Performance Analysis of Fogging Cooled Gas Turbine with Regeneration and Reheat under Different Climatic Conditions

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1 Proceedings of the ASME 204 Power Conference POWER204 July 28-3, 204, Baltimore, Maryland, USA POWER Performance Analysis of Fogging Cooled Gas Turbine with Regeneration and Reheat under Different Climatic Conditions Maryam Besharati-Givi and Xianchang Li Department of Mechanical Engineering Lamar University Beaumont, TX 7770 ABSTRACT One of the most efficient power plants in past decades is the gas turbine. Significant studies have been conducted on the effects of various design and operation parameters on gas turbine performance. However, it is still a challenge to find the optimal operating parameters for the best performance. One of the important parameters is the climate at which the gas turbine operates. The current study is on the performance of gas turbine with cooling, focusing on the effect of inlet air humidity and ambient temperature. The overall efficiency will be altered through inlet cooling and regenerative heating. Other components such as reheating are accompanied. The cooling is also compared to the chiller inlet cooling, in which the performance of coefficient (COP) varies with temperature difference. The analysis is conducted by using Aspen Plus software. The results indicate that a combination of cooling and regeneration as well as reheating can substantially improve the system thermal efficiency. Compared to the system with only cooling, the efficiency increases by 24.5 % when both regeneration and reheat systems are combined to the ging cooled system. NOMENCLATURE AFR COP c p K m Air fuel mass flow ratio Coefficient of performance Specific heat capacity (kj/kg K) Constant Mass (kg) Dry air mass flow rate (kg/s) Mass flow arte of water vapor (kg/s) Compressor power (kw) Cooling system power (kw) Net power (kw) Turbine power (kw) Rate of thermal energy (kj/s) T Temperature ( C) Ambient temperature ( C) TIT Turbine inlet temperature ( C) Volume flow rate ( ) Specific humidity (kg/kg of dry air) Rate of work into the system (kj/s) Density ( ) Total efficiency of system INTRODUCTION One of the most efficient power plants in past decades is the gas turbine. Numerous studies have been conducted on the effects of various design and operation parameters on gas turbine performance. However, it is still a challenge to find the optimal operating parameters for the best performance, and one of the important parameters is the climate at which the gas turbine operates. Chaker and Meher-Homji [] reported that the turbine power decreases about % for every C increase in ambient temperature. An inlet cooling system can lower the inlet temperature to increase the air density. As a result, the turbine operates with the higher mass flow rate, which produces more power. Different types of inlet cooling have been discussed by Ibrahim et al. [2] such as evaporative cooler, indirect mechanical refrigeration system, mechanical refrigeration with ice storage, direct mechanical refrigeration system, refrigeration system with chilled water storage, and absorption chiller inlet air cooling system. One of the practical inlet cooling methods is cooling because of a smaller pressure drop in the inlet air stream. Relatively it is not that expensive to install and repair. The cooling can be also Copyright 204 by ASME

2 combined with the regenerative and reheat systems to improve the overall efficiency of the gas turbine system. In addition, cooling can be combined with other inlet cooling methods such as absorption chiller to reduce the cooling capacity needed for very hot weather [3] FEEDFUEL 789 FUEL2 4 EXT RAAIR CC AIR-OUT CC-OUT GAST CC2 CC2-OUT In general, can be generated by an array of specially designed high pressure nozzles so that the water droplets can be distributed into the air evenly. To reduce the potential water droplet conglomeration some grids such as trash screen can be used (Chaker et al. [4]). Therefore, the cooling duct has to be designed in a way to evaporate the droplets effectively small droplets and enough residence time. Based on Chaker et al. [5], droplets bigger than 30 microns in high humidity regions can make evaporation difficult. There are some risks of blade erosion when the droplets enter the compressor. Fog cooling is based on the principle of evaporative cooling, which states that water evaporates and cools the air by the latent heat of vaporization. This technique is highly effective because a relative humidity up to 00% is attained at the gas turbine air inlet. On the other hand, the structural modification in inlet filtering system, operation, maintenance, and water treatments are important issues for the cost of inlet cooling. The amount of water required