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Thermoeconomic analysis of mixed gas–steam cycles
Applied Thermal Engineering 22 (2002) 1±21
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Thermoeconomic analysis of mixed gas±steam cycles Alberto Traverso, Aristide F. Massardo * Dipartimento di Macchine, Sistemi Energetici e Trasporti, Universita di Genova, Via Montallegro 1, 16145 Genoa, Italy Received 10 January 2001; accepted 21 June 2001
Abstract In this paper the direct thermoeconomic analysis approach developed by the authors [ASME Paper 95CTP-38; ASME Cogen Turbo Power Conference, Wien, 23=25 August, 1995] is applied to the assessment of the thermoeconomic performance of mixed gas±steam cycles such as the steam injected cycle (steam injected gas turbine, STIG), regenerated water injected (RWI) cycle, and humid air turbine (HAT) or evaporative cycle. All the simulations were carried using the thermo-economic modular program (TEMP) code developed at the University of Genoa [ASME Trans., J. Engng. Gas Turbine Power 119 (1997) 885; Thermo-economic and environmental optimisation of energy systems, Tesi di Dottorato, Universita di Genova (DIMSET), 1997] and carefully tested here, mainly for the HAT cycle and saturator, using the experimental data provided by the HAT pilot-plant operating at the Lund University, Sweden [Theoretical and experimental evaluation of the EvGT-process, Thesis for Degree of Licentiate in Engineering, Lund Institute of Technology, Sweden, 1999; Evaporative cycles ± in theory and in practice, Doctoral Thesis, Lund Institute of Technology, Sweden, 2000]. Three dierent mixed cycles (STIG, RWI, and HAT) are analysed in detail together with an additional fourth layout proposed by the authors [Thermoeconomic analysis of STIG, RWI and HAT cycles with carbon dioxide (CO2 ) emissions penalty, Tesi di laurea, Universita di Genova (DIMSET), 2000], named HAWIT, humid air water injection turbine, that appears to be the most attractive solution. The thermoeconomic results of mixed cycles are presented here for the ®rst time in open literature. These results are compared to the data of a conventional two-pressure level combined cycle considered as representative of the state of the art of high eciency conversion systems. A new representation proposed by the authors [ASME Trans., J. Engng. Gas Turbine Power 122 (2000)], such as cost of electricity versus cycle eciency or internal rate of return versus electric eciency, is used to demonstrate the main features of these types of innovative energy plants. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thermoeconomics; Mixed gas±steam turbine cycles; Saturator
*
Corresponding author. Tel.: +39-10-3532444; fax: +39-10-3532566. E-mail address: [email protected] (A.F. Massardo).
1359-4311/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 1 ) 0 0 0 6 4 - 3
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Nomenclature AC aftercooler CC combined cycle CC2LP Two-pressure level combined cycle speci®c heat cp ECO economiser EVAP evaporative cycle GT gas turbine h speci®c enthalpy (kJ/kg) HAT humid air turbine cycle HAWIT humid air water injected turbine cycle HPC high pressure compressor HRSG heat recovery steam generator IC intercooler ICR intercooled regenerated cycle IRR internal rate of return LHV low heating value (kJ/kg) LPC low pressure compressor m_ mass ¯ow rate (kg/s) p pressure (bar) P power (kW) PEC purchased equipment cost ($) psat =p rsat REC recuperator RWI regenerated water injected cycle SAT saturator STIG steam injected gas turbine T temperature °C x mass fraction ~x molar fraction b pressure ratio g eciency Subscripts 1 saturator inlet air 2 saturator outlet air 3 saturator inlet water 4 saturator outlet water bo bleed-o c compressor corr corrected da dry air
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ex in m max n sat w
3
exergy compressor inlet mechanical maximum net saturation water
1. Introduction The diusion of GTs for power generation, the rise of fuel cost and the increasing attention to energy plant environmental emissions have increased power industry interest in advanced cycles such as mixed gas±steam cycles. Among these, the ®rst to be studied and set up was the steam injected cycle (STIG) that, nowadays, boasts numerous applications, mainly for cogenerative purposes [8]. The RWI cycle is emerging as a possible high eciency cycle for land engines, as some important industries propose speci®cally designed GTs [9]. However, the most innovative mixed cycle is certainly the HAT cycle, proposed by Rao [10], studied world-wide from the theoretical point of view for its interesting thermodynamic performance. Recently the ®rst operating HAT cycle test facility was set up at the University of Lund [4,5], after the installation of a Volvo VT600 microturbine operated in simple, regenerated and ®nally evaporative (EVAP) (or HAT) cycle. The test facility clearly demonstrated the HAT operation, providing considerable information on the SAT design and performance, system management, etc. In open literature several authors [11±16] have studied the thermodynamic features and characteristics of mixed cycles, showing their promising performance. Nevertheless no ®nancialeconomic assessments of these types of cycles have been presented till now, probably due to the diculty ®nding detailed cost information. This fact induced the authors to apply direct thermoeconomic analysis to mixed cycles, aiming to highlight their commercial competitiveness, a necessary prelude for their success in the energy market especially for distributed power generation in the range of 0.5±50 MW. Therefore the main targets of this work are to: · · · ·
Upgrade the calculation model for mixed-cycle-oriented applications. Verify the HAT cycle SAT performance. Assess the thermodynamic performance of RWI and HAT cycles. Compare the thermoeconomic characteristics of mixed cycles (STIG, RWI, HAT) to a standard combined gas±steam cycle in the 50 MW range, also taking information from previous thermodynamic analyses [11,15,16].
2. Mixed gas±steam cycles The introduction of water into the airstream after ¯ue gas heat recovery is a very promising technique for obtaining an increase in GT eciency and speci®c work, with the consequent
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greater plant compactness, and allows cogeneration applications [8,17]. Moreover, because of the growing interest in power plant emissions, mixed gas±steam cycles take advantage of the high moisture level in the combustion to cut down NOx and CO emissions [4,8,18]. The analysed plants are brie¯y discussed in the following, which points out the most important features of the dierent solutions considered. 2.1. Steam injected gas turbine cycle The steam injected cycle (STIG) exploits sensible heat from ¯ue gases to generate steam afterwards introduced into the airstream. Thus, in this cycle the steam turbine and condenser are not necessary as in a combined plant, with unquestionable bene®ts in terms of simpli®ed plant scheme, construction time, and investment saving. A typical arrangement is considered here for the STIG plant, consisting of a steam injected gas turbine and a single-pressure level HRSG (ECO, evaporator and superheater). The generated superheated steam is then injected into the GT combustion chamber and/or downstream. 2.2. Regenerated water injected cycle The RWI cycle is conceptually derived from the regenerated intercooled cycle (ICR) [7] that, although oering greater eciency than the GT simple cycle, cannot achieve the thermodynamic performance of mixed cycles. The RWI cycle corrects the ICR scheme with the introduction of an additional water heat recovery system downstream from the REC. The pressurised hot water is injected downstream from the HPC, in order to decrease the compressed air temperature (aftercooling): this solution improves the heat exchange inside the REC, balancing the heat capacity of the two streams. In the RWI cycle the intercooling can be carried out by two dierent types of ICs: (i) surface; (ii) mixing. The second option is chosen in order to keep the plant lay out as simple and cheap as possible (Fig. 1). Certainly the surface IC would allow better thermodynamic performance but would be accompanied by higher investment costs and plant complexity. From a thermodynamic point of view, the RWI plant is mainly limited by three factors: (a) high irreversibility in the gas±liquid water mixing process; (b) water mass ¯ow rate limited by gas
Fig. 1. Conceptual plant scheme of intercooled water injection cycles (RWI).
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saturation; (c) high steam mass fraction and temperature in the exhaust. These reasons justify the lower maximum eciency achieved by RWI cycles compared to the other mixed cycles. 2.3. Humid air turbine cycle Since the ®rst HAT cycle was advanced by Rao in 1989 [10], several authors have carried out studies to ®nd the optimum plant con®guration and operating parameters for this innovative and rather complex cycle [12,15,16]. In this paper the analysed HAT layout follows the one presented in Refs. [15,16]. There are many analogies between RWI and HAT cycles: (i) both are ICR cycles with the introduction of water into the airstream, with consequent NOx emission reduction [4,8,18]; (ii) both have at least one heat exchanger downstream from the REC in order to optimise the heat recovery from ¯ue gases; (iii) both realise compressed air aftercooling, carried out either by liquid water injection (RWI) or by heat exchangers (HAT). The HAT superiority lies in making the aftercooling and the water mixing separately and in a more reversible way, through the use of a direct contact heat exchanger, like the SAT. Moreover, the HAT cycle does not require high quality water: conceptually it could work even with seaquality water [5]. On the other hand STIG and RWI cycles require very high quality water. In this work the authors propose and analyse a simpli®ed version of the HAT plant layout of Fig. 2, called HAWIT, humid air water injected turbine, with the number of the heat exchangers reduced from seven to four. In the scheme, shown in Fig. 3, the intercooling is obtained by water injection, as in the RWI cycles. Since the injected water must be of high quality, a further economiser (called ECO1) has to be introduced in order to pre-heat such supply water before its
Fig. 2. Conceptual plant scheme of intercooled humid air cycles (HAT).
