Crosswords1 min ago
Generating Power
With all the controversy over fracking, nuclear power, etc, etc., with a good deal of the earth covered in water, it seems to me that with a little innovation, we have potentially all the power we'll ever need. Why can't we harness the power of the sea?
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@ Khandro This is a pretty basic info resource, but is quite illuminating, I think. In it, they discuss the use of water wheels to generate electricity. The conclusion appears to be that it simply cannot generate sufficiently large quantities of electricity for anything other than a local solution.
From the article linked to;
"Another run-of-the-river design uses a traditional water wheel on a floating platform to capture the kinetic force of the moving river. While this approach is inexpensive and easy to implement, it doesn't produce much power. The entire Amazon River, if harnessed this way, would produce only 650 MW of power."
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(I haven't checked the math they use for the power output calculation, but it sounds about right.)
Any medium term power solution for global energy requirements is going to take a mixture of solutions, ranging from increased energy saving and energy efficiency systems, through to renewable energy, nuclear fission in the shorter term and hopefully nuclear fusion in the medium to long term.
Fracking, despite some of the issues surrounding it, will inevitably play a part; Given the available volumes around the globe, I cannot see any country ignoring such an energy resource, even given the possible impact it might have on climate change.
From the article linked to;
"Another run-of-the-river design uses a traditional water wheel on a floating platform to capture the kinetic force of the moving river. While this approach is inexpensive and easy to implement, it doesn't produce much power. The entire Amazon River, if harnessed this way, would produce only 650 MW of power."
http://
(I haven't checked the math they use for the power output calculation, but it sounds about right.)
Any medium term power solution for global energy requirements is going to take a mixture of solutions, ranging from increased energy saving and energy efficiency systems, through to renewable energy, nuclear fission in the shorter term and hopefully nuclear fusion in the medium to long term.
Fracking, despite some of the issues surrounding it, will inevitably play a part; Given the available volumes around the globe, I cannot see any country ignoring such an energy resource, even given the possible impact it might have on climate change.
Thanks to SD and LG. So the taking out of energy from a flow of water results in its slowing down, therefore making the installation of another system close by unfeasible, which I guess would figure.
I like the link LG, and love the waterwheel illustration, also;
"Meanwhile, the reservoirs or lakes are used for boating and fishing, and often the rivers beyond the dams provide opportunities for whitewater rafting and kayaking." makes it all sound a very civilised method of energy production.
I like the link LG, and love the waterwheel illustration, also;
"Meanwhile, the reservoirs or lakes are used for boating and fishing, and often the rivers beyond the dams provide opportunities for whitewater rafting and kayaking." makes it all sound a very civilised method of energy production.
Some posters on here do not understand the significance of the first law of thermodynamics
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Put simply this states that energy can never be created or destroyed , it can only be changed from one form to another.
If we put a water wheel / dynamo on a river it changes some of the energy in the river into electricity but the energy removed means the river slows down.
The same applies to wind turbines , they slow down the wind, and even to tidal power stations, they slow down the Moon which means it moves further away from Earth. ( see bert_h's post)
There is only a finite amount of energy in the Universe, so it the real scientific world there can never be limitless power.
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Put simply this states that energy can never be created or destroyed , it can only be changed from one form to another.
If we put a water wheel / dynamo on a river it changes some of the energy in the river into electricity but the energy removed means the river slows down.
The same applies to wind turbines , they slow down the wind, and even to tidal power stations, they slow down the Moon which means it moves further away from Earth. ( see bert_h's post)
There is only a finite amount of energy in the Universe, so it the real scientific world there can never be limitless power.
Tidal energy does not peak twice a day. At any given point the tidal velocity reaches a maximum nearly 4 times per day. Around the British Isles there are high and low tides at all times of the day.For example at Southampton low water spring tide is at about 6:00 and 18:00 but the same day it is at about midday and midnight at Milford Haven. Thus there would be a seamless supply of energy if thr generators were placed appropriately, whether tidal reservoirs or low head turbines were used.
EDDIE51; //If we put a water wheel / dynamo on a river it changes some of the energy in the river into electricity but the energy removed means the river slows down.// I can see that, but not permanently for the rest of its course; surely if a river moving at 10MPH relinquishes its speed because of a wheel to say, 7MPH, gravity will allow it to return to its former speed (and power) after a certain distance, to be used again. I can't see what effect this could have on the moon.
Why this would have an effect on the Moon (albeit a small one) has been mentioned already, but loosely you can picture the Moon as someone turning a crank, which represents the tides. The crank is easy to turn if there is no resistance (such as the natural tidal frictions), and harder to turn if you add more resistance (such as constantly drawing yet more energy out by a machine). The picture misses a lot of detail, but captures the essence -- I think -- of what is going on.
In practice, even if we drew on tidal energy as much as possible it would add little to the natural forces that affect lunar motion, so while there would be an effect it would not be all that significant.
A similar principle exists in the case of rivers. The speed of a river varies anyway due to friction with the riverbanks and riverbed, but for the sake of argument let us ignore that and allow for a river that is 5 metres wide and 1 deep flowing at 2 metres per second = about 20 kW of energy (probably less, but never mind). If you extracted, say, half of this energy, that is equivalent to reducing the speed to about 1.4 metres per second (probably more, because we can't be 100% efficient).
Friction forces are difficult to estimate, but typically they are enough to ensure that the river barely accelerates at all. This is because even if you allow only for gravity with no friction at all, a normal slope of say 2 degree or so gives an acceleration of 0.3m/s^2 (ish), which is not a lot (but on its own would lead to very huge river speeds!). This acceleration is anyhow reduced, in most cases, to effectively zero, so that a river would take several miles to get back "up to speed". This will necessarily change the behaviour of the river in the affected stretch, and it may even be the case that for some rivers, there would not be enough distance to regain all the speed.
