[acsinuk (OP)]

 It was reported last September by the Guardian that the department of energy and climate change had commissioned a report from the Crown Estates on the available of tidal energy around the UK shore lines.
The report revealed that there was the potential of extract 153,000 MW of energy. The question is how can we get at this energy?  Surely, wind power is good but is it an unpredictable source and we need to keep the grid running at peak evening times not just when the wind blows.
CliveS

[Phractality]

I assume the survey counts all possible sources of tidal power, not just those that are economically feasible and eco-friendly. If you don't count the schemes that would disrupt navigation, pose a hazard to marine wildlife and alter delicate estuary habitat, the total is probably far smaller.

With a tidal barrage system, you have a dam across the inlet to a river or bay. At high tide, you allow water to flow into the waterway thru turbines. When the water level is equally high on both sides of the dam, you briefly open gates for navigation. Then you close the gates and wait for the tide to fall. At low tide, you let water to flow out of the waterway thru the turbines. When the water level is equally low on both sides of the dam, you briefly open gates for navigation. Then you close the gates and wait for the tide to rise.

The effect of such a system is to limit navigation and alter the pattern of tides in the estuaries connected to the waterway. In the estuaries, the high tide is lower, and the low tide is higher, and water flows faster between extended periods of stagnation. The effect on wildlife is uncertain. What is certain is that estuaries are the nurseries of the marine fishing industry.

Tidal stream projects place giant turbines in places where tide flows strongly, without damming the stream. Each turbine produces a tiny change in the water level on each side. The more power you harness, the more you restrict the flow. Conceivably, you could place enough turbines across a major strait to get a significant alteration of the current and water levels. The main danger to wildlife is the turbines, themselves. Just as wind turbines kill thousands of hapless birds, poorly designed water turbines can slice and dice fish, crustaceans and marine mammals. Making the turbines more wildlife friendly is likely to make them much more expensive.

A tidal lagoon project is similar to a tidal barrage, except that it may create a lagoon in the middle of open water, away from any shore, river or estuary. Vessels would have to navigate around it, but they would not be restricted from entering or leaving harbors. A system that has flexible walls supported by an inflated top perimeter would have a low initial cost, compared to a rigid dam, but it would only harness power when releasing water thru the turbines at low tide; it would not generate power during the rising tide. Such a system, with eco-friendly turbines, looks promising to me.

When calculating the time required for a tidal power project to pay back its initial investment, we should deduct the loss to the fishing industry from the income produced by selling the power. And don't forget to include the increased price of seafood at the market. You will be paying a hidden cost of power when you buy seafood.

[acsinuk (OP)]

Thanks for the prompt reply.
I think I agree with you that the most environmentally friendly approach is to use a flexible barrage to somehow create a tidal water level differential to drive an extra low head turbine or water wheel.
Have you given any thought as to how to build a flexible barrage and secure it to the sea bed to avoid leakage yet?
CliveS

[CliffordK]

I've thought that an excellent place for a tidal barrage (dam) would be Baja California (sorry UK).  But, one would certainly have broad effects on the wildlife. 

It would likely have a positive effect on some species, and a negative effect on other species.  One could choose to exclude various large predators from the bay, potentially improving fishing.  Migratory species could have significant problems, and no doubt at least one species would be unable to cope with such a dam.

For shipping, one could always put locks in the dams.  However, I believe that twice a day the water levels inbound and outbound would be equal, so one should be able to open shipping up at the locks for brief periods.  The complexity of the lock system would depend on the actual shipping in the area.  One could potentially still generate power with the water differential within a lock.

Anyway, there are many ocean features where one could generate significant power such as underwater currents, and the "gulf stream".  However, given the caustic nature of the ocean, and distances involved, it may be difficult to make it economically feasible.  Although, one might think of some of the US Dams built by the government, and still being "paid for" a half century later.  Perhaps governments need to put more effort into long-term planning.

[evan_au]

Tidal power is more predictable than wind power, since tidal power operates on a roughly daily cycle linked to the rotation of the earth and another roughly 29-day cycle linked to the orbit of the moon.