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Fig. 3. Conceptual plant scheme of intercooled humid air water injected cycles (HAWIT).
mixing with the water leaving the SAT. As further shown, this solution allows high eciency and low investment cost to be matched, achieving in this way the best thermoeconomic performance among all the energy plants presented here. 3. Calculation model All the results described here are calculated using thermo-economic modular program (T E M P ) [2,3], speci®cally upgraded to predict the performance of complex mixed cycles [6,20]. At the present 64 modules are available, and they allow the thermoeconomic analysis of a large number of energy cycles to be obtained such as steam, GT, combined, and advanced cycles (mixed gas±steam cycles, fuel cells (SOFC and MCFC) and hybrid cycles, partial oxidation cycles, chemical recovery cycles [7,21,22]). In this work some new modules have been created, such as the SAT model for the HAT cycle brie¯y illustrated here, while a complete description is reported in Ref. [6]. Two main SAT types can be used in HAT cycles: plate towers and packing towers. In the former the contact between the air and water ¯ows is carried out by subsequent steps, because the liquid falls from one plate to the next, the latter exploits an internal packing in order to enhance heat and mass exchange surface between gas and water. The packing towers are characterised by lower pressure drops and lower costs, which would bene®t HAT cycle applications. The SAT can operate with any clean and ®ltered water source as long as the dissolved substances at the water outlet remain below their precipitation concentration at the operating conditions. The water quality is maintained via a combination of the water treatment system and the SAT blow down to purge impurity [13]. The main feature of the SAT lies in its air±water mixing capability with an exergy loss minimisation, owing to the low internal temperature dierences.
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Fig. 4. SAT installed at the University of Lund and its conceptual scheme.
Imposing outlet gas saturation, the steam mass ¯ow rate m_ w2 and mass fraction xw2 for the steam±air mixture assume the following formulations (Fig. 4): 1
rsat m_ 1 Mw 1 rsat M1
1
rsat xw1 1 rsat ~xw1
m_ w2 xw2
~xw1
1
m_ 1 m_ 2
2
~xw1
where rsat ~xw2 psatp T2 2 .
Outlet gas saturation imposition is in accordance with SAT theoretical calculations reported in Ref. [19] and experimental evaluations published in Ref. [5]. 4. Test case To validate the new version of T E M P code for mixed gas±steam cycles (v. 6.0), the HAT pilotplant working at the Lund University was used as a test case (see Figs. 4 and 5), as the experimental data were available in Refs. [4,5]. The Evaporative Gas Turbine project involves Swedish Universities and important European gas turbine companies: the main target was the installation of and experimentation with an EVAP cycle, the ®rst working HAT cycle in the world. Fig. 6 reports the plant layout, with the SAT and the ¯ue gas condenser: the latter carries out cycle water consumption recovery through the ¯ue gas condensation. To better understand the results some important additional notes are necessary: · The aim of the air bleed o system is to have the same expansion line in the expander for both the operated recuperative dry cycle and the operated EVAP cycle at the same load point: in this
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Fig. 5. Volvo GT VT600 installed at the University of Lund: (1) GT, (2) expander outlet, (3) REC.
way politropic expansion eciency is constant and avoiding compressor surge danger. In fact the turbine mass ¯ow rate, increased by water evaporation carried out by the SAT, would enhance compressor pressure ratio, operating the GT at constant speed (constant rpm) and constant Tmax . This bleed-o causes a decrease of plant eciency (g), that has to be corrected as follows: Pn Pc mm__ boin gm 3 gcorr g Pn · The ``CC cooler'' component simulates just the modi®ed combustion chamber cooling, realised by compressed air. According to experimental data, plant thermodynamic performance is: net power: 420 kW; corrected LHV eciency gcorr : 30.28%. Observing eciency rise from the simple cycle (18.5%) to the regenerated cycle (22%) and the EVAP one (30%), at ®xed power (420 kW), it is very interesting to note an increase of 11.5% points. By extrapolation from the experimental evaluations to the nominal power of 600 kW, an eciency increase of 13% points can be observed, from 22% in the simple cycle to 35% in the EVAP one. Analysing the plant thermodynamic performance in detail the theoretical results shown in Fig. 6 are always in good agreement with the experimental data. Only at the SAT air outlet section is a dierence (1.9°C) between the measured and the calculated temperature evident. This point has been carefully analysed, as discussed in Ref. [5], because experimental evaluations seem to require a relative humidity greater than 100% (104±105%). In the present work a ®rst attempt to improve the agreement between theoretical and experimental data has been pursued modifying the humid air calculation through the introduction of the steam compressibility factor (z) into the T E M P code [20]. Unfortunately, the improvement was only 0.4°C (the temperature dierence is now 1.9°C instead of 2.3°C without the use of the compressibility factor), not enough to properly match the experimental data. To justify the humidity level higher than 100%, the thermodynamic transformation in the SAT needs to be more accurately investigated in the near future from both the experimental and theoretical points of view [23].
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Fig. 6. HAT pilot-plant scheme working at the University of Lund.
However, this modelling problem is mainly concerned with the SAT project and design, while it just marginally aects the thermoeconomic analysis carried out in this paper. Therefore, it is possible to use the T E M P code (tested for the thermodynamic aspect) for the complete thermoeconomic analysis of mixed cycles, including detailed economic and ®nancial assessments. 5. Results of optimised cycles The results of the calculations are now presented for the layouts shown in Figs. 1±3 and for a single-pressure level STIG cycle, with particular attention on the thermoeconomic features of each plant. In order to consider a reference term from the thermoeconomic comparison point of view, the results of a thermoeconomic analysis carried out for a conventional two-pressure level CC plant were utilised.
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The size of all the chosen cycles is 50 MW and each one is studied for two dierent maximum temperatures (Tmax ): 1200°C and 1400°C; all the plants are optimised in order to maximise LHV eciency. 5.1. Thermodynamic analysis Table 1 reports the main thermodynamic assumptions used in the present analysis: they do not represent the highest technological level reached by GTs. These choices are justi®ed by the main target of this work: a thermoeconomic analysis of the commercial competitiveness of mixed cycles. Table 1 Main thermodynamic assumptions GT Compressor isentropic eciency Combustion chamber pressure drop Combustion eciency Tmax Expansor isentropic eciency Mechanical eciency
0.86 4% 0.996 1200±1400°C 0.88 0.98
Gas±water/steam heat exchangers Subcooling DT Pinch point DT Approach DT for gas/water heat exchangers Approach DT for superheater Heat loss Gas pressure drop Water pressure drop
>5°C >5°C >5±10°C >15°C 1% 2% 8%
REC Heat exchanger eciency Hot side pressure drop Cold side pressure drop Heat loss
0.8 3% 2% 1%
SAT Enthalpy pinch point Gas pressure drop
>20 kJ/kgdryair 0.7%
Alternator and auxiliaries Electric eciency Pump hydraulic eciency Pump mechanical eciency
0.985 0.83 0.9
Fuel Chemical composition
Type volume fraction CH4 N2
LHV
44,243 kJ/kg
93% 7%
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Fig. 7. Cycle LHV eciency versus speci®c work, for Tmax 1200°C, 1400°C.
