Microturbine Generators



INTRODUCTIONAs energy demands increase and the associated costs increasing with demand, newer energy alternatives are becoming more important to society and also consumers want an uninterrupted and economical electric power. Recently, distributed generation (DG) has become an attractive method of providing electricity to consumers and retailers. In addition, from the viewpoint of economic feasibility, the costs of installing generators and producing the electricity can be comparatively inexpensive using the DG method.One of DG sources is Microturbine Generation systems. Microturbine generator systems are those generator systems equipped with small combustion turbines approximately the size of a refrigerator with outputs of 25kW to 500kW.They operate at a high speed generally in the range of 50,000 to 120,000rpm.Electric power is produced in the range of 1400-4000Hz.They are most suitable for small to medium-sized commercial and industrial loads. The microturbine provides input mechanical energy for the generator system which is converted by the generator to electrical energy. The electrical energy is later converted to normal supply frequency and passed through the transformer, is delivered to the distribution system and the local load. Fig 1.Block diagram of a microturbine generator system The microturbine generators come under the Distributed Energy Resources. Device category. Those devices enable renewable energies utilization and more efficient utilization of waste heat in combined heat and power (CHP) applications and lowering emissions. Unlike traditional backup generators, microturbine generators are designed to operate for extended periods of time and require little maintenance. They can supply customer’s base-load requirements or can be used for standby, peak shaving and cogeneration applications. As microturbine generators don’t have reciprocating parts, there is no need of lubricating and all. Some microturbines even utilize air bearings and air cooling, thereby completely eliminating the need to change and dispose of hazardous liquid lubricants and coolants. In any case, microturbines are similar to major power plants, able to run for extended periods at full power output, and require little scheduled maintenance compared with traditional reciprocating engine generators of similar size. This makes them ideal for stationary prime power applications. The combustion process in a microturbine is continuous and clean burning, similar to modern gas turbine power plants. Microturbine manufacturers have deployed state of the art lean-burn combustion technology to control emissions without the need for expensive catalytic exhaust treatment equipment or chemicals. MICROTURBINE GENERATORMicroturbine generators(MTG) are small, high speed power plants that are usually include the turbine, compressor and power electronics to deliver the power to the grid. These small power plants typically operate on natural gas. Future units may have the potential to use lower energy fuels such as gas produced from landfill or digester gas. Microturbine generators are classified into two types:Unrecuperated (simple cycle) microturbine generators.Recuperated microturbine generators.2.1 UNRECUPERATED MTGIn a simple cycle or unrecuperated systems the compressed air is mixed with fuel and burned under constant pressure conditions. The resulting hot gas is allowed to expand through a turbine to perform work. Simple cycle MTGs have lower efficiency at around 15%, but also lower capital costs, higher reliability and more heat available for co-generation applications than recuperated units.2.2 RECUPERATED MTG Recuperated units use a thin sheet-metal heat exchanger that recovers some of the heat from an exhaust stream (1,200?F) and transfers it to the incoming air stream, boosting the temperature of the air stream (around 300?F) supplied to the combustor. Further exhaust heat recovery can be used in a co-generation configuration. 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. Depending on the microturbine operating parameters, recuperators can more than double machine efficiency. 3. TECHNICAL BACKGROUNDThe entire microturbine generator system can be divided into three primary sub-systems:3.1 Mechanical The mechanical system comprises the turbine, generator, compressor and recuperator. The compressor-turbine package is the heart of the microturbine generator system. They are commonly mounted on a single shaft along with the electric generator. Two bearings support the single shaft. The microturbine generator system produces electrical power via a high speed generator turning on the single turbo-compressor shaft. The high-speed generator of the single-shaft design employs a permanent magnet (typically Samarium Cobalt) alternator, and requires that the high frequency AC output (about 1400Hz-4000Hz) be converted to 50Hz for the general use. They operate at cool, clean, low-vibration, environment and offers 160,000 hours of normal service.Fig 2. Internal view showing the parts of an MTG Block3.1.1 Generator/Gearbox The standard Power Works (NREC’s microturbine) package incorporates a single-stage helical gear set to transfer power from the turbine to the 3600 RPM generator. The low-torque, highsliding- velocity results in exceptional