Introduction

water turbines

Water turbines are basically fairly simple systems.  They consist of the following components:

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 generator shaft - a steel shaft that connects the runner to the generator generator - a small electric generator that creates the electricity exit valve - a tube or shute that returns the water to the stream it came fro powerhouse - a small shed or enclosure to protect the water turbine and generator from the 

elements

Impulse vs. Reaction Turbines

Water turbines are also often classified as being either impulse turbines or reaction turbines.  In a reaction turbine the runners are fully immersed in water and are enclosed in a pressure casing.  The runner blades are angled so that pressure differences across them create lift forces, like those on aircraft wings, and the lift forces cause the runner to rotate.

In an impulse turbine the runner operates in air, and is turned by one or multiple jets of water which make contract with the blade.  A nozzle converts the pressurized low velocity water into a high speed jet much like you might use with a garden hose nozzle. The nozzle is aligned so that it provides maximum force on the blades. 

Types of Turbines

There are many kinds of micro hydro turbine designs. Typical microhydro generators have outputs of 10 kilowatts (kW) or less and can generate either DC or AC current depending upon the design. You will often hear water turbines referred to as either Pelton or Turgo turbines.  These terms have to do with the structure of the water wheel inside the turbine. 

Turgo Turbines

Pictured here is a Turgo style wheel. A Turgo turbine is an impulse type of turbine in which a jet of water strikes the turbine blades.  The structure of a Turgo wheel is much like that of airplane turbine in which the hub is surrounded by a series of curved vanes. These vanes catch the water as it flows through the turbine causing the hub and  shaft to turn.  Turgo turbines are designed for higher speeds than Pelton turbines and usually have smaller diameters.

Pelton Turbines

A Pelton turbine is also an impulse turbine but in this type of turbine the hub is surrounded by a series of cups or buckets which catch the water. The buckets are split into two halves so that the central area does not act as a dead spot incapable of deflecting water away from the oncoming jet.  The cutaway on the lower lips allows the following bucket to move further before cutting off the jet propelling the bucket ahead of it.  This also permits a smoother entrance of the bucket into the water jet. 

Cross-Flow Turbines

A cross-flow turbine, also sometimes called a Michell-Banki turbine (from the name of the manufacturer) is a turbine that uses a drum shaped runner much like the wheel on an old paddle wheel steamboat.  A vertical rectangular nozzle is used with this type of turbine to drive a jet of water along the full length of the runner. One advantage of this type of turbine is that it can be used in situations where you have significant flow but not enough head pressure to use a high head turbine. 

Francis Turbine

The Francis type of turbine is a reaction type of turbine in which the entire wheel assembly is immersed in water and surrounded by a pressure casing.  In a Francis turbine the pressure casing is spiral shaped and is tapered to distribute water uniformly around the entire perimeter of the runner.  It uses guide vanes to ensure that water is fed into the runners at the correct angle.

Propeller Turbine

A propeller turbine is just what its name implies.  It uses a runner shaped just like a boat propeller to turn the generator.  The propeller usually has six vanes.  A variation of the propeller turbine is the Kaplan turbine in which the pitch of the propeller blades is adjustable.  This type of turbine is often used in large hydroelectric plants.  An advantage of propeller type of turbines is that they can be used in very low head conditions provided there is enough flow.

Selecting the Best Type of Turbine

Which type of water turbine is best for a particular situation often depends on the amount of head (water pressure) you will have in your location and whether you want to suspend the turbine in the water  (reaction) or whether you want to use jets of water (impulse).  By looking at these factors together you can get some indication of what type of turbine design will work best:

  micro turbines.doc (Size: 397 KB / Downloads: 1115) 

  micro turbines.ppt (Size: 524 KB / Downloads: 1454) 

  What is a Microturbine.doc (Size: 693 KB / Downloads: 839) INTRODUCTIONOne is emerging from perhaps the most deliberate and least colourful engineering fields of all: gas turbine engineering. Gas turbines are internal combustion engines, like the ones that drive cars, except that they use a rotating shaft or rotor instead of pistons "reciprocating" in cylinders. This makes their operation smooth and steady, which lowers maintenance costs and increases reliability. Though they became practical only sixty years ago, today gas turbines are one of the keystone technologies of the civilization. As jet engines, they deliver most of our air transport, while stationary gas turbines are responsible for an increasing fraction of our electrical power generation.Partly because of this critical role, gas turbine engineers tend to innovate one tiny step at a time. In a field where liability exposures and development costs both can run into nine and ten figures, any kind of sweeping enthusiasm makes people nervous. Still, that doesn't mean engineers can't dream on their own time. In the spring of 1994, when a MIT turbine engineer named Alan Epstein found himself sitting in a jury pool, he started to think about what it would take to build the smallest possible jet engine. He concluded that in theory the device could be shrunk a lot, perhaps to the size of a collar button. If you attached a microgenerator to the turbine, essentially creating a tiny power plant, the combination would act like a battery, making power at twenty to fifty times the rate of anything you could get at the hardware store. (Because there is much more energy per gram in burning hydrocarbons than in the electrochemicals that usually go in batteries.) Depending on how much fuel came with the turbine, a laptop might run for months on a single charge; a cellphone, for half a 