The highest eciencies are reached by the reference CC (see Fig. 7) that, contrary to mixed gas± air cycles, does not suer the exergy loss caused by gas±water mixing, has a steam condensation temperature of about 30°C and has a lower steam mass fraction in the exhaust stream. As far as speci®c work is concerned, mixed cycles achieve the highest values, therefore characterising themselves as compact and cheap plants, as discussed in the next paragraph. The comparison between the mixed cycles shows that the HAT cycle achieves the higher eciencies for all the pressure ratios considered here (Fig. 7), with a water/air ratio about constant (at the same maximum cycle temperature) and always lower than 25%, as shown in Fig. 8. This fact is very interesting, because it means that a thermodynamically optimised HAT cycle requires an almost ®xed water/air ratio that does not depend on the pressure ratio, once the cycle Tmax is ®xed: that is an absolutely useful result for the future design and project of GTs working on the HAT cycle. RWI, HAWIT and HAT cycles, compared to the STIG cycle, suer a lower eciency drop, decreasing the pressure ratio: for Tmax 1200°C they lose <2% points, passing from b 30 to b 10, instead of almost 5% points for the STIG cycle. This is a very attractive feature for small±medium size applications, characterised by low pressure ratios. In these conditions, the STIG carries out very high speci®c work values, as a consequence of the large amount of water
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Fig. 8. Mixed cycle speci®c water consumptions versus water±air ratio.
consumed. Unfortunately, it would require a speci®c GT design, in order to allow a much greater mass ¯ow rate in the expander to avoid compressor surge. In comparison to the other mixed cycles, from a thermodynamic point of view, the HAWIT cycle is in an intermediate position between the HAT and the RWI cycles, achieving better eciencies than the STIG cycles. Its speci®c work values are nevertheless similar to HAT ones, because of the greater amount of water consumed. Fig. 8 shows the opposite behaviour of RWI, HAWIT cycles and STIG one for water consumption: increasing the pressure ratio the former realises higher and higher water/air ratio while the STIG water/air ratio decreases. As far as the calculated best con®gurations are concerned, the eciency and speci®c work increases, in comparison with the original simple cycle (Fig. 9), do not vary much with Tmax and can be quanti®ed as reported in Table 2. One of the most interesting aspects of the study of mixed cycles is the presentation of the exergy analysis, particularly for the mixing losses. The complete exergetic analysis of the considered mixed cycles is reported in Ref. [6] while just the air±water mixing contribution to the total exergy losses is presented here in Fig. 10. It is possible to note that the SAT allows a more reversible
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Fig. 9. Simple cycle GT LHV eciency versus speci®c work, for Tmax 1200°C, 1400°C. Table 2 Eciency and speci®c work average increases from the original simple cycle RWI (b 30) STIG (b 30) HAWIT (b 30) HAT (b 30) CC (b 18)
Eciency increase (% points)
Speci®c work increase (%)
9 10.5 11 13 14
100 85 110 120 55
Fig. 10. Water±air mixing exergetic losses in the highest LHV eciency con®gurations for mixed cycles. All the values are expressed as a percentage of the maximum mixing exergy loss of RWI with Tmax 1200°C.
introduction of the water in the airstream: this is the main reason for HAT and HAWIT higher thermodynamic performance. Besides the mixing exergetic losses, the inferior RWI eciency, when compared to the other mixed cycles, is due to the water injection limit derived from gas saturation. Such a limit can be
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Fig. 11. LHV eciency with aftercooling water injection increase over compressed air saturation for RWI cycle with Tmax 1200°C and b 30. Power percentual raise and water/air ratio are reported at the LHV eciency maximum point.
overcomed by injecting a greater amount of water into the AC. In fact in this case the RWI ef®ciency rises because of the greater heat recovery in the REC (Fig. 11), with a maximum bene®t of about 0.5% points. Nevertheless this device could cause some structural problems in the REC, because it would become a variable temperature evaporator on the colder side (Fig. 12). 5.2. Thermoeconomic analysis Thermoeconomics is an analytic and complex approach for energy system analysis developed by several authors. Thermoeconomic analysis, integrated with the exergoeconomic analysis, allows a better understanding of system exergetic and monetary loss locations inside the plant and permits plant optimisation to minimise dierent thermoeconomic objective functions.
Fig. 12. REC temperature pro®le for the RWI cycle of Fig. 11, at the point of maximum eciency.
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Table 3 Main economic assumptions In¯ation Nominal escalation rate of PEC Nominal escalation rate of fuel cost
2.5% 2.5% 3.0%
Construction initial year (1 January) Construction time Plant economic life (book life) Plant life for tax purposes
2000 2 years 20 years 10 years
Debts ± ®nancing fraction Preferred stocks ± ®nancing fraction Common equities ± ®nancing fraction Debts ± required annual return Preferred stocks ± required annual return Common equities ± required annual return
50% 15% 35% 5.5% 6% 6.5%
Average income tax rate
30%
Fuel price (natural gas) Demineralised water price Sale price of electric power
4.0E 6 $/kJ 0.5 $/m3 1.32E 5 $/kJ
In this paper only whole plant thermoeconomic results are reported, without facing the internal thermoeconomic analysis, that is, however, indispensable for the right interpretation of the causes of the published results. The complete thermoeconomic analysis is reported by the authors in Refs. [1,6,24,25]. Table 3 illustrates the main economic assumptions, maintained constant for all the analysed plants: therefore they represent a compromise between CCs and mixed cycles data. In addition, the operating and maintenance cost was taken as equal to 4% of PEC. An equivalent working time of 8000 h/year was used. From a thermoeconomic point of view, a cost taking into account all water treatments must be assigned to supply water: in the framework of this paper a water cost of 0.5 $=m3 is assumed for all the cycles, even if for the HAT cycle it could be lower because it does not require high quality water like STIG and RWI cycles, since the SAT operates as a distillation tower. Economic considerations do not include SAT cost, for which the necessary economic data are still not available in literature: however, a way to determine a maximum allowable cost for such a device is suggested here. Figs. 13 and 14 show the energy cost and the IRR versus plant eciency. The reference CC (Tmax 1200, 1400°C, b 18), although it reaches the highest eciencies (51% and 52.8%), can not compete with mixed cycles for the medium size plant considered here (50 MW), because of higher capital costs due mainly to the heat recovery system (two-pressure levels), steam turbine and condenser purchase costs. On the other hand, mixed cycles are able to have low investment costs with high conversion eciencies, even for small sizes. Moreover, because of the presence of a parallel open cycle of water near the airstream, mixed cycles are proper for cogeneration applications too: the economic
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Fig. 13. Cycle energy costs in ®rst operating year (2002).
in¯uence of this opportunity is nevertheless not considered in this paper, but it can be easily handled using the T E M P code. As far as energy costs in the ®rst working year are concerned (Fig. 13), all the mixed cycles show the lowest values in a pressure ratio range of 15±20, that is slightly over the maximum speci®c work point of the original simple cycles. The consequent eciency improvement brought about by rising compressor pressure ratio is not enough to allow the energy costs to decrease, because of the important increase in capital costs, mainly concerned with GT purchase cost. The lowest energy costs are shown by the HAWIT cycle for both the considered maximum temperatures, while HAT and RWI energy costs are similar but a little higher. The STIG cycle is in an intermediate position between the CCs and the other mixed cycles, and shows the lowest thermoeconomic performance among the last. This is due to its higher capital cost than the RWI cycle, not balanced by a sucient conversion eciency increase as for the HAT cycle. Referring to the IRR (Fig. 14), the best values are always shown by the HAWIT cycle (more than 18% and 22% respectively), that demonstrate its excellent economic performance due to the best compromise between capital costs and high eciencies, for the 50 MW size considered.
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Fig. 14. Cycle IRR.
Among the other mixed cycles, the RWI cycle beats the HAT cycle by at least 1±2% points for Tmax 1200°C, because of its lower investment cost. This dierence is not present for Tmax 1400°C, because the HAT cycle better exploits the increase of the cycle maximum temperature in terms of conversion eciency improvement. Fig. 15 reports RWI and HAT energy costs during plant economic life. While for Tmax 1200°C a break-even point comes out after six working years, for Tmax 1400°C the HAT cycle always shows the lowest energy costs. It is interesting to note how the dierence between the two compared costs varies during the plant lifetime with an increase in the HAT advantage, because its higher eciency makes it less sensible to fuel cost rises. In fact, when comparing HAT and RWI plants for Tmax 1200°C with a greater in¯ation rate (from 2.5% to 5%), the break-even point comes earlier, after four working years. If the same comparison were carried out between HAWIT and RWI cycles, no break-even point would emerge because the former would always show lower energy costs for both the considered maximum cycle temperatures. Since at the moment it is very dicult to de®ne SAT purchase cost, a criterion to ®nd its maximum allowable cost is now discussed. In a free energy market environment the IRR, illustrated in Fig. 14, should not always be considered as the most important investment parameter
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Fig. 15. Energy cost during plant life (2002±2021) of RWI and HAT cycles with Tmax 1200°C, 1400°C and b 30 for two economic scenarios: (a) in¯ation rate 2.5%, fuel escalation rate 3%; (b) in¯ation rate 5%, fuel escalation rate 6%.
because all the plants could not succeed in selling their energy for 8000 h each year, as supposed. In fact plant competitiveness is well represented by its energy cost, that is the lowest possible sale price of the energy generated. Therefore, the graphics reported in Fig. 15 are suitable for a maximum SAT cost evaluation that cancels out HAT higher competitiveness in comparison with the RWI cycle. The dierence between the two curves represents a dierent speci®c gross pro®t during each year of the plant working life. Using the in¯ation rate to discount the total gross pro®t dierence between the two plants over their entire economic life to the ®nal construction year, it is possible to calculate a SAT maximum cost that makes such a dierence equal to zero. Applying this method to the HAT and HAWIT cycles, compared with RWI one, their closest competitor, considering the best eciency con®guration for all the plants, the results shown in Table 4 can be achieved. It is worth noting that the margin for the SAT cost is considerable: in fact the SAT does not require an innovative technology, being formed mainly by a pressurised tank operating in a temperature range of 100±200°C and in a pressure range of 1±3 MPa, with packing inside similar to that in EVAP towers. Moreover, if the proposed method were applied using the STIG cycle instead of the RWI cycle as a reference, still greater margins would result.
