The Design of a Multi Propeller Turbine Unit - Case Study Example

This research paper describes the design of a multi propeller turbine unit that is able to generate power in a low head power installation that is micro hydroelectric in nature. For the turbine to be appropriate for remotely located areas, the system needs to have a simple design. In this research, we shall explore the micro hydroelectric power system and analyze the most appropriate design turbine for low head generators. Introduction In this research, a small community requires the creation of a working concept of the renewable electricity of their area. What is required is a design of the power plant that is able to generate 2.5 Kilowatts of electric power. This power will mainly be supplied to their houses. The layout of the location is as shown in the figure below. Figure 1 Objectives 1. To create a conceptual design for an appropriate turbine in a low head micro hydroelectric project. 2. To determine the most appropriate turbine to be used to generate electricity for micro hydroelectric projects. Literature review 1. Micro hydroelectric power The process of generating electricity from water flowing in a stream I normally free from pollution and the energy produced is usually renewable [1]. The energy that is contained in falling water is usually lost through turbulence in the form of heat which is similar to raising the water temperature by a quarter of a degree for every 100 m of the height dropped. A turbine and generator then transforms a part of this energy into electric power which is eventually released into the environment as heat [8]. Micro systems produced power less than 10 kilowatts and have a simple, cheap and robust design. They are normally used to supply electricity to houses, small villages, small factories and farms. In remotely located areas away from the national grid, micro hydroelectric power can be very suitable [6]. The most important requirements of for micro hydroelectric power projects are safety, reliability and low cost of operation. They should have a high level of safety, very reliability and easy to maintain and repair. In order to satisfy the system, the micro hydroelectric power must be simple in design. Simplicity in design ensures that it operates at minimal cost [4]. An electronic controller can be added into the system in order to ensure that the turbine runs continuously at maximum power. This improves the simplicity and efficiency of the turbine. Some notable impacts that come up as a result of construction of Micro hydroelectric power plants include disruption of the stream flow and the site works involved during the construction phase [2]. In summary, a micro hydroelectric power consists of the following fundamental components [3]. a) The water turbine b) A control mechanism c) Electricity distribution lines In order to develop a micro hydroelectric power, the following features must be available. a) An intake to divert the flow of the stream. b) A pipeline to carry the water from the intake c) A fore bay tank and gravel trap to filter the debris that may damage the turbine. d) A penstock pipe to transport water into the powerhouse. e) A power house to protect the equipment and conversion of power into electricity. 2. The pelton wheel turbine Turbines are normally used to convert energy that is in the form of falling water into power. A turbine has the following parts [6]: a) An intake shaft: This is a tube that connects to the penstock and brings water into the turbine from the stream. b) Water nozzle: This is a nozzle that shoots a water jet into the system. c) Runner: This is a wheel which catches water a it is flowing and causes the wheel to spin. d) Generator shaft: This is a shaft connecting the runner and the generator. e) Generator: This is a component that creates electricity. f) Exit valve: This is a tube that allows water to return to the stream. g) Powerhouse: This is a shed that protects the water turbine. The selection of turbines is normally dependent on the characteristics of the site including: The available head, the rate of flow, the required power and the speed desired to run the generator. Turbines can be classified as either reaction or impulse [12]. The most appropriate turbine will be comprised of a runner that is drum shaped with two parallel discs that are connected together close to their rims by a series of cured blades. It also has a runner shaft that is horizontal to the ground [5]. The runner is comprised of a horizontal shaft with 18-37 blades. The edges of the blade are sharpened in order to reduce the resistance from the flow of water. 3. Low head generators Low head turbines normally have heads of approximately 2-30 meters and a flow rate of approximately 0.3-100 m3/s. there are basically two types of low head turbines which include the axial generators and the radial (centrifugal) generators [4]. Low head generators are most appropriate for landscapes that appear flat and gently sloping. Water in these landscapes flows at a low velocity and hence there is need to raise the level of water using a dam or weir. This helps to increase the velocity of the water as it flows along [7]. It is important to take the design of the system into consideration since the efficiency of the system is dependent on the head factors. An incorrect design can lead to the destruction of the net head as well as the performance of the project [9]. Low head generators are greatly affected by algae and grass in the water sources and unmonitored interference can lead to power loss. The runner diameter controls the risk potential of the plant and the higher the runner diameter, the higher the level of risk involved [9]. The efficiency of the generator can be maintained at approximately 35% of the design flow. Calculations involved Sketches of the turbine layout Figure 2: layout of the turbine NOTES The pipe is of 0.2m diameter The pipe should be laid within a reasonable level below the ground level The overall head is 5.2 m. Figure 3: Design of the pelton turbine Critical analysis The pelton wheel turbine normally consists of one or more stages that are located on the rear part of the generator. The turbines can either be classified as being