Performance of Pelton Turbine for Hydroelectric Generation in Varying Design Parameters
Water power is a renewable energy source which has great potential in replacing fossil energy for generating electricity. The aim of this research is to analyze the influence of vertical distance of water source (water head) and nozzle diameter on the electrical power generated by Pelton turbine. It used Pelton turbine type with 22 buckets (vanes) which employed a PMG 200 watts generator with 1 : 2 pulley transmission system. Four different values of nozzle diameter and three different values of water head were chosen as the design parameters of the turbine. The electrical power was measured in three replications for each combination of the design parameters. The research showed that water head and nozzle diameter significantly affect the power generated by the Pelton turbine. The higher the water head from the surface, the more power generated. It was found that the electric power linearly increases with the increasing of nozzle diameter. However, it reaches the peak in 9 mm nozzle diameter and is getting lower in a larger diameter. The highest electric power of 16.89 watt is observed by adjusting the water head on 4.6 m with 9 mm nozzle diameter. Those design parameters are able can produce a rotation speed at 320 rpm in the generator. By identifying the appropriate parameters, it is possible to have more power generated by the water turbine used for hydroelectric power generation plant.
Identifying the optimum water flow and number of buckets in laboratory-scale Pelton turbine using Central Composite Design (CCD)
Pelton turbines were a type of water turbines which, principally, turned the water kinetic energy into electrical energy. The working principle of Pelton turbines was to utilize water power to produce turbine power. This paper reported the optimization of design parameters for laboratory-scale Pelton turbine using the central composite design (CCD) as the response surface methodology (RSM). This research obtained the quadratic polynomial equations for the turbine power. The independent variables were water flow (0.11 m3/s; 0.13 m3/s; 0.15 m3/s) and number of buckets (13; 15; 17 pieces). The results, based on the RSM, showed that the most influential parameter on the turbine power was the number of buckets. The optimum turbine power was 24 kW on 0,16 m3/s water flow and 18 buckets of RSM.
Jet quality and Pelton efficiency
The paper gives a short overview on contents and goals of a research project on jets of Pelton turbines performed by the Hochschule Lucerne and the Ecole Centrale de Lyon in collaboration with ANDRITZ Hydro. Effects governing the quality of jets in Pelton plants are discussed, such as: the jet dispersion, which is defined by the widening of the jet with distance from the nozzle and includes the air water mixture on the jet’s surface, the jet deviation, which is defined by the deviation of the jets centerline from the theoretical axis, deformation described by the out of roundness of the jet, rotation of the jet, the turbulence level, secondary flows leading to streaks on the jet, entrapped air bubbles causing sudden expansions on the jet surface and finally droplets or splashing water impinging on the free jet in the housing of the turbine. With case studies in power plants the potential for improvement of efficiency is pointed out. The combination of thermodynamic efficiency measurement, flow visualization and CFD simulations allowed in the investigated cases good interpretation. The images taken of the jet in the power plants allowed quantitative evaluation of the jet diameter and its dispersion and eventually deviation. A clear relationship of increased jet dispersion and decreased efficiency could be found. Furthermore, a direct correlation of upstream bends and jet dispersion was found. This effect is induced by secondary flows. 1. Importance of jet quality for Pelton turbines Pelton turbines belong to the class of action turbines. The kinetic energy of the jet is reduced to a minimum when passing through the buckets of the runner. This deceleration occurs within a short distance within the buckets. The pressures needed for the deceleration drive the runner and effect the shaft torque for turbine operation. The order of magnitude of the deceleration may be typically 10 000 m/s or 1000 times the acceleration due to gravity. The long experience in developing Pelton turbines led to an optimization of this process and maximum efficiencies close to 92% and under favorable conditions even higher efficiencies are reached. Necessary condition for such high efficiencies is a high quality jet impinging on the Pelton buckets. The jet quality can be described with the uniformity of the velocity distribution within the jet, the absence of secondary flows and the compactness of the jet. Villacorta [1] was the first to measure velocity distributions within the jet and to investigate the jet dispersion with laboratory experiments and field tests. Recently pressure distributions within the buckets were analyzed by Perrig [2] Switzerland's hydro machinery park encompasses more than 196 Pelton turbines with power of more than 5 MW installed in a total of 107 hydro power plants. About 70 percent of hydro electricity generated in plants with storage capacity is produced with Pelton turbines, machines which are also especially suited for peak power generation (network regulation). With respect to the total hydroelectric generation in Switzerland Pelton turbines have a share of about 40 percent. 