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Experimental investigation on the off-design performance of a small-sized humid air turbine cycle
Applied Thermal Engineering 51 (2013) 166e176
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Experimental investigation on the off-design performance of a small-sized humid air turbine cycle Chenyu Wei, Shusheng Zang* Turbomachinery Institute, School of Mechanical Engineering, Shanghai Jiao Tong University, No.800 Dongchuan Road, Shanghai 200240, PR China
h i g h l i g h t s < We built a flexible small-size test rig of HAT cycle gas turbine and the real test data were reported and analyzed. < The HAT cycle with higher humidity ratio exhibits higher thermal efficiency and lower NOx emission. < Good agreement between theoretical and experimental results. < Increasing the humidity ratio can move the compressor running line away from the surge line. < Introducing a recuperator to the existing HAT cycle will greatly increase cycle efficiency.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 April 2012 Accepted 4 August 2012 Available online 8 September 2012
This research aimed to study the improvement of the gas turbine performance of a humid air turbine (HAT) cycle at low pressure ratio and at low turbine inlet temperature (TIT). To achieve this goal, an offdesign performance test investigation was conducted on a small-sized, two-shaft gas turbine test rig. The test rig consisted of a centrifugal compressor, a centripetal turbine, an individual direct flow flame tube, a free power turbine, a dynamometer, and a saturator with structured packing. Two different conditions were considered for the test investigation: in Case I, the control system kept the fuel flow constant at 57 kg/h, and in Case II, the turbine inlet temperature was kept constant at 665 C. In Case I, when the air humidity ratio increased from 30 g/kg dry air (DA) to 43 g/kg DA, the power output increased by 3 kW. At the same time, the turbine inlet temperature decreased by 19 C, and the NOx emissions were reduced from 25 ppm to 16 ppm. In Case II, when the air humidity ratio increased from 48 g/kg DA to 57 g/kg DA, the power output increased by 9.5 kW. Based on the actual gas turbine parts, characteristics, and test conditions, the off-design performance of the HAT cycle was calculated. Upon comparing the measured and calculated results, the HAT cycle was found to perform better than the two-shaft cycle in terms of specific work, efficiency, and specific fuel consumption. The effect of performance improvement became more obvious as the air humidity ratio increased. Under the same inlet air flow, turbine inlet temperature, and power output, the surge margin on compressor curves became enlarged as the humidity ratio increased. The off-design performance of a HAT cycle with regenerator was also investigated. The results show that the highest efficiency can be increased by 3.1%, which will greatly improve the gas turbine performance. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Humid air turbine (HAT) cycle Experimental investigation Humidity ratio Off-design performance NOx emission Performance on compressor curves
1. Introduction The humid air turbine (HAT) cycle is one of the most advanced gas turbine cycles that is known for its high efficiency and low emission, as well as for its competitive costs, particularly in smalland medium-capacity gas turbines [1]. Numerous researchers have
* Corresponding author. E-mail address: [email protected] (S. Zang). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.08.061
conducted thermodynamic analyses of HAT cycles. Jonsson and Yan [2] presented a detailed literature review of humidified cycles and showed the potential of this cycle. Gallo [3], Traverso [4], and Nyberg [5] presented the performance of a complete HAT cycle consisting of an intercooler, an aftercooler, a recuperator, and an economizer. In Ref. [6], a general thermodynamic assessment of a micro humid air cycle in the range of 100e500 kW was presented. Wang et al. [7] compared the off-design performance of the HAT cycle and that of four other gas turbine cycles, namely, the simple cycle, the recuperated cycle, the recuperated water injected cycle,
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and steam injection gas turbine cycle. Only a few laboratories around the world have demonstrated and tested the HAT cycle from an experimental point of view. Supported by its industrial partner Alstom Power Generation Ltd., Lund University [8] was the first to demonstrate the entire humid air cycle successfully. They ran an actual HAT cycle using the existing microturbine VT600 with a capacity of 600 kW and an efficiency of 22% in a simple cycle. Based on the measurements on the pilot plant, the HAT cycle was found to be capable of remarkably enhancing efficiency by up to 35%. An experimental study of a pressurized humidification tower with structured packing was carried out by Traverso [9] and Pedemonte [10]. In their work, a test rig was built and a wide range of experimental data was collected to analyze the saturator behavior. Moreover, they introduced the measurement of humidity ratio at the saturator outlet. Afterward, an almost unique industrial experiment was carried out by Hitachi Ltd., as reported in Refs. [11e14]. They built an innovative configuration of a HAT cycle that substituted the water atomization cooling (WAC) system for the intercooler system of the HAT cycle. Their experimental results showed that the electrical efficiency exceeded 40% in the HAT cycle. In more recent studies [15], Hitachi Ltd. reported developing a 150 kW microturbine system that applied the HAT cycle. A prototype machine for a next-generation microturbine system incorporating a simplified HAT cycle and a WAC system had been developed for laboratory evaluation. The results of WAC and HAT performance tests showed that electrical efficiency increased by 3.0%, and the electrical output increased by 20%. Small-capacity gas turbines, especially microturbines, are usually operated at low pressure ratio and at low TIT. However, few researches have focused on the performance of HAT cycle at low pressure ratio and at low TIT, and relevant experimental data are still scarce. Regarding the analyses of the off-design performance of HAT cycles, the improved cycle performance in various pressure ratios and TITs after using HAT technology has been discussed. However, only a few studies have focused on the influence of humidity ratio at the saturator outlet on the overall performance of the gas turbine. In addition, some characteristics of the components, such as the efficiencies of the compressors and the turbines, are always assumed to be constant, when these efficiency values actually vary with changes in working conditions. Moreover, only a few comparisons between the measured and calculated results have been made. In the current study, a small test rig of a simplified two-shaft HAT cycle gas turbine was built at the TMI laboratory of Shanghai Jiaotong University to experimentally evaluate the performance of the HAT cycle at low pressure ratio and at low TIT. The test results of the existing HAT cycle test rig were first analyzed. Second, the offdesign performance of the HAT cycle and the operation behaviors on compressor curves were calculated using the characteristics of the actual components. The measured and calculated results were then compared. The effects of the humidity ratio on the operation characteristics of the HAT cycle were also studied. Finally, system optimization was discussed upon adding a recuperator to the present HAT cycle system.
a pump is mixed with hot recycled water from the bottom of the saturator, and is then sent into the aftercooler. The aftercooler functions in two ways: it cools the compressed air and it heats the water. The water from the aftercooler then mixes with the water from the economizer, increasing its temperature before entering the top of the saturator. The heat exchanger system maximizes the temperature of the water entering the saturator, as doing so improves the heat and mass transfer to the air and ultimately improves the performance of the gas turbine. 2.1. Test rig description A flexible test rig, as shown in Fig. 2, was built at the TMI laboratory of Shanghai Jiaotong University to verify the feasibility of the HAT system in a two-shaft gas turbine. The trial operation began in March 2011. In the layout of the two-shaft gas turbine, a one-stage centripetal high-pressure turbine drives a one-stage centrifugal compressor, the combination of which acts as a gas generator for a one-stage axial flow power turbine. An individual direct flow flame tube is assembled, and an electric eddy current dynamometer is driven by the power turbine through a reduction gearbox. The core of the rig is a saturation tower, the structured internal packing of which is used because of the relatively small pressure drop. A set of electric heating devices acting as an economizer is provided to preheat the make-up water. Table 1 shows the design specifications of the main components of the test rig. The design of the gas turbine is outlined in detail in Ref. [16]; here only a brief summary is given. The HP turbine inlet gas temperature (750 C) is the maximum temperature. In the gas turbine starting system, the air is provided by a centrifugal compressor driven by a 400 kW variable frequency alternating current motor. The present test rig was conducted in a preliminary laboratory study phase; thus, the aftercooler and the recuperator in the two-shaft HAT system were not used. 2.2. Control system and measurement A remote control system is developed in the test rig independently. The signal acquisition and feedback are manipulated using a SIMATIC S7-300 PLC control system, which is a modular automation system by Siemens Technologies. Fig. 3 shows the control panel for the monitoring and control system of the two-shaft HAT cycle. The control panel comprises the gas turbine monitor, the
2. Two-shaft HAT test system Fig. 1 shows a schematic diagram of the two-shaft arrangement of the HAT system. In this cycle, the air is compressed and cooled, and then sent to the saturator, where hot water is allowed to evaporate and to mix with air. In the recuperator, the exhaust gas from the power turbine pre-heats the humid air before entering the combustion chamber. The economizer also recovers heat from the exhaust gas to the water. The make-up water furnished through
167
Fig. 1. Schematic diagram of the two-shaft HAT cycle.