depends on the ambient temperature and relative humidity. The water for application requires treatment like demineralization and silica removal. There are some risks for assumption of 00% evaporation. In practical case, some water droplets may enter the compressor and cause blade erosion. Therefore, in practical case, the cooling system can have a target relative humidity less than 00% in addition to the approaches to trap the droplets AIR-IN 0 WAT ER-IN W= Temp eratu r e ( C) Pr essu re ( b ar) OUT -C Vo lu me Flo w Rate ( cu m /h r ) Mo lar Flo w Rate ( k mo l/h r ) Q Du ty ( Gcal/h r ) W Po wer (k W ) CC2-IN T -C-OUT 548 Mass Flo w Rate (k g /h r) W=-4930 Q=0 COM MIXER Q=0 W=-4595 GAST GAS-IN HEAT EX Q=5 596 GAS-OUT Figure : Schematic of gas turbine cycle in Aspen plus The models of thermodynamic properties in Aspen Plus are the Peng Robinson cubic equation of state and the BostonMathias alpha function (PR-BM). The sequential modular process simulation is used in this study. Moreover, air is assumed to be composed of 79% nitrogen and 2% oxygen. The fuel is assumed to be pure methane (considered as natural gas) and burned 00% in the combustion chamber. While the heat and pressure losses through the pipelines are simplified, the pressure loss in the combustion chamber is considered. The water is added in the mixture module to make the air evaporate from ambient relative humidity to 00% humidity and cool down the air in the compressor inlet. Some extra air is provided to the compressor because of density change. The air volume flow rate is fixed. The additional water amount is calculated based on the air humidity change and dry air mass flow rate as follows: In fact, cooling can be operated with a temperature anywhere between the web bulb and ambient temperatures. Therefore, both the relative humidity and the inlet temperature of the compressor are the key factors for power production with cooling. Suggestions are given in [6] for selection of climatic design point. This paper examines the performance of gas turbine power plants with cooling, focusing on the effect of inlet air humidity and ambient temperature. Inlet chiller cooling is considered to compare with inlet cooling. Other components such as regeneration and reheat are also accompanied to improve the system performance. () where w is the specific humidity of the ambient air and w2 is the specific humidity after the ging is applied. In other literature, such as Dawoud et al. [7], the water needed for cooling is calculated based on the 98% wet bulb temperature effectiveness, which is defined by the Kraneis [8]. In this way, the final temperature will be a few tenths of a degree higher than that of the theoretical web bulb temperature. The reason is mainly due to the limitation in the technology and safety. METHODOLOGY The heat exchanger (or the recuperator) for regeneration is designed in such a way that the temperature of gas turbine outlet is reduced to the proper temperature value while its main purpose is to increase the compressed air temperature before the combustion chamber. In this section the structure created in Aspen Plus is first described and then parts related to operation components are explained. The simple mathematic model is presented and the parameters of a base case under study are listed. Figure is an example schematic of the cooled system with reheat and regeneration that is simulated in Aspen Plus. The evaporative cooling system is designed in Aspen by using a mixture block (or a mixer) and the regeneration system is modeled by using a heat exchanger, which is also called recuperator. To include reheat, the turbine needs to be split into two parts. A proper isentropic efficiency is set to the split turbines so that the overall efficiency remains the same as the case without reheat. Note that cooling in this paper is compared to the chiller cooling. In most literature for calculation of the chiller cooling power, fixed COPs are considered. In this paper, the performance of the chiller cooling is based on the following equations, which provide different COPs when the reduction of temperature is different. In general, for a chiller cooling system, the coefficient of performance can be written as: 2 Copyright 204 by ASME

