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.

## Operation

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.

## Baseload

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”*.

## References

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 –

- Haishan Tidal Power Plant
- TIDAL BARRAGE POWER PLANTS:TWO-BASIN PROJECTS
- Derby Tidal Power Preliminary Design Report

## Scotland

### The Solway Firth

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

##### Almorness Tidal Energy Scheme

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.

### Sea 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.

Simple.

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.

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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?

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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.

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David Mackay had almost exactly your schema, save he adds pumps.

http://www.withouthotair.com/cG/page_321.shtml

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Thanks for the reference which I’ve now added to my post. MacKay actually considers the option of double lagoon generation

without requiring any pumps whatsoeverwhere he says“These toppings-up and emptyings could be done either passively through sluices, or”.Actually, “passively” works perfectly and best so MacKay’s discussion of pumps in this context is an unwelcome distraction in my opinion.

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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.

http://www.inference.phy.cam.ac.uk/sustainable/book/tex/Lagoons.pdf

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Some slightly more real world physical considerations are discussed in this paper by David Prandle:

https://www.researchgate.net/profile/David_Prandle/publication/245211836_Simple_theory_for_designing_tidal_power_schemes/links/56cdc28408ae059e37532f95.pdf

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