microturbine full report
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One 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 and implimentation 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 and implimentation 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 and implimentation 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 and implimentation required such innovations constantly, radical ideas too new for anyone to be expert on them.
Over the next seven years the project and implimentation 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 and implimentation has inspired a whole R&D sector in micropower devices. The Defense Department alone is funding well over a dozen project and implimentations, 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.)
It 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).
There 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 DESIGN
Conventionally 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 . 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 MECHANICS
Considerations 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.
Air 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 . Work is now beginning on a hydrocarbon fueled catalytic combustor
BEARINGS AND ROTOR DYNAMICS
Low 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.
A 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 and implimentation, 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.
The 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 30Ã‚Âº. 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.
Torque 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).
To 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).
The 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 FABRICATION
The 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.
Microturbines 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 and implimentation 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 century
The 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
1. A.F. Massardo and C.F.McDonald, Microturbine for high-efficiency electrical generation, Transactions of ASME for gas turbine and power, January 2002.
2. E.Utrainen and B.Sunden,Evaluation of the heat transfer surfaces for Microturbine Recuperator, Transactions of ASME for Gas turbine and power, July 2002.
3. Lue Frrchette, Stuart A.Jacobson, Kenneth S.Breuer, Fedric F.Ehric, Reza ghodssi, Ravi khanna, Martin A.Schmidt and Alan H.Epstein,Demonstration of microfabricated high speed turbine supported on gas bearings, solid- state Sensor and actuator workshop, Hilton Head, June 4-8,200
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Microturbines are becoming wide spread for distributed power and combined heat and power applications. They range from handheld units producing less than a kilowatt to commercial sized systems that produce tens or hundreds of kilowatts. They are also known as "turbo alternators", or "gensets". Part of their success is due to advances in electronics, which allow unattended operation and interfacing with the commercial power grid. Electronic power switching technology eliminates the need for the generator to be synchronized with the power grid. This allows, for example, the generator to be integrated with the turbine shaft, and to double as the starter motor. Microturbine systems have many advantages over piston engine generators, such as higher power density (with respect to footprint and weight), extremely low emissions and few, or just one, moving part. They accept most commercial fuels, such as natural gas, propane, diesel and kerosene. The are also able to produce renewable energy when fueled with biogas from landfills and sewage treatment plants. Microturbine designs usually consist of a single stage radial compressor, a single stage radial turbine and a recuperator.Typical micro turbine efficiencies are 25 to 35 percent. When in a combined heat and power cogeneration system, efficiencies of greater than 80 percent are commonly achieved.
Microturbines are a new type of combustion turbine being used for stationary energy generation applications. They are small combustion turbines, approximately the size of a refrigerator, with outputs of 25kw to 500kw, and can be located on sites with space limitation for power production. Microturbines are composed of a compressor, combustor, turbine, alternator, recuperator, and generator. Waste heat recovery can be used in combined heat and power system to achieve energy efficiency levels greater than 80%. In addition to power generation micro turbines offer an efficient and clean solution to direct mechanical drive markets such as compression and air conditioning. Since making their commercial debut a mere five years ago, microturbines have installed with considerable success in office and apartment building, hotels and motels. Supermarkets, school and college, office and industrial parks, small industries, and numerous other facilities both in the US and abroard.They provide not only electricity, but the thermal energy to provide for all heating and cooling needs.
WHAT IS A MICROTURBINE
Microturbines are small combustion turbines approximately the size of a refrigerator with outputs of 25kw to 500kw. They evolved from automotive and truck turbochargers, auxiliary power units for airplanes, and small jet engines and are comprised of a compressor, combustor, turbine, alternator, recuperator, and a refrigerator. The engine itself is about the size of a beer keg. The most popular models have just one moving partsâ€a shaft with a turbine wheel on one end , a permanent magnet generator on other end, and an air compressor wheel in the middle. This assembly rotates at up to 96,000 rpm. At that speed, traditional oil-lubricated bearings are severely challenged. Accordingly the most popular micro turbine engines use air bearing to float the shaft.
