Power Generation/Hydro Power

Review: Lesson 2

The previous Lesson discussed the steam power station. Here are some points you need to remember from lesson 2.

Schematic Arrangement of steam power station.
Types of cooling sytems for a steam power station.
Location & Efficiency of steam power station.

Preview: Lesson 3

This Lesson is about Hydro Power station. The student/User is expected to understand the following at the end of the lesson.

Basics:
Arrangements:
Location:
Pumped storage scheme:

Definition, advantages & disadvantages.
components that make up a steam power station.
Factors influencing selection of construction site.
Overview & comparison to Hydro-power station.

Figure 1:Krasnoyarsk hydroelectric station in Russia ( Click on image to view full size image )

Introduction: Hydro Power station

A hydro power station uses potential energy of water at high level for generating electrical energy.

This power station is generally located in hilly areas where dams can be built conveniently and large water reservoirs can be obtained. This kind of power station can be used to produce large amounts of electrical energy. In most countries these power stations are used as peak load power stations. This is because they can be started and stopped easily and fast.

Operation

The water from the dam is lead to the water turbine through the penstock. Here the hydraulic energy of water is converted to rotational mechanical energy by the turbine. The turbine is connected to the generator through the turbine shaft and hence mechanical energy is converted into electrical energy by the generator.

Pros & Cons: what this power station presents


Advantages	Disadvantages
Requires no fuel, thus called clean power station	Very high capital cost for dam construction
Small running charges and no need for specialised manpower	Uncertainty about availability of huge amounts of water
Simple construction & requires less maintenance	Skilled personel required for construction
Very robust & has long life	High cost of transmission line as plant is located in hilly areas.
Also used for flood control and irrigation	Impacts native watershed ecology

Future generations will want to depend more on this type of electricity generating power station (and other renewable energy sources), due to a fast increasing depletion of fuels(Coal). There are a number of construction projects currently underway for this kind of power station around the world.

Figure 2:Hydroelectric dam cross-section diagram ( Click on image to view full size image )

Hydo-electric power stations boast a simple design and construction method that is very robust and reliable(when done properly). The following sub topic discusses the most important constituents of this kind of power station (as shown in figures 2 & 3).

Constituents of Hydro-electric power station

Reservoir: This is where water is stored for use as and when needed. The type and arrangement depends on topography of the site.	Penstock: This is a conduit (conduits) that carry water to the turbines. They are made of reinforced concrete or steel. A surge tank is installed next to each penstock for over flow control and protection of penstock from bursting.
Water turbine: Water turbines are used to convert hydraulic energy of flowing water into rotational mechanical energy. Figure 3 is an example of the make of a typical water turbine.	Generator: This machine is used to convert rotational mechanical energy transferred from the turbine through the shaft, into electrical energy. the produced electrical energy is transmitted to the transformer for long distance transmission.

Location of Hydro-electric power station: influencing factors

Availablility of water: Adequate water must be available at good head.	Cost and Type of Land: Land should be available at reasonable price. The bearing capacity of the land should be enough to withstand huge structures & equipment.
Storage of water: A dam must be constructed to store water in order to deal with variations of water availability during the year.	Transportaion facilities: The site should be accessible by rail and(or) road for ease in transporting equipment & machinery.

This schematic diagram must be properly understood. It is the basis upon which Hydro-electric power station designs are done. The individual power station complexity may differ slightly to the schematic and yet over and above that will use the same principle.

Figure 3:Hydraulic turbine and Electrical generator diagram ( Click on image to view full size image )

Figure 4:Pumped storage scheme power station diagram ( Click on image to view full size image )

Pumped storage schemes are a convenient way of storing large quantities of energy which can be used during emergency or peaking times.

Operation:

During off-Peak hours, the plant draws electric energy from the electrical grid & uses that to pump water to the upper reservoir.

When Peak time comes, the water from the upper reservoir is released & electric energy is generated in the lower reservoir. This cycle is repeated daily.

By their nature, pumped storage schemes cannot be used as base load power stations. These are strictly used for peak time supply as they can be brought on-line quickly.

Comparison:

Hydro power plant	Pumped storage plant
Once water passes through penstock & turbine, it is released into the river	Water is re-used by pumping and generative action of the scheme.
Can be used for irrigation & flood control	Can be used for pumping water from readly available areas to areas in need of water
Can still be used as base load station	can not be used as base load station as it can only generate for limited hours

As evident from above table, both power stations are very desirable for use that goes outside of electrical energy generation confines. This schematic diagram must be properly understood. it is the basis upon which pumped-storage scheme power station designs are done. the individual power station complexity may differ slightly to the schematic and yet over and above that will use the same principle.

Below are equations used for calculations involving this kind of power station:

Weight of the water head:

 $W=(Volume\times density)=V{\rho }$  (in Kg) ... Equation 3.1

The potential energy of the water head:

 $E_{potential}=mgh$ (in Joules) ... Equation 3.2

Thus the equivelent electrical energy will be:

 $E_{Electrical}={\frac {E_{potential}}{36\times 10_{.}^{5}J}}$ (in kWh) ... Equation 3.3

Electrical enegry can also be expressed as:

 $E_{Electrical}={\frac {WH{\eta }_{overall}}{(3600.sec\times 1000)}}$ (in kWh) ... Equation 3.4

Where:

m = mass (in Kg)
g = constant ≈ $9.8m.s^{-2}$
h/H = water head height (metres)
ρ = density of water ( $m^{3}$ )

Below are furhter equations used for calculations involving this kind of power station:

Gross plant capacity (G.C):

 $G.C={\frac {E_{potential}}{sec}}$  (in Watts) ... Equation 3.5

The firm capacity (F.C) will therefore be:

 $F.C=G.C\times {\eta }$ (in Watts) ... Equation 3.6

Thus the yearly gross out put:

 $Y.G.O=F.C\times 8760Hrs$ (in kWh) ... Equation 3.7

We can also estimate the volume of flow per annum:

$V_{annum}=Catchment_{.}Area\times Annual_{.}Rainfall\times Yield_{.}Factor$

(in kWh) ... Equation 3.8

Where:

η = Efficiency
m = mass (in Kg)
g = constant ≈ $9.8m.s^{-2}$
h/H = water head height (metres)
ρ = density of water ( $m^{3}$ )

it is imp part of the power plant

References

This resource is prepared from Lecture notes by Thuvack.
V.K Mehta & Rohit Mehta :- Principles of Power systems (1st ed.). S.CHAND .ISBN 81-219-2496-0