Table 4 Maximum allowable SAT cost HAT (Tmax 1200, b 30) HAWIT (Tmax 1200, b 30) HAT (Tmax 1400, b 30) HAWIT (Tmax 1400, b 30)
GT cost ($)
Maximum SAT cost (% GT cost)
3,958,000 3,887,000 3,365,000 3,316,000
12% 52% 64% 89%
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6. Conclusions In this paper the prediction of thermoeconomic performance of mixed water±air cycles has been outlined using the T E M P code, an advanced program for thermoeconomic analysis of complex energy plants. A comparison with a reference two-pressure level CC has been carried out, showing mixed cycles, and especially HAT and HAWIT cycles, attractive thermoeconomic features, for the considered size of 50 MW. The main conclusions of this work are: · ·
·
· ·
·
· · ·
code, developed by the authors and tested with the data of the EVAP pilot-plant installed at Lund University, Sweden, is a robust, ecient, reliable program for thermoeconomic analysis also for mixed cycles. SAT experimental data reported in Refs. [4,5] show a relative humidity greater than 100% (104± 105%) in the air leaving the SAT. The introduction of the steam compressibility factor (z) is not enough to explain such an incongruence, that needs to be more accurately investigated in the near future. When the mixed cycles are compared with a reference CC on a thermodynamic base they suer mixing exergetic losses (Fig. 10), eectively reduced in the HAT and HAWIT cycles by the SAT. In fact, these cycles provide the highest eciencies among the mixed cycles, as shown in Fig. 7. Mixed gas±steam cycles represents a fair investment opportunity, because they match high speci®c works and high eciencies with relatively low capital costs, especially for small±medium size systems. The HAWIT cycle presented here (Fig. 3), derived from the HAT cycle (Fig. 2) and speci®cally developed for small±medium size GTs, pursues a decrease of investment cost through the reduction of the heat exchanger number. This fact allows the best thermoeconomic performance to be achieved among all the plants analysed here, despite the lower eciencies in comparison with the original HAT cycle. A method to evaluate the maximum allowable cost of the SAT for HAT and HAWIT cycles is discussed. It focuses on plant life energy cost, which well represents the commercial competitiveness in a free energy market. Applying such a method to HAT and HAWIT cycles, compared to the RWI one, their closest competitor, shows that they have a great economic margin that allows them to be designated as the best layouts for power generation in the 50 MW range. The method introduced here for the SAT cost evaluation could be extended in the future to the analysis of cycles based on devices without not well-de®ned costs, such as partial oxidisers, steam reformers, fuel cells, etc. Thanks to the steam high mass fraction entering the combustion chamber, the mixed gas±steam cycles propose themselves as low-NOx and low-CO emission energy plants, as already demonstrated for STIG cycles [8] and for the Lund pilot-plant [5]. In the opinion of the authors, mixed cycle economic competitiveness, in comparison with conventional energy plants such as CCs, enhances decreasing plant size. In particular mixed cycles could become extremely competitive for sizes under 10 MW, where thermodynamic performance of the bottoming steam cycle quickly decreases.
TEMP
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It is interesting to note that the present study, carried out in an academic environment as the natural prosecution of previous thermoeconomic analyses on gas, steam, combined, advanced GT based systems, should be an important step for future thermoeconomic investigations carried out in an industrial frame too. Acknowledgements The authors wish to thank Professor Tord Torisson ± Lund University (S) for permission to publish the data and some inherent pictures concerning the HAT pilot-plant. This work has been sponsored by MURST of Italy, grant ``Co®nanziamento 1999'' Prot. 9909167799_003. References [1] A. Agazzani, A. Massardo, A. Satta, Thermoeconomic analysis of complex steam plants, ASME Paper 95-CTP-38, ASME Cogen Turbo Power Conference, Wien, 23/25 August, 1995. [2] A. Agazzani, A.F. Massardo, A Tool for Thermoeconomic Analysis and Optimization of Gas, Steam, and combined plants, ASME Transactions, Journal of Engineering for Gas Turbine and Power 119 (1997) 885±892. [3] A. Agazzani, Thermo-economic and environmental optimisation of energy systems, Tesi di Dottorato, Universit a di Genova (DIMSET), 1997. [4] T. Lindquist, Theoretical and experimental evaluation of the EvGT-process, Thesis for Degree of Licentiate in Engineering, Lund Institute of Technology, Sweden, 1999. [5] P.M. Rosen, Evaporative Cycles-in Theory and in Practice, Doctoral Thesis, Lund Institute of Technology, Sweden, 2000. [6] A. Traverso, Thermoeconomic analysis of STIG, RWI and HAT cycles with carbon dioxide (CO2 ) emissions penalty, Tesi di laurea, Universita di Genova (DIMSET), 2000. [7] A.F. Massardo, M. Scial o, Thermoeconomic analysis of gas turbine based cycle, ASME Transactions, Journal of Engineering for Gas Turbine and Power 122 (2000) 664±671. [8] J.B. Burnham, et al., Development; installation and operating results of a steam injection system (STIG) in a general electric LM5000 gas generator, ASME Paper 86-GT-231, 1986. [9] V. de Biasi, LM6000 Sprint design enhanced to increase power and eciency, Gas Turbine World, July±August 2000. [10] A.D. Rao, Process for producing power, US patent no. 4'829'763, 1989. [11] A.D. Rao, et al., A comparison of humid air turbine (HAT) and combined-cycle power plants, EPRI Report IE7300, Project 2999-7, Final Report, 1991. [12] Y. Xiao, R. Lin, R. Cai, System optimization of humid air turbine cycle, ASME Paper 94-GT-240, 1994. [13] M. Nakhamkin, E.C. Swensen, J.M. Wilson, G. Gaul, M. Polsky, The cascaded humi®ed advanced turbine (CHAT), ASME Paper 95-CTP-5, 1995. [14] M. Nakhamkin, E.C. Swensen, M. Polsky, G. Touchton, A. Cohn, CHAT technology: An alternative approach to achieve advanced turbine systems eciencies with present combustion turbine technology, ASME Paper 97-GT142, 1997. [15] E. Macchi, S. Consonni, G. Lozza, P. Chiesa, An assessment of the thermodynamic performance of mixed gas± steam cycles: Part A ± Intercooled and steam injected cycles, ASME Transactions, Journal of Engineering for Gas Turbine and Power 117 (1995) 489±498. [16] P. Chiesa, G. Lozza, E. Macchi, S. Consonni, An assessment of the thermodynamic performance of mixed gas± steam cycles: Part B ± Water injected and HAT cycles, ASME Transactions, Journal of Engineering for Gas Turbine and Power 117 (1995) 499±508.
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[17] G. Lindgren, et al., The HAT cycle, a possible future for power and cogeneration, Proceedings of the 1992 FLOWERS Congress, Florence, Italy, 1992, pp. 125±141. [18] A. Bhargava, M. Colket, W. Sowa, K. Casleton, D. Maloney, An experimental and modelling study of humid air premixed ¯ames, ASME Transactions, Journal of Engineering for Gas Turbine and Power 122 (2000) 405±411. [19] P. Chiesa, Thermodynamic analysis of humid air turbine cycles using natural gas or coal gasi®cation fuel, Tesi di Dottorato, Politecnico di Milano, 1995 (in Italian). [20] A.F. Massardo, A. Traverso, T E M P user's handbook. Version 6.0, TN 010-2000, DIMSET Universit a di Genova, 2000. [21] A.F. Massardo, F. Lubelli, Internal reforming solid oxide fuel cell ± gas turbine combined cycles (IRSOFC-GT) Part A: Cell model and cycle thermodynamic analysis, ASME Transactions, Journal of Engineering for Gas Turbines and Power 122 (2000) 27±35. [22] A. Massardo, B. Bosio, Assessment of molten carbonate fuel cell models and integration with gas and steam cycles, ASME Paper 2000-GT-0174, and ASME Transactions, in press. [23] T. Torisson, Personal Communication, Lund University, September 2000. [24] R. Borchiellini, A. Massardo, M. Santarelli, An analytical procedure for the carbon tax evaluation, December 1999, Energy Conversion and Management Journal (2000) 1509±1531. [25] A. Massardo, M. Santarelli, R. Borchiellini, Carbon exergy tax (CET): Impact on conventional energy systems design and its contribution to advanced systems utilisation, ECOS 2000, Twentee, The Netherlands, July 2000.