Not only is the turbine turning at high rpm, so is the generator. The generator in turn produces a high frequency electrical output, which is then converted by power electronics unit to grid â€œcompatible 400-to-480-volts alternating current, 10-to-60 hertz.3phase power.
Microturbine offer a number of potential advantages compared to other technologies for small-scale power generation. These advantages include a small number of moving parts, compact size, light-weight, greater efficiency, lower emission, lower electricity cost, and opportunities to utilize waste fuels. They have the potential to be located on sites with space limitation for the production of power. Waste heat recovery can be used with these systems to achieve efficiencies greater than 80%.
There is very definitely a trend toward installing microturbine system onsiteâ€not only for generating electric power. But also for meeting site heating and cooling needs. Such microturbine configuration are called combined heat and power, or combined cooling, heat and power (cogeneration) system. The core idea is this: when burning a fuel in a micro turbine unit, donâ„¢t just use the resulting heated gases to spin a turbine and generate electricity. There is still a huge amount of thermal energy in the turbine exhaust. Donâ„¢t waste that valuable energy to the atmosphereâ€which is what they do in most central power plants (because there is no use for the heat in remote areas).
Instead, use a heat exchanger to capture much of that thermal energy and use it to meet all the heating and cooling needs of the site. When a microturbine unit is arranged in CHP or CCHP mode, heat from the turbine stack is captured and used to meet some or all the heating and cooling needs of the facility. This makes for much more efficient fuel use. Instead of just using 35% of thermal energy released during fuel combustion (as with a traditional central power plant), with CHP and CCHP one would be using 65% or more of the fuels thermal energy. This realization is a major reason the federal Department of Energy has been strongly encouraging the advance of onsite power generation with CHP and CCHP.
The 30-kilowatt model of Microturbine is very versatile, being able to burn several gaseous or liquid fuelsâ€natural gas, propane, biogas, diesel, and kerosene.
Commercial Available Yes (limited)
Size Range 25-500 kW
Fuel Natural gas, hydrogen, propane, and diesel.
Efficiency 20-30% (recuperated)
Environmental low (<9-50 ppm) NOx
Other features Cogeneration (50-80 C water)
Commercial Status Small volume production, commercial prototypes.
BASIC COMPONENTS OF MICROTURBINE
The basic components of a microturbine are the compressor, turbine generator, and recuperator. The heart of the microturbine is the compressor-turbine package, which is commonly mounted on a single shaft along with the electric generator. Two bearings support the Microturbines Single shaft. The single moving part of the one-shaft design has the potential for reducing maintenance needs and enhancing overall reliability. There are also two-shaft versions, in which the turbine on the first shaft directly drives the compressor while a power turbine on the second shaft drives a gearbox and conventional electrical generator producing 60 Hz power. The two shaft design features more moving parts but does not require complicated power electronics to convert high frequency AC power output to 60 Hz. Moderate to large-size gas turbines use multi-stage axial flow turbines and compressors, in which the gas flows along the axis of the shaft and is compressed and expanded in multiple stages. However, micro turbine turbo machinery is based on single-stage radial flow compressors and turbines. Radial flow turbo machinery handles the small volumetric flows of air and combustion products with reasonably high component efficiency.1 Large-size axial flow turbines and compressors are typically more efficient than radial flow components. However, in the size range of microturbines -- 0.5 to 5 lbs/second of air/gas flow -- radial flow components offer minimum surface and end wall losses and provide the highest efficiency. In micro turbines, the turbo compressor shaft generally turns at high rotational speed, about96, 000 rpm in the case of a 30 kW machine and about 80,000 rpm in a 75 kW machine. One 45kW model on the market turns at 116,000 rpm. There is no single rotational speed-power size rule, as the specific turbine and compressor design characteristics strongly influence the physical size of components and consequently rotational speed. For a specific aerodynamic design, as the power rating decreases, the shaft speed increases, hence the high shaft speed of the small micro turbines.
