# Why is hydroelectric power only practical in certain places?

It seems that you can only generate hydroelectric power at certain areas where you construct massive dams, but I don’t understand why this is.

Why couldn’t you just take a running river and set up turbines to be spun by the river every 10 feet? Or whatever the optimal spacing is. It seems like you could take any river that has constant flow that has no need for shipping and just extract power at every inch of the river by having a continuous series of turbine generators spinning in the water - sort of like water windmills.

Why isn’t this done?

It doesn’t sound very efficient. A damn means you can use the entire flow of a river to create power. And the height difference has gotta help too. A ton of water dropping 20 metres is going to give out a lot of energy.

Turbines in a river wouldn’t be able to generate much power, because if they start resisting the water flow to any great degree, the water would just go around it.

But I’m no expert, what do I know?!

Also it is probably more efficient to have several large turbines producing electricity then, I would assume, thousands of smaller turbines along the length of a river.

It’s basically…thermodynamics I guess. The energy in the water is consumed by one dam, and it takes a height difference across distance to regain the energy. The Blackstone river is just down the hill from me, where the Industrial Revolution started here in America. Even back then there were disputes over mills using up the river’s energy.

The maximum energy available is proportional to M*h, where M is the mass of the water, and h is the the difference in height between where water goes into your energy extraction device, and where it leaves it. For instance between the top of a dam and the exit pipe of a turbine.

What about the speed of the water in a flowing river then? Well if you want to extract that power your extraction device has to slow the water down, which makes it a dam. If you don’t want to flood the nearby countryside such a device would have to be small relative to the size of the river.

In my very first engineering class in college (many, many years ago) we had to design a hydroelectric power supply for a small hunting cabin. Our theoretical imaginary cabin happened to have a small stream running next to it. Being environmentally conscious little engineering students, we first tried to generate enough power without damming up the stream, but it just didn’t work. We couldn’t get enough power. Then we constructed a little imaginary dam, and even though our dam wasn’t that large, it still allowed us to generate enough power for this small hunting cabin.

The key is water height, not flow. The stream still had the same amount of water flowing through it, but by increasing the height at our generator, we were able to get significantly more power out of it.

People think hydroelectric power is good for the environment because it doesn’t throw out pollutants like a coal plant or radiation like a nuke plant, but damming up streams and rivers is very bad for the environment. Dams alter the water level throughout the waterway, and interfere with wildlife that moves up and down through that waterway, preventing fish from reaching their spawning grounds, for example.

Most places now have laws that prevent you from damming up a stream, even if it is on your property. If you want to construct a dam of some sort, you now often have to do an environmental impact study of some sort to prove that it won’t negatively affect the local wildlife.

I’m not an engineer, so please bear with me. I took a class that talked about the (emerging?) field of microhydro power, small-scale hydroelectric systems using special turbines designed for low-flow/low-head rivers and streams; they’re also designed to not reduce the flow rate of the river by any more than 50%, and unlike regular dams, none of the water is stored for more than a few minutes and the stream returns to full power just a few hundred feet or so from the facility.

Did the scenario you describe already take all that into consideration?

I’m still not quite getting it - a running river has so much energy in the water moving downstream. I’m thinking about something that looks like a windmill, only with the arms turned flat so the water pushed them. You’d have a giant turbine spinning all day, just like a windmill. And you could have a “farm” of them by putting them all along the river. Why wouldn’t having the power of the rushing water rotating the turbines generate enough power?

Is it because that even a turbine which had paddles that were small relative to the size of the river woud slow the flow enough that the water levels upstream would raise and flood?

I mean - take the Amazon river as an extreme example. There’s so much energy in that water flowing. Couldn’t we just stick thousands of turbines in the water there and harness it? I’m just not getting why if getting a big turbine spinning in the wind can generate enough power to be worthwhile, getting a big turbine spinning by much more powerful (compared to wind) water currents can’t.

Are you sure this is an engineering problem and not a political/economic one? If it’s cheaper to go with large-scale hydro or fossil fuels, that’s what most people will use.

The entire early industrial revolution was started by water power. The dark satanic mills, and so forth. I wonder how much power could be generated just by reactivating old water-mills.

I have seen proposals for these micro hydro plants. I don’t remember seeing numbers for the capital per kilowatt. Yes, it can be done. Is it a good place to put our money? So many people don’t worry about how much these things cost.

Yes, we have many old small dams that once generated power. Often they are already near the grid. I drive by an old one that has a coal fired plant next to it.

i think that dams provide height where there isn’t that naturally and provide a constant predictable flow for large hydro.

micro hydro can be done without dams though the flow might be concentrated.