Most waterwheels therefore only operate on a small scale. Any larger, and you might stop the river entirely, or at least seriously change its motion and properties. This is environmentally damaging, potentially, and certainly unsustainable.
In practice, even if we drew on tidal energy as much as possible it would add little to the natural forces that affect lunar motion, so while there would be an effect it would not be all that significant.
A similar principle exists in the case of rivers. The speed of a river varies anyway due to friction with the riverbanks and riverbed, but for the sake of argument let us ignore that and allow for a river that is 5 metres wide and 1 deep flowing at 2 metres per second = about 20 kW of energy (probably less, but never mind). If you extracted, say, half of this energy, that is equivalent to reducing the speed to about 1.4 metres per second (probably more, because we can't be 100% efficient).
Friction forces are difficult to estimate, but typically they are enough to ensure that the river barely accelerates at all. This is because even if you allow only for gravity with no friction at all, a normal slope of say 2 degree or so gives an acceleration of 0.3m/s^2 (ish), which is not a lot (but on its own would lead to very huge river speeds!). This acceleration is anyhow reduced, in most cases, to effectively zero, so that a river would take several miles to get back "up to speed". This will necessarily change the behaviour of the river in the affected stretch, and it may even be the case that for some rivers, there would not be enough distance to regain all the speed.
Most waterwheels therefore only operate on a small scale. Any larger, and you might stop the river entirely, or at least seriously change its motion and properties. This is environmentally damaging, potentially, and certainly unsustainable.
The kinetic energy in a flowing river is converted into heat by friction, converting that energy into electricity means that that heat is transferred elsewhere ie to someone's house. The same applies to the tides, most tidal kinetic energy in the sea becomes heat energy due to friction and warms up the sea. Extracting that energy will result in a very very slightly cooler sea, which might not be a bad thing. Any effect on the moon's orbit will be miniscule.
jim&jom :-) Much appreciated answers, this really fascinates me (I live on the bank of a river) I didn't before this thread realise the effect of abstracting energy from a river was slowing it down to such an extent, and I can see how a further sequence of wheels would be compoundly less and less effective. What of dams though; if the Turbine in a hydro-electric dam is powered by the pressure from the backed-water, isn't the pressurised water leaving the dam after driving the turbine(s) the same amount as is entering the lake behind it ? and assuming the surrounding topography allowed, couldn't a second dam be constructed?
If you're happy with the environmental damage, then yes, you can build as many dams as the land allows. What is required for a dam is that you have enough space to have a higher/ lower effect going on, so that the water can fall and return to its original path, so for particularly long rivers with a lot of valleys, you could build a lot of dams along the river with some decent energy returns, and indeed this has already been done on occasion. The Yangtze river has two dams currently, one capable of generating about 22 GW of power, and the other a more modest 3 GW; both are inside the 30 largest HEP stations in the world.
Unfortunately the dams are blamed (I don't know how correctly) for contributing to the extinction of the baiji dolphin; the larger Three Gorges dam has destabilised some of the surrounding land (increased risk of landslides); a potential increased risk of flooding downstream; and, perhaps less seriously, displaced over 1 million people and flooded some historically important sites.
If you are prepared, however, to allow for such effects, then a river can support as many dams as the topography allows -- presumably because you are effectively adding potential energy to the river, by creating a high lake that was not there, and then taking it away again. There are plans to build a further four dams on the Jinsha/ Yangtze rivers, and there are going to be sixteen dams at least on the Jinsha once all work has been completed.
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Unfortunately the dams are blamed (I don't know how correctly) for contributing to the extinction of the baiji dolphin; the larger Three Gorges dam has destabilised some of the surrounding land (increased risk of landslides); a potential increased risk of flooding downstream; and, perhaps less seriously, displaced over 1 million people and flooded some historically important sites.
If you are prepared, however, to allow for such effects, then a river can support as many dams as the topography allows -- presumably because you are effectively adding potential energy to the river, by creating a high lake that was not there, and then taking it away again. There are plans to build a further four dams on the Jinsha/ Yangtze rivers, and there are going to be sixteen dams at least on the Jinsha once all work has been completed.
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Khandro- There are two main factors to consider when calculating the amount of energy which can be extracted from water. Firstly the mass of water, secondly the height of the water above the turbine.
The gravitational potential energy of the water is the mass (in kilograms) multiplied by the height (in metres) multiplied by Earth's gravitational field strength (approx. 10 Newtons per kilogram)
The water turbine will convert the gravitational potential energy into electrical energy via a generator. Of course the turbine and generator are not 100% efficient at converting energy.
The water exiting the turbine will flow slower (have less kinetic energy) than it would if the turbine wasn't in place.
Power is the energy converted per second (in Watts). If the water behind a dam is used at a high rate (eg. many tonnes per second) then the power will be correspondingly high.
The gravitational potential energy of the water is the mass (in kilograms) multiplied by the height (in metres) multiplied by Earth's gravitational field strength (approx. 10 Newtons per kilogram)
The water turbine will convert the gravitational potential energy into electrical energy via a generator. Of course the turbine and generator are not 100% efficient at converting energy.
The water exiting the turbine will flow slower (have less kinetic energy) than it would if the turbine wasn't in place.
Power is the energy converted per second (in Watts). If the water behind a dam is used at a high rate (eg. many tonnes per second) then the power will be correspondingly high.
On the subject of power generation and the continuing controversies of fracking and nuclear and all the rest of it, George Monbiot wrote a recent article in the Grauniad which you might find interesting;
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This news article appeared on the BBC, may be of interest:
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