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Hydro-electric generation from river beds


cetus
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why do they flood enviroments to build hydro electic dams

when they could just build them in to the river beds

Do you want this post split off as it's own thread, possibly in Engineering?


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This post was split from the thread Global Warming at the request of the thread starter.

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why do they flood enviroments to build hydro electic dams

when they could just build them in to the river beds

 

Because you can generate more energy that way. Hydro plants use the vertical drop of the water rather than the motion in a river. The potential energy is mgh, which is converted to kinetic energy as it falls (1/2mv^2), and you can extract some fraction of that. The kinetic energy in a river is much smaller because the speed is small, and you don't have access to all of the water to turn your turbine — if you came anywhere close to doing that, the river wouldn't be navigable. In a dam, a much larger fraction of the water can be directed through the turbines. Also, I think turbines are more efficient at higher speeds.

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A dam solves lots of other practical issues.

 

For the use of a generator in a riverbed, there is the problem that the water has to be flowing at the exact level to match the turbine. The water level in a river goes up and down all the time; so unless you want to be moving the (usually extremely large) turbines all the time, they won't be operating the majority of the time.

 

With a dam, you can force the water to flow only on the exact "sweet spot" for the turbines. You can store excess water for use during the summer (when the river normally goes dry). During a flood, you can still operate the turbines; while letting the excess water go through the floodgates (if the turbines were only in the river, they would then be under water).

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i meant under water turbines to meet the under currunt it would work for a small community

also dams dams may solve problems like floods and large scale electricity

bu destroy lots of preasuse farm land pasture land woods ect

dont destroy more land for electricity and money

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i meant under water turbines to meet the under currunt it would work for a small community

also dams dams may solve problems like floods and large scale electricity

bu destroy lots of preasuse farm land pasture land woods ect

dont destroy more land for electricity and money

 

I'm not sure underwater turbines would work all that well, even for a small community. The efficiency is going to be low and there would the corresponding problems associated with river stages (flooding and drought would cause problems). There are going to be problems associagted with mud and debris clogging things up that are generally less of a problem with dams. But it is certainly possible to do, as long as you are willing to accept these costs (which is probably why this isn't seen very often).

 

I do agree with you regarding the loss of land though it should at least be pointed out that a lake has been gained which compensates for the lost farms and forest.

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but a lake can only be use for fishing and sailing swimming would be to dangerous

and also what good a lake

globle warming (note this is the starter tread of globle warming) is being pervented by trees that are being destroyed by the flooding for these lakes

plus the scotts dont like them as they are stealing from there lands

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As far as I'm aware, a device for utilizing the natural flow of a river has already been invented.

The device Cetus proposed sounds rather like an under-shot water wheel. One where the flat paddles of the wheel dip into the water just above the riverbed; used to power small industries like saw mills etc.

 

it's successor was the over-shot water wheel which produced a lot more torque and could power a huge textiles mill. It - like the hydro-electric damn - could use a stored 'head' of water if necessary.

 

Although water wheel look rather quaint these days, they can hardly meet the needs of today's power consumption

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here is a system that uses the low head of a river very efficiently...

 

 

The modern hydrodynamic turbulent water circulation will be eliminated as good as possible. On the other hand we developed the world wide first so called gravitation water vortex power plant which uses the rotation energy of a single gigantic water vortex. It is a milestone in the hydrodyamic development because in the past we needed energy to aerate water - now we have a water aeration process which produce electric energy. Already in the first operation year the invention of the Austrian engineer Franz Zotlöterer produced 50.000kWh - electric energy for 14 average european households. The prototype plant supply the public electricity network with current. You can find the gravitation water vortex power plant on the millstream in Obergrafendorf (10km southwest from St.Pölten in Austria in Central Europa). It is public and can be reached on the cycle track along the pielach river:

 

Presently used falling height 1,3m

Presently used flow rate 1m³/s

Diameter of the rotation tank 5,5m

Hydraulical power 13kW

Elektrical power 8kW

Effectiveness of the turbine 80% at 3/3, 83% at 2/3 and 76% at 1/3 of the maximum flow rate

Turbine speed 25rpm

Investment minus financial support around 40.000€

Joyful working capacity of 50.000kWh in the first year of operating - since February 2006 the actually total production of electricity is over 120.000kWh

 

 

Pictures and the rest of article can be found here...