The peak of tidal power does not always occur at the peak of demand, but it would frequently occur at the "shoulder" of the demand curve, or be able to be shifted onto the shoulder of the demand curve (with some loss in power output).

In any case, using tidal power reduces the usage of non-renewable sources like coal.
There is always some risk from storms, which can produce large swells in areas exposed to the open sea.

[acsinuk (OP)]

Yes, in fact if you have several tidal schemes at various locations it is possible to get around 75% of the combined rated energy output because the high tide at different places is at different times. The ideal gap would be 3 hours.
CliveS

[Atomic-S]

Another way to harness the tides is to replace the lagoon with a float of the same surface area. A large hollow pad would float on the surface anywhere at sea, and would be connected by a rod to a machine anchored to the seafloor, and as the tide rose and fell, the float also would, transmitting the motion to a generator. The energy output should be comparable to that with the lagoon-and-turbine method because the same amount of water is being displaced over the same height in both cases. The advantage of the float method is that it can be put just about anywhere at sea, avoiding sensitive areas.

[CliffordK]

harness the tides ... with a float.... A large hollow pad would float on the surface anywhere at sea, and would be connected by a rod to a machine anchored to the seafloor, and as the tide rose and fell, the float also would, transmitting the motion to a generator.

Of course you would need power transmission lines so it would need to be placed reasonably close to the shoreline.  Salt and barnacles, of course are always an issue at sea.

Could you build your float as a submarine, say 100 feet below the surface, where it would act as both a tidal energy capture device, as well as an artificial reef, and also would be out of sight, and would be relatively protected from storms.  Even subsurface, it should still have some floating changes with the tides.

Off and on there have been proposals to build floating cities.  Perhaps such tidal energy could become a major power source for a floating community.  Draft of the vessel shouldn't be an issue.  Storms, of course, are issues, but one might be able to move the city out of the path of the worst storms, although moving a floating city might take a significant amount of months worth of the energy salvaged from tides, so it shouldn't be done frequently.

Currents might be a problem, but one should be able to use angled anchor lines, with limited slop for tides, and a vertical generator line.  Adding a weight might allow bidirectional generation, with a short rigid segment.

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[David Cooper]

No one should be pushing for any of the more environmentally destructive schemes of this kind to go ahead until other ideas have been taken as far as they can go. Energy conservation is the most important place to start, eliminating as much wasted energy as possible, and most energy is currently wasted.

[imatfaal]

Agree with David that conservation, efficiency, and usage are key areas to address; much easier to make savings there than generate cheap green energy.  Although I see no reason that this shouldnt be in tandem with new schemes - just that the new ideas cannot be seen as an alternative to cutting our baseline usage.

on the tidal issue - Geezer and BC wrote some great posts about tidal energy a year or so ago.  You gotta do the maths; sometimes the energy returns really don't match up with common-sense expectations - and that's the theoretical maximum return, not the actual return after countless losses and inefficiency within the generation process. 

http://www.thenakedscientists.com/forum/index.php?topic=41578.0

[CliffordK]

I agree that power conservation is key, and foresee major decreases in lightbulb power requirements over the next decade or so.  However, in general, I only foresee the Global energy usage going up over the next century.  And people won't be willing to "give up" what they already have.  Cutting fossil fuel power generation means building new power generation facilities that are not fossil fuel driven.  And this will have to be done using a variety of technologies from rooftop solar to wind to hydroelectric and ocean energy sources.

As far as a functioning tidal Barrage, there is one in Rance France.

According to Wikipedia, it peaks at about 240MW, with an average of about 64MW power generation.  This puts its output equivalent to a small to moderate sized dam.

The reason I like Baja California/Mexico for the location of a large tidal barrage is that the gulf covers about 160,000 km², relatively narrow at the end.  Consider area when designing a floating tidal generator. 

Since the Mediterranean is more or less East-West oriented, the tides would get substantially dissipated over the area, as cause less flow than would otherwise be expected through Gibraltar. 

Baja, however, is more or less North-South oriented, so the tides for the whole area would be created synchronously creating substantial water flow and volume displacement at the opening, and throughout the gulf.

