Closed Cycle Gas Turbine

Closed cycle gas turbine is divided into two types, namely gas turbine with direct closed cycle and gas turbines with   indirect closed cycle. In direct closed cycle gas turbine, cooler gas is heated in the reactor and expands through turbine, cooled in heat exchanger and it is compressed back into reactor. This cycle can also use other gas. There is not radioactive gas be discharged into atmosphere in normal operation. The most suitable fluid for this cycle is helium.

Figure 1: Direct Closed Cycle Gas Turbine

While on indirect closed cycle gas turbine, gas turbine with this cycle is combination of indirect open-cycle turbines and direct closed-cycle turbine, because reactor separate from working fluid by heat exchanger. While working gas was throwing heat into the atmosphere through heat exchanger. Primary coolant material that is usually used is water or helium gas.

Figure 2: Indirect Closed Cycle Gas Turbine

In this closed cycle, working fluid is not connected to surrounding atmosphere; thereby its purity can be maintained. This is very advantageous in terms of prevention of damage caused by erosion and corrosion. In this system can also be used with high pressure air up to 40 atm as the installation of steam, but the works did not experience phase change.

The advantages in closed cycle gas turbine are:

1. For the same power turbine has a smaller size
2. Can be used on high-pressure system
3. The use of fuel can be more efficient 

Reaction Water Turbine

The blades of reaction water turbine have special profile that results in decreased water pressure during flow through the blade. This different pressure exerts force on the blade so that the runner (turbine part that rotates) can rotate. Turbine generator that works based on this principle is grouped as reaction water turbine. Runner reaction water turbine is fully immersed in water and in the turbine house.

Reaction water turbine can be classified as two types:

1. Francis water turbine

2. Kaplan and propeller water turbine

Kaplan-Propeller Water Turbine

Kaplan and Propeller turbines are included in reaction turbine type with axial flow. Kaplan and Propeller turbines is composed of the propeller as the boat. The propeller usually has three to six blades.

Figure 1: Kaplan and Propeller Water Turbine
(source: http://en.wikipedia.org/wiki/Kaplan_turbine)

Francis Water Turbine

Francis water turbine is one of reaction turbine type. A Francis water turbine is installed between the source of high pressure water at the entrance side and low water pressure at the exit side. Francis water turbine uses the blade steering. Steering blades directs incoming water tangentially. The steering blades of Francis water turbine may be fixed steering blade or adjustable steering blade which can adjust angle of blade. For use on various conditions of water flow using an adjustable steering blade is the right choice.

Figure 1: Francis Water Turbine
(Source: http://en.wikipedia.org/wiki/francis_turbine)

Figure 2: Skecth of Francis Water Turbine

(Source: http://lingolex.com/bilc/engine.html)

Crossflow Water Turbine

Crossflow water turbine is one of impulse water turbine type and can be also known as the Michell-Banki turbine. Michell-Banki is discoverer of this turbine type. Crossflow water turbine can be also called as Osberger turbine because of Osberger is company that manufactures Crossflow water turbine. Crossflow water turbine can be operated at discharge 20 liters/sec up to 10 m³/sec and head between 1 m up to 200 m.

Crossflow water turbines use rectangular nozzle which has width suitable the width of runner. Jets of water enter turbine and hit blades resulting in conversion of kinetic energy into mechanical energy. Water flows out hit the blade and provide energy (lower than at admission) and then leave the turbine. Runner turbine is made of several blades that are attached on a pair of parallel plate.

Figure 1: Crossflow Water Turbine
(source: http://europa.eu.int/en/comm/dg17/hydro/layman2.pdf)

Turgo Water Turbines

Turgo water turbines can operate on the head 30 m up to 300 m. As Pelton water turbine, Turgo water turbine is categorized in impulse water turbine type, but its blades is different. Jets of water from the nozzle hit the blades at angle 20^o.

Rotational speed of turgo water turbine is greater than the pelton water turbine. As a result, the transmission can be ​​possible distributed directly from the turbine to the generator thereby increasing total efficiency and lowering cost of maintenance.

Figure 1: The Blades and Nozzles of Turgo Water Turbine

Pelton Water Turbines

Pelton water turbines are grouped in water turbine impulse. Pelton water turbine consists of a set of running blades that are rotated by jets of water which is sprayed from one or more device called as nozzle. Pelton water turbine is one of water turbine type that has highest efficiency. Pelton water turbine is suitable for high head.

Figure 1: Pelton Water Turbine
(Source: http://en.wikipedia.org/wiki/pelton_wheel)

Form of turbine blade consists of two symmetrical parts. Blade is formed so that the jets of water will hit the middle of blades and jets of water will be turned in both directions so that it can reverse the jets of water well and release the blade from side forces. For turbines with high power, water spray system is divided by multiple nozzles. Thus the diameter of water jet can be reduced and bucket blade will be smaller. Pelton water turbines for large-scale plant require approximately 150 meters head but for the micro-scale 20 meters head is adequate.