www.elsevier.com/locate/apthermeng
Thermoeconomic analysis of mixed gas±steam cycles Alberto Traverso, Aristide F. Massardo * Dipartimento di Macchine, Sistemi Energetici e Trasporti, Universita di Genova, Via Montallegro 1, 16145 Genoa, Italy Received 10 January 2001; accepted 21 June 2001
Abstract In this paper the direct thermoeconomic analysis approach developed by the authors [ASME Paper 95CTP-38; ASME Cogen Turbo Power Conference, Wien, 23=25 August, 1995] is applied to the assessment of the thermoeconomic performance of mixed gas±steam cycles such as the steam injected cycle (steam injected gas turbine, STIG), regenerated water injected (RWI) cycle, and humid air turbine (HAT) or evaporative cycle. All the simulations were carried using the thermo-economic modular program (TEMP) code developed at the University of Genoa [ASME Trans., J. Engng. Gas Turbine Power 119 (1997) 885; Thermo-economic and environmental optimisation of energy systems, Tesi di Dottorato, Universita di Genova (DIMSET), 1997] and carefully tested here, mainly for the HAT cycle and saturator, using the experimental data provided by the HAT pilot-plant operating at the Lund University, Sweden [Theoretical and experimental evaluation of the EvGT-process, Thesis for Degree of Licentiate in Engineering, Lund Institute of Technology, Sweden, 1999; Evaporative cycles ± in theory and in practice, Doctoral Thesis, Lund Institute of Technology, Sweden, 2000]. Three dierent mixed cycles (STIG, RWI, and HAT) are analysed in detail together with an additional fourth layout proposed by the authors [Thermoeconomic analysis of STIG, RWI and HAT cycles with carbon dioxide (CO2 ) emissions penalty, Tesi di laurea, Universita di Genova (DIMSET), 2000], named HAWIT, humid air water injection turbine, that appears to be the most attractive solution. The thermoeconomic results of mixed cycles are presented here for the ®rst time in open literature. These results are compared to the data of a conventional two-pressure level combined cycle considered as representative of the state of the art of high eciency conversion systems. A new representation proposed by the authors [ASME Trans., J. Engng. Gas Turbine Power 122 (2000)], such as cost of electricity versus cycle eciency or internal rate of return versus electric eciency, is used to demonstrate the main features of these types of innovative energy plants. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thermoeconomics; Mixed gas±steam turbine cycles; Saturator
*
Corresponding author. Tel.: +39-10-3532444; fax: +39-10-3532566. E-mail address: [email protected] (A.F. Massardo).
1359-4311/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 1 ) 0 0 0 6 4 - 3
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Nomenclature AC aftercooler CC combined cycle CC2LP Two-pressure level combined cycle speci®c heat cp ECO economiser EVAP evaporative cycle GT gas turbine h speci®c enthalpy (kJ/kg) HAT humid air turbine cycle HAWIT humid air water injected turbine cycle HPC high pressure compressor HRSG heat recovery steam generator IC intercooler ICR intercooled regenerated cycle IRR internal rate of return LHV low heating value (kJ/kg) LPC low pressure compressor m_ mass ¯ow rate (kg/s) p pressure (bar) P power (kW) PEC purchased equipment cost ($) psat =p rsat REC recuperator RWI regenerated water injected cycle SAT saturator STIG steam injected gas turbine T temperature °C x mass fraction ~x molar fraction b pressure ratio g eciency Subscripts 1 saturator inlet air 2 saturator outlet air 3 saturator inlet water 4 saturator outlet water bo bleed-o c compressor corr corrected da dry air
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ex in m max n sat w
3
exergy compressor inlet mechanical maximum net saturation water
1. Introduction The diusion of GTs for power generation, the rise of fuel cost and the increasing attention to energy plant environmental emissions have increased power industry interest in advanced cycles such as mixed gas±steam cycles. Among these, the ®rst to be studied and set up was the steam injected cycle (STIG) that, nowadays, boasts numerous applications, mainly for cogenerative purposes [8]. The RWI cycle is emerging as a possible high eciency cycle for land engines, as some important industries propose speci®cally designed GTs [9]. However, the most innovative mixed cycle is certainly the HAT cycle, proposed by Rao [10], studied world-wide from the theoretical point of view for its interesting thermodynamic performance. Recently the ®rst operating HAT cycle test facility was set up at the University of Lund [4,5], after the installation of a Volvo VT600 microturbine operated in simple, regenerated and ®nally evaporative (EVAP) (or HAT) cycle. The test facility clearly demonstrated the HAT operation, providing considerable information on the SAT design and performance, system management, etc. In open literature several authors [11±16] have studied the thermodynamic features and characteristics of mixed cycles, showing their promising performance. Nevertheless no ®nancialeconomic assessments of these types of cycles have been presented till now, probably due to the diculty ®nding detailed cost information. This fact induced the authors to apply direct thermoeconomic analysis to mixed cycles, aiming to highlight their commercial competitiveness, a necessary prelude for their success in the energy market especially for distributed power generation in the range of 0.5±50 MW. Therefore the main targets of this work are to: · · · ·
Upgrade the calculation model for mixed-cycle-oriented applications. Verify the HAT cycle SAT performance. Assess the thermodynamic performance of RWI and HAT cycles. Compare the thermoeconomic characteristics of mixed cycles (STIG, RWI, HAT) to a standard combined gas±steam cycle in the 50 MW range, also taking information from previous thermodynamic analyses [11,15,16].
2. Mixed gas±steam cycles The introduction of water into the airstream after ¯ue gas heat recovery is a very promising technique for obtaining an increase in GT eciency and speci®c work, with the consequent
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greater plant compactness, and allows cogeneration applications [8,17]. Moreover, because of the growing interest in power plant emissions, mixed gas±steam cycles take advantage of the high moisture level in the combustion to cut down NOx and CO emissions [4,8,18]. The analysed plants are brie¯y discussed in the following, which points out the most important features of the dierent solutions considered. 2.1. Steam injected gas turbine cycle The steam injected cycle (STIG) exploits sensible heat from ¯ue gases to generate steam afterwards introduced into the airstream. Thus, in this cycle the steam turbine and condenser are not necessary as in a combined plant, with unquestionable bene®ts in terms of simpli®ed plant scheme, construction time, and investment saving. A typical arrangement is considered here for the STIG plant, consisting of a steam injected gas turbine and a single-pressure level HRSG (ECO, evaporator and superheater). The generated superheated steam is then injected into the GT combustion chamber and/or downstream. 2.2. Regenerated water injected cycle The RWI cycle is conceptually derived from the regenerated intercooled cycle (ICR) [7] that, although oering greater eciency than the GT simple cycle, cannot achieve the thermodynamic performance of mixed cycles. The RWI cycle corrects the ICR scheme with the introduction of an additional water heat recovery system downstream from the REC. The pressurised hot water is injected downstream from the HPC, in order to decrease the compressed air temperature (aftercooling): this solution improves the heat exchange inside the REC, balancing the heat capacity of the two streams. In the RWI cycle the intercooling can be carried out by two dierent types of ICs: (i) surface; (ii) mixing. The second option is chosen in order to keep the plant lay out as simple and cheap as possible (Fig. 1). Certainly the surface IC would allow better thermodynamic performance but would be accompanied by higher investment costs and plant complexity. From a thermodynamic point of view, the RWI plant is mainly limited by three factors: (a) high irreversibility in the gas±liquid water mixing process; (b) water mass ¯ow rate limited by gas
Fig. 1. Conceptual plant scheme of intercooled water injection cycles (RWI).
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5
saturation; (c) high steam mass fraction and temperature in the exhaust. These reasons justify the lower maximum eciency achieved by RWI cycles compared to the other mixed cycles. 2.3. Humid air turbine cycle Since the ®rst HAT cycle was advanced by Rao in 1989 [10], several authors have carried out studies to ®nd the optimum plant con®guration and operating parameters for this innovative and rather complex cycle [12,15,16]. In this paper the analysed HAT layout follows the one presented in Refs. [15,16]. There are many analogies between RWI and HAT cycles: (i) both are ICR cycles with the introduction of water into the airstream, with consequent NOx emission reduction [4,8,18]; (ii) both have at least one heat exchanger downstream from the REC in order to optimise the heat recovery from ¯ue gases; (iii) both realise compressed air aftercooling, carried out either by liquid water injection (RWI) or by heat exchangers (HAT). The HAT superiority lies in making the aftercooling and the water mixing separately and in a more reversible way, through the use of a direct contact heat exchanger, like the SAT. Moreover, the HAT cycle does not require high quality water: conceptually it could work even with seaquality water [5]. On the other hand STIG and RWI cycles require very high quality water. In this work the authors propose and analyse a simpli®ed version of the HAT plant layout of Fig. 2, called HAWIT, humid air water injected turbine, with the number of the heat exchangers reduced from seven to four. In the scheme, shown in Fig. 3, the intercooling is obtained by water injection, as in the RWI cycles. Since the injected water must be of high quality, a further economiser (called ECO1) has to be introduced in order to pre-heat such supply water before its
Fig. 2. Conceptual plant scheme of intercooled humid air cycles (HAT).
