Question about hydroelectric dams...

[navigator]

It seems science accepts these as facts... energy comes from the sun, is a finite amount and cannot be created or destroyed.

 

A lake has a finite amount of potential energy in the water, which was placed there by the sun. Man has learned how, by increasing the head(distance the water drops), size and shape of the penstock(inlet tube), to increase the amount of potential energy by just using simple mechanics. Where does that inreased amount of potential energy, developed by man, fit into the law of energy conservation?

[SH3RL0CK]

The total potential energy in the lake will not change. However, the energy that can be converted into energy that man can use has increased because of technological developments.

[GDG]

It seems science accepts these as facts... energy comes from the sun, is a finite amount and cannot be created or destroyed.

 

A lake has a finite amount of potential energy in the water, which was placed there by the sun. Man has learned how, by increasing the head(distance the water drops), size and shape of the penstock(inlet tube), to increase the amount of potential energy by just using simple mechanics. Where does that inreased amount of potential energy, developed by man, fit into the law of energy conservation?

 

First, energy does not come only from the Sun.

 

Second, if you are talking about turning tubines in a hydroelectric dam, the energy is coming from gravitational potential energy, as water flows from a place of higher potential to a place of lower potential. Technological improvements have only increased the efficiency of the energy extraction, not increased the amount of energy available.

[navigator]

A different perspective...

 

Previous to daming a river the water flow has x amount of potential energy, man comes and builds a dam with a 100' head and increases the potential energy, then builds a larger dam with a 1000' head again increasing the head and the potential energy.

 

Are you guys telling me that the increase in energy is from more efficient conversion and not the mechanical design that increases the kinetic energy of the water?

[J.C.MacSwell]

A different perspective...

 

Previous to daming a river the water flow has x amount of potential energy, man comes and builds a dam with a 100' head and increases the potential energy, then builds a larger dam with a 1000' head again increasing the head and the potential energy.

 

Are you guys telling me that the increase in energy is from more efficient conversion and not the mechanical design that increases the kinetic energy of the water?

 

Yes. The mechanical design and dam are how you go about making it more efficient. No point in building a 1000 foot high dam if you have no potential to fill it.

[Sisyphus]

If you increase a 100 foot dam to a 1000 foot dam and usefully extract energy from the entire drop, then you're just making use of kinetic energy that you weren't before, namely that 900 foot drop of the water falling from the height of the top of the new dam to the top of the height of the old dam. Presumably this previously would have been over the course of the river upstream, or as falling rainwater, depending on the situation.

[Klaynos]

The dam can never be higher than the source... or if it is purely rain filled then the clouds... Nor can it be lower than the sea (assuming you've not dug a big hole for the water to go into which you would then have to keep pumping the water out of which would cost more energy than you gained.

 

The max potential energy change is from cloud to sea level...

 

Most of the energy 'increases' are made by reducing losses.

[navigator]

First, energy does not come only from the Sun.

 

Second, if you are talking about turning tubines in a hydroelectric dam, the energy is coming from gravitational potential energy, as water flows from a place of higher potential to a place of lower potential. Technological improvements have only increased the efficiency of the energy extraction, not increased the amount of energy available.

 

What is the difference between mechanical energy and gravitational potential energy?

 

Is gravity not a type of fuel for mechanical energy?

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The dam can never be higher than the source... or if it is purely rain filled then the clouds... Nor can it be lower than the sea (assuming you've not dug a big hole for the water to go into which you would then have to keep pumping the water out of which would cost more energy than you gained.

 

The max potential energy change is from cloud to sea level...

 

Most of the energy 'increases' are made by reducing losses.

 

Assuming there is no wave action or wind, the potential energy at the suface of any lake or the ocean is the same, zero.

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If you increase a 100 foot dam to a 1000 foot dam and usefully extract energy from the entire drop, then you're just making use of kinetic energy that you weren't before, namely that 900 foot drop of the water falling from the height of the top of the new dam to the top of the height of the old dam. Presumably this previously would have been over the course of the river upstream, or as falling rainwater, depending on the situation.

