Hydro Electric Plants - Classification, Advantages and Disadvantages

Classification of Hydro Electric Plants
The classification of hydro electric plants based upon:
(a) Quantity of water available
(b) Available head
(c) Nature of load


The classification according to Quantity of water available is

(i) Run-off river plants without pondage:

These plants do not store water; the plant uses water as it comes. The plant can use water as and when available. Since these plants depend for their generating capacity primarily on the rate of flow of water, during rainy season high flow rate may mean some quantity of water to go as waste while during low run-off periods, due to low flow rates, the generating capacity will be low.

(ii) Run-off river plants with pondage:

In these plants pondage permits storage of water during off peak periods and use of this water during peak periods. Depending on the size of pondage provided it may be possible to cope with hour to hour fluctuations. This type of plant can be used on parts of the load curve as required, and is more useful than a plant without storage or pondage.

When providing pondage tail race conditions should be such that floods do not raise tail-race water level, thus reducing the head on the plant and impairing its effectiveness. This type of plant is comparatively more reliable and its generating capacity is less dependent on available rate of flow of water.

(iii) Reservoir Plants:

A reservoir plant is that which has a reservoir of such size as to permit carrying over storage from wet season to the next dry season. Water is stored behind the dam and is available to the plant with control as required. Such a plant has better capacity and can be used efficiently throughout the year. Its firm capacity can be increased and can be used either as a base load plant or as a peak load plant as required. It can also be used on any portion of the load curve as required. Majority of the hydroelectric plants are of this type.


The classification according to availability of water head is

(i) Low-Head (less than 30 meters) Hydro electric plants:

"Low head" hydro-electric plants are power plants which generally utilize heads of only a few meters or less. Power plants of this type may utilize a low dam or weir to channel water, or no dam and simply use the "run of the river". Run of the river generating stations cannot store water, thus their electric output varies with seasonal flows of water in a river. A large volume of water must pass through a low head hydro plant's turbines in order to produce a useful amount of power. Hydro-electric facilities with a capacity of less than about 25 MW (1 MW = 1,000,000 Watts) are generally referred to as "small hydro", although hydro-electric technology is basically the same regardless of generating capacity. 

(ii) Medium-head (30 meters - 300 meters) hydro electric plants:

These plants consist of a large dam in a mountainous area which creates a huge reservoir. The Grand Coulee Dam on the Columbia River in Washington (108 meters high, 1270 meters wide, 9450 MW) and the Hoover Dam on the Colorado River in Arizona/Nevada (220 meters high, 380 meters wide, 2000 MW) are good examples. These dams are true engineering marvels. In fact, the American Society of Civil Engineers as designated Hoover Dam as one of the seven civil engineering wonders of the modern world, but the massive lakes created by these dams are a graphic example of our ability to manipulate the environment - for better or worse. Dams are also used for flood control, irrigation, recreation, and often are the main source of potable water for many communities. Hydroelectric development is also possible in areas such as Niagra Falls where natural elevation changes can be used.


(iii) High-head hydro electric plants:

"High head" power plants are the most common and generally utilize a dam to store water at an increased elevation. The use of a dam to impound water also provides the capability of storing water during rainy periods and releasing it during dry periods. This results in the consistent and reliable production of electricity, able to meet demand. Heads for this type of power plant may be greater than 1000 m. Most large hydro-electric facilities are of the high head variety. High head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet the electrical demand on a distribution system.






The classification according to nature of load is

(i) Base load plants:

A base load power plant is one that provides a steady flow of power regardless of total power demand by the grid. These plants run at all times through the year except in the case of repairs or scheduled maintenance.

Power plants are designated base load based on their low cost generation, efficiency and safety at set outputs. Base load power plants do not change production to match power consumption demands since it is always cheaper to run them rather than running high cost combined cycle plants or combustion turbines. Typically these plants are large enough to provide a majority of the power used by a grid, making them slow to fire up and cool down. Thus, they are more effective when used continuously to cover the power base load required by the grid.

Each base load power plant on a grid is allotted a specific amount of the base load power demand to handle. The base load power is determined by the load duration curve of the system. For a typical power system, rule of thumb states that the base load power is usually 35-40% of the maximum load during the year. Load factor of such plants is high.

Fluctuations, peaks or spikes in customer power demand are handled by smaller and more responsive types of power plants.

