All of us know that Power Generating Station is one where electrical power is generated. There are various types of Power Generating Station like Thermal Power Plant, Nuclear Power Plant, Solar Power Plant, Hydro Power Plant etc. All these power generating stations or plant use energy from different source to produce electrical power e.g. Thermal Power Plant use Coal, Nuclear Power Plant uses UO2 (Uranium Oxide) pallets encapsulated in Fuel Bundles and Solar Power Plant uses Solar energy. Hydro power plant are those kind of power generating stations which utilizes the potential /kinetic energy of hydro or water to generate electrical power. Most of us think that for Hydro Power Plant Dam is must, but it is not true, in Run-of-River type Hydro Power Plant a channel is made from a portion of a river to drive turbine which may not require the use of a dam. There are around 80,000 dams in USA out of which only 24,00 dams are used for generation of power and rest are used for other purposes like irrigation, flood control etc.
Hydro power plant is the most widely used form of renewable energy. Once a hydro power plant is constructed, the plant produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO2) than fossil fuel powered energy plants. As per CEA report, the total installed capacity of hydro power plant in India is 42,783.42 MW as on 31st March 2016 with 194 stations and 676 units.
Some Interesting Facts about Hydro Power Plant
- Hydro Power Plant is normally used as a peak load station, because it can be readily stopped and started.
- Hydro Power Plant do not need auxiliary power for its start up. This is a great advantage of Hydro Power Plant. Normally power plants like thermal or nuclear need auxiliary power supply for their start-up which is taken from Grid. In the event of Grid failure (assuming all plants tripped), only Hydro Power Plant can feed auxiliary power supply to other thermal and nuclear plants to meet their auxiliary supplies for their start-up.
- Efficiency of Hydro Power Plant is around 95% whereas for thermal and nuclear plant it’s only around 36%.
Types of Hydro Power Plant
On the basis of working principle and construction, Hydro Power Plant can be categorized into three type viz. Impoundment, Diversion and Pumped Storage.
Impoundment Type Hydro Power Plant
The most common type of hydroelectric power plant is an impoundment type. An impoundment facility, typically a large hydro power system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through penstock and finally led to turbine which in turn rotates a generator to produce electricity. A typical Impoundment facility is shown in figure below.
An Impoundment type hydro power plant uses the potential energy of the water retained in the dam to drive a water turbine which in turn drives an electric generator. The available energy of water thus depends on the head of the water above the turbine and the volume of water flowing through it.
Turbines used in such plant are usually reaction types whose blades are fully submerged in the water flow. Figure below shows the turbine used in Impoundment type hydro plant.
Output Power Calculation
As we know that,
Potential energy = mass x acceleration due to gravity x height
Suppose the height of dam is h meter and water is flowing at a rate of Q m3/sec and the efficiency of turbine generator set is η.
Thus, rate of change of potential energy water = (Volume x Density) x 9.8 x h
As, density of Water = 103 Kg / m3 and rate of change of energy is called power. Thus
Electrical Power Output = η x 103 x Q x 9.8 x h Watt
Thus we observe that for Impoundment type hydro power plant, electrical power output is directly proportional to the height of the dam. But the height of dam cannot be made so high because of limitation imposed by the height of river bed. Therefore it is usual practice to install turbine generator set well below the ground level to get more water head i.e. h.
Again, suppose the flow of water Q = 1 m3/sec and height of dam = 1 m
Efficiency of turbine Generator set = 100 %
Then, electrical power output = 9.8 kW ………………..(1)
Diversion Type Hydro Power Plant
In diversion or sometimes called run-of-river type hydro power plant, a channel is made out of portion of a river through a canal or penstock to drive turbine. Water from a fast flowing river or stream is diverted to a turbine which drives the generator to produce electricity as shown in figure below.
In such plant, the head of water is zero. Mind here that, head is the height of standing water. As there is no dam in such plants hence head is zero. Hence it is the kinetic energy of water which is converted to mechanical energy by turbine and ultimately this mechanical energy is converted into electrical energy by Generator. The power output for such plant therefore depends on the quantity of water flowing through the turbine and the square of its velocity.
Impulse turbines which are only partially submerged are more commonly employed in fast flowing run of river installations while in deeper slower flowing rivers, submerged Kaplan turbines may be used to extract the energy from the water flow.
Run of River type plants are typically used for smaller generating stations having capacity less than 10 MW.
Output Power Calculation
Let us assume that the cross sectional area of the channel is A m2 and water is flowing with a speed of v m/sec and at a flow rate of Q m3/sec.
Volume of Water flowing per second Q = Cross Sectional Area x Velocity
Mass of water flowing per second (m) = Volume x density
= (Av) x 103
Let the Kinetic Energy of Water impinging the turbine blades per second be K.E.
K.E = (mv2) / 2
= (103 Av3) / 2
This K.E is the power of flowing water impinging on the turbine blades. Thus the output power from the generator, assuming turbine generator set to have an efficiency of η will be,
Electrical Power Output = (103 ηAv3) / 2
Suppose, the flow of water Q = 1 m3/sec, velocity of water = 1 m, Cross Sectional Area of Channel = 1 m2 and efficiency of turbine Generator set = 100 %
Then, electrical power output = 0.5 kW ……………………..(2)
Carefully observe equation (1) and equation (2), you will notice that the for the same flow of water and efficiency of turbine generator set, the power output for hydro power plant using dam is around 20th times that of the power output for diversion or Run-of-River type hydro power plant.
Pumped Storage Type Hydro Power Plant
Pumped storage type of hydro power plant works like a battery, storing the electricity generated by other power sources like solar, wind, and nuclear for later use. It stores energy by pumping water uphill to a reservoir at higher elevation from a second reservoir at a lower elevation. When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir and turns a turbine, generating electricity.
Classification of Hydro Power Plant on the basis of Size
Hydro Power plant can again be classified into three types i.e. Large, Small and Micro on the basis of their installed capacity. Hydro Power plant having capacity more than 30 MW is kept under Large Plants. Similarly if the capacity of plant is less than 10 MW then it is kept under Small whereas if the capacity is below 100 kW then it is called Micro Plant. Micro hydro power plant can produce enough energy to power up homes, village, farms or ranch.