year. Given the insatiable appetite our portable gizmos have for batteries, the microturbine project suddenly became very interesting. The U.S. Army, which badly wants to reduce the weight carried by their "soldier systems", agreed to write the checks. By 1995 the microturbine project was humming along. Unlike a conventional gas turbine design job, where each member is a world-class expert on one (but only one) phase of the process, all the researchers on this project were starting from the same place: how to make engines less than a hundreth the size of a conventional turbine design. For instance, for a gas turbine to work well, the tips of its rotors have to turn at about the speed of sound, or five hundred meters a second. The smaller the diameter of a turbine, the faster the rotor has to spin to move its tips at that speed. A conventional jet engine can get there with a few tens of thousands of revolutions a minute. The microturbine had to do much better: closer to two million rpm, or twenty thousand revolutions a second. This awe-inspiring number raised all kinds of questions. For one: How was the rotor going to be attached The usual solution to this problem would be some sort of bearing, but what material could handle that level of abuse And even if such a substance existed, how would you make the bearings or keep them in place Eventually, after many failures, the team discovered clever ways for the rotor to use its blistering speed to lift itself up during operation, essentially making it fly in place, so that no material bearings were needed. The project required such innovations constantly, radical ideas too new for anyone to be expert on them. Over the next seven years the project made amazing progress, considering that designing a conventional jet engine usually takes five years. Today actual working models exist, though the microturbine is not quite ready to be handed over to a manufacturer. (One of the remaining problems is exactly how to cool the exhaust to a level comfortable for consumer use .The success of the microturbine project has inspired a whole R&D sector in micropower devices. The Defense Department alone is funding well over a dozen projects, from microfuel cells and micropiston engines to microrockets. The University of Wisconsin is even looking at a micronuclear reactor. (One of the attractions is that tiny jet engines deliver ten times the thrust per unit weight of a conventional turbine, which means the huge cost airplanes now pay to haul their engines around might be radically reduced.) LITERATURE REVIEWTHERMODYNAMIC CONSIDERATIONSIt is influenced by fluid and structural mechanics, and by material, electrical and Thermal power systems encompass multitude of technical disciplines. The architecture of the overall system is determined by thermodynamics while the design of the systemâ„¢s components fabrication concerns, the physical constraints on the design of the mechanical and electrical components are often different at micro scale than at more familiar sizes so that the optimal component and system designs are different as well. Most thermodynamic systems in common use today are variations of the Brayton (air), Rankine (vapour, Otto, or Diesel cycles. The Brayton power cycle (gas turbine) was selected for the initial investigation based on relative considerations of power density, simplicity of fabrication, ease of initial demonstration, ultimate efficiency, and thermal anisotropy. A conventional, macroscopic gas turbine engine consists of a compressor, a combustion chamber, and turbine (driven by the combustion exhaust) that powers the compressor, and can drive machinery such as an electric generator. The residual enthalpy in the exhaust stream provides thrust. A macro scale gas turbine with a meter diameter air intake generates power on the order of 100 MW. Thus, tens of watts would be produced when such a device is scaled to millimeter size if the power per unit 

of airflow is maintained. When based on rotating machinery, such power density requires (1) combustor exit temperatures of 1300-1700 K; (2) rotor peripheral speeds of 300-600 m/s and thus rotating structures centrifugally stressed to several hundred MPa (the power density of both fluid and electrical machines scales with the square of the speed, as does the rotor material centrifugal stress); low friction bearings; high geometric tolerances and tight clearances between rotating and static parts; and thermal isolation of the hot and cold sections. These thermodynamic considerations are no different at micro- than at macroscale. But, the physics influencing the design of the components does change with scale, so that the optimal detailed designs can be quite different. Examples include the viscous forces in the fluid (larger at microscale), usable strength of materials (larger), surface area to volume ratios (larger), chemical reaction times (invariant), realizable electric field strength (higher), and manufacturing constraints (planar geometries).ENGINE DESIGNThere are many thermodynamic and architectural design choices in a device as complex as a gas turbine engine. These involve trade-offs among fabrication difficulty, structural design, heat transfer, fluid mechanics, and electrical performance. Given that the primary goal is to demonstrate â€œ that a high power density MEMS heat engine physically reliable, the design philosophy adopted is that the first engine will be as simple as possible, trading performance for simplicity. For Example, the addition of a heat exchanger transferring heat from the turbine exhaust to the compressor discharge fluid (a recuperated cycle) offers many benefits including reduced fuel consumption and relaxed turbo machinery performance requirements, but it introduces additional design and fabrication complexity. Thus, the baseline design is a simple cycle gas turbine generator. While this engine is the simplest of gas turbines, it is an extremely complex and sophisticated MEMS device. Arriving at a satisfactory design requires heavy dependence on simulation of the mechanical, thermo fluid, and electrical behavior to achieve the required levels of component performance and integration .The baseline engine design is illustrated in Figure 1.The engine consists of a supersonic radial flow compressor and turbine connected by a hollow shaft. Gaseous H2 fuel is injected at the compressor exit and mixes with air as it flows radially outward to the flame holders. The combustor discharges radially inward to the turbine whose exhaust turns 90 degrees to exit the engine nozzle. A thin film electric induction starter-generator is mounted on a shroud over the compressor blades and is cooled by compressor discharge air. Cooling air is also used to thermally isolate the compressor from the combustor and turbine. The rotor is supported on air bearings. The following sections briefly discuss component design considerations.MATERIALS AND MECHANICAL DESIGNConventionally sized engines, constructed from titanium and heavily cooled nickel and cobalt-based super alloys, are stress-limited in the rotating components. Nonmetallic such as silicon (Si), silicon carbide (SiC), and silicon nitride (Si3N4) offer substantial improvement in strength-to-density ratio and temperature capability, but large parts with acceptable properties have proven difficult to manufacture from these materials. However, they are readily available in essentially flaw-free form for micro scale fabrication so that significantly superior material performance is available for micro-heat engines than can now be realized in conventionally- sized devices. In addition, because of the small length scales required here, material which are unsuitable for a large heat engine due to thermal shock considerations (e.g. aluminum oxide), would be usable in a micro engine given a fabrication technology [1]. Silicon is suitable for the compressor (600 K) but cannot operate at the combustor discharge temperature needed (1300-1700 K) without cooling. SiC can operate uncooled but SiC fabrication technology is much less developed than that for Si. The baseline design assumes 

