Tidal Barrage plans gathering pace at Liverpool

[AE-NMidlands]

I understood the latest option for the Scottish lochs was going to use submerged turbines at narrow sections.
Not heard anything more recently though.
Moxi

That would be the sea lochs, like the experimental one at the mouth of Strangford Lough, I suspect. The flows at Connel Ferry have to be seen to be believed, so it's a good idea, as long as it doesn't mince too many fish...

[Mart]

Hi Moxi, I'm not sure you are correct about generating the same power/energy. With hydro it's not just the amount of water, but the head differential that determines power and energy. So X ML's at 2m head would generate roughly twice as much as X ML's at 1m head.

That's why all these designs are based on holding back the water for as much of the flood or ebb phase as possible, to maximise the head, before generation starts.

So for instance, you point to a 1hr slack period, so that means 4 x 5hr periods of movement, assuming a 1m head is available throughout, so a generation comparable to 5 (1m x 5hr) for our purposes. But the idea of these tidal laggons/bays is to maximise the head, to maximise power and energy. If I'm doing this correct, and I doubt I am, then a 4m to 10m head would generate considerably more over the 3.5hr window that these schemes are designed for. So let's say 4m head for 3.5hrs, but as the water rises or drops to match the outside height, so will the head, so on average across the 3.5hrs it will half, that gives us, I think 3.5hrs x 4m /2 = 7 (v's 5), and over 10m we get 17.5 (v's 5). Perhaps an average of 12.25 v's 5. [Further correction, I suppose in the low head example, much of the first hour may be lost waiting for the 1m head to develop, and half of the fifth hour as the differential diminishes. So it may be more like 7 v's 3.5 and 17.5 v's 3.5.]

Apologies in advance, as I suspect I may have gotten much of this wrong. Edit - The more I think about this, I wonder if I'm getting lost confused over something here, and we may not be discussing/describing the same thing.

[Moxi]

Hi Mart

Low head lower power but a longer operating time frame as the impounded water passes the turbine at a slower rate, high head higher power but shorter operating time because the volume of impounded water is passing through the turbines faster than the water at 1 m head.

You can’t get something for nothing in this world Mart the laws of preservation won’t allow it

Moxi

[Moxi]

Oh apart from sausage rolls and coffee ….. apparently

Moxi

[Mart]

Hi Mart

Low head lower power but a longer operating time frame as the impounded water passes the turbine at a slower rate, high head higher power but shorter operating time because the volume of impounded water is passing through the turbines faster than the water at 1 m head.

You can’t get something for nothing in this world Mart the laws of preservation won’t allow it

Moxi

Yep, that's why I included time in my ponderings too. So 5hrs minus the time to create a 1m head, v's the 3.5hrs of the max head plan. I made the power/energy difference about 3:1. [Ranging from about 2:1 to 5:1 depending on tidal range across the season.]

I'm not sure where I'm going wrong. I appreciate your laws of preservation given it's the same amount of water, but with hydro generation you also need to consider the relative head, don't you? So the same amount of water, at a higher head would generate more energy in total, even if it's over a shorter time period, such as 3.5hrs v's 4 to 5hrs.

[dan_b]

Nice!

Oh apart from sausage rolls and coffee ….. apparently

Moxi

[Moxi]

Hi Mart

Low head lower power but a longer operating time frame as the impounded water passes the turbine at a slower rate, high head higher power but shorter operating time because the volume of impounded water is passing through the turbines faster than the water at 1 m head.

You can’t get something for nothing in this world Mart the laws of preservation won’t allow it

Moxi

Yep, that's why I included time in my ponderings too. So 5hrs minus the time to create a 1m head, v's the 3.5hrs of the max head plan. I made the power/energy difference about 3:1. [Ranging from about 2:1 to 5:1 depending on tidal range across the season.]

I'm not sure where I'm going wrong. I appreciate your laws of preservation given it's the same amount of water, but with hydro generation you also need to consider the relative head, don't you? So the same amount of water, at a higher head would generate more energy in total, even if it's over a shorter time period, such as 3.5hrs v's 4 to 5hrs.

Hi Mart

Don’t forget if you have higher head you either have to size the pipe work so that friction loses are minimised which is akin to keeping a sensibly low flow velocity, typically less than 1.2m/s and ideally closer to 0.76m/s.

If you pursue high velocity to spin the hydro turbine faster then you will incur friction losses in the pipe work which is most likely the reason your larger turbines look better as the loss isn’t being incorporated.

I would expect the turbines in the very lowest of low head applications to Archimedean to ensure sufficient surface area to squeeze out the available hydraulic power but on other barrages low head reversing hydro turbines would work fine and be a preferred choice.

Moxi

[Mart]

Hi Mart

Low head lower power but a longer operating time frame as the impounded water passes the turbine at a slower rate, high head higher power but shorter operating time because the volume of impounded water is passing through the turbines faster than the water at 1 m head.

You can’t get something for nothing in this world Mart the laws of preservation won’t allow it

Moxi

Yep, that's why I included time in my ponderings too. So 5hrs minus the time to create a 1m head, v's the 3.5hrs of the max head plan. I made the power/energy difference about 3:1. [Ranging from about 2:1 to 5:1 depending on tidal range across the season.]

I'm not sure where I'm going wrong. I appreciate your laws of preservation given it's the same amount of water, but with hydro generation you also need to consider the relative head, don't you? So the same amount of water, at a higher head would generate more energy in total, even if it's over a shorter time period, such as 3.5hrs v's 4 to 5hrs.

Hi Mart

Don’t forget if you have higher head you either have to size the pipe work so that friction loses are minimised which is akin to keeping a sensibly low flow velocity, typically less than 1.2m/s and ideally closer to 0.76m/s.

If you pursue high velocity to spin the hydro turbine faster then you will incur friction losses in the pipe work which is most likely the reason your larger turbines look better as the loss isn’t being incorporated.

I would expect the turbines in the very lowest of low head applications to Archimedean to ensure sufficient surface area to squeeze out the available hydraulic power but on other barrages low head reversing hydro turbines would work fine and be a preferred choice.

Moxi

Thanks, all interesting. I'm now fascinated to know what the potential difference in generation is, and how it works out economically.

I also now want a sausage roll, which must mean I'm easily seduced by either a) the power of suggestion ..... or b) sausage rolls.

[Moxi]

I think it’s a fascinating subject and would love to be involved in the detailed design construction and operation of a tidal barrage, but equally I am getting more interested in the free standing underwater arrays which although less efficient might be cheaper to deploy in sufficient numbers to make them more easy to deploy around the uk coast without the need for the massive civil infrastructure.

What is clear is that this energy source is ripe for development off the back of wind to help close the energy supply reliability from renewables loop.

Ps I can’t have a sausage roll as I’m only on 1500 calories a day at the minute

Moxi