The microturbine produces electrical power either via a high-speed generator turning on the single turbo-compressor shaft or with a separate power turbine driving a gearbox and conventional 3,600 rpm generator. The high-speed generator of the single-shaft design employs permanent magnet (typically Samarium-Cobalt) alternator, and requires that the high frequency output (about 1,600 Hz for a 30 kW machine) be converted to 60 Hz for general use. This power conditioning involves rectifying the high frequency AC to DC, and then inverting the DC to 60 Hz AC. Power conversion comes with an efficiency penalty (approximately five percent).To start-up a single shaft design, the generator acts as a motor turning the turbo-compressor shaft until sufficient rpm is reached to start the combustor. Full start-up requires several minutes. If the system is operating independent of the grid (black starting), a power storage unit (typically battery UPS) is used to power the generator for start-up.
Recuperators are heat exchangers that use the hot turbine exhaust gas (typically around 1,200Ã‚ÂºF)to preheat the compressed air (typically around 300Ã‚ÂºF) going into the combustor, thereby reducing the fuel needed to heat the compressed air to turbine inlet temperature. Depending onmicroturbine operating parameters, recuperators can more than double machine efficiency. However, since there is increased pressure drop in both the compressed air and turbine exhaust sides of the recuperator, power output typically declines 10 to 15% from that attainable without the recuperator. Recuperators also lower the temperature of the micro turbine exhaust, reducing the micro turbineâ„¢s effectiveness in CHP applications.
Microturbines operate on either oil-lubricated or air bearings, which support the shaft(s). Oil lubricated bearings are mechanical bearings and come in three main forms â€œ high-speed metal roller, floating sleeve, and ceramic surface. The latter typically offer the most attractive benefits in terms of life, operating temperature, and lubricant flow. While they are a well-established technology, they require an oil pump, oil filtering system, and liquid cooling that add tomicroturbine cost and maintenance. In addition, the exhaust from machines featuring oil lubricated bearings may not be useable for direct space heating in cogeneration configurations due to the potential for contamination. Since the oil never comes in direct contact with hot combustion products, as is the case in small reciprocating engines, it is believed that there liability of such a lubrication system is more typical of ship propulsion diesel systems (which have separate bearings and cylinder lubrication systems) and automotive transmissions than cylinder lubrication in automotive engines.. Air bearings have been in service on airplane cabin cooling systems for many years. They allow the turbine to spin on a thin layer of air, so friction is low and rpm is high. No oil or oil pump is needed. Air bearings offer simplicity of operation without the cost, reliability concerns, maintenance requirements, or power drain of an oil supply and filtering system. Concern does exist for the reliability of air bearings under numerous and repeated starts due to metal on metal friction during startup, shutdown, and load changes. Reliability depends largely on individual manufacturers' quality control methodology more than on design engineering, and will only be proven after significant experience with substantial numbers of units with long numbers of operating hours and on/off cycles.
As discussed, single-shaft micro turbines feature digital power controllers to convert the high frequency AC power produced by the generator into usable electricity. The high frequency AC is rectified to DC, inverted back to 60 or 50 Hz AC, and then filtered to reduce harmonic distortion. This is a critical component in the single-shaft microturbine design and represents significant design challenges, specifically in matching turbine output to the required load. To allow for transients and voltage spikes, power electronics designs are generally able to handle seven times the nominal voltage. Most microturbine power electronics are generating three phase electricity. Electronic components also direct all of the operating and startup functions. Microturbines are generally equipped with controls that allow the unit to be operated in parallel or independent of
the grid, and internally incorporate many of the grid and system protection features required for interconnect. The controls also allow for remote monitoring and operation.
