APPLICATIONS OF TURBINES-HYDROELECTRICPOWER PLANTS

HOW DO HYDROELECTRIC POWER PLANTS WORK?• Hydroelectric and coal-fired power plants produce

electricity in a similar way.

• In both cases a power source is used to turn a propeller-like piece called a turbine, which then turns a metal shaft in an electric generator, which is the motor that produces electricity.

• A coal-fired power plant uses steam  to turn the turbine blades; whereas a hydroelectric plant uses falling water to turn the turbine.

• The most important part of the hydroelectric power plant is the dam, which acts as the water reservoir.

The basic components of a conventional hydropower plant are:• Dam - Most hydropower plants rely on a dam that holds back water

• Intake - Gates on the dam open and gravity pulls the water through the penstock, a pipeline that leads to the turbine.

• Turbine - The water strikes and turns the large blades of a turbine, which is attached to a generator above it by way of a shaft.

• Generators - As the turbine blades turn, so do a series of magnets inside the generator. Giant magnets rotate past copper coils, producing alternating current

• Transformer - The transformer inside the powerhouse takes the AC and converts it to higher-voltage current.

• Power lines - Out of every power plant come four wires: the three phases of power being produced simultaneously plus a neutral to all three.

TYPICAL HYDROELECTRIC POWER PLANT

THE ROLE OF TURBINES IN POWER GENERATION

• Turbines are versatile and can be used in a number of applications such as steam turbines used at coal-burning electricity plants to liquid water turbines used at hydro-electric plants.

• Large scale electrical energy production largely depends on the use of turbines. Nearly all of the world's power that is supplied to a major grid is produced by turbines. 

• The simplistic design, versatility, and efficiency of turbines allow for its widespread use in electrical power generation. 

WHAT ARE TURBINES• A turbine is a simple device

with few parts that uses flowing fluids (liquids or gases) to produce electrical energy.

• Fluid is forced across blades mounted on a shaft, which causes the shaft to turn. 

• The energy produced from the shaft rotation is collected by a generator which converts the motion to electrical energy using a magnetic field.

TURBINES USED IN HYDRO POWER PLANTS

• The type of hydropower turbine selected for a project is based on the head and the flow or volume of water at the site.

• Other deciding factors include how deep the turbine must be set, efficiency, and cost.

• There are two main types of hydro turbines: impulse and reaction.

IMPULSE TURBINES• The impulse turbine generally uses the velocity of the

water to move the runner and discharges to atmospheric pressure.

• The water stream hits each bucket on the runner.

• There is no suction on the down side of the turbine, and the water flows out the bottom of the turbine housing after hitting the runner.

• An impulse turbine is generally suitable for high head, low flow applications.

• Impulse turbines consist of: Pelton turbine, Turgo wheel and cross flow turbine.

PELTON TURBINES• Pelton turbines are used on

medium to high head hydropower sites with heads from 20 metres up to more than 300 metres.

• Power outputs can range from a few kW up to tens of MW’s on the largest utility-scale Pelton systems.

• Because the operating head is high, the flow rate tends to be low, ranging from 5 litres/second on the smallest systems up to 1 m3/s on larger systems.

• Compared to the power produced Pelton turbines are relatively compact, and because the flow rates are relatively low the associated pipework is relatively small.

• This means that a Pelton turbine itself is generally easier to install than an equivalently-powered Kaplan turbine.

• But Pelton turbines need a high pressure water supply from a penstock pipe, and designing and installing the penstock pipe is normally more challenging and is more expensive than the turbine.

• Once through the spear-jet the water impinges on the rotor and transfers around 97% of its kinetic energy into the rotational energy of the rotor. To achieve this transfer of energy the buckets are precisely designed to minimise all losses.

• The surface of the buckets is normally highly polished to minimise drag and the rotor itself is finely balanced.

• The small ‘cut-out’ in the tip of the bucket is to ensure that the next bucket doesn’t cut through the jet of water on the previous bucket prematurely as the rotor rotates.

• Once the water jet leaves the rotor it falls to bottom of the turbine casing and returns to the river through a discharge pipe.