3 RESULTS AND DISCUSSION (2) Case : Fogging cooling system and base case in which the total heat transfer rate can be given by For this case the ambient temperature is set to 30 C. The efficiency, compressor and turbine powers of a ging cooled gas turbine are investigated with different relative humidity. As the base case, the relative humidity is changed from 0 to 80% at 30 C ambient temperature without a cooling system. The ging cooled system includes the use of water that evaporates in the air and increases the relative humidity from a given condition (0 to 80%) to 00%. Some extra air is needed to keep the air volume flow rate constant. (3) where and are the mass flow rate and specific heat capacity for the air, respectively. and stand for the ambient temperature and the inlet compressor temperature designed, respectively. For a reversed Carnot cycle, (4) Figure 2 shows the system efficiency for different cases in different relative humidity. The efficiency does not change significantly with relative humidity for the base case (around 33.2%) and it is slightly higher for a lower relative humidity. For a ging cooled system, when the humidity is high only a small amount of water is evaporated to lower the temperature; consequently, the efficiency is relatively lower. The efficiency increases when the humidity decreases because there is more room for the temperature reduction. Alternatively, the chiller cooling system is used and the temperature is reduced the same as the ging system for a given relative humidity, achieving 00% relative humidity. The efficiency increases with decrease of the relative humidity that causes lower temperature after cooling. The slope of efficiency versus relative humidity decreases when the relative humidity is low because of higher cooling power. Therefore, the required chiller power becomes, (5) where mass flow rate is replaced by the volume flow rate of the inlet air, (in ), and its density,. Considering the lower COP for an actual cooling system, a constant, K, is defined as 4, which means the actual cycle has one fourth of the COP of the reversed Carnot Cycle. (6) The overall gas turbine efficiency is defined as following (7) The heat transfer rate into the system can be given by (8) where LHV is the lower heating value of methane, which is taken as 000 kj/kg. The fuel mass flow rate is calculated in a way that turbine inlet temperature (TIT) is fixed to C. Note that the TIT chosen here is only for a typical case. In fact, the TIT can increase to 700 C with some new materials and technology (Hirano [9]). Table lists the parameters and conditions for the gas turbine under study. ging base case chiller Table : Detailed condition and parameters of gas turbine Parameters Ambient temperature ( C) Ambient pressure (atm) Air volume flow rate ( ) TIT ( C) Fuel temperature( C) Fuel pressure (atm) Combustion chamber pressure loss Compressor pressure ratio Compressor isentropic efficiency Turbine isentropic efficiency Figure 2: of gas turbine with cooling, chiller cooler system, and the base case at 30 C ambient temperature Baseline % Figure 3 shows the cooling process on the Psychometric chart for a specific ambient temperature, 30 C, which is lowered from different relative humidity 20% and 80% to 00% humidity. The black line shows the cooling. The red line represents the chiller cooling system, which cools down the air to the same temperature as ging system as cases reported in Fig. 2. In real chiller cooling cases the red line can reach to the saturation temperature or cool down the air to an even lower temperature. For comparison purpose in this study, the temperature reduction is assumed to be the same. 3 Copyright 204 by ASME

4 Figure 3: Psychometric chart showing evaporative and chiller cooling systems in different relative humidity, 20% and 80% at 30 C ambient temperature Figure 4 presents the compressor power input for the ging cooled system and the base case. The compressor work increases as the relative humidity decreases because the volume flow rate is fixed, and the higher mass flow rate of air is compressed with lower temperature and humidity. When compared with the base case the compressor work has a higher value for the ging cooled system due to the higher mass flow of air. However, the change of the compressor power is very small only about 0 kw at the most, which is about 0.2% the power input. Compressor Power(kW) ging base case Figure 4: Compressor power input for the ging cooled system and the base case at 30 C ambient temperature with different relative humidity The turbine power output is given in Fig. 5, which shows a significant increase when cooling is used for lower relative humidity. As seen in the figure, the maximum change is about 600 kw, which represents a change of 7.4%, comparing with the base case. Furthermore, the base case does not show any significant change in turbine power production for different relative humidity. Turbine Power[kW] ging base case Figure 5: Turbine power output for the ging cooled system and the base case at 30 C ambient temperature with different relative humidity Case 2: Fog cooling with reheat and regenerative heating For a better performance, gas turbine systems, especially those land-based units, are usually not operated on a simple Brayton cycle. Reheat and regeneration are commonly applied. Figures 6 and 7 compare the