(ii) Peak load plants:
Power plants for electricity generation which, due to their operational and economic properties, are used to cover the peak load. Gas turbines and storage and pumped storage power plants are used as peak load power plants. The efficiency of such plants is around 60 -70%.


Advantages of hydroelectric plants
· Operation, running and maintenance costs are low.
· Once the dam is built, the energy is virtually free.
· No fuel is burnt and the plant is quite neat & clean.
· No waste or pollution produced.
· Generating plants have a long lifetime.
· Much more reliable than wind, solar or wave power.
· Water can be stored above the dam ready to cope with peaks in demand.
· Unscheduled breakdowns are relatively infrequent and short in duration since the equipment is relatively simple.
· Hydroelectric turbine-generators can be started and put "on-line" very rapidly.
· Electricity can be generated constantly


Disadvantages of hydroelectric plants
· Very land-use oriented and may flood large regions.
· The dams are very expensive to build. However, many dams are also used for flood control or irrigation, so building costs can be shared.
· Capital cost of generators, civil engineering works and cost of transmission lines is very high.
· Water quality and quantity downstream can be affected, which can have an impact on plant life.
· Finding a suitable site can be difficult - the impact on residents and the environment may be unacceptable.
· Fish migration is restricted.
· fish health affected by water temperature change and insertion of excess nitrogen into water at spillways
· available water and its temperature may be affected
· reservoirs alter silt-flow patterns


How Hydropower Plant works

A hydroelectric power plant harnesses the energy found in moving or still water and converts it into electricity.

Moving water, such as a river or a waterfall, has mechanical energy. ‘Mechanical energy is the energy that is possessed by an object due to its motion or stored energy of position.’ This means that an object has mechanical energy if it’s in motion or has the potential to do work (the movement of matter from one location to another,) based on its position. The energy of motion is called kinetic energy and the stored energy of position is called potential energy. Water has both the ability and the potential to do work. Therefore, water contains mechanical energy (the ability to do work), kinetic energy (in moving water, the energy based on movement), and potential energy (the potential to do work.)

The potential and kinetic/mechanical energy in water is harnessed by creating a system to efficiently process the water and create electricity from it. A hydroelectric power plant has eleven main components. The first component is a dam.

The dam is usually built on a large river that has a drop in elevation, so as to use the forces of gravity to aid in the process of creating electricity. A dam is built to trap water, usually in a valley where there is an existing lake. An artificial storage reservoir is formed by constructing a dam across a river. Notice that the dam is much thicker at the bottom than at the top, because the pressure of the water increases with depth. 

The area behind the dam where water is stored is called the reservoir. The water there is called gravitational potential energy. The water is in a stored position above the rest of the dam facility so as to allow gravity to carry the water down to the turbines. Because this higher altitude is different than where the water would naturally be, the water is considered to be at an altered equilibrium. These results in gravitational potential energy, or, “the stored energy of position possessed by an object.” The water has the potential to do work because of the position it is in (above the turbines, in this case.)

Gravity will force the water to fall to a lower position through the intake and the control gate. They are built on the inside of the dam. When the gate is opened, the water from the reservoir goes through the intake and becomes translational kinetic energy as it falls through the next main part of the system: the penstock. Translational kinetic energy is the energy due to motion from one location to another. The water is falling (moving) from the reservoir towards the turbines through the penstock.

The intake shown in figure includes the head works which are the structures at the intake of conduits, tunnels or flumes. These structures include blooms, screens or trash - racks, sluices to divert and prevent entry of debris and ice in to the turbines. Booms prevent the ice and floating logs from going in to the intake by diverting them to a bypass chute. Screens or trash-racks(shown in fig) are fitted directly at the intake to prevent the debris from going in to the take. Debris cleaning devices should also be fitted on the trash-racks. Intake structures can be classified in to high pressure intakes used in case of large storage reservoirs and low pressure intakes used in case of small ponds. The use of providing these structures at the intake is, water only enters and flows through the penstock which strikes the turbine.

Control gates arrangement is provided with Spillways. Spillway is constructed to act as a safety valve. It discharges the overflow water to the downstream side when the reservoir is full. These are generally constructed of concrete and provided with water discharge opening, shut off by metal control gates. By changing the degree to which the gates are opened, the discharge of the head water to the tail race can be regulated in order to maintain water level in reservoir.

