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Post by mrsonde on Jan 28, 2014 10:27:13 GMT 1
Hmmm...don't really understand what you mean by "the water it uses". Presumably the water it doesn't pump uphill simply passes through into the stream, doesn't it? What's the problem? As for the inefficiency - that can only be due to worn sealants or valves. In a closed system, like the one I'm proposing here - water from the river or the sea drops down to turn a turbine, then passes through a pipe to a chamber and then onto the ram pump - it will eventually all get returned to the original source. Nowhere else for it to go. And - don't forget, there's no power expenditure involved, so whatever the inefficiency, it hardly matters. The same is said by proponents of wind turbines of course, except in that case the inefficiency produces wear and tear and, I strongly suspect, a very costly reduction in the machine's lifetime. There's some wear and tear on a ram pump, but it's negligible. My neighbour's pump used a great plug of granite for its valve, and had lasted for 120 years.
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Post by jonjel on Jan 28, 2014 11:35:09 GMT 1
I see where you are coming from mrsonde, but I was under the impression (and have just looked it up) that you need a big flow to get any sensible 'lift'. For a micro system for irrigation or to provide drinking water it would be fine and I am sure they are widely used for that, but to produce power could be a different scenario.
And I don't really understand your addition of a turbine. Surely if you have enough head for a turbine then you simply dispense with the ram pump and either generate the power with the turbine or indeed pump water with the turbine? Or of course you use a water wheel?
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Post by mrsonde on Jan 28, 2014 12:16:10 GMT 1
Ermmm...the lift is a function of the size of the pump, and the refractive chamber, I think. The rate of pumping no doubt is proportional to flow - because that will determine the force of the refraction wave. Again, I think. Alan will probably be able to clarify, I'm not an engineer.
It does seem conclusively demonstrated that the ancient Egyptians used them to build the Great Pyramid, however - the chambers and tunnels beneath work as a ram pump, when accurately modelled. That was fed by a leit to the Nile - quite a slow flowing river. At least, it is today.
The power - I think I haven't explained myself clearly. The power would be produced by a turbine, in the normal hydroelectric manner. The ram pump merely recycles the water that has dropped. If you don't do that, you can't open the sluice again - your reservoir would simply fill up. You see? Your "shaft" - you could make it as huge as you like, as with the barrage schemes, or any modern dam - needs to be evacuated. And a lot of pressure, so a lot of flow. Pumping with the turbine - you're wasting power that would generate electricity. A ram pump needs no external power. And is much more powerful than a water wheel.
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Post by buckleymanor1 on Jan 28, 2014 18:35:05 GMT 1
The only energy you can get from a float is its maximum potential energy mgh where m is the mass of the float g is g and h is the tidal range. The power you can get from a turbine is proportional to the cube of the flow rate and the swept area of the rotor. For a given mass of construction material, you will get a heck of a lot more energy per tide cycle from a turbine if you can restrict the water flow to maximise the tidal flow rate - at the limit, it depends not on the mass of the machine but on the mass of water that flows past it. Imagine an average tidal flow of 1 m/s (about 1.5 kt) across a square meter of turbine sweep. That's one tonne per second at 1 m/s. You would be hard pressed to find a vertical tidal range speed of 1 m/s anywhere. Won't the energy you get from a float depend on it's size.You don't need a verticle tidal range speed of 1m/s if your float is large enough.You could have a tenth of that range and still produce the same amount of energy or more than a turbine with a tonne of water per second sweeping over it.There will be more material used but an ideal float is mainly air. Restricting tidal flow for turbines is down to choice sites and or expensive materials. |
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Post by jonjel on Jan 29, 2014 10:23:23 GMT 1
How weird is that.
I was thinking much the same in the shower this morning.
Although it is (now) obvious that you get more power from a turbine in a tidal flow a float will work in zero flow. As long as there is a good tidal range it will work regardless of any flow.
