Giving a boost to Iran
According to estimates about 20 per cent (1900 MW) of gas turbine capacity in Iran is lost during the summer months. This article presents a study that quantifies the impact of air inlet cooling on gas turbines installed at the Lowshan power plant.
There are more than 170 gas turbine units in Iran with a total capacity of around 9500 MW. However, the power output of the units is about 80 per cent of their rated capacity in the summer. This means that around 1900 MW is lost during the hot season. With the growing demand of electricity and the gas turbine power degradation in summer, one solution is to enhance the gas turbine power output using inlet air cooling systems. A study carried out at the Lowshan power plant assessed the impacts of inlet air cooling on two gas turbines.
Lowshan city is located in the north of Iran near the Caspian Sea. Two V93.1 gas turbines with a rated power of 62 MW produce electricity at the power plant. The performance curve of the gas turbines shows that for each one-degree centigrade increase in ambient air temperature, the power output will decrease by 0.76 per cent.
There are five months of the year (June – October) when the ambient temperature is higher than ISO conditions (15˚C) in Lowshan City.
Figure 1. V93.1 performance curve
The maximum, the average and the minimum temperatures are 37°C, 25.8°C and 17°C, respectively. There is an average 10.8°C difference between the ambient temperature and the ISO condition. The maximum relative humidity is 79 per cent, the average is 52.8 per cent and the minimum is 35 per cent.
Various gas turbine inlet air cooling technologies can be used to enhance the performance and the output power of gas turbines. These include: evaporative cooling; mechanical refrigeration cooling; chilled water thermal energy storage and absorption chiller cooling.
Evaporative cooling: These systems use the vaporization latent heat of water in an adiabatic air saturation process to reduce the dry bulb temperature to the wet bulb temperature. Therefore, their success in reducing the high air temperature depends on the relative humidity of the ambient air. Although these types of systems are economical, they are suitable for hot dry climates rather than hot humid conditions.
Figure 2.Variation of temperature during the hot months
Mechanical refrigeration cooling: The mechanical refrigeration cooling system involves the use of the vapour compression refrigeration equipment, which is commonly employed in the commercial air-conditioning or industrial refrigeration systems. The inlet air to the gas turbine is cooled in a heat exchanger using the chilled water or the refrigeration fluid itself. The equipment is simpler and has less operation and maintenance costs than the absorption chiller. However, its capital cost is still relatively high. The parasitic power needed for the operation of the refrigeration system is typically 25 to 30 per cent of the incremental power increase obtained from the gas turbine.
Chilled water thermal energy storage: Chilled water storage systems use the sensible heat capacity of water to store the cooling energy. Water is cooled by a chiller and stored in a tank for later use to meet the cooling needs. This system is used during the peak loads when power is highly valued (5 to 7 hours per day). Chillers are sized to run during non-peak demand times (perhaps 17 to 19 hours per day).
Figure 3. Psychometric diagram of air cooling process
Absorption chiller cooling: In this system, the hot flue gas from the gas turbine exhaust is used to generate steam in a heat recovery steam generator (HRSG). The steam is used in a double-effect lithium-bromide absorption chiller to produce the chilled water. A compact heat exchanger is designed for installation at the compressor inlet duct. The chilled water from the absorption chiller flows through the heat exchanger and cools the inlet air. This system was used in the Lowshan study.
Absorption chiller capacity selection has important effects on the refrigeration cycle components and economical benefits. It is therefore necessary to define the optimum chiller capacity.
To calculate the cooling load, it is necessary to estimate the amount of energy per hour that is used to change the temperature and humidity from a given condition to the ISO temperature of 15°C and the relative humidity of 100 per cent. Therefore the chiller capacity is a function of the temperature and the relative humidity of air. Some of the cooling load is used to decrease the ambient temperature until the relative humidity reaches 100 per cent. The remaining cooling load is used to reduce the air temperature below the dew point. Figure 3 shows the cooling process on a psychometric diagram.