 

http://www.zotloeterer.com/our_company/water_vortex_engineering/water_vortex_power_plant.php

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It generates about 150 kW. A big dam can generate more than a thousand times more power.

 

On a comparative scale it is very efficient. No dam, the type your reffering to, would come close to generating 150 kw with a 1.3m head and low water volume. Thats not to mention the affect on the ecological system is almost zero, do that with a dam.

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The OP asked why we build hydro-electric dams instead of wheels in rivers. That question has been addressed. It's about output and efficiency.

 

Now, why not just pop up some solar panels? They don't hurt the fish at all.

It's good to think about smarter solutions, but important that the proposals scale to the need.

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On a comparative scale it is very efficient. No dam, the type your reffering to, would come close to generating 150 kw with a 1.3m head and low water volume. Thats not to mention the affect on the ecological system is almost zero, do that with a dam.

 

That's the point, though, of why dams are used — more power. I don't think anyone has claimed that there is no ecological impact.

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On a comparative scale it is very efficient. No dam, the type your reffering to, would come close to generating 150 kw with a 1.3m head and low water volume. Thats not to mention the affect on the ecological system is almost zero, do that with a dam.

About the water-vortex-plant

Per volume and head (head = difference in height) of water, hydro dams are close to optimal. Hydro dams run at 80% efficiency (the rest is friction). You simply cannot get much more out of it... You can get the 20% extra if you use frictionless moving parts and superconductors and such... it's all a bit science fiction.

So, I'm not sure of this water-vortex-plant, but I saw different data to start with:

# Presently used falling height 1,3m

# Presently used flow rate 1m³/s

# Diameter of the rotation tank 5,5m

# Hydraulical power 13kW

 

Which is all correct: [math]Power = flow * head * g[/math], so [math] Power = 1000 * 1.3 * 9.81 = 13 kW[/math]

 

Apparently these guys have zero friction... although later they claim that their effectiveness is "over 70%" (i.e. not 100%), which puts it at the same efficiency, or slightly lower, as standard hydro dams.

 

The "150 kW" was another value, for another system, with another flow and/or head.

 

Still, I am not sure about the ecological impact of this thing: it still has large steel pieces moving around, effectively blocking the path of the fishes.

 

About the topic in general

Any system that has a less severe ecological impact will also run at lower efficiency, because they all have a bypass of some kind.

Whenever water is just flowing past the power station without pushing on some rotor, wheel, turbine or propeller, its potential energy goes to waste.

Of course, the fish like this, because heavy steel equipment and fish never combined very well, unless you want fish-soup.

 

But the bottom line is: hydro dams are so popular because they're the most efficient.

 

Please note that the other type of hydro-power (using propellers under water) is also used: as tidal energy. Why? Because at the coast you have no constant difference in height like in a river. The available energy is kinetic energy, not potential energy.

Edited by CaptainPanic
I found more info :-)
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Now, why not just pop up some solar panels? They don't hurt the fish at all.

It's good to think about smarter solutions, but important that the proposals scale to the need.

 

The two main reasons, I should think, would be magnitude of output and consistency/controllability. The Hoover Dam has put out 2 gigawatts of clean, consistent energy for the last 70 years (at the cost of localized screwing with the river's ecosystem). That's a role that it solar would find extremely difficult to fulfill.


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Please note that the other type of hydro-power (using propellers under water) is also used: as tidal energy. Why? Because at the coast you have no constant difference in height like in a river. The available energy is kinetic energy, not potential energy.

 

There are lots of different types of tidal power, and I expect we'll be making a lot more use of it in the next few decades. I think what you're talking about are the ones make use of horizontal flow, like "underwater wind turbines." The flow is more consistent and predictable than wind and, because its water instead of air, a lot more energy can be extracted. I've read somewhere that New Zealand was looking into the technology for the strait between the north and south islands, as the current is strong and broad enough there that they could basically power the whole country, and expand as needed. The downsides being that the ocean environment is very harsh, and maintenance would likely be difficult.

