PARAMETRIC STUDY OF GAS TURBINE CYCLE COUPLED WITH VAPOR COMPRESSION REFRIGERATION CYCLE FOR INTAKE AIR COOLING

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 9, September 2018, pp , Article ID: IJMET_09_09_029 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed PARAMETRIC STUDY OF GAS TURBINE CYCLE COUPLED WITH VAPOR COMPRESSION REFRIGERATION CYCLE FOR INTAKE AIR COOLING Adnan Abdulla Ateeq Thermal Mechanical Engineering, Southern Technical University, Iraq-Basra Safaa Hameed Faisal Thermal Mechanical Engineering, Southern Technical University, Iraq-Basra Hanadi Mahmood Ali Thermal Mechanical Engineering, Southern Technical University, Iraq-Basra ABSTRACT In this paper, a parametric study has been made on the combined operation of a gas turbine cycle (GTC) and vapor compression cycle (VCC) for cooling the inlet air during hot climate. The study analyzes the effect of the most important operating parameters such as ambient temperature, compressor pressure ratio, firing temperature, condenser, and evaporator. Results reveal that there is an obvious drop off for the most of GTC performance characteristics while operating during high ambient temperatures. The thermal efficiency and power output decrease linearly with the increase of the ambient temperature. The energy analysis shows that adopting the VCC to enhance the GTC could improve the power output by 3.792% to 35.97%, while the thermal efficiency could be better by 0.75% to 7.1%. Also, the results indicate that there is an optimum value for the pressure ratio that causes the power output and thermal efficiency to be a maximum. The optimum pressure ratio increases always by increasing the firing temperature. Keywords: Power plant, Gas turbine; Vapor compression cycle; intake air cooling; Energy analysis. Cite this Article: Adnan Abdulla Ateeq, Safaa Hameed Faisal and Hanadi Mahmood Ali, Parametric Study of Gas Turbine Cycle Coupled with Vapor Compression Refrigeration Cycle for Intake Air Cooling, International Journal of Mechanical Engineering and Technology, 9(9), 2018, pp editor@iaeme.com

2 Parametric Study of Gas Turbine Cycle Coupled with Vapor Compression Refrigeration Cycle for Intake Air Cooling 1. INTRODUCTION The gas turbine engine is one of the most widely used electric power generating technologies around the world nowadays. The open GTC is a type of internal combustion engine that converts the chemical energy of fuels to mechanical energy that drives the generator to produce electrical power. It composed of three main components namely the compressor, combustion chamber, and turbine. It characterized with low installation cost and do not require a large space of installation compared with the steam turbine power plant. Moreover, the gas turbine has a high production power per size. Conversely, a disadvantage of the gas turbine power plant is its performance degradation due to high ambient temperatures (Cohen et al. 1972). One solution to overcome this problem is to cool the intake air before entering the air compressor. Actually, there are different ways that can be adopted to cool the compressor intake air and one of these is the VCC. In fact, investigation the GTC performance is very essential to clarify the possible enhancement opportunities. As gas turbine technology is widely used for electric power production, it has been studied and analyzed by several research projects intentionally to recover the performance. Rahman et al. (2011) developed a parametric study for a gas turbine power plant. They found that the ambient temperatures, compression ratio, as well as the isentropic efficiencies are strongly affecting the thermal efficiency. De Sa and Al Zubaidy (2011) studied the impact of varying ambient temperature on the gas turbine performance. They showed that for every 1K rise in ambient temperature above the ISO conditions, the gas turbine loses 0.1% in terms of thermal efficiency and 1.47 MW useful power output. Mohapatra and Sanjay (2014) studied the energy analysis of simple and combined GTC with inlet air cooling using vapor compression and vapor absorption cooling. Results indicated that vapor compression cooling improves the efficiency of GT cycle by 4.88 % and net output by %. Comodi et al. (2015) studied the boosting of micro gas turbine performance in the hot weathers by using VCC. It has been shown that the gain in electric power was up 8% with respect to the ISO operating conditions while the thermal efficiency increased by 1.5%. Faisal et al. (2017) performed energy and exergy study of Rumaila Basra gas turbine power plant. They found that plant lost its performance remarkably during the hot season at both full and part load operating conditions. Kamal et al. (2017) discussed the feasibility of turbine inlet air cooling in Malaysia climate using mechanical chillers. They claimed that this modification is effective for power augmentation in Malaysia by 27.5% to 32.11% editor@iaeme.com

