7 Hydropower As An Energy Source

Prof. Arun Kumar

1. HYDRO POWER

Hydropower is a renewable energy source where power is derived from the energy of water moving from higher to lower elevations. It is derived from a greek word ‘Hydro’ for water. So hydropower is the energy contained in water (ie electricity from water). Hydropower is a renewable energy source where power is derived from the energy of water using its gravitational force when it is flowing or falling. It is a proven, mature, predictable and cost competitive technology. Hydropower has among the best conversion efficiencies of all known energy sources (about 90% efficiency, water to wire). It requires relatively high initial investment, but has a long lifespan with very low operation and maintenance costs. In the past, hydropower has acted as a catalyst for economic and social development by providing both energy and water management services, and it can continue to do so in the future. Hydro storage capacity can mitigate freshwater scarcity by providing security during lean flows and drought for drinking water supply, irrigation, flood control and navigation services. Hydropower plants do not consume the water that drives the turbines. The water, after power generation, is available for various other essential uses. Hydropower can serve both in large, centralized and small, isolated grids, and small-scale hydropower is an option for rural electrification.

2. HISTORICAL BACKGROUND OF HYDRO POWER GENERATION

The development of hydro power in India started almost in the pace of world’s first hydro-electric installation in 1882 (at Appleton USA). The early phase of hydro power installation were small hydro only and played a crucial role. The 130 kW installation in Sidrapong (Darjeeling) in the year 1897 was the first hydroelectric installation in India. Few other installations e.g. Shivasamundram in Mysore (2000 kW) and Chamba (40 kW) in 1902, Mohra (450 kW) in 1905, Galogi in Mussoorie (3000 kW) in 1907, Jammu (800 kW) and Karteri (997 kW) in 1908, Pullivasal (400 kW) in 1910 and Jubbal (50 kW) in 1911 and Chhaba (1750 kW) in Shimla in 1913 are the known SHP stations still working. These plants were used primarily for lighting purposes in the important towns.

Major handicap of small hydroelectric stations in these days was high voltage transmission lines which were not year developed and their distances from the load centre were to incur heavy line losses. With the development of high voltage transmission lines in the early part of the twentieth century a shift also occurred-from small hydro plants serving local electricity markets to large scale plants feeding into extensive distributing grids. Bhakra, Rihand, Hirakund, Periyar, Koyna, Saravathi and Machkund are some large hydropower projects executed in early days. The same trend was observed in thermal plants fired by coal and later by petroleum fuels.

3. HYDROPOWER POTENTIAL

The head is relatively constant in run-of-river schemes except for variation in friction losses with the varying discharge. In irrigation canal or dam toe based scheme head also vary depending season of release of water as water level in the dam varies throughout the year. Weighted average is found out to design the head of turbine. The design head is so selected that turbine is operated to the maximum time giving optimum energy generation. Energy per year is calculated as below:

E = P . time (in hours 8760 per year) in kiloWatt hour

For the run of river hydropower scheme, it is useful to know the variation of flow over the year to select the most appropriate turbine configuration and estimated power generation. Flow variation presented in the form of a flow duration curve is the most useful form. The firm power and secondary power could be easily estimated from the flow duration curve.

Power potential may not be necessary developed fully as it all depends upon the economics of power project, power demand, grid characteristics. If the small hydro station is connected to the grid, the entire energy generated can be used by the system. For isolated operation load forecast is necessary.

The potential for energy production in a hydropower plant is determined by the following parameters, which are dependent on the hydrology, topography and design of the power plant:

The amount of water available;

Water loss due to flood spill, bypass requirements or leakage;

The difference in head between upstream intake and downstream outlet;

Hydraulic losses in water transport due to friction and velocity change; and The efficiency in energy conversion of electromechanical equipment.

In addition, some energy losses occur in the water conductor system from diversion point (intake) to turbine. These losses, called head loss, reduce the head and hence the energy potential for the power plant. These losses can be classified either as friction losses or singular losses. Friction losses depend mainly on water velocity and the roughness in tunnels, pipelines and penstocks.