HOW MICROTURBINE WORKS
Microturbine engine has only one moving part, basically a shift. At one end of that shaft is a turbine wheel; at the opposite end of the shaft is a permanent magnet electric generator; and positioned at the mid point of that shaft is an air impeller wheel (ie; an air compressor) for drawing ambient air , compressing it , then pumping it into combustor. Fuel is then injected into the combustor, where it then mixes with compressed air. Combustion occurs and the resulting gasses expand and rush out through the turbine, spinning it to a very high rpm.
This whole microturbine system is packaged in an enclosure not much bigger than a refrigeratorâ€about 7 feet tall, 2.5 feet wide and 6.5 feet deep. Ambient air is first drawn into the microturbine system enclosure, filtered, then passed over the electric generator, which is kept cool by this passing air. Next, the air is drawn into the impeller (or compressor), which compresses the air before pumping it into the combustor
Now, a part of that compressed â€œair stream exiting the impeller (compressor) is diverted to the air bearing. The microturbine shaft in effect now rides on a thin film of compressed airâ€this being in the thin annular space between the rotating shaft and the stationary bearing housing
TYPES OF MICROTURBINE
Microturbine are classified by the physical arrangement of the component parts; single Shaft, simple cycle, or recuperated, inter-cooled, and reheat. The machines generally rotate over 40000 revolutions per minute. The bearing selection â€œoil or air- is dependent on usage .A single shaft microturbine with high rotating speeds of 90000 to 120,000 revolutions per minute is the more common design ,as it is simpler and less expensive to built. Conversely, the spilt shaft is necessary for machine drive applications, which does not require an inverter to change the frequency of the AC power.
Microturbine generator can also be divided into two general classes:
Unrecuperated (simple cycle) micro turbineâ€in a simple cycle, or unrecuperated, turbine. Compressed air is mixed with fuel and burned under constant pressure condition. The resulting hot gas is allowed to expand through a turbine to perform work. Simple cycle microturbines have lower efficiencies at around 15%, but also lower capital costs, higher reliability, and more heat available for cogeneration application than recuperated unit.
Recuperated microturbinesâ€recuperated units use a sheet metal heat exchanger that recovers some of the heat from an exhaust stream and transfers it to the combustor. Further exhaust heat recovery can be used in a cogeneration configuration. The figures below illustrate a recuperated microturbine system. The fuel-energy-to electrical-conversion efficiencies are in the range of 20 to 30%. In addition, recuperated units can produce 30 to 40% fuel savings from preheating.
CHARACTERITICS OF MICROTURBINES
Some of primary applications for microturbine include:
Distributed generationâ€stand â€œalone, on site applications remote from power grids.
Quality power and reliabilityâ€reduced frequency variation, voltage transients, surges, dips, or other disruptions.
Stand by powerâ€used in the event of an outage, as back up to electric grid.
Peak shavingâ€the use of microturbines during times when electric use and demand charges are high.
Boost powerâ€boost localized generation capacity and on more remote grids.
Low cost energyâ€the use of microturbines as base load primary power that is less expensive to produce locally than it is to produce from the electric utility
Combined heat and power (cogeneration )â€increase the efficiency of on-site power generation by using the waste heat for existing thermal process.
DISTRIBUTED ENERGY GENERATION
Energy is produced on a large scale in large thermal and hydro electric power plants and is then distributed to the users through network of lines called the power grid. These plants meet the need of consumers over a large area. In distributed energy generation on the other hand involves the on site generation of small scale power. On-site power generation means power is generated right where it is needed.
Advantages of Distributed Generation
As energy need not be transmitted there is no need of any large transmission infrastructure. Thus the losses during power transmission are greatly reduced. The combined heat and power (CHP) technology can be applied to micro turbines to increase its efficiency. This lowers emission and operating cost by reducing losses and increasing efficiency. From a companyâ„¢s point of view, it gives greater control, choice and flexibility in meeting needs for power and heat energy.
