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The role of the powerhouse in a small hydropower scheme is to cover the electromechanical equipment that converts the potential energy of water into electricity, from the weather hardships. The size and shape of the building , is determined by the number, type and power of the turbo-generators. Their configuration, the scheme head and the geomorphology of the site also determine the powerhouse aspect.
The electromechanical equipment found in the powerhouse is:
â€¢ Inlet gate or valve
â€¢ Speed increaser (if needed)
â€¢ Control system
â€¢ Condenser, switchgear
â€¢ Protection systems
â€¢ DC emergency supply
â€¢ Power and current transformers
In order to reduce the environmental impact the powerhouse can be entirely submerged. In this way the level of sound is sensibly reduced and the visual impact is nil.
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This figure is a schematic view of an integral intake indoor powerhouse suitable for low head schemes. The substructure is part of the weir and embodies the power intake with its trashrack, the vertical axis Kaplan turbine coupled to the generator, the draft tube and the tailrace.
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Powerhouses are more conventional with an entrance for the penstock and a tailrace in medium and high head schemes. This kind of powerhouse can be also built underground.
Also the powerhouse can be at the base of an existing dam, where the water arrives through an existing bottom outlet or an intake tower.
5.2 Hydraulic turbines
The hydraulic turbine transforms the waters potential energy to mechanical rotational energy.
Types and configuration :
In the turbine the potential energy in water is converted into mechanical energy, by one of two fundamental and basically different mechanisms:
1. The water pressure applys a force applied on the face of the runner blades, which decreases as it proceeds through the turbine. Turbines that operate in this way are called reaction turbines. The turbine casing, with the runner fully immersed in water, must be strong enough to withstand the operating pressure. In this category the most important turbines are Francis, Kaplan and Bulb turbines.
2. The water pressure is converted into kinetic energy before entering the runner. The kinetic energy is in the form of a high-speed jet that strikes the buckets, mounted on the periphery of the runner. Turbines that operate in this way are called impulse turbines. The most importante impulse turbines are Pelton , Turgo , Cross -Flow turbine (Banki - Michell)
The hydraulic power at disposition of the turbine is given by:
Ph=ÏQ âˆ™gH [W] (5.1)
ÏQ = mass flow rate [kg/s]
Ï = water specific density [kg/m3]
Q = Discharge [m3/s]
gH = specific hydraulic energy of machine [J/kg]
g = acceleration due to gravity [m/s2]
H = "net head"
The mechanical output of the turbine is given by:
Pmec=Ph â‹…Î· [W] (5.2)
Î· = turbine efficiency C:\Users\GRYG\Desktop\Licenta\poze hidrocentrla\bazine.bmp
The specific hydraulic energy of machine is defined as follows:
Where: gH = specific hydraulic energy of machine [J/kg]
px = pressure in section x [Pa]
cx = water velocity in section x [m/s]
zx= elevation of the section x [m]
The subscripts 1 and 2 define the upstream and downstream measurement section of the turbine. They are defined by IEC standards.
The net head is defined as:
5.2.1 Impulse turbines
184.108.40.206 Pelton Turbines
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Figure 5.4 Figure 5.5
Pelton turbines are impulse turbines where one or more jets put pressure on a wheel carrying on its periphery a large number of buckets. Each jet issues water through a nozzle with a needle valve to control the flow. They are only used for high heads from 60 m to more than 1 000 m. The axes of the nozzles are in the plan of the runner. In case of an emergency, stop of the turbine, the jet may be diverted by a deflector so that it does not impinge on the buckets and the runner cannot reach runaway speed. In this way the needle valve can be closed very slowly, so that overpressure surge in the pipeline is kept to an acceptable level (max 1.15 static pressure).
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Figure 5.5 - Pelton nozzle
A typical installation of Pelton turbines consists of the following elements:
1. - Entrance elbow
2. - Injector: it is the distributor of the Pelton turbines. It transforms pressure energy into kinetic energy.
