The Three Gorges Dam (Part 3)

Posted on Thu 08/14/2008 by


Even though this project has a history dating back 90 years, the addition of the ability to generate hydro electric power would have been something more recent than that, and the ability not to be able to do this was an opportunity too good to pass up, as China strives to industrialise and bring electricity to its vast populace, most of which have no access to electricity at all.
The addition of the hydro power capability makes this the largest hydro electric plant on Earth, and also the largest producer of electricity at a single site.

Just the facts alone regarding this hydro power, even though of a technical nature, make for interesting reading. Why they are interesting is in the comparison with traditional coal fired power production.

This project has a total nameplate capacity of 22,500 MW and when all generators come on line in 2011, it will pump 100 TerraWattHours (TWH) of power into the surrounding grid. This number probably sounds meaningless, but when taken in context, and compared to power production in the US, it amounts to just a little less than half the total hydro power produced in the US, and makes up just under 2.5% of the total power production from all sources in the whole of the US.
The generators have been gradually coming on line since 2003, and currently the numbers of generators on line are producing 85% of the expected overall total.
The interesting part of this is that power already produced, has been contributing towards paying off the overall cost of the whole project. Even though the project is not due to be completed until 2011, so far returns from the sale of the produced electricity have paid off one quarter of the total cost of the project, that total being $US30 Billion. So, when some people say that the project is an economic white elephant that was not economically viable from the getgo, there is every possibility that it will be more than half paid for before the overall project is even completed.

One of the Francis turbines for The Three Gorges hydro electric project. Image from Voith-Siemens is a Wikimedia Commons image. Click on image to open in a larger window.

This image is of a Francis Turbine, the most efficient form of water driven turbine to drive the generator attached to its shaft. Each of these turbines was constructed by a German Company, Voith-Siemens. I suppose the easiest way to explain it is that it is similar to a multi bladed propellor. Instead of an engine driving the propellor, the flowing water drives the this turbine. Only the section with the blades is in the water flow.
The pressure of the water behind the dam wall provides the driving force of the water. Each of these turbines is mounted inside a tunnel and the water flows down the tunnel, forced from behind by the immense pressure. The tunnel narrows as it approaches the turbine, increasing the speed of the water flow, which can sometimes be as high as 80MPH. The lower, widest part of the turbine is almost 34 feet in diameter and the turbine rotates at 75 RPM.
These turbines, being continuously in the flow of water, may be subject to some wear, but this is also something that is contentious. Environmentalists say that the vast scope of the project will tend to silt up the river behind the dam. If there was no dam, this topsoil would just flow down river and eventually be washed out to sea, or to silt up the delta at the mouth of the river where it flows into the East China Sea. With the dam in place, that silt now backs up behind the dam. This soil, if sucked into the water tunnels will impact upon the blades of the turbine, but as this happens with every water driven turbine, the effects have been selectively quoted here by environmentalists to support their own agenda.

The turbine drives the generator through a Constant Speed Device that keeps the generator revolving at the same speed all the time. With conventional power plants the turbine and generator are mounted on the horizontal plane, while with nearly all hydro turbine/generator combinations they are mounted vertically.

With conventional steam powered turbines, be they coal fired or nuclear plants, the turbines are more complex. With those coal fired plants, the turbines are in three stages, one for superheated steam, the second for the intermediate steam as it cools slightly, and the third for steam as it cools further, before the cooling steam reverts back to hot water, where it is recycled back to the coal fired furnace/boiler complex, and some also is passed into the atmosphere as those huge white clouds you see coming out of the large concave stacks at power plants. The similar applies for nuclear plants where the nuclear reaction is used to boil water to steam to drive the multi stage turbines in those complexes.

With hydro electric generation, just the flow of water drives the turbine shown in the above image, and that’s all there is. It is a far less complex system. The fact that the whole turbine/generator complex is mounted vertically also makes for less inherent problems. With conventional steam powered plants with their horizontally mounted units, they must be kept rotating, otherwise that immense weight of the generator could tend to bend the shaft, rendering the unit inoperable.

Mounted vertically, the only strain is on the main load bearing, and in the case of the units here at The Three Gorges, this bearing has a load factor of around 5200 tons.

The generators themselves weigh in the vicinity of 6000 tons. This is where the electricity is generated. The shaft from the turbine is attached to a large rotor which rotates inside of the stator.

In a much simplified form, the electricity is generated like this. Both rotor and stator have electrical windings, that rotor using a small amount of electrical power to generate an immense electromagnetic field in the vast number of windings. The Stator also has an immense number of windings as well. The rotating magnetic field generated by the rotor generates electricity in the stator, and this is the transmitted power.

Each stator has an inside and outer diameter of 60 and 70 feet respectively, and is 11 feet from top to bottom, making them the largest stators in the World.

Each unit alone generates 700MW of power, and this is enough to power a large city.

There are 32 of these generators.

As an example, and I understand that this is not really comparing ‘oranges with oranges’, as the technology has advanced considerably, the generators at the Hoover Dam are around a third to a half the size of these, and these ones at the Three Gorges Dam produce seven times the total power of each unit at the Hoover Dam.  At Hoover, greater generating capacity was added in the Sixties to that originally constructed in 1935, doubling the number of generators, and the power output. At Hoover now, there are 16 generators producing 1860MW in total. This same amount of power at The Three Gorges is produced from two and half of the 32 generators.

The power generated from the project, once fully operational in 2011 will be transmitted as far afield as Shanghai, one thousand miles downstream from this site, and will supply power to 9 Provinces. Prior to construction, it was perceived that this project alone would supply as much as 10% of China’s total power. However, the speed at which China is industrialsing means that when fully operational, that number has been reduced to just on 3%.

There are plans for a further four dam projects upstream and another one downstream. When they come on line over the next ten years they will add a further 38,000MW of power to China’s grid, and this amounts to double what this project alone contributes. what this effectively means is that China is constructing enough hydro electric projects equivalent to 30 large coal fired plants. True, they are also constructing those coal fired plants at a huge rate, but projects of this scope mean that a lot less of those coal fired plants will need to be constructed. The benefits also lie in the fact that disastrous floods will also be mitigated, and people in those areas will benefit in more ways than just being able to have access to a reliable and constant supply of electricity, something they have never had.

That may seem to be a lot of dams on this one river, the Yangtze, but consider this. Not counting this project and those five I mentioned, there are already 8 dams upstream, and a further 8 dams downstream on this river.

In the next post, I’ll run a comparison with these projects to coal fired equivalents.