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Fig. 2. Layout of the two-shaft HAT test rig.
saturator monitor, the ignition control system, and the valve control system. A WOODWARD LQ3 liquid fuel-metering valve, which provides reliable interfaces between electronic engine control systems and gas turbines, is used to precisely control the fuel flow rate supplied to the combustor chamber. Type K thermocouples, which are all standardized prior to testing, are installed to monitor the inlet and outlet temperature of the equipment. Two vibrating sensors are installed on both shafts of the air-supply compressor and the gas turbine compressor to monitor the vibration caused by mechanical failure or even by surge. One of the most complex problems of saturation towers is the accurate measurement of the air humidity ratio at the saturation tower outlet. Even the smallest difference in water mass flow between inlet and outlet may severely affect the precision of the resulting consumption. According to the procedure employed in Ref. [9], water consumption is estimated by averaging over a certain period of time the feed water that has to be supplied to the system to restore the level at the bottom of the tower. This system provides a mean value over a time period and guarantees high accuracy.
~ and the isentropic efficiency (h) as a function of the corratio ðmÞ ~ Þ: rected air mass flow (p) and corrected rotational speed ðn
~ ¼ f1 ðp; n ~Þ m h ¼ f2 ðp; n~ Þ
(1)
Such functions could be numerically evaluated through a twodimensional interpolation on digitized maps [17]. The corrected ~ and corrected rotational speed ðn ~ Þ are expressed in mass flow ðmÞ Eqs. (2) and (3), respectively:
~ ¼ m
pffiffiffiffiffiffiffiffiffiffiffiffiffi m T=Tref p=pref
(2)
n ~ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi n T=Tref
(3)
where Tref and pref are the reference temperature and pressure, generally at ISO conditions of 15 C and 1.013 bar. The overall turbine performance could be evaluated using the same approach. Humidity ratio
3. Calculation model 3.1. Modeling of HAT cycle The performance of the gas turbine was calculated using a simulation program called SJGT, which is the result of an ongoing development at the Turbomachinery Institute of Shanghai Jiaotong University. The code was developed in Matlab-SimulinkÒ. The main components, namely, a compressor, a combustor, two turbines, and a simplified saturator, were introduced for the present study of the HAT cycle. A recuperator built for further study will be discussed in Section 5. The following three features were considered for real HAT cycle performance analysis: Component characteristics of compressor and turbines The performance of compressor was evaluated using a compressor map. The compressor map provided the pressure
The humidity ratio is defined by:
d ¼ 1000
mv Mv pv pv ¼ 1000 ¼ 622 mDA MDA pDA pHA pv
(4)
Table 1 Design specifications of the main test rig equipment. Item Two-shaft gas turbine Gas generator shaft Compressor Combustor HP turbine Power turbine Gearbox Saturator Electric heating device
Rated power output Rated rotational speed Pressure ratio Isentropic efficiency Combustion efficiency Inlet gas temperature Isentropic efficiency Isentropic efficiency Transmission ratio Packing type Rated power
Unit
Value
kW rpm e % % C % % e e kW
25 57,500 3.0 79 95 750 82 80 4.67 Stainless steel Intalox 150
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169
Fig. 3. Monitoring and control system of the two-shaft HAT cycle.
where m and M are the mass and molar mass, respectively. The vapor partial pressure pv could be given by:
pv ¼ f$MINðpsv ; pHA Þ
target of this work. These choices also corresponded with the test operation condition for model validation.
(5) 4. Results and discussion
where f is the relative humidity ratio collected by the hygrometer. The pressure of saturation vapor psv is expressed in terms of the saturator outlet air temperature THA as [18]:
2 T 7:21275 þ 3:981 0:745 HA þ1:05 647:3 3 T 647:3 0:745 HA $ 1 THA 647:3
psv ¼ 221:2$exp
(6)
The test campaign aimed to obtain the test data from the twoshaft HAT cycle performance and to discuss the influence of humidity ratio at the outlet of the saturator on the HAT cycle performance within the capabilities of the test rig itself. 4.1. Test results The operation tests for the two-shaft HAT cycle were carried out in two conditions.
Humid gas mixture In the present study, the humid gas was considered the ideal mixture of gas and water vapor. Using Dalton’s law, the partial pressure of the vapor in humid gas is expressed by:
pv ¼ cv $ pv þ pg
mv =Mv mv =Mv þ mg =Mg
(7) Compressor
(8)
The enthalpy of humid gas mixture could be obtained from:
hmix ¼
mg hg þ mv hv mg þ mv
Item Ambient conditions
where the mole fraction cv is given by:
cv ¼
Table 2 Main thermodynamic assumptions.
(9)
Combustor chamber HP turbine
Power turbine Saturator Fuel oil
Unit
Value
Pressure kPa C Temperature Relative humidity % Inlet pressure loss % Isentropic efficiency % Mechanical efficiency % Air leakage (at the compressor outlet) % Pressure loss % Efficiency % C Inlet gas temperature Isentropic efficiency % Mechanical efficiency % Isentropic efficiency % Mechanical efficiency % Pressure loss % LHV kJ/kg
101.3 45 60 1 72e78 99 2 4 95 650e700 73e78 99 73e77 98 0.5 42,700
Heat exchanger efficiency Hot side pressure loss Cold side pressure loss
92 3 2
3.2. Calculation assumptions Recuperator
Table 2 shows the main thermodynamic assumptions used in the present analysis. These choices were justified by the main
% % %
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Fig. 4. Test with same fuel flow rate.
Case I: constant fuel flow operation As shown in Fig. 4, the fuel flow was kept constant at 57 kg/h. The operation was divided into two stages. In the first stage, the humidity ratio was increased rapidly from 30 g/kg dry air (DA) to 37 g/kg DA in 6 min. In the second stage, the humidity ratio was increased gradually from 37 g/kg DA to 43.5 g/kg DA in the next 22 min. Fig. 5 shows the behavior of the TIT and the power output. In the first stage, the TIT decreased rapidly from 678 C to 668 C, and the power output increased by 1.4 kW immediately after water was added to the saturator. The effect of humid air on the gas turbine cycle was reflected promptly without delay. The TIT decreased by 17 C, and the power output increased by 3 kW when the humidity ratio increased from 30 g/kg DA to 43.5 g/kg DA during the whole operation. An increase in humidity ratio caused a decrease in combustion temperature, which helped to reduce the thermal stress in the combustion chamber and the turbines. Although the TIT decreased, a slight increase in power output could still be
noticed because of the higher mass flow over the turbines. Evidently, the humidity ratio has an important effect on the performance of gas turbines in the HAT cycle. Fig. 6 shows the performance of NOx emissions. NOx emissions decreased from 25 ppm to 16 ppm when the humidity ratio increased from 30 g/kg DA to 43.5 g/kg DA. Clearly, the humidity ratio of humid air at the outlet of the saturator is an important factor that affects the flame temperature in the combustor chamber. Lower NOx emission was also found to occur along with the lower flame temperature caused by higher humidity ratio. Case II: constant TIT operation As shown in Fig. 7, the TIT was kept constant at about 665 C during operation by regulating the fuel flow from 69 kg/h to 102 kg/h when the humidity ratio was increased from 48 g/kg DA to 57 g/kg DA. Fig. 8 shows the behavior of the cycle performance. An enormous increase of 9.5 kW in the total power output was seen after the air was humidified by the saturator. Such behavior signifies
Fig. 5. TIT and power output performance.
