Efficiency and Effectiveness

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GENERAL INTRODUCTION:  as suggested in the composite diagram above, scoping for future ‘tidal range’ schemes is now in transition from older text-book variants of low-flow generation  to the newer idea of high-flow, two-way generation.  The key issue is: how can plant designs handle of order 3x greater flow rates efficiently and effectively enough to control both storage and release of tidal energy? Use of the term ‘tidal energy storage and release’ is here suggested in order to encourage clarity over all the components for properly integrated plant design briefs.

SUMMARY: it is easy to overlook that in high-flow, two-way tidal range schemes, small variations in hydraulic efficiency will mean relatively big differences in water release, affecting overall energy extraction to a surprising extent.  Two thought experiments using figures comparable to those at La Rance in Brittany in the 1970’s are here used to show why, despite improvements to the headline efficiency of plant, use of singly mounted bulb turbines is unlikely to correct this problem satisfactorily.  The 2010 UK government sponsored Atkins/Rolls-Royce Severn Embryonic Technologies Scheme study outlined a potentially better alternative format, broadly similar to the counter-positioned, contra-rotating (CPCR) format that the author had put forward at the outset of SETS in 2008. Because the Swansea Bay lagoon is well placed to serve as the generic pilot scheme then envisaged, it is argued that this must be properly studied for trial there. With a potential 50 GW installed capacity, catalysing efforts to adapt and mitigate, realistic UK market potential dwarfs other marine renewables.  Regional devolution and the Paris Agreement combine to make early, multi-level debate and well-informed collaborative modelling essential.

THOUGHT EXPERIMENT No 1: the outcomes tabled below assume a choice of three ordinary, low head one-way hydro units to drain a single reservoir, from which all the water comes for power generation. The operating head is fixed, and reservoir evacuation time in the L side column arbitrary, with values chosen for ease of comparison with low-flow, two-way tidal generation at La Rance (see below), also applying Newton’s Law that from rest v = √(2gs).

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THOUGHT EXPERIMENT No 2:  to extend Experiment No 1, suppose the owner wants to double the reservoir’s design size, and a supplier offers to run a ‘free’ cross-flow turbine in parallel with a reaction one.  Average efficiency is 87%, only 6% less than the reaction turbine’s, so the offer looks good – until the owner sees that water is inevitably going to be shunted away from the more efficient plant to the less efficient one in a ratio of nearly 5:8 over each usage cycle, making the reservoir run out early.  So the more efficient reaction machine can use only 78% of the water it would have used if still left working alone, while perversely the cross-flow turbine is doubly rewarded, both for its inefficiency and for its incontinence. At 489 units the overall output per cycle is just 46% more than that for a single reaction machine, while most water now discharges at almost 4 m/sec, eroding the streambed and making a downstream lake dubiously viable as a community trade-off.  He must insist on two non-wasteful reaction machines.  If business plans could merit a bigger reservoir, or reservoirs, similar arguments would apply even more strongly.


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KEY TO DIAGRAM: from the classic 1979 paper on two-way generation at La Rance

  • Niveau bassin/mer = level of basin/sea
  • Remplssage/vidage = filling/emptying period
  • Attente/pompage = hold/pumping period
  • Turbinage = generation period (nb. green directional arrows are added to show maximum operating head  during generation periods. Mean operating heads are somewhat lower than this, albeit variation is not great.)

NOTE ON MATHEMATICS:  Clearly, it would be helpful to submit assumptions implicit in the analogies made here to more formal treatment.  A relevant feature of the tidal sine wave is the long quasi-linear stretch, both on ebb & flood phases, allowing ‘reservoir emptying’ to occur with only minor changes to operating head.

    1. Because all 24 turbines were in full use both ways, the above diagram reflects generic bulb turbine function. Flood generation flow gradients as drawn are approximately 25% steeper than those during ebb generation despite 1/3rd reductions in operating head, e.g. from 5m to 3.3m. Such ‘leaky reverse generation’ represents inability to control discharge adequately to sustain comparable area-under-curve outputs. The end result at La Rance was that along with shorter reverse generation periods (e.g. 2.3 hrs vs. 3.6 hrs in the above), energy extracted on flood generation was ~50% of that on ebb – far less than spreadsheet comparison of headline efficiency (81% vs. 93%) would suggest.
    2. Bulb turbine manufacturers claim to have improved reverse hydraulic efficiency to 87%. vs. 93% forwards. But this still allows ~50% net deficits through head reduction and/or faster discharge (see AEP diagram below L). Meanwhile variable speed advances, while welcome, are irrelevant to comparison as they can benefit any technology.
    3. Setting bulb turbines in alternate directions, to share the same operating head while in opposite generation modes, broadly parallels Thought Experiment No 2. The intention here (to prevent the rises in mean basin level shown in diagrams above & below L) is good. But the shunting effects of even this degree of hydraulic wastage stand to help reduce overall energy extraction by no less, while adding in the unnecessary impacts of unevenly fast discharge.
    4. A novel counter-positioned, contra-rotating (CPCR) twin-turbine format was proposed by Rolls-Royce in the Severn Embryonic Technology Scheme published by DECC in 2010, offering 90%(+) across-the-range bi-directional hydraulic efficiencies for barrages and lagoons. This format is fish-friendly, works up to a 5m head, and offers much greater pumping efficiency (70% vs. 40% max. in bulb turbines) to help ‘naturalise’ basin water level excursion while raising net energy extraction. Such a unique win/win situation is generic, allowing reduced mudflat exposure during spring tides in estuary schemes, and of beach in open coast ones, to be compensated for on-site during mid-to-neap tides.
    5. Following the reasoning of Thought Experiment No 2, a good pilot tidal range scheme must demonstrate the widest spectrum of such economic, environmental and social benefits, helping to open up sustainable 21st C tidal energy markets rather than create a false dichotomy between the parallel needs of UK estuarial and open-coast schemes. Best taxpayer value must signpost a proactively science-driven, economically and socially honest UK and worldwide approach to creating and sustaining markets fully sensitive to the challenges of coastal adaptation and mitigation.

CONCLUSIONS: To make tidal range fit for 21st C purposes the broad quality issues over flow management must govern plant design.

sa new 5AEP work at Swansea Bay (left) indicates that certain basic flaws with bulb turbines remain unresolved. To raise mean basin level may preclude the very low-lying inhabited UK coastal areas most urgently needing help with mixed tidal, fluvial and pluvial flood risk.

Diagram redrawn from ‘Swansea Bay tidal power-plant: bi-directional bulb pump turbine with variable speed’ by Krogl M et al, made available in December 2015

By contrast, SA GRAPH 3 NEWpatterns of generation with a CPCR format (right) could satisfy all requirements bar off-the-shelf availability.

The UK government holds the keys to resume R&D essential to create a coherent world market based on the only suitable technology, and must be encouraged to use them.



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For references please email me and visit the IRISH SEA 2050’ website.