either an impulse or reaction turbine. In the impulse turbines, the water normally expands in the nozzle and then it flows over the moving blades of the turbine [9]. The blades are then able to convert the kinetic energy into mechanical energy and also the flow of water to the next stage towards the exit. In reaction turbines, the pressure usually drops in the blades as the passage on the blade varies in a continuous pattern [2]. To understand how the pelton wheel turbine operates, it is important to define the velocity triangles. Velocity triangles Water that has an absolute velocity C1 and making an angle α1 usually enters the nozzle and leaves at a velocity C2 making an angle α2 with the axial direction of flow. The angle on the rotor blade is then chosen to suit the direction of the water β2. The water at this instant has velocity V2 and at an angle β3. The magnitude and the direction of the absolute velocity at the point of exit can be found by addition of vectors [6]. Figure 4: Velocity triangles From the velocity triangle, we have the flow equation Comparing the pelton wheel turbine to the other types of radial turbines, it is noted that the main difference is usually encountered in the manner in which the water flows through the compressor and the turbine. In radial turbines, the flow of the water at the inlet is radial to the shaft while the flow is parallel in pelton wheel turbines [10]. In centrifugal compressors, the water usually receives more energy as it accelerates at a continuously increasing diameter. The energy of the velocity is then converted into pressure when slowed down by the diffuser. The water provides an increasing pressure in pelton wheel turbine, a factor that is usually dependent on the number of stages and the number of intermediate stators in the turbine [11]. While designing the turbine stage, there are some restrictions that normally arise from the view point of the blade stress. The speed of the blade is limited by the blade stress. The performance of the turbine is controlled by the compressibility of the turbine and the stress achieved. They both limit the mass flow in the system [12]. The pelton wheel turbines are usually composed of compressors that are aero foil based and they rotate about a given axis. The operation principal of the compressor is that the fluid flows in a parallel plane to the axis of rotation. The pelton wheel turbine is capable of achieving a high pressure ratio on a single shaft. The amount of energy that is transferred in a single stage is usually limited [10]. The ease of putting the flows stages together can lead to pressure ratios of up to a ratio of 6:1 or even higher. The pelton wheel turbine can therefore consist of a number of stages. One of the stages normally consists of a row of rotor blades and it is then followed by a row of stator valves. A row of the outlet Guide valves is usually needed on the downstream in order for the system to carry the structural load. Efficiency of the pelton wheel turbine Two forms of efficiency can be realized in the pelton wheel turbine and these include: a) Isentropic efficiency: This is the ratio of the ideal specific work input to the actual definition of the isentropic system. The isentropic efficiency falls as the ratio increases. b) Polytrophic efficiency: This is the efficiency of an infinitesimal small compression step. The polytrophic efficiency falls as the ratio increases. Sizing parameters in pelton wheel turbine a) Various parameters are used to define the components and the functionality of the pelton wheel turbine and these include: b) The mean inlet mach number: The preferred range of this parameter is 0.4-0.6. c) Tip relative mach number: This occurs on the first stage inlet of the turbine. The absolute velocity of the water is usually considered as being axial and taken to be axial across the annulus. The preferred conservation values for this parameter are 0.8-1.3. d) The stage loading: This is the measure of the amount of work that is demanded of the compressor. e) The pressure ratio and the number of stages present: The achievable pressure ratio for any given number of stages. It is governed by good efficiency. f) Hade angle: This is the inner or outer angle annulus line to the axial of the turbine. It can go up to a maximum of 10 degrees but the most appropriate value is less than 5 degrees. g) The velocity and axial velocity ratio: The velocity ratio is the ratio of the axial velocity to the speed of the blade on the line of pitch. It ranges between 0.5 and 0.75. h) Aspect ratio: This is the ratio of the height to the valve. It ranges between 105 and 3.5 design values. i) Exit mach number and the swirl angle: This value of the mach number must be minimized in order to prevent loss of pressure on the downstream side. The exit mach number should not exceed 0.35 and the exit swirl angle should not be less than 10 degrees. Losses encountered in axial flow turbines a) Frictional losses: This occurs as a result of friction of the fluid in both stationary and rotating blades passages. The losses occur due to skin friction and the separation of the bounding layer. The amount of frictional loss is dependent on the factor of friction, the length of the flow passage and the square of the fluid velocity. b) Incidence losses: During the off design conditions, the direction of the relative velocity of the fluid at the inlet does not match with the blade angle of the inlet. The fluid does not enter the passage of the blade smoothly by gliding along the surface of the blade. c) Clearance and leakage losses: The clearance losses normally depend on the diameter of the impeller and the static pressure of the tip of the impeller. A larger diameter is very necessary for a higher speed at the periphery. Advantages of pelton wheel turbine a) The frontal area of the pelton wheel turbine is lower for a given mass flow and the pressure ratio. b) The weight is less because of the lower engine diameter. c) For the mass flow rates that are greater than 5 kg/s, the compressor has a much greater isentropic efficiency. d) The magnitude for the weight increases with the rate of mass flow. e) As a result of the difficulties experienced during the manufacturing process, there