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Fig. 6. NOx emission performance.
that higher power output can be obtained at the same TIT in a HAT cycle than in a general two-shaft cycle. These results indicate that gas turbine performance can be improved without the restriction of a higher TIT dependent on the development of material technology. 4.2. Off-design performance analysis The off-design performance of the two-shaft HAT cycle calculated through SJGT is shown in Figs. 9e11. Two fixed TITs (700 C and 650 C) were selected for analysis because more than 90% test points were operated between these two TITs. Three humidity ratio levels of the humid air, namely, 30, 43, and 55 g/kg DA, were chosen for analyses. The humidity ratio of 30 g/kg DA was considered as the general humidity ratio, which could be obtained in the cycle without humidification by Eqs. (4)e(6). Fig. 9 shows the relationships between the pressure ratio and the specific work. Two groups of curves are shown in this figure;
the upper three curves indicate the performance when TIT is 700 C, and the lower three curves indicate the performance when TIT is 650 C. More specific work was obtained with a higher TIT when the humidity ratio was equal. Specific work was augmented with the increase of pressure ratio, e.g., both of the solid lines represent a humidity ratio of 55 g/kg DA, and the specific work changes from 11.4 kJ/kg to 22 kJ/kg at a pressure ratio of 2.8 when the TIT changes from 650 C to 700 C. Fig. 9 shows that an optimal pressure ratio achieving optimum specific work in each curve exists, and that the value of optimal pressure ratio increases with TIT. The diagram also shows that, as expected, the humidity ratio is one of the most important parameters in the HAT cycle: the higher the humidity ratio is, the greater the augmentation of the specific work is. Compared with the gas turbine cycle without HAT, e.g., at a TIT of 700 C, the specific work increased from 15.5 kJ/kg to 22 kJ/kg, signifying that more power output can be obtained because of the higher amount of evaporated water in the saturation tower.
Fig. 7. Test with same TIT.
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Fig. 8. TIT and power output performance.
The specific work is one of the primary performance parameters in the HAT test. Six points marked with a pentagram (P1eP6), as shown in Fig. 9, were the test cases selected for analysis. The setting values of the six test cases are indicated in Table 3. Points 1 and 2 were both operated at a TIT of about 700 C. P1 approximately indicated the maximum power output during the test at a higher pressure ratio (2.64) and humidity ratio (55 g/kg DA). P2 showed the status just before air humidification at a lower pressure ratio (1.69). The other four points (P3eP6) were run at a TIT of about 650 C. P3, compared with P4, P5, and P6, offered relatively higher specific work because of higher pressure ratio (2.35) and higher humidity ratio (55.1 g/kg DA). With regard to P4eP6, P5 achieved 1.29 kW more specific work than P6, as its pressure ratio was only 0.04 higher than that of P6. P4 reached 0.36 kJ/kg more specific work than P6 even though its pressure ratio was 0.09 lower than that of P6. These differences were observed because P4 had a higher humidity ratio (47.5 g/kg DA) than P5 (43.7 g/kg DA) and P6
(37.5 g/kg DA). Evidently, humidity ratio greatly influences the behavior of specific work in the HAT cycle. Fig. 9 compares the measured and calculated specific work at off-design operation of the two-shaft HAT test rig. As the results were in agreement, the calculation model is therefore effective, and the simulation tool can be further used to evaluate the performance of the HAT cycle with a recuperator. Fig. 10 shows the cycle efficiency performance of the HAT configuration described above. An optimal pressure ratio achieving optimum efficiency can be found in each curve, and its value increases with TIT, e.g., an optimum efficiency of 2.94% is achieved at a pressure ratio of 2.8, and the efficiency reaches a maximum of 2.1% at a pressure ratio of 2.5, indicated by the two solid lines in Fig. 10. Efficiency clearly grows with TIT, and the temperature should be as high as possible. Based on the solid line in Fig. 10, the efficiency increased by 1.27% when the TIT increased from 650 C to 700 C with the same pressure ratio of 2.8 and humidity ratio of
Fig. 9. Specific work performance.
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Fig. 10. Cycle efficiency performance.
Fig. 11. SFC performance.
55 g/kg DA. In addition to the TIT, the humidity ratio also demonstrated its influence on cycle efficiency. An increase of humidity ratio from 30 g/kg DA to 55 g/kg DA can enhance the efficiency by 0.95% under the same temperature of 700 C and the same pressure ratio of 2.8. Fig. 11 displays the specific fuel consumption (SFC) performance in the HAT cycle. As seen in the figure, lower SFC results from higher TIT. Moreover, the increase of humidity ratio reduces the SFC, with the effect being more significant in lower pressure ratio. By comparing the HAT cycle with a humidity ratio of 55 g/kg DA to the cycle without HAT, one can see that the SFC decreases from 28.3 kg/kW h to 12.7 kg/kW h at a pressure ratio of 1.6. Such case indicates that the HAT cycle with higher humidity ratio exhibits higher thermal efficiency. 4.3. Performance analysis on compressor characteristic curves Fig. 12 shows the characteristic curves of the centrifugal compressor in the test. The horizontal axis shows the corrected
mass flow rate, and the vertical axis shows the pressure ratio. Five curves, representing different corrected rotational speeds, and three dotted lines, representing different isentropic efficiencies of compressor, are shown in Fig. 12; the surge line is plotted as the dot-dash line. Two equilibrium running lines (L1 and L2) are obtained in Fig. 12, and all the work points on both lines are operated under the same TIT of 665 C, corresponding to the second test condition (Case II). The upper running line (L1) is run in a general
Table 3 Test conditions of test cases. ID
Humidity ratio (g/kg DA)
Pressure ratio
Specific work (kJ/kg)
P1 P2 P3 P4 P5 P6
50 30 55.1 47.5 43.7 37.5
2.64 1.69 2.35 1.79 1.92 1.88
21.96 7.95 12.09 6.89 7.82 6.53
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Fig. 12. Comparison of equilibrium running lines on the compressor characteristic curves.
two-shaft gas turbine with a humidity ratio of 30 g/kg DA. The lower running line (L2) is operated in a HAT cycle with a higher humidity ratio of 55 g/kg DA. Each pair of points on both running lines with the same inlet conditions, such as inlet temperature, pressure, and air flow, achieves the same power output. The working points on L2, compared with the corresponding points on L1, are operated at lower pressure ratios and lower corrected rotational speeds, although they are run at lower compressor efficiencies. The test results of the second test condition were compared with the predicted results and plotted as small circles in Fig. 12. The test running line was run from low to high pressure ratio at a TIT of 665 C; the humidity ratio was increased from 48 g/kg DA to 57 g/kg DA. The test points were located between L1 and L2 when the
pressure ratio was in the range of 2.1e2.3 because the humidity ratio was less than 55 g/kg DA. When the pressure ratio was over 2.4, the test points were operated below L2 because of the higher humidity ratio. Increasing the humidity ratio can move the compressor running line away from the surge line. The increase of surge margin improves operation safety, especially when the engine is operated under low conditions. 4.4. Performance of HAT with recuperator The existing test rig is planned to be extended with a recuperator to utilize exhaust gas energy and raise the total efficiency of the cycle. The characteristics of the recuperator are listed in Table 2. The
Fig. 13. Difference in specific work when the recuperator is used.
