Memorandum 4
Submission from the University of Liverpool
EXECUTIVE SUMMARY
This submission aims to bring back to full attention
the substantial potential role of tidal barrage solutions for
renewable energy generation in the UK. It is demonstrated here
that installations on as few as eight major estuaries should be
capable of meeting 10-12% of present electricity demand (possibly
over 15% with a more ambitious scheme on the Severn) this employing
fully proven technology. This far exceeds the potential of tidal
"stream" turbine or practicable "lagoon systems
much vaunted by funding agencies over recent times. It also brings
attention to an ongoing study investigating the tidal power potential
in the North West of England.
TAPPING THE
UK TIDAL POWER
POTENTIAL
1. The medium to long-term procurement of
energy and the related issue of climate change is set to remain
at the top of government and public agendas, both nationally and
internationally, for some time to come. No clear vision has yet
emerged for a sustainable global energy future and the combination
of rapid growth in both economies and populations in the developing
world are set to place extreme pressure on fossil fuel reserves.
It seems inevitable, therefore, that as the 21st century evolves,
ever greater utilisation of renewable energy resources must be
made if the means for modern living are to be preserved. From
the perspective of the global community, it is argued that it
will ultimately become an obligation for all societies to properly
and fully exploit the natural energy resources at their disposal
for the common good.
2. 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 with a somewhat lower degree of unpredictability
but with necessary technological advances still outstanding at
present. Even more exclusive, however, is the potential for tidal
energy extraction from around the UK coastline. The most attractive
locations for harnessing tidal power are estuaries with a high
tidal range for barrages and other areas with large tidal currents
(eg straits and headlands) for free-standing tidal stream turbines.
Pertinent here is the fact that tidal barrage solutions, drawing
on established low-head hydropower technology, are fully proven.
The La Rance scheme in France is now in its 39th year of operation
(Cottillon, 1978; Pierre, 1993).
3. Of about 500-1,000TWh/year of energy
potentially available worldwide (Baker 1991), Hammons (1993) estimated
the UK to hold 50TWh/year, representing 48% of the European resource,
and few sites worldwide are as close to electricity users and
the transmission grid as those in the UK. Following from a series
of government funded studies commissioned by UKAEA in the 1980s,
Rufford (1986) identified 16 UK estuaries where tidal barrages
would be capable of procuring 44TWh/year and Baker (1986) identified
further sites suitable for small-scale installations. In fact
the bulk of this energy yield would accrue from eight major estuaries,
in rank order of scale, the Severn, Solway Firth, Morecambe Bay,
Wash, Humber, Thames, Mersey and Dee (see also Baker, 1991).
4. In the context of the future UK energy
mix, it is worth noting that the earlier estimates of UK tidal
barrage potential amounted to approximately 20% of UK electricity
need in the late 1980s and today could offer in the region of
15% (DTI, 2005), with the added benefit (over wind and wave based
renewables) of predictable availability. In addition to barrage
solutions to tidal energy capture, there is also more modest scope
for tidal-stream energy generation using submerged rotors, either
free standing or as part of a "tidal fence", these extracting
from the kinetic energy of the tidal flows. With attention inevitably
to be placed upon reduced energy consumption and demand management,
a future tidal power contribution at 20%+ of UK electricity demand
would appear realistic.
5. Although all tidal energy generation
is intermittent locally, covering about 10-11 hours per day, normally
in two pulses synchronised with the approximately 12 hour tidal
cycle, tidal phase lag around the coastline provides an opportunity
for the grid input window to be extended to closer to 24 hours.
With its complete predictability, and operating in a mix with
thermal, hydropower and nuclear production as well as thermal
renewables, an effective base-load role should be attainable.
6. The case for a tidal barrage in the Severn
estuary, with the highest tidal range in Europe, has and is being
actively promoted by the Severn Tidal Power Group with increasing
influential support. This scheme alone, (the smaller "inner"
of two earlier options [Baker, 1991]), would be capable of meeting
about 5-6% of current UK electricity need (Watson & Shaw,
2007).
7. The estuaries of the North West of England
offer fully complementary potential to the Severn by virtue of
the tidal phase lag, as will be illustrated below. The Dee, Mersey,
Ribble and Wyre estuaries, Morecambe Bay and the Solway Firth
all have a macro-tidal range. Based on the earlier studies (Baker,
1991) a total installed capacity of 12GW was estimated (Ribble
excluded), with a potential energy yield of at least 17.5TWh/year,
approximately 6% of UK national need and by inference a sizeable
proportion of the North West's electricity demand. Of all potential
UK sites, the Mersey with a very narrow mouth, and therefore needing
a relatively short barrage length (MBC 1992), could offer power
production at the lowest unit cost of all UK sites (Baker 1991).