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Post by buckleymanor1 on Jan 29, 2014 14:08:32 GMT 1
How weird is that. I was thinking much the same in the shower this morning. Although it is (now) obvious that you get more power from a turbine in a tidal flow a float will work in zero flow. As long as there is a good tidal range it will work regardless of any flow. That is not strictly true.Let us suppose the float is spread out over a large area in other words it is big.The tidal range is only one foot and tethered to the sea bed. The amount of water dissplaced by the float will be dependent on the floats total area x one foot. The energy produced will be equivalent to that area of that water whatever it's size falling or riseing one foot.If you let it fall or rise all at once it could power a generator with more power than a tidal turbine depending on the floats size. |
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Post by alancalverd on Feb 1, 2014 9:30:08 GMT 1
How weird is that. I was thinking much the same in the shower this morning. Although it is (now) obvious that you get more power from a turbine in a tidal flow a float will work in zero flow. As long as there is a good tidal range it will work regardless of any flow. You can't have a tidal range without a tidal flow! And the float only generates power when it is moving, i.e. when the tide is flowing. Now just imagine a flat dumbell with a very narrow neck between two huge pans. In order to produce a tiny difference in water levels between the pans, you need a huge flow through the connecting neck. Which makes more sense - to build a float the size of the pan, or a turbine the size of the neck? Nature has provided us with plenty of dumbells in the shape of tidal rivers and channels, but the number of economically viable tidal range sites seems to be about 3 and they have already been exploited. In fact, the Rance barrage is simply used to control the flow. Whether you use a float or a turbine, the "raw" power output is inherently sinusoidal, but with a barrage you can moderate the flow to get a more constant or on-demand output from a turbine. |
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Post by rsmith7 on Feb 1, 2014 11:33:54 GMT 1
All very interesting and we all agree that energy can be extracted from the tide but the real question is at what cost?
If the tide can't be harnessed for similar costs as coal, gas and nuclear then why bother. My figures arrived at an installation cost of £325/W if current costs can be reduced by a factor of ten.
Now that's orders of magnitude more than conventional power so isn't tidal energy just pie in the sky?
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Post by alancalverd on Feb 4, 2014 9:47:55 GMT 1
Tides and waves have been with us for ever, and mechanical engineering goes back to Stonehenge and earlier, but AFAIK there are less than a dozen commercially operational TP stations generating electricity in the world, mostly around the UK. It just doesn't pay. Interestingly there are plenty of abandoned tide mills from previous centuries - direct extraction of mechanical energy is marginally more economic than a horse. I don't know of any grid-connected wave power installations but some navigational buoys use WP to keep the lights working: given the difficulty of maintaining a buoy and the penalties of failure, that makes economic sense.
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Post by principled on Feb 4, 2014 18:45:16 GMT 1
Alan, I thought that things on the UK on the TP front would be very quiet, but in doing some checking I see that two are planned in Wales. One near Anglesey over the next two years. Very small at 10MW. The other (2016-17) is a barrage in Swansea Bay which promises a respectable 400GWh per year. Mind you, that's still less than the Rance station in N. France at 540GWh. Wiki says that the latter's costs have now been recovered (50 years!) and the cost of electricity is now (according to Wiki) 1.8c per kWh, versus 2.5c per kWh for nuclear. Both are dwarfed by the potential of the Severn Barrage which is estimated (again according to Wiki) at 17 TWh per year.
The last time I spoke to anyone "loosely" connected to barrage plans (about 10 years ago), I was told that a "half barrage" with a double row of bi-directional turbines had been muted. This was to overcome some of the environmentalists fears about a full barrage. Not sure how far that idea got from the drawing board, I suspect as far as the waste bin! It will be interesting to see if minds are changed when or if the Swansea barrage is built.
The rest of the proposed large projects seem to be in Russia. I wonder if any will see the light of day?
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Post by rsmith7 on Feb 5, 2014 13:49:55 GMT 1
I wonder if the eye-watering cost of dredging silt build up is factored into the figures for barrages.
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Post by jonjel on Feb 5, 2014 16:32:56 GMT 1
Silt dear boy, silt!
Can't possibly happen as we all know that when fresh water from the Severn meets salt sea water the silt never ever drops out due to the difference in SG. Or is it the other way round...
Just the tiny difference in tidal flow completely covered most of the putcher salmon fisheries on the East bank of the river. Probably, but totally denied by the authorities caused by the building of the second bridge, and a large drainage pipe complete with concrete stabilisation which sticks out into the river
From fishing on rock or mud only perhaps a foot deep it is now mud around 10' thick in places, so totally unusable.
My patch so I know it rather well.
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Post by rsmith7 on Feb 8, 2014 17:10:23 GMT 1
Yes, the obvious problem of silting will probably be "overlooked" in the costings of barrages given the renewables advocate's habit of imaginative accounting where wind is concerned.
Another 10% rise in electricity prices this week. Great.
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Post by rsmith7 on Feb 8, 2014 17:15:10 GMT 1
If anyone's wondering, my profile picture is of the "Oyster" wave energy device lying on Lyness Pier in Orkney ... completely wrecked.
It was described in the press as "a great success"...
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