• To get a higher flow rate, hence more power through a single pelton rotor it is possible to have multiple spear-jets, six jets normally considered to be the maximum.

• Pelton turbines can reach up to 95% efficiency, and even on ‘micro’ scale systems 90% peak efficiency is achievable.

• With more spear-jets the Pelton turbine would operate at high efficiency over an even wider flow range.

• Thus pelton wheels are only ideal for low power installations.

TURGO TURBINES• Turgo turbines were

developed by Gilkes in 1919 and are a development of the Pelton turbine.

• They can handle a higher flow rate than a physically similar-sized Pelton turbine and the rotor is slightly cheaper to manufacture.

• In most other respects they are very similar to Pelton turbines.

• The main physical difference is that the water jet strikes one side of the rotor and exits from the opposite side.

CROSS FLOW TURBINES• Crossflow turbines are widely used on small hydro sites with typical

power outputs from 5 kW up to 100 kW till 3MWon the very largest systems.

• They will work on net heads from just 1.75 metres all of the way to 200 metres, though there are more appropriate turbine choices for sites with heads above 40 metres.

• They will work on average annual flows as low as 40 litres/second up to 5 m3/s, though on the higher flow rates there may be better other turbine types to consider.

• The water flows over and under the inlet guide-vane which directs flow to ensure that the water hits the rotor at the correct angle for maximum efficiency.

• . Most of the power is extracted by the upper blades (roughly 75%) and the remaining 25% by the lower blades.

• One of the advantages of a crossflow turbine is that it is self-cleaning to a degree.

• The water exits the rotor and falls into the draft tube, which can be 1/3rd of net head.

• In good quality crossflows the guide-vanes fit the turbine casing with such precision that they can stop the water flow 100%.

• The peak efficiency shown of 86% is a little high: 82% is more realistic for a high quality crossflow turbine.

• Crossflows are available in a number of rotor diameters, normally in 100 mm steps from 100 mm to 500 mm.

• The smaller diameters are for higher-head sites.

REACTION TURBINES

• A reaction turbine develops power from the combined action of pressure and moving water.

• The runner is placed directly in the water stream flowing over the blades rather than striking each individually.

• Reaction turbines are generally used for sites with lower head and higher flows than compared with the impulse turbines.

• It includes propeller turbines such as bulb turbine, straflo, tube turbine, kaplan turine,etc., and francis turbines.

PROPELLER TURBINES

• A propeller turbine generally has a runner with three to six blades in which the water contacts all of the blades constantly.

• Through the pipe, the pressure is constant; if it isn't, the runner would be out of balance. The pitch of the blades may be fixed or adjustable.

• There are several different types of propeller turbines:• BULB TURBINE: The turbine and generator are a

sealed unit placed directly in the water stream.• STRAFLO: The generator is attached directly to the

perimeter of the turbine.• TUBE TURBINE: The penstock bends just before or

after the runner, allowing a straight line connection to the generator.

KAPLAN TURBINE• A Kaplan turbine is

basically a propeller with adjustable blades inside a tube.

• It is an axial-flow turbine, which means that the flow direction does not change as it crosses the rotor.

• The inlet guide-vanes can be opened and closed to regulate the amount of flow that can pass through the turbine.

• The rotor blade pitch is also adjustable, from a flat profile for very low flows to a heavily-pitched profile for high flows.

• Kaplan turbines could technically work across a wide range of heads and flow rates, but are relatively expensive.

• They are used on sites with net heads from 1.5 to 30 metres and peak flow rates from 3 m3/s to 30 m3/s.

• Such systems would have power outputs ranging from 75 kW up to 1 MW.

• The smallest good quality Kaplan turbines available have rotor diameters of 600 mm and the largest rotors available have 3 to 5 metre diameters.

• There are variants of Kaplan turbines that only have adjustable inlet guide-vanes or adjustable rotor blades, which are known as semi-Kaplan’s.

• Although the performance of semi-Kaplan’s is compromised when operating across a wide flow range.