8. In this region of the Eastern Irish Sea,
exploitable tidal stream resources have also been identified to
the north of Anglesey and to the north of the Isle of Man, with
more localised resources in the approaches to Morecambe Bay and
the Solway Firth (DTI, 2004). In the estuarial situation, however,
it is unlikely that tidal stream options can come close to the
energy yield of barrage alternatives. Recent assessments for the
Mersey (www.merseytidalpower.co.uk) offer estimates of 40-100
GWh for tidal stream arrays, contrasting with 1200 GWh estimated
for a barrage, at an equivalent location. In a similar vein, whilst
tidal lagoons are often mooted as a viable alternative to estuary
barrages, offering a similar operational function, it is highly
unlikely that they could be realised at a comparable scale and
remain competitive on cost against the major barrage schemes cited
above.
9. It should be noted that a barrage solution
attempts merely to delay the natural motion of the tidal flux
as sea level changes: holding back the release of water as tide
level subsides under "ebb generation" so that "head"
(water level) difference is sufficient for turbine operation;
deferring the entry of rising tidal flow into the inner estuary
basin for "flood generation"; or "dual mode",
a combination of both. Each mode has some restricting effect,
so reducing the range of tidal variation within the basin, ebb
generation solutions generally uplifting mean water levels, "flood"
reducing mean levels and dual mode resulting in little change.
A degree of environmental modification is, therefore, inevitable,
but this does not necessarily imply serious degradation from a
physical or ecological perspective, though issues related to protection
of habitats would inevitably need to be confronted.
10. 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. Thus, a
fully holistic treatment of overall cost-benefit is imperative
for robust decision-making. It is suggested that, to date, this
position has been inadequately addressed in the formulation of
energy strategy, especially in respect of barrages' potential
strategic roles in flood defence and transportation planning.
It follows, therefore, that apart from the direct appraisal of
energy capture, other complementary investigations must be sufficiently
advanced to enable proper input in decision-making in respect
of these "secondary" functions, as well as the various
adverse issues, such as sediment regime change, impact on navigation
and environmental modification.
11. It is important that robust estimates
of the realisable UK tidal energy reserves be established so that
they can properly be assimilated into future energy planning (accepting
the 10-15 year time horizons necessary). Thereby, rational implementation
might be initiated as and when concerns over energy price, security,
or carbon emissions dictate. Furthermore, it is considered paramount
that this energy potential be fully appreciated when planning
application is received for alternative schemes, which might compromise
maximum exploitation of the renewable resource. Such instances
might arise, for example, should a tidal stream array or tidal
fence installation be promoted where the barrage option remains
viable and for which a substantially increased energy capture
might be expected.
12. Following this line of argument, there
now remains a need to re-appraise the earlier study estimates
of potential barrage energy yield and to further this detailed
technical scrutiny with assessment of the various operational
mode options (ebb, flood or dual) and in conjunctive action, to
firmly establish the scope for an extended (near 24 hour) generation
window and a potential base-load role within the electricity grid.
13. This submission offers some new insight
in this respect, and aims also to bring attention to an ongoing
study "Tapping the tidal power potential of the Eastern Irish
Sea" being conducted jointly by the University of Liverpool
and Proudman Oceanographic Laboratory. Project aims are summarised
in the Appendix.
14. At this early phase of the project, it is
possible to offer only preliminary findings on the potential for
large scale energy procurement from estuary barrages. This draws
on energy generation routines developed for the project (figure
1) and applied to the base data on the estuary bathymetries, barrage
lines and tidal regimes taken from the 1980s' literature (later
phases will use more precise and updated inputs).
Figure 1
SCREEN IMAGE SHOWING: TOP—TURBINE PERFORMANCE
CHARACTERISTICS; MIDDLE—TIDAL (GREEN) AND BASIN (BLUE) LEVEL
VARIATIONS; AND BOTTOM—POWER OUTPUTS. [UNATTRIBUTED EXAMPLE
FOR ILLUSTRATION ONLY]
15. Figure 2, over the page, illustrates
potential outcomes from the introduction of the eight major barrage
schemes considered earlier (Baker, 1991). These show the combined
power outputs, from the favoured ebb-generation using double regulated
axial flow turbines (after Baker, 1991), at each of the barrages.
It is immediately apparent that they form essentially two distinct
"co-phase" focused groups, the Severn/Wash/Humber and
the Solway/Morecambe/Mersey/Dee, with the Thames lying somewhere
in between.
16. As far as possible an attempt has been
made to consider equivalent barrage power schemes to those adopted
in the earlier studies (ie similar number and size of turbines
and sluices and generator capacities), though limitations in detail
available in the literature led to the need for assumptions and
compromises, the technical details of which are not given here,
but which will be fully explained in future publications.