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Fig. 3. Conceptual plant scheme of intercooled humid air water injected cycles (HAWIT).
mixing with the water leaving the SAT. As further shown, this solution allows high eciency and low investment cost to be matched, achieving in this way the best thermoeconomic performance among all the energy plants presented here. 3. Calculation model All the results described here are calculated using thermo-economic modular program (T E M P ) [2,3], speci®cally upgraded to predict the performance of complex mixed cycles [6,20]. At the present 64 modules are available, and they allow the thermoeconomic analysis of a large number of energy cycles to be obtained such as steam, GT, combined, and advanced cycles (mixed gas±steam cycles, fuel cells (SOFC and MCFC) and hybrid cycles, partial oxidation cycles, chemical recovery cycles [7,21,22]). In this work some new modules have been created, such as the SAT model for the HAT cycle brie¯y illustrated here, while a complete description is reported in Ref. [6]. Two main SAT types can be used in HAT cycles: plate towers and packing towers. In the former the contact between the air and water ¯ows is carried out by subsequent steps, because the liquid falls from one plate to the next, the latter exploits an internal packing in order to enhance heat and mass exchange surface between gas and water. The packing towers are characterised by lower pressure drops and lower costs, which would bene®t HAT cycle applications. The SAT can operate with any clean and ®ltered water source as long as the dissolved substances at the water outlet remain below their precipitation concentration at the operating conditions. The water quality is maintained via a combination of the water treatment system and the SAT blow down to purge impurity [13]. The main feature of the SAT lies in its air±water mixing capability with an exergy loss minimisation, owing to the low internal temperature dierences.
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Fig. 4. SAT installed at the University of Lund and its conceptual scheme.
Imposing outlet gas saturation, the steam mass ¯ow rate m_ w2 and mass fraction xw2 for the steam±air mixture assume the following formulations (Fig. 4): 1
rsat m_ 1 Mw 1 rsat M1
1
rsat xw1 1 rsat ~xw1
m_ w2 xw2
~xw1
1
m_ 1 m_ 2
2
~xw1
where rsat ~xw2 psatp T2 2 .
Outlet gas saturation imposition is in accordance with SAT theoretical calculations reported in Ref. [19] and experimental evaluations published in Ref. [5]. 4. Test case To validate the new version of T E M P code for mixed gas±steam cycles (v. 6.0), the HAT pilotplant working at the Lund University was used as a test case (see Figs. 4 and 5), as the experimental data were available in Refs. [4,5]. The Evaporative Gas Turbine project involves Swedish Universities and important European gas turbine companies: the main target was the installation of and experimentation with an EVAP cycle, the ®rst working HAT cycle in the world. Fig. 6 reports the plant layout, with the SAT and the ¯ue gas condenser: the latter carries out cycle water consumption recovery through the ¯ue gas condensation. To better understand the results some important additional notes are necessary: · The aim of the air bleed o system is to have the same expansion line in the expander for both the operated recuperative dry cycle and the operated EVAP cycle at the same load point: in this
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Fig. 5. Volvo GT VT600 installed at the University of Lund: (1) GT, (2) expander outlet, (3) REC.
way politropic expansion eciency is constant and avoiding compressor surge danger. In fact the turbine mass ¯ow rate, increased by water evaporation carried out by the SAT, would enhance compressor pressure ratio, operating the GT at constant speed (constant rpm) and constant Tmax . This bleed-o causes a decrease of plant eciency (g), that has to be corrected as follows: Pn Pc mm__ boin gm 3 gcorr g Pn · The ``CC cooler'' component simulates just the modi®ed combustion chamber cooling, realised by compressed air. According to experimental data, plant thermodynamic performance is: net power: 420 kW; corrected LHV eciency gcorr : 30.28%. Observing eciency rise from the simple cycle (18.5%) to the regenerated cycle (22%) and the EVAP one (30%), at ®xed power (420 kW), it is very interesting to note an increase of 11.5% points. By extrapolation from the experimental evaluations to the nominal power of 600 kW, an eciency increase of 13% points can be observed, from 22% in the simple cycle to 35% in the EVAP one. Analysing the plant thermodynamic performance in detail the theoretical results shown in Fig. 6 are always in good agreement with the experimental data. Only at the SAT air outlet section is a dierence (1.9°C) between the measured and the calculated temperature evident. This point has been carefully analysed, as discussed in Ref. [5], because experimental evaluations seem to require a relative humidity greater than 100% (104±105%). In the present work a ®rst attempt to improve the agreement between theoretical and experimental data has been pursued modifying the humid air calculation through the introduction of the steam compressibility factor (z) into the T E M P code [20]. Unfortunately, the improvement was only 0.4°C (the temperature dierence is now 1.9°C instead of 2.3°C without the use of the compressibility factor), not enough to properly match the experimental data. To justify the humidity level higher than 100%, the thermodynamic transformation in the SAT needs to be more accurately investigated in the near future from both the experimental and theoretical points of view [23].
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Fig. 6. HAT pilot-plant scheme working at the University of Lund.
However, this modelling problem is mainly concerned with the SAT project and design, while it just marginally aects the thermoeconomic analysis carried out in this paper. Therefore, it is possible to use the T E M P code (tested for the thermodynamic aspect) for the complete thermoeconomic analysis of mixed cycles, including detailed economic and ®nancial assessments. 5. Results of optimised cycles The results of the calculations are now presented for the layouts shown in Figs. 1±3 and for a single-pressure level STIG cycle, with particular attention on the thermoeconomic features of each plant. In order to consider a reference term from the thermoeconomic comparison point of view, the results of a thermoeconomic analysis carried out for a conventional two-pressure level CC plant were utilised.
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The size of all the chosen cycles is 50 MW and each one is studied for two dierent maximum temperatures (Tmax ): 1200°C and 1400°C; all the plants are optimised in order to maximise LHV eciency. 5.1. Thermodynamic analysis Table 1 reports the main thermodynamic assumptions used in the present analysis: they do not represent the highest technological level reached by GTs. These choices are justi®ed by the main target of this work: a thermoeconomic analysis of the commercial competitiveness of mixed cycles. Table 1 Main thermodynamic assumptions GT Compressor isentropic eciency Combustion chamber pressure drop Combustion eciency Tmax Expansor isentropic eciency Mechanical eciency
0.86 4% 0.996 1200±1400°C 0.88 0.98
Gas±water/steam heat exchangers Subcooling DT Pinch point DT Approach DT for gas/water heat exchangers Approach DT for superheater Heat loss Gas pressure drop Water pressure drop
>5°C >5°C >5±10°C >15°C 1% 2% 8%
REC Heat exchanger eciency Hot side pressure drop Cold side pressure drop Heat loss
0.8 3% 2% 1%
SAT Enthalpy pinch point Gas pressure drop
>20 kJ/kgdryair 0.7%
Alternator and auxiliaries Electric eciency Pump hydraulic eciency Pump mechanical eciency
0.985 0.83 0.9
Fuel Chemical composition
Type volume fraction CH4 N2
LHV
44,243 kJ/kg
93% 7%
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Fig. 7. Cycle LHV eciency versus speci®c work, for Tmax 1200°C, 1400°C.