The penstock is a long shaft that carries the water towards the turbines where the kinetic energy becomes mechanical energy. The force of the water is used to turn the turbines that turn the generator shaft. The turning of this shaft is known as rotational kinetic energy because the energy of the moving water is used to rotate the generator shaft. The work that is done by the water to turn the turbines is mechanical energy. This energy powers the generators, which are very important parts of the hydroelectric power plant; they convert the energy of water into electricity. Most plants contain several generators to maximize electricity production.

The generators are comprised of four basic components: the shaft, the excitor, the rotor, and the stator. The turning of the turbines powers the excitor to send an electrical current to the rotor. The rotor is a series of large electromagnets that spins inside a tightly wound coil of copper wire, called the stator. “A voltage is induced in the moving conductors by an effect called electromagnetic induction.” The electromagnetic induction caused by the spinning electromagnets inside the wires causes electrons to move, creating electricity. The kinetic/mechanical energy in the spinning turbines turns into electrical energy as the generators function.

The transformer, another component, takes the alternating current and converts it into higher-voltage current. The electrical current generated in the generators is sent to a wire coil in the transformer. This is electrical energy. Another coil is located very close to first one and the fluctuating magnetic field in the first coil will cut through the air to the second coil without the current. The amount of turns in the second coil is proportional to the amount of voltage that is created. If there are twice as many turns on the second coil as there are on the first one, the voltage produced will be twice as much as that on the first coil. This transference of electrical current is electrical energy. It goes from the generators to one coil, and then is transferred through an electromagnetic field onto the second coil. That current is then sent by means of power lines to the public as electricity.

Now, the water that turned the turbines flows through the pipelines (translational kinetic energy, because the energy in the water is being moved,) called tailraces and enters the river through the outflow. The water is back to being kinetic/mechanical/potential energy as it is in the river and has to potential to have the energy harnessed for use as it flows along (movement.)

Electrical terms associated with hydropower engineering:
Electrical power generated or consumed by any source is usually measured in units of Kilowatt-hour (kWh). The power generated by hydropower plants are normally connected to the national power grid from which the various withdrawals are made at different places, for different purposes. The national power grid also obtains power generated by the non-hydropower generating units like thermal, nuclear, etc. The power consumed at various points from the grid is usually termed as electrical load expressed in Kilo-Watt (KW) or Mega-Watt (MW).

The load of a city varies throughout the day and at certain time reaches the highest value (usually in the evening for most Indian cities), called the Peak load or Peak demand. The load for a day at a point of the national power grid may be plotted with time to represent what is known as Daily Load Curve. Some other terms associated with hydropower engineering are as follows:

Load factor
This is the ratio of average load over a certain time period and the maximum load during that time. The period of time could be a day, a week, a month or a year. For example, the daily load factor is the ratio of the average load may be obtained by calculating the total energy consumed during 24 hours (finding the area below the load vs. time graph) and then divided by 24. Load factor is usually expressed as a percentage.

Installed capacity
For a hydro electric plant, this is the total capacity of all the generating units installed in the power station. However all the units may not run together for all the time.

Capacity factor
This is the ratio of the average output of the hydroelectric plant for a given period of time to the plant installed capacity. The average output of a plant may be obtained for any time period, like a day, a week, a month or a year. The daily average output may be obtained by calculating the total energy produced during 24hours divided by 24. For a hydroelectric plant, the capacity factor normally varies between 0.25 and 0.75.

Utilization factor
Throughout the day or any given time period, a hydroelectric plant power production goes on varying, depending upon the demand in the power grid and the power necessary to be produced to balance it. The maximum production during the time divided by the installed capacity gives the utilization factor for the plant during that time. The value of utilization factor usually varies from 0.4 to 0.9 for a hydroelectric plant depending upon the plant installed capacity, load factor and storage.

Firm (primary) power
This is the amount of power that is the minimum produced by a hydro-power plant during a certain period of time. It depends upon whether storage is available or not for the plant since a plant without storage like run-of-river plants would produce power as per the minimum stream flow. For a plant with storage, the minimum power produced is likely to be more since some of the stored water would also be used for power generation when there is low flow in the river.

Secondary Power
This is the power produced by a hydropower plant over and above the firm power.