heads. The work has demonstrated that with unconstrained basin uplifts an energy gain up to about 40%
may be achieved with reasonable assumptions on turbine and pump efficiencies – in contrast to published figures of
up to 325% increases (MacKay 2009).
2. Lagoons – potential energy gains and cost effectiveness
During the NWDA/JC study, Lagoons were explored only briefly, which led to a rough estimate that the cost of energy
would be (at least) 40% more expensive than a comparable estuary barrage scheme. Areas under investigations are –
More detailed examination of how lagoon construction costs will scale with crest height.
Exploration of whether the pumping gains described by MacKay offset the cost increase relative to barrages.
3. Enhancements to the 0-D ‘Generation’ software and resultant studies
The 0-D ‘Generation’ software written during the Joule project has subsequently been enhanced to allow input of
defined head levels for start and end of turbining, allowing more precise control of (simulated) operations. In addition,
the software is in the process of being enhanced to allow simulation of tidal ‘reefs’ or ‘fences’. Ongoing study areas are:
Energy gains realisable by alternative technologies such as ‘reefs and ‘fences’.
Investigate whether a modified dual mode operation focussed on maintaining high water levels is cost effective, as
this may provide a better operating model for shallow estuaries.
4. 2-D Studies
The 2-D model is receiving further validation and will also be exploited further. The inconsistent Mersey and Dee
energy calculations are being investigated. Other areas under investigation are –
Providing more realistic estimates of the realisable energy resource from tidal stream farms in the Irish Sea.
Individual barrages will be explored within the model to identify the performance and external effects of each. It
will then become clear whether barrage effects are simply additive, or whether there are more complex non-linear
effects from conjunctive operation.
CONCLUDING REMARKS
1. Barrages on the Solway Firth, Morecambe Bay, Mersey and Dee, operating in ebb-only generation with 1xDoEn
turbine provision could meet about 5% of UK demand. These should be no more than 70% more expensive in unit
energy costs when compared to that achievable from the Severn.
2. 2-D modelling significantly alters the energy predictions from the 0-D modelling, so demonstrating the necessity
of the more rigorous approach with major schemes operating in conjunction.
3. Whilst power production between the North West estuaries and the Severn is fully complementary in ebb-mode
operation, the NWDA/JC project demonstrates that dual-mode (two-way) operation would give rise to
synchronised and higher peak power pulses for the electricity grid to handle.
4. From the 33TWh/year arising from the conjunctive operation covered here, could be added at least a further 20
TWh/year from other UK schemes (Wash, Humber, Thames, Bridgewater Bay, North Wales Coast Attached
Lagoons). In addition, further exploitation of pump assisted impoundment could be utilised to provide a total
resource exceeding 15% of UK electricity demand. Tidal stream array developments are expected to be able to
provide at least 5% of UK electricity demand, and hence tidal renewables ought to be capable of providing a 20%
contribution to the UK power grid.
ACKNOWLEDGEMENTS
The original study was undertaken as part of project JIRP 106/03 funded over the period 2006-2008 by the Northwest
Regional Development Agency through the Joule Centre. Subsequent developments ongoing are being funded by an
EPSRC Doctoral Training Award to the first author.
REFERENCES
Baker AC (1991). Tidal Power, IEE Energy Series 5, Peter Peregrinus Ltd, London.
Burrows R et al. (2009, (a) ). Tapping the Tidal Potential of the Eastern Irish Sea, Final Report, Joule Project
JIRP106/03, University of Liverpool and Proudman Oceanographic Laboratory,
<www.liv.ac.uk/engdept/tidalpower>.
Burrows R, Walkington IA, Yates NC, Hedges TS, Li M, Zhou JG, Chen DY, Wolf J, Holt J and Proctor R, 2009 (b)
‘Tidal energy potential in UK waters’, Proc ICE, Maritime Engineering, 162, MA4, 155-164.
Burrows R, Walkington IA, Yates NC, Hedges TS, Wolf J and Holt J, 2010. ‘The tidal range energy potential of the
west coast of the United Kingdom’, Applied Ocean Research (in press).
Department of Energy (DoEn) (1989). The Potential for Tidal Energy from Small Estuaries, ETSU TID 4048P1.
Department of Trade and Industry (DTI) (2004). Atlas of Marine Renewable Energy Resources: Technical Report,
Report no. R1106, ABP Marine Environmental Research Ltd.
MacKay (2009) Sustainable Energy – Without the Hot Air, UIT Ltd, Cambridge.
Prandle (1984) Simple theory for designing tidal power schemes, Adv. Water Resources, Volume 7.
United Kingdom Atomic Energy Authority (UKAEA) (1980). Preliminary Survey of Tidal Energy of UK Estuaries,
Severn Tidal Power Report STP-102, Binnie & Partners.
United Kingdom Atomic Energy Authority (UKAEA) (1984). Preliminary Survey of Small Scale Tidal Energy, Severn
Tidal Power Report STP-4035 C, Binnie & Partners.
Walkington I and Burrows R, 2010. ‘Modelling tidal stream power potential’, Applied Ocean Research (in press).
Wolf J, Walkington IA, Holt J and Burrows R, 2009. ‘Environmental Impacts of Tidal Power Schemes’, Proc ICE,
Maritime Engineering, 162, MA4, 165-177.
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