Analysis of regulation methods of a combined heat and power plant based on gas turbines

Energy 72 (2014) 574e589

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Analysis of regulation methods of a combined heat and power plant based on gas turbines nez-Espadafor Aguilar*, R. Rodríguez Quintero, E. Carvajal Trujillo, Francisco Jime Miguel Torres García Escuela T ecnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos, s/n, 41092 Sevilla, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 February 2014 Received in revised form 7 May 2014 Accepted 21 May 2014 Available online 16 June 2014

This paper addresses the study of eight different regulation methods on a cogeneration plant integrated by two gas turbines model LM2500 coupled to an HRSG (heat recovery steam generator), which provides steam at high and medium pressure and thermal oil. This study is aimed at showing the capacity of each regulation method meaning the range of thermal power that is available for a given electrical power. This responds to one of the main problems raised by the cogeneration systems: satisfying both electrical and thermal power demands which might vary and not be coupled, simultaneously. The performance parameters, as energetic efficiency and energy saving potential are also evaluated, which enables the assessment and comparison of the cogeneration plant responses under the different regulation methods. The results obtained allow a grading of the regulation methods based on three parameters: regulation capacity, efficiency and energy saving potential. This grading enables to rule out those regulations methods that perform worst results or might damage the Gas Turbine. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Combined heat and power Part-load Cogeneration plant regulation

1. Introduction Combine heat and power plants have become a key issue for the European policies aimed at the challenges posed by the increase dependence of energy imports, the scarcity of energy resources and the climate change. The Energy and Climate package sets out the targets known as the “20e20e20” [1], meaning three objectives for 2020: (i) 20% reduction in EU (European Union) greenhouse gas emissions from 1990 levels; (ii) raising the share of EU energy consumption produced from renewable resources to 20%; (iii) 20% improvement in the EU's energy efficiency. Cogeneration e the simultaneous utilization of heat and power from a single fuel or energy source e plays an important role on these policies due to its significant potential for saving primary energy. The European Union institutions are aware that this potential is largely untapped in the Member States, despite the previous Directive 2004/8/EC [2] to foster CHP (combine heat and power). Hence, the Directive 2012/ 27/EU [3] on energy efficiency was adopted to further promote this technology, among other objectives. Moreover, the advantages of cogeneration are widely agreed beyond the EU boundaries. The * Corresponding author. Tel.: þ34 954487245; fax: þ34 954487243. nez-Espadafor E-mail addresses: [email protected], [email protected] (F. Jime Aguilar). http://dx.doi.org/10.1016/j.energy.2014.05.083 0360-5442/© 2014 Elsevier Ltd. All rights reserved.

International Energy Agency has addressed the assessment of the CHP through cost-benefit analysis in two reports released in 2008 and 2009 [4,5]. Some of the main conclusions were that CHP can reduce CO2 emissions arising from new generation in 2015 by more than 4% (170 Mt/year), while in 2030 this saving increases to more than 10% (950 Mt/year), while reducing power sector investments by USD795 billion over the next 20 years, around 7% of total projected power sector investment over the period 2005e2030, by mean of reduced need for transmission and distribution network investment, and displacement of higher-cost generation plant. On the other hand, cogeneration must face the problem of supplying two divergent and variable energy demands at the same time. Heat and electrical demands both from industrial and residential consumers fluctuate in a wide range of loads. The heat demand varies daily and seasonally, as a function of the heating demand. In the Mediterranean climate, the thermal power demand in a typical hospital might vary from 200 kW in summer months to 900 kW in winter months [6]. Electrical energy demand also varies daily and seasonally, bound to the needs of light and space heatingcooling. The cogeneration plant shall respond to both electrical and thermal load profiles, whose curves and magnitudes differ, simultaneously and at suitable efficiency ratios. The study of the different methods of regulation goes through the capability of the cogeneration plant to satisfy both demands at good performance ratios.

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Nomenclature A av C CHP Cpg Cpw EES ESI EU GSP GT Hadd

heat exchanger area average HRSG (heat recovery steam generator) parameter combine heat and power gas specific heat water/steam specific heat Engineering Equation Solver energy saving indicator European Union Gas turbine Simulation Program gas turbine additional thermal power demanded by the bleeding method HRSG heat recovery steam generator i load condition for validation IAT inlet air throttle K2 constant of evaporator formulation LHV lower heating value N rotor speed Nc corrected rotor speed NTU number of transfer units OAT outlet air throttle p parameter validated P total compressor inlet pressure in Nc and wc equations Pr total pressure ratio Pref reference pressure Prsurge_line total pressure ratio at surge line Prrunning_line total pressure ratio at running line Q thermal power exchanged in the heat exchangers R specific gas constant Rref reference specific gas constant T temperature Tref reference temperature t1 entry gas temperature to the evaporator t2 exit gas temperature from the evaporator