thermal efficiency and net power with different combination of options. The ambient temperature is set to 30 C. For all the four different cases, cooling is adopted. It can be observed from that the case with reheat and cooling produces the highest net power while the efficiency of this combination is the lowest. The reason is that the exhaust temperature is higher when reheat is applied. In a reheat system, if the heat absorption temperature (or the TIT) increases, it helps the increase of efficiency. The highest efficiency is observed when cooling is combined with both reheat and regeneration systems, and the highest efficiency can be up to 45%. In this combination, the outlet temperature of regeneration is designed to be 465 C and the polytrophic efficiency of turbines is set as to ensure the overall efficiency of the two turbines will be 0.87 as specified in Table. Note that a combination of reheat with cogeneration from Khaliq and Kaushik [0] also shows a high efficiency. However, for cogeneration the efficiency is not only for power output. Although the case with ging and regeneration has almost the same net power as that with ging only, it presents a higher efficiency because it uses a lower mass flow of fuel. The high efficiency can also be explained by the lower average temperature of the exhaust gas. Based on the cycle analysis in thermodynamics, the bigger the upper and lower temperature difference, the higher the thermal efficiency is. In theory, the net power of the cases with and without regeneration should have the same net power. For example, the comparison can be between the case with ging and the case with ging plus regeneration. The small difference shown in the figure is due to the smaller amount of fuel for the cases with regeneration. Because of the higher air temperature at the combustor inlet when regeneration is applied, the fuel needed is reduced to achieve the same TIT. However, the 4 Copyright 204 by ASME

5 efficiency for the cases with regeneration is higher. When the pressure loss in the recuperator is considered, which in this study is 0.5% the total pressure on the cold side, the net power difference becomes more significant. Figure 6: for combination of ging cooled system with reheating and regeneration at 30 C ambient temperature Net Power [kw] reheat + regeneration +regeneratio n+reheat +reheat + regeneration +regeneratio n+reheat Figure 7: Net power for combination of ging cooled system with reheating and regeneration at 30 C ambient temperature Table 2 presents more results to compare the cases with and without cooling. All other operation conditions and parameters are as those in Figs. 6 and 7. It can be seen that the cases with cooling have a higher net power, which is due to higher air mass flow rate. The efficiency does not change significantly when cooling is considered. The reason is that cooling lowers the air temperature but more fuel is needed to reach the same TIT. Table 2: Net power and efficiency for cases with and without cooling Cycles Net Power Reheat Fog + reheat Regeneration Fog + regeneration Reheat+ regeneration Fog + reheat+ regeneration After examining the results in Table 2, the three cases with higher efficiency are analyzed for different levels of relative humidity. The efficiency, net power, and air-fuel mass flow ratio are calculated. The case with cooling and reheat is eliminated because of high temperature outlet of turbine and low efficiency, which means high consumption of fuel. As seen in Fig. 8 the efficiency increases by 8% [=( )/34.5] when the recuperator with pressure losses is added to the gas turbine system with cooling. Note the -only case, which has an efficiency of 34.5% with zero relative humidity, is not listed in Table 2. Furthermore, the efficiency increases by 24.5% when the ging cooled system is combined with both regeneration and reheating. For all the cases in Fig. 8, the efficiency shows a slight increase when the relative humidity decreases, which is consistent with the cycle of ging only as discussed earlier (see Fig. 2) regeneration +regeneration+reheat Figure 8: of gas turbine for ging cooled system and cases with regeneration and reheat at 30 C ambient temperature Figure 9 gives the net power for the three cases. In theory, the net power should be almost the same for the systems with ging only and with both ging and regeneration. The pressure loss in recuperation and less fuel in combustion chamber are the reason for lower net power for the case with regeneration. When reheat is considered, the net power increases by about 770 kw, which represents a significant impact. For the system with regeneration, the air before the combustion chamber is heated. Since the turbine inlet temperature is fixed, the lower mass flow rate of fuel is used; consequently, the efficiency becomes higher. As for the impact of the relative humidity, all the three cases follow the same trend. The net power increases by 475 kw when the relative humidity decreases from 80% to 0%, which is more significant when compared to the impact of relative humidity on the thermal efficiency. 5 Copyright 204 by ASME

