There are two main types of moving water - these are either rivers or tides. In both cases the moving water is used to turn turbines. The energy in the flowing water is ultimately derived from the sun, and is therefore constantly being renewed. Tidal power is very limited in use, since there are only a very few locations in the world where it is economically viable, therefor most hydropower comes in the form of a dam of some style which likely spans across a river.
The damming of a river results in a reservoir of water accumulating behind the dam. This reservoir builds up to a certain water depth, the depth of the water from the top of the water surface to the height of the generator is called the head. The higher the head, the greater the potential energy of the water trapped behind the dam (from the equation PE = mgh). As the water travels through a passage in the dam, (called the penstock) it flows past some sort of wheel that turns a shaft that will in turn, spin the turbine generating electricity.
As with most dams, the height of the head can vary with season, there is some variability in the amount of electricity generated. However, you can also control how much water is flowing across the wheels turning the turbines, so you can adjust when the maximum power output is over the course of the day. This is important as the electricity demand fluctuates quite frequently.
There are a number of designs used that will use the water to actual turn the turbine. Listed below are the main ones used.
1. The simplest and design that has been in use for thousands of years, is the undershot or breast wheel. In this case the water pushes on flanges and runs along the bottom of the wheel. This is a relatively low efficiency design, and is used only in small and low head applications in modern times.
2. The overshot wheel is similar to the undershot, except the water comes across the top of the wheel. This design has also been around for as long as we have used water power, but it is a more efficient design.
3. Reaction wheel, in this case the water turns a propeller - this is the most efficient of designs, and it used in most modern medium to high head applications. This uses Newton's third Law to turn the impeller.
4. Often modern turbines also use an impulse wheel, which is similar to an overshot or undershot wheel, except the wheel is turned on its side, and the water is directed at high speeds at the blades or flanges.
Compared to other renewable sources, hydropower is much more reliable, you can tell from exactly how big the head on the reservoir is, how much electricity can be generated. You can also control the generation process by opening or closing more water flow to turbines. Big hydro station with big reservoirs can generate significant amounts of power (big projects like James Bay in Canada or Three Gorges Dam in China can produce amounts of electricity on the scale of multiple nuclear generators). Hydro plants although the initial investment can be high, are cheap to run. Thus they are able to offer quite a promising potential to private business and goverments alike.
Head is the height of the water above the level of the generator. The higher the head the greater the gravitational potential energy. The head is usually referred to as low (few meters high), medium, or high. The dam is what forces the water to collect into a reservoir. The taller the dam the higher the reservoir can build up, and the greater the head. The head behind the dam changes with the time of year depending on how much water has built up. In times of drought the head can be lowered considerably. In times of heavy rains, the head may be so high that flood gates have to be opened to allow for excess water to escape.
Depending on the head height there are a few different ways to set up the turbines.
This can apply to commercial sized turbines but it more likely realtes to the people
interested in producing their own power through micro hydro.
In a turbine that uses a wheel type design you can either have an undershot wheel,
where the water runs under the wheel to turn it. In an overshot
wheel the water pours over the wheel to turn it. An overshot wheel
will actually provide more energy (it has a higher efficiency), but engineering
considerations play a large role in determining the type of wheel you should use.
Additionally the height of the head is a large determining factor that plays a role.
Basically there is the:
Basic undershot wheel - 20- 40% efficiency
Breast undershot Wheel - 65% efficiency
Overshot wheel - 85% efficiency
In the turbine there are two basic designs - these have to do with how the water
turns the turbine.
Impulse turbine - the force of the water actually pushes the wheel around.
Reaction turbines - use Newton's third law to turn the turbine.
(3rd law - for every action there is an equal and opposite reaction) this is
the same principal along which rockets work. Most modern medium to high head
dams use this type of turbine to generate electricity.
- The building of a dam requires that a reservoir be built up. This is one the major environmental
problems with a dam as it floods up to several hundred kilometers backwards, destroying ecosystems.
- To build a hydro electric facility takes a significant amount of capital and this
is a difficult task to overcome for some countries as well as people interested in microhydro
where a dam is necessary.
- Hydropower is limited to use where there are rivers with an appropriate geology
to support the dam. Also the river has to have sufficient flow to make the hydropower
economical.
- Another limitation is locating where a dam can be built. You need
flowing water of a great enough scope to build up a reasonable reservoir. You
also need a solid geology under the river bed, in order to support the dam and
the forces of water on the dam.
- When you build a dam, you are changing the natural water flow. Water normally
carries small particles and dissolved material with it. This will accumulate
behind the dam in the resevoir, this can slowly fill the reservoir in, destroying
your power generation capability. Any kind of pollution in the river also builds
up in the reservoir. In creating the reservoir you are also flooding land. This
can lead to materials that are toxic being dissolved in the water. As well any
vegetation covered by the water will break down and this can lead to the release
of large amounts of gases like methane.
- Dams since they change the pattern of the water flow, also change the nature of
aquatic life. In particular they can block the migration of species like salmon
up river to their spawning grounds. This can be avoided to some extent through
the use of fish ladders and elevators, but the presence of a dam, will change the
environment and what lives there
Hydro and Micro Hydro have a very prosperous future as this world currently relies heavily on the electricity that flowing water generates. If you really look, water is used a lot, for the coolant in nuclear reactors to the tansfer medium of steam in most generating facilities. We use water quite a bit and therefor is a very powerful commodity to control.