MICROTURBINES

                                                      ABSTRACT

              Distributed generation is one of the important field of research now a days. Market prospect for microturbine for distributed power generation and their associated high grade heat extremely encouraging. Therefore microturbines are becoming a point of study. Presented paper tries to discus the microturbine concept, technology description gives technical and practical background through basic process and thermodynamic cycle. Various components of microturbine and their performance is briefly analyzed.

             To improve this characteristics and efficiency the points like development of chemically recuperated gas turbine is added. Microturbine economics is a big question .Fuel used for microturbine gives list of all possible fuel to be used. Manufacturers and availability of microturbine is one important discussed point. Application gives idea about customer range, customer targeted stand by power, hybrid   electric vehicles, CHP operation, etc. At the end conclusion is drawn for the feasibility study of microturbine                                         

  TECHNOLOGY   DISCRIPTION OF MICROTURBINES

2.1 Basic Processes

                               Microturbines are small gas turbines, most of which feature an internal heat exchanger called a   recuperator. In a microturbine, a radial flow (centrifugal) compressor compresses the inlet air that is then preheated in the recuperator using heat from the turbine exhaust. Next, the heated air from the recuperator mixes with fuel in the combustor and hot combustion gas expands through the expansion and power turbines. The expansion turbine turns the compressor and, in single shaft models, turns the generator as well. Two-shaft models use the compressor drive turbine’s exhaust to power a second turbine that drives the generator. Finally, the recuperator uses the exhaust of the power turbine to preheat the air from the compressor. Single-shaft models generally operate at speeds over 60,000 revolutions per minute (rpm) and generate electrical power of high frequency, and of variable frequency (alternating current --AC).  This power is rectified to direct current (DC) and then inverted to 60 hertz (Hz) for U.S. commercial use. In the two-shaft version, the power turbine connects via a gearbox to a generator that produces power at 60 Hz. Some manufacturers offer units producing 50 Hz for use in countries where 50 Hz is standard, such as in Europe and parts of Asia.

2.2 Thermodynamic Cycle

Microturbines operate on the same thermodynamic cycle, known as the Brayton cycle, as larger gas turbines. In this cycle, atmospheric air is compressed, heated, and then expanded, with the excess power produced by the expander (also called the turbine) over that consumed by the compressor used for power generation. The power produced by an expansion turbine and consumed by a compressor is proportional to the absolute temperature of the gas passing through those devices. Consequently, it is advantageous to operate the expansion turbine at the highest practical temperature consistent with economic materials and to operate the     compressor with inlet airflow at as low a temperature as possible. Higher temperature and pressure ratios result in higher efficiency and specific power. Thus, the general trend in gas turbine advancement has been towards a combination of higher temperatures and pressures. However, microturbine inlet temperatures are generally limited to 1,800ºF or below the use of relatively inexpensive materials for the turbine wheel, and to maintain pressure ratios at a comparatively low 3.5 to 4.0.

2.3 Basic Components

2.3.1 Turbo-Compressor Package

                                           The basic components of a microturbine are the compressor, turbine generator, and recuperator   Figure 2.3.1. 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 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, microturbine turbo machinery is based on single-stage radial flow compressor and turbines. Rotary vane and scroll compression are the most commonly used technology in the microturbine industry. Second generation gas compressor technologies are in development or being introduced. That may reduce costs and target on-board application Rotary vane compression technology offers a wide range of gaseous fuel flexibility Parasitic loads vary based on type of gas and inlet  pressures available, general rule 4 to 6% for natural gas and 10 to 15% for bio gas.

2.3.2 Generator

                    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 a permanent magnet (typically Samarium-Cobalt) alternator, and requires that the high frequency AC 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 a battery UPS) is used to power the generator for start-up.

2.3.3 Recuperators

                    Recuperators are heat exchangers that use the hot turbine exhaust gas (typically around 1,200ºF) to preheat the compressed air (typically around 300ºF) going into the combustor, there by reducing the fuel needed to heat the compressed air to turbine inlet temperature. Depending on microturbine 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 microturbine exhaust, reducing the micro turbine’s effectiveness in CHP applications.

2.3.5 Air bearings

                 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.

2.3.6 Power Electronics

.              The high frequency AC is rectified to DC, inverted back to 60 or 50 Hz AC, and then filtered to reduce harmonic distortion.. To allow for transients and voltage spikes, power electronics designs are generally able to handle seven times the nominal voltage. Most icroturbine power electronics are generating three phase electricity. Electronic components also direct all of the operating and startup functions.

                                                                 

                    FIG 2.3.1 MICROTURBINE BASED CHP SYSTEM ( SINGLE SHAFT )

                               

             

FIG  2.3.2   BASIC  PARTS OF MICROTURBINE

                                                                     

                                                                        
                                                                       

 FIG 2.3.3 MICROTURBINE CONSTRUCTION

Chapter 3

                                  

Design Characteristics 0f microturbines

                                  Thermal output: Microturbines produce thermal output at   temperatures     in the  400 to 600°F range, suitable for supplying a   variety of building  thermal needs.