uncooled SiC for simplicity but a cooled Si design is also under study. The individual components are being demonstrated in Si while SiC manufacturing technology is being developed. Since the properties of such materials are a strongly influenced by the details of their fabrication, material testing is an integral part of this program .g/sec implies airfoil and passage heights on the order of 200-300 microns as in figure 2.Deep reactive ion etching was used to produce the turbine shown in Figure 3, which has a 4mm rotor diameter and 200 micron span blades. The rim of the 300-micron thick disk serves as a journal bearing. This unit is a rotor dynamics test piece. With the addition of a generator on the back surface of the disc, it becomes an 80-watt turbine generator. Also, using only known process steps, a strawman process simulation yields wafers of completed engines, including a freely turning rotor, without additional assembly. It is a complex and aggressive process requiring 7 aligned wafer bonds, 20 lithography steps, and the deposition of 9 thin film layers.TURBO MACHINERY AND FLUID MECHANICSConsiderations of engine thermodynamic efficiency, combustor performance, and turbine viscous losses suggest that compressor pressure ratio should be relatively high. Since both the pressure ratio and the centrifugal stress in the rotor scale with the square of the peripheral Mach number, the pressure ratio per stage of compression is set by the allowable material stress. Material property values in the literature are consistent with a 500 m/s rotor tip speed, which was therefore adopted as a baseline. A 4:1 pressure ratio compressor has been designed to operate at this speed. Current fabrication technology largely restricts complex curvatures to in plane, which inhibits the use of the high degree of three-dimensionality typically employed in centrifugal turbomachinery to improve efficiency and reduce material stresses. However, the usable material strength is higher at microscale. Also, this flow regime is unusual in that it is supersonic (Mach 1.4) but laminar (Reynolds number 20,000). Three-dimensional fluid calculations suggest that this machine should achieve an adiabatic efficiency of about 70%. To facilitate detailed measurement of the turbomachinery fluid mechanics, a 75:1 geometrically scaled up test rig has been built. It operates at the same Mach and Reynolds numbers as the microturbomachinery.COMBUSTIONAir breathing combustion requires fuel injection (and evaporation if a liquid), fuel-air mixing, and chemical reaction of the mixed reactants. The time required for these processes (the combustor residence time) sets the combustor volume. In large engines, the residence time is typically 5-10 ms. Most of this is for fuel mixing; chemical reaction times are a few hundred microseconds or less. In order to expedite the engine development process, hydrogen was selected as the baseline engineâ„¢s fuel. Hydrogen offers rapid mixing and chemical reaction times, and flammability over a wide range of fuel-to-air ratios. By operating at a low fuel-to-air ratio, the peak combustor temperature can be reduced to levels compatible with uncooled SiC construction (1600 K), eliminating the requirement for the complicated cooling geometries needed on large engines. A combustor with the geometry of Figure 1 has been built and tested. It has demonstrated the predicted levels of performance over a wide range of temperatures and mixture ratios. The data agree with numerical simulations that suggest that complete combustion can still be achieved with a factor of two reductions in combustor volume [2]. Work is now beginning on a hydrocarbon fueled catalytic combustorBEARINGS AND ROTOR DYNAMICSLow friction bearings are required to support the rotor against fluid and electrical forces, rotor dynamics, and externally applied accelerations while operating at speeds of over two million rpm. Gas film, electrical, and hybrid gas electrical bearing concepts were examined. Gas bearings were 