Gas Turbine regulation for cogeneration purposes is directly related to part-load performance. Any change on mechanical power, in this case due to regulation requirement, produces also a change on gas flow and exhaust gas temperature and therefore on cogeneration capability. In this sense, the analysis of regulation methods focused on cogeneration plants involve the part-load performance study. The work of Haglind and Elmegaard [7] shows a methodology for part-load performance prediction in Gas Turbines that has been adapted for this study. Different regulation methods applied to district heating are studied by C. Carcasci and N. A. Colitto Cormacchione [8] which examined the performance of gas turbine CHP systems for different operating conditions when providing heat to a district heating system. Several models of gas turbines are studied under the conditions of a heat load profile and different regulation methods: inlet guide vane angle and variable stator vane angle, GT (gas turbine) mass fuel control, compressor bleeding, backflow, inlet air throttling and exhaust boiler bypass. Several parameters and indicators are assessed in order to determine the optimal choice to fit the heat load profile, depending on the ambient temperature. The response of the compressor to the methods backflow and IAT (inlet air throttle) is not described. Alternative methods as air inlet heating nez-Espadafor et al. [9] fall out the scope of this study. F. Jime analyzed three feasible regulation methods to study the potential

575

Tbackflow compressor outlet temperature in backflow method Tbleeding compressor outlet temperature in bleeding method Tgas entry HRSG gas flow entry temperature TIT turbine inlet temperature Tref reference temperature ts saturation temperature of the steam Tsteamentry entry steam temperature to the super heater Tsteamout exit steam temperature from the super heater U overall heat transfer coefficient VAN turbine variable area nozzle VGV compressor variable guide vane w air flow wbleeding air flow to the combustion chamber in the bleeding method wc corrected air flow wcomp_backflow air flow the compressor inlet in the backflow method wcomp_bleeding air flow the compressor inlet in the bleeding method We electrical power provided by the GT (gas turbine) wf mass fuel flow wg mass gas flow Wth thermal power provided by the HRSG ww mass steam flow g heat capacity ratio gref reference heat capacity ratio DPbackflowdifference between the compressor outlet and inlet pressure in the backflow method DPbleeding difference between the compressor outlet and inlet pressure in the bleeding method 3 heat exchanger efficiency hc isentropic compressor efficiency hele electrical efficiency henergy global plant energy efficiency hth thermal efficiency rair air density

energy savings and the increase of profitability with the lowest transformation cost. The object of this study is a real cogeneration plant at different climate conditions. The regulation methods are Gas Turbine Power turbine regulation, Inlet gas turbine temperature and supplementary firing. An economic analysis is also carried out taking into account the Spanish legal framework. F. Haglind [10] quantified the effects of variable geometry on the gas turbine part load performance in two different configurations, two-shaft and single-shaft. Finally, the regulation by mean of heat storage in a district heating system was modelled and studied by A. Pini Prato et al. [11]. This study is aimed at showing the capability of feasible regulation methods to broaden the map of thermal and electrical loads, together with the performance parameters, which enables the evaluation and optimization of the cogeneration plant response and the effectiveness of each method. The cogeneration plant is integrated by two gas turbines model LM2500 coupled to an HRSG (heat recovery steam generator), which provides steam at high and medium pressure and thermal oil. The regulation methods considered in this work are the following:  Turbine inlet temperature  Air inlet heating, with three different thermal sources  Compressor inlet throttle

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Compressor outlet throttle Back-flow Bleeding Compressor variable guide vanes (VGVs) Turbine VAN (variable area nozzle)

This work considers the performance of the plant from the point of view of power generation and efficiency eelectrical and thermal- and the effect of the different regulation methods considered on the gas turbine running line and operating points. This allows to find out additional limits to some of the regulation methods studied. The analysis has been performed using GSP (Gas turbine Simulation Program) [12] and EES (Engineering Equation Solver) [13]. The GSP model has been validated with the real Gas Turbine data for the LM2500 un-modified [14]. ‘LM2500 Marine Gas Turbine Performance data’ provided by General Electric is the document that gathers steady-state performance data for that gas turbine. The data are provided up to the maximum capacity of the engine, and are based on an average engine in the new and clean condition. The engine performance reflects operation up to a gas generator corrected speed of 10,050 rpm, or to a power turbine inlet temperature of 1158 K [14]. 2. Cogeneration plant description The components of the cogeneration plant studied are two gas turbines LM2500 coupled to a Heat recovery steam generator equipped with afterburning. Fig. 1 depicts the scheme of the cogeneration plant. The LM2500 gas turbine is arranged in a two-shaft configuration, see Fig. 2: a gas generator followed by a power turbine, whose main features at ISO (International Organization for Standardization) conditions and 3000 rpm are the following:  Nominal power: 24,293 kW  Nominal heat rate: 10,086 kJ/kWh  Nominal exhaust gas flow: 254.6 t/h This is a two-shaft design, with the gas generator mechanically uncoupled from the power turbine. This arrangement enables the power turbine to operate at the continuous speed of 3000 rpm regardless the speed of the gas generator. Compressor has 16 axial

stages and the high and low pressure turbines feature two and six stages respectively. Both gas turbines are coupled to a natural circulation heat recovery steam generator the HRSG that recovers the waste heat from the exhaust gases, comprising ten sections of evaporators, economizers and super heaters with two boilers for steam and one economizer for heating oil, Therminol 68 [15]. In the HRSG, superheated steam is generated at two pressures (10 and 64 bar) and thermal oil is heated from 290  C to 345  C. All these products are sold to the chemical industries near the plant. Table 1 shows generated flows for steam and oil from HRSG at ISO conditions when HRSG inlet gas flow 500 t/h at 555  C. 2.1. Gas turbine model description The real data of the gas turbine provided by the manufacturer have been simulated through the GSP. Some regulation methods were also simulated by mean of the components provided by this software. GSP is an off-line component-based modelling environment for gas turbines developed by the NLR (National Aerospace Laboratory, Amsterdam). Simulation with GSP is based on zerodimensional modelling of the processes in the different gas turbine components with aero-thermodynamic relations and steady state characteristics from component maps. Air and gas properties, thermodynamically averaged over the flow cross-areas are used in the calculations [12]. GSP is an “off-design” model. A predefined design point is calculated first from a set of user specified design point data. The deviation from the design point is calculated by solving a set of nonlinear differential equations. GSP calculates engine performance and gas condition changes across the components using the following equations:       

equations for conservation of mass equations for conservation of energy the perfect gas state equation the isentropic flow equation equations for conservation of momentum of gas flow the equation for rotor inertia effects equations for heat flux between the gas path, material and ambient environment.