design-life margins. At the conditions specified for the PSOFC, the gear and bearing life exceed one million hours. A commercial 2-pole 3600 RPM induction generator is standard with the Power Works package, and for a production version of the proposed system would be the probable choice. The manufacturer predicts a B10 life of 160,000 hours for normal service. The generator has been conservatively selected and operates in a cool, clean, low-vibration environment. For cold weather and extended peaking-power operation, a higher power rated generator can be provided. An optional synchronous generator can also be substituted for grid-isolated operation, as proposed in connection with the current experimental program.3.1.2 Combustor The combustor proposed for the integrated PSOFC package would be a modification of the standard patented Power Works (NREC’s microturbine) design, originally developed in 1990 in collaboration with SoCal Gas. It has consistently demonstrated NOx levels below 9ppmv, with exceptionally good turndown stability and proven durability. Departure from the standard Power Works (NREC’s microturbine) design is needed to limit combustor pressure loss during unfired operation. Combustor inlet temperature under these conditions will be in the vicinity of 1600F, whereas the current running condition is around 1200F. The design change needed to accommodate this difference is straightforward, and is roughly a matter of increasing the effective flow area of the combustor.3.1.3 Recuperator Recuperator is a heat exchanger which transfers heat from the exhaust gas to the discharge air before it enters the combustor to reduce the amount of fuel required to raise the discharge air temperature to that required by the turbine.3.1.4 TurbineThere are two kinds of turbines, high speed single shaft turbine and split shaft turbines. All are small gas turbines. Fig 3. Isometric view of an MTG3.2 ElectricalThe electrical system includes main control software, inverter and power firmware.3.2.1 Engine controllerEngine controller is a digital system which controls the entire process of the microturbine generator. They provide the provision of automated starting and all. And we can set the delay using this system. They will also locate the fault occurred and perform the safety functions & speed can be controlled. Engine controller will reduce the power output produced if the engine is running near its maximum permitted temperature. They also have the ability to interact with the other parts of the generator control systems.3.2.2 Power Conditioning SystemWe know the power output of a microturbine generator will be between the frequency ranges 1.5-4 kHz. For our usage it must have to be converted to the useable standard mode. Fig 4. Simplified diagram of a power conditioning systemThe power conditioning system converts the unregulated, variable-frequency output of the generator into a high quality, regulated waveform. The waveform quality surpasses the general utility standards and is suitable for supplying sensitive equipment. Output voltage and frequency are adjustable between desirable ranges, allowing the system to be easily configured for the operation anywhere.We know that the electrical output from the MTG will have a frequency in the range of 1,400-4,000Hz. The high frequency power from the generator is introduced into an inverter where it is converted into dc before the inverter followed by it can reconstruct a three-phase voltage supply at a lower frequency required for the grid connections.In the figure, we can see that an MTG feeding 3-phase power to a rectifier and the dc is then fed to a high frequency, a single-phase inverter so that a compact, high frequency transformer can be used. The secondary of the transformer feeds an ac/ac converter that takes the single phase, high frequency voltage to produce a 3-phase voltage at a frequency and phase needed to make a direct connection to the grid. The circuit has following advantages:The use of a transformer for robust isolation.The high frequency inverter permits the use of compact, high frequency transformers.The use a transformer permits the easy addition of other isolated loads and supplies via additional windings and taps.The circuit eliminates the need for static transfer switches.Ancillary services can be provided with control software changes and additional hardware.Adding additional hardware is easier.3.2.3 Power ControllerThey are mostly on-board, pc-based, a processor linked to pc, etc., depending on constraints and factors such as MTG packaging, desired versatility, type of available features, and the sophistication/maturity of the system design. A power controller control and co-ordinates the operation of the power conditioning circuit by ensuring that the functions such as voltage following, current following, phase matching, harmonic suppression, etc are performed reliably and at high efficiency. 