94104330 InTech Micro Gas Turbines

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7 Micro Gas Turbines 1Dipartimento di Energetica – Università Politecnica delle Marche 2Università degli Studi e-Campus Italy Flavio Caresana1, Gabriele Comodi1, Leonardo Pelagalli1 and Sandro Vagni2 1. Introduction Conventional gas turbines (GTs) range from a size of one or a few MWe to more than 350 MWe (GTW, 2009). Those at the small end of the range are commonly used in industrial applications, for mechanical or onsite electrical power production, while the larger ones are usually install
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  7 Micro Gas Turbines Flavio Caresana 1 , Gabriele Comodi 1 ,Leonardo Pelagalli 1 and Sandro Vagni 2   1 Dipartimento di Energetica – Università Politecnica delle Marche 2 Università degli Studi e-CampusItaly 1. Introduction Conventional gas turbines (GTs) range from a size of one or a few MWe to more than350 MWe (GTW, 2009). Those at the small end of the range are commonly used in industrialapplications, for mechanical or onsite electrical power production, while the larger ones areusually installed in large-scale electrical power plants, often in combined cycle plants, andare typically located far away from the consuming region.In the future distributed energy systems based on small local power plants are likely tospread; since they lie close to the final users, they reduce electrical transport losses, andmake thermal energy recovery profitable both in energy-related and in economic terms(Papermans et al., 2005; IEA, 2002). These benefits explain the increasing interest in small-size generation systems.Recently, gas turbines < 1 MWe, defined as micro gas turbines (MGTs), have appeared onthe market. MGTs are different from large GTs and cannot therefore be considered merelyas their smaller versions. Their advantages as distributed energy systems lie in their lowenvironmental impact in terms of pollutants and in their competitive operation andmaintenance (O&M) costs. MGTs appear to be particularly well suited for service sector,household and small industrial applications (Macchi et al., 2005; Zogg et al., 2007). 2. The technology of Micro Gas Turbines The small power size of MGTs entails implications that affect the whole structure. Inparticular the low gas mass flow rate is reflected in machine size and rotational speed: thesmaller the former, the greater the latter. MGTs therefore differ significantly from GTs,mainly in (i) the type of turbomachines used; (ii) the presence of a regenerator; and (iii) thehigh rotational speed, which is independent of grid frequency. In fact unlike GTs, MGTscommonly use high-revving, single-stage radial turbomachines rather than multi-stage axialones, to achieve greater compactness and low manufacturing costs. As a consequence of thehigh rotational speed, the electrical current is generated at high frequency and is thenconverted to the grid frequency value (50 or 60 Hz) by power electronics. Theturbocompressor and turbine are usually fitted on the same shaft as the electrical generator,which also serves as the starting motor. Single-stage radial machines afford limitedcompression ratios and need a regenerative cycle to attain satisfactory electrical efficiency.  Gas Turbines 146 Therefore a regenerator is usually installed between the compressor and the combustionchamber. Figures 1 and 2 show, respectively, the layout and corresponding thermodynamiccycle of a typical cogeneration MGT. EGGCGTHRBRCC 234567 PE ElectricityFuelExhaustsWaterInWaterOut BPV   PE Power Electronics CC Combustion ChamberEG Electrical Generator R RegeneratorGC Gas Compressor BPV ByPass ValveGT Gas Turbine HRB Heat Recovery Boiler  Fig. 1. Layout of a typical cogeneration MGT 010020030040050060070080090010003.63.844.24.44.64.855.25.4s (kJ/(kg K))    t   (   °   C   )  Fig. 2. MGT regenerative Brayton-Joule cycleThe ambient air (1, in both figures) is compressed by the centrifugal compressor; it thenenters the regenerator (2), where it is preheated by the exhausts coming from the turbine,and is conveyed from the regenerator (3) to the combustion chamber, where it is used in the1764532  Micro Gas Turbines 147 combustion process to achieve the design turbine inlet temperature (4). The hot gases thenexpand through the turbine (5) and enter the regenerator. Given their fairly hightemperature at the power unit exit (6), the exhausts can be sent to a heat recovery boiler(HRB), where they are used to heat water, before being discharged to the flue (7). In thisconfiguration combined heat and power (CHP) production increases fuel energy conversionefficiency. When the thermal power demand is lower than the power that can be recoveredfrom the exhausts, part of the fumes is conveyed directly to the chimney (7) via a bypassvalve (BPV). The core power unit is fitted with auxiliary systems that include (i) fuel, (ii)lubrication, (iii) cooling, and (iv) control systems. The fuel feeding system compresses thefuel to the required injection pressure and regulates its flow to the combustion chamberaccording to the current operating condition. The lubrication system delivers oil to therolling components, with the dual effect of reducing friction and removing heat. The coolingsystem keeps the operational temperatures of the different components, lubrication oilincluded, in the design ranges. The cooling fluid can be air, water, or both. The function ofthe electronic control system is to monitor MGT operation through continuous, real timechecking of its main operational parameters. 3. Operation modes MGTs can usually operate in two modes:1.   Non-cogeneration (electricity production only): the MGT provides the electrical powerrequired by the user and all the available thermal power is discharged to the flue.2.   Cogeneration (combined production of electrical and thermal energy): the MGTproduces the electrical and thermal power required by the user. MGTs operating incogeneration mode can usually be set to work with electrical or with thermal powerpriority. a.   Electrical priority operating mode In this operating mode the MGT produces the electrical power set by the user,while heat production is regulated by the BPV installed before the HRB. This is notan energy efficiency-optimized operating mode, because in conditions of highelectrical and low thermal power demand a considerable amount of the recoverableheat is discharged to the flue.  b.   Thermal priority operating mode Thermal priority operation involves complete closure of the MGT bypass valve, sothat all the exhaust gases from the regenerator pass through the HRB for thermalpower recovery. Thermal power production is regulated by setting the electricalpower. This mode maximizes MGT efficiency in all operating conditions. 4. Performance and emissions The considerations made so far apply to most MGTs. The data presented below have beenobtained from theoretical studies and experimental testing of a specific machine, aTurbec T100 PH (Turbec, 2002), which the authors have been using for their research workfor several years (Caresana et al., 2006). With due caution, these findings can be extended tomost MGTs. In this section, the performance and emissions of a real MGT-based plant arereported and some criticalities connected to MGT behaviour highlighted.The main performance parameters of an MGT are:  Gas Turbines 148 ã   electrical power el P ; ã   thermal power th P ; ã   electrical efficiency el η  , defined as: elel f  PmLHV  η  =  (1) ã   thermal efficiency th η  , defined as: thth f  PmLHV  η  =  (2) ã   total efficiency tot η  , defined as: elthtotelth f  PPmLHV  η η η  += = +  (3)where  f  m  and LHV  are the mass flow rate and the Lower Heating Value of the fuel,respectively.Since electrical power is the main final output, we have represented the dependence of theother performance parameters on P el (Figures 3-7). Unless specified otherwise, theexperimental data refer to ISO ambient conditions, i.e. temperature and relative humidity(R.H.) equal to 15 °C and 60 % respectively (ISO, 1989). 182022242628303230405060708090100110Electrical power (kW)    E   l  e  c   t  r   i  c  a   l  e   f   f   i  c   i  e  n  c  y   (   %  Fig. 3. Electrical efficiencyFigure 3 plots the trend of the electrical efficiency, which is consistently high from thenominal power down to a partial load of about 70 %, with a maximum slightly > 29 %around 80 kWe. Figures 4 and 5 report the thermal power and total efficiency data,respectively, for different degrees of BPV opening, calculated as the ratio between thethermal power recovered and that which can be recovered at the nominal power. The testswere conducted at a constant water flow rate of 2 l/s entering the HRB at a temperature of50 °C.
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