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Improving the Mechanical Efficiency of a Pelton Wheel Impulse Turbine at Low Head During Operation

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Improving the Mechanical Efficiency of a Pelton Wheel Impulse Turbine at Low Head During Operation International Journal of Engineering Research and Technology. ISSN 0974-3154, Volume 13, Number 7 (2020), pp. 1508-1515 © International Research Publication House. http://www.irphouse.com Improving the mechanical efficiency of a Pelton Wheel impulse turbine at Low head during operation P. B. Sob Department of Mechanical Engineering, Faculty of Engineering and Technology, Vaal University of Technology, Vanderbijlpark 1900, Private Bag X021, South Africa. Abstract proportional to the height and amount of water level [1-13]. This water must strikes the buckets at very high heads for the In this paper an approach to improve the performance of a buckets to move at a very high speed for high electrical Pelton wheel impulse turbine at very low energy head was efficiency to be generated during operation. This water flow investigated for efficient and stable power generation during from the dam through the penstock into the jet which strikes the electrical power generation. During operation gravitational vanes that rotates the turbines and mechanical energy is energy of the elevated water into mechanical energy is being converted into electrical energy [1-7]. The efficiency of the converted into electrical energy by water that strikes the vanes system is usually very high if the water level in the dam is very which rotate the runner for an electromagnetic force (emf) to high and very low if the water level in the dam is very low [1- be generated which produced electricity. This require a process 10]. Therefore the energy generated depend on the water level of kinetic energy produced by the water jet which is directed in the dam, which is directly related to the height of water in the tangentially to the buckets of the Pelton wheel and usually the jet dam which translate to the potential energy and kinetic energy energy is used to propelled the rim of the buckets for power [1-17]. generation. Usually this process is more convenient for high water head application as it required high kinetic and potential When the moving fluid acts on the van blades there is a reaction energy from the source during operation due to high energy of flow from the van blade and this imparted the rotational heads during operation. For over the year’s hydroelectric power energy of the rotor which moves in a magnetic field from the generation is reported to have low energy and efficiency during impulse turbine [1-13]. In the current study the water from the low heads since the buckets speed is very low and the kinetic dam must flow at a very low head to the impulse turbine for the energy is very low to power the wheel. Most often materials Pelton wheel to generate electricity [1-13]. The Pelton wheel is used to design the vanes and runner are very heavy which designed as a tangential free flow jet impulse turbine which increases the weight of the vanes and the runner during uses a jet nozzle usually under very high heads to hit the vanes operation. For the runner and the vanes to rotates faster even at that rotates at a very high speed for the relevant kinetic energy low head the weight of the vanes and runner must be minimise to be produced to power the shaft of the turbine. The jet for the runner and vanes to have the desire high speed even at momentum striking the runner is affected and the velocity of very low energy head during operation. In the current study the water at the tangential point of entry is assumed to be zero and weight of the runner and vanes are reduced to test the energy all the produced kinetic energy are converted to mechanical produced at low head and the strength of the vanes and runner energy to produce high mechanical, hydraulic and over tested if it was carry the load at the reduced weight during efficiency. This require velocity of the turbines which is operation. The discharge produced by the jets was able to affected by the impact of the jet discharge and the weight of the power the vanes and the runner at very low head. Consequently, vanes and runner that have been reduced for efficient speed this designed Pelton wheel turbine will open a new horizon for even at very low head of water supply. hydro power plants in central and West Africa during dry The Pelton wheel experience great velocity along the turbine season when energy generation is very low, since sufficient reaction since the impulse turbines operating principle is based energy can be generated even at very low energy head due to on the concept of velocity due to direction of fluid from the jet. the low weight of the vanes and runner. During operating the turbine rotates and generate energy that Keywords: Pelton-wheel, performance, kinetic energy, drive the velocity of water in the jet from the nozzle into the potential energy, gravitation and Jet-propulsion buckets which rotates the shaft for electricity to be generated. In most of the current study the weight of the vanes and runner are very heavy. This heavy weight can only rotate faster if water 1. INTRODUCTION strikes the vanes at very high head. Therefore for low energy head the power generated is very low since the runner and Hydroelectricity is very common in West and Central Africa vanes cannot rotates to generate the desire energy. The when energy of a falling water strikes the buckets which rotates characteristic describes newton’s second law of motion as the a runner and convert mechanical energy into electrical energy turbine spins and remove kinetic energy of the fluid. There is a [1-15]. Usually the amount of stored hydroelectricity is directly change in pressure due to change in velocity head by the 1508 International Journal of Engineering Research and Technology. ISSN 0974-3154, Volume 13, Number 7 (2020), pp. 1508-1515 © International Research Publication House. http://www.irphouse.com accelerating fluid through the nozzle. This has to deal with gives the relationship between mechanical power produced at force which deals with weight of the moving body. The high the turbine shaft in watts (P), the hydraulic efficiency of the pressure of water is channel through the penstock to the nozzle turbine (Ƞ), the density of the water in kg/m3 (ρ), the which strikes the vanes to rotate the shaft. Due to modification acceleration due to gravity in m/s2 (g), the volume flow rate of the Pelton wheel for over the years by designing the buckets passing through the turbine in m3/s (Q) and the effective to release water that will increase the speed of the runner to pressure head (H) of water across the turbine in meter which produce power [7-17] in conventional Pelton wheel. The jet corresponds to the head of water supply to power the runner for release water which strikes the buckets or vanes and the vanes mechanical power to be generated is given as rotate the runner which convert the water energy into mechanical energy. The vanes are design that they deflect water 푃 = ƞ휌푄푔퐻 (1) away from the jet and for over the year’s improvement on the Pelton wheel has been on the vanes design to improve the efficiency of the jet Pelton wheel during operation [1-8]. From equation [1], the mechanical power produced at the shaft Usually the nozzle design in the penstock is to increase the depend greatly on effective pressure head (H). The model of the discharge of water which increases the kinetic energy of water effective pressure head in a turbine can be derive from the net in the penstock which increase the speed of vanes to increase head or the effective head available at the inlet of the turbine. the speed of the runner [1-10]. Therefore this is an indication Generally as water flow from the head race to the turbine, a loss that increasing the design of the Pelton when and vanes always of head due to friction between the water and penstock usually increases the efficiency of the impulse turbine increases occurs. There are other minor energy losses due to bending, pipe whenever there is an increase in speed of the runner. Therefore fittings and entrance losses of the penstock which also affect the vanes must rotate at high speed if the runner must run at the net head but these minor losses are ignored in the expression fast speed to increase energy and the system efficiency. From of the net head given as the expression of the hydraulic system 푃 = ƞ휌푄푔퐻, where, P is the mechanical power produced at the turbine shaft in watts. Ƞ is the hydraulic efficiency of the turbine. ρ is the density of the (2) water in kg/m3 . g is the acceleration due to gravity in m/s2 . Q is the volume flow rate passing through the turbine in m3/s. H is the effective pressure head of water across the turbine in Where H is the net head in meter, 퐻푔 is the gross head in meter, meter which corresponds to the head of water supply to power 푓 is the coefficient of friction L is the length of the penstock in the runner for mechanical power to be generated [1-17]. It must meter, 푉 is the velocity of flow in penstock in m/s and 퐷 is the be recall that in any Pelton wheel the mechanical power can be diameter of the penstock in meter. This net head affect the all improve or modify modelling the given equation parameters by efficiency of the Pelton wheel such as hydraulic efficiency, means of changing Ƞ, Q and H inputs as g and ρ are constants volumetric efficiency, mechanical efficiency and overall parameter during the operation of the Pelton wheel when power efficiency.
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