3 Adnan Abdulla Ateeq, Safaa Hameed Faisal and Hanadi Mahmood Ali 2. THEORETICAL MODEL Figure 1 shows a schematic of a simple GTC coupled with VCC. Both cycles are represented on the T-S and P-H diagrams. The plant components are modeled before and after cooling based on energy principle editor@iaeme.com

4 Parametric Study of Gas Turbine Cycle Coupled with Vapor Compression Refrigeration Cycle for Intake Air Cooling The following assumptions are made in the present analyses: Steady state operation for all cycle s components. The processes in the turbine, compressors, combustion chamber, and expansion device are adiabatic. The potential and kinetic energy are neglected. Air and combustion gases are modeled as ideal gases with variable properties. Pressure losses are considered in the compressor inlet, combustion chamber, and turbine exit, and it is not considered in the VCC. The refrigerant in the VCC leaves the evaporator and condenser as saturated vapor and liquid respectively. Compressed air is extracted at the final compressor stage Basic equations Based on the first law of thermodynamics, the energy conservation equation applied to open system is given by (Cengel and Boles, 2006): The mass flow rate at inlet and exit are restricted by the mass conservation equation: 2.2. Analysis of the air compressor The air pressure at compressor inlet is given by: The air pressure at compressor exit is calculated using the compressor pressure ratio by: The enthalpy and entropy of air at the inlet are found using inlet pressure and temperature. When the compression process is isentropic, then the entropy at the inlet is equal to the entropy at the exit. Using the entropy and pressure, the other isentropic exit properties can be calculated. The actual exit enthalpy is given by (Cengel and Boles, 2006): The work needed to drive the air compressor is: ( ) ( ) 2.3. Analysis of combustion chamber In the combustion chamber, fuel is burned with compressed air. For a specified firing temperature for combustion products, the fuel mass flow rate needed is: The actual combustion reaction equation written for one kg of compressed air is given by (Faisal et al, 2017): editor@iaeme.com

5 Adnan Abdulla Ateeq, Safaa Hameed Faisal and Hanadi Mahmood Ali The actual fuel to air ratio is calculated with the aid of stoichiometric fuel to air ratio by: The stoichiometric reaction deals with combustion process where there is no excess air. The chemical reaction equation that represents the stoichiometric reaction is (Faisal et al, 2017): [ ] The above equation is written for any fuel. Essentially, two important calculations are gained from the stoichiometric combustion equation. The first one is stoichiometric fuel to air ratio given by (Faisal et al, 2017): ( ) The stoichiometric kmoles of air is found by usual atomic balance for Eq. (10). The other important calculation is the enthalpy of the stoichiometric products of combustions. The amount of stoichiometric products will not be affected by supplying extra air for the combustion process. The value of equivalence ratio, which is a measure for excess air, is calculated using the total enthalpy concept. This principle gives the following relation (Faisal, 2002; Sullivan, 1975): [ ( )] The heat input to the plant is: [ ( )] 2.4. Analysis of turbine The pressure at the inlet and the outlet of the turbine can be calculated as: ( ) In case of isentropic expansion, then the entropy of combustion products at the turbine inlet is equal to the entropy at the turbine exit. By using this property along with exhaust pressure at state five, the other isentropic properties can be calculated. The actual enthalpy is given by (Cengel and Boles, 2006): ( ) This property together with the exhaust pressure at state five is used to evaluate the other actual properties. The output power from the turbine is calculated as: editor@iaeme.com