92 of the 107 Swiss Pelton hydro power plants were built before 1970. Many of them did undergo major refurbishment in the meantime mostly only runner and control system replacement. In order to minimize the investment costs, housings and injectors of the turbines are usually not replaced. Thus, it can be stated that in most of the plants, the jet quality corresponds to the state of the art in Pelton design, before Villacorta [1] wrote his thesis in 1972. Before initiating the project, which will be described in the following, strong indications was found that in a great number of Pelton plants, efficiency gains in the order of 0.5 to 1 percent could be achieved by improving the jet quality as it is described in case studies by Staubli [3, 4] and Peron [5]. The state of the art of today’s procedures for Pelton rehabilitation projects focusing not only on runner and control system but also on jet quality is described by Parkinson [6] 2. The project and its topics In order to consolidate existing knowledge on jet properties, to acquire new understanding and to find ways to predict the quality of jets, a research project was initiated by ANDRITZ Hydro, in collaboration with the Hochschule Luzern, Switzerland, and the Ecole Centrale de Lyon, France. External funding of the research activities was provided by Swisselectric-Research. The project encompasses work packages with field tests, laboratory measurements, CFD and theoretical studies. The theoretical background showed to be rather complex since not only local Froude, Weber and Reynolds numbers have influence on the jet properties, but also the entire upstream flow history. In the upstream flow, secondary flow due to bends, bifurcations and elements within the flow (e.g. internal or external servomotors for the needle operation within the injector) hardly decay due to the usually high Reynolds number occurring in hydro power plants. These secondary flows influence the topology of the jet and lead to deformation (surface), deviation (jet axis) and rotation of the jet's deformations. Secondary flow also causes increased dispersion of the jet diameter. The closer the elements are to the nozzle exit, e.g. needle and mouth piece, the more important their influence on the jet development becomes. Old designs of injectors often show smaller needle and mouth piece angles compared to more recent designs, an indicator that the jet quality might not be optimum. Computational Fluid dynamics (CFD) shows to be a valuable tool for prediction of flow disturbances and secondary flows. With free surface models even jet deviations and surface deformations can be predicted. During the project, a serie of power plants were analyzed using CFD simulation. Upstream unsteadiness of the flow reflects on the jet surface. Such unsteadiness may be of stochastic nature, e.g. due to the flow turbulence generated in the penstock or due to deterministic phenomena such as pressure wave propagations or periodic vortex formations at cavities, bifurcations or support structures (Kármán vortices). Fig 1: Definition of dispersion and deviation The jet dispersion is defined by the increase of the jet diameter with distance from the nozzle. Included in the definition of the jet diameter is the air water mixture surrounding the core of the jet. The deviation is defined by the difference of the jet centre line from the theoretical axis. Once the jet has exited the nozzle, further effects may negatively influence the jet and its dispersion, such as: ventilated droplets or water centrifuged from the bucket cut outs, splashing water being diverted within the housing or from deflectors onto the jet. Also interference with other jets or injectors may occur. Furthermore, highly pressurized air bubbles may eventually be enclosed in the oncoming water. When expanding after the nozzle exit this will lead to a kind of explosions observed on the jet surface. During the starting process of Pelton turbines such enclosed air and the resulting explosions are quite common. The effect of larger amounts of air can be easily heard outside of the housing during start-ups. During normal operation this noise disappears. 3. Investigated Pelton plants In the Swiss power plants listed in table 1, thermodynamic efficiency measurements and jet visualizations were performed during the project. The jet dispersion and deviation could be quantitatively determined from the visualization in most of the cases. Clear correlation between turbine efficiency and jet quality was observed. Hydro Power Plant (HPP) Type Power Discharge Head Theoretical jet diameter (at nominal and 100% needle openings) [MW] [m/s] [m] [mm] Sedrun twin horizontal turbines, 2-nozzles each 51.6 10.1 588 182 Rabiusa-Realta horizontal, 2-nozzles 13 3 498 147 Soazza horizontal, 2-nozzles 40 7 704 195 Table 1: Main data of the investigated power plants The installation of equipment for prototype visualization is delicate since the best positioning of camera and lighting instrumentation can not be found on the basis of trial and error but must be based on experience due to the inaccessibility of the equipment. Furthermore, the mechanical forces of possible water impingement on the camera and lighting instrumentation requires a rigid installation. Housings for camera and lights have to be waterproof and measures must be taken to avoid condensation building up on the lenses. In order to achieve acceptable image quality under the adverse circumstances present in the housing of an operating Pelton turbine, special equipment is necessary. The camera housing and the stroboscopic lights were mounted within protecting housings in the shelter of the injector and cut-in deflector and could be adjusted at different distances from the nozzle exit with a stepping motor. All equipment could be operated from outside of the housing. 3.1 HPP Sedrun Figure 2 shows the result of the thermodynamic efficiency measurements which have been performed in the HPP Sedrun, with twin horizontal turbines, 2-nozzles each. The blue curve is representing the efficiencies measured for the 2 nozzle operation and the green and red points measurements with only one nozzle in operation. The needle opening of the one nozzle operation corresponds approximately to the opening at 0.9 of the maximum turbine output with two nozzles in operation. Thus efficiencies of 0.45 output (1 nozzle) have to be compared with the efficiency at 0.9 output (2 nozzles) and we observe that the green points of the lower injector reach almost the efficiency of the two nozzle operation. In contrast the upper injector lies about 0.5 percent lower. 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Turbine output [-] Tu rb in e ef fic ie nc y [-] 0. 5% 2-nozzle operation 1-nozzle operation, upper injector 1-nozzle operation, lower injector Fig.2: Turbine efficiency measu