The total efficiency of a hydropower plant is determined by the sum of these three loss components. Hydraulic losses can be reduced by increasing the turbine capacity or by increasing the reservoir capacity to get better regulation of the flow. Head losses can be reduced by increasing the area of headrace and tailrace, by decreasing the roughness in these and by avoiding too many changes in flow velocity and direction. Different turbine types have quite different efficiency profiles when the turbine discharge deviates from the optimal value.

4. TECHNOLOGY

Head and also installed capacity (size) are often presented as criteria for the classification of hydropower plants. A broad range of hydropower systems, classified by project type, system, head or purpose, can be designed to suit particular needs and site-specific conditions. Hydro power in India is broadly classified in Micro, Mini, Small, Medium and Large Hydro systems based on plan capacity. Micro is up to and below 100 kW, mini is up to 2 MW, small is up to 25 MW, medium is up to 100 MW and large is above 100 MW. Further hydropower also classified based on head available as Ultra Low Head is below 3 metres, Low Head is less than 40 metres, Medium Head is above 40 metres and High head is above 75 metres.

Hydropower plants are often classified in three main categories according to operation and type of flow. Run-of-river (RoR), storage (reservoir) and pumped storage HPPs all vary from the very small to the very large scale, depending on the hydrology and topography of the watershed. In addition, there is a fourth category called in-stream technology, which is a young and less-developed technology.

4.1 Run of River Scheme

These are hydropower schemes in which the available water is diverted from stream for hydro power generation (Figure 1). Such a hydropower plant generally includes some short-term storage (hourly, daily, or weekly), allowing for some adaptations to the demand profile. Run-of-river hydropower plants are normally operated as base-load power plants. A portion of river water might be diverted to a channel, pipe line (penstock) to convey the water to hydraulic turbine which is connected to an electricity generator. Installation of small RoR plants is relatively cheap and has in general only minor environmental impacts.

4.2 Reservoir

In order to reduce the dependence on the variability of inflow, many hydropower plants comprise reservoirs where the generating stations are located at the dam toe or further downstream through tunnel or pipelines as per the electricity or downstream water demand (Figure 2). Such reservoirs are often situated in river valleys. Most of the time these dams are constructed in the river for desired use like irrigation, drinking, flood control.

4.3 Pumped-storage

Pumped-storage plants pump water into an upper storage basin during off-peak hours using surplus electricity from baseload power plants and reverse flow to generate electricity during the daily peak load period (Figure 3). It is considered to be one of the most efficient technologies available for energy storage. The excess electricity in the grid is usually the generation of the thermal power plants or other renewable energy e.g. wind and solar.

4.4 Instream technology using existing facilities

To optimise existing facilities like weirs, barrages, canals or falls, small turbines can be installed for electricity generation (Figure 4). Smaller falls are also available on cooling water return channels on thermal power stations, drinking water and sewage channel after treatment. These are basically functioning like a run-of-river scheme

5. COMPONENTS OF HYDRO POWER PLANT

A hydro power plant comprises of following components

Civil components

Hydro electrical components comprising of hydro mechanical and hydro electrical equipment

5.1. Civil Components

5.1.1 Diversion structure or reservoir

The function of diversion structure is to divert the water required for power generation to the sedimentation tank through feeder channel whereas the function of reservoir is to store the water from surplus period to deficit period. Reservoir or diversion structure may be built on surface (weir &barrage, inflatable rubber dam). The structure designed should be safe against sliding, overturning and piping.

5.1.2 Desilting Device

For run of river type of schemes, desilting devices are provided in the water conductor systemas diverted water carries sediments as may damage the turbine parts, penstock and other components due to abrasion.In the desilting devices the incoming velocity of water is lowered by increasing the cross sectional area of desilting device thereby effectively allowing the sediment of particle.

5.1.3     Power Channel

The power channel is the one which carries water from source to forebay tank. The main function of power channel is for water conveyance, and sometimes to take the surge of water. The power channel may be lined with concrete to prevent the loss of water due to seepage. The shape of the power channel may be of rectangular or trapezoidal.

