Hydroelectricity is based on the power of gravity. Water in rivers and streams flows downwards towards the sea; as the water passes through a hydropower system, the energy in the water drives a turbine which turns a generator and energy is produced.
The power of the system will depend on the strength of the water passing through, as well as the efficiency of the system.
There are three standard types of hydropower systems. The first is a standard ‘run of river’ system, which uses the existing flow of the river. Water is typically redirected to pass through the turbine and the water is passed back into the river or stream.
While this system is the most straightforward, it also has the disadvantage of being entirely dependent on the river strength. If your river goes dry due to drought, your system will not run.
However, due to its simplicity it is also the most typical to use for domestic or community systems.
The second type is a storage system, or dam, which is the most common form. Dams are used for large-scale hydroelectricity projects around the world, but can also be used for smaller systems.
A reservoir stores the water from the river and lets it through gradually. This offers a greater degree of control because the system can still work if the river runs dry.
The final type is a pumped system, which uses cheaper, off-peak energy to pump stored water back up to a higher point to generate energy at peak times.
Is domestic hydropower practical?
Hydropower’s practicality depends entirely on your access to running water. But, even if you have a river or stream nearby, it doesn’t mean you can automatically consider hydropower.
If you think you may be eligible, you should contact a certified hydropower installer who can take a look at your site. Whether it is suitable or not will depend not only on your location and access, but also how steeply the river flows and how much water passes through.
You should also consider the seasons. Your river’s lowest level will determine how feasible your site is more than your river’s highest level. This in turn will vary year by year depending on rainfall levels.
The Energy Saving Trust recommends hydropower as an excellent community development project. This will lower installation costs and give you greater flexibility around installation.
How much does it cost?
The cost of a hydroelectricity system depends almost entirely on its size and where you put it.
The costs are likely to be significant either way. A typical 5kW system to power one home will cost around £25,000, but it could be more or less. The good news is that once the system is installed it requires very little upkeep.
How much energy the system will generate, and hence what your savings will be, is even harder to estimate as it will depend not only on the system, but also how long it is able to operate at full efficiency, which in turn depends on the water levels in your area.
What are the advantages and disadvantages of hydroelectricity?
The pros include:
- A clean, renewable resource perfect for a wet climate like the UK
- Almost maintenance free; a system life expectancy of up to 50 years
The cons include:
- Installation costs are high
- Suitability depends entirely on location and other factors
- Energy generated can be easy to predict, but will be highly seasonal
So just how do we get electricity from water? Actually, hydroelectric and coal-fired power plants produce electricity in a similar way. In both cases a power source is used to turn a propeller-like piece called a turbine, which then turns a metal shaft in an electric generator, which is the motor that produces electricity. A coal-fired power plant uses steam to turn the turbine blades; whereas a hydroelectric plant uses falling water to turn the turbine. The results are the same.
Take a look at this diagram (courtesy of the Tennessee Valley Authority) of a hydroelectric power plant to see the details:
The theory is to build a dam on a large river that has a large drop in elevation (there are not many hydroelectric plants in Kansas or Florida). The dam stores lots of water behind it in the reservoir. Near the bottom of the dam wall there is the water intake. Gravity causes it to fall through the penstock inside the dam. At the end of the penstock there is a turbine propellor, which is turned by the moving water. The shaft from the turbine goes up into the generator, which produces the power. Power lines are connected to the generator that carry electricity to your home and mine. The water continues past the propellor through the tailrace into the river past the dam. By the way, it is not a good idea to be playing in the water right below a dam when water is released!
A turbine and generator produce the electricity
Diagram of a hydroelectric turbine and generator. (Credit: U.S. Army Corps of Engineers)
As to how this generator works, the Corps of Engineers explains it this way:
“A hydraulic turbine converts the energy of flowing water into mechanical energy. A hydroelectric generator converts this mechanical energy into electricity. The operation of a generator is based on the principles discovered by Faraday. He found that when a magnet is moved past a conductor, it causes electricity to flow. In a large generator, electromagnets are made by circulating direct current through loops of wire wound around stacks of magnetic steel laminations. These are called field poles, and are mounted on the perimeter of the rotor. The rotor is attached to the turbine shaft, and rotates at a fixed speed. When the rotor turns, it causes the field poles (the electromagnets) to move past the conductors mounted in the stator. This, in turn, causes electricity to flow and a voltage to develop at the generator output terminals.”
Pumped storage: Reusing water for peak electricity demand
Demand for electricity is not “flat” and constant. Demand goes up and down during the day, and overnight there is less need for electricity in homes, businesses, and other facilities. For example, here in Atlanta, Georgia at 5:00 PM on a hot August weekend day, you can bet there is a huge demand for electricity to run millions of air conditioners! But, 12 hours later at 5:00 AM …. not so much. Hydroelectric plants are more efficient at providing for peak power demands during short periods than are fossil-fuel and nuclear power plants, and one way of doing that is by using “pumped storage”, which reuses the same water more than once.
Pumped storage is a method of keeping water in reserve for peak period power demands by pumping water that has already flowed through the turbines back up a storage pool above the power plant at a time when customer demand for energy is low, such as during the middle of the night. The water is then allowed to flow back through the turbine-generators at times when demand is high and a heavy load is placed on the system.
Pumped storage: Reusing water for peak electricity demand
The reservoir acts much like a battery, storing power in the form of water when demands are low and producing maximum power during daily and seasonal peak periods. An advantage of pumped storage is that hydroelectric generating units are able to start up quickly and make rapid adjustments in output. They operate efficiently when used for one hour or several hours. Because pumped storage reservoirs are relatively small, construction costs are generally low compared with conventional hydropower facilities.