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Fig. 14. Difference in cycle efficiency when the recuperator is used.
Fig. 15. Difference in SFC when the recuperator is used.
thermodynamic performance of the HAT cycle with a recuperator is calculated through SJGT. The working condition is set in accordance with the points on the top curve in Fig. 9, which is at a TIT of 700 C and a humidity ratio of 55 g/kg DA. Thus, the calculation results are also comparatively analyzed based on the same curve. Fig. 13 shows the difference in behavior of specific work when the recuperator is used. The difference decreases as the pressure ratio is increased. When the pressure ratio is greater than 2.5, the specific work obtained by the HAT cycle with a recuperator is even less than that obtained by the HAT cycle without recuperator. Some specific work will be sacrificed at a higher pressure ratio in the recuperated cycle. These results are in agreement with the findings of Gallo et al. [3]. Fig. 14 shows the effect on cycle efficiency when a recuperator is added. When the recuperator is used, the cycle efficiency is strongly increased at the pressure ratio from 1.6 to 2.8, reaching a maximum increase of 3.1% at a pressure ratio of 1.9. Evidently, the exhaust gas energy is properly utilized in the recuperated cycle, and the recuperator greatly affects the total cycle efficiency. Fig. 15 displays the difference in SFC between the HAT cycle with a recuperator and the one without a recuperator. As seen from the
figure, the SFC performance improves when a recuperator is added to the HAT cycle at a compressor pressure ratio ranging from 1.6 to 2.8. The maximum decrease of SFC (5.1 kg/kW h) can be observed at a low pressure ratio of 1.6. From a thermodynamics viewpoint, although some specific work is sacrificed at a higher pressure ratio, introducing a recuperator to the existing HAT cycle will greatly increase cycle efficiency and improve SFC performance. Hence, adding a recuperator to the existing HAT cycle is necessary. 5. Conclusions The results from this work are summarized as follows: A small-sized test rig of a two-shaft HAT cycle gas turbine was built to experimentally evaluate the thermodynamic performance and emission behavior of the HAT cycle at low pressure ratio and at low TIT. The experimental results demonstrate that the effect of humid air is reflected promptly on the gas turbine cycle, and that the humidity ratio has an important effect on the performance of gas turbine in the HAT cycle.
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When the fuel consumption was fixed, an increase in humidity ratio from 30 g/kg DA to 43.5 g/kg DA caused a decrease in combustion temperature by 17 C, which helped reduce thermal stress in the combustion chamber and the turbines. Although the TIT decreased, a slight increase of 3 kW in power output could be noticed because of the higher mass flow over the turbines. In addition, the NOx emission decreased from 25 ppm to 16 ppm when the humidity ratio increased from 30 g/kg DA to 43.5 g/kg DA. Lower NOx emission occurs along with the lower flame temperature caused by higher humidity ratio. When the TIT was maintained at 665 C, an increase of 9.5 kW in the total power output was found after the air was humidified by a saturator with an increase of humidity ratio from 48 g/kg DA to 55 g/kg DA. Therefore, gas turbine performance can be improved without the restriction of a higher TIT dependent on the development of material technology. The results of the off-design performance of the HAT cycle calculated using SJGT matched the actual test results well. The results demonstrate that the higher humidity ratio is, the greater the augmentation in the specific work and in the cycle efficiency. Increasing the humidity ratio reduces the SFC, and the effect is more significant in lower pressure ratio. Therefore, the HAT cycle with higher humidity ratio exhibits higher thermal efficiency. A comparative analysis was carried out on the performance of two equilibrium running lines with different humidity ratios on compressor characteristic curves. When the gas turbine runs with the same inlet air flow, turbine inlet temperature, and power output, the working points on the running line with higher humidity ratio, although run at lower compressor efficiencies, can be operated at lower pressure ratios and lower corrected rotational speeds. Increasing the humidity ratio can move the compressor running line away from the surge line. The increase of surge margin will improve operation safety, especially when the engine is operated under low conditions. To perfect the HAT cycle, the existing test rig is planned to be extended with a recuperator. From a thermodynamics viewpoint, although some specific work is sacrificed at a higher pressure ratio, introducing a recuperator to the existing HAT cycle will greatly increase cycle efficiency and improve SFC performance. Hence, adding a recuperator to the existing HAT cycle is necessary. References [1] M. Nakhamkin, R. Pelini, M. Patel, Power augmentation of heavy duty and two-shaft small and medium capacity combustion turbines with application of humid air injection and dry air injection technologies, in: Proceedings of ASME POWER 2004, POWER2004-52095 (2004). [2] M. Jonsson, J. Yan, Humidified gas turbinesda review of proposed and implemented cycles, Energy 30 (2005) 1013e1078. [3] W. Gallo, A comparison between the HAT cycle and other gas-turbine based cycles: efficiency, specific power and water consumption, Energy Convers. Manag. 38 (15e17) (1997) 1595e1604. [4] A. Traverso, A. Massardo, Thermoeconomic analysis of mixed gas-steam cycles, Appl. Therm. Eng. 22 (2002) 1e21. [5] B. Nyberg, M. Thern, Thermodynamic studies of a HAT cycle and its components, Appl. Energy 89 (1) (2012) 315e321.
[6] J. Parente, A. Traverso, A. Massardo, Micro humid air cycle part A: thermodynamic and technical aspects, in: Proceedings of ASME Turbo Expo. 2003, GT2003-38326 (2003). [7] B. Wang, S. Zhang, Y. Xiao, Steady-state off-design performance of humid air turbine cycle, in: Proceedings of ASME Turbo Expo. 2002, GT2007-27350 (2007). [8] T. Lindquist, M. Thern, T. Torisson, Experimental and theoretical results of a humidification tower in an evaporative gas turbine cycle pilot plant, in: Proceedings of ASME Turbo Expo. 2002, GT2002-30127 (2002). [9] A. Traverso, Humidification tower for humid air gas turbine cycles: experimental analysis, Energy 35 (2010) 894e901. [10] A. Pedemonte, A. Traverso, A. Massardo, Experimental analysis of pressurised humidification tower for humid air gas turbine cycles. Part A: experimental campaign, Appl. Therm. Eng. 28 (2008) 1711e1725. [11] H. Araki, S. Higuchi, S. Marushima, Design study of a humidification tower for the advanced humid air turbine system, ASME J. Eng. Gas Turbines Power 128 (2006) 543e550. [12] H. Araki, Y. Katagiri, S. Marushima, Experimental results of humidification and water recovery of the advanced humid air turbine system pilot plant, in: Proceedings of the International Gas Turbine Congress 2007, Tokyo, C2007 Tokyo TS-012 (2007). [13] H. Kuroki, S. Hatamiya, T. Shibata, Development of elemental technologies for advanced humid air turbine system, ASME J. Eng. Gas Turbines Power 130 (2008) 031701-1e031701-6. [14] H. Araki, T. Koganezawa, C. Myouren, Experimental and analytical study on the operation characteristics of the AHAT system, in: Proceedings of ASME Turbo Expo. 2011, GT2011-45168 (2011). [15] S. Nakano, T. Kishibe, H. Araki, Development of a 150 kW microturbine system which applies the humid air turbine cycle, in: Proceedings of ASME Turbo Expo. 2007, GT2007-28192 (2007). [16] Operation and Maintenance Instructions Manual for JTH160-10A Turbocharger. [17] Characteristic Curves of MTM 60 Gas Generator Compressor. [18] J. Yan, Engineering Thermodynamics, second ed., 1981, pp. 243e246.