3. - Nozzle
4. - Needle valve: it is moved longitudinally. The fuze and the needle of the injector usually are constructed of very hard steel.
5. - Actuator: it moves the injector by oil pressure.
6. - Regulator
7. - Control of the baffle plate
8. - Baffle plate: it serves to control the turbine's movement.
9. - Spurt
10. - Bun
11. - Blades or spoons.
12. - Turbine's brake by water spurt: the small spurt of 25mm of diameter in this case acts on the back of the blades and restrains the bun. Without it, the bun would continue turning more and more slowly damaging the bun.
Figure 5.6 - Pelton turbine
Once identified the Pelton turbines component elements, and known the respective functions, it is easily understood its operation.
The successive energy transformation is made in the following way. The dammed water gravitational potential energy, or pressure energy up to the nozzles holes, becomes, practically without losses, kinetic energy, when leaving the water through this holes in form of free jets, to a speed that corresponds to all the useful jump height, being referred this one, for Pelton turbines concrete case, to the considered jets centre.
It has the maximum kinetic energy in the moment when the water tangentially impacts on the bun, pushing its buckets, being obtained the wanted mechanical work.
The buckets concave forms make the water jet direction change, flowing it, already without appreciable energy, for the lateral borders, without any later incidence on the following buckets. This way, the water jet transmits its kinetic energy to the bun, where it is transformed instantly into mechanical energy.
The needle valve, governed by the speed regulator, closes more or less the nozzle exit hole, being able to modify the water flow that flows through it, with the aim of maintaining the bun speed constant, being avoided rushing or revolutions number reduction, by respectively decrease or increase of the load requested to the generator.
The edge that divides each bucket in two symmetrical parts, cuts the water jet, cutting it in two fluid sheets, theoretically of the same flow, throwing each one toward the corresponding concavity. Such a disposition allows to counteract the axial pushes originated in the bun, balancing pressures on it, when changing, symmetrical and oppositely, both water sheets senses.
The Pelton turbine can have one or more nozzles.Those who have one or two nozzles can have horizontal or vertical axis, and those with three or more nozzles have vertical axis. The maximum number of nozzles is 6.
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Figure 5.7 - horizontal axis turbine Figure 5.8 - vertical axis turbine
The efficiency of a Pelton turbine is good from 30% to 100% of the maximum discharge for a one-jet turbine and from 10% to 100% for a multi-jet one.
220.127.116.11 Turgo Turbine
The Turgo turbine can operate under a head in the range of 50-250 m. Like the Pelton, it is an impulse turbine, however its buckets are shaped differently and the jet of water strikes the plane of its runner at an angle of 20°. Water enters the runner through one side of the runner disk and emerges from the other . It can operate between 20% and 100% of the maximal design flow.
Figure 5.9 - turgo turbine nozzle
It is a turbine used for micro hydro power plants with lower head than the Pelton turbines and greater water flow.
Unlike the Pelton turbine which has its installed flow limited by the condition that reflected water from one cup should not interfere with the adjacent one, the Turgo turbine is not affected by such problems. As a result it has a higher rotational speed and can be joint directly to the generator shaft, eliminating speed multipliers.C:\Users\GRYG\Desktop\Licenta\poze hidrocentrla\Turgo.bmp
The efficiency is lower than for the Pelton and Francis turbines.
Compared to the Pelton, a Turgo turbine has a higher rotational speed for the same flow and head.
A Turgo can be an alternative to the Francis when the flow strongly varies or in case of long penstocks, as the deflector allows avoidance of runaway speed in the case of load rejection and the resulting water hammer that can occur with a Francis.
18.104.22.168 Cross-flow turbines (Banki - Michell)
This impulse turbine, also known as Banki-Michell is used for a wide range of heads overlapping those of Kaplan, Francis and Pelton. It can operate with heads between 5 and 200 m.