The highest eciencies are reached by the reference CC (see Fig. 7) that, contrary to mixed gas± air cycles, does not suer the exergy loss caused by gas±water mixing, has a steam condensation temperature of about 30°C and has a lower steam mass fraction in the exhaust stream. As far as speci®c work is concerned, mixed cycles achieve the highest values, therefore characterising themselves as compact and cheap plants, as discussed in the next paragraph. The comparison between the mixed cycles shows that the HAT cycle achieves the higher eciencies for all the pressure ratios considered here (Fig. 7), with a water/air ratio about constant (at the same maximum cycle temperature) and always lower than 25%, as shown in Fig. 8. This fact is very interesting, because it means that a thermodynamically optimised HAT cycle requires an almost ®xed water/air ratio that does not depend on the pressure ratio, once the cycle Tmax is ®xed: that is an absolutely useful result for the future design and project of GTs working on the HAT cycle. RWI, HAWIT and HAT cycles, compared to the STIG cycle, suer a lower eciency drop, decreasing the pressure ratio: for Tmax 1200°C they lose <2% points, passing from b 30 to b 10, instead of almost 5% points for the STIG cycle. This is a very attractive feature for small±medium size applications, characterised by low pressure ratios. In these conditions, the STIG carries out very high speci®c work values, as a consequence of the large amount of water
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Fig. 8. Mixed cycle speci®c water consumptions versus water±air ratio.
consumed. Unfortunately, it would require a speci®c GT design, in order to allow a much greater mass ¯ow rate in the expander to avoid compressor surge. In comparison to the other mixed cycles, from a thermodynamic point of view, the HAWIT cycle is in an intermediate position between the HAT and the RWI cycles, achieving better eciencies than the STIG cycles. Its speci®c work values are nevertheless similar to HAT ones, because of the greater amount of water consumed. Fig. 8 shows the opposite behaviour of RWI, HAWIT cycles and STIG one for water consumption: increasing the pressure ratio the former realises higher and higher water/air ratio while the STIG water/air ratio decreases. As far as the calculated best con®gurations are concerned, the eciency and speci®c work increases, in comparison with the original simple cycle (Fig. 9), do not vary much with Tmax and can be quanti®ed as reported in Table 2. One of the most interesting aspects of the study of mixed cycles is the presentation of the exergy analysis, particularly for the mixing losses. The complete exergetic analysis of the considered mixed cycles is reported in Ref. [6] while just the air±water mixing contribution to the total exergy losses is presented here in Fig. 10. It is possible to note that the SAT allows a more reversible
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Fig. 9. Simple cycle GT LHV eciency versus speci®c work, for Tmax 1200°C, 1400°C. Table 2 Eciency and speci®c work average increases from the original simple cycle RWI (b 30) STIG (b 30) HAWIT (b 30) HAT (b 30) CC (b 18)
Eciency increase (% points)
Speci®c work increase (%)
9 10.5 11 13 14
100 85 110 120 55
Fig. 10. Water±air mixing exergetic losses in the highest LHV eciency con®gurations for mixed cycles. All the values are expressed as a percentage of the maximum mixing exergy loss of RWI with Tmax 1200°C.
introduction of the water in the airstream: this is the main reason for HAT and HAWIT higher thermodynamic performance. Besides the mixing exergetic losses, the inferior RWI eciency, when compared to the other mixed cycles, is due to the water injection limit derived from gas saturation. Such a limit can be
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Fig. 11. LHV eciency with aftercooling water injection increase over compressed air saturation for RWI cycle with Tmax 1200°C and b 30. Power percentual raise and water/air ratio are reported at the LHV eciency maximum point.
overcomed by injecting a greater amount of water into the AC. In fact in this case the RWI ef®ciency rises because of the greater heat recovery in the REC (Fig. 11), with a maximum bene®t of about 0.5% points. Nevertheless this device could cause some structural problems in the REC, because it would become a variable temperature evaporator on the colder side (Fig. 12). 5.2. Thermoeconomic analysis Thermoeconomics is an analytic and complex approach for energy system analysis developed by several authors. Thermoeconomic analysis, integrated with the exergoeconomic analysis, allows a better understanding of system exergetic and monetary loss locations inside the plant and permits plant optimisation to minimise dierent thermoeconomic objective functions.
Fig. 12. REC temperature pro®le for the RWI cycle of Fig. 11, at the point of maximum eciency.
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Table 3 Main economic assumptions In¯ation Nominal escalation rate of PEC Nominal escalation rate of fuel cost
2.5% 2.5% 3.0%
Construction initial year (1 January) Construction time Plant economic life (book life) Plant life for tax purposes
2000 2 years 20 years 10 years
Debts ± ®nancing fraction Preferred stocks ± ®nancing fraction Common equities ± ®nancing fraction Debts ± required annual return Preferred stocks ± required annual return Common equities ± required annual return
50% 15% 35% 5.5% 6% 6.5%
Average income tax rate
30%
Fuel price (natural gas) Demineralised water price Sale price of electric power
4.0E 6 $/kJ 0.5 $/m3 1.32E 5 $/kJ
In this paper only whole plant thermoeconomic results are reported, without facing the internal thermoeconomic analysis, that is, however, indispensable for the right interpretation of the causes of the published results. The complete thermoeconomic analysis is reported by the authors in Refs. [1,6,24,25]. Table 3 illustrates the main economic assumptions, maintained constant for all the analysed plants: therefore they represent a compromise between CCs and mixed cycles data. In addition, the operating and maintenance cost was taken as equal to 4% of PEC. An equivalent working time of 8000 h/year was used. From a thermoeconomic point of view, a cost taking into account all water treatments must be assigned to supply water: in the framework of this paper a water cost of 0.5 $=m3 is assumed for all the cycles, even if for the HAT cycle it could be lower because it does not require high quality water like STIG and RWI cycles, since the SAT operates as a distillation tower. Economic considerations do not include SAT cost, for which the necessary economic data are still not available in literature: however, a way to determine a maximum allowable cost for such a device is suggested here. Figs. 13 and 14 show the energy cost and the IRR versus plant eciency. The reference CC (Tmax 1200, 1400°C, b 18), although it reaches the highest eciencies (51% and 52.8%), can not compete with mixed cycles for the medium size plant considered here (50 MW), because of higher capital costs due mainly to the heat recovery system (two-pressure levels), steam turbine and condenser purchase costs. On the other hand, mixed cycles are able to have low investment costs with high conversion eciencies, even for small sizes. Moreover, because of the presence of a parallel open cycle of water near the airstream, mixed cycles are proper for cogeneration applications too: the economic
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Fig. 13. Cycle energy costs in ®rst operating year (2002).
in¯uence of this opportunity is nevertheless not considered in this paper, but it can be easily handled using the T E M P code. As far as energy costs in the ®rst working year are concerned (Fig. 13), all the mixed cycles show the lowest values in a pressure ratio range of 15±20, that is slightly over the maximum speci®c work point of the original simple cycles. The consequent eciency improvement brought about by rising compressor pressure ratio is not enough to allow the energy costs to decrease, because of the important increase in capital costs, mainly concerned with GT purchase cost. The lowest energy costs are shown by the HAWIT cycle for both the considered maximum temperatures, while HAT and RWI energy costs are similar but a little higher. The STIG cycle is in an intermediate position between the CCs and the other mixed cycles, and shows the lowest thermoeconomic performance among the last. This is due to its higher capital cost than the RWI cycle, not balanced by a sucient conversion eciency increase as for the HAT cycle. Referring to the IRR (Fig. 14), the best values are always shown by the HAWIT cycle (more than 18% and 22% respectively), that demonstrate its excellent economic performance due to the best compromise between capital costs and high eciencies, for the 50 MW size considered.
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Fig. 14. Cycle IRR.
Among the other mixed cycles, the RWI cycle beats the HAT cycle by at least 1±2% points for Tmax 1200°C, because of its lower investment cost. This dierence is not present for Tmax 1400°C, because the HAT cycle better exploits the increase of the cycle maximum temperature in terms of conversion eciency improvement. Fig. 15 reports RWI and HAT energy costs during plant economic life. While for Tmax 1200°C a break-even point comes out after six working years, for Tmax 1400°C the HAT cycle always shows the lowest energy costs. It is interesting to note how the dierence between the two compared costs varies during the plant lifetime with an increase in the HAT advantage, because its higher eciency makes it less sensible to fuel cost rises. In fact, when comparing HAT and RWI plants for Tmax 1200°C with a greater in¯ation rate (from 2.5% to 5%), the break-even point comes earlier, after four working years. If the same comparison were carried out between HAWIT and RWI cycles, no break-even point would emerge because the former would always show lower energy costs for both the considered maximum cycle temperatures. Since at the moment it is very dicult to de®ne SAT purchase cost, a criterion to ®nd its maximum allowable cost is now discussed. In a free energy market environment the IRR, illustrated in Fig. 14, should not always be considered as the most important investment parameter
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Fig. 15. Energy cost during plant life (2002±2021) of RWI and HAT cycles with Tmax 1200°C, 1400°C and b 30 for two economic scenarios: (a) in¯ation rate 2.5%, fuel escalation rate 3%; (b) in¯ation rate 5%, fuel escalation rate 6%.
because all the plants could not succeed in selling their energy for 8000 h each year, as supposed. In fact plant competitiveness is well represented by its energy cost, that is the lowest possible sale price of the energy generated. Therefore, the graphics reported in Fig. 15 are suitable for a maximum SAT cost evaluation that cancels out HAT higher competitiveness in comparison with the RWI cycle. The dierence between the two curves represents a dierent speci®c gross pro®t during each year of the plant working life. Using the in¯ation rate to discount the total gross pro®t dierence between the two plants over their entire economic life to the ®nal construction year, it is possible to calculate a SAT maximum cost that makes such a dierence equal to zero. Applying this method to the HAT and HAWIT cycles, compared with RWI one, their closest competitor, considering the best eciency con®guration for all the plants, the results shown in Table 4 can be achieved. It is worth noting that the margin for the SAT cost is considerable: in fact the SAT does not require an innovative technology, being formed mainly by a pressurised tank operating in a temperature range of 100±200°C and in a pressure range of 1±3 MPa, with packing inside similar to that in EVAP towers. Moreover, if the proposed method were applied using the STIG cycle instead of the RWI cycle as a reference, still greater margins would result.
