Water Turbine Impulse

The potential energy of water converted into kinetic energy at the nozzle. The water exit nozzle that has high speed hit the water turbine blades. After hitting the blade, direction of flow velocity changes resulting change in momentum (impulse). As a result, the water turbine wheel will rotate.

Water turbine impulse is has same pressure with atmospheric because the flow of water coming out of the nozzle is equal to the surrounding atmospheric pressure. All energy due to height place and pressure when enter into the turbine blade path is converted into velocity energy.

Water turbine impulse type can be classified as the following below:

1. Pelton Water Turbine
2. Turgo Water Turbine
3. Cross flow Water Turbine

Turbine Type Selection for Micro-Hydro Power Plant

Basically, the selection of turbine type for micro-hydro power plant is same with the selection of the conventional hydropower turbines (water turbine) type which ever existed. Basic selection as the driving turbine type generators on micro hydro power plant must first know the amount of head (meters), water discharge (m3/second), and its magnitude turbine rotational speed (n). Turbine rotation speed is obtained by knowing the velocity of water that will enter the turbine blades, by changing a linear velocity to the speed of the circumference (centrifugal) on the turbine shaft speed is called the circumference equation as following below:

U1 = D . π . n

Where:

U1       = Circumferential speed ​​(m/s)

D         = Diameter wheel turbine (m)

n          = Cycle turbine (rpm)

The selection rotation speed can be determined as high as possible, because with a high rotational speed, moment coupling will be obtained is small, small shaft, and small turbine wheel diameter, so it will make the size smaller generators. Circumferential speed (U1) is increasing with the growing cycle. The next very important to know in turbine plan is to determine the specific speed (nq) that will be crucial in planning the type of turbine that will be used in the micro-hydro power plant. Large specific speed (nq) can be obtained by the following formula:

nq = n. (V^0.5 / H^0.75)

Where:

n = Number of rotation (rpm)

V = water capacity (m³/second)

H = Head / water fall height (m)

Besides using the above formula, the value can also be obtained by using the chart specific speed below after the following parameter has been known such as head, turbine rotation, and water capacity. After know the specific speed, the type of turbine will be used can be determined whether to be used for propeller turbine, pelton, cross flow or the other. Determination of turbine type for micro-hydro power plant can also be directly known through the chart below follows the known value of specific speed of the calculation method above.

Figure 1: The Selection of Turbine Type for Micro Hydro Power Plant-1

Figure 2: The Selection of Turbine Type for Micro Hydro Power Plant-2

Tub Waterwheel/Water Turbine

Tub waterwheel / water turbine is the waterwheel which has wheel that was placed horizontally and its blades angled towards the vertical line.  Tub waterwheel / water turbine type can be made ​​smaller than the type of overshot waterwheel and undershot waterwheel type. Because the direction of force from water flows on sideways then, the energy received by the Tub waterwheel is the potential and kinetic energy.

The following below is the advantages and disadvantages of using the tub water wheel or water turbine:

The advantages of tub water wheel:

1. Tub waterwheel / water turbine have more compact construction.
2. Faster rotation speeds.

The disadvantages of tub water wheel:

1. Tub waterwheel / water turbine does not generate big power.
2. Because the smaller components, so Tub waterwheel / water turbine requires more rigorous level of accuracy.

Figure 1: Tube Waterwheel / Water Turbine

Breastshot Waterwheel/Water Turbine

Breastshot waterwheel / water turbine is a blend of overshot waterwheel and undershot waterwheel type that are seen from the received energy. High-fall distance does not exceed the diameter of waterwheel, the direction of water flow that drives the paddle wheel around the axis of the shaft waterwheel. Breastshot waterwheel type improves the performance of undershot waterwheel type.

Figure 1: Breastshot Waterwheel /Water Turbine

The following below are the advantages and disadvantages of using a breastshot waterwheel / water turbine:

Advantages of breastshot waterwheel / water turbine:

1. Breastshot waterwheel / water turbine type is more efficient than undershot waterwheel type.
2. When compared to overshot waterwheel, this type has shorter of fall heightl.
3. Breastshot waterwheel can be applied on a flat stream of water resources.

Disadvantages of breastshot waterwheel / water turbine:

1. Blades of this type are not flat like the undershot type but more complicated.
2. Required dams on stream flow flat.
3. Efficiency is smaller than the overshot waterwheel type.

Undershot Waterwheel/Water Turbine

Undershot waterwheel / water turbine works when running water hit the wall of blades located at the bottom of waterwheel. Undershot waterwheel type does not have the additional advantage of the head. This type is suitable mounted on the shallow water on a flat area. This type is also called the "Vitruvian". Here the water flows opposite with the direction of blades that rotates waterwheel.

Figure 1: Undershot Waterwheel/Water Turbine

The following below is advantages and disadvantages of using undershot waterwheel / water turbine:

Advantages of undershot waterwheel:

1. Simpler construction.
2. More economical.
3. Undershot waterwheel is easy to move.

Disadvantages of undershot waterwheel:

1. Undershot waterwheel / water turbine has small efficiency.
2. The power which is generated is relatively small.

Overshot Waterwheel/Water Turbine

Overshot waterwheel / water turbine works when the water that flows down into the upper edge of blades, and gravity of water will rotates paddle wheel (water turbine). Overshot waterwheel is waterwheel which is most widely used compared to other types of water wheel.

Figure 1: Overshot Waterwheel/Water Turbine

The following below is the advantages and disadvantages of using the overshot water wheel or water turbine:

The advantages of water wheel:

1. High level of efficiency can reach 85%.
2. Does not require a heavy flow.
3. Construction is simple.
4. Easy to maintenance.
5. The technology is simple and easy to implement in an isolated area.

The disadvantages of water wheel:

1. Because the flow of water from above, so reservoir water or dam water usually requires more investment.
2. Overshot waterwheel cannot be applied to high speed-rotation machine.
3. Overshot waterwheel requires a broader space for placement.
4. The power will be generated is relatively small.

Water Energy for Turbine Generator

Water is a source of energy which is cheap and relatively easy to obtain. Because of water is stored potential energy (on the water falls) and kinetic energy (on water flow). Hydropower is the energy obtained from water flow.

Water energy can be utilized and used in the form of mechanical energy and electrical energy for turbine generator. Utilization of water energy is mostly done by using a water wheel or water turbine which utilizes the presence of a waterfall or water flow in rivers.

Since the early 18th century water turbine is widely used as the driving flour mills, sawmills and textile machinery. The amount of hydropower available from a water source depends on the size of head and debit water. In connection with the reservoir water, so head is the height difference between the water level in the reservoir and the water out of the waterwheel / water turbines. The total energy available from a water reservoir is a potential energy of water, namely:

E = m g h

Where:

E          = potential energy of water (Joule)

m         = mass of water (kg)

h          = head (m)

g          = acceleration of gravitation (m/s²)

Power is energy per unit time (E/t), so from the equation above can determine equation as follow:

E/t = m g h / t

By replace / doing substitution P to (E/t) and substitute (ρ Q) to (m/t), so can be taken the equation below:

P = ρ Q g h

Where:

P          = potential power of water (watt)

Q         = debit flow of water (m³/s)

ρ          = density of water (kg/m³)

In addition to utilizing hydropower falling water for water turbine generator, the power can be obtained from flat water flow. In this case the available energy is kinetic energy:

E = 0.5 m v²

Where:

v          = velocity of water (m/s)

Available power of water is expressed as the following equation:

P = 0.5 ρ Q v²

The other equation can be obtained with using continuity equation Q=Av, so:

P = 0.5 ρ A v³

Where:

A         = cross area of water flow (m²)