Hydroelectric Power Plants: Principles of Operation

Hydroelectric power plants convert the potential energy of stored water or kinetic energy of running water into electric power. Hydroelectric power plants are renewable sources of energy as the water available is self-replenishing and there are no carbon emissions in the process. In this article, we’ll discuss the details and basic operations of a hydroelectric power plant. How different parameters are calculated, different components of a hydroelectric power plant.

The Hydrologic Cycle: 

The universe follows the ‘conservation of energy’ principle. Therefore, if we extract electrical energy from Hydroelectric power plants then it means that some source is there to feed the energy into the cycle to keep it running. That source of energy is Sun. 

In the Hydrologic cycle (also called water cycle), evaporation takes place above the large water bodies such as oceans. The vapors rise and form clouds which are driven through winds. Once the clouds reach areas of higher altitudes and lower temperatures, waterfalls in the form of rain. The water is collected in large water reservoirs having higher altitude than sea level. This water head can be used to generate electrical energy out of it. Nonetheless, the water completes the cycle after eventually becoming part of the ocean. The cycle repeats.

You may check a comprehensive article on the basic components and operation of a Thermal power plant.

Calculation of Power Output in a Hydroelectric Power Plants

Water at height contains potential energy given by: 

𝑃𝐸 = 𝑚𝑔𝐻 = 𝜌Δ𝑉𝑔𝐻 

𝑤ℎ𝑒𝑟𝑒 𝑃𝐸 𝑖𝑠 𝑡ℎ𝑒 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟.  With no losses in the watercourse, 

With losses such as Intake structure losses, friction losses in conduit, elbow losses, and valve losses. 

𝑃_𝑑= 9.81𝜂𝑄𝐻 𝑘𝑊 

𝑤ℎ𝑒𝑟𝑒 𝜂 𝑟𝑒𝑝𝑟𝑒𝑠𝑒𝑛𝑡𝑠 𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑖𝑒𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑐𝑜𝑢𝑟𝑠𝑒, 𝑡𝑢𝑟𝑏𝑖𝑛𝑒 𝑎𝑛𝑑 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑜𝑟 

Selection of Site for Hydroelectric Power Plants

Selecting site for hydroelectric power plant these factors should have to consider: excess water availability, high gross head, water storage reservoir, cost of transmission lines, Accessibility of the site to transport equipment.

Measurement of Water Flow Rate

For the measurement of volume flow rate some there are the following methods:

1-Bucket Method: Water of stream is diverted to fill a bucket of known volume. The time of filling is measured. Flow rate is calculated by dividing the volume by filling time. This method is applicable for small streams.  

2-Gauging Weir: A weir is designed at right angles to the direction of stream flow.  

𝑄 = 1.8(𝑊 − 0.2ℎ)ℎ³

3-Velocity-area Method: The velocity of water is measured, and the approximate area of the conduit is also measured. Using the formula given below, the flow rate is computed. 

You may check a comprehensive article on the basic components and operation of gas turbine power plants.

Head Measurement Methods for Hydroelectric Power Plants

For head measurement following methods are used:

1. Water Pressure Method: To estimate the head, a water filled hose of appropriate length, with no air trapped inside it, is stretched between the intake and proposed turbine site. The lower end of the pipe is fitted with an accurate pressure gauge. To compute the gross head in feet, the pressure reading in psi is multiplied by 2.31 ft/psi.
2. Direct Distance measurement: In this, a surveyor’s transit or a contractor’s leveler mounted on a tripod can be used to measure the head with high accuracy.But this method is slow and laborious.   

• Hydrology: Deals with processes governing deletion and replenishment of water resources. 
• 𝐼𝑛𝑓𝑙𝑜𝑤 = 𝑂𝑢𝑡𝑓𝑙𝑜𝑤 + 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑆𝑡𝑜𝑟𝑎𝑔𝑒 
• Hydrograph: Graph showing discharge of flowing water with respect to time. They indicate power available from streams at different times a day, week or year. 

Major Components of a Hydroelectric Power Plant

Reservoir

A natural or artificial reservoir accumulates water from the catchment area. For example, a lake in the mountains is the natural reservoir. Its purpose is to regulate water for power generation. an artificial reservoir is created by building a DAM across a river. 

Dam types:

1. Gravity Dam
2. Arch Dam
3. Earth filled dam
   Tarbella dam is the largest earth-filled dam in the world. 

Headrace:

The Headrace is used to convey water from upstream diversion weirs to forebays through open channels known as headrace. 

Forebay:

A small reservoir constructed by widening water carrying canal at its end. Water from forebay is distributed through penstocks into turbines. It acts as a settling basin for suspended sediments.

Intake structures:

Intake structures consist of booms, trash racks, sluice gates, and valves. Booms are floating chains of end-to-end tied logs to divert ice, weeds, logs, animal carcasses, etc., into bypass chutes to protect turbines from damage. Gates are installed to control the flow released into the penstock. Trash racks made of steel bars and screens are used to prevent debris from flowing into the penstock. To reduce head losses, they need to be often cleaned mechanically or manually. 