5.1.4    Forebay Tank or surge tank

The forebay is a small storage pond located in between power channel and penstocks. It provides immediate water demand on starting the generating units and required water seal over the penstock inlet against air entrainment. Surge tank is the storage with possibility of allowing the surge to more up and down without causing any penstock. Suitable spillway is provided on one side of forebay to dispose of safely the excess inflows during load rejection.

5.1.5    Penstock

Pressure conduits carrying water from forebay/surge tank to the power house are termed as penstock. Penstocks are required to bear maximum water pressure including water hammer which occurs due to sudden closure of inlet valve, they are quite costly and are a very important part of the water conductor system. The penstock material can be of steel, glass fiber reinforced plastic or concrete.

5.1.6    Power House

The functions of the power house are to support and house the generating units (turbine and its auxiliary units) and their accessories against weather and other human activities. It can be of surface type or underground type also.

5.1.7     Tail Race Channel

After passing through the turbines, the water is discharged back into the stream through a short channel called tailrace. The tailrace channel to be designed ion such a way that minimum tail water level is required to maintain safe suction head for smooth operation of turbine.

5.2 Hydro Mechanical Equipment

Hydro mechanical equipment comprises of Hydro turbine, gear box and gates & valves.

5.2.1     Hydro Turbine

Hydro Turbine is a fluid machine used for converting hydro potential available in water into mechanical power and then utilize this power for driving the electric generator in power plant. Turbines can be either reaction or impulse types (Figure 5). The turbines type indicates the manner in which the water causes the turbine runner to rotate. Reaction turbine operates with their runners fully flooded and develops torque because of the reaction of water pressure against runner blades. Reaction turbines are classified as Francis (mixed flow) or axial flow. Axial flow turbines are available with both fixed blades (Propeller) and variable pitch blades (Kaplan). Both axial flow (Propeller & Kaplan) and Francis turbines may be mounted either horizontally or vertically. Additionally, Propeller turbines may be slant mounted.

Second type of turbines are Impulse turbines and operate with their runner in air and convert the water’s pressure energy into kinetic energy of a jet that impinges onto the runner buckets to develop torque. Pelton turbines are impulse turbines and installed at high head.

5.2.2 Speed Governors

A speed governor is a combination of devices and mechanisms, which detect speed deviation and convert it into a change in servomotor position. A speed-sensing element detects the deviation from the set point; this deviation signal is converted and amplified to excite an actuator, hydraulic or electric, that controls the water flow to the turbine. In a Francis turbine, where there is a reduction in water flow needed to rotate the wicket-gates. For this, a powerful governor is required to overcome the hydraulic and frictional forces and to maintain the wicket-gates in a partially closed position or to close them completely.

Several types of governors are available, varying from old fashioned purely mechanical to mechanical-hydraulic to electrical-hydraulic and mechanical-electrical.

5.2.3  Speed Increasers

When the turbine and the generator operate at the same speed and can be placed so that their shafts are in line, direct coupling is the right solution; virtually no power losses are incurred and maintenance is minimal. Turbine manufactures recommend the type of coupling to be used, either rigid or flexible although a flexible, coupling that can tolerate certain misalignment, is usually recommended.

In many instances, particularly in low head schemes, turbines run at less than 400 rpm, requiring a speed increaser to meet the 750-1000 rpm of standard alternators. In the range of powers contemplated in small hydro schemes, this solution is often more economical than the use of a custom made alternator.