Fig. 1. Scheme of cogeneration plant.

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577

Fuel

Toutlet compressor Poutlet compressor

Tinlet power turbine Pinlet power turbine

Combustion chamber

Compressor

Power turbine Generator

Tinlet, Pinlet

Texhaust

Air Inlet

Gas generator Fig. 2. Gas turbine scheme.

From these equations, a set of NDEs (non-linear differential equations) is arranged and solved by the GSP solver. Since gas turbine off-design models are particularly non-linear, and therefore GSP has its own NewtoneRaphson based solver optimized for gas turbine models. The model operating point is defined by a number of states collected in a state vector. The number of NDEs equals the number of states and the deviation from a valid solution is represented by the error vector which holds the error variables. The GSP solver iterates towards the solution where all errors are zero within the user specified tolerance [16]. NLR's GSP software has been applied to work out a model able to simulate the data of the GT LM2500 provided by the manufacturer, see GT configuration at Fig. 2. The modelling environment of GSP is based on engine components whose parameters are modified to fit the nominal conditions, at a first stage of the modelling and the off-design conditions as a result of a further fine-tuning of the parameters. The simulation needed the adjustment of the following parameters of each component:  Inlet: air flow at design conditions and pressure drop. Temperature and Pressure at ambient conditions.  Compressor (16 Stages): it was simulated by one compressor component whose parameters were tuned to match the real data (rotor speed, pressure ratio, corrected air flow and efficiency at design conditions).  Fuel control: fuel flow at design conditions  Combustion: type of fuel, constant pressure specific heat, fuel LHV (lower heating value) at reference temperature, C/H ratio. Combustion efficiency.  Gas generator turbine: efficiency at design conditions.  Power turbine: efficiency at design conditions. Rotor speed.  Load control: table including the steady state off-design series (real values of load provided by the manufacturer).

Both real data and model results are shown in Fig. 3 for comparison, in a scale 0e1 related to the nominal real point of each parameter. Real data are depicted by points and model results as pointlines. The parameters validated have been the following:       

Air flow Fuel consumption Compressor rotor speed Temperature outlet compressor Pressure outlet compressor Temperature inlet power turbine Temperature exhaust gases The error has been calculated as described in Eq. (1)

errorp;i




real  model

p;i p;i
¼



realp;i

Where: errorp;i ¼ error of the parameter p at i conditions realp;i ¼ real value of the parameter p at i conditions modelp;i ¼ model result of the parameter p at i conditions p ¼ each parameter validated i ¼ each load of the steady state series within the range 746e24,293 kW provided by the Gas Turbine manufacturer. The average error (errorp;av ) of each parameter validated was calculated according the Eq. (2).

errorp;av

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n X 1u ¼ t error2p;i n

(2)

i¼1

Table 1 HRSG production at ISO conditions.

Steam 64 bar Steam 10 bar Thermal oil

(1)

Production (t/h)

Outlet temperature ( C)

40.7 13.7 395.0

480 187 345

As it is displayed in Table 2, the simulation performed by mean of GSP results in a model able to match a wide range of parameters at load conditions within the range 750e24,292 kW, with a maximum average error of 5.3% in the parameter worst fitted and minimum average error of 1.5% in the best fitted one.

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578 1

0.9

Pi part load/Pi full load

0.8

0.7

0.6 Air flow Fuel flow Compressor speed Poutlet compressor Toutlet compressor Tinlet power turbine T exhaust gases

0.5

0.4

0.3 5000

7000

9000

11000

13000

15000

17000

19000

21000

23000

25000

We (kW)

Fig. 3. Real data and model results 0e1 scale.

The compressor map and the running line simulated through the GSP model at a range load conditions between the nominal load and 750 kW is shown in Fig. 4. This figure shows total pressure ratio versus corrected mass flow for different corrected compressor speed, both defined at (3) and (4), and compressor isentropic efficiency (hc). Red (in the web version) line defines the compressor surge limit. As it can be appreciated, running line is far enough from surge line as it was expected. This surge margin guarantees the smooth and safe operation of the gas turbine under transient operational conditions or when gas turbine deteriorates due to running hours. The corrected air flow Wc is calculated according Eq. (3).

Wc ¼

W

qffiffiffiffiffiffi T Tref

P Pref

pffiffiffiffiffiffiffiffiffiffiffiffiffi R$gref pffiffiffiffiffiffiffiffiffiffiffiffiffi Rref $g

(3)

(4)

g$T$R gref $Tref $Rref

In order to develop a model for HRSG prediction at full load and part load, plant data of HRSG at full load was filtered, discarding anomalous or non-valid data. There are not operational data or characteristic curves available to provide HRSG performance at part load conditions. Therefore, a model for HRSG part load performance was implemented based on the NTU (number of transfer units) method. 2.2.1. Evaporator Global heat transfer model for the evaporator is reduced to the Eq. (5).

ln

t1  ts U$A ¼ t2  ts wg $Cpg

(5)

Because this HRSG uses water tube evaporators, overall heat transfer coefficient (U) is proportional to wg0.6, where wg is the exhaust gas flow [17]. Including this in Eq. (6) yields:

The corrected rotor speed Nc is calculated according Eq. (4).