                  

Fuel flexibility:       Microturbines can operate using a number of different fuels:                                 

                                 Sour gases (high sulfur, low Btu content), and liquid fuels such                               

                                 as gasoline, kerosene, natural gas and diesel fuel/heating oil.

Life                          Design life is estimated to be in the 40,000 to 80,000 hour range.             

Size  range:              Microturbines available and under development are sized                                                                                                                              

                                  From 25 to 350 KW

Emissions:              Low inlet temperatures and high fuel-to-air ratios result in   NOx                                                                                                                              

                                 Emissions of less than 10 parts per million (ppm) when

                                  Running on natural gas               

Modularity:            Units may be connected in parallel to serve larger loads and     

                                 Provide power reliability

                      

Dimensions:            About 12 cubic feet.        

Chapter 4

Microturbines and distributed generation

  Distributed generation, a concept first promoted by Thomas Edison in the 19^th century, is rewiring the way facility. Operators and environmental mangers think about how electric power can be produced and distributed. For decades, energy users have waited for the promise of fuel cells, solar panels, and wind turbines to translate into reliable and economically viable sources of power. The table shown below compares the microturbines with other D.G.  resources. Microturbines are quietly delivering on those promises and proving to be a supplement to traditional forms of power generation.    Moving away from 100% dependence on the utility power grid to having an onsite microturbine power supplement is, admittedly, a Para diagram shift. But for progressive environment mangers worldwide, microturbines are quickly becoming an energy management solution that saves money, resources, and the environment in one compact and scalable package- is it stationary or mobile, remote or interconnected with the utility                

                                                                       

Chapter 5

Economics of Microturbines[

                                        Micro turbine capital costs range 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 vary significantly by location but generally add 30-50% to the total installed cost. Micro turbine 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, micro turbine 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 micro turbine 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.

Micro turbine Cost
Capital Cost	$700-$1,100/kW
O&M Cost	$0.005-0.016/kW
Maintenance Interval	5,000-8,000 hrs

Chapter 6

Fuel Flexibility of microturbines[2][3]

             Microturbines are small power plants operate on natural gas, diesel, gasoline or other similar high-energy, fossil fuel. However, research is progressing on using lower grade; lower energy fuels such as gas produced from biomass to power the microturbine. This gas, called biogas, is a combustible gas derived from decomposing biological wastes that have undergone conversion by biological decomposition called anaerobic digestion or by thermal decomposition in a gasifier which is called pyrolysis.

In a forest, a gasifier could be used to convert wood chips and pine needles to a biogas on site. By making modifications, the turbine will be able to utilize low pressure fuels with lower energy content than traditional fuels. Natural gas-fired turbines have fuel with a heating value of 1,000 British thermal units per cubic foot. Biogases typically have between 10 and 20 percent of the heating value of fossil fuels. The thrust of current research is concentrated on fuel flexibility. The goal is to modify microturbines so they can utilize low energy, low pressure biogases. In order to do this, a key change is to add a catalytic combustor. An added benefit of the catalytic combustor is that it will eliminate the formation of nitrogen oxides, a technology breakthrough. These modified microturbines have been nicknamed "Flex-microturbines".                                        

Chapter 7

              

                    MIROTURBINES TEST PROCEDURE & INSTALLATION

7.1 Test Procedures [3]

                                        To fully evaluate the MTGs, a series of tests were developed. Testing of MTGs has been   categorized into the following phases:

• Installation and Startup.
• Operation and Maintenance.
• Performance.

      

7.2 Installation and Startup Procedures

                                  Each MTG delivered to the test site was inspected and confirmed to include:

·         Operating instructions.

·         Repair parts or a recommended spare parts list.

·         Consumable supplies.

·         Troubleshooting and maintenance procedures/guides.

·         Drawings and diagrams sufficient to support maintenance.

7.3 Performance Procedure

      For the test program, MTGs were operated for as long as practicable at the full load the units were capable of producing under ambient conditions. Daily operating parameters: fuel flow, ambient air pressure, temperature and humidity, energy output, operating temperatures, and pressures were recorded. The recorded MTG parameters were used to determine heat rate and   efficiency, gross and net peak kilowatts, operating hours, capacity factor and availability. Capacity factor for an operating period is the ratio of the actual kilowatt hours generated to the maximum potential kilowatt hours for the rating of the unit. Availability for an operating period is the ratio of the hours the unit was not restricted from operation due to maintenance or repairs unit to the maximum possible hours of operation. Peak gross power is defined as the peak power output by the MTG inverter.