 

 

The potential energy of the water in the resevoir 1000' above the turbines is x. After the water drops through the turbines producing mechanical energy and collects in the lower resevoir, has the potential energy decreased, or does it remain the same?

Edited May 29, 2009 by navigator
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[Sisyphus]

What is the difference between mechanical energy and gravitational potential energy?

 

Is gravitational potential energy not a type of fuel for mechanical energy?

 

It is not too off to think of gravitational potential energy as "fuel" for kinetic energy, no.

 

Assuming there is no wave action or wind, the potential energy at the suface of any lake or the ocean is the same, zero.

 

The gravitational potential energy is proportional to the weight of the water and to the height which it can fall. With water, it is reasonable to measure it from sea level, since that's the lowest you can practically get it to fall to. (It's not absolute quantity, though. You can measure it to the center of the Earth, the center of the Sun, etc.) A lake, then, is a store of potential energy, since it can flow downhill to the ocean. Its motion is kinetic energy, and is defined as 0.5mv^2. Waves would be kinetic energy, but they aren't really relevant to this example.

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The potential energy of the water in the resevoir 1000' above the turbines is x. After the water drops through the turbines producing mechanical energy and collects in the lower resevoir, has the potential energy decreased, or does it remain the same?

 

The potential energy has decreased, because it's lower. Basically, the sun gives the water its potential energy by lifting it out of the water and up into the clouds, where it has its maximum. As it falls, first as rain, then over land as a river, it is steadily giving up potential energy.

Edited May 29, 2009 by Sisyphus
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[navigator]

It is not too off to think of gravitational potential energy as "fuel" for kinetic energy, no.

 

 

 

The gravitational potential energy is proportional to the weight of the water and to the height which it can fall. With water, it is reasonable to measure it from sea level, since that's the lowest you can practically get it to fall to. (It's not absolute quantity, though. You can measure it to the center of the Earth, the center of the Sun, etc.) A lake, then, is a store of potential energy, since it can flow downhill to the ocean. Its motion is kinetic energy, and is defined as 0.5mv^2. Waves would be kinetic energy, but they aren't really relevant to this example.

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The potential energy has decreased, because it's lower. Basically, the sun gives the water its potential energy by lifting it out of the water and up into the clouds, where it has its maximum. As it falls, first as rain, then over land as a river, it is steadily giving up potential energy.

 

So the potential energy has decreased, due to its height relative to sea level, not because some was converted into mechanical energy as it passed across the turbines causing them to spin?

[swansont]

So the potential energy has decreased, due to its height relative to sea level, not because some was converted into mechanical energy as it passed across the turbines causing them to spin?

 

Yes. The potential energy is solely due to the position. What you have done with it is convert some of it to work, but you didn't have to.

[navigator]

Yes. The potential energy is solely due to the position. What you have done with it is convert some of it to work, but you didn't have to.

 

In order to convert all forms of energy into mechanical energy you lose some of the original form of potential energy, but you just said the loss of potential energy is soley due to the position and not the conversion???

 

You have changed the definition of convert...

[Sisyphus]

In order to convert all forms of energy into mechanical energy you lose some of the original form of potential energy, but you just said the loss of potential energy is soley due to the position and not the conversion???

 

You have changed the definition of convert...

 

No. It's always doing work as it loses potential energy by moving to a lower position, it's just a matter of useful (to us) work vs. non-useful work.

[navigator]

No. It's always doing work as it loses potential energy by moving to a lower position, it's just a matter of useful (to us) work vs. non-useful work.

 

Ok, but can you not see the unique difference in the mechanical energy developed by a hydroelectric dam and the mechanical energy that is produced by expending any other form of potential energy in order to convert it into mechanical energy?

[SH3RL0CK]

Another way to look at it is by the conservation of energy (energy cannot be created or destroyed, only moved).

 

The energy starts by the sunlight evaporating water vapor from the ocean to re-form in clouds. This results in rain falling, eventually returning back to the ocean.