KAPLAN TURBINE IN POWER PLANTS

FRANCIS TURBINE• It was named after James

Bicheno Francis (1815-1892), the American engineer who invented the apparatus in 1849.

• The Francis turbine is a reaction turbine designed to operate fully submerged.

• It has a runner with fixed buckets (vanes), usually nine or more.

• Water is introduced just above the runner and all around it and then falls through, causing it to spin.

• Francis turbines have the highest efficiencies of all hydro electric turbines – typically over 95 per cent.

• In addition to this high efficiency, they can cover a wide range of heads between 30 m and 300 m, and have the widest and highest range of power outputs.

• Large scale turbines used in dams are capable of delivering over 500 MW of power from a head of water of around 100 metres with efficiencies of up to 95%

ARCHIMEDEAN SCREWS • Archimedean screws are

relatively newcomers to the small-scale hydro world having only been used over the last ten years.

• They have been particularly used in sewage treatment works.

• They can work efficiently on heads as low as 1 metre, though are not generally used on heads less than 1.5 m (for economic reasons).

• The water enters the screw at the top and the weight of the water pushes on the helical flights, allowing the water to fall to the lower level and causing the screw to rotate.

• Single screws can work on heads up to 8 metres, but above this multiple screws are generally used.

• The maximum flow rate through an Archimedean screw is determined by the screw diameter.

• The smallest screws are just 1 metre diameter and can pass 250 litres / second and larger screws are 5 metres in diameter with a maximum flow rate of around 14.5 m3/s.

• In terms of power output, the very smallest Archimedean screws can produce as little as 5 kW, and the largest 500 kW.

• Archimedean screws typically rotate at around 26 rpm, but when the top of the screw is connected to a gearbox, the speed increases to 750-1500 rpm to make it compatible with standard generators.

• Though they rotate relatively slow they can splash water around, which is reduced by the use of a splash guard.

• Good quality Archimedean screws have a design life of 30 years

• A significant advantage of Archimedean screws is their debris tolerance.

• The low rotational speed and large flow-passage dimensions of Archimedean screws also allow fish to pass downstream through the screw in relative safety.

HOW MUCH POWER COULD THESE PLANTS GENERATE?• The maximum power output is entirely dependent on how

much head and flow is available at the site.

• Power output is given by: P = m x g x Hnet x System efficiency

Where, P = Power, measured in Watts (W), m = Mass flow rate in kg/s, g = the gravitational constant, which is 9.81 m/s2, Hnet = the net head.

• For a ‘typical’ small hydro system the turbine efficiency would be 85%, drive efficiency 95% and generator efficiency 93%, so the overall system efficiency would be 0.85 x 0.95 x 0.93 = 0.751 or 75.1%.

• Therefore for a head of 2.25m, a flow rate of 3 m3/s, the maximum power output of the system=3,000x9.81x2.25x0.751=49.7kW.

CONSTRUCTION COSTMaximum Power

Output Estimated Project Cost £ / kW installed

5 kW £100k £20k25 kW £190k £7.5k50 kW £330k £6.5k

100 kW £560k £5.6k250 kW £1.13M £4.5k500 kW £1.86M £3.7k

OPERATING COSTMaximum Power Output

Estimated Annual Operational Costs

5 kW £2,20025 kW £4,00050 kW £6,300

100 kW £11,000250 kW £25,000500 kW £48,300

WHAT MAKES A GOOD HYDRO SITE?

• Good head

• Good flow

• Simple site layout with main parts close together

• Good grid connection

• Good site access

• Single ownership of the site, or cooperative neighbours

• Not too many environmental sensitivities

WHAT ARE THE MINIMUM HEAD AND FLOW NEEDED

• The table shows the minimum annual mean (i.e. average) flow rate required for a given head to generate a maximum power output of 25 kW

Low-head

Hydropower Sites

High-head

Hydropower Sites

Max. Power Output

GrossHead

2 m

GrossHead

5 m

GrossHead

10 m

GrossHead

25 m

GrossHead

50 m

GrossHead

100 m

25 kW 1.9 m3/s0.75 m3/s

0.38 m3/s

0.15 m3/s

75 litres/s

38 litres/s

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