3.3 Fuel systemMicroturbine generator have fuel flexibility and are capable of using alternative fuels including natural gas, diesel, ethanol, landfill gas and other bio-mass derived liquids & gases. The microturbine generators are fitted with fuel boosters which reduce the fuel consumption. For 2 kW power, the machine consumes only 25 icfm.Fig 5. Complete view of an MTG4. WORKINGMechanically the microturbine generator is a single shaft gas turbine with the entire compressor, power turbine and the permanent magnet generator being mounted on the same shaft. The power turbine drives the generator which produces the electrical power and speed of rotation of this power turbine is from 50000-120,000 rpm.During engine operation, air is drawn into the compressor unit through an air filter. The air filter will filter out unwanted components in the air. The compressor unit will then compress in taken air and raises its pressure to a heavy value. The high pressure air then is introduced into a recuperator arrangement where the heat exchanging process takes place. Inside the recuperator, the exhaust air from the turbine after burning the fuel, possessing a temperature around 650 degrees Celsius will then transfer the heat to the compressed air and thereby increase the temperature by 200 degree Celsius.Fig 6. Figure illustrating the working of an MTGNow, the hot air is passed into the combustion chamber. Simultaneously the fuel which is also get compressed in a gas compressor is introduced and mixed with high temperature air and due to this burning of fuel will occur, producing high temperature gas or steam. This gas is then taken into the power turbine by means of a nozzle. As a result the thermal energy holding by the gas is used effectively to rotate the turbine to high speed.Thus the generator which is coupled to the turbine wheel is get rotated and eventually the electrical power is produced at higher frequencies which is later get regulated.Fig 7. Working processes5. MACHINE PERFORMANCE TESTSVarious tests have been performed on a microturbine generator to evaluate its performance, maintenance requirements and all.5.1 Endurance TestIn this test program, microturbine generator will be operated as long as practicable at normal load. Daily operating parameters such as fuel flow, air pressure, temperature, humidity, power produced, operating temperature and pressure are noted & verified.5.2 Transient ResponseMicroturbine generator should be capable to respond adequately to load changes. For the units that are not capable to operate on isolated bus will operate parallel with system grid. Changes in the system load will be picked up by the grid and noted by microturbine generator units. Load changes on these microturbine generator units will be accomplished by manually setting load using a control system arrangement.5.3 Noise MeasurementAmbient noise levels will be measured using a handheld noise meter. Each unit will be operated independently to acquire the noise measurements during operations, it is found that the microturbine generator have the least noise level as compared to other generator sets and is around 63 db.5.4 Emission measurementThe exhaust of the microturbine generator is subjected to emission tests. Additionally, periodic measurements with available handheld equipment would be made to determine trends and any condition of degradation that may occur with operating hours.Microturbine generators have the least NOx emission, which is the main factor behind the global warming. The amount of NOx emitted is only 7ppm whereas it is too higher for the conventional generator sets. A microturbine generator will produce only .564 kg of CO2 per kW of electricity. That’s why we prefer this technology of power production.5.5 Peak Load Gross and NetPeak load gross and net measurements will be taken with a BMI meter or equivalent recorder that measures power. For units without compressors, or compressors that are externally powered, the net output must be determined by subtracting the external power requirements to sustain MTG operation. Results of this test will yield performance characteristics such as efficiency, heat rate, fuel consumption and operating hours. Comparisons will be made to manufacturer specifications.6. ECONOMIC ASPECTSThe capital cost for a microturbine generator is estimated as 700-1000 $/kW which include all the hardware, associated manuals, software and all. Adding heat recovery increases the cost by 75-350$/kW. Installation costs vary significantly by location but generally add 30-50% to the total installed costs.Microturbine manufacturers are targeting a future cost below 650$/kW. This appears to be feasible if the market expands and sales volume increases.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, a once-a-year maintenance schedule should suffice. Most manufacturers are targeting maintenance intervals of 5,000-8,000 hours.Maintenance costs for microturbine units are still based on forecasts with minimal real-life situations. Estimates range from $0.005-$0.016 per kWh, which would be comparable to that for small reciprocating engine systems.Table 1ECONOMICS OF AN MTGType of costCost(in dollars)Capital cost$700-$1000/kWOperational & maintenance cost$.005-$.016/kW7. CHARACTERISTICS OF AN MTG7.1 AestheticsImproves sightlines and views with off-the-grid systems, which eliminate the need for overhead power lines.7.2 Cost-EffectiveEnables cost savings by reducing the peak demand at a facility, therefore lowering demand charges.7.3 FunctionalProvides better power reliability and quality, especially for those in areas where brownouts, surges, etc. are common or utility power is less