Notation d: humidity ratio (g/kg DA) DA: dry air LHV: low heating value (kJ/kg) m: mass flow (kg/s) ~ corrected mass flow (kg/s) m: M: molar mass (g/mol) n: rotational speed (rad/s) b : corrected rotational speed (rad/s) n P: pressure (kPa) SFC: specific fuel consumption (kg/kWh) T: temperature (K) TIT: turbine inlet temperature ( C) p: pressure ratio h: efficiency c: mole fraction f: relative humidity Abbreviations AC: aftercooler C: compressor CC: combustion chamber ECO: economizer HAT: humid air turbine HPT: high-pressure turbine PT: power turbine REC: recuperator SAT: saturator Subscripts g: gas HA: humid air mix: humid gas mixture ref: reference sv: saturation vapor v: vapor
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental investigation on the off-design performance of a small-sized humid air turbine cycle Chenyu Wei, Shusheng Zang* Turbomachinery Institute, School of Mechanical Engineering, Shanghai Jiao Tong University, No.800 Dongchuan Road, Shanghai 200240, PR China
h i g h l i g h t s < We built a flexible small-size test rig of HAT cycle gas turbine and the real test data were reported and analyzed. < The HAT cycle with higher humidity ratio exhibits higher thermal efficiency and lower NOx emission. < Good agreement between theoretical and experimental results. < Increasing the humidity ratio can move the compressor running line away from the surge line. < Introducing a recuperator to the existing HAT cycle will greatly increase cycle efficiency.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 April 2012 Accepted 4 August 2012 Available online 8 September 2012
This research aimed to study the improvement of the gas turbine performance of a humid air turbine (HAT) cycle at low pressure ratio and at low turbine inlet temperature (TIT). To achieve this goal, an offdesign performance test investigation was conducted on a small-sized, two-shaft gas turbine test rig. The test rig consisted of a centrifugal compressor, a centripetal turbine, an individual direct flow flame tube, a free power turbine, a dynamometer, and a saturator with structured packing. Two different conditions were considered for the test investigation: in Case I, the control system kept the fuel flow constant at 57 kg/h, and in Case II, the turbine inlet temperature was kept constant at 665 C. In Case I, when the air humidity ratio increased from 30 g/kg dry air (DA) to 43 g/kg DA, the power output increased by 3 kW. At the same time, the turbine inlet temperature decreased by 19 C, and the NOx emissions were reduced from 25 ppm to 16 ppm. In Case II, when the air humidity ratio increased from 48 g/kg DA to 57 g/kg DA, the power output increased by 9.5 kW. Based on the actual gas turbine parts, characteristics, and test conditions, the off-design performance of the HAT cycle was calculated. Upon comparing the measured and calculated results, the HAT cycle was found to perform better than the two-shaft cycle in terms of specific work, efficiency, and specific fuel consumption. The effect of performance improvement became more obvious as the air humidity ratio increased. Under the same inlet air flow, turbine inlet temperature, and power output, the surge margin on compressor curves became enlarged as the humidity ratio increased. The off-design performance of a HAT cycle with regenerator was also investigated. The results show that the highest efficiency can be increased by 3.1%, which will greatly improve the gas turbine performance. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Humid air turbine (HAT) cycle Experimental investigation Humidity ratio Off-design performance NOx emission Performance on compressor curves
1. Introduction The humid air turbine (HAT) cycle is one of the most advanced gas turbine cycles that is known for its high efficiency and low emission, as well as for its competitive costs, particularly in smalland medium-capacity gas turbines [1]. Numerous researchers have
* Corresponding author. E-mail address: [email protected] (S. Zang). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.08.061
conducted thermodynamic analyses of HAT cycles. Jonsson and Yan [2] presented a detailed literature review of humidified cycles and showed the potential of this cycle. Gallo [3], Traverso [4], and Nyberg [5] presented the performance of a complete HAT cycle consisting of an intercooler, an aftercooler, a recuperator, and an economizer. In Ref. [6], a general thermodynamic assessment of a micro humid air cycle in the range of 100e500 kW was presented. Wang et al. [7] compared the off-design performance of the HAT cycle and that of four other gas turbine cycles, namely, the simple cycle, the recuperated cycle, the recuperated water injected cycle,
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and steam injection gas turbine cycle. Only a few laboratories around the world have demonstrated and tested the HAT cycle from an experimental point of view. Supported by its industrial partner Alstom Power Generation Ltd., Lund University [8] was the first to demonstrate the entire humid air cycle successfully. They ran an actual HAT cycle using the existing microturbine VT600 with a capacity of 600 kW and an efficiency of 22% in a simple cycle. Based on the measurements on the pilot plant, the HAT cycle was found to be capable of remarkably enhancing efficiency by up to 35%. An experimental study of a pressurized humidification tower with structured packing was carried out by Traverso [9] and Pedemonte [10]. In their work, a test rig was built and a wide range of experimental data was collected to analyze the saturator behavior. Moreover, they introduced the measurement of humidity ratio at the saturator outlet. Afterward, an almost unique industrial experiment was carried out by Hitachi Ltd., as reported in Refs. [11e14]. They built an innovative configuration of a HAT cycle that substituted the water atomization cooling (WAC) system for the intercooler system of the HAT cycle. Their experimental results showed that the electrical efficiency exceeded 40% in the HAT cycle. In more recent studies [15], Hitachi Ltd. reported developing a 150 kW microturbine system that applied the HAT cycle. A prototype machine for a next-generation microturbine system incorporating a simplified HAT cycle and a WAC system had been developed for laboratory evaluation. The results of WAC and HAT performance tests showed that electrical efficiency increased by 3.0%, and the electrical output increased by 20%. Small-capacity gas turbines, especially microturbines, are usually operated at low pressure ratio and at low TIT. However, few researches have focused on the performance of HAT cycle at low pressure ratio and at low TIT, and relevant experimental data are still scarce. Regarding the analyses of the off-design performance of HAT cycles, the improved cycle performance in various pressure ratios and TITs after using HAT technology has been discussed. However, only a few studies have focused on the influence of humidity ratio at the saturator outlet on the overall performance of the gas turbine. In addition, some characteristics of the components, such as the efficiencies of the compressors and the turbines, are always assumed to be constant, when these efficiency values actually vary with changes in working conditions. Moreover, only a few comparisons between the measured and calculated results have been made. In the current study, a small test rig of a simplified two-shaft HAT cycle gas turbine was built at the TMI laboratory of Shanghai Jiaotong University to experimentally evaluate the performance of the HAT cycle at low pressure ratio and at low TIT. The test results of the existing HAT cycle test rig were first analyzed. Second, the offdesign performance of the HAT cycle and the operation behaviors on compressor curves were calculated using the characteristics of the actual components. The measured and calculated results were then compared. The effects of the humidity ratio on the operation characteristics of the HAT cycle were also studied. Finally, system optimization was discussed upon adding a recuperator to the present HAT cycle system.