                                                        Chapter 8

  

             Development of Chemically Recuperated Micro gas turbine [1]       

outline of the chemically Recuperated Gas Turbine (CRGT) System

              Figure 8.1 shows a block diagram of a CRGT system. In an MGT system, turbine exhaust temperature is about 600°C, and the power generation efficiency is increased by heat recovery of air recuperator, but in CRGT system, the reformer recovers the turbine heat first. As shown in Fig. 8.1 .The equipment after the reformer is the air recuperator, and next is the evaporator, but the system works if sequence of lie equipment after the reformer is arranged in accordance with layout of the engine system of micro gas turbines The amount of   heat recovered by the reformer is determined by enthalpy change of chemical reaction which converts fuel and steam into hydrogen rich gas. The principal reactions in a reformer are expressed as follows:

                                                                                                                                

                  The reaction rate of water gas shift is larger than that of steam reforming, and it is regarded as equilibrium. Thus, the molar stoicometric ratio between methane and steam (i.e., steam carbon ratio, SIC) in the above reactions is 2.0, but SI\C is operated from 3 to 4 in order to avoid deposition of carbon by Boudouard’s reaction expressed as follows:

                        The steam reforming reaction occurs partially below 600°C, methane of 100% is not converted into hydrogen-rich gas. The maximum conversion of methane is limited thermodynamically, which is influenced  by physical conditions: temperature T, total pressure P. and SI\C   For example, equilibrium conversion is about 35% under the conditions of ‘I’ 500°C, P=0.4 MPa, and S/C = 4.0. The reaction pressure P in the reformer is higher than that in the combustor and it ranges between 0.3 and 0.5 Mpa. The more temperature or S/C increases, the more equilibrium conversion increases. Therefore, it is preferable that temperature and SIC in the reformer are set high to enlarge heat recovery.                   

However, configurations of the gas turbine and the evaporator restrict the amount of steam, and S/C has an upper limit. In the gas turbine, surge limit of the compressor determines the maximum flow rate including steam through the turbine. In the evaporator, the upper limit of    steam generation is determined because the pinch point temperature difference becomes critical. Hence, the maximum S/C is set to approximately 7 Besides, the temperature in the reformer   is influenced by the configuration of the reformer because the reaction occurs in the reformer exchanging heat with turbine exhaust. Hence, the design of the reformer is very important

 Figure 8.1 shows the principal stream conditions of a chemically recuperated MGT based on a commercial 75 kW MTG system using natural gas as fuel. Figure8.2 shows a schematic illustration of heat balance in the system .Figure 8.2 also shows each energy percentage of natural gas LHV (lower heating value) .. In the system shown in Fig. 8.1, as much steam possible is generated to enlarge heat recovery by large conversion and S/C is set to 6.3. The heat duty of the reformer is estimated considering reactions as equilibrium, and the reformer uses exhaust heat from 656°C to 561°C. The mole fraction of hydrogen is the reformer output is 24%, and the conversion rises to 51%. The heat recovery at the reformer is 82.4 kW, which recovers 25.6% of fuel LHV. The air   recuperator is placed after reformer and it heats compressed air until 520°C using exhaust heat from 561°C to 274°C. This recovers 40.5 kW. The evaporator uses the bottom of exhaust heat from 274°C to 121°C. According to this system analysis, the output and the efficiency were expected to improve up to 98 kW and 30.4%, respectively, comparing with 75 kW and 28% of the original MGT.

                                                   

                      

                          Fig 8.1 The principle stream conditions for CRGT system                       

                      

                                                                                      

               Fig 8.2 heat balance in CRGT system expressed as % of natural gas LHV

Chapter 9

Applications of Microturbines [4]

Combined heat and power (co-generation) (chp)
 Waste heat from the microturbine 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 greenhouse to grow plants; water can be ducted to provide central heating in buildings in winter. Thermal hosts can be found easier because the heat produced by each microturbine unit is so much smaller than that by a large power station.

Distributed power generation
Hospitals, hotels, factories and holiday resorts can install distributed power systems on site to supplement power supplied by grid. Also, electricity can be generated at remote sites without grid access. Distributed generation provides a wide range of services to consumers and utilities, including standby generation, peak shaving capability, baseload generation and co-generation.

Hybrid (microturbine connected to high speed alternator):[4]
  In hybrid vehicle applications, the power produced by a microturbine is converted into electricity by a high-speed 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.

Hybrid vehicle (microturbine and fuel):

       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 microturbine’s turbine stage. If the fuel cell produces enough heat, the microturbine can generate additional wer. For the hybrid combination, efficiency is expected to be as much as 60% and emissions less than 1.0 ppm NOx, with negligible Sox and other pollutants.

Chapter 10

Conclusion

                                                    

                                                      The drawbacks of centralized power generation and shortage of power leading to concept of Distributed generation (DG).DG tends to several advantages and concept of DG is more feasible. Microturbine is the application of DG .The history of IC engine . Shows several year research works for today’s better result. Therefore microturbine is tomorrow’s world. Microturbine can use low grade of fuel very effectively like waste gases, sour gases etc.

                                           Thus microturbine gives chance of low fuel cost and less emission. The dimensions of Microturbine comparatively small by which it can be installed at field where power is consumed. It has few efficiency problems. Due to chemical recuperation the thermal efficiency increases sharply. Microturbine is also effective in CHP operation .It is having  problem of   Starting time and that’s why it fails as standby power generator compared to IC engines. In India the microturbine is quite useful. The power shortage effect can be solved using microturbine, using fuels like biogas, etc .But in India the technology is still underdevelopment so the present seminar is an honest attempt to introduce microturbine technology in India for solving the problem of power generation in future.