As naita said, the available energy is dependent on height and volume. If you have a lot of both, it is practical. Otherwise, you will have to use some unproven technology like this.

The height of water drop in many large rivers is much less than you might think. The Amazon drops only 80 feet from hundreds of miles upstream (Manacapuru) to the ocean. The Mississippi drops only 300 ft from the Ohio River to the Gulf of Mexico. And to capture enough of this energy, you have to funnel the water thru a narrow channel, interrupting navigation of boats and fish.

Putting a turbine in the middle of a river will not work well without creating a dam or channel because water will take the path of least resistance, and there is less resistance going around your turbine than going thru it.

You can generate power using “run-of-river” hydro turbines. They have a different design from conventional turbines.

In broad terms, if you have a good height difference (conventional dammed hydro power) you use a “reaction turbine” (e.g. a Francis turbine) meaning there is a change in pressure within the turbine, whereas if you have little or no height difference you use an “impulse turbine” (e.g. a Kaplan turbine), which recovers the kinetic energy associated with the speed of the water. This is similar to the difference between a wind turbine and a windmill.

My company owns and operates a number of these small run-of-river plant on small rivers in the west of Ireland. Their output is only in the 200-300 kW range. By contrast, our conventional hydro plant at Ardnacrusha generates up to 90 MW at one site.

Additionally, Ardnacrusha and similar dammed hydro provides a valuable service to the transmission system. The dammed water represents a large amount of stored energy that can be called upon, allowing the plant to be dispatched and the power to be generated when it is most needed. This helps to reduce the cost of power at peak times.

Run-of-river plant, by contrast, is non-dispatchable, the output being determined by the flow of water in the river on the day.

In practice, while “pure” run-of-river plant does exist, there is very often a certain amount of water sequestered upstream for use in the turbine, so the boundary between run-of-river and dammed becomes a little blurred.

I have some right outside my apartment, in the East River (NYC). They are powered by the tide and generate power for the grocery store. They had to study the first turbines for 18 months before they could put more in, to make sure they weren’t sucking in all the fish and grinding them up. I think they swivel, to face the direction of the tide.

The OP is describing extracting energy from the kinetic energy due to the velocity of the water. Dams extract energy from the potential energy of the water in its height.

Someone can check my numbers, but it takes sqrt(2/g) = 0.45 s to drop one meter. The velocity is then gt = 4.4 m/s. So the energy available from a 1 meter drop in height (a very short dam) is the same as slowing water from 4.4 m/s to 0. If you only want to reduce the velocity to half its original velocity, you need to start with 5.1 m/s, and slow to 2.55 m/s.

5 m/s seems like a fast flow rate to me, and it’s only providing as much energy as a 1 meter dam.

Rivers move very slowly. This must be true or they wouldn’t be navigable by unpowered boats. The flow is steady, but the push is small.

Old fashioned flour and saw mills used the technique you’re talking about. And generated enough power to grind some wheat.

Generating concentrated power calls for concentrated water. That’s why dams to raise the water’s height and give it some oomph are the universal solution. Nobody would give you permission to line every single foot of both banks of every river to get the same effect. Not would you do it because of the horrendous inefficiencies.

For any river or stream, at a particular width, depth, and flow of water, over a particular distance along the river, there is a limited amount of energy that can be extracted no matter how many turbines, waterwheels, or other devices you use.

Yes, it can and is being done, but does anybody have a \$/kw figure?

We have an unlimited supply of free power. All we need is the money to collect it.

I hope I am not straying too far off topic to give some figures. The 11 April Forbes has an article on Japan recovering from their disasters. It includes some figures on what it would cost to replace the power from the atomic plants.

Solar parabolic troughs covering 80,000 acres
\$ 40 billion
Wind 6,000 turbines covering 100,000 acres
\$18 billion
Coal 15 modern coal-fired power plants
30 billion Natural gas 8 modern combined cycle power plants 7 billion
Nuclear 6 new reactors
\$ 24 billion

I am surprised how well wind fares, second only to natural gas with its high fuel costs. We can build wind cheaper than coal?

I am sure the Japanese will be forced to select choices minimizing land usage. Can you imagine Mount Fugi ringed with wind turbines?

Perhaps we need to think more about wind. I have imagined a turbine at the top of every high tension tower. The electric company already owns the land, easy to connect to the grid, and it can’t be much more of an eyesore. Likely the towers aren’t built to support the additional loads.

We have the space, and mostly need power on hot summer afternoons, often windy times. I find it unlikely that New Mexico, Wyoming, and off shore Maine would frequently be becalmed at the same time. They do grind up birds. They work although the wind is free to go around them.

bird kills by wind machines is dwarfed by those killed by buildings.