Table 4 Maximum allowable SAT cost HAT (Tmax 1200, b 30) HAWIT (Tmax 1200, b 30) HAT (Tmax 1400, b 30) HAWIT (Tmax 1400, b 30)
GT cost ($)
Maximum SAT cost (% GT cost)
3,958,000 3,887,000 3,365,000 3,316,000
12% 52% 64% 89%
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6. Conclusions In this paper the prediction of thermoeconomic performance of mixed water±air cycles has been outlined using the T E M P code, an advanced program for thermoeconomic analysis of complex energy plants. A comparison with a reference two-pressure level CC has been carried out, showing mixed cycles, and especially HAT and HAWIT cycles, attractive thermoeconomic features, for the considered size of 50 MW. The main conclusions of this work are: · ·
·
· ·
·
· · ·
code, developed by the authors and tested with the data of the EVAP pilot-plant installed at Lund University, Sweden, is a robust, ecient, reliable program for thermoeconomic analysis also for mixed cycles. SAT experimental data reported in Refs. [4,5] show a relative humidity greater than 100% (104± 105%) in the air leaving the SAT. The introduction of the steam compressibility factor (z) is not enough to explain such an incongruence, that needs to be more accurately investigated in the near future. When the mixed cycles are compared with a reference CC on a thermodynamic base they suer mixing exergetic losses (Fig. 10), eectively reduced in the HAT and HAWIT cycles by the SAT. In fact, these cycles provide the highest eciencies among the mixed cycles, as shown in Fig. 7. Mixed gas±steam cycles represents a fair investment opportunity, because they match high speci®c works and high eciencies with relatively low capital costs, especially for small±medium size systems. The HAWIT cycle presented here (Fig. 3), derived from the HAT cycle (Fig. 2) and speci®cally developed for small±medium size GTs, pursues a decrease of investment cost through the reduction of the heat exchanger number. This fact allows the best thermoeconomic performance to be achieved among all the plants analysed here, despite the lower eciencies in comparison with the original HAT cycle. A method to evaluate the maximum allowable cost of the SAT for HAT and HAWIT cycles is discussed. It focuses on plant life energy cost, which well represents the commercial competitiveness in a free energy market. Applying such a method to HAT and HAWIT cycles, compared to the RWI one, their closest competitor, shows that they have a great economic margin that allows them to be designated as the best layouts for power generation in the 50 MW range. The method introduced here for the SAT cost evaluation could be extended in the future to the analysis of cycles based on devices without not well-de®ned costs, such as partial oxidisers, steam reformers, fuel cells, etc. Thanks to the steam high mass fraction entering the combustion chamber, the mixed gas±steam cycles propose themselves as low-NOx and low-CO emission energy plants, as already demonstrated for STIG cycles [8] and for the Lund pilot-plant [5]. In the opinion of the authors, mixed cycle economic competitiveness, in comparison with conventional energy plants such as CCs, enhances decreasing plant size. In particular mixed cycles could become extremely competitive for sizes under 10 MW, where thermodynamic performance of the bottoming steam cycle quickly decreases.
TEMP
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It is interesting to note that the present study, carried out in an academic environment as the natural prosecution of previous thermoeconomic analyses on gas, steam, combined, advanced GT based systems, should be an important step for future thermoeconomic investigations carried out in an industrial frame too. Acknowledgements The authors wish to thank Professor Tord Torisson ± Lund University (S) for permission to publish the data and some inherent pictures concerning the HAT pilot-plant. This work has been sponsored by MURST of Italy, grant ``Co®nanziamento 1999'' Prot. 9909167799_003. References [1] A. Agazzani, A. Massardo, A. Satta, Thermoeconomic analysis of complex steam plants, ASME Paper 95-CTP-38, ASME Cogen Turbo Power Conference, Wien, 23/25 August, 1995. [2] A. Agazzani, A.F. Massardo, A Tool for Thermoeconomic Analysis and Optimization of Gas, Steam, and combined plants, ASME Transactions, Journal of Engineering for Gas Turbine and Power 119 (1997) 885±892. [3] A. Agazzani, Thermo-economic and environmental optimisation of energy systems, Tesi di Dottorato, Universit a di Genova (DIMSET), 1997. [4] T. Lindquist, Theoretical and experimental evaluation of the EvGT-process, Thesis for Degree of Licentiate in Engineering, Lund Institute of Technology, Sweden, 1999. [5] P.M. Rosen, Evaporative Cycles-in Theory and in Practice, Doctoral Thesis, Lund Institute of Technology, Sweden, 2000. [6] A. Traverso, Thermoeconomic analysis of STIG, RWI and HAT cycles with carbon dioxide (CO2 ) emissions penalty, Tesi di laurea, Universita di Genova (DIMSET), 2000. [7] A.F. Massardo, M. Scial o, Thermoeconomic analysis of gas turbine based cycle, ASME Transactions, Journal of Engineering for Gas Turbine and Power 122 (2000) 664±671. [8] J.B. Burnham, et al., Development; installation and operating results of a steam injection system (STIG) in a general electric LM5000 gas generator, ASME Paper 86-GT-231, 1986. [9] V. de Biasi, LM6000 Sprint design enhanced to increase power and eciency, Gas Turbine World, July±August 2000. [10] A.D. Rao, Process for producing power, US patent no. 4'829'763, 1989. [11] A.D. Rao, et al., A comparison of humid air turbine (HAT) and combined-cycle power plants, EPRI Report IE7300, Project 2999-7, Final Report, 1991. [12] Y. Xiao, R. Lin, R. Cai, System optimization of humid air turbine cycle, ASME Paper 94-GT-240, 1994. [13] M. Nakhamkin, E.C. Swensen, J.M. Wilson, G. Gaul, M. Polsky, The cascaded humi®ed advanced turbine (CHAT), ASME Paper 95-CTP-5, 1995. [14] M. Nakhamkin, E.C. Swensen, M. Polsky, G. Touchton, A. Cohn, CHAT technology: An alternative approach to achieve advanced turbine systems eciencies with present combustion turbine technology, ASME Paper 97-GT142, 1997. [15] E. Macchi, S. Consonni, G. Lozza, P. Chiesa, An assessment of the thermodynamic performance of mixed gas± steam cycles: Part A ± Intercooled and steam injected cycles, ASME Transactions, Journal of Engineering for Gas Turbine and Power 117 (1995) 489±498. [16] P. Chiesa, G. Lozza, E. Macchi, S. Consonni, An assessment of the thermodynamic performance of mixed gas± steam cycles: Part B ± Water injected and HAT cycles, ASME Transactions, Journal of Engineering for Gas Turbine and Power 117 (1995) 499±508.
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[17] G. Lindgren, et al., The HAT cycle, a possible future for power and cogeneration, Proceedings of the 1992 FLOWERS Congress, Florence, Italy, 1992, pp. 125±141. [18] A. Bhargava, M. Colket, W. Sowa, K. Casleton, D. Maloney, An experimental and modelling study of humid air premixed ¯ames, ASME Transactions, Journal of Engineering for Gas Turbine and Power 122 (2000) 405±411. [19] P. Chiesa, Thermodynamic analysis of humid air turbine cycles using natural gas or coal gasi®cation fuel, Tesi di Dottorato, Politecnico di Milano, 1995 (in Italian). [20] A.F. Massardo, A. Traverso, T E M P user's handbook. Version 6.0, TN 010-2000, DIMSET Universit a di Genova, 2000. [21] A.F. Massardo, F. Lubelli, Internal reforming solid oxide fuel cell ± gas turbine combined cycles (IRSOFC-GT) Part A: Cell model and cycle thermodynamic analysis, ASME Transactions, Journal of Engineering for Gas Turbines and Power 122 (2000) 27±35. [22] A. Massardo, B. Bosio, Assessment of molten carbonate fuel cell models and integration with gas and steam cycles, ASME Paper 2000-GT-0174, and ASME Transactions, in press. [23] T. Torisson, Personal Communication, Lund University, September 2000. [24] R. Borchiellini, A. Massardo, M. Santarelli, An analytical procedure for the carbon tax evaluation, December 1999, Energy Conversion and Management Journal (2000) 1509±1531. [25] A. Massardo, M. Santarelli, R. Borchiellini, Carbon exergy tax (CET): Impact on conventional energy systems design and its contribution to advanced systems utilisation, ECOS 2000, Twentee, The Netherlands, July 2000.