2.2 Improvement of Gas Turbine Thermal Efficiency
2.2.2 Major Improvement of Gas Turbine Thermal Efficiency
Reheat: An effective method for substantially increasing the thermal efficiency without a significant increase in turbine inlet temperature is the application of the well known reheat principle as a classical method for thermal efficiency augmentation. Although this standard efficiency improvement method is routinely applied in steam turbine power generation, it did not find its way into the aircraft and the power generation gas turbine design. As discussed in Chapter 1, the Compressed Air Energy Storage (CAES) gas turbine was the first to utilize the reheat concept. The gas turbine has two combustion chambers designed and manufactured by Brown Boveri & Cie (BBC), installed in Huntorf Germany and commissioned 1978. The successful operation of this gas turbine was the basis for a major technology change that has lead to a new gas turbine type GT24/26 by the Swiss manufacturer ABB. The development of this new engine started 1990 and completed 1995. The gas turbine GT24 designed for 60Hz market (USA) and GT26 for 50Hz (Europe). Figure 2.6 shows a schematic arrangement of the gas turbine 24/26. The application of the reheat concept, increased the efficiency of GT-9 from 32% to 40.5%.
The T-s-diagram of the gas turbine GT24/26 compared to a conventional baseline design is presented in Fig. 2.7. Starting with the blue hatched baseline process, Fig. 2.7(a), the red hatched reheat process, Fig. 2.7(b) exhibits an addition to the area enclosed by the baseline T-s diagram. This addition translates into a substantial increase of the efficiency as shown in Fig. 2.8. It shows quantitatively the efficiency improvement using the reheat concept. The blue curve represents the efficiency of a relatively advanced GT up to 1986. The green curve exhibits the efficiency of a generic reheat gas turbine.
Inlet Compressor Bypass
Combustor 1
Reheat turbine Combustor 2
Diffuser Multi-stage turbine
Fig. 2.6: Schematic arrangement of the gas turbine GT-24/26. The application of the reheat concept increased the efficiency of GT-9 from 32% to 40.5%.
2
3
BL 4BL
T3BL BL
a
2
1
BL
b T3BL
4BL 3BL
2 3
5
6
Entropy s
Baseline GT (BL) GTwith reheat
Temperature T
Entropy s
Temperature T
Fig. 2.7: The T-s-diagram of a gas turbine with a reheat stage compared with a conventional baseline design; (a) baseline GT, (b) GT with a reheat turbine stage and two combustion chambers.
The green curve includes the predicted efficiency of 40.5% already reported in [1]
and [2]. It should be noted that the turbine inlet temperatures for the baseline GT and the GT with reheat are the same. This is a very important aspect that enables a gas turbine design with a high efficiency but at a lower temperature for example 1200 oC. The major efficiency increase shown in Fig. 2.8, gave rise to further increase by introducing a new technology discussed in the following section.
GT- with reheat stage Concept realization: ABB Optimum presure ratio:π= 40 Design pressure ratioπ= 30 Predicted ηth= 40.5%
GT-9 efficiency improvement:Δη= 8.5%
ηthatπDesign
Baseline GT Baseline GT GT-9:ηth=32%
TIT = 1200C
for both GTs ηthatπopt
π η
th0 10 20 30 40 50
0.2 0.3 0.4 0.5
Fig. 2.8: Major efficiency improvement using the reheat concept.
D1 N1
B V1
SIC
S
14
T1
7 1
C3 C2
6 4
3 2
C1 T2 T3 T4
13
UHEGT-SCH CC = Combustion Chamber C = i compressor stage groupBV = Bypass valve D = Exit diffuser FT = Fuel tank
i
i
TS N F i th
i turbine stage G = Generator Load
Shaft Speed sensor
= i
i th
= S
Fuel valve i= V
= Fuel flow
= Gas flow
= Air flow
= Signal flow
Simulation Schematic of the UHEGT
8 11 12
N = Inlet nozzle
SIC 9 SIC
5
C4
10 2V F
V3 1 F
V V F
B 1 B 1V
Fig. 2.9: Schematic diagram of an Ultra High Efficiency gas turbine.
New Technology: Major efficiency improvement requires a major technology change. As shown in [3], and [4], major improvement can be achieved by using the UHEGT-technology (Ultra High Efficiency Gas Turbine technology) that utilizes stator internal combustion. This technology eliminates the combustion chambers altogether and places the combustion process inside the stator and rotor blade passages. Figures 2.9 and 2.10 show the T-s-diagram and the schematic and a composition of the UHEGT-components
Entropy s
BL
4
Temperature T
1 a
2BL
UHEGT
3BL 3 T
2 5 4
3
Temperature T
10
BL
Entropy s
BL
1 2
UHEGT b
4BL 3BL
8
9 T
3 5 2
4
3BL
10 9
Fig. 2.10: T-s-diagrams for the Ultra High Efficiency Gas Turbine, (a) with constant TIT, (b) with variable TIT.
As shown in Fig. 2.9, a ir enters the inlet nozzle and is compressed through the LPC (C1 and C2), IPC (C3) and HPC(C4). It leaves the compressor at a pressure ratio of 40/1. The high pressure air enters the first turbine stator with an array of fuel injection nozzles that are distributed circumferentially. A portion of the total fuel mass flow is injected into the axial gap upstream of the first stator row. Combustion starts upstream of the first stator row and the stator blade channels. Combustion gas exits the first stator row entering the first rotor row the rotational motion of the rotor causes a strong mixing of unburned fuel particles with the rest of combustion gas resulting in a complete combustion. After exiting the first rotor row the lean combustion gas enters the second stator row, where the second portion of the fuel is added. After completing the same combustion` process in the second stage, combustion gas enters the third stage turbine, where the injection process takes place similar to the first to stages. The process of fuel injection and subsequent combustion is associated with the simultaneous energy extraction from the combustion gas and generation of turbine shaft power. The process is shown in Fig. 2.10. As in Fig. 2.7, the above T-s-diagram includes the baseline process (hatched blue area and the addition of sequential reheat-expansion process (hatched red). Figure 2.10(a) represents the process with the same TIT after each combustion. Based on the design requirement, the TIT may have a variable distribution.
Figure 2.11 shows the comparison of UHEGT with GT24/26 and a baseline Gas turbine. In Fig. 2.11 gas turbines with three and four stator internal combustion, UHEGT-3S and UHEGT-4S, respectively are shown . The turbine inlet temperature TIT, for all cycles is the same and equals to 1200 C. As discussed in Chapter 1, Fig.
1.16, for UHEGT-3S a thermal efficiency above 45% is calculated. This exhibits an increase of at least 5% above the efficiency of the most advanced gas turbine engine which is close to 40%. Increasing the number of stator internal combustion to 4, curve labeled with UHEGT-4S, can raise the efficiency above 47% which is an enormous efficiency increase compared to any existing gas turbine engine. It should be noted that UHEGT-concept substantially improves the thermal efficiency of gas turbines, where the pressure ratio is optimized corresponding to the turbine inlet temperature.
This gives UHEGT a wide range of applications from small to large size engines.