6 Net Power [kw] Figure 9: Turbine power with ging cooled system and ging with regeneration and reheating systems for 30 C ambient temperature Figure 0 shows the air fuel mass flow ratio (AFR) for four different cases, including the base case. AFR is defined as the ratio of air mass flow to fuel mass flow rate and varies based on the design of gas turbines and other conditions. For the base case the air fuel ratio does not change much when the relative humidity varies. For all the other three cases, the air fuel ratio increases when the relative humidity increases due to the big amount of air needed to keep the TIT constant. Among the four cases, the system with and regeneration has the highest air-fuel ratio and the system with ging only has the lowest air-fuel ratio. Air Fuel Mass Flow Ratio regeneration +regeneration+reheat base case + regeneration +regeneration+reheat Figure 0: Air fuel mass flow ratio of gas turbine with ging only, ging plus regeneration and reheating as well as the base case at 30 C ambient temperature Case 3: Fog cooling for different relative humidity at different ambient temperatures 20, 30 and 40 C In the previous sections, the ambient temperature is set to 30 C. To examine the performance of gas turbines with cooling at different ambient temperatures, this section presents the results for two other temperatures, i.e., 20 and 40 C, with different relative humidity. Figure 3 shows the higher efficiency for lower ambient temperature. The increase of efficiency has a bigger slope with the reduction of relative humidity for ambient temperature of 40 C when compared to 20 C because the change of mass flow rate in different relative humidity is higher in lower ambient temperatures. In another word, the difference of thermal efficiency becomes bigger at higher relative humidity Figure : of gas turbine with cooling for 20, 30 and 40 C ambient temperature in different relative humidity The ambient temperature also affects the compressor work, which is calculated with a constant volume flow rate and density change. Lower air temperature results in higher compressor work as seen in Fig. 2. Furthermore, lower relative humidity shows more compressor work. However, the change of the compressor power input is relatively small for all these cases (~ 0.2%). Compressor Power[kW] T=40 C T=30 C T=20 C T=40 C T=30 C T=20 C Figure 2: Compressor power of gas turbine with ging cooled system for 20, 30 and 40 C ambient temperature in different relative humidity Similarly the turbine power output can be affected by both ambient temperature and relative humidity. The turbine power has a higher value for lower inlet relative humidity because of more temperature reduction and denser air after the ging. The power output at lower ambient temperature is higher when compared to the case with a higher ambient temperature as seen in Fig Copyright 204 by ASME

7 Turbine Power(kW) T=40 C T=30 C T=20 C Figure 3: Turbine power of gas turbine with ging cooled system for 20, 30 and 40 C ambient temperature in different relative humidity Case 4: Fog cooling with different relative humidity and temperature reduction The performance of ging cooled gas turbine systems are studied for different ambient temperatures and relative humidity in this section. Table 3 shows the temperature of cooling system when water evaporates and the relative humidity reaches to 00% or the web-bulb temperature. The highest temperature reduction (22.4 o C) occurs when the relative humidity is zero and the ambient temperature is 35 o C (35-2.6=22.4). The temperature reduction is directly linked to the performance of cooling Temperature( C) RH0% RH40% RH80% Figure 4: of gas turbine with ging cooled system for different relative humidity and ambient temperatures Figure 5 compares the compressor work of a ging cooled gas turbine system with temperature reduction as opposed to the base case, where no ging system exists. The trend for both the cases is the same: the compressor work is higher for lower temperature and relative humidity. However, the overall power input is higher for the ging cooled system than the case without ging because of the higher air mass flow rate for the ging cooled case. Table 3: Wet-bulb temperature with different relative humidity and ambient temperatures 0% RH 40% RH 80% RH 5 C 3.4 C 8.3 C 2.9 C 25 C 8.2 C 6.2 C 22.4 C 35 C 2.6 C 23.7 C 32 C As seen in Fig. 4, the efficiency is higher when the temperature is low and the relative humidity is low. The efficiency