dependable.Provides power to remote applications where traditional transmission and distribution lines are not an option such as construction sites and offshore facilities.Can be an alternative to diesel generators for on-site power for mission critical functions (e.g., communications centers).Possesses combined heat and power capabilities.Reduces upstream overload of transmission lines..Optimizes utilization of existing grid assets—including potential to free up transmission assets for increased wheeling capacity.Improves grid reliability.Facilitates faster permitting than transmission line upgrades.Can be located on sites with space limitations for the production of power.7.4 ProductiveProvides high-quality power for sensitive applications.Responds faster to new power demands—as capacity additions can be made more quickly.Facilitates less capital tied up in unproductive assets—as the modular nature of microturbines means capacity additions and reductions can be made in small increments, closely matched with demand, instead of constructing central power plants sized to meet estimated future (rather than current) demand.Stand-by power decreases downtime, enabling employees to resume working.Produces less noise than reciprocating engines.7.5 Secure/SafeStrengthens energy security.Stand-by power provides quick recovery after an event.7.6 SustainableProduces the lowest emission of any noncatalyzed fossil fuel combustion system.Has a small footprint, minimizing site disturbance.Reduces or defers infrastructure (line and substation) upgrades.For recuperated microturbine, possesses higher energy conversion efficiencies than central generation.Enables more effective energy and load management.8. ADVANTAGES AND DISADVANTAGES7.1 AdvantagesMTG has small number of moving parts, therefore maintenance is comparably less.It has compact size.Most of the parts are light weight.Good efficiency.Low emission & less noise and vibration than reciprocating systems.Can utilize waste fuels.Strengthens energy security.Cheap and easy installation.Wide range of benefits in terms of operational and fuel flexibility, service performance and maintainability.7.2 DisadvantagesLow power output & efficiency with higher ambient temperaturesTime-variable electrical and thermal demand distorts MTG’s energy balance sometimes leading to larger fuel requirement.9. APPLICATIONSMicroturbines can be used for stand-by power, power quality and reliability, peak shaving, and cogeneration applications. In addition, because microturbines are being developed to utilize a variety of fuels, they are being used for resource recovery and landfill gas applications. Microturbines are well suited for small commercial building establishments such as: restaurants, hotels/motels, small offices, retail stores, and many others.1. MTG’s are excellent power generators for use in combined heat and power (CHP) systems. Their low maintenance and clean exhaust make them a reliable choice for base load CHP applications. Integrating hot water heat recovery into the microturbine package has proven cost effective, and a growing number of commercial installations are saving money using this technology. Not only do microturbines provide this cost saving performance day in and day out, but their value is further increased when the cost for traditional backup generation is eliminated. By considering the CHP system installed in Radisson Hotel in Santa- Maria, California, we can examine effectiveness of MTG based systems. Two C60-ICHP systems are installed at the Radisson Hotel in Santa Maria, California. In this application, the hot water output is used for several different purposes. One use is for domestic hot water for the hotel guests. This thermal load is highest in the morning, and then increases again later in the day. A second use is for laundry service. This is highest during the working day. The third use is for building heat. This load is seasonal and steady during the day when outside temperatures are low.The two C60-ICHP systems are set to operate in parallel with the electric grid, and Electric Priority mode is used. In this CHP mode, the electric power output for each microturbine is set at the desired level. For this Radisson hotel with 188 rooms, electric power is normally set for maximum from each microturbine during the day. This is below the building’s peak electric demand, and power does not flow back into the electric utility grid. While the microturbines work to maintain their programmed electrical outputs, the exhaust diverters automatically adjust to accommodate the changing thermal requirements of the hotel. This example shows how the flexible control capabilities of the C60-ICHP allow simple integration with a building with changing thermal requirements. The two C60-ICHP systems are set to operate 24 hours per day. The operating scheme was selected to match the thermal requirements of the hotel, provide the maximum electric energy, and reduce time-of-use demand charges from the local electric utility. This results in maximum financial benefit to the hotel, and helps to offload the utility when power is needed most by other customers. Powerhouse Energy supplied the ICHP systems to the hotel and managed the installation, system start-up and continuous operation.ICHP application qualified