EFFECT OF INLET AIR COOLING ON GAS TURBINE PERFORMANCE

Transcription

1 EFFECT OF INLET AIR COOLING ON GAS TURBINE PERFORMANCE WAIEL KAMAL ELSAIED 1,*, ZAINAL AMBRI BIN ABDUL KARIM 2,* Universiti Teknologi PETRONAS Bandar Seri Iskandar, Tronoh, Perak, Malaysia Abstract The performance of gas turbine is always rated to ISO standard condition of 15 o C and relative humidity (RH) of 60%. When the gas turbine is operated at ambient condition of relatively low temperature of 24 o C and RH of 60%, there is a potential decrease of power output by about 6.3%, accompanied by a 1.8% drop in thermal efficiency and a 1.8% increase in specific fuel consumption when compared to the performance at ISO standard condition. Operating at high ambient temperature of 35 o C and RH 60% when compared to ISO standard conditions, the potential power output of gas turbine decrease by about 13.65%, accompanied by a 4.7% drop in thermal efficiency and a 4.7% increase in specific fuel consumption. Another factor influencing in the power output of a gas turbine is the relative humidity of the ambient air, 10% increases in RH decrease the power output by 0.21%, accompanied by a 0.21% drop in thermal efficiency and a 0.21% increase in specific fuel consumption. The purpose of this paper is to study the effect of cooling inlet air in gas turbine. Keywords Gas turbine, power output, inlet air cooling 1. Introduction The power output and efficiency of a gas turbine plant depends among others, on the temperature of inlet air. The efficiency and power output of a gas turbine during hot condition is less than power output during cold condition. The performance of the plant efficiency is decreasing as the ambient temperature increases, due to the inverse relation between air density and temperature. Cooling the inlet air of gas turbine, decreases the temperature which increases the air density, hence increasing the mass flow rate. Therefore, ability to cool the inlet air will facilitate the production of consistent gas turbine power output throughout the year, irrespective of the 1

2 changes in ambient temperature. Also cooling the inlet air increases the mass flow of air into the gas turbine and at the exhaust outlet. The increased exhaust mass flow increases steam production in the heat recovery steam generator downstream of the gas turbine due to higher energy availability in the exhaust gas. For the same power output, decreasing the inlet air has the effect of decreasing the fuel consumption. Kakaras [1] reported that the gas turbine output and efficiency is a strong function of the ambient air temperature. Depending on the gas turbine type, power output is reduced by a percentage between 5 to 10 percent of the ISO-rated power output (15 o C) for every 10 K increase in ambient air temperature. At the same time the specific heat consumption increases by a percentage between 1.5 and 4 percent. Lamfon [2] investigated the performance of a 23.7 MW gas turbine plant operated at ambient temperature of 30 to 45 o C. The net power output is improved by 11 percent when the gas turbine engine is supplied with cold air at the inlet. At the ambient temperature of 30 o C the net power output increases by 11 percent at ISO-rated condition, accompanied by a 2 percent rise in thermal efficiency and a drop in specific fuel consumption of 2 percent. Mohanty [3] presented that by increasing the inlet air temperature from the ISOrated condition to a temperature of 30 o C, would result in a 10 percent decrease in the net power output. For gas turbine of smaller capacities, this decreased in power output can be even greater. He also indicated that a rise in the ambient temperature by 1 o C resulted in 1 percent drop of the gas turbine rated capacity. Ameri [4] reported that in a 16.6 MW gas turbine when the ambient temperature decrease from 34.2 o C to ISO-rated condition, the average power output can be increased by as much as 11.3 percent. He also indicated for each 1 o C increase in ambient air temperature, the power output will decrease by 0.74 percent. Boonnasa [5] presented the results from the study of combined cycle power plant operated in Bangkok. The results showed that decreasing temperature from 35 o C to ISO-rated condition increase the power output of a gas turbine by 10.6 percent and the combined cycle power plant by 6.24 percent annually. The gas turbine was rated at MW. Aihazmy [6] reported that an average power output increment of 0.57 percent for each 1 o C drop in inlet temperature. The power output is increased by 10 percent during cold humid conditions and by 18 percent during hot humid condition. Dawoud [7] presented the results from the study of gas turbine plant in two locations in Oman. The results showed that fogging cooling is accompanied with 11.4 percent more electrical energy in comparison with evaporative cooling in both locations. On the other hand, 2

3 absorption cooling offers 40 percent and 55 percent more energy than fogging cooling. 2. Gas turbine cycle The data used for the analysis is obtained from the manufacturer data sheet of TAURUS 60 gas turbine model. TAURUS 60 is a simple gas turbine, the nominal performance at ISO condition (15 o C and 60% RH), power output is 5670 KW, heat rate KJ/KW.hr, exhaust temperature 783 K and natural gas fuel flow, no inlet and exhaust losses and no accessory losses. 3. Basic equation for calculation The amount of water vapor in the air can be obtained by the humidity ratio. The humidity ratio is define as the ratio of the mass of water vapor to the mass of dry air, and is denoted by the symbol ω [8]. ω= (0.622 Ø p g1 ) / (p 1 - Ø p g1 ) (1) Where ω is the humidity ratio (the ratio of the mass of water vapor to the mass of air), Ø is the relative humidity, p 1 is pressure at the air intake and p g1 the saturation pressure of water at the intake temperature The total enthalpy of atmospheric air is the sum of the enthalpies of dry air and the water vapor, h= h a + ω h g (2) Where h a the enthalpy of dry air and h g is the enthalpy of water vapor. The enthalpy of water vapor can be determined approximately from [8], h g = T (3) Using the relation for ideal gases T o2 = (T 2 -T 1 ) / η c + T 1 (4) Where T o2 is the temperature of the air leaving the compressor having an isentropic efficiency η c. At the combustion chamber, the energy balance can be determine by applying the first law of thermodynamics such that, Heat supplied by fuel = heat gain by burning gases M f LHV= [M a (1+ ω) +M f ] [Cp g (T 3 -T o2 ) + ω (h 3 -h o2 )] (5) Where M f is mass flow rate of fuel, M a mass flow rate of air, LHV is the lower heating value of fuel, Cp g is specific heat at constant pressure of gases, T 3 is turbine inlet temperature, h o2 is the enthalpy of water vapor at compressor outlet and h 3 is the enthalpy of water vapor at turbine inlet. The power at the compressor can be estimated using the first law of thermodynamics as follows, W c = M a (1+ ω) [Cp a (T o2 -T 1 ) + ω (h o2 -h 1 )] (6) Where W c is the compressor work and Cp a is specific heat at constant pressure of inlet air. 3

