PERFORMANCE IMPROVEMENT OF GAS TURBINE CYCLES

Abstract
The gas turbines are generally used for large scale power generation. The basic gas turbine cycle has low thermal
efficiency, so it is important to look for improved gas turbine based cycles. The inlet air cooling helps in
increasing the performance of gas turbines. Another method for increasing the performance has been to introduce
a high amount of water or steam at various points in the cycle. There are several methods suggested and some are
already in operation. All of them offer increased performance and increased specific output compared to a dry
gas turbine cycle. The water addition in the gas turbine cycle also helps in reducing exhaust emissions. The work
presents the effect of inlet air cooling on gas turbine performance and thermodynamic assessment of various
advanced gas turbine cycles. Performance maps are obtained for these cycles for a range of pressure ratio and
turbine inlet temperature. The humid air turbine (HAT) is compared with combined cycle, steam injected cycle
(STIG), simple cycle and inter-cooled recuperated cycle. The performance of HAT cycle is found to be best for
all the range of pressure ratios considered here. The amount of water required for HAT and STIG cycles is also
discussed.
Keywords : Absorption Chiller, Humid Air Turbine, Combined Cycle Gas Turbine
Introduction
The demand of energy in the developing regions of the world, particularly in Asia, has witnessed pronounced
increase in the recent past. According to a report of International Energy Outlook 2004, the world net electricity
consumption is expected to nearly double over the next two decades. Much of the growth in new electricity
demand is expected to come from countries of the developing world. Therefore, it is important to find improved
technologies for power generations that have high efficiency and specific power output, low emissions of
pollutants, low investment, and low operating and maintenance cost for a sustainable use of available fuels.
Industrial gas turbines are one of the well established technologies for power generation. The advantage of gas
turbines lies in the fact that they have high power/weight ratios and low pollutant emissions compared to
reciprocating engines.
The introduction of combined cycles boosted the efficiency of gas turbine cycles. However, combined cycle
power plants have relatively high investment costs and require high-grade steam to be generated from the flue
gases. Due to this combined cycle plants are often built around big gas turbines that are optimized for
applications with high exhaust gas temperatures. For smaller gas turbines as well as gas turbines with high
pressure ratios and low exhaust gas temperatures, some of the proposed schemes for performance improvements
are:
• Injection of water in the inlet air
• Injection of water mist or steam in the compressor during the compression process.
• Massive stream injection in the combustion chamber and turbine
Some of the gas turbine cycles using the above modifications are:
• Evaporative regenerative gas turbine cycle
• Inter cooled recuperative gas turbine cycle (ICRGT).
Performance Improvement of Gas Turbine Cycles 23
• Steam injected gas turbine cycle (STIG).
• Humid air turbine (HAT).
The objective of the present work is to assess the thermodynamic performance of the above advanced gas turbine
cycles for a typical gas turbine cycle power plant with a power output in excess of 200 MW. Typical component
merit indices have been chosen and performance assessed for several combinations of design parameters and
representative inlet conditions. A computer code has been developed using realistic models for various gas
turbine processes including combustion.
Various methods to improve gas turbine efficiency and power output are as follows:
Gas Turbine Inlet Air Cooling
The turbine inlet air cooling methods can be divided into two categories a) Cooling with wetted media and b)
Cooling with chillers
Evaporative Cooling
In this process water is distributed over pads of fibers through which the air passes to be humidified. Spray
intercoolers or fogging systems were also used to cool the inlet air. De Lucia et al(1995) reported that
evaporative inlet-cooling is economical and simple, but suitable for only dry hot climates. He concluded that
evaporative inlet cooling could enhance power by 2–4% depending on the weather. Bassily(2001) presented the
effects of the turbine’s inlet-temperature, ambient temperature, and relative humidity on the performance of the
recuperated gas-turbine cycle with evaporative inlet cooling and the intercooler reheat regenerative gas-turbine
cycle with indirect evaporative inlet cooling.
Cooling with Absorption Chiller
Chillers can increase the gas turbine power output by 15-20% and efficiency by 1-2%. Ait-Ali(2001) presented
the concept of inlet air refrigeration to boost the power output from the gas turbine. The absorption chiller works
on the principle of vapor absorption refrigeration cycle. The main advantage of this chiller lies in the fact that the
inlet air can be cooled down to a specific temperature for a wide range of ambient air temperatures and, therefore
the power output of a gas turbine remains more or less constant, independent of ambient air conditions. The low
grade exhaust energy can be used to drive the chiller. The chilled water (≈5o C), produced by the absorption
system, is passed through the inlet air cooler, which is an indirect type air to water heat exchanger.
A typical absorption chiller with a capacity of 3000 refrigeration tons and a COP of 0.70 is taken for the current
study. This absorption system uses the waste heat to produce steam, required by the chiller.
Advanced Gas Turbine Cycles
The gas turbine cycles with exhaust heat recovery are generally known as advanced cycles. Some of the
advanced gas turbine cycles are discussed by Heppenstall(1998). Heat recovery schemes are one of the most
important ways of increasing the efficiency of the power generation process. Some of the advanced gas turbines
are 1) Gas to gas recuperated cycles. 2)Steam Injected Gas turbine cycles 3) Evaporative Regenerative Gas
Turbine Cycle 4) Humid Aur Turbines and 5) combined cycle power plants. Some are discussed below
Steam Injected Gas Turbine Cycle (STIG)
The exhaust gas from the turbine is used as an energy source in a heat recovery steam generator (HRSG) where
energy is transferred from the exhaust gases to the boiler feed water. The high pressure steam is generated from
HRSG. The steam is then injected into the combustion chamber. Injection of steam increases the mass flow rate
through the expander and so the power output and the efficiency of the turbine increase. Steam injection also
helps in reducing the NOx emissions from the gas turbine. The amount of steam generated in the HRSG depends
upon the pinch point of the boiler. Due to this pinch point and the turbine outlet temperature, the HRSG cannot
utilize all the heat available in the flue gas to generate steam.
Evaporative Regenerative Gas Turbine Cycle (ERGT)
In this cycle heated water is injected in the compressed air down stream from the LPC (Low Pressure
Compressor) and HPC(High Pressure Compressor) in order to inter-cool and after-cool the compressed air.
This improves the heat exchange inside the recuperator. The mass of water added and the fact that water has a
higher specific heat than air results in a gas turbine with a higher specific power output and a lower heat rate as
24 Advances in Energy Research (AER – 2006)
compared to simple cycle. Care must be taken to be certain that all the water sprayed into the air is evaporated.
The maximum amount of water is limited to that which results in the air being saturated at the evaporator exit or
when the temperature at the evaporator exit is equal to the temperature of the water being sprayed into the
evaporator.
Humid Air Turbine (HAT)
The HAT cycle is the most advanced recuperated gas turbine cycle. The HAT cycle was introduced by
Rao(1989).The advantage of HAT cycle over ERGT cycle lies in making the after-cooling and the water mixing
separate and in a more reversible way, through the use of a direct contact heat exchanger, like the humidifier. In
this cycle air is humidified prior to its entry in the recuperator. In this cycle the air is compressed in a low
pressure compressor, inter-cooled with water, compressed to final pressure and then after-cooled. The air is then
passed through the humidifier, where hot water is allowed to evaporate and to mix with water. The relative
humidity in the humidifier increases from the bottom to the top, where it leaves the humidifier at almost
saturated conditions. The purpose of humidifier is to increase the mass flow and the enthalpy of the compressed
air. The humidifier makes it possible to recover energy below the boiling point of the water.



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