for the state’s PUC Self- Generation Incentive Program rebate of 30% on the total installed cost. The expected energy savings are very good, with a calculated average savings of about $5,528 a month or $66,336 per year. This savings to the hotel is net of natural gas, projected lifecycle maintenance costs, and project financing. Total installed cost was $185,000. The operating availability of the ICHP systems, including start up, commissioning, and service response time has been better than 95% to date.2. Another example of CHP system is Capstone microturbine installation at Inns of America in Carlsbad, California, was completed in August 2002 by California Power Partners (Calpwr). It included a Capstone C60 with fuel gas booster and separate hot water heat recovery module. As for the Radisson, this microCHP system provides both thermal and electric base load for this hotel. Energy savings were estimated at 40%. This lowered the daily per room energy cost by $4.00 – a significant portion of the hotel’s profit margin.While this installation is saving the hotel owner money every day, there is a unique attribute that provided even more value than anyone envisioned when the decision to purchase this system was made. The hotel owner decided to purchase Capstone’s dual mode version microturbine, with the capability to provide power even when the electric utility is not available. The logic was to avoid the cost of a traditional backup generator, thereby improving the economics of this project. Such traditional diesel backup generators are designed and permitted to operate only for short periods of time in case of a utility outage. But microturbines certified by the California Air Resources Board can operate continuously without the need for local air permits. In October of 2003, Southern California was ravaged by multiple wildfires that lasted days and crippled the state with huge property and personal losses. Carlsbad, where the hotel is located, and the nearby San Diego region were especially hard hit. Power lines were shut down, and many homes and businesses. During this time, the Inns of America lights remained on, powered by the Capstone 60kW microturbine. In support of the local community, the Inns donated a number of rooms for people who, and the Inns of America became an emergency base of operations for several groups. The result for the hotel was increased business and a strengthened relationship with the community – things that were never directly considered in their original decision to install a microCHP system.3. McDonald's restaurant in Chicago, Illinois, gets most of its electricity from a natural-gas- powered microturbine, cutting $1,500 off its total monthly power bill.4. The Chesapeake Building on the University of Maryland campus, College Park, Maryland has a cooling, heating, and power (CHP) system consisting of microturbines, chiller, and stack that uses waste heat to cool and heat the building, significantly increasing system efficiency. Fig 8. Chesapeake Building CHP system, University of Maryland10. CONCLUSIONThus this new scheme of power generation is having ample importance in the present era where we are paying a great attention and care for environment friendly power generations. The power generation using a microturbine is becoming popular in North America, Europe because of its ecofriendly nature along with descent power delivery on considering both efficiency and economics. MTG’s continue to find economic application in a growing market. Integration of hot water heat recovery, absorption chilling, and backup power functions makes for simple solutions that save money and increase power reliability, with the added social benefits of clean emissions, reduced greenhouse gas production, and more efficient use of our limited natural resources. The development of microturbine technology for transportation applications is also in progress. Automotive companies are interested in microturbines as a lightweight and efficient fossil-fuel-based energy source for hybrid electric vehicles, especially buses.Other ongoing developments to improve microturbine generator design, lower costs, and increase performance in order to produce a competitive distributed generation product include heat recovery/cogeneration, fuel flexibility, and hybrid systems (e.g., fuel cell/microturbine, flywheel/microturbine).Manufacturers are moving toward packaging MTGs with integrated heat recovery equipment to lower both the cost of installation and operation. Moreover, this is a clean source of electrical power.A variety of energy consumers that are already using MTG due to its high reliability & low operating cost, neglecting its high initial cost. Undoubtedly this technology will conquer the energy sector in the near future, on considering the present environmental scenario.11. REFERENCES1. D.K.Nicholas & Kevin.P.Loving, ASSESSMENT OF MICROTURBINE GENERATORS, IEEE 2003.2. Amer Al- Hinai & Ali Feliachi, Dynamic Model of Microturbine Used As a Distributed Generator, West Virginia University, 20063. Stephanle.L.Hamilton, MICROTURBINE GENERATOR PROGRAMME, Hawaii Intnl. Conference on System Sciences, 2000.4. Microturbine Power Conversion Technology, R.H.Staunton & B.Ozpineci.5. ................
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