6 Parametric Study of Gas Turbine Cycle Coupled with Vapor Compression Refrigeration Cycle for Intake Air Cooling ( )( ) 2.5. Analysis of evaporator system The evaporator system could contain a secondary working fluid e.g. water which transfers heat from the intake air to the VCC. The first step in performing the energy analysis of VCC is the calculating the cooling loa1d. Considering only the sensible heat, the cooling load required to cool the compressor intake air is (Cengel and Boles, 2006): The heat absorbed by the refrigerant is: ( ) ( ) The exit enthalpy from the evaporator is the saturated vapor enthalpy at the evaporator temperature. Equating Eq. (18) and (19), the refrigerant mass flow rate is estimated as: ( ) ( ) 2.6. Analysis refrigerant compressor Like the air compressor in GTC, the real refrigerant enthalpy of compressor exit can be calculated using the isentropic efficiency definition as: ( ) Using isentropic entropy and the refrigerant compressor exit pressure, the refrigerant isentropic properties at compressor exit can be found. The compression power is given by: ( ) 2.7. Analysis of condenser The heat rejected in the condenser can be calculated using energy balance for the condenser as follows (Cengel and Boles, 2006): ( ) The exit enthalpy from the condenser is the saturated liquid enthalpy evaluated at the condenser temperature Analysis of expansion device The energy balance of the expansion device is: 2.9. Overall plant performance characteristics The net output power from the plant is calculated as: editor@iaeme.com

7 Adnan Abdulla Ateeq, Safaa Hameed Faisal and Hanadi Mahmood Ali The exhaust heat rejected by GTC to the environment is: ( ) The thermal efficiency of the GTC is the ratio of net output power to the input heat, i.e.: 3. RESULTS AND DISCUSSION The mathematical model presented in this study was set in a computer code written with Engineering Equation Solver (EES). Table 1 presents the assumptions and reference data adopted in the study. The range and typical values of input parameters taken are given in Table 2. Table 1 Assumptions and reference data GTC ISO conditions 15, kpa Fuel condition 25, 2500kPa = 10 kpa 0.04* P3 Compressor air extraction 13% VCC Refrigerant R Table 2 Input Data Range and Typical Values Data for GTC Fuel Type 15, 2500 kpa ISO Air Flow rate kg/sec Range ( ) with typical values of Firing Temperatures 1000,1100,1124, and 1200 Ambient Temperature Range (5-55) with typical values of 35, 45, and 55 Compressor Pressure Ratio Range (5-25) with typical value 12.6 Data for VCC Condenser Temperature Range (35-55) with typical values of 35 Evaporator Temperature Range (-5 to +5) with typical values of 5 Fig. 2 shows how the output power is affected by the increase in ambient temperature. The figure reveals that the output power suffers a clear drop as the ambient temperature getting a rise. For the entire temperature range, the power is decreased from MW to 90.2 MW which indicates that there is approximately 0.9 MW drop in power for each 1 C rise in ambient temperature. The main reason for decreasing the output power is directly related to the air mass flow rate. By increasing the ambient temperature, the air density is decreased and thereby the mass flow rate is decreased. Besides, the rise in ambient temperature causes an editor@iaeme.com