N Nc ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2.2. Heat recovery steam generator description

ln

t1  ts K ¼ 2 t2  ts w0:4 g

(6)

Table 2 Data from the manufacture of realp,i at the values of shaft i, and errorp,av for each parameter p. Shaft i (kW)

Fuel flow (kg/s)

Air flow (kg/s)

Compressor speed (rpm)

Toutlet compressor (K)

Tinlet power turbine (K)

Texhaust gases (K)

Poutlet compressor (bar)

24,293 24,235 22,371 20,507 18,643 16,778 14,914 13,050 11,186 7457 3729 2237 746 errorp;av ð%Þ

1.590 1.587 1.454 1.333 1.222 1.122 1.025 0.927 0.829 0.629 0.424 0.343 0.249 5.293

69.95 69.89 68.83 67.16 64.94 62.01 58.76 55.46 51.87 43.72 33.55 28.34 22.77 3.205

10,050 9979 9456 9171 8992 8816 8658 8522 8341 8044 7656 7415 7067 1.481

759.3 757.9 731.8 711.6 694.1 679.7 666.7 653.1 638.3 602.7 554.9 529.7 496.8 4.041

1145.4 1144.4 1092.2 1049.8 1015.0 990.1 967.7 941.8 916.4 859.3 795.8 775.9 728.7 2.221

851.5 850.9 813.4 785.1 763.8 751.8 742.9 732.1 722.9 703.9 692.2 701.1 695.4 2.913

18.96 18.92 18.13 17.28 16.37 15.39 14.38 13.36 12.30 9.99 7.32 6.07 4.70 2.707

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Fig. 4. Compressor map.

where K2 is calculated from HRSG operational data at full load.

Where NTU and C are calculated as the following expressions Eqs. (9) and (10):

2.2.2. Economizers The overall heat transfer coefficient (U) is calculated from the geometric data of the HRSG using the Briggs and Young's correlation for circular or helical finned tubes [17]. The geometric data is given in Table 3: Heat power transferred in the economizer is approximated by Eq. (7).

U$A Cmin   w$Cp min  C¼ w$Cp max

    Q ¼ 3$ wg Cp min $ tg1  tw1

(7)

Counter flow type economizers are used; therefore efficiency 3 is modelled as Eq. (8):

1  exp½NTU$ð1CÞ 3¼ 1  C$exp½NTU$ð1CÞ

(8)

(9)

(10)

where ðw$Cp Þmin and ðw$Cp Þmax corresponds to steam and gas properties, respectively. The previous equations together with the following ones solve the unknown parameters for each economizer:

Q ¼ ww $Cpw $ðtw2  tw1  Þ Q ¼ wg $Cpg $ tg1  tg2

(11)

2.2.3. Super heater The super heater has been considered at constant efficiency, equal to efficiency at full load conditions, and is given by Eq. (12).

Table 3 Geometric data of economizers to calculate U coefficient.

Design pressure (bar) Design temperature ( C) Surface area (m2) Tube outer diameter (mm) Tube wall thickness (mm) Material Number of tubes wide Longitudinal tubes Transverse pitch (mm) Longitudinal pitch (mm) Fins per inch Fin material Fin height (mm) Fin thickness (mm)

NTU ¼

High-pressure economizer 1

High-pressure economizer 2

High-pressure economizer 3

75.2

75.2

75.2

371

371

371

1523 31.8

3047 31.8

4371 31.8

3.2

3.2

3.2

SA-192 46

SA-192 46

SA-192 44

2 76.25

4 76.25

4 79.8

66.04

66.04

69.06

5.5 Carbon steel 16 1.27

5.5 Carbon steel 16 1.27

5.5 Carbon steel 16 1.27

3¼

Tsteamout  Tsteamentry Tgasentry  Tsteamentry

(12)

2.2.4. HRSG model results The quality of the HRSG model has been tested comparing full load simulation results (continuous line) to full load operational data (discontinuous line), shown in Fig. 5. It is relevant to notice that the HRSG model does not fit at 100% to real plant data but for the purpose of this work e the comparison of feasible gas turbine regulation methods for CHP plant- it is considered valid. The TeWth diagram is also created for part load situations, shown in Fig. 6 for 70% of the maximum thermal power. 3. Regulation methods The different regulation methods considered in this study are depicted in the diagrams within Fig. 7.

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Fig. 5. HRSG TeWth diagram at the maximum thermal power.

Each regulation method is briefly described below. 3.1. TIT (turbine inlet temperature) The temperature at the turbine inlet is reduced by reducing the amount of fuel burnt in the combustion chamber, resulting in a decrease of both electrical and thermal power. The latter is calculated by the product of mass flow by exhaust gas enthalpy. This method is the one that provides the baseline to evaluate the performance of the rest of regulation methods.

reducing the net mass flow through the combustion chamber and turbine section. The backflow results in a lower pressure ratio in the engine, a higher turbine outlet temperature and a lower mass flow at turbine outlet. 3.6. Bleeding The air is bled from at the end of the compressor path. The results obtained by this method are similar to back-flow method, but a lower performance is reached since the inlet air is not preheated by the air bled and therefore there is a reduction of the air temperature at combustion chamber inlet what increases a lot fuel consumption.