a pump is mixed with hot recycled water from the bottom of the saturator, and is then sent into the aftercooler. The aftercooler functions in two ways: it cools the compressed air and it heats the water. The water from the aftercooler then mixes with the water from the economizer, increasing its temperature before entering the top of the saturator. The heat exchanger system maximizes the temperature of the water entering the saturator, as doing so improves the heat and mass transfer to the air and ultimately improves the performance of the gas turbine. 2.1. Test rig description A flexible test rig, as shown in Fig. 2, was built at the TMI laboratory of Shanghai Jiaotong University to verify the feasibility of the HAT system in a two-shaft gas turbine. The trial operation began in March 2011. In the layout of the two-shaft gas turbine, a one-stage centripetal high-pressure turbine drives a one-stage centrifugal compressor, the combination of which acts as a gas generator for a one-stage axial flow power turbine. An individual direct flow flame tube is assembled, and an electric eddy current dynamometer is driven by the power turbine through a reduction gearbox. The core of the rig is a saturation tower, the structured internal packing of which is used because of the relatively small pressure drop. A set of electric heating devices acting as an economizer is provided to preheat the make-up water. Table 1 shows the design specifications of the main components of the test rig. The design of the gas turbine is outlined in detail in Ref. [16]; here only a brief summary is given. The HP turbine inlet gas temperature (750 C) is the maximum temperature. In the gas turbine starting system, the air is provided by a centrifugal compressor driven by a 400 kW variable frequency alternating current motor. The present test rig was conducted in a preliminary laboratory study phase; thus, the aftercooler and the recuperator in the two-shaft HAT system were not used. 2.2. Control system and measurement A remote control system is developed in the test rig independently. The signal acquisition and feedback are manipulated using a SIMATIC S7-300 PLC control system, which is a modular automation system by Siemens Technologies. Fig. 3 shows the control panel for the monitoring and control system of the two-shaft HAT cycle. The control panel comprises the gas turbine monitor, the
2. Two-shaft HAT test system Fig. 1 shows a schematic diagram of the two-shaft arrangement of the HAT system. In this cycle, the air is compressed and cooled, and then sent to the saturator, where hot water is allowed to evaporate and to mix with air. In the recuperator, the exhaust gas from the power turbine pre-heats the humid air before entering the combustion chamber. The economizer also recovers heat from the exhaust gas to the water. The make-up water furnished through
167
Fig. 1. Schematic diagram of the two-shaft HAT cycle.
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Fig. 2. Layout of the two-shaft HAT test rig.
saturator monitor, the ignition control system, and the valve control system. A WOODWARD LQ3 liquid fuel-metering valve, which provides reliable interfaces between electronic engine control systems and gas turbines, is used to precisely control the fuel flow rate supplied to the combustor chamber. Type K thermocouples, which are all standardized prior to testing, are installed to monitor the inlet and outlet temperature of the equipment. Two vibrating sensors are installed on both shafts of the air-supply compressor and the gas turbine compressor to monitor the vibration caused by mechanical failure or even by surge. One of the most complex problems of saturation towers is the accurate measurement of the air humidity ratio at the saturation tower outlet. Even the smallest difference in water mass flow between inlet and outlet may severely affect the precision of the resulting consumption. According to the procedure employed in Ref. [9], water consumption is estimated by averaging over a certain period of time the feed water that has to be supplied to the system to restore the level at the bottom of the tower. This system provides a mean value over a time period and guarantees high accuracy.
~ and the isentropic efficiency (h) as a function of the corratio ðmÞ ~ Þ: rected air mass flow (p) and corrected rotational speed ðn
~ ¼ f1 ðp; n ~Þ m h ¼ f2 ðp; n~ Þ
(1)
Such functions could be numerically evaluated through a twodimensional interpolation on digitized maps [17]. The corrected ~ and corrected rotational speed ðn ~ Þ are expressed in mass flow ðmÞ Eqs. (2) and (3), respectively:
~ ¼ m
pffiffiffiffiffiffiffiffiffiffiffiffiffi m T=Tref p=pref
(2)
n ~ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi n T=Tref
(3)
where Tref and pref are the reference temperature and pressure, generally at ISO conditions of 15 C and 1.013 bar. The overall turbine performance could be evaluated using the same approach. Humidity ratio
3. Calculation model 3.1. Modeling of HAT cycle The performance of the gas turbine was calculated using a simulation program called SJGT, which is the result of an ongoing development at the Turbomachinery Institute of Shanghai Jiaotong University. The code was developed in Matlab-SimulinkÒ. The main components, namely, a compressor, a combustor, two turbines, and a simplified saturator, were introduced for the present study of the HAT cycle. A recuperator built for further study will be discussed in Section 5. The following three features were considered for real HAT cycle performance analysis: Component characteristics of compressor and turbines The performance of compressor was evaluated using a compressor map. The compressor map provided the pressure
The humidity ratio is defined by:
d ¼ 1000
mv Mv pv pv ¼ 1000 ¼ 622 mDA MDA pDA pHA pv
(4)
Table 1 Design specifications of the main test rig equipment. Item Two-shaft gas turbine Gas generator shaft Compressor Combustor HP turbine Power turbine Gearbox Saturator Electric heating device
Rated power output Rated rotational speed Pressure ratio Isentropic efficiency Combustion efficiency Inlet gas temperature Isentropic efficiency Isentropic efficiency Transmission ratio Packing type Rated power
Unit
Value
kW rpm e % % C % % e e kW
25 57,500 3.0 79 95 750 82 80 4.67 Stainless steel Intalox 150
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169
Fig. 3. Monitoring and control system of the two-shaft HAT cycle.
where m and M are the mass and molar mass, respectively. The vapor partial pressure pv could be given by:
pv ¼ f$MINðpsv ; pHA Þ
target of this work. These choices also corresponded with the test operation condition for model validation.
(5) 4. Results and discussion
where f is the relative humidity ratio collected by the hygrometer. The pressure of saturation vapor psv is expressed in terms of the saturator outlet air temperature THA as [18]:
2 T 7:21275 þ 3:981 0:745 HA þ1:05 647:3 3 T 647:3 0:745 HA $ 1 THA 647:3
psv ¼ 221:2$exp
(6)
The test campaign aimed to obtain the test data from the twoshaft HAT cycle performance and to discuss the influence of humidity ratio at the outlet of the saturator on the HAT cycle performance within the capabilities of the test rig itself. 4.1. Test results The operation tests for the two-shaft HAT cycle were carried out in two conditions.
Humid gas mixture In the present study, the humid gas was considered the ideal mixture of gas and water vapor. Using Dalton’s law, the partial pressure of the vapor in humid gas is expressed by:
pv ¼ cv $ pv þ pg
mv =Mv mv =Mv þ mg =Mg
(7) Compressor
(8)
The enthalpy of humid gas mixture could be obtained from:
hmix ¼
mg hg þ mv hv mg þ mv
Item Ambient conditions
where the mole fraction cv is given by:
cv ¼
Table 2 Main thermodynamic assumptions.
(9)
Combustor chamber HP turbine
Power turbine Saturator Fuel oil
Unit
Value
Pressure kPa C Temperature Relative humidity % Inlet pressure loss % Isentropic efficiency % Mechanical efficiency % Air leakage (at the compressor outlet) % Pressure loss % Efficiency % C Inlet gas temperature Isentropic efficiency % Mechanical efficiency % Isentropic efficiency % Mechanical efficiency % Pressure loss % LHV kJ/kg
101.3 45 60 1 72e78 99 2 4 95 650e700 73e78 99 73e77 98 0.5 42,700
Heat exchanger efficiency Hot side pressure loss Cold side pressure loss
92 3 2
3.2. Calculation assumptions Recuperator
Table 2 shows the main thermodynamic assumptions used in the present analysis. These choices were justified by the main
% % %
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Fig. 4. Test with same fuel flow rate.
