Although the performance of the gas turbine is not especially attractive compared with the efficiencies possible in Diesel and Petrol engine power plants, a simple gas turbine has advantages in weight, size and vibration compared to the engine and in size and cost compared to small steam plant. It is also superior to both in quantity of water used, for the simple gas turbine plant uses almost no cooling water.
Even if the components used in the gas turbine power plants are improved in design, the efficiency and the specific output of the simple gas turbine cycle are quite low. The efficiency handicap is surmountable, at the expense of adding complexity to the gas turbine plant.
The principle refinements which accomplish this are:
(a) Regeneration
ADVERTISEMENTS:
(b) Intercooling and
(c) Reheating.
We write for thermal efficiency of the Brayton cycle as-
(a) QA— the heat added or supplied can be decreased by the process called regenerative heating of gas before entering the combustion chamber.
(b) Compressor work Wc can be decreased when multistage compressors with intercooling are used.
(c) Turbine work-output can be increased by using a multistage turbine with reheating of the gas in between the two stages.
Thus the modifications in the simple gas turbine cycle required for improving the performance of the cycle are:
Methods to Improve the Efficiency of Gas Turbine
Methods # 1. Regeneration:
We observe that the temperature of the turbine exhaust at (4), Fig 34.8, is higher than the temperature at the end of compression at 2, and, therefore, we may think of applying Ericsson’s notion of regeneration. In this event, the exhaust at 4 gives up heat to the air at 2. This transfer of heat is called Regenerative heating and the heat exchanger used for this is called Regenerator. This results in cooling of final exhaust gases and thus a reduction in heat rejection takes place. 
Theoretically, if the heat exchanger were large enough and flow were slow enough, the air from the compressor could be heated reversibly to temperature 4 at state b, while the exhaust cools to temperature 2 at state a. Some of the formerly discharged heat h4 – ha is exchanged within the system and the heat to the sink is now only ha – h1.
Moreover, it is necessary to add only the heat equal to h3 – hb instead of h3 – h2 as before. Consequently less fuel is needed and this additional piece of equipment should materially increase the efficiency of the ideal cycle. From the figure we find the thermal efficiency for a constant mass of 1 kg as-
With a fixed initial temperature T1 the above equation shows that with a regenerator, the thermal efficiency increase as T3, increases and the thermal efficiency decreases as the pressure ratio increases.
ADVERTISEMENTS:
Without the regenerator, the cycle efficiency increases as the pressure ratio increases.
Effectness of Regenerator:
Figure 34.10 is the T-S diagram for imperfect regeneration with fluid friction in turbine.
1-2 Isentropic compression of air in compressor.
ADVERTISEMENTS:
1-2′ Actual compression.
3-4 Isentropic expansion of gases in turbine.
3-4′ Actual expansion of gases in turbine.
If regeneration is perfect then
d is the state of air before combustion chamber
c is the state of gases before the gases are exhausted
and in this case heat given by gases from turbine is equal to the heat given by gases from turbine is equal to the heat gained by air in the regenerator.
But the actual state of air will be d’ because the regenerator will not be perfect. Consequently the regenerator will not be 100 % efficient. The performance of the regenerator is given by the Effectiveness of the regenerator. It is defined as-
Methods # 2. Intercooling:
Net work of the gas turbine cycle can be increased either by reducing the compressor work or increasing the turbine work. For decreasing the compressor work, advantage is taken of the nature of the constant pressure curves of h-s and T-S diagrams. The vertical distance between any two constant pressure lines goes on decreasing to the left and goes on increasing to the right.
If the compression is achieved in two or more stages, the air delivered by the 1st stage of the compressor, is cooled, on its way to the next stage. This cooling of air in between the two stages is called intercooling. When the air is cooled to the temperature of air entering any stage, intercooling is called perfect intercooling.
1- 2 Isentropic compression in first stage
2- 3 Intercooling between the stages
3-4 Isentropic compression in the second stage
1-5 Isentropic compression without intercooling
Vertical distance between 3-4 is less than the vertical distance 2-5 and therefore,
[(1-2) + (3-4)] < (1-5)
∴ The compression work is reduced while the turbine work remains same when other data remains same.