selected for the baseline engine based on superior load bearing capability and relative ease of fabrication. A journal bearing supports the radial loads and thrust plates support the axial loads. The physical regime that the microgas bearings operate in is unusual in several regards: the peripheral speed of the bearing is transonic so compressibility effects are important; the ratio of inertial to viscous forces (Reynolds number) is high; the surface area of the bearing is very large compared to the mass of the rotor; and the journal length-to-diameter ratio is quite low. The net effect of these influences is a journal bearing well outside existing theory and empirical design practice. Magnitude higher than the critical frequency (spring-mass damper equivalent) of the rotating system. Sub critical operation would require submicron-operating clearances, which are difficult to fabricate and incur viscous losses greater than the engine power output. The design adopted uses a ten-micron journal gap to reduce losses to a few watts but is linearly unstable at some speeds. Numerical simulations indicated, however, that this design would operate satisfactorily in a nonlinear limit cycle. Turbine-driven rotor dynamic test rigs have been constructed both at 1:1 microscale (Figure 3) and at 26:1 macroscale (to facilitate detailed instrumentation). Preliminary data confirm that the rotor does operate in a stable limit cycle. As a precaution, an electric damper is being designed to augment the bearing stability should it prove desirable.ELECTRICAL MACHINERYA motor-generator starts the gas turbine and produces the electrical power output. Integrating the motor-generator within the engine offers the advantages of mechanical simplicity since no additional bearings or structure are required over that needed for the engine and cooling air is available. Either electric or magnetic machines could be used. Here, an electric machine was chosen due to considerations of power density, ease of microfabrication, and high-temperature and high-speed operation. The baseline design is a 180-pole planar electric induction machine mounted on the shroud of the compressor rotor. Simulations suggest that such a machine can produce on the order of 20-40 watts with an electrical efficiency in excess of 80%. The major source of loss in the machine is viscous drag in the rotor-stator gap.In a first phase of the project, the problem has been scaled down to a turbine powered by compressed air. Compressor, combustion chamber, and generator have been left out and will be addressed in a later phase. The micro turbine is a single-stage axial impulse turbine. Expansion of the gas takes place in the stationary nozzles and not between the rotor blades. This type of turbine has been chosen because of its simple construction.Figure 1 shows an exploded view and an assembly of the microturbine design. The compressed air enters via a standard pneumatic connector (1) and expands over the stationary nozzles (3) where it is deflected in a direction tangential to the turbine rotor (5). After the air has passed the rotor blades, it leaves the device through the openings in the outlet disc (6). Screwing the pneumatic connector in the housing (8) presses the stationary nozzle disc against a shoulder in the housing. The rotor blades, wheel and axis are one monolithic part. The rotor is supported by two ball bearings (4), one mounted in the stationary nozzle disc and one mounted in the outlet disc. The outlet disc is locked in the housing by a circlip (7).Figure 2: Microturbine design.The diameter of the turbine rotor is 10 mm. The housing has a diameter of 15 mm and is 25 mm long. All parts, except pneumatic connector and circlip, are made of stainless steel. The nozzles are designed for subsonic flow, so have a converging cross-section. Sonic speed is reached for a relative supply pressure of 1 bar. The exit losses are minimal when turbine is designed for a u/c1 ratio of 0.5, with u the circumferential speed and c1 the absolute speed at the nozzle exit. At 1 bar, c1 reaches 

sonic speed resulting in optimal turbine speed of 420,000 rpm. As this is too high for the bearings, the turbine has been designed for a u/c1 ratio of 0.25, and is operated below its optimal speed of 210,000 rpm.TURBINE PRODUCTIONThe different parts of the turbine are produced by turning and EDM .The nozzle disc and rotor are the most complex parts. In a first step, their cylindrical surfaces are machined on a lathe. In a second step, the nozzles and blades are created by die-sinking EDM as illustrated for the rotor in figure 3. The rotor is clamped in a rotary head, which is indexed with steps of 30º. A prismatic copper electrode with a cross-section having the shape of the air channels between the blades is sunk into the turbine wheel by EDM. The electrode is produced by wire-EDM. One of the problems during the production of the turbine blades is electrode wear. This wear is difficult to predict and not uniform across the electrode. This problem has been solved by cutting away the lower edge of the electrode by wire-EDM at regular intervals. As the electrode is prismatic, the shape after shortening remains the same. Figure 4 shows a subassembly of nozzle disc, rotor, and bearings.

Fig 3: Machining of the rotor blades by EDM. Fig4:Subassembly of nozzle disc, turbine rotor, and bearings. 

MECHANICAL OUTPUTTorque and power of the turbine have been tested up to a speed of 100,000 rpm. For this purpose, a 30 mm diameter brass wheel has been fixed to the turbine axis. An optical sensor measures the rotation of the wheel in a contact less way: two vanes on the wheel interrupt the optical path of a photo sensor. The turbine is tested by switching on the pressure and accelerating the turbine until it reaches its maximum speed. The torque is then derived from the acceleration and the moment of inertia of the wheel and turbine rotor. As the turbine passes through the whole speed range, acceleration, torque and power are know as a function of speed.When the turbine is rotating at full speed, the pressure is switched off and a new measurement is done while the turbine slows down. This gives the friction torque as a function of speed. Friction mainly occurs between the wheel with vanes and the surrounding air. The friction torque and power are added to the results of the acceleration test to obtain the total torque and power of the turbine.Fig 5 and 6 show torque and mechanical power as a function of speed for different supply pressures up to 1 bar. The maximum torque and power are respectively 3.7 Nmm and 28 W. The dashed lines represent the friction losses determined with the deceleration test.