Figure 5.11 - cross-flow turbine
TheÂ cross-flow turbine,Â Banki-Michell turbine, orÂ Ossberger turbineÂ was developed by the AustralianÂ Anthony Michell, the HungarianÂ Donát BánkiÂ and the German Fritz Ossberger. Michell obtainedÂ patentsÂ for his turbine design in 1903, and the manufacturing company Weymouth made it for many years.
Unlike mostÂ water turbines, which have axial or radial flows, in a cross-flow turbine the water passes through the turbine transversely, or across the turbine blades. As with aÂ water wheel, the water is admitted at the turbine's edge. After passing the runner, it leaves on the opposite side. Going through the runner twice provides additionalÂ efficiency. When the water leaves the runner, it also helps clean the runner of small debris and pollution. The cross-flow turbine is a low-speed machine that is well suited for locations with a low head but high flow.
Most practical cross-flow turbines have two nozzles, arranged so that the water flows do not interfere.
Cross-flow turbines are often constructed as two turbines of different capacity that share the same shaft. The turbine wheels are the same diameter, but different lengths to handle different volumes at the same pressure. The subdivided wheels are usually built with volumes in ratios of 1:2. The subdivided regulating unit, the guide vane system in the turbine's upstream section, provides flexible operation, with 33, 66 or 100% output, depending on the flow.
This simple design of the cross- flow turbine makes it cheap and easy to repair in case of runner brakes due to the important mechanical stresses.
The Cross-flow turbines have low efficiency compared to other turbines and the important loss of head due to the clearance between the runner and the downstream level should be taken into consideration when dealing with low and medium heads. Moreover, high head cross-flow runners may have some troubles with reliability due to high mechanical stress.
It is an interesting alternative when one has enough water, defined power needs and low investment possibilities, such as for rural electrification programs.
5.2.2 Reaction Turbines
22.214.171.124 Francis Turbine
They are known as overpressure turbines as pressure is variable in the bun areas, or total admission ones since this is subjected to the water direct influence in all their periphery. They are also known as radial-axialÂ turbines andÂ reactionÂ turbines, concepts that will be enlarged conveniently.
The application field is very broad, given the technological advance gotten in the construction of this type of turbines. They can be used in different height jumps inside a wide range of flows (between 2 and 200 m3/s approximately).
We will consider the following classification, in function of the bunÂ specific speedÂ whose number of revolutions per minute depends on the jump characteristics.
-Â SlowÂ Francis turbine. For great height jumps (around 200 m. or more).
-Â Â NormalÂ Francis turbine. Indicated in half height jumps (between 200 and 20 m.)
-Â Â Quick and extra-quickÂ Francis turbines. Appropriate to small height jumps (inferior to 20 m.).
Francis turbines, are of very good performance, but only among certain margins (for 60% and 100% of the maximum flow), being one of the reasons why several units are settled in each power station, to the object that none works, individually, below values of 60% of the total load.
As Pelton turbines, Francis turbines can be installed with the shaft in horizontal, or vertical position, being this last disposition the most widespread as it has been broadly experienced, especially in great power sets.
Figure 5.12 - vertical shaft turbine Figure 5.13 - horizontal shaft turbine
The list of fundamental components, considering as a reference, whenever it is feasible, the turbine water circulation sense, is the following:
-Â Â Â Â Â Â Â Â Â Â SpiralÂ case
-Â Â Â Â Â Â Â Â Â Â Wicket Gate
-Â Â Â Â Â Â Â Â Â Â Turbine runner
-Â Â Â Â Â Â Â Â Â Â Draft tube
-Â Â Â Â Â Â Â Â Â Â Shaft
-Â Â Â Â Â Â Â Â Â Â Turbine shaft
-Â Â Â Â Â Â Â Â Â Â Turbine guide bearing
-Â Â Â Â Â Â Â Â Â Â Push bearing
- Shift ring
Figure 5.14 - Francis turbine section
Considering the Francis turbines components constructive aspects, its operation is easily understood.