∴ Network = Wt – Wci
= Constant – reduces Wc
> Wt – Wc
Where Wc = h5 – h1
Wci = Compression work with intercooling.
When the intercooling is perfect and when the intermediate pressure is the geometric mean (P2 = √p1 x p3) then the compression work is minimum.
Usually water-cooled surface coolers are employed. Low and high pressure stages must necessarily separately encased.
Figure 34.12 shows intercooling and fluid friction during compression. The diagram is self-explanatory the same working substance flows through the different components of the plant again and again, receiving heat and rejecting heat at the approximate events in the cycle.
If the entire flow comes from the atmosphere and is returned to the atmosphere, the turbine is said to work on the Open Cycle.
The basic difference between open and closed cycle gas of turbines is in the method of heating the air after compression. In case of an open cycle gas turbine, the fuel is burned in the air itself to increase its temperature i.e., fuel is mixing with air and then the products of combustion are passed on to the atmosphere through turbine. For the next cycle, a fresh supply of air is taken in the compressor and the processes are repeated.
On the other hand, in a closed cycle gas turbine, the same air or working substance is circulated over and over again. The working substance is heated in a heat exchanger where a separate hot gas is obtained by burning the fuel in the supply of additional air in a combustion chamber. The heat exchanger is of the shell and tube type so that working substance does not come into contact with the products of combustion.
Most of the gas turbines in use are open cycle plants. But the recent developments made the closed-cycle plants work producing more than 1500 kW and having gas turbine inlet gas temperature of 800°C and having a thermal efficiency of the order of 30 %.
At state 1, cold gas enters a compressor, where shaft work is to be done on the compressor to increase the pressure and temperature. The gas leaves the compressor at the state 2. This gas enters the heater (heat exchanger) where heat is supplied at constant pressure. The temperature of the gas increases and the gas leaves the heater at state 3. This gas from the heater enters the gas turbine where it is expanded to the lower pressure so that the temperature is reduced, shaft work is produced. Part of this shaft work is utilized to run the compressor and the rest is supplied to the load or the useful power or net-power.
Methods # 3. Reheating and Reheat Cycle:
Another variation in the simple Brayton cycle is the use of two turbines instead of one, in which case, one turbine is used to drive the compressor and the other produces the network output. The shafts of the turbines may collinear or as shown in Fig. 34.14. In between these two turbine we may or may not arrange to bum more fuel by installing another combustor as shown.
Reheating is the increase of temperature of partially expanded gas by burning more fuel, in a device called Reheater. Figure 34.14 shows reheating and regeneration.
Methods # 4. Gas Turbine (Brayton Cycle) with all Intercooling, Reheating and Regeneration:
All the modifications to the simple cycle may be applied separately or together. They are capable of raising the plant efficiency to over 30%, thereby erasing any advantage of fuel efficiency possessed by Diesel or condensing steam plant.
A schematic diagram of modified Brayton cycle incorporating intercooling, reheating and regeneration is shown in Fig. 34.15.
Methods # 5. Effect of Modifications on the Performance of Brayton Cycle-Gas Turbine:
Various graphs showing the effect of intercooling, reheating and regeneration on the cycle efficiency are given in Fig. 34.17.
These graphs give the nature of the curves only.
A—Simple cycle.
B—Simple cycle with regeneration.
C—Simple cycle with intercooling and regeneration.
D—Simple Open cycle with regenerator, re-heater and inter-cooling.
Combine effect of intercooling, reheating and regeneration on gas turbine cycle on thermal efficiency and specific output for different maximum temperatures are shown in Fig. 34.18 (a) and (b).
Methods # 6. Water Injection:
This is one of the methods of improving the performance of the gas turbine is to inject water into the working air at the entrance to the compressor. This way the compressed air is cooled by absorbing, from air, the latent heat of vaporisation of water.
By injecting of water, the total mass flow of the working medium is increased by the mass of the injected water and hence the power output of the cycle is increased, the work ratio is also increased in addition to the lowering of the air rate.
Water injection system is commonly used as a power book for take off and emergencies on the jet propelled air-craft. It is possible for marine or land based gas turbines to use water injection for long duration or continuous water injection. The water to be injected should be pure otherwise it may cause corrosion or deposits on the blades.