Penstock:

The penstock is a conduit to supply water from forebay to turbines under pressure. It may be made of steel, cast steel, or RCC. If the distance between forebay and turbines is short, each turbine will have a separate penstock. Otherwise, a single penstock may feed two or more turbines. Different types of valves such as butterfly and needle-type are installed in the penstock for flow control. In addition, air vents installed on the penstock prevent it from collapsing when the pressure inside it drops too low.

Surge tank:

When turbine gates are suddenly closed/opened by governor action to admit less water/more water as demand on the generator decreases (increases), the decelerating (accelerating) water inside penstock generates a pressure wave (negative pressure wave) that propagates upward to the nearest open water surface. This is called water hammer and surge tanks are installed to avoid this because it may lead to a collapse of the penstock. 

Types of Hydro Turbines

Hydro turbines convert water’s kinetic energy to rotational energy. Different types of designs are available that include  

Impulse Turbines

Pelton wheel, Turgo Turbine, Crossflow turbine 

Reaction Turbines

Francis Turbine, Kaplan Turbine, Propeller Turbine, Bulb Turbine, etc.

Fixed pitch propeller turbine: Due to fixed blades, part flow efficiencies are poor. They are cost-effective.

Bulb turbines: used for extremely low heads, comes with generator encapsulated and bulb like structure

Draft Tube and Tail Race of Hydroelectric Power Plants

Water that has passed through the runners moves next through a carefully designed vertical channel called draft tube. Draft tube recovers the kinetic energy of water by decelerating it before it exits the tailrace thus improving the hydraulic efficiency of the turbine. There are two types of draft tubes:   

1. Elbow type                    2. Straight type

Power house:

The building that houses alternators, turbines, transformers, and accessories for their control and protection. Two categories; Surfaced based and underground  

Alternators:

Its windings are made of laminated steel sheets with slots. They have two types: salient poles and round rotors

Governor:

A governor mechanism is an elaborate control system that senses the variations in the frequency of the generator, compares it with the reference frequency (i.e., 50 Hz/60 Hz) and then adjusts the speed of turbine through various means to offset the frequency variations

Classification of Hydroelectric Power Plants

Based on the nature of load:  

1. Baseload      2. Intermediate load           3. Peak load           

Base on the Head

Based on the Scale of Hydroelectric Power Plants:

Advantages of Hydroelectric Power Plants:   

One of the major advantages is that the ‘fuel’ used is Water which is self-replenishing. Moreover, it requires no transportation like coal or oil. The same water can be used for drinking and agriculture. The system is highly efficient (95%). Also, it has very long life (50 years almost). It is a quick start/shutdown system. The only costs are operational and maintenance. Side benefit is that it helps in flood control, fisheries and gives recreational sites.  

Drawbacks of Hydroelectric Power Plants:  

One of major drawbacks is that it requires high initial capital. Also, the construction period is high. Large portion of area is submerged under water. It can also lead to environmental and social problems. Availability of water varies from year to year. Also there is issue of Siltation that could limit the life of dam.  

Modern Large Sized Hydroelectric Power Plants

The Dam of Three Gorges in China, currently the largest (18,300 MW, target power 22,500 MW). Itaipu Dam, on the border between Brazil and Paraguay, until recently the largest dam (14,000 MW). Grand Coulee Dam (Columbia River)   (6800 MW).  Mangla Dam:    1150 MW. Tarbela Power Station:  4888MW 

In Pakistan, due to seasonal variation different dam power production varies. For example, Tarbela is from November to June when the capacity reduces to as low as about 1,350 MW against the maximum of ~4,500 MW during high head period i.e. August to September. WAPDA’s hydel generating capability varies between 2,414 MW and 6,902 MW over the cycle of a year.

Pakistan’s Hydel Power Potential

There is a lot of potential of hydropower in Pakistan i.e 44334 MW in Indus river, 8027 MW in Jhelum river, and more.  There are also a lot of future projects of hydel power across all over Pakistan which can fill up energy deficiency. Some projects are Neelum Jhelum project  (installed capacity 969 MW),  Diamer Basha dam project (4500 MW), Bunji hydropower project  (7100 MW), Dasu Hydropower (4320 MW, 201m Head)

Kalabagh Dam Controversy:  

The main objective of the project is to compensate for storage lost due to silting up of existing reservoirs. Its secondary objective is to generate a large amount of low-cost hydroelectric power near major load centers. 

Potential Positive Impacts:  

• Hydroelectric power  
• Irrigation water  
• Reduction of flood losses  
• Municipal and Industrial impacts  
• Net benefits to fisher & recreational facilities  
• River navigation etc. 

Apprehensions of Some Provinces:   

• No surplus water to fill Kalabagh reservoir 
• The project would render Sindh into a desert 
• Seawater intrusion in Indus delta would get highlighted  
• Fish production and drinking water supply below Kotri would be affected Apprehensions of KPK:  
• Population dislocation and resettlement costs 
• Fertile cultivable land would be submerged 
• Land acquisition  
• Greenhouse gas emissions

Conclusion

I hope you’ve liked our comprehensive article on the working principle and operation of hydroelectric power plants. You may also like our detailed article on the working and different components of a thermal power plant and 10 must-learn software for an electrical power engineer.

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