5.3 HYDRO ELECTRICAL EQUIPMENT

It comprises of Electrical generators, power and instrument Transformers, circuit breakers and relays, etc.,

5.3.1 Electrical generator

It converts the mechanical energy available from hydro turbine into electrical energy by means of electromechanical energy conversion process. Turbine speed governs the design of generator. The synchronous speed of generator is given by

Ns = 120 f/P

Where P is number of poles and f is the frequency of system

Mostly two types of generators are used in electrical power systems which are Synchronous generators: They are equipped with a DC electric or permanent magnet excitation system (rotating or static) associated with a voltage regulator to control the output voltage before the generator is connected to the grid. They supply the reactive energy required by the power system when the generator is connected to the grid. Synchronous generators can run isolated from the grid and produce power since excitation is not grid-dependent

Asynchronous generators: They are simple squirrel-cage induction motors with no possibility of voltage regulation and running at a speed directly related to system frequency. They draw their excitation current from the grid, absorbing reactive energy by their own magnetism. Adding a bank of capacitors can compensate for the absorbed reactive energy. They cannot generate when disconnected from the grid because are incapable of providing their own excitation current. However, they are used in very small stand-alone applications as a cheap solution when the required quality of the electricity supply is not very high.

In synchronous generator speed (Ns) is kept constant whereas in Asynchronous generator speed (Ns) is varying.

5.3.2  Transformers

It is a static device used to transfer electrical energy from its primary to secondary side by stepping up or steeping down the voltage. Step up transformers are used at power stations to step up the generated voltage for the purpose of transmission, transmitting higher voltage over long distance has many advantages like low power transmission loss, reduction in cable and insulator size, low cost and further.

Step down transformer are used at distribution end to step down the voltage according to requirement of users.

5.3.3  Instrument transformer

It is a measuring instrument used to measure the high voltage and high current in electrical power systems. These are categorised into two types which are Current transformers Voltage transformers

5.3.4 Circuit breakers

It is a protective device which protects the electrical networks from faulty condition. The main purpose of circuit breaker is to isolate the faulty parts from the networks so that fault is being localised. Earlier fuse was used now it’s being replaced by circuit breakers.

5.3.5 Relay

It is also a protective device which used to sense the abnormal condition in the electrical system. Whenever abnormal condition occurs relay operates by closing its contacts to give the signal to circuit breaker for opening circuit breaker contacts there by fault is isolated.

6. Powerhouse

In a hydro power scheme the role of the powerhouse is to protect the electromechanical equipment that converts the potential energy of water into electricity from the weather and provide a place for carrying out operation and maintenance activities. The number, type and power of the generating units, their configuration, the scheme head and the topographical conditions of the site determine the shape and size of the building. The layout may differ from project to project as per site conditions. The powerhouse shall comprise of machine hall having main hydro generating equipment (turbine, governor and generator), service bay to carrying out erection and maintenance activities, control room having control panels, relays etc. and tailrace for water exit.

Turbine

Speed increaser (if needed) Generator

Control system

Condenser, switchgear Protection systems
DC emergency supply

Power and current transformers etc.

7. Advantages of Hydro Power

• Clean fuel: Hydropower is fueled by water
• The SHPs (Small / mini / micro hydel projects) projects have potential to meet power requirements of remote and isolated areas – most attractive renewable source of grid quality power generation
• Conserve fossil fuels
• substitute thermal power
• hereby reducing carbon emissions.
• Hydropower relies on the water cycle, which is driven by the sun, thus it’s a renewable power source.
• Hydropower is generally available as needed; engineers can control the flow of water through the turbines to produce electricity on demand.
• Hydropower plants provide benefits in addition to clean electricity.– water supply and – flood control- Impoundment  hydropower creates reservoirs that offer a variety of recreational activities : fishing, swimming, and boating.

It requires relatively high initial investment, but has a long lifespan with very low operation and maintenance costs.

7. Disadvantages of Hydro Power

The local social and environmental impacts of hydropower projects vary depending on the project’s type, size and local conditions. Some impacts such as changes in flow and water quality of river, barriers to fish migration, impact on biological diversity, displacement of people due to impoundments. But reservoirs provides, beyond electricity supply, multiple beneficial services. While lifecycle assessments indicate very low carbon emissions, there is currently no consensus on the issue of land use change-related net emissions from reservoirs.

In the past, hydropower has acted as a catalyst for economic and social development by providing both energy and water management services, and it can continue to do so in the future. Hydropower can serve both in large, centralized and small, isolated grids, and small-scale hydropower is an option for rural electrification.