Figure 5: Torque as a function of speed and supply pressure. Figure 6: Mechanical power of the turbine. At 1 bar, the turbine consumes 8 Nm3/h of compressed air, which corresponds to a power consumption of 152 W when assuming an ideal isentropic expansion. This means that the mechanical efficiency of the turbine lies around 18 %. Figure 7 shows the turbine efficiency as a function of speed for different supply pressures.The Ëœdipsâ„¢ in the characteristics at high speed are caused by the measurement method as they always occur at the maximal speed, even for different loads and pressures. In reality, power and efficiency increase further with speed to reach their maxima theoretically at 210,000 rpm (for 1 bar). These speeds can be reached using a smaller load. Figure 7: Efficiency of the turbine (compressed air to mechanical power). 

ELECTRICAL OUTPUTTo measure the electrical power output of the system, the generator is connected to a variable 3-phase load consisting of 3 potentiometers (range 2 kW, 10 turns). In contrast with the mechanical tests, the electrical tests are performed at constant speed. The speed of the turbine, which is measured from the frequency of the generator voltage, is controlled by varying the load. Figure 8 shows the electrical power measured for different supply pressures and speeds. At a pressure of 1 bar, the maximal electrical power is 16 W and is reached at a speed of 100,000 rpm. Measurements show that the airflow and input power depend only on the supply pressure and not on speed or load. Therefore, the input power is the same as in the mechanical test at 1 bar, i.e. 152 W. Figure 9 shows the total efficiency (compressed air to electricity) as a function of speed and for different supply pressures. The maximal total efficiency is 10.5 % and is reached at a speed of 100,000 rpm.

Figure 8: Electrical power generated by the total system (turbine plus generator). Figure 9: Total efficiency (compressed air to electricity). SANKEYS DIAGRAMThe energy flow and the different losses are illustrated in the Sankey diagram shown in figure 10. The diagram is generated for a supply pressure of 1 bar and a speed of 100,000 rpm. This corresponds to the working point at which the maximal electrical power and maximal total efficiency are reached. Input power, mechanical power, electrical power and the combination of ventilation losses (6) and bearing friction (7) are measured values. This last value (6 + 7) is obtained with a deceleration test of the turbine without generator and without external load. The loss associated with the leak flow around the turbine wheel (2) and the exit losses (8) are calculated from the known air speeds. The expansion losses (1), incidence losses (4) and blade profile losses (5) are calculated using friction and loss coefficients known from large turbines and may be less accurate. The generator losses (10) are derived from the manufacturerâ„¢s data sheets. The obstruction losses (3) and the losses in the coupling (9) are derived as the difference between the calculated and measured values.The major losses are the blade profile losses and the exit losses. The large blade profile losses can be explained by the increased friction in miniature systems (large surface-to-volume ratio and low Reynolds numbers). The high exit losses can be explained by the low u/c1 ratio (0.25 instead of 0.5 in the optimal case). Additionally, the turbine operates below its optimal speed because the ball bearings limit the speed. Both factors result in higher air speeds at the turbine exit, and thus higher exit losses.Figure 10: Sankey diagram for a supply pressure of 1 bar and a speed of 100,000 rpm. CHALLENGES IN DESIGN AND FABRICATIONThe Microturbine presents challenges in the mechanical and electrical engineering disciplines of fluid dynamics, structural mechanics, bearing and rotor dynamics, combustion and electrical machinery design. Then comes difficulties involved in fabrication, heat transfer, structural design and electrical performance .The challenges also includes the need of small bearings and in manufacturing components .the turbine blades may come across with hydrogen burning, so it should be ceramic blade with micro ion etching .The challenges also there for cooling the systems. There will be a chance of high centrifugal stress, which will effect the life of micro turbine during high speed of rotation. Here there is the chance of heat losses due to the high surface to volume ratio, which will effects the efficiency and performance of microturbine.APPLICATIONS OF MICROTURBINE

Microturbines are suited to meet the energy needs of small users such as schools, apartments, restaurants, offices and small businesses. The Microturbine coupled with solid oxide fuel cell can be used in supermarkets, factories, and military developing nations. This can be applied for heating, drying, cooling, desalination and several others. They include space vehicles, electronic devices, unmanned aircrafts. The microturbines can be used in remote areas. This is because of small size. Microturbines are used when a high quality, energy density energy is needed.CONCLUSIONMicroturbines and miniature thermal devices pose unique challenges and opportunities for combustion in small volume. The principal difficulties are associated with limited residual time and heat transfer losses due to high surface to volume ratio. This paper addresses a preliminary analysis of Microturbine .The microturbine is in early stages of pre-production and is still in the developmental phase .The coupling of microturbine with a high temperature fuel cell (SOFC â€œ solid oxide fuel cell) is one of them .If the waste heat is used the overall fuel utilization efficiency can be increased. Major features, parameters and performance of the microturbine are discussed here. Fully understanding these and identifying the solutions, it is key to the future establishing of an optimum overall system. In the case of the microturbine changes will be minor as they enter production on a large scale within the next year or so, there is an extensive efforts are expanded to reduce unit cost .It is reasonable to project that a high performance and cost effective hybrid plant, with high reliability, will be ready for commercial service in the middle of the first decade of the twenty centuryFUTURE WORKThe first goal is to increase the efficiency of the turbine, mainly by decreasing the exit losses. This can be reached in two ways: introducing air bearings, which allow much higher speeds, or decreasing the speed by using a multiple-stage design. In the long term, a compressor and a combustion chamber will be added to finally come to a micro-generator REFERENCES

Introduction: (Initial Observation)For many years, energy of moving water has been used to grind grains and run machinery such as wood working, knitting and cutting machinery. Many of these places are now preserved as museums. With modern technology, the energy of water is not directly used to do do the job. Instead, water energy is used to produce electricity and the electricity is used to run different machinery.  