I suppose the question is what are the tradeoffs that we are willing to take in order to wean the world from petroleum fuels?

[evan_au]

Some years ago I was introduced to the concept of "Negawatts": The power plant capacity you don't need to build because of economy measures which reduce power consumption.

The most expensive generating capacity is the capacity you only need to use just a few days of the year. Traditionally this was due to peak power consumption on the hottest day of the year (or coldest, depending on your climate). However, with increasing renewables, this may soon be on a hot, still, cloudy day with a half moon (which eliminates windpower and reduces solar & tidal power output).

The concept of "SmartGrids" is that by communicating power capacity information to the consuming equipment (perhaps by way of dynamic pricing), consumer equipment can back off power consumption by a few percent, and avoid blackouts. Steps are being made in this direction, but it does face both technical and psychological obstacles; probably the most significant one is people changing their mindset from "me" to "we".

[CliffordK]

I'm sure that peak power is a huge problem for electric grids.  Perhaps a little less so for natural gas grids.  Of course, the majority of our natural gas is also a fossil fuel.

Perhaps that may be an issue with things like smart thermostats which may cause power usage peaks around 7AM and 5PM. 

Around here, our utility companies are public entities...  which I thought was "normal" until I moved elsewhere for some a while.  They have invested in power reduction and personal alternative energy programs for years.  While they may not get revenue from a person adding additional insulation to a house, it does reduce the expenses for the company with having to invest in new very expensive power generation projects.

Fortunately AC usage around here is low, and peak water flow for hydroelectric is in the winter coinciding with peak energy demands.

[acsinuk (OP)]

Thanks Clifford
What I am really looking for is a way to generate tidal range power without building a large dam.  There is an idea that tidal dynamics may do the trick as the tide each side of a headland may be one metre difference in height.  I am thinking of a sort of environmentally friendly floating barrage which need only hold back a metre head of water and some sort of tidal water wheel to skim the energy off the top rather than a turbine.    Do you think it could work if constructed out into an estuary?                              CliveS

[Atomic-S]

If you are thinking of blocking only the upper few metres of water, I do not think that will do any good.

[acsinuk (OP)]

No, I am working on the idea of using a zigzag folding gate that is raised and lowered by floating pontoons each side.  The ends slides up and down in a groove on the piled piers driven into the estuary bed.  Above the gate is a water wheel that spins as the water flows through it.
CliveS

[Dawe]

There are now hundreds of off shore wind turbines in use now taking energy from above the water as the foundations and cabling is already in place why are they not also used for tidal energy as well.
I have an idea of a vertical turbine rotating around the bottom of the supporting tower with the length dependent on the depth of the water, the vertical blades are hinged so they open with the tidal flow but are closed by the flow on the opposite side so the blades would have an elliptical orbit  of the outer edge and would work on any direction of flow.
It would then remain to get the drive to the generator, this could be magnetic and keep the tower intact, see scetch.

[CliffordK]

With 6 hours for tides to rise, and 6 hours to fall, that would be one slow moving turbine, and water may simply try to go around the blades.

Putting the blades into an enclosed structure, one might force the water through the turbine.  However, it would still be slow.

Likely there is much more benefit on a small scale by using waves or ocean currents.

One could increase the flow rate through the turbine by creating essentially a funnel or tank above the turbine.

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This is essentially what dams do, using natural terrain to form the "funnel".

However, one could potentially make the tank open at the bottom, and closed at the top, using an air turbine.  This would decrease the exposure of the turbine to the caustic sea environment, and may be safer for sea life, although there would still be a risk of animals getting lost in the tank and unable to find their way back out.

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[Bored chemist]

Just for the record.
"Studies have estimated the UK’s total theoretical tidal range resource at between 25 and 30GWs – enough to supply around 12% of current UK electricity demand. "
from the decc's web page
https://www.gov.uk/wave-and-tidal-energy-part-of-the-uks-energy-mix

Somewhat different from the 153GW in the OP.

[acsinuk (OP)]

Yes, but if the peak is only around 60 then it would be almost 50% and what is more it could supply an even higher percentage during the rest of the time.