In most of the cases, the installation of this type of turbines, is carried out in power stations for whose water feeding the existence of a reservoir is required. Another particularity in these turbines location, resides in that their essential group, that is to say, spiral chamber-distributor-bun-aspiration tube, is, generally, at an inferior level regarding the level reached by the water in its exit toward the downstream river bed.
Therefore, a continuous water column presence, between the different levels of the mentioned ends, dam and water exit, deducing that the turbine is completely full with water. According to other installation dispositions, especially in very little height jumps, we could interpret that it is submerged, such it is the case of not having spiral chamber, being installed the bun inside anÂ open chamber, usually of concrete, directly connected with the reservoir water intake area.
The dammed water gravitational potential energy, becomes kinetic energy in its way toward the distributor, where, in its exit, it has energy in kinetic and pressure form, being, the bun water entrance speed, inferior to which would correspond to jump height, due to the abrupt direction changes in its journey.
Centring us in the distributor's area, we can add that the water, when flowing through the spiral chamber fixed shovels and the distributor's guiding shovels, diminishes its pressure, acquiring speed and, under such conditions, it causes the bun rotation, when reflecting through its paddles, on which the rest of the existent water masses pressure acts, endowed, in turn, of kinetic energy.
The aspiration tube produces a depression in the bun exit or, said in other terms, a suction.
126.96.36.199 Kaplan and propeller turbines
Kaplan and propeller turbines are axial-flow reaction turbines; generally used for low heads from 2 to 40 m. The Kaplan turbine has adjustable runner blades and may or may not have adjustable guide- vanes. If both blades and guide-vanes are adjustable it is described as "double-regulated". If the guide-vanes are fixed it is "single-regulated". Fixed runner blade Kaplan turbines are called propeller turbines. They are used when both flow and head remain practically constant, which is a characteristic that makes them unusual in small hydropower schemes.
Figure 5.15 - Kaplan turbine Figure 5.16 - Kaplan turbine
Figure 5.17 - Kaplan turbine section
The double regulation allows, at any time, for the adaptation of the runner and guide vanes coupling to any head or discharge variation. It is the most flexible Kaplan turbine that can work between 15% and 100% of the maximum design discharge. Single regulated Kaplan allows a good adaptation to varying available flow but is less flexible in the case of important head variation. They can work between 30% and 100% of the maximum design discharge.
The double-regulated Kaplan is a vertical axis machine with a spiral case and a radial guide vane configuration. The flow enters in a radial manner inward and makes a right angle turn before entering the runner in an axial direction. The control system is designed so that the variation in blade angle is coupled with the guide-vanes setting in order to obtain the best efficiency over a wide range of flows and heads. The blades can rotate with the turbine in operation, through links connected to a vertical rod sliding inside the hollow turbine axis.
Kaplan turbines are certainly the machines that allow the most number of possible configurations. The selection is particularly critical in low-head schemes where, in order to be profitable, large discharges must be handled. When contemplating schemes with a head between 2 and 5 m, and a discharge between 10 and 100 m3/sec, runners with 1.6 - 3.2 metres diameter are required, coupled through a speed increaser to a generator. The hydraulic conduits in general, and water intakes in particular, are very large and require very large civil works with a cost that generally exceeds the cost of the electromechanical equipment.
In order to reduce the overall cost (civil works plus equipment) and more specifically the cost of the civil works, several configurations have been devised that nowadays are considered as classic.
The selection criteria for such turbines are well known:
â€¢ Range of discharges
â€¢ Net head
â€¢ Geomorphology of the terrain
â€¢ Environmental requirements (both visual and sonic)
â€¢ Labour cost
The configurations differ by how the flow goes through the turbine (axial, radial, or mixed), the turbine closing system (gate or siphon), and the speed increaser type (parallel gears, right angle drive, belt drive).
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Figure 5.18 - Cross section of a Kaplan siphon Figure 5.19 - Cross section of a vertical
power plant Kaplan power plant
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Figure 5.20 - Cross section of a Kaplan inverse Figure 5.21 - Cross section of a inclined
siphon power plant Kaplan power plant