Since the flow of water is seasonal, dams are made to control the water flow and produce electricity all year long. With dams, storing water is storing energy.

In this project you will make a working model of a water turbine and calculate the amount of electrical energy that can be produced by your water turbine. You can make a

water turbine using wood. So wood working skills, some tools and adult supervision and help is required for this project. 

Information Gathering:Gather information about your project. If you are a basic or advanced member of ScienceProject.com, your project advisor may prepare the initial information that you need and enter them in this section. In any case it is necessary for you to read additional books, magazines or ask professionals who might know in order to learn more about the subject of your research. Keep track of where you got your information from. 

Water turbines

Photo: A giant Francis reaction turbine (the orange wheel at the top) being lowered into position at the Grand Coulee Dam in Washington State, USA. Water flows past the angled blades, pushing them around and turning the shaft to which they're attached. The shaft spins an electricity generator that makes power. Photo by courtesy of US Bureau of Reclamation.

Water wheels, which date back over 2000 years to the time of the ancient Greeks, were the original water turbines. Today, the same principle is used to make electricity in hydroelectric power plants. The basic idea of hydroelectric power is that you dam a river to harness its energy. Instead of the river flowing freely downhill from its hill or mountain source toward the sea, you make it fall through a height (called a head) so it picks up speed (in other words, so its potential energy is converted to kinetic energy), then channel it through a pipe called a penstock past a turbine and generator. Hydroelectricity is effectively a three-step energy conversion:

The river's original potential energy (which it has because it starts from high ground) is turned into kinetic energy when the water falls through a height.

The kinetic energy in the moving water is converted into mechanical energy by a water turbine.

The spinning water turbine drives a generator that turns the mechanical energy into electrical energy.

Different kinds of water turbine are used depending on the geography of the area, how much water is available (the flow), and the distance over which it can be made to fall (the head). Some hydroelectric plants use bucket-like impulse turbines (typically Pelton wheels); others use Francis, Kaplan, or Deriaz reaction turbines. The type of turbine is chosen carefully to extract the maximum amount of energy from the water.

Components

These are the components or parts for each of most water mills.

Water WheelThe Water Wheel draws the power for the mill from a current of water.

The Water Wheel transmits the energy trough a shaft to other parts of a mill to do a work or run an electric generator. 

Transmission of energy can be done using gears, pulleys and belts.

The purpose of this project is to build a small wooden water wheel and connect it to a bicycle generator in order to produce electricity.

Identify Variables:When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other. 

Variables that may affect the production of electricity are design variables such as size of the water wheel and the shape of buckets made on the wheel. Design variables affect the speed and torque of the wheel. On the other hand higher torque and speed of the water wheel, when transferred to a generator, produces a higher amount of electricity.

Hypothesis:Based on your gathered information, make an educated guess about the answer to your question or the result of your experiment.  

My hypothesis is that the force of a wooden water wheel with 2 feet diameter can be transmitted to a small generator (directly or using belts and pulleys) to produce electricity. 

Experiment Design:Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a "control." A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral "reference point" for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a "controlled experiment." 

So you want to make a water wheel and use it's energy to run an electric generator. There are many ways that you can design your project. For example you may construct a wooden water wheel and connect it to a bicycle generator to produce electricity. Or you may convert an existing bicycle wheel to a water wheel by connecting aluminum or plastic cups to that. 

Start by drawing your design and doing the calculations. Imagine that you want to give your drawings to a builder and he is supposed to complete the project without your supervision or advise. Preparing the drawings and doing the calculations is what engineers do. So this is your chance to test your engineering skills. 

How big should your water wheel be? It depends on the generator that you use. The generator needs certain torque (rotational force) and certain rotational speed in order to produce electricity. Your water wheel should be able to produce enough force or you will get no electricity. Test your generator to see how much force do you need to run it efficiently and produce electricity. Use a volt meter to see what is the voltage produced by your generator at different speeds. You may use pulleys or gears to increase the rotational speed (Number of turns per minute). Results of Experiment (Observation):Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental "runs." During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered "raw data" since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results. Calculations:If you do any calculation for your project, write your calculations in this section.

Summery of Results:Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did. Related Questions & Answers:What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are

making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.

Kaplan turbine and electrical generator cut-away view.

The runner of the small water turbine

A water turbine is a rotary engine that takes energy from moving water.

Water turbines were developed in the 19th century and were widely used for industrial power prior to electrical grids. Now they are mostly used for electric power generation. They harness a clean and renewable energy source.

HistoryWater wheels have been used for thousands of years for industrial power. Their main shortcoming is size, which limits the flow rate and head that can be harnessed. The migration from water wheels to modern turbines took about one hundred years. Development occurred during the Industrial revolution, using scientific principles and methods. They also made extensive use of new materials and manufacturing methods developed at the time.