3.2. Inlet air heating 3.7. VGV (variable guide vane) The air inlet temperature is increased prior to compression by mean of (i) the exhaust gases before they are led to the HRGS, (ii) the high pressure steam produced in the HRGS and (iii) the medium pressure steam produced in the HRGS. The increase of the inlet temperature causes an air flow decrease and a reduction of the pressure ratio, and thus the exhaust gases temperature raises.

Several gas turbines have along the first stages of the compression line variable setting stator vanes. This power reduction system is called Variable Guide Vane VGV. It has been simulated applying the equations and values provided by F. Haglind [10]. 3.8. VAN (variable area nozzle)

3.3. Compressor inlet throttle The air at the compressor inlet is throttled prior to compression, making that the compressor section of the engine works with a higher pressure ratio than that of the expansion ratio in the turbine.

In some gas turbines, it is common to use a power turbine with VAN load control. This regulation method decreases the air flow in the power turbine by shrinking the nozzle area, besides a reduction of the turbine efficiency. It has been simulated applying the equations and values provided by F. Haglind [10].

3.4. Compressor outlet throttle 4. Gas turbine operation limits The air at the compressor inlet is throttled after the compression, prior to the combustion chamber, resulting in a lower pressure ratio at the expansion turbine and a higher temperature of the exhaust gases. This regulation method is very limited by the surge line of the compressor that quickly is crossed by the operation line as throttle valve reduces effective area. 3.5. Back-flow A fraction of the air at the compressor discharge is back flowed to the compressor inlet, thus increasing the temperature at the inlet and

The regulation methods allow to broaden the operation range of the cogeneration plant but this range is constrained due to the operation limits set by the characteristics of the GT. These limits are the following:     

Compressor surge Compressor outlet temperature Gas generator speed Temperature limits at gas generator turbine inlet Temperature limits at the power turbine outlet

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Fig. 6. HRSG TeWth diagram at 70% of the maximum thermal power.

4.1. Compressor limits The operation limits of the compressor are defined by 1) The running line whose pressure ratios (Pr) meet the Eq. (13), according to the operation limit suggested by N.A. Cumpsty [18] to prevent any likely compressor damages due to working conditions close to the surge line:

Prsurge_line  Prrunning_line ¼ 20% Prrunning_line

(13)

Fig. 8 shows the running line, surge line and 20% limit line on the compressor map. The regulation method based on compressor outlet throttle is the most limited by this restriction. It is worth to mention that the 20% limit line accounts for non-stationary operational conditions that are not compatible with stationary simulations. Accordingly, with this surge margin it is guarantee that conditions as transient throttle changes, such as occur when the gas turbine is accelerated; transient geometry deviations such as tip and axial clearance changes following speed variations; and compressor mechanical damage including fouling or blade erosion, do not induce compressor surge. 2) The maximum rotor speed (10,050 rpm) as set in the LM2500 manufacturer data [14]. Maximum gas generator speed is limited by manufacturer which has determined the yield and burst speed of each turbine disc and accordingly it has defined a speed above which the integrity of the disc is compromised. 3) The maximum compressor outlet temperature is set to 520  C, which is the compressor outlet temperature at maximum power conditions and ambient air temperature equal to 54  C, as set in the LM2500 manufacturer data [14]. This limit affects mainly to the air inlet heating regulation methods. 4.2. Turbine limits One of the upper operation limits set by the manufacturer is defined by the maximum attainable gas inlet temperature to the

power turbine, which can reach values up to 1158 K [14], and the maximum rotor speed that for the gas generator is the same as the compressor. The regulation methods allow to increase or, depending on the method, to diminish the reduction of thermal power at turbine exist when electrical power is reduced. This is accomplished in most of the regulation methods by reducing turbine expansion ratio and therefore increasing turbine exhaust temperature. Because the LM2500 has not been designed for cogeneration applications the last low pressure turbine is not refrigerated and therefore exhaust temperature has to be limited to that compatible with allowable blades and discs creep temperature. This limit is considered to be equal to the maximum temperature value that the LM2500 is able to reach according to the manufacturer data (918 K) [14]. This limit affects to almost all the regulation methods and if the low pressure turbine exhaust temperature could be increased the range of attainable thermal power would increase accordingly. Furthermore, maximum gas generator turbine inlet temperature is set at 1460 K which is the maximum temperature reached for the Gas Turbine at any operational condition. This temperature value is provided by the GSP simulation model of the LM2500, since outlet combustion chamber temperature is not within the manufacturer data. The temperature limit is a function of the disc or blade creep temperature and it cannot be surpassed at any operational condition for long operating life. 5. Results for compressor inlet throttle regulation method Although the eight regulation methods shown at Fig. 7 have been fully simulated, for the sake of simplicity the results of the Compressor inlet throttle method is shown here. 5.1. Effects on the air flow and pressure ratio In Fig. 9 the values of the air flow versus the electrical power delivered by the GT are pictured. Each coloured line represents the results at constant Turbine inlet temperature (gas generator), and the points of each line represent the positions of the

582

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Fig. 7. Diagrams of the regulation methods.

throttle valve. In this case each point is the ratio compressor inlet pressure/ambient pressure in the range 1e0.6. The black line plots the results of TIT regulation method, which is the one taken as baseline to compare the results and performances with the other methods. As expected, closing the compressor inlet throttle causes the reduction of air flow and electrical power, as shown in Fig. 9. In Fig. 10 the values of corrected air flow and pressure ratio that correspond to the points in Fig. 9 are depicted over the compressor map. Closing compressor inlet throttle valve produces a decrease of the corrected air flow and thus of the pressure ratio, that moves the compressor running line towards the surge line. As it can be appreciated all the operational points remain within the 20% surge limit defined previously.