Selected strength and weaknesses of microturbine technology are:
Small number of moving parts
Good efficiencies in cogeneration
Can utilize waste fuels
Long maintenance interval
Less noise than reciprocating engines
Low fuel to electricity efficiencies
Loss of power output and efficiency with higher ambient temperature and elevation.
APPLICATIONS OF MICROTURBINES
Microturbines are being increasingly preferred over reciprocating engines in many applications. These include:
Combined heat and power (co-generation)
Waste heat from the micro turbine can be transferred via a heat exchanger to produce steam or provide hot water for local area. The hot water can be used in a green house to grow plants; water can duct to provide central heating in building in winter. Thermal hosts can found easier because the the produced by each microturbine unit is so much that by a large power station.
Distributed power generation
Electricity is generated locally to meet demand in the local area, for example a small townâ„¢s electricity supply. This can relieve congestion of the distribution network or power grid. Hospitals, hotels, factories and holiday resorts can install distributed power at remote sites without grid access.
Distributed generation provides a wide range of services to consumers and utilities, including standby generation, peak shaving capability, base load generation and co-generation.
The waste heat from the generator can be used to create for the sterilization of medical equipment as well as for laundry purposes, like the daily changing of bed linen. It can also act as backup power supply, which is critical for the smooth functioning of various life-supporting equipments.
Microturbines can also be used in remote areas where there is no access to electricity. It could provide electricity for research station in the middle of a jungle or desert, where there is no ready access to diesel supplies but is located near gas wells.
Hybrid vehicle( microturbine to high speed alternator)
In hybrid vehicle applications, the power produced by a microturbine is converted into electricity by a high â€œspeed alternator. The power is used to drive electric motors connected to the wheels. Any excess energy is directed to an energy storage system such as batteries or flywheels. The operating mode of the hybrid approaches ranges from battery-primary systems where the microturbine can be a Ëœbattery chargerâ„¢, to engine-primary system where the batteries help the micro turbine meet peak power needs, e.g. during acceleration.
Hybrid vehicle (microturbine and fuel cell together)
A hybrid combination of micro turbines with fuel cells can increase overall system efficiencies. Hybrid systems take advantage of an increase in fuel cell efficiency with an increase in operating pressure. The microturbine compressor stage is used to provide this pressure. The fuel cell produces heat along with power, and this heat energy is used to drive the microturbines turbine stage. If the fuel cell produces enough heat the micro turbine can generate additional power. For the hybrid combination, efficiency is expected to be as much as 60% and emission less than 1.0 ppm NOx, with negligible SOx and other application.
ECONOMICS OF MICROTURBINES
Microturbine capital costs ranges from $700 -$1,100\kW. These costs include all hardware, associated manuals, software, and initial training. Adding heat recovery increases the cost by $75-$350\kW. Installation costs very significantly by location but generally add 30-50%to the total installed cost.
Microturbine manufacturers are targeting a future cost below $650\kW. This appears to be feasible if the market expands and sales volumes increase.
With fewer moving parts, microturbine vendors hope the units can provide higher reliability than conventional reciprocating generating technologies. Manufacturers expect that initial units will require more unexpected visits, but as the products mature, once-a- year maintenance schedule should suffice. Most manufacturers are targeting maintenance intervals of 5,000-8,000 hours.
Maintenance costs for micro turbine units are still based on forecasts with minimal real-life situation. Estimates range from $0.005-$0.016 per kWh, which would be comparable to that for small reciprocating engine systems.
Capital cost $700-$1100\Kw
O&M Cost $0.005-0.016\kw
Maintenance Interval 5,000-8,000hrs
ADVANCED MICROTURBINE PROGRAM
The Advanced Microturbine Program is a six-year program for FY 2000-2006 with a Government is investment of over $60 Million. End-use applications for the program are open and include stationary power applications in industrial, commercial, and institutional sectors. The program includes competitive solicitation (s) for engine conceptual design, and sensors and controls. Technology evaluations and demonstrations are also part of the program.