8 Parametric Study of Gas Turbine Cycle Coupled with Vapor Compression Refrigeration Cycle for Intake Air Cooling increase in the specific enthalpy difference for the air compressor. This certainly makes the specific power consumed by the compressor be more. The figure also indicates that adopting VCC for cooling the inlet air will improve the net output power in the range of 3.792% to 35.97%. The slight decrease in net output power after using this cooling option is due to the power needed to drive the cooling system. Figure 2 Output power versus ambient temperature before and after cooling Fig. 3 explains the variation of thermal efficiency with ambient temperature. The figure reveals that the drop in thermal efficiency is 36.08% that occurs at 20 C to 33.33% at 55 C. This point to approximately 0.08% drops in efficiency for each 1 C rise in ambient temperature. Actually, this behavior is due to two opposite factors. Firstly, the decrease in output power caused certainly a decrease in thermal efficiency. On the other hand, an increase in the ambient temperature will decrease the fuel mass flow rate due to decreasing the air mass flow given that the firing temperature is held constant. In fact, for each firing temperature, there is a constant fuel to air ratio. This factor will tend to increase the thermal efficiency. However, since the fuel mass flow rate is very low, then its effect on the thermal efficiency is found to be insignificant. Generally, it is observed that the percentage decrease in thermal efficiency due to the higher temperature is lower than that for power output. When the inlet air to the compressor is cooled to the ISO temperature by connecting the VCC system, then the thermal efficiency will be increased in the range of 0.75% to 7.1%. Figure 3 Thermal efficiency versus ambient temperature before and after cooling Fig. 4 shows the variation of output power with the pressure ratio at different firing temperatures and a typical value of ambient temperature. The figure reveals a rise in output power with an increase in pressure ratio. As seen in the figure, the increase continuous just to a limited range of pressure ratios. After that, the output power suffers a reduction in their values. That is meant there is an optimum value for the pressure ratio that causes the power editor@iaeme.com

9 Adnan Abdulla Ateeq, Safaa Hameed Faisal and Hanadi Mahmood Ali output to be a maximum. This fact is clarified in fig. 5 in which the optimum pressure ratio is drawn as a function of firing temperature. This behavior is attributed to one important factor which is the compressor work. Any increase in the pressure ratio will certainly increase both turbine power and compressor power at a constant mass flow rate of air. When the compressor pressure ratio is at a lower value, then the net output power increases due to little compressor work. But, if the pressure ratio exceeds the critical value, then the net output power will drop as a result of higher power required to drive the compressor. Concerning the effect of firing temperature, fig. 4 shows a positive effect of increasing the firing temperature on the power output from the cycle. As a matter of fact, it is impossible to increase the firing temperature to excessive values since it implies a higher quality of materials. The effect of firing temperature is related to the turbine power which is directly proportional to the firing temperature. By attaching the VCC to recover the compressor intake air back to ISO temperature, then the output power will be improved for all the range of pressure ratios and selected firing temperatures. Figure 4 Variation of output power of plant versus pressure ratio at different firing temperature before and after cooling Figure 5 Optimum pressure ratio for thermal efficiency and output power versus firing temperature before and after cooling Fig. 6 shows the effect of pressure ratio on the thermal efficiency at different firing temperatures before and after using the VCC. In a similar manner to output power, the thermal efficiency rises with increasing the pressure ratio and continues in this manner until the pressure ratio reaches a critical value and then it begins to decrease at higher values of pressure ratios. An optimal value for the pressure ratio that causes the thermal efficiency to be a maximum is shown in fig.5. Actually, this behavior of the curve depends directly on the behavior of output power. Moreover, the efficiency is inversely proportional to the fuel mass flow rate, which decreases by increasing the pressure ratio. That is mainly due to increase in editor@iaeme.com