5.2. Effects on power turbine inlet and outlet temperatures Fig. 11 shows the power turbine inlet temperature values that correspond to the points represented in Fig. 9 where each coloured line represents TIT constant value and each point a position of the inlet throttle valve. The power turbine inlet temperature increases slightly as the compressor inlet valve is closed, since the exhaust pressure ratio reduces at constant TIT. Fig. 12 shows the power turbine exhaust gas temperature that correspond to the points represented in Fig. 9, built as an analogue chart, where each coloured line represents constant TIT and each point a position of the throttle valve. Exhaust GT temperature follows the same tendency than the power turbine temperature but more intense, due to the reduction of the expansion ratio when closing throttle valve. In fact, it is

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583

25 10050 Nc 9576 9097 20

8618 15

Pr

8139 0.87 10

7661

0.86

7182

0.8

6703

0.78

5

surge line

0.83

Running line

Etac

lim 20%

0 10

20

30

40

50

60

70

80

Wc (kg/s)

Fig. 8. Compressor map including limit of 20% surge line.

appreciated that the exhaust temperature limit blocks the regulation range when exhaust gases get to the limit of 918 K. 5.3. Effects on the regulation capacity Fig. 13 shows thermal power provided by the HRSG versus electrical power delivered by the GT. Each coloured line represents results at constant Turbine inlet temperature (gas generator) and the points of each line represent the throttle valve position. The black line plots outputs for regulation by mean of only TIT temperature, which is the basic regulation method taken as baseline to compare the performance with the other methods. At Fig. 13 the line named “Boundary points” comprises the last point of each line at constant TIT and represents the maximum

thermal power per each electrical power. These boundary points are used to build the chart where the regulation capacities of all methods are shown for comparison, as it is further explained in the following section. 6. Comparative analysis of the regulation methods The comparative analysis of the regulation methods studied is provided building a graphic representation of the boundary points of the charts Thermal power versus Electrical power for each method, as depicted in Fig. 14. Each coloured line represents the boundary points for each regulation method. Fig. 14 allows the comparison of the regulation capacity provided by all the regulation methods, meaning the range of thermal power that is available

150

140

1460 K

130

1430 K 120

1397 K 1364 K

air flow (kg/s)

110

1329 K 100

1293 K 1255 K

90

1218 K 80

1129 K Regulation TIT

70

60 5000

15000

25000

35000

45000

We (kW) Fig. 9. Air flow versus electrical power for compressor inlet throttle method.

55000

F. Jimenez-Espadafor Aguilar et al. / Energy 72 (2014) 574e589

584

surge line

25

1460 K 1430 K 20

1397 K 1364 K 15

1329 K

Pr

1293 K Regulation TIT

10

1255 K 1218 K 5

1129 K 1054 K 1007 K

0 0

10

20

30

40

50

60

70

80

Wc (kg/s)

lim 20% surge line

Fig. 10. Pressure ratio versus air flow corrected over compressor map for compressor inlet throttle method.

for a given electrical power. This range is enclosed by the boundary points for each regulation method that it is determined by the Gas Turbine limits. Fig. 14 has been constructed by mean of the thermal power versus electrical power results of each regulation method, taking the boundary points of each chart. The boundary points comprise:  The points of the line that corresponds to the maximum TIT equal to 1460 K  The last point of each constant TIT line per each regulation method, as shown in Fig. 13 for the Compressor inlet throttle method. Fig. 14 summarizes the cogeneration regulation capability of the methods shown in Fig. 7 and highlights the huge difference between the conventional regulation method based only on TIT

control and the rest. As it can be seen, the Gas Turbine regulated by the Turbine Inlet Temperature method gives a line on the thermal power versus electrical power chart and therefore it is not able to supply, simultaneously, a combination of thermal and electrical power out of that line. On the contrary, the other regulation methods of Fig. 7 project thermal and electrical power on a map being able to supply a range of both type of power. Considering only energy issues, the energetic efficiency can be calculated according the Eq. (14) in terms of power:

henergy ¼

Wth þ Wel ¼ hele þ hth wf $LHV

(14)

Furthermore, a chart is built to represent the energetic efficiency values that correspond to the boundary points displayed in Fig. 14. The results are pictured in Fig. 15. It can be observed that several regulation methods have pick efficiency between 79% and 80% and

1200

1100

1460 K 1430 K 1397 K

1000

Tit power turbine (K)

1364 K 1329 K

900

1293 K 1255 K 800

1218 K 1129 K Regulation TIT Tlimit

700

600 5000

15000

25000

35000

45000

We (kW) Fig. 11. Power turbine inlet temperature versus We for compressor inlet throttle method.