is a practical upper limit of about 0.8 m on the diameter of the impeller hence there is a capability of efficient mass flow and pressure ratio. Disadvantages of pelton wheel turbine a) The effect of roughness on the surface of the disc and the impeller of the turbine result into high friction which minimizes the efficiency of the turbine. b) In case of fluctuations in the flow of the water from the sources, the functionality of the turbine is greatly felt and hence requires a continuous supply of water. c) The effect of losses such as mechanical losses and power losses is a great problem within the turbine. Mechanical losses occur at the bearings and seals while power losses occurring as a result of the leakage of the fluid between the back surface of the impeller hub plate and the casing of the turbine. The pelton wheel turbine The pelton wheel turbine is composed of a runner that is drum shaped with two parallel discs that are connected together close to their rims by a series of cured blades. It also has a runner shaft that is horizontal to the ground [10]. The runner is comprised of a horizontal shaft with 18-37 blades. The edges of the blade are sharpened in order to reduce the resistance from the flow of water. The ends of the blades are usually welded onto disks in order to create a cage rather than using bars [9]. The blades in these turbines are normally trough blades. During the operations of the axial flow turbine, the jet of water that flows across the cross section of the rectangular section twice the blades of the rotor that are arranged at the periphery of the cylindrical runner. Water flow through the turbine usually strikes the periphery blades towards the center of the turbine [6]. Most of the kinetic energy that is generated is lost and this energy then crosses the runner from the inside position to the outside. The fact that the water goes through the runner twice usually gives additional efficiency to the turbine. While the water is leaving the runner, it usually clears the debris collected in the device [3]. Due to the low speed as the water flows, it is well suited for locations that have a low head. The effective head that usually drives the axial flow runner can be increased by inducing a partial vacuum within the casing. It is important that the valve is carefully designed in order to avoid situations of water holding back or the runner submerging into the water. The efficiency of the axial flow turbines is determined by the sophistication of the design [5]. Simple designs of the axial flow turbine usually achieve an efficiency of 60-70%. There are two major attractions that make the axial flow turbines very efficient. First it is designed to fit a number of heads and power ratios. Secondly, the turbines have simple fabrications that are easily integrated into systems. The housing of the turbine is made of mild steel plates. This gives the turbine a design and layout that enable the turbine to minimize the effects of vibrations and noise levels [9]. The control of the flow is normally done using a guide vane. The shaft of the guide vane is normally parallel to the shaft of the rotor. The guide vane normally guides water to the runner and usually controls the amount of water that enters the runner. The most appropriate design speed of the turbine is normally dependent on the power rating of the turbine, the type of the turbine and the site head [6]. Conclusion In conclusion, the research has made is clear that there is an economical way of generating electric power in areas that are located in remote regions very far away from the national grid. With the availability of a source of water, an axial flow turbine can be used as a low head generator to generate electricity for use in the farms and houses. The design of the turbine is very important and will determine the efficiency of the whole plant. Recommendation In order to improve the flow of water, it is recommended that the roughness that occurs on blades resulting into friction be reduced. In order to lessen the friction and increase the efficiency of the turbine, more investment in the pressure die in the casting of the turbine should be emphasized. Welding of the blades to the runners should also be done using the electron beam and the laser beam to reduce the friction. It is also recommended that the axial runner and the blades be cut using the CNC laser machine or the CNC plasma in order to reduce the jagged points resulting from other methods of cutting hence increasing the roughness of the surface. To improve the flow of water, it can also be recommended that the surface area of the casing is increased to reduce collision and friction within the turbine hence increasing the efficiency of the turbine. It would also be prudent to maintain a given level of water in the stream in order to maintain a steady flow of water in the turbine. References [1] Renewable Energy Policy Network for the 21st Century (REN21), Renewables Global Status 2005 Update, Paris, 2005. [2] White, David, Use of Low Head Hydroelectric Generators in Wastewater Treatment Facilities, the Cooper Union for the Advancement of Science and Arts (2010). [3] Balino, Design Pelton Bucket Runner. viewed on 13th February 2015. [4] John F Douglas, Janus M Gaiseric, John A Swaffied, Lynn B Jack, Fifth edition. Fluid Mechanics (2005). [5] R.K. Bansal, A textbook of fluid mechanics and hydraulic machines: (in S.I. units), (2005). [6] HydropowerAxco Generators for hydropower applications ( 2012), retrieved from viewed on 13th February 2015. [7] John F Douglas, Janus M Gasiorek, John A Swaffied, Lynn B Jack. Fifth edition. Fluid Mechanics (2005). [8] Micro-hydro Design Manual, A Harvey & A Brown, ITDG Publishing, 2002. [9] Small Hydro as an Energy Option for Rural Areas of Perú by Teodoro Sanches ITDG Latin America: retrieved from http://www.itdg.org.pe/Programas/energia/articulos/shaaeofra.pdf viewed on 13th February 2015. [10] Greenstone (2012), Community Development Library, Pelton Turbine viewed on 13th February 2015. [11] Development book shop (2012), Micro-hydro Pelton Turbine Manual viewed on 13th February 2015. [12] Bright hub (2012), Hydraulic Turbines: Francis Turbine viewed on 13th February 2015. Read More