415
11.4 Performance of Power Gas Turbines with Enhanced Cycles
used for a superheating part in the steam cycle. With current machines (2014) with
steam cooling, the combined-cycle efficiency is above 61 %.
Steam cooling has for a long time been considered as an essential way to reach
60 % efficiency of combined steam and gas turbines. With respect to technology
steps, it has become customary to denote gas turbine types with subsequent letters of
the alphabet. Current classes are E, F, G and H (A, B and C are older types, but some
still exist). Class E refers to machines with convection and impingement cooling,
originally with a turbine inlet temperature (TIT) around 1100 °C and reaching about
50 % combined-cycle efficiency. Modern versions of these machines reach a TIT of
1250 °C due to use of thermal barrier coatings (TBC) and directionally solidified
blades. Class E machines are very robust and are typically used with crude fuels,
which cause deposits in the turbine, making film cooling impossible. Class F means
machines with film cooling, TBC and directionally solidified blades. These allow
TIT in the order of 1350 °C and reach above 55 % combined-cycle efficiency, typically around 57–58 %. Class H meant up to some years ago machines with closed
loop steam cooling both in stationary and rotating parts, advanced TBC and singlecrystal blades in the first stage of the turbine part. These machines allow TIT in
the order of 1450 °C, even 1500 °C, and reach combined-cycle efficiency above
60 %. The most recent turbine with closed-loop steam cooling is the GE 9HA from
General Electric. The version 02 reaches 470 MW and 41.5 % efficiency in simplecycle mode and 710 MW and 61.4 % efficiency in combined-cycle operation. Class
G machines have closed-loop steam cooling only in stationary parts, which means
steam cooling of the liners with can-annular combustion chambers, the transition
pieces and the vanes of the first stages of the turbine part. These machines have a
TIT up to 1500 °C, similarly to class H machines, and reach 58–59 % combinedcycle efficiency. Steam cooling is still used by the manufacturers General Electric
and Mitsubishi, but has been abandoned by Siemens. The reason is that the role of
combined gas and steam turbine power plants has moved somewhat from base-load
plants to mid-load plants. By mid load we mean plants which follow the variation
of the electricity consumption on the time scale of the day. The need for more mid
load comes from the growing fraction of electricity produced by wind turbines and
solar cells. Combined steam and gas turbines with steam cooling of the gas turbine
have a large thermal inertia. This is avoided with pure air cooling. In the past years,
much effort has been made by manufacturers to decrease the start-up time of gas
turbines. With current F-class machines, the time needed for reaching full power of
the gas turbine (not including the steam turbine) starting from standstill has been
reduced to about 10 min, which was about 30 min up to 10 years ago. This is not
possible with big thermal inertia. It is possible to obtain above 60 % combined-cycle
efficiency with air-cooled machines. This is proved by the SGT5-8000H machine
by Siemens. Siemens describes the machine as H-class, although there is no steam
cooling, to express the advanced technology level. The improvement with respect
to the F-class comes from improved component design. Compressor efficiency is
better due to improved blade design. Turbine efficiency is better, mainly due to a
clearance control system (displacement of the shaft in axial direction). This improves the simple-cycle efficiency from 39.5 % of the SGT5-4000F to 40 % with
11.4 Performance of Power Gas Turbines with Enhanced Cycles
used for a superheating part in the steam cycle. With current machines (2014) with
steam cooling, the combined-cycle efficiency is above 61 %.
Steam cooling has for a long time been considered as an essential way to reach
60 % efficiency of combined steam and gas turbines. With respect to technology
steps, it has become customary to denote gas turbine types with subsequent letters of
the alphabet. Current classes are E, F, G and H (A, B and C are older types, but some
still exist). Class E refers to machines with convection and impingement cooling,
originally with a turbine inlet temperature (TIT) around 1100 °C and reaching about
50 % combined-cycle efficiency. Modern versions of these machines reach a TIT of
1250 °C due to use of thermal barrier coatings (TBC) and directionally solidified
blades. Class E machines are very robust and are typically used with crude fuels,
which cause deposits in the turbine, making film cooