Double Tidal Lagoon Baseload Scheme

Double Tidal Lagoon Baseload Scheme - Plan View
Double Tidal Lagoon Baseload Scheme – Plan View

I propose a renewable energy scheme where a tidal lagoon is partitioned into a ‘high’ lagoon and a ‘low’ lagoon by a dividing wall, which houses turbines which continuously generate power as sea water flows from the high lagoon to the low lagoon.

Double Tidal Lagoon Baseload Scheme - Cross Section View
Double Tidal Lagoon Baseload Scheme, Cross-section


At high tide, the sea-gates of the high lagoon are opened and the high lagoon is filled up to high tide level.

When the ebb tide begins, the sea-gates of the high lagoon are closed and remain closed until the next high tide.

At low tide, the sea-gates of the low lagoon are opened and the low lagoon is emptied to low tide level.

When the flood tide begins, the sea-gates of the low lagoon are closed and remain closed until the next low tide.

The sea-gates are functionally identical to one-way flap valves and may be engineered as such.


The Double Tidal Lagoon Baseload Scheme delivers a genuine baseload generation capability which can’t be delivered by inferior single tidal lagoon schemes as proposed by Tidal Lagoon PLC, as explained in the critical review in Energy Matters, “Swansea Bay Tidal Lagoon and Baseload Tidal Generation in the UK”.


A couple of days after posting this, a comment below was kind enough to provide a reference to David J C MacKay’s “Sustainable Energy – without the hot air”, pages 320/321 – “Getting “always-on” tidal power by using two basins”

“These toppings-up and emptyings could be done either passively through sluices, or …” – David J C MacKay

So MacKay’s “passively through sluices” “two basins” scheme is indeed absolutely equivalent to my double lagoon proposal here.

See also –


Scotland's tidal ranges
Scotland’s tidal ranges

The Solway Firth

The Solway Firth is the best location for Scottish tidal lagoon plans because that’s where Scotland’s highest tides are.

The Solway Firth
The Solway Firth
Almorness Tidal Energy Scheme
Almorness Tidal Energy Scheme
Almorness Tidal Energy Scheme – Map

The Almorness Tidal Energy Scheme is my outline design concept intended to serve only as an example of possible Double Tidal Lagoon Baseload Schemes. Points to note are

  • the River Urr empties into the high lagoon, adding to generation capacity.
  • dredging the estuary mud out of the lagoons, especially the low lagoon and around the turbine house would likely be necessary for satisfactory performance
  • there should be a drainage canal to redirect water flow to prevent drainage into the low lagoon
  • the lagoon walls would obstruct sea-going navigation to the Urr estuary harbour unless a lock for boats was built into the high lagoon sea wall to enable (admittedly delayed) navigation.

Scottish north-west coastSea Lochs

Whilst the tides on Scotland’s north-west coast aren’t so high, there do seem to be quite a number of suitable sea-lochs there that could relatively easily be barraged to exploit tidal energy, somewhat in the style of a tidal lagoon but without having to build much in the way of lagoon walls, nature having done most of the work already.



12 thoughts on “Double Tidal Lagoon Baseload Scheme”

  1. I had this idea about 5 years ago. It makes such sense I can’t imagine anyone considering anything else. The benefit of providing continual power is extremely valuable as it eliminates the requirement for back-up.


  2. Your baseload scheme makes technical sense.

    The financial competition to a baseload dual lagoon scheme is not a single lagoon alone, but a single lagoon plus grid battery storage. (which you can determine precisely).

    Have you worked out the efficiency for your scheme compared with the more standard single lagoon of double the size of the individual lagoons in your baseload scheme?

    Liked by 1 person

  3. I suggest you analyse your scheme by showing the proposed cycle of operations in more detail. For simplicity, assume the tide is a sawtooth between 1 and 0 and consider values every 0.1 or 0.05 step in the tide over a complete cycle once stable operation is established. Show columns for the water height in each lagoon, and assume a constant rate of water flow (not instantaneous) whenever a tide gate is open, in the direction appropriate for the difference in levels either side of the gate: show flow through each gate and the turbines. The power generated will be proportional to the difference in levels between lagoons in each period, and the flow volume between them. The wall has a length 5L if we assume an oblong construction to contain an area L^2 per lagoon, compared with length 4L to contain an area 2L^2 for a single lagoon.

    You may then like to consider what happens in a neap tide, where the water levels vary between 0.25 and 0.75.
    Assume the C is at high tide Max, and A is filled while B is “empty” at Min, and gates are closed. Turbine flow is started at rate r, while the C ebbs at rate c. The level of C is thus Max-ct and of A is Max-rt and of B is Min+rt. Power generated at time t is (Max-rt-Min-rt)rk, where k is a constant reflecting the area of the semi lagoon (ignoring reduction in turbine efficiency as head reduces). It falls to zero when Max-rt=Min+rt, when the C is at Max-ct. If cr, then it is below, and it becomes possible to lower the level in B at rate g by opening its gate until it catches up with the tide, or at rate c when it has done so, assuming g>c. However, the potential of this flow is wasted because there is no turbine in the gate. But it does make it possible to resume generating and emptying A assuming g>r, albeit the head and power generated now depends on g-r. At low tide, the level in A is Max-r if we scale t to be 1 for the half cycle, while the level in B is hopefully Min, and we shut its gate. You cannot refill A at this point without pumping, but you can go on emptying it until the level in B matches. Refilling of A can only commence once the level of C is higher, and cannot be faster than c without pumping. Meanwhile the level in B continues to rise….

    Your move.


      1. You shouldn’t get too concerned about pumps – they’re employed because they increase the net energy output, albeit they create a demand spike when in use. The flow through a sluice is given by kAsqrt(2gh) where k is a dimensionless constant less than 1 reflecting the efficiency of the sluice orifice design, g is 9.81 m/s^2, and h is the head (i.e. difference in water levels), and A is the area of the sluice opening. Dimensional check [L^2][LT^-2.L]^1/2=L^3.T^-1. Of course, you could simply lower the entire wall to sea bed level and try creating a tsunami…

        However, you have yet to rise to the challenge of calculating the sustainable rate of energy production using your scheme and how it might vary between spring and neap tides – or find whether Mackay is indeed right that a very much higher wall is required (with possible excavation into the sea bed to provide the low level, below lowest low tide, in the second lagoon) in order to provide continuous energy, and that pumping is beneficial. Do check his diagrams of levels carefully, but bear in mind his scheme doesn’t begin to account for engineering and cost realities.


        1. pumps – they’re employed because they increase the net energy output

          Primarily, yes, but that’s not the only reason. They also allow the tide height to remain the same as it was before. Without pumping the tides will be somewhat lower than before. This has enormous inplications for the tidal zone ecology, so it’s much easier to deal with environmental concerns/objections, and get everyone on-board, if the scheme includes pumping to previous levels.


  4. I wrote in the following terms to the Welsh Government on 18th September 2017


    I caution the authorities to delay before final approval of funding and construction of these tidal lagoon schemes so that the plans’ proposed inferior single lagoon design can be upgraded to the superior double lagoon design, as explained in the following quote from my Scottish Scientist blog post –

    I would hate any delay to put these projects in jeopardy and I’d rather see the less-than-optimal single lagoon design built than lose this project altogether.

    I’m a friend of these tidal lagoon projects but as a friend, I think a bit of tough love is needed so as to insist on the better double lagoon option.”

    I received the following email in reply to an earlier email of mine to the Welsh Government.

    Department for Economy and Infrastructure, Welsh Government
    2 October 2017
    Dear Sir/Madam
    Thank you for your email of 18 September regarding the concept of a double lagoon to the First Minister and Cabinet Secretaries. I have been asked to respond.
    The Welsh Government is aware of this concept and it was considered thoroughly during the Severn Tidal Power study, completed in October 2010.
    Charles Hendry’s report on lagoons recommended a competitive approach for future lagoon developments after the first, less than 500MW, project. Competition will provide an opportunity for more efficient and cost effective designs to be put forward.
    Your comments have been noted and your email has been passed to officials, so they can consider his comments as part of their work.
    Yours sincerely
    Gwenllian Roberts
    Deputy Director – Energy and Steel

    A google search for “Severn Tidal Power” study turned this up –

    and from page 194, this

    11.2.3 Generic Application of Multiple Basin Solutions
    A good account of the application of twin basin (or linked basin) developments is provided in Elements of Tidal Electric Engineering (Clark, 2007). Clark discusses the simple linked-basin concept and more complex double and triple basin schemes.
    These are described below.
    Simple linked-basin concept
    The linked basin concept, attributed to Decoeur and discussed in Clark (2007) is shown in Figure 11.3.

    The scheme is divided into a smaller ‘high’ basin and a larger ‘low’ basin, separated by a dyke and powerhouse. Each basin is connected to the sea by sluices. Generation occurs continuously with flow from the high basin to the low basin through one way turbines.
    When sea and high basin are both at high water level, the sluices between the high basin and the sea are closed. The high basin then discharges through the turbines to the low basin as the sea level falls. When there is equilibrium between the low basin level and the falling sea level, the sluices between the low basin and the sea open until low water when the sluices are closed. When the falling water levels in the high basin reach the rising sea levels on the flood tide, the sluices between the sea and the high basin open until high water. Then the cycle is repeated.

    So yes that’s the same concept but I don’t see any good reason for not implementing such “more efficient and cost effective designs” for the first project at Swansea Bay. Why not get it right first time? Why settle for second best just because it is the first project? Just because Charles Hendry’s report says so? That’s not a good reason, is it?


    1. I received the following email in reply to same email which was sent to UK Government also.

      Dear Scottish Scientist,
      Thank you for your email dated 17 September, where you suggest dual tidal lagoon designs should be considered.
      Double lagoons could be an option for larger lagoons but would be dependent on the energy market and the value associated with generating continual power against the significant increase in capital cost of a dual lagoon scheme.
      An independent review of tidal lagoons was led by Charles Hendry and published earlier this year. The Government is currently assessing the recommendations of the Hendry Review.
      The issues are complex, as they relate to an untried technology in the marine environment. It is important we understand the costs and challenges posed by this technology in relation to its possible role in the UK’s energy mix and potential environmental impact.
      The Government will publish a response to the Hendry review in due course.
      Yours sincerely

      I replied in the following terms –

      The Double Tidal Lagoon should be the preferred option for lagoons of all sizes, smaller as well as “larger”.

      The double lagoon offers operability in either the valuable dispatchable power-on-demand mode or continual baseload power mode – neither of which the single lagoon design can do.

      My dual lagoon scheme does not imply a “significant increase in capital cost” compared to the single lagoon design offered by Tidal Lagoons Plc for Swansea Bay.

      Admittedly, the double lagoon needs an additional partitioning wall between the high and low lagoons, whose length as a percentage of the equivalent single lagoon design depends on the particular layout of the scheme but for example consider a single lagoon whose walls describe a circle which can be converted to a double lagoon by adding a diameter partition where the increase in the total wall length is only equal to the ratio of a diameter to a circumference of a circle or 1/PI or about 32%.

      The cost per length of wall is not fixed but it depends on design, if it is fit for purpose or whether it is more expensive than it needs to be because it has been over engineered.

      To take our example again, the cost of a wall of length 132% can be no more than the cost of a wall of length 100% if the cost per length of the wall of length 132% can be reduced by a factor of 100/132 or about 75%.

      The cost per wall of the Tidal Lagoon Plc’s proposed Swansea Bay plan can easily be reduced to 75% or less because their plan proposes lagoon walls which would be greatly over-engineered for the lagoon wall purpose so as to function in addition as a superfluous and expensive road and footpath for recreational purposes.

      A simple lagoon wall that is not also a road and footpath will have a lower cost per length and that’s how to make a longer wall cost the same or less than a shorter over-engineered wall.

      The Charles Hendry “independent review of tidal lagoons” is fatally flawed for the following reason.

      Hendry concludes – “it offers limited dispatchability” – oblivious to (or in denial of) the fact that the double lagoon design offers good dispatchability, unlike the single lagoon which cannot dispatch any power at all whenever the level of the water inside the lagoon is the same as the level of sea outside the lagoon.

      If the UK is to have a new Tidal Lagoon Authority then its head or commissioner or “Tidal Tsar” should not be Hendry because he has failed to conclude that the single lagoon design is not appropriate even for a pathfinder or first, smaller demonstration tidal lagoon project. That conclusion at least is dangerously misleading.

      It is therefore disappointing for me to read in your reply on behalf of the UK BEIS Minister mentioning the “Hendry Review” as if that matters when really it shouldn’t matter, at all.


  5. As the Welsh report points out, double lagoons have been considered. And the existing lagoon people have modelled various multi-segment lagoon designs too. When I went to a talk by their chief engineer a couple of years ago they were keen to get more modelling expertise in the company to further explore these ideas, so if you volunteered some modelling effort they might well still be interested.

    I’ve not done the sums, but as pointed out in another reply you need to model this properly to work out how much less overall generation you get in exchange for continuous generation, and how the gate-sizes vary. There is the advantage of optimising for a one-way turbine, but that’s only a few percent, and you get reduced head quite a lot of the time, so I’m fairly sure the output is significantly lower.

    But these sums are not obvious (for example top-up pumping gets you back more energy than you used, which seems unlikely at first glance)


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