17. The operation strategy depicted in figure
2 is that configured to provide the widest generation window on
each barrage. The simulation has been undertaken for 28 tides
representing a spring-neap-spring series, shown in part (a), whilst
(b) and (c) show the power produced over two-day periods from
the neap and spring phases respectively.
18. Observations arising and implications:
— The North West group of estuary barrages
would operate in a complementary fashion to the Severn (and "phase-aligned"
Wash and Humber). It should be noted that only approximate estimates
of tidal phase have been used herein, based mostly on records
from nearest ports and so slight adjustments to the synchronisation
might be expected from a more refined analysis.
— By judicious use of pumping to enhance
water capture around high tide (essentially short-term "pumped
storage") and optimal conjunctive operation of the individual
schemes, it would seem possible that the power dip between the
Severn group outputs and the following NW Group peaks might be
smoothed out.
— It appears less likely that such
action could eliminate the major daily trough, during which only
the Thames makes a significant contribution. Other potential estuary
barrage or "lagoon" locations, for example around the
East coast of Scotland, may be worthy of future consideration,
or else different modes of operation may need consideration. "Flood
generation" or "dual-mode" operation, whilst generally
less efficient in energy conversion than "ebb generation",
may provide the added flexibility necessary to provide a significant
24-hr (continuous) output to the grid. The ongoing "Tapping
the tidal power potential of the Eastern Irish Sea" study
should go some way towards appraisal of these possibilities.
— Whilst, therefore, the ability to
offer a balanced daily supply remains unproven at this point,
it is clear that substantial contributions to daily electricity
demands could be made. From this preliminary analysis, it appears
that for much of the day, tidal power contributions of close to
6GW could be provided during "springs", falling to around
2GW during "neaps". These figures should be set against
typical power demands in summer ranging, approximately, from 25-40GW
and in winter from 30-50GW.
— The annual energy output from this
"maximum generation window" operation simulation is
29.4 TWh, an alternative "maximum power" operation yields
36.1 TWh, these figures representing about 10% and 12 % of UK
annual demand, respectively. The more ambitious outer Severn option
(Baker, 1991) would be required to lift output above 15%.
— The practicability of rapid introduction
of such large power inputs to the grid will need careful attention,
though this has recently been broached by the proponents of the
Severn barrage (Watson & Shaw, 2007)
— It is clear that a phased introduction
of the schemes in pairs could enable an incremental increase in
capacity whilst preserving a reasonable power balance across the
generation window, ie pairing the Severn and Solway, Morecambe
Bay and Wash, and Humber with Mersey/Dee.
— Whilst it is appreciated that the
economics are likely to play a major part in any progression of
these major tidal power proposals, it is reassuring to note that
the unit cost estimates made in the 1980s varied by little more
than a factor of 2, with the Severn and Mersey lowest and the
Thames highest (Baker, 1991).
Figure 2
SUMMATIVE PLOTS OF POWER OUTPUTS FROM MULTIPLE
TIDAL BARRAGES (PROVISIONAL)
a) 28 tide spring-neap-spring series
b) 2-day segment from "neaps"
c) 2-day segment from "springs"
REFERENCES
Baker A C, 1986 The development of functions
relating cost and performance of tidal power schemes and their
application to small-scale sites, in Tidal Power, Thomas
Telford, London.
Baker A C, 1991, Tidal Power, Energy
Policy, 19(8), 792-797.
Cottillon J, 1978 La Rance tidal power station-review
and comments. Proc Colston Symp on Tidal Energy, Bristol, Scientechnia,
46-66.
DTI, 2004 Atlas of Marine Renewable Energy
Resources: Technical Report. Report no. R1106, ABP Marine
Environmental Research Ltd.
Hammons J H, 1993 Tidal Power, Proc IEEE,
81(3), 419-433.
Mersey Barrage Company, 1992 Tidal power from
the River Mersey: A feasibility study Stage III, MBC, 401
Pierre J, 1993 Tidal energy: promising projects—La
Rance—a successful industrial scale experiment, Proc IEEE
Trans Energy Conversion, 8(3), 552-558.
Rufford N, 1986 Tidal power still in the running,
New Civil Engineer, 22 May 1986, 12.
Shaw, T L, 1980 An environmental appraisal of
tidal power stations: with particular reference to the Severn
barrage.
Shaw (ed), London: Pitman Advanced Publishing
Program, 220pp.
Watson M J & Shaw T L, 2007 Energy generation
from a Severn barrage prior to full commissioning, Proc ICE Engineering
Sustainability, 160, March, ES1, 35-39.
|