 

When the water is at a high(er) altitude, it has a lot of potential energy. As it flows down the river, it converts this into kinetic energy (i.e. the river has a current flow). When you place a turbine into this flow, the turbine will spin and the current flow slows down a bit to maintain the conservation of energy. Much of the stream also flows outside the turbine.

 

Now that we are getting power from the river (by slowing down the water current), how do we maximize the energy into the turbine? Lets stop the river flow altogether so that it all must flow through the turbine, thus increasing the efficiency. A good way to do this is with a dam so that all the potential energy is captured. A second dam (or larger dam) can capture more of the potential energy.

 

The maximum theoretical (and impractical) dam would be from the cloud tops to sea level so that all the energy that the sun provided to evaporate the ocean water would be recaptured as the water returns to the sea.

[Sisyphus]

Ok, but can you not see the unique difference in the mechanical energy developed by a hydroelectric dam and the mechanical energy that is produced by expending any other form of potential energy in order to convert it into mechanical energy?

 

I don't know what you mean by "developed" as opposed to "produced."

[navigator]

 

Now that we are getting power from the river (by slowing down the water current), how do we maximize the energy into the turbine? Lets stop the river flow altogether so that it all must flow through the turbine, thus increasing the efficiency. A good way to do this is with a dam so that all the potential energy is captured. A second dam (or larger dam) can capture more of the potential energy.

 

 

You guys are throwing words around paying little attention to their definition.

 

Lets take two dams, one with the turbine developing mechanical energy, the other with no mechaical objects in the path of the water, just the open penstock directly into the lower resevoir which is identical to the system with the turbines lower resevoir. If I captured some of the potential energy from the falling the water hitting the turbines,then potentials, once all the water has collected into the two lower resevoirs, would be different.

[SH3RL0CK]

There is energy in the waterfall regardless of the presence of a turbine. Where do you suppose this energy goes?

 

It get converted into heat (by the friction of the water on the surrounding concrete) and mechanical energy. When a turbine is in place, some of the energy that heats up the surrounding concrete and some of the mechanical energy of the flowing water turns the turbine.

[navigator]

I don't know what you mean by "developed" as opposed to "produced."

 

You produce something by using somwthing or a combination of things to make something better, but once you make it the materials used cannot be re-used, unless the production is deconstructed.

 

To develop something, you bring forth something from something else, but the primary thing remains in its original form and can be used to develop that thing again or even something else.

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There is energy in the waterfall regardless of the presence of a turbine. Where do you suppose this energy goes?

 

It get converted into heat (by the friction of the water on the surrounding concrete) and mechanical energy. When a turbine is in place, some of the energy that heats up the surrounding concrete and some of the mechanical energy of the flowing water turns the turbine.

 

Exactly my point! Regardless if I put turbines in the path of the falling water the potential energy at the bottom is the same, nothing is converted!

 

I am not concerned with the loses due to friction, I am interested in the excess mechanical energy and how science reconciles this fact with the law of energy conservation. The 2000 megawatts in the hoover dam comes from somewhere. If it was converted from the potential energy in the water, then why does the potentail at the bottom not reflect a loss?

[SH3RL0CK]

 

Exactly my point! Regardless if I put turbines in the path of the falling water the potential energy at the bottom is the same, nothing is converted!

 

 

Not true. The potential energy stored at the top of the dam is converted into kinetic energy in the waterfall. This kinetic energy is converted into heat (by friction) and mechanical energy (by moving things such as sand, rocks, or a turbine). With the inclusion of a turbine, we minimize the heat and maximize the mechanical energy to our advantage.

[navigator]

Not true. The potential energy stored at the top of the dam is converted into kinetic energy in the waterfall. This kinetic energy is converted into heat (by friction) and mechanical energy (by moving things such as sand, rocks, or a turbine). With the inclusion of a turbine, we minimize the heat and maximize the mechanical energy to our advantage.