10 Parametric Study of Gas Turbine Cycle Coupled with Vapor Compression Refrigeration Cycle for Intake Air Cooling compressor exit temperature which reduces the fuel mass flow rate needed to attain the required firing temperature in the combustion chamber. Generally, for all the selected firing temperatures it is found that the optimum pressure ratio that causes the power output to be highest is lesser than that causes the thermal efficiency to be maximum. This fact is indicated clearly in fig. 5. Returning to the effect of firing temperature on the thermal efficiency, the thermal efficiency always increases by increasing the firing temperature for any selected pressure ratio. The increase in firing temperature will result in the increase of the fuel mass flow rate required. This rise in fuel mass flow rate inversely affect the thermal efficiency, but an increase in firing temperature will certainly increase the turbine power as described in the section concerning output power. The increase in turbine power is greater than the increase in input energy as fuel and the final result is an increase in the thermal efficiency. By adopting the VCC system, the thermal efficiency will be improved significantly for all the pressure ratio range and selected firing temperature. Figure 6 Variation of thermal efficiency of GT power plant cycle with pressure ratio at different firing temperatures Concerning the effect of VCC parameters, firstly, the basic item for the VCC that must be evaluated is the cooling load. Fig. 7 shows the variation of the required cooling load with the ambient air temperature. This curve is drawn for constant relative humidity at inlet and outlet from the cooling coil, i.e. just sensible cooling load is considered. Certainly, when the ambient air temperature increases, then the required cooling load increases due to the severe temperature difference. Figure 7 Required cooling load for compressor inlet air as a function of ambient temperature editor@iaeme.com

11 Adnan Abdulla Ateeq, Safaa Hameed Faisal and Hanadi Mahmood Ali Fig. 8 shows the effect of evaporator temperature on the output power at different ambient temperature. The figure shows that any increase in the evaporator temperature is accompanied by a slight increase in output power. This increase in the net output power is due to the drop in power consumed by the refrigerant compressor. The later will be higher in low evaporator temperature due to the excessive load applied on the refrigerant compressor. Of course, the output power will be decreased when the ambient temperature rises and this fact is discussed before. For example, at ambient temperature 55 C the output power is increased from MW at -5 C to MW at 5 C which shows that there is about 0.11 MW increase in output power for each 1 C increase in evaporator temperature. Figure 8 Variation of output power with evaporator temperature Fig. 9 shows the effect of evaporator temperature on the thermal efficiency at different ambient temperatures. Actually, it is found that the increase in evaporator temperature causes an insignificant increase in thermal efficiency. On the other hand, the figure shows that an increase in the ambient temperature will decrease thermal efficiency as discussed before. For example, at ambient temperature 55 C the thermal efficiency is improved from 35.41% at - 5 C to 35.72% at 5 C which indicates that there is about 0.031% increase in thermal efficiency for each 1 C increase in evaporator temperature. Figure 9 Variation of thermal efficiency with evaporator temperature Fig. 10 shows that the output power suffers a drop as the condenser temperature getting rise. For example, at an ambient temperature of 35 C the power is decreased from MW at condenser temperature of 35 C to MW at condenser temperature of 55 C which shows that there is about MW drop in net power for each 1 C rise in condenser temperature. The main reason for decreasing the output power is directly related to the power consumed in refrigerant compressor which is increased with increase in condenser temperature editor@iaeme.com

12 Parametric Study of Gas Turbine Cycle Coupled with Vapor Compression Refrigeration Cycle for Intake Air Cooling Figure 10 Variation of output power with condenser temperature The thermal efficiency is affected by the change in the condenser temperature. However, it is found that any increase in condenser temperature is accompanied by a slight decrease in thermal efficiency. That is clearly shown in fig. 11. The main reason of this drop in thermal efficiency is due to the decrease in net output power which directly linked to the power consumed in water chiller compressor. For example, at ambient temperature 55 C the thermal efficiency is decreased from 35.72% to 35.07% as condenser temperature increases from 35 C to 55 C which indicates that there is about % drop in thermal efficiency for each 1 C increase in condenser temperature. Figure 11 Variation of thermal efficiency with condenser temperature 4. CONCLUSIONS There are many methods to augment the performance of GTC operating under higher ambient temperatures. One of these methods is using the VCC to cool the compressor inlet temperature. In this work, a thermodynamic model that consists of a GTC coupled to VCC is presented. The affect of important operating parameters has been investigated and the following conclusions are reached: Ambient temperature has a clear effect of the performance of the GTC operation. The studied performance specifications are found to be declined with the increase of the ambient temperature. There is an optimum value for the pressure ratio that causes the power output and thermal efficiency to be a maximum. This value increases always by increasing the firing temperature editor@iaeme.com