line has a small slope for 0% relative humidity. The reason is that there is a large room to reduce the temperature by evaporating water. For a higher relative humidity, the amount of water that can be added in is small. Figure 5: Compressor power of gas turbine with ging cooled system for different relative humidity and ambient temperatures Figure 6 shows the result of turbine power output of the ging cooled systems for different temperature and relative humidity. The turbine power has a higher value for lower temperatures because of the fixed volume flow rate passing through the compressor. A lower temperature means a higher mass flow rate of air. Furthermore, with a lower relative humidity the turbine power is greater, which is because more water evaporates in a ging cooled system and bigger temperature reduction occurs. These results are compatible to those in Jones and Jacobs []. 7 Copyright 204 by ASME

8 REFERENCES Turbine Power(kW) 9200 RH0% RH40% RH80% [] Chaker, M., Meher-Homji, C. B., 2002, Inlet Fogging of Gas Turbine Engines: Climatic Analysis of Gas Turbine Evaporative Cooling Potential of International Location, Proceeding of ASME Turbo Expo 2002, NO:2002-GT30559 [2] Ibrahim, T. K., Rahman, M. M., and Abdalla, A. N., 20, Improvement of Gas Turbine Performance Based on Inlet Air Cooling Systems: A technical review, International Journal of Physical Sciences, Vol.6, pp [3] Meher-Homji, C. B., Mee, T. R., 2000, Inlet Fogging of Gas Turbine Engines-Part B:Practical Considerations, Control, and O&M Aspects, Proceeding of ASME Turbo Expo 2000, NO:2000-GT-308 [4] Chaker, M., Meher-Homji, C. B., Mee III, T., 2002, Inlet Fogging of Gas Turbine Engines- Part C: Fog Behavior in Inlet Duct, CFD Analysis and Wind Tunnel Experiments, Proceedings of ASME Turbo Expo, Amsterdam, The Netherlands, 2002-GT [5] Chaker, M., Meher-Homji, C. B., Mee III, T., 2002, Inlet Fogging of Gas Turbine Engines- Part A: Fog Droplet and Practical Considerations, Heat Transfer and Practical Considerations, Proceedings of ASME Turbo Expo, Amsterdam, 2002-GT [6] Chaker, M., Meher-Homji, C. B., 202, Selection of Climatic Design Points for Gas Turbine Power Augmentation, Journal of Engineering for Gas Turbine and Power, Vol. 34/ [7] Dawoud, B., Zurigat, Y. H., and Bortmany, J., 2005, Thermodynamic Assessment of Power Requirements and Impact of Different Gas-Turbine Inlet Air Cooling Techniques at Two Different Locations in Oman, Applied Thermal Engineering. Vol. 25, pp [8] Kraneis, W., 2000, The Increased Importance of Evaporative Coolers for Gas Turbine and CombinedCycle Power Plants, Munters Euroform GmbH Aachen, VGB Power Tech 9 [9] Hirano, K., 2005, Application of Eutectic Composites to Gas Turbine System and Fundamental Fracture Properties Up to 700 C, Journal of the European Ceramic Society, Vol. 25, pp [0] Khaliq, A., Kaushik, S. C., 2004, Thermodynamic Performance Evaluation of Combustion Gas Turbine Cogeneration System with Reheat, Applied Thermal Engineering, Vol. 24, pp [] Jones, C., Jacobs, J. A., 2000, Economic and Technical Considerations for Combined-Cycle performanceenhancement Options, GE Power Systems, GER-4200, Schenectady, NY Temperature( C) 45 Figure 6: Turbine power of gas turbine with ging cooled system for different relative humidity and ambient temperatures CONCLUSIONS Fogging cooled gas turbine systems are investigated at different levels of ambient temperature and humidity. The gas turbine system is enhanced by adding regenerative heating and reheat components. The overall thermal efficiency and net power improvements are the main consideration in different climatic conditions. When compared to a chiller inlet cooler, the cooling with the same temperature reduction can increase the system efficiency more due to the power required for the chiller. For cooling with regeneration, the efficiency increases by 8% as compared to cooling alone. The efficiency increases 24.5% when a ging cooled system is combined with both regeneration and reheating systems when compared to the ging cooled system. The air fuel mass flow ratio has a higher value for the ging cooled system with regeneration when compared to a ging cooled gas turbine system. When the temperature changes from 5 to 45 C, the gas turbine efficiency change is approximately % for 0% relative humidity, and it furthermore changes about.5% for 40% relative humidity. Note that this study shows the performance of cooling in respect of thermodynamics. In practice, cooling may be subject to the local climate conditions. In addition, droplet conglomeration and incomplete evaporation may take some of the droplets into the compressor, which can lead to blade erosion. Risk and cost analyses are always needed. ACKNOWLEDGEMENT This study is partially supported by Entergy Energy Foundation. 8 Copyright 204 by ASME