Case I: constant fuel flow operation As shown in Fig. 4, the fuel flow was kept constant at 57 kg/h. The operation was divided into two stages. In the first stage, the humidity ratio was increased rapidly from 30 g/kg dry air (DA) to 37 g/kg DA in 6 min. In the second stage, the humidity ratio was increased gradually from 37 g/kg DA to 43.5 g/kg DA in the next 22 min. Fig. 5 shows the behavior of the TIT and the power output. In the first stage, the TIT decreased rapidly from 678 C to 668 C, and the power output increased by 1.4 kW immediately after water was added to the saturator. The effect of humid air on the gas turbine cycle was reflected promptly without delay. The TIT decreased by 17 C, and the power output increased by 3 kW when the humidity ratio increased from 30 g/kg DA to 43.5 g/kg DA during the whole operation. An increase in humidity ratio caused a decrease in combustion temperature, which helped to reduce the thermal stress in the combustion chamber and the turbines. Although the TIT decreased, a slight increase in power output could still be
noticed because of the higher mass flow over the turbines. Evidently, the humidity ratio has an important effect on the performance of gas turbines in the HAT cycle. Fig. 6 shows the performance of NOx emissions. NOx emissions decreased from 25 ppm to 16 ppm when the humidity ratio increased from 30 g/kg DA to 43.5 g/kg DA. Clearly, the humidity ratio of humid air at the outlet of the saturator is an important factor that affects the flame temperature in the combustor chamber. Lower NOx emission was also found to occur along with the lower flame temperature caused by higher humidity ratio. Case II: constant TIT operation As shown in Fig. 7, the TIT was kept constant at about 665 C during operation by regulating the fuel flow from 69 kg/h to 102 kg/h when the humidity ratio was increased from 48 g/kg DA to 57 g/kg DA. Fig. 8 shows the behavior of the cycle performance. An enormous increase of 9.5 kW in the total power output was seen after the air was humidified by the saturator. Such behavior signifies
Fig. 5. TIT and power output performance.
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Fig. 6. NOx emission performance.
that higher power output can be obtained at the same TIT in a HAT cycle than in a general two-shaft cycle. These results indicate that gas turbine performance can be improved without the restriction of a higher TIT dependent on the development of material technology. 4.2. Off-design performance analysis The off-design performance of the two-shaft HAT cycle calculated through SJGT is shown in Figs. 9e11. Two fixed TITs (700 C and 650 C) were selected for analysis because more than 90% test points were operated between these two TITs. Three humidity ratio levels of the humid air, namely, 30, 43, and 55 g/kg DA, were chosen for analyses. The humidity ratio of 30 g/kg DA was considered as the general humidity ratio, which could be obtained in the cycle without humidification by Eqs. (4)e(6). Fig. 9 shows the relationships between the pressure ratio and the specific work. Two groups of curves are shown in this figure;
the upper three curves indicate the performance when TIT is 700 C, and the lower three curves indicate the performance when TIT is 650 C. More specific work was obtained with a higher TIT when the humidity ratio was equal. Specific work was augmented with the increase of pressure ratio, e.g., both of the solid lines represent a humidity ratio of 55 g/kg DA, and the specific work changes from 11.4 kJ/kg to 22 kJ/kg at a pressure ratio of 2.8 when the TIT changes from 650 C to 700 C. Fig. 9 shows that an optimal pressure ratio achieving optimum specific work in each curve exists, and that the value of optimal pressure ratio increases with TIT. The diagram also shows that, as expected, the humidity ratio is one of the most important parameters in the HAT cycle: the higher the humidity ratio is, the greater the augmentation of the specific work is. Compared with the gas turbine cycle without HAT, e.g., at a TIT of 700 C, the specific work increased from 15.5 kJ/kg to 22 kJ/kg, signifying that more power output can be obtained because of the higher amount of evaporated water in the saturation tower.
Fig. 7. Test with same TIT.
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Fig. 8. TIT and power output performance.
The specific work is one of the primary performance parameters in the HAT test. Six points marked with a pentagram (P1eP6), as shown in Fig. 9, were the test cases selected for analysis. The setting values of the six test cases are indicated in Table 3. Points 1 and 2 were both operated at a TIT of about 700 C. P1 approximately indicated the maximum power output during the test at a higher pressure ratio (2.64) and humidity ratio (55 g/kg DA). P2 showed the status just before air humidification at a lower pressure ratio (1.69). The other four points (P3eP6) were run at a TIT of about 650 C. P3, compared with P4, P5, and P6, offered relatively higher specific work because of higher pressure ratio (2.35) and higher humidity ratio (55.1 g/kg DA). With regard to P4eP6, P5 achieved 1.29 kW more specific work than P6, as its pressure ratio was only 0.04 higher than that of P6. P4 reached 0.36 kJ/kg more specific work than P6 even though its pressure ratio was 0.09 lower than that of P6. These differences were observed because P4 had a higher humidity ratio (47.5 g/kg DA) than P5 (43.7 g/kg DA) and P6
(37.5 g/kg DA). Evidently, humidity ratio greatly influences the behavior of specific work in the HAT cycle. Fig. 9 compares the measured and calculated specific work at off-design operation of the two-shaft HAT test rig. As the results were in agreement, the calculation model is therefore effective, and the simulation tool can be further used to evaluate the performance of the HAT cycle with a recuperator. Fig. 10 shows the cycle efficiency performance of the HAT configuration described above. An optimal pressure ratio achieving optimum efficiency can be found in each curve, and its value increases with TIT, e.g., an optimum efficiency of 2.94% is achieved at a pressure ratio of 2.8, and the efficiency reaches a maximum of 2.1% at a pressure ratio of 2.5, indicated by the two solid lines in Fig. 10. Efficiency clearly grows with TIT, and the temperature should be as high as possible. Based on the solid line in Fig. 10, the efficiency increased by 1.27% when the TIT increased from 650 C to 700 C with the same pressure ratio of 2.8 and humidity ratio of
Fig. 9. Specific work performance.
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173
Fig. 10. Cycle efficiency performance.
Fig. 11. SFC performance.
55 g/kg DA. In addition to the TIT, the humidity ratio also demonstrated its influence on cycle efficiency. An increase of humidity ratio from 30 g/kg DA to 55 g/kg DA can enhance the efficiency by 0.95% under the same temperature of 700 C and the same pressure ratio of 2.8. Fig. 11 displays the specific fuel consumption (SFC) performance in the HAT cycle. As seen in the figure, lower SFC results from higher TIT. Moreover, the increase of humidity ratio reduces the SFC, with the effect being more significant in lower pressure ratio. By comparing the HAT cycle with a humidity ratio of 55 g/kg DA to the cycle without HAT, one can see that the SFC decreases from 28.3 kg/kW h to 12.7 kg/kW h at a pressure ratio of 1.6. Such case indicates that the HAT cycle with higher humidity ratio exhibits higher thermal efficiency. 4.3. Performance analysis on compressor characteristic curves Fig. 12 shows the characteristic curves of the centrifugal compressor in the test. The horizontal axis shows the corrected
mass flow rate, and the vertical axis shows the pressure ratio. Five curves, representing different corrected rotational speeds, and three dotted lines, representing different isentropic efficiencies of compressor, are shown in Fig. 12; the surge line is plotted as the dot-dash line. Two equilibrium running lines (L1 and L2) are obtained in Fig. 12, and all the work points on both lines are operated under the same TIT of 665 C, corresponding to the second test condition (Case II). The upper running line (L1) is run in a general
Table 3 Test conditions of test cases. ID
Humidity ratio (g/kg DA)
Pressure ratio
Specific work (kJ/kg)
P1 P2 P3 P4 P5 P6
50 30 55.1 47.5 43.7 37.5
2.64 1.69 2.35 1.79 1.92 1.88
21.96 7.95 12.09 6.89 7.82 6.53
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Fig. 12. Comparison of equilibrium running lines on the compressor characteristic curves.