Swirl

The word turbine was introduced by the French engineer Claude Burdin in the early 19th century and is derived from the Latin word for "whirling" or a "vortex". The main difference between early water turbines and water wheels is a swirl component of the water which passes energy to a spinning rotor. This additional component of motion allowed the turbine to be smaller than a water wheel of the same power. They could process more water by spinning faster and could harness much greater heads. (Later, impulse turbines were developed which didn't use swirl).

Time line

Roman turbine mill at Chemtou, Tunisia. The tangential water inflow of the millrace made the submerged horizontal wheel in the shaft turn like a true turbine.[1]

A Francis turbine runner, rated at nearly one million hp (750 MW), being installed at the Grand Coulee Dam, United States.

A propeller-type runner rated 28,000 hp (21 MW)

The earliest known water turbines date to the Roman Empire. Two helix-turbine mill sites of almost identical design were found at Chemtou and Testour, modern-day Tunisia, dating to the late 3rd or early 4th century AD. The horizontal water wheel with angled blades was installed at the bottom of a water-filled, circular shaft. The water from the mill-race entered tangentially the pit, creating a swirling water column which made the fully submerged wheel act like a true turbine.[1]

Ján Andrej Segner developed a reactive water turbine in the mid-18th century. It had a horizontal axis and was a precursor to modern water turbines. It is a very simple machine that is still produced today for use in small hydro sites. Segner worked with Euler on some of the early mathematical theories of turbine design.

In 1820, Jean-Victor Poncelet developed an inward-flow turbine.

In 1826, Benoit Fourneyron developed an outward-flow turbine. This was an efficient machine (~80%) that sent water through a runner with blades curved in one dimension. The stationary outlet also had curved guides.

In 1844, Uriah A. Boyden developed an outward flow turbine that improved on the performance of the Fourneyron turbine. Its runner shape was similar to that of a Francis turbine.

In 1849, James B. Francis improved the inward flow reaction turbine to over 90% efficiency. He also conducted sophisticated tests and developed engineering methods for water turbine design. The Francis turbine, named for him, is the first modern water turbine. It is still the most widely used water turbine in the world today. The Francis turbine is also called a radial flow turbine, since water flows from the outer circumference towards the centre of runner.

Inward flow water turbines have a better mechanical arrangement and all modern reaction water turbines are of this design. As the water swirls inward, it accelerates, and transfers energy to the runner. Water pressure decreases to atmospheric, or in some cases subatmospheric, as the water passes through the turbine blades and loses energy.

Around 1890, the modern fluid bearing was invented, now universally used to support heavy water turbine spindles. As of 2002, fluid bearings appear to have a mean time between failures of more than 1300 years.

Around 1913, Viktor Kaplan created the Kaplan turbine, a propeller-type machine. It was an evolution of the Francis turbine but revolutionized the ability to develop low-head hydro sites.

A new concept

Figure from Pelton's original patent (October 1880)Main article: Pelton wheel

All common water machines until the late 19th century (including water wheels) were basically reaction machines; water pressure head acted on the machine and produced work. A reaction turbine needs to fully contain the water during energy transfer.

In 1866, California millwright Samuel Knight invented a machine that took the impulse system to a new level.[2][3] Inspired by the high pressure jet systems used in hydraulic mining in the gold fields, Knight developed a bucketed wheel which captured the energy of a free jet, which had converted a high head (hundreds of vertical feet in a pipe or penstock) of water to

kinetic energy. This is called an impulse or tangential turbine. The water's velocity, roughly twice the velocity of the bucket periphery, does a u-turn in the bucket and drops out of the runner at low velocity.

In 1879, Lester Pelton (1829-1908), experimenting with a Knight Wheel, developed a double bucket design, which exhausted the water to the side, eliminating some energy loss of the Knight wheel which exhausted some water back against the center of the wheel. In about 1895, William Doble improved on Pelton's half-cylindrical bucket form with an elliptical bucket that included a cut in it to allow the jet a cleaner bucket entry. This is the modern form of the Pelton turbine which today achieves up to 92% efficiency. Pelton had been quite an effective promoter of his design and although Doble took over the Pelton company he did not change the name to Doble because it had brand name recognition.

Turgo and Crossflow turbines were later impulse designs.

Theory of operationFlowing water is directed on to the blades of a turbine runner, creating a force on the blades. Since the runner is spinning, the force acts through a distance (force acting through a distance is the definition of work). In this way, energy is transferred from the water flow to the turbine

Water turbines are divided into two groups; reaction turbines and impulse turbines.

The precise shape of water turbine blades is a function of the supply pressure of water, and the type of impeller selected.

Reaction turbines

Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow.

Newton's third law describes the transfer of energy for reaction turbines.

Most water turbines in use are reaction turbines and are used in low (<30m/98 ft) and medium (30-300m/98–984 ft) head applications. In reaction turbine pressure drop occurs in both fixed and moving blades.

Impulse turbines

Impulse turbines change the velocity of a water jet. The jet pushes on the turbine's curved blades which changes the direction of the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy.

Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.

Newton's second law describes the transfer of energy for impulse turbines.

Impulse turbines are most often used in very high (>300m/984 ft) head applications .