 

Two Identical systems, two identical upper and two identical lower resevoirs. Same height same amount of water, everything is identical. Open both penstocks and release all the water in both systems. When the water collects in the bottom of the 2 resevoirs the potential energy is less, but it is identical in both lower resevoirs. You can line up as many turbines as possible in one of the systems, leaving the other just a waterfall. You can increase the output by adding more turbines etc. But the potential in the lower resevoir will always end up the same. No energy has been converted, the loss in potential is relative to its height, not the output of the dam.

 

No potential energy is converted, it remains the same in the lower resevoir regardless of how many turbines you put and megawatts you develop as the water falls.

[SH3RL0CK]

Two Identical systems, two identical upper and two identical lower resevoirs. Same height same amount of water, everything is identical. Open both penstocks and release all the water in both systems. When the water collects in the bottom of the 2 resevoirs the potential energy is less, but it is identical in both lower resevoirs. You can line up as many turbines as possible in one of the systems, leaving the other just a waterfall. You can increase the output by adding more turbines etc. But the potential in the lower resevoir will always end up the same. No energy has been converted, the loss in potential is relative to its height, not the output of the dam.

 

No potential energy is converted, it remains the same in the lower resevoir regardless of how many turbines you put and megawatts you develop as the water falls.

 

 

The same total energy in the water is lost from both reservoirs. In the case where there are turbines, this energy went to a form that can be used by people. In the case where there are no turbines, this energy went elsewhere... I think perhaps you misunderstand "potential energy"

 

http://dictionary.reference.com/browse/potential%20energy

 

–noun Physics. the energy of a body or a system with respect to the position of the body or the arrangement of the particles of the system.

 

Notice how position and arrangement is important. In the case of the reserviors, the final stage is lower than the first stage and thus at a lower potential energy.

 

I'm loving dictionary.com today...

[navigator]

The same total energy in the water is lost from both reservoirs. In the case where there are turbines, this energy went to a form that can be used by people. In the case where there are no turbines, this energy went elsewhere... I think perhaps you misunderstand "potential energy"

 

http://dictionary.reference.com/browse/potential%20energy

 

 

 

Notice how position and arrangement is important. In the case of the reserviors, the final stage is lower than the first stage and thus at a lower potential energy.

 

I'm loving dictionary.com today...

 

If the same energy in both systems is lost, it is due to many factors; a turbine in the path of the water is not one of them. To convert potential energy it would be lost moving through the turbine causing it to spin. This can be stated as scientific fact, not theory, because the potential energy in the lower resevoirs remains equal, regardless of how much mechanical energy you develop from the falling water.

 

I am not sure how else to say it. A hydroelectric dam does not convert the suns energy. Once the H20 reaches the clouds gravity takes over. Gravity is a not a form of energy. It is a force that can be used as a fuel for mechanical energy, but because it requires some type of matter or vehicle to show itself it cannot be a form of energy, it is an invisible force.

 

All forms of energy lose potential whenever they are converted into mechanical energy, but the losses in a hydroelectric dam system are not associated in anyway with the output. The output can be increased and the potential will always remain the same in the lower resevoir.

[Sisyphus]

That's factually incorrect, navigator. A dam cannot extract more energy than the potential. When part of that potential is converted into useful work, less non-useful work is accomplished. The total energy remains constant in both cases.

[navigator]

That's factually incorrect, navigator. A dam cannot extract more energy than the potential. When part of that potential is converted into useful work, less non-useful work is accomplished. The total energy remains constant in both cases.

 

The fact that the potential in the lower resevoir remains the same makes it factually correct.

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To say that a hydroelectric converts the waters potential into mechanical energy is factually inncorrect, otherwise the lost potential in the conversion would show up as a loss in the amount of potential left in the water when it reaches the lower resevoir. But when compared to a identical system with no turbine the potential is always the same.

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A dam cannot extract more energy than the potential.

 

I agree with you here.

 

But you can extract the energy without affecting the potential left at the bottom.

 

This is not converting energy, it is developing it.