two-shaft gas turbine with a humidity ratio of 30 g/kg DA. The lower running line (L2) is operated in a HAT cycle with a higher humidity ratio of 55 g/kg DA. Each pair of points on both running lines with the same inlet conditions, such as inlet temperature, pressure, and air flow, achieves the same power output. The working points on L2, compared with the corresponding points on L1, are operated at lower pressure ratios and lower corrected rotational speeds, although they are run at lower compressor efficiencies. The test results of the second test condition were compared with the predicted results and plotted as small circles in Fig. 12. The test running line was run from low to high pressure ratio at a TIT of 665 C; the humidity ratio was increased from 48 g/kg DA to 57 g/kg DA. The test points were located between L1 and L2 when the
pressure ratio was in the range of 2.1e2.3 because the humidity ratio was less than 55 g/kg DA. When the pressure ratio was over 2.4, the test points were operated below L2 because of the higher humidity ratio. Increasing the humidity ratio can move the compressor running line away from the surge line. The increase of surge margin improves operation safety, especially when the engine is operated under low conditions. 4.4. Performance of HAT with recuperator The existing test rig is planned to be extended with a recuperator to utilize exhaust gas energy and raise the total efficiency of the cycle. The characteristics of the recuperator are listed in Table 2. The
Fig. 13. Difference in specific work when the recuperator is used.
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Fig. 14. Difference in cycle efficiency when the recuperator is used.
Fig. 15. Difference in SFC when the recuperator is used.
thermodynamic performance of the HAT cycle with a recuperator is calculated through SJGT. The working condition is set in accordance with the points on the top curve in Fig. 9, which is at a TIT of 700 C and a humidity ratio of 55 g/kg DA. Thus, the calculation results are also comparatively analyzed based on the same curve. Fig. 13 shows the difference in behavior of specific work when the recuperator is used. The difference decreases as the pressure ratio is increased. When the pressure ratio is greater than 2.5, the specific work obtained by the HAT cycle with a recuperator is even less than that obtained by the HAT cycle without recuperator. Some specific work will be sacrificed at a higher pressure ratio in the recuperated cycle. These results are in agreement with the findings of Gallo et al. [3]. Fig. 14 shows the effect on cycle efficiency when a recuperator is added. When the recuperator is used, the cycle efficiency is strongly increased at the pressure ratio from 1.6 to 2.8, reaching a maximum increase of 3.1% at a pressure ratio of 1.9. Evidently, the exhaust gas energy is properly utilized in the recuperated cycle, and the recuperator greatly affects the total cycle efficiency. Fig. 15 displays the difference in SFC between the HAT cycle with a recuperator and the one without a recuperator. As seen from the
figure, the SFC performance improves when a recuperator is added to the HAT cycle at a compressor pressure ratio ranging from 1.6 to 2.8. The maximum decrease of SFC (5.1 kg/kW h) can be observed at a low pressure ratio of 1.6. From a thermodynamics viewpoint, although some specific work is sacrificed at a higher pressure ratio, introducing a recuperator to the existing HAT cycle will greatly increase cycle efficiency and improve SFC performance. Hence, adding a recuperator to the existing HAT cycle is necessary. 5. Conclusions The results from this work are summarized as follows: A small-sized test rig of a two-shaft HAT cycle gas turbine was built to experimentally evaluate the thermodynamic performance and emission behavior of the HAT cycle at low pressure ratio and at low TIT. The experimental results demonstrate that the effect of humid air is reflected promptly on the gas turbine cycle, and that the humidity ratio has an important effect on the performance of gas turbine in the HAT cycle.
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When the fuel consumption was fixed, an increase in humidity ratio from 30 g/kg DA to 43.5 g/kg DA caused a decrease in combustion temperature by 17 C, which helped reduce thermal stress in the combustion chamber and the turbines. Although the TIT decreased, a slight increase of 3 kW in power output could be noticed because of the higher mass flow over the turbines. In addition, the NOx emission decreased from 25 ppm to 16 ppm when the humidity ratio increased from 30 g/kg DA to 43.5 g/kg DA. Lower NOx emission occurs along with the lower flame temperature caused by higher humidity ratio. When the TIT was maintained at 665 C, an increase of 9.5 kW in the total power output was found after the air was humidified by a saturator with an increase of humidity ratio from 48 g/kg DA to 55 g/kg DA. Therefore, gas turbine performance can be improved without the restriction of a higher TIT dependent on the development of material technology. The results of the off-design performance of the HAT cycle calculated using SJGT matched the actual test results well. The results demonstrate that the higher humidity ratio is, the greater the augmentation in the specific work and in the cycle efficiency. Increasing the humidity ratio reduces the SFC, and the effect is more significant in lower pressure ratio. Therefore, the HAT cycle with higher humidity ratio exhibits higher thermal efficiency. A comparative analysis was carried out on the performance of two equilibrium running lines with different humidity ratios on compressor characteristic curves. When the gas turbine runs with the same inlet air flow, turbine inlet temperature, and power output, the working points on the running line with higher humidity ratio, although run at lower compressor efficiencies, can be operated at lower pressure ratios and lower corrected rotational speeds. Increasing the humidity ratio can move the compressor running line away from the surge line. The increase of surge margin will improve operation safety, especially when the engine is operated under low conditions. To perfect the HAT cycle, the existing test rig is planned to be extended with a recuperator. From a thermodynamics viewpoint, although some specific work is sacrificed at a higher pressure ratio, introducing a recuperator to the existing HAT cycle will greatly increase cycle efficiency and improve SFC performance. Hence, adding a recuperator to the existing HAT cycle is necessary. References [1] M. Nakhamkin, R. Pelini, M. Patel, Power augmentation of heavy duty and two-shaft small and medium capacity combustion turbines with application of humid air injection and dry air injection technologies, in: Proceedings of ASME POWER 2004, POWER2004-52095 (2004). [2] M. Jonsson, J. Yan, Humidified gas turbinesda review of proposed and implemented cycles, Energy 30 (2005) 1013e1078. [3] W. Gallo, A comparison between the HAT cycle and other gas-turbine based cycles: efficiency, specific power and water consumption, Energy Convers. Manag. 38 (15e17) (1997) 1595e1604. [4] A. Traverso, A. Massardo, Thermoeconomic analysis of mixed gas-steam cycles, Appl. Therm. Eng. 22 (2002) 1e21. [5] B. Nyberg, M. Thern, Thermodynamic studies of a HAT cycle and its components, Appl. Energy 89 (1) (2012) 315e321.
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Notation d: humidity ratio (g/kg DA) DA: dry air LHV: low heating value (kJ/kg) m: mass flow (kg/s) ~ corrected mass flow (kg/s) m: M: molar mass (g/mol) n: rotational speed (rad/s) b : corrected rotational speed (rad/s) n P: pressure (kPa) SFC: specific fuel consumption (kg/kWh) T: temperature (K) TIT: turbine inlet temperature ( C) p: pressure ratio h: efficiency c: mole fraction f: relative humidity Abbreviations AC: aftercooler C: compressor CC: combustion chamber ECO: economizer HAT: humid air turbine HPT: high-pressure turbine PT: power turbine REC: recuperator SAT: saturator Subscripts g: gas HA: humid air mix: humid gas mixture ref: reference sv: saturation vapor v: vapor