The maximum cooling load (qmax), which is needed to cool the air to 15°C, is about 13 490 kW throughout the year. To select the optimum load of chiller among the different calculated values, the average (qave) is used. This load is the average of the different cooling loads that are calculated for the different working hours during a year. The average cooling load is 5475 kW.
Figure 4. Produced MWh versus the cooling load
The gas turbine power output augmentation and the decrease in the inlet air temperature are calculated. Figure 4 shows the variation of produced electricity using the absorption chiller system versus the chiller’s cooling load.
The optimization study has shown that the optimum cooling load is calculated to be 5500 kW. The power output of the gas turbine will increase by 9.6 per cent on a typical hot day. The generated electricity is around 15 750 MWh during a year based on this cooling load.
Inlet air cooling equipment
There are three basic components in an inlet air cooling which uses the absorption chiller:
1) Absorption chiller
2) Compact heat exchanger
3) Heat recovery steam generator (HRSG).
Two lithium-bromide absorption chillers are selected for each gas turbine unit. The capacity of each chiller is 2800 kW. Lowshan City is far from the sea, therefore a wet cooling tower is used for the absorption chiller cooling system.
A compact heat exchanger is designed for the installation at the compressor inlet duct. The heat exchanger is made of the fin-tube configuration. The heat transfer surface is around 7000 m2. The cooling coil is designed to achieve the minimum pressure drop. The inlet air pressure drop in this heat exchanger is calculated to be 114 mm H2O. This pressure drop has no sensitive effect on the gas turbine power output.
The HRSG is designed to deliver saturated steam at a pressure of 8 bar. The components of the HRSG are economizer, evaporator and drum. Heat transfer surfaces of economizer and evaporator are 68 m2 and 244 m2 respectively.
The pressure drop in the HRSG is calculated to be 107.3 mm H2O. There are cooling water pumps, chilled water pumps, condensate pumps and feed water pumps for the heat recovery and the refrigeration cycles.
This method requires a suitable control system to keep the inlet air temperature at the stable temperature. In addition, it demands a transformer to provide the necessary electricity for the system.
The cost of an inlet cooling system is often evaluated in terms of $/kW. This can be misleading because the output power enhancement as the result of the inlet air cooling method varies with the ambient temperature. The favoured method of evaluating the economic feasibility of a cooling system is through the cost-benefit analysis in which the additional revenues are calculated as a result of additional electricity production in MWh.
The net electricity production increase is around 15 000 MWh a year, creating an extra income of $375 000, based on the current electricity price in Iran which is 2.5 cents/kWh for off-peak hours.
For the economic calculation we have considered 17 per cent for the domestic interest rate, 7 per cent for the foreign interest rate and 20 years for the equipment life.
After taking into account the capital cost (including installation) for the absorption chiller inlet air cooling system; auxiliary power consumption and annual operating and maintenance costs of the system components, the final economic results are shown in Table 1.
The economic analysis has shown that the capital cost of the system is estimated to be around $213 per maximum increased kilowatt compared to the gas turbine installation cost of $275/kW in Iran. The COE (cost of electricity) is calculated to be 1.63 cents/kWh. This cost is less than the current cost of industrial electricity in Iran which is 2.5 cents/kWh for off-peak hours. The rate of return (ROR) is 21.4 per cent for this project. The payback period is calculated to be 4.6 years.
It has been shown that the average power output can be increased by as much as 9.6 per cent. The maximum power augmentation is around 7 MW. The electricity production is increased by 15 000 MWh per year.
As Iran has many gas turbines with the same ambient conditions, implementing inlet air-cooling systems using absorption chiller technology to provide peak demand would be highly beneficial. MEE
A fuller version of this paper was presented at Power-Gen Middle East 2004, Bahrain. “The V93.1 gas turbine power augmentation using inlet air cooling”, by Mohammad Ameri, Seyed Hossein Hejazi, Vahid Zarafshani, Power & Water University of Technology, Tehran, Iran