55000

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585

950

900

1460 K

T exhaust gases (K)

850

1430 K 1397 K

800

1364 K 1329 K 750

1293 K 1255 K 700

1218 K 1129 K

650

600 5000

Regulation TIT Tlimit 15000

25000

35000

45000

55000

We (kW) Fig. 12. Temperature exhaust gases versus We for compressor inlet throttle method.

that except of bleeding, any regulation method has better efficiency than only TIT control. According to Eq. (14), the values of the electrical efficiency (hele) and the thermal efficiency (hth) can be represented separately as Fig. 16 shows. hele are represented by line-solid markers and hth by line-non solid markers. Directive 2004/8/EC [2] define an indicator of the primary energy saving within the frame of cogeneration plants, that is given by Eq. (15):

ESI ¼

Wel helref

þ hWth  wf $Hp

(15)

thref

Wel helref

þ hWth

thref

where helref and hthref are the reference efficiencies for the separate generation of electrical and thermal power (0.38 and 0.89 respectively). This indicator compares the energy saved when thermal

and electric power are produced by a cogeneration plant relative to the separated production of both. Therefore is a good index of the energy saving due to a cogeneration technology. Fig. 17 represents the energy saving indicator values for the boundary points versus the electrical power. Fig. 14 shows that the regulation methods Backflow, Bleeding, Compressor inlet throttle and VGV bring about similar regulation capacities, slightly higher in the case of VGV, since this method produced a lower decrease of the air flow reaching the same exhaust gas temperatures. The VAN method causes a higher decrease of the air flow to get the turbine temperature limits, thus a lower regulation capacity is achieved. A lower regulation capacity is provided by the air inlet heating methods, which factor in the need of thermal power that is

60000

50000

1460 K 1430 K

Wth (kW)

40000

1397 K 1364 K 1329 K

30000

1293 K 1255 K 20000

1218 K 1129 K 10000

regulation TIT Boundary points

0 5000

15000

25000

35000

45000

We (kW)

Fig. 13. Wth versus We for compressor inlet throttle method.

55000

F. Jimenez-Espadafor Aguilar et al. / Energy 72 (2014) 574e589

586 60000

55000

50000

45000

Wth (kW)

40000

35000

30000

25000

20000 0

10000

20000

30000

40000

50000

60000

We (kW) Turbine Inlet temperature VGV bleeding Compressor outlet throttle

Compressor inlet throttle Air inlet heating exhaust gases VAN

Backflow Air inlet heating high P steam Air inlet heating medium P steam

Fig. 14. Boundary points of thermal power (Wth) versus electrical power (We) for each regulation method.

supplied by the cogeneration system itself. The medium pressure steam methods shows an even lower capacity from the 20,000 kWe load, since the medium pressure steam flow does not suffice to reach the set air inlet temperatures at certain part load conditions. The worst method in terms of regulation capacity is the compressor outlet throttle, as expected. The 20% surge line limit that defines the safe side of the compressor means the major constraint for this method, as shown in Fig. 18. At Fig. 18 can be observed that just first positions of this regulation method fall within the safe zone set by the compressor limit, so the exhaust gas temperature keep far from the exhaust turbine temperature limit, see Fig. 19. The points within the grey area fall out the 20% surge line limit: they are plotted to illustrate the results of this method though they are not taken into account.

In an analogue comparison shown in Fig. 15, the energetic efficiency values follow the patterns as the regulation capacities, meaning that higher henergy figures are obtained in the methods that show higher regulation capacities, except for two methods: Bleeding and VAN. Bleeding method features the lowest henergy values, in contrast to the regulation capacity that it performs. This is caused by two factors: B The temperature reduction at compressor outlet when compared with backflow method. The consequence of the heat wasted due to the bleeding brings about an additional demand of fuel for getting TIT, and thus a further decrease of the electrical efficiency.

82.00%

80.00%

78.00%

ηenergy

76.00%

74.00%

72.00%

70.00% 0

10000

20000

30000

40000

50000

60000

We (kW) Turbine inlet temperature VGV Bleeding Compressor outlet throttle

Compressor inlet throttle Air inlet heating exhaust gases VAN

Backflow Air inlet heating high P steam Air inlet heating medium P steam

Fig. 15. Boundary points of energetic efficiency versus electrical power (We) for each regulation method.

F. Jimenez-Espadafor Aguilar et al. / Energy 72 (2014) 574e589 40.00%

587 60.00% 58.00%

35.00%

56.00% 54.00%

30.00%

50.00%

ηele

25.00%

ηth

52.00%

48.00% 20.00%

46.00% 44.00%

15.00%

42.00% 10.00% 0

10000

20000

30000

40000

50000

40.00% 60000

We (kW)

Compressor inlet throttle

Backflow

VGV

Air inlet heating

Bleeding

VAN

Compressor outlet throttle

Turbine inlet temperature

Fig. 16. hele and hth versus We for all regulation methods of Fig. 7.

the highest thermal efficiencies. On the other hand, VAN shows the highest henergy values, which does neither fit the rule of thumb of the regulation capacity. This method results in the optimized figures of electrical and thermal efficiencies, as shown in Fig. 16, in such way that thermal efficiencies reach an optimal range of values and also the electrical efficiencies. In addition, it is also observed that the regulation method by mean of the TIT is the one that achieves the best valued of electrical efficiency, which makes sense considering that the GT is designed to work under those conditions. The rest of regulation methods increase the thermal efficiency at the same ratio that their electrical efficiencies are decreased, with the exceptions already explained. The Compressor outlet throttle method shows an electrical performance that is close to the TIT method, because the limit due to the surge line does not allow much range of regulation, and thus the thermal efficiency is very low, but also because the running line moves towards an area of higher efficiency in the compressor map, see Fig. 18.