13 Adnan Abdulla Ateeq, Safaa Hameed Faisal and Hanadi Mahmood Ali Increasing firing temperature has a positive effect on the performance of GTC and its value is restricted due to requirement of high quality materials. Increasing evaporator temperature has a positive effect on the performance of GTC. On other hand, increasing condenser temperature has negative impact on the performance. Enhancing GTC by using VCC could improve the power output by range 3.792% to 35.97%. While the thermal efficiency could be improved by range 0.75% to 7.1%. REFERENCES [1] Cengel, A.Y., Boles, M.A., 2006, Thermodynamics Engineering Approach, Fifth Edition, McGraw Hill Companies, New York, USA. [2] Cohen, H., Rogers, G.F.C. & Sarravanmuttoo, H.I.I., 1972 Gas Turbine Theory, Longman Group Ltd, London, UK. [3] Comodi, G., Renzi, M., Caresana, F., and Pelagalli, L., 2015, Enhancing micro gas turbine performance in hot climates through inlet air cooling vapor compression technique, Applied Energy, 147, pp [4] De Sa, A., and Al Zubaidy, S., 2011, Gas turbine performance at varying ambient temperature, Applied Thermal Engineering, 31, pp [5] Faisal, S.H., 2002, Utilization of Nontraditional Gaseous Fuels in Gas Turbine Based Power plants, M.Sc. Thesis, Basra University. [6] Faisal, S. H., Al-Mudhaffar, M., and Khetar, A. D., 2017, Energy and exergy analysis of Rumaila Basra gas turbine power plant during hot season, Basra Journal for Engineering Science, 17, pp [7] Kamal, S. N. O., Salim, D. A., Fouzi, M. S. M., Khai, D.T. H., and Yusof, M. K. Y., Feasibility study of turbine inlet air cooling using mechanical chillers in malaysia climate, International Conference on Alternative Energy in Developing Countries and Emerging Economies AEDCEE, Bangkok, Thailand, May, [8] Mohapatra, A. K, and Sanjay, Thermodynamic assessment of impact of inlet air cooling techniques on gas turbine and combined cycle performance, Energy, 68, pp: , [9] Rahman, M. M., Ibrahim, T. K., and Abdalla, A. N., 2011, Thermodynamic performance analysis of gas-turbine power plant, International Journal of Physical Sciences, 6, pp [10] Sullivan, D.A., 1975, Gas turbine combustor analysis, ASME Journal of Engineering for Power, 97, pp NOMENCLATURE English symbols Number of kmoles (kmole). General fuel formula. Fuel to air ratio (kg f /kg a ). Specific enthalpy (kj/kg). Lower heating value (kj/kg f ). Molecular weight (kg/kmole). Mass flow rate (kg/s). Pressure (kpa). Pressure ratio (-). Heat power (kw). Specific entropy (kj/kg.k). Temperature (K) editor@iaeme.com

14 Parametric Study of Gas Turbine Cycle Coupled with Vapor Compression Refrigeration Cycle for Intake Air Cooling Power (kw). Fraction of compressed air extraction. Greek symbols Thermal efficiency (-). Δ Difference (-). Equivalence ratio (-). Subscripts Points at gas turbine and VCC cycles. Air. Air compressor. Actual. Combustion chamber. Condenser. Evaporator. Exhaust. Expansion device. Fuel. Generator. Gas turbine. Inlet and exit. Input and output. Isentropic. Mechanical. Net value. Products of combustion. Refrigerant. Refrigerant compressor. Stoichiometric products. Stoichiometric. Turbine. Abbreviations EES Engineering equation solver. GTC Gas turbine cycle ISO International standards organization. VCC Vapor compression cycle