 

There are other designs, of course, making use of vertical motion. Like huge barges that extract energy from both rising and falling with the tides.

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There are lots of different types of tidal power, and I expect we'll be making a lot more use of it in the next few decades. I think what you're talking about are the ones make use of horizontal flow, like "underwater wind turbines." The flow is more consistent and predictable than wind and, because its water instead of air, a lot more energy can be extracted. I've read somewhere that New Zealand was looking into the technology for the strait between the north and south islands, as the current is strong and broad enough there that they could basically power the whole country, and expand as needed. The downsides being that the ocean environment is very harsh, and maintenance would likely be difficult.

 

There are other designs, of course, making use of vertical motion. Like huge barges that extract energy from both rising and falling with the tides.

 

Yep. I think that's the only option if you want to build something "into the river bed" (as written in the 1st post). You cannot block the flow, so you must convert kinetic energy, rather than potential energy.

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Now, why not just pop up some solar panels? They don't hurt the fish at all.

It's good to think about smarter solutions, but important that the proposals scale to the need.

 

but it would not be efficant in countrys with little sun light like england or france.

the turbines under the water provide energy to a small comuntity but stop the dead and destrution of natural land scapes meaning you could build them in any old river with little consequences to consider.

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I know, and agree with your point (as well as the well articulated one by Sisyphus above). However, let me clarify my point.

 

If you're worried about fish, find another energy source which doesn't require big mechanical whirly-gigs to sit in the water. That's the larger point I was making.

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That's the point, though, of why dams are used — more power. I don't think anyone has claimed that there is no ecological impact.

 

I agree, I was just giving an example that technology is advancing and river bed hydro-electric system, similiar to the power generating capabilities of a dam, may not be too far in the future.:)

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Now, why not just pop up some solar panels? They don't hurt the fish at all.

It's good to think about smarter solutions, but important that the proposals scale to the need.

 

but it would not be efficant in countrys with little sun light like england or france.

the turbines under the water provide energy to a small comuntity but stop the dead and destrution of natural land scapes meaning you could build them in any old river with little consequences to consider.

 

The underwater turbines may still kill fish, and they reduce the depth of the river, so larger ships can get into trouble (if there are any ships on a river).

 

Solar panels obviously reduce space for plants and nature.

 

Wind power, to me, seems one of the best options... but even wind power does not have zero ecological impact, and people complain that windturbines are ugly (personally, I disagree with that - then again, I'm Dutch, and us Dutch have a thing with windmills :D)

 

But in the end: if all 6.7 billion humans want energy, something else has to go away so that we can construct something new. Zero ecological impact is just not possible.

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for the fish to die they will ahve to be very samll to swimm throught a grate that protects the trubine from rubbish

 

Thanks for pointing out this compromise and why a dam has an advantage over just the current flow in the river. For the turbine to not get clogged with debris (such as a very large tree; or an old tire) it needs to have a grate.

 

Across a river, the grate will become clogged with debris; though it is cheaper and easier to clear the grate than replace turbine blades. As this occurs, the efficiency is reduced because there will be less water flow through the grate. Essentially, you now have to pay someone to clean the grate all the time; and it entirely likely that you will need to shut down the turbine while you do so.

 

With a dam, the floating debris can simply stay on top of the water; above the turbine gates. The heavy debris can simply stay at the bottom; well below the turbine gates. There are very few things that naturally float in the middle; so the grate will remain clean for a very long time in comparison. Consequently, there is much less of a problem with debris clogging things up when using a dam.

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yes there are still some hitchs but it is still a lot cheaper then dam cost

people have to weekly ceack the dam for cracks so a massive wave is not produced this has to be done under water

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I agree with Sisyphus: hydro electricity from dams is cheaper than the river bed turbines. In fact, hydro electricity is cheaper than electricity from fossil fuels.

 

If these river bed turbines were as cheap as the dams, then all rivers would be full of these things (given the capitalist world we live in). The fact are however that companies and governments are motivated to build extra dams, but nobody invests in these river bed turbines.

 

That could be because they're not fully developed, but that's not true. Such turbines are extensively researched and they exist for all sizes and water flows.

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