4 engine efficiency power output(kw) The power produced by the turbine is evaluated from the following relation, W t = [M a (1+ ω) + M f ] [Cp g (T 3 -T o4 ) + ω (h 3 -h o4 )] (7) Where W t is the turbine power, T o4 is actual turbine outlet temperature, h o4 is the enthalpy of water vapor at turbine outlet. The energy of exhaust gas, G is obtained by, G = [M a (1+ ω) + M f ] [Cp G (T o4 -T 1 ) + ω (h o4 -h 1 )] (8) Where Cp G is specific heat at constant pressure of gases at T o4 Hence, the net power W n obtained from the gas turbine power plant is, W n = W t - W c (9) The thermal efficiency η th of the gas turbine power plant can be calculated as, η th = W n / M f LHV (10) 4. Results and discussion The power output and efficiency of the cycle were calculated for various ambient air temperatures, for relative humidity of 60% and 100%, and the results of the analysis are presented in Fig.1 and Fig.2 respectively. It can be observed from these figures that the effect of temperature variation on both the power output and efficiency of the cycle is pronounced. A rise in the ambient temperature by 1 o C result 0.75% drop from gas turbine rated capacity Fig.1. Power output variation vs inlet air temperature Fig.2. Efficiency variation vs inlet air temperature When the low ambient temperature of 24 o C and 60% RH decreased to ISO standard condition, the power output increases by about 6.3%, accompanied by a 1.8% rise in thermal efficiency. When the high ambient temperature of 35 o C and 60% RH decreased to ISO standard condition, the power output increases by about 13.65%, accompanied by a 4.7% rise in thermal efficiency. 4

5 specific fuel consumption (Kg/KW.hr) energy of exhaust gas (KW) The specific fuel consumption (SFC) of gas turbine decrease when the ambient temperature decreases, Fig.3 shows that at low ambient temperature of 24 o C decreases to ISO standard condition, the specific fuel consumption drop by 1.8% and at high ambient temperature of 35 o C decreases to ISO standard condition the specific fuel consumption drop by a 4.7% Fig.3. Specific fuel consumption variation vs inlet air temperature By reducing the intake air temperature, the flow rate of the exhaust gas will correspondingly increase. This means that more heat can be recovered from the exhaust gas at a lower intake air temperature. Fig.4 shows at the low ambient temperature of 24 o C decreases to ISO standard condition, the energy of exhaust gases increases by about 3.55% and at the high ambient temperature of 35 o C decreases to ISO standard condition, the energy of exhaust gases increases by about 8.3% x Fig.4. Energy of exhaust gas variation vs inlet air temperature Another factor influencing in the performance of a gas turbine is the relative humidity of the ambient air. This effect has also been analyzed and found that, the efficiency and power output of gas turbine decreases as the ambient relative humidity increases. The results presented in all the figures show that a 10% increase in relative humidity causes 0.21% drop in both the power output and efficiency, accompanied by a 0.21% increase in specific fuel consumption. It is to be anticipated that this rising trend in increases of thermal efficiency of the gas turbine engine would continue for operations at part load conditions. 5. Conclusion The performance of gas turbine successfully improved by decreasing the temperature of inlet air. Reducing the temperature from ambient condition to ISO standard condition could help to increase the power output between 6.3% at low ambient temperature and 13.65% at high ambient temperature. 5

6 References [1] Kakaras, E. (2006), Inlet Air Cooling Methods for Gas Turbine Based Power Plant, ASME vol.128, pp [2] Lamfon, J.N.(1998), Modeling and Simulation of Combined Gas Turbine Engine and heat Pipe System for Waste Heat Recovery and Utilization, ENERGY CONVERS vol.39, pp Locations in Oman, APPLIED THERMAL ENGINEERING vol.25, pp [8] Cengel, Boles. (2006), Thermodynamics An Engineering Approach, 5th ed. SI units [3] Mohanty, B. (1995), Enhancing Gas Turbine Performance By Intake Air Cooling Using an Absorption Chiller, HEAT RECOVERY SYSTEMS & CHP vol.15, pp [4] Ameri, M. (2004), The Study of Capacity Enhancement of The Chabahar Gas Turbine Installation Using an Absorption Chiller, APPLIED THERMAL ENGINEERING vol.24, pp [5] Boonnasa, S. (2006), Performance Improvement of The Combined Cycle Power Plant By Intake Air Cooling Using an Absorption Chiller, ENERGY vol.31, pp [6] Alhazmy, M.M. (2004), Augmentation of Gas Turbine Performance Using Air Coolers, APPLIED THERMAL ENGINEERING vol.24, PP [7] Dawaud, B. (2005), Thermodynamic Assessment of Power Requirements and Iimpact of Different Gas-Turbine Inlet Air Cooling Techniques at Two 6