Planned activities for this program focus on the following performance targets for the next generation of ultra-lean, high efficiency microturbine product design:
High efficiency: Fuel- electricity conversion efficiency of at least 40%.
Environment : NOx< 7ppm (natural gas)
Durability: 11,000 hrs of reliable operation between major overhauls and a service life of at least 45,000 hrs.
Cost of power: System costs< $500/kW, costs of electricity that are competitive with the alternatives (including grid) for market applications.
Fuel flexibility: Options for using for using multiple fuels including diesel, ethanol, landfill gas, and bio-fuels.
The leading microturbine manufacturers are
1. Bowman power systems
2. Capstone Turbine Cooperation
3. Elliott energy systems
4. Turbec AB
5. Ingersoll-Rand Company
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04-03-2010, 11:11 AM
Having a PC and related electronic equipment in majority of homes today is accepted without question. In the same vein, having a personal turbine in the home could also be taken for granted for the next generation to assure a constant and reliable source of electrical power. Microturbines are small combustion turbines approximately the size of a refrigerator with outputs of 3 Kw to 350Kw.They operates on the same principle as a jet engine but can use a variety of commercially available fuels, such as natural gas, diesel, and propane. Most microturbines are comprised of a compressor, combustion chamber, turbine, alternator, recuperator (a device that captures waste heat to improve the efficiency of the compressor stage), and generator. Microturbines offer a number of potential advantages compared to other technologies for small-scale power generation. These advantages include a small number of moving parts, compact size, light-weight, greater efficiency, lower emissions, lower electricity costs, and opportunities to utilize waste fuels. They have the potential to be located on sites with space limitations for the production of power. Waste heat recovery can be used with these systems to achieve efficiencies greater than 80%.In addition, microturbines offer an efficient and clean solution to direct mechanical drive markets such as compression and air-conditioning.
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Microturbines.docx (Size: 246.96 KB / Downloads: 48)
Microturbines work like jet engines but produce electricity instead of thrust.
(Courtesy of Capstone Turbine Corp.)
Microturbines are a relatively new distributed generation technology being used for stationary energy generation applications. They are a type of combustion turbine that produces both heat and electricity on a relatively small scale. Microturbines offer several potential advantages compared to other technologies for small-scale power generation, including: a small number of moving parts, compact size, lightweight, greater efficiency, lower emissions, lower electricity costs, and opportunities to utilize waste fuels. Waste heat recovery can also be used with these systems to achieve efficiencies greater than 80%.
Because of their small size, relatively low capital costs, expected low operations and maintenance costs, and automatic electronic control, microturbines are expected to capture a significant share of the distributed generation market. In addition, microturbines offer an efficient and clean solution to direct mechanical drive markets such as compression and air-conditioning.
Types of Microturbines
Microturbines are classified by the physical arrangement of the component parts: single shaft or two-shaft, simple cycle, or recuperated, inter-cooled, and reheat. The machines generally rotate over 40,000 revolutions per minute. The bearing selection—oil or air—is dependent on usage. A single shaft microturbine with high rotating speeds of 90,000 to 120,000 revolutions per minute is the more common design, as it is simpler and less expensive to build. Conversely, the split shaft is necessary for machine drive applications, which does not require an inverter to change the frequency of the AC power.
• Distributed generation—stand-alone, on-site applications remote from power grids
• Quality power and reliability—reduced frequency variations, voltage transients, surges, dips, or other disruptions
• Stand-by power—used in the event of an outage, as a back-up to the electric grid
• Peak shaving—the use of microturbines during times when electric use and demand charges are high
• Boost power—boost localized generation capacity and on more remote grids
• Low-cost energy—the use of microturbines as base load or primary power that is less expensive to produce locally than it is to purchase from the electric utility
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