World Academy of Science, Engineering and Technology 83 2013 Improving the Efficiency of Pelton Wheel and Cross-Flow Micro Hydro Power Plants Loice K Gudukeya, Charles Mbohwa Abstract—The research investigates hydro power plant II. RESEARCH OBJECTIVE efficiency with a view to improving the power output while keeping To investigate options for increasing the turbine efficiencies the overall project cost per kilowatt produced within an acceptable range. It reviews the commonly used Pelton and Cross-flow turbines of Pelton wheel and Cross-flow turbines targeting the which are employed in the region for micro-hydro power plants. improvement of the overall micro hydro system efficiency Turbine parameters such as surface texture, material used and while keeping the project financially feasible. fabrication processes are dealt with the intention of increasing the efficiency by 20 to 25 percent for the micro hydro-power plants. III. OVERVIEW OF MICRO-HYDRO POWER PLANT Hydro power is the harnessing of energy from falling water, Keywords—Hydro, power plant, efficiency, manufacture. such as water falling through steep mountain rivers. The energy in flowing water is converted into useful mechanical I. INTRODUCTION power by means of a water wheel or a turbine [3]. The ICRO-HYDRO power plants are an attractive option for mechanical power from the turbine can be converted into Mproviding electricity in off grid areas of the country. electricity using an alternator or a generator. The Pelton and Cross-flow turbines are predominantly used Hydro power systems that are classified as “micro” have the for these projects as they are cheaper to construct for this form power generation capacity of less than 100kW. They are of renewable energy. Current level of efficiency is estimated relatively small power sources that may be used to supply to be 60%, thus allowing for improvements on the overall power to a small group of users or communities, who are efficiency of whole micro-hydro system. At 60% turbine independent of the general electricity supply grid [5]. efficiency micro-hydro schemes seem to be underutilising A micro-hydropower system (MHS) has the following resources. Communities are benefitting by having their components [6]: business centres, clinics and schools powered. However more • A water turbine that converts the energy of flowing or electrical power can be attained without increasing the falling water into mechanical energy. This drives an resources but by only increasing the turbine efficiencies [4]. alternator, which then generates electrical power This can ensure that more households in the catchment area • A control mechanism in the form of electronic load get more than just lighting, but are able to use other devices controller to provide stable electrical power. such as refrigerators, stoves etc in their houses. • Electrical distribution lines To develop an MHS the following features are needed as given in Figures 1 and 2 [7]: • an intake or weir to divert stream flow from the water course; • a headrace, the canal or pipeline to carry the water to the forebay from the intake; • a forebay tank and trash rack (gravel trap) to filter debris and prevent it from being drawn into the turbine at the penstock pipe intake; • a penstock (pipe) to transport the water to the powerhouse. This may be set up above the ground surface or underground depending on the topography of the site. At rocky sites, penstocks are supported above ground on concrete blocks called Anchors or Saddle (Pier) supports. The saddle supports are provided along the straight length at regular intervals and anchors are provided at horizontal L K Gudukeya is with the Mechanical Engineering Department, University and vertical bends along the alignment of the penstock. of Zimbabwe, Box MP 167, Mt Pleasant, Harare (phone: 00263 773 286 349; These are designed to carry the thickness and the email: [email protected]). C Mbohwa is with the Quality and Operations Management Department, diameter of the penstock University of Johannesburg, Auckland Park Kingsway, South Africa (phone: 0027 78 207 1516; email: [email protected]). 1047 World Academy of Science, Engineering and Technology 83 2013 • a powerhouse, being the building that accommodates and protects the electro-mechanical equipment, (turbine and generator), that convert the power of the water into electricity; • a tailrace through which the water is released back to the river or stream without causing erosion; Fig. 3 Concept of the head [9] No power conversion system delivers 100% useful power as some power is lost by the system itself in the form of Fig 1 Components of a micro-hydro power system [8] friction, heating, noise etc. The efficiency of the system needs to be taken into account, since all the equipment used to convert the power from one form to another do so at less than 100% efficiency. Fig 4 shows the power losses and efficiencies within a micro-hydro power system [5]: Fig 2 The inside of a power house [8] A. Power Generation A hydro scheme requires water flow and a drop in height (head) to produce useful power (Fig 3). It is a power conversion system, absorbing power in the form of head and flow, and delivering power in the form of electricity. To measure the potential power and energy on a site the first step is to determine the flow rate and the head through which the water falls. These two parameters are defined thus: • Flow rate (Q) is the quantity of water flowing past a point at a given time. Q is measured in cubic metres per second (m3/s) • Head (H) is the vertical height from the level where the water enters the penstock at forebay tank to the level of turbine centerline. The typical unit of measurement is metres (m). 1048 World Academy of Science, Engineering and Technology 83 2013 where Pactual is the actual power produced (kW) 3 ρwater is the density of water (kg/m ) Q is the flow in the penstock pipe (m3/s) g is the acceleration due to gravity (9.81m/s2) hgross is the total vertical drop from intake to turbine (m) x x x x x The turbine efficiency is typically the lowest of the component efficiencies [5]. Hence it has the highest chances of being improved and once this has been done, the overall system may be improved. B. The turbine The turbines convert energy in the form of falling water into rotating shaft power. They basically consist of the following components [5]: • intake shaft - a tube that connects to the piping or penstock which brings the water into the turbine • water nozzle - a nozzle which shoots a jet of water (Impulse type of turbines only) • runner - a wheel which catches the water as it flows in causing the wheel to turn (spin) • generator shaft - a shaft that connects the runner to the generator • generator - a unit that creates the electricity • exit valve - a tube or shute that returns the water to the stream from where it came • powerhouse - a small shed or enclosure to protect the Fig. 4 Power losses and efficiencies in a micro-hydro system [5] water turbine and generator from the elements Typical system component efficiencies are shown in Table Selection of the turbine depends on the site characteristics, I: the main factors being the head available, the flow rate and the power required as well as the speed at which the turbine is TABLE I TYPICAL SYSTEM COMPONENT EFFICIENCIES [5] desired to run the generator [4]. All turbines have a power- System Component Efficiency speed characteristic and an efficiency-speed characteristic. For a particular head they will tend to run most efficiently at a Canal 95% particular speed, and require a particular flow rate. Penstock 90% Water turbines are often classified as being either Reaction Turbine 60 -80% or Impulse turbines. In a reaction turbine, the runners are fully immersed in water and are enclosed in a pressure casing. In an Generator 85% impulse turbine, the runner operates in air and is turned by one Step-up and down 96% or more jets of water which make contact with the blade. The transformers water remains at atmospheric pressure before and after Transmission 90% making contact with the runner blades. In this case, pressure energy is converted into the kinetic energy of a high speed jet of water in the form of a nozzle. Then it is converted to To determine the actual power output from the MHS the rotation energy in contact with the runner blades by deflection following equation that includes the system efficiency factor, of the water and change of momentum. The nozzle is aligned ηtotal is used: so that it provides maximum force on the blades. An impulse 1049 World Academy of Science, Engineering and Technology 83 2013 turbine needs a casing only to control splashing and to protect A. Characteristics of Pelton Wheel Cups/Buckets against accidents. The central part of each bucket has a splitter ridge. This is Impulse turbines are cheaper than Reaction turbines the part that the water jet hits first. This splitter ridge should because no specialist pressure casing and no carefully be sharp and smooth, so that it cleanly splits the jet into two engineered clearances are needed. These fabrication halves. The force on the bucket comes from it catching the constraints make impulse turbines more attractive for use in water and taking out as much of the water’s momentum as it micro-hydro systems. Other advantages of using impulse can. The more the water’s momentum absorbed by each turbines over reaction turbines in micro-hydro systems are that bucket, the better the turbine efficiency. Getting a good torque impulse turbines are more tolerant to sand and other particles from this force requires the force to act at as large a radius as in the water.