Figure 2: Pelton Water Turbine Blade

Figure 3: Pelton Water Turbine Nozzle
(Source: http://europa.eu.int/en/comm/dg17/hydro/layman2.pdf)

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)

How Steam Turbine Work

Gas Steam turbine is a first mover which converts potential energy of steam into kinetic energy then this kinetic energy is converted into mechanical energy in the form of rotation turbine shaft. Turbine shaft will be connected to driven mechanism directly or with the help of gear reduction. Steam turbine can be used in various industrial fields, for power plant and transportation.

To convert potential energy of steam into mechanical energy in the form of shat rotation shaft can be done in various ways, so steam turbine generally consists of three main types as following below:

1. Steam turbine impulse
2. Steam turbine reaction
3. Steam turbine combined (impulse-reaction)

During expansion process of steam in the turbine, some major losses occur during this process both inside loss and outside loss. Heat losses will decrease velocity, decrease pressure, decrease cycle efficiency and finally decrease power generator that will be generated by an electric generator.

Turbine Blade Design

The turbine blades are installed on turbine wheel that rotates on the surface. The turbine blades design move together with turbine wheel, so turbine blade design can be called as motion blade or path blade.  At turbine wheel may be there are some motion blades, each blade row consist of blades which are arranged circle on turbine wheel, each blades has same shape and dimension.

Turbine which has single row motion blade is called as single level turbine and turbine which has some rows motion blades is called as multi-level turbine. At multi-level turbine, fluid work flow through first row blades, second row blades, third row blades and so on. Working fluid in multi-level turbine before flow from one motion blade to other motion blade will flow through the row blades that are united with turbine house.

The blades which are united and do not rotate can be called as fixed or stationary blades. Stationary turbine blade has function to flow working fluid into motion blades and has function also as nozzle. In the multi-level turbine, expansion process of working fluid is performed gradually. So working fluid will flow from one level to next level where each one level consists of one row stationary turbine blade and one row motion turbine blade.

The using of multi-level turbine is designed to increase efficiency. The gap between turbine house and top of turbine blade should be designed as narrow as possible to make fluid energy can be converted maximally into useful work.

Radial Flow Turbine Generator

A lot of turbine types in the world, one of turbine types is classified according to fluid flow like radial flow turbine generator. Radial flow turbine generator is the turbine which has direction of fluid flow perpendicular to axis of turbine shaft. In the radial flow turbine generator, expansion fluid from initial pressure to the last pressure occurs in all of rotating blades.

Radial flow turbine is generally used for small flow rate. The advantageous of radial flow turbine generator are radial flow turbine is cheaper and simpler for manufacturing than axial flow turbine generator. For example are small gas turbine installations in the field of automotive and fire pump that can be removable. Figure 1 below shows the characteristic of radial flow turbine generator.

Figure 1: Characteristic of Radial Flow Turbine generator

Axial Flow Turbine Generator

Turbine generator has some types of turbines, one of them is type turbine based on fluid flow such as axial flow turbine generator. Axial flow turbine generator is the turbine which has direction of fluid flow parallel to axis of turbine shaft. Axial flow turbine is generally used for large capacity and power because it has more advantageous than radial flow turbine generator as follows:

1. Axial flow turbine has better efficiency than radial flow turbine.
2. Comparison of pressure (rp) can be made higher.
3. Construction of axial flow turbine generator is lighter and does not require large room.

Figure 1: Axial Flow Turbine Generator
(Source: Gas Turbine Engineering Handbook - Meherwan P. Boyce)

Axial flow turbine can be classified based on energy conversion system as follow:

1. Axial reaction turbine

Axial reaction turbine is axial turbine which has expansion process not only occur in motion blade but also expanded evenly so decrease in heat content is distributed evenly on all level.

2. Axial action (impulse) turbine

Axial action (impulse) turbine is axial turbine which has expansion process occur only in stationary blade and velocity energy is converted into mechanism energy in turbine blades without occurrence of expansion in motion blade.

Types of Turbines

There are some types of turbines in the world. Types of turbines can be classified as follow:

1. Type turbine based on working cycle:

1.      Open cycle turbine generator

2.      Closed cycle turbine generator

3.      Combined cycle turbine generator

2. Type turbine based on construction

1.      Single shaft turbine generator

2.      Multi shaft turbine generator

3. Type turbine based on fluid flow

1.      Axial flow turbine generator

2.      Radial flow turbine generator

4. Type turbine based on working fluid

1.      Gas turbine generator

2.      Steam turbine generator

3.      Water turbine generator

5. Type turbine based on pressure

1.      Low pressure turbine generator

2.      Middle pressure turbine generator

3.      High pressure turbine generator

4.      Very high pressure turbine generator

5.      Supercritical pressure turbine generator

6. Type turbine based on amount of cylinder:

1.      Single cylinder turbine generator

2.      Double cylinder turbine generator

3.      Triple cylinder turbine generator

4.      Multi cylinder turbine generator

7. Type turbine based on amount level of pressure:

1.      Single level pressure or some levels speed turbine generator.

2.      Impulse and many levels turbine generator

8. Type turbine based on method of setting:

1.      Turbine generator with throttling setting method

2.      Turbine generator nozzle setting method

3.      Turbine generator with by-pass governing method

9. Type turbine based on use:

1.      Turbine generator with constant / stationer rotation which is used to drive alternator.

2.      Turbine generator with flexible and has variable of speed to drive turbo power, pump, etc.

3.      Turbine generator non stationery which is used to steam machine, ship and locomotive.

Gas Turbine Generator Design

Gas turbine generator is a first mover that is designed to convert potential energy into kinetic energy and the gas kinetic energy is then converted into mechanical energy in the form of rotation turbine shaft. Turbine shaft is connected by driven mechanism directly or with help of reduction gears. Gas turbine can be designed for industries purpose such as manufacturing process and transportation. Gas turbine can be also designed to drive electric generator in power plant.

Figure 1: recuperative medium–sized Industrial Gas Turbine (Courtesy Solar Turbines Incorporated)

Design gas turbine use gas as its working fluid. The simplest gas turbine system consists of generator, combustion chamber, and compressor. The compressor compresses air from outside to be high pressure air and fed to combustion chamber. Natural gas is burnt together with compressed air in the combustion chamber. Hot gas out of the burner or reactor can be used directly as a working fluid which flowed into turbine to drive rotor which is connected to electric generator.

Figure 2: Installation Diagram of Gas Turbine

Gas Turbine Multi Shaft

Gas turbine multi shaft is used to resist variation of load and torque. The first turbine shaft is coupled directly to axial shaft. High pressure turbine has function to drive compressor and supply hot gas for low pressure turbine. Low pressure turbine rotate electric generator. Gas turbine multi shaft is also used for central power and industry. This gas turbine is planned to operate at different rotation without using reduction gear.

Figure 1: Diagram Gas Turbine Multi Shaft

Caption:

C         = compressor

S          = shaft

LPT     = low pressure turbine

HPT     = high pressure turbine

G         = generator

CC       = combustion chamber

Gas Turbine Components

Gas turbine has main components as follow: compressor, combustion chamber, gas turbine, load gear and generator.

1. Compressor

Compressor serves to suck outside air or atmospheric air and then compressed to get higher pressure.

2. Combustion chamber

Combustion chamber is a chamber or room which is used to perform combustion process with gas as its fuel and the gaseous combustion product will be used to drive turbine. Fuel will be mixed with compressed air and burnt together when there is spark of flame from igniter.

3. Gas turbine

Gas turbine serves to change kinetic energy which is stored in the gases of combustion into useful energy.

4. Generator

Generator function is to change mechanical energy which is generated by gas turbine into electrical energy.

Gas Turbine Cycle

Gas turbine generator can be divided into some classifications. One of its classifications is based on cycle.  The following below is classification of gas turbine generator based on cycle:

1. Open Cycle Gas Turbine

In this cycle, gas from combustion process after passed and expanded through turbine generator, will be directly discharged into atmosphere. This installation has simple structure which consists of compressor, furnace and turbine as a drive of compressor and load. The structure and composition of gas turbine installation with open cycle can be seen in the Figure 1.

Figure 1: Open Cycle Gas Turbine Generator

Caption:

F          = furnace

C         = compressor

T          = turbine

G         = generator

2. Closed Cycle Gas Turbine

Gas turbine can be designed with closed system. In this cycle, the working fluid is not connected to surrounding atmosphere, thereby can be maintained its purity. This is advantageous in terms of prevention of damage caused by erosion and corrosion. This system can be also used in high pressure up to 40 atm like installation of steam generator but its work does not experience phase change. The scheme closed cycle gas turbine installation can be seen in Figure 2.

Figure 2: Closed Cycle Gas Turbine Generator

Caption:

F          = furnace

C         = compressor

T          = turbine

G         = generator

HE       = heat exchanger

Closed cycle gas turbine has more complicated construction because it need larger heater and inter cooler equipment before enter compressor. The following below is advantageous of closed cycle gas turbine:

-          For the same power, this turbine has smaller size

-          Can work at high pressure

-          More efficient in the use of fuel

3. Combined Cycle Gas Turbine

Because of the energy is lost along with waste flue gas, it has made several attempts to use flue gas by adding some kind of new process after additional equipment. Therefore, the energy which should be discharged can be utilized again for particular process and thus can improve efficiency of the system. But along with additional process and equipment, the required of investment cost will be higher. In terms of economic, gas turbine with combined cycle has some advantageous if gas turbine is performed in base/primary load and continuously.