B For the same fraction of air flow bled, the Bleeding method attains lower exhaust gases temperature than the Backflow method, because the drop of the pressure ratio is lower. The Bleeding method needs a fraction of 16% bled to reach the limit of the exhaust gases temperature, while the Backflow method just needs a fraction of 9%. That means that the boundary point of the bleeding method requests a higher air flow inlet, and thus an additional work of the compressor that increases the fuel demand. This increase of fuel demand makes the bleeding method perform the lowest electrical efficiency when compared to the other methods. Fig. 16 shows the thermal and electrical efficiencies for the regulation methods and, as expected, higher thermal efficiencies that some regulation methods perform, match to lower values of electrical efficiencies. In the case of Bleeding, this method shows the lowest electrical efficiencies that do not correspond to

35.00%

30.00%

25.00%

ESI

20.00%

15.00%

10.00%

5.00%

0.00% 0

10000

20000

30000

40000

50000

60000

We (kW) Compressor inlet throttle

Backflow

VGV

Air inlet heating exhaust gases

Bleeding

VAN

Compressor outlet throttle

Turbine inlet temperature

Fig. 17. Boundary points of energy saving indicator versus electrical power (We) for each regulation method.

F. Jimenez-Espadafor Aguilar et al. / Energy 72 (2014) 574e589

588

surge line

25

1460 K 1430 K 20

1397 K 1364 K 15

1329 K

Pr

1293 K 1255 K

10

1218 K 1129 K 5

1054 K 1007 K Regulation TIT

0 0

10

20

30

40

50

60

70

Wc (kg/s)

80

lim 20% surge line

Fig. 18. Pressure ratio versus air flow corrected over compressor map for compressor outlet throttle method.

The energy saving potentials are plotted in Fig. 17, and it is worth to highlight that all the regulation methods achieve ESI (energy saving indicator) values equal or lower to the TIT method, but the VAN method, whose ESI values are above the ones of TIT method, and the air inlet heating method (just some points). According to the expression of ESI and these results, the energy saving potential seems to be linked to the electrical efficiency, which is also one of the conclusions reached by other authors [19]. 7. Conclusions The regulation methods shown in Fig. 7 have proven their capacity to generate a thermal versus electrical power map, in front of the regulation method based only on TIT temperature, as it can be seen in Fig. 14. These solutions broaden the capacity of Gas Turbine for cogeneration purposes and increase the attractive of GT for island system not connected to a general electrical grid.

The comparative analysis of the results provided by the regulation methods has assessed different parameters that allow the grading of these methods based on the regulation capacity, the performance and the energy saving potential. VGV method shows an optimized behaviour from an overall perspective: the regulation capacity provided by this method is similar to Backflow and Compressor inlet throttle, but it is slightly higher in the boundary points that correspond to the line at TIT equal to 1460 K. VGV reaches the second best efficiency values (79.1% maximum) after VAN method and the energy saving indicator attains similar figures to Backflow and Compressor inlet throttle. Backflow and Compressor inlet throttle result in similar regulation capacities, energetic efficiencies (78.9% and 78.8% maximum, respectively) and energy saving potential, albeit the choice between these two options needs to consider other technical issues that might hinder the implementation of these methods, such as additional equipment and the space available in the gas turbine.

Fig. 19. Temperature exhaust gases versus We for compressor outlet throttle method.

F. Jimenez-Espadafor Aguilar et al. / Energy 72 (2014) 574e589

As noted from the analysis above, the VAN method shows the best results in terms of efficiency (79.9% maximum), but the regulation capacity does not achieve the broadest range of thermal power for the air flow reduction that this method produces. However, the results of the energy saving indicator are above the ones provided by the TIT method, in contrast to the rest of methods, so VAN method might be considered an optimal option in those cases that do not precise a very wide regulation capacity. Air inlet heating methods do not provide good results in terms of regulation capacity and energetic efficiency (77.6% maximum), but the energy saving indicator reaches similar values than the TIT method. Bleeding method allows a wide regulation but in detriment of the energy efficiency (it decreased from 77.0% at fraction bled equal to zero to 74.0% at fraction bled equal to 16%) and energy saving parameters. The weak results in those parameters put this regulation method in a difficult position when compared to Backflow and Compressor outlet throttle, but its technical implementation may be really straightforward since the fraction bled needed is 16% for the maximum regulation capacity. Finally, the Compressor outlet throttle method is very constraint due to fast displacement of the operation line towards the surge line, resulting in the lowest regulation capacity. This method should be ruled out since it seriously jeopardizes the compressor functioning. The regulation capacity is very limited, and for its maximum regulation capacity, it attains a lower value of energy efficiency (77.0%) than the air inlet heating methods (77.4%). As overall conclusion, the VGV method is the one providing the best results in the regulation capacity. The maximum regulation capacity is produced at 26 MWe, where the thermal power range that the regulation methods can provide is 33.7e47.9 MW. This range means that VGV method attains thermal powers 39% above the thermal power allowed by the baseline method. The energetic efficiency associated increases 6% (74e79%), and VGV achieves the second best values after VAN method. Acknowledgements The authors would like to express their gratitude to National Aerospace Laboratory (NLR) and especially to Oscar Kogenhop for providing valuable information on the GSP program.

589

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