Power

The power available in a stream of water is;

where:

power (J/s or watts) turbine efficiency density of water (kg/m³) acceleration of gravity (9.81 m/s²) head (m). For still water, this is the difference in height between the inlet and

outlet surfaces. Moving water has an additional component added to account for the kinetic energy of the flow. The total head equals the pressure head plus velocity head.

= flow rate (m³/s)

Pumped storage

Some water turbines are designed for pumped storage hydroelectricity. They can reverse flow and operate as a pump to fill a high reservoir during off-peak electrical hours, and then revert to a turbine for power generation during peak electrical demand. This type of turbine is usually a Deriaz or Francis in design.

Efficiency

Large modern water turbines operate at mechanical efficiencies greater than 90% (not to be confused with thermodynamic efficiency).

Types of water turbines

Various types of water turbine runners. From left to right: Pelton Wheel, two types of Francis Turbine and Kaplan Turbine

Reaction turbines:

Francis Kaplan, Propeller, Bulb, Tube, Straflo

Tyson Gorlov

Impulse turbine

Waterwheel Pelton Turgo Crossflow (also known as the Michell-Banki or Ossberger turbine) Jonval turbine Reverse overshot water-wheel Archimedes' screw turbine

Design and application

Turbine selection is based mostly on the available water head, and less so on the available flow rate. In general, impulse turbines are used for high head sites, and reaction turbines are used for low head sites. Kaplan turbines with adjustable blade pitch are well-adapted to wide ranges of flow or head conditions, since their peak efficiency can be achieved over a wide range of flow conditions.

Small turbines (mostly under 10 MW) may have horizontal shafts, and even fairly large bulb-type turbines up to 100 MW or so may be horizontal. Very large Francis and Kaplan machines usually have vertical shafts because this makes best use of the available head, and makes installation of a generator more economical. Pelton wheels may be either vertical or horizontal shaft machines because the size of the machine is so much less than the available head. Some impulse turbines use multiple water jets per runner to increase specific speed and balance shaft thrust.

Typical range of heads

• Hydraulic wheel turbine• Archimedes' screw turbine• Kaplan

0.2 < H < 4   (H = head in m)1 < H < 102 < H < 40

• Francis• Pelton• Turgo

10 < H < 35050 < H < 130050 < H < 250

Specific speed

Main article: Specific speed

The specific speed of a turbine characterizes the turbine's shape in a way that is not related to its size. This allows a new turbine design to be scaled from an existing design of known performance. The specific speed is also the main criteria for matching a specific hydro site with the correct turbine type. The specific speed is the speed with which the turbine turns for a particular discharge Q, with unit head and thereby is able to produce unit power.

Affinity laws

Affinity Laws allow the output of a turbine to be predicted based on model tests. A miniature replica of a proposed design, about one foot (0.3 m) in diameter, can be tested and the laboratory measurements applied to the final application with high confidence. Affinity laws are derived by requiring similitude between the test model and the application.

Flow through the turbine is controlled either by a large valve or by wicket gates arranged around the outside of the turbine runner. Differential head and flow can be plotted for a number of different values of gate opening, producing a hill diagram used to show the efficiency of the turbine at varying conditions.

Runaway speed

The runaway speed of a water turbine is its speed at full flow, and no shaft load. The turbine will be designed to survive the mechanical forces of this speed. The manufacturer will supply the runaway speed rating.

Maintenance

A Francis turbine at the end of its life showing cavitation pitting, fatigue cracking and a catastrophic failure. Earlier repair jobs that used stainless steel weld rods are visible.

Turbines are designed to run for decades with very little maintenance of the main elements; overhaul intervals are on the order of several years. Maintenance of the runners and parts exposed to water include removal, inspection, and repair of worn parts.

Normal wear and tear includes pitting from cavitation, fatigue cracking, and abrasion from suspended solids in the water. Steel elements are repaired by welding, usually with stainless steel rods. Damaged areas are cut or ground out, then welded back up to their original or an improved profile. Old turbine runners may have a significant amount of stainless steel added this way by the end of their lifetime. Elaborate welding procedures may be used to achieve the highest quality repairs.[4]

Other elements requiring inspection and repair during overhauls include bearings, packing box and shaft sleeves, servomotors, cooling systems for the bearings and generator coils, seal rings, wicket gate linkage elements and all surfaces.[5]

Environmental impactMain article: Environmental impacts of reservoirs

Water turbines are generally considered a clean power producer, as the turbine causes essentially no change to the water. They use a renewable energy source and are designed to operate for decades. They produce significant amounts of the world's electrical supply.

Historically there have also been negative consequences, mostly associated with the dams normally required for power production. Dams alter the natural ecology of rivers, potentially killing fish, stopping migrations, and disrupting peoples' livelihoods. For example, American Indian tribes in the Pacific Northwest had livelihoods built around salmon fishing, but aggressive dam-building destroyed their way of life. Dams also cause less obvious, but potentially serious consequences, including increased evaporation of water (especially in arid regions), build up of silt behind the dam, and changes to water temperature and flow patterns. Some people[who?] believe that it is possible to construct hydropower systems that divert fish and other organisms away from turbine intakes without significant damage or loss of power; historical performance of diversion structures have been poor. In the United States, it is now illegal to block the migration of fish, for example the endangered great white sturgeon in North America, so fish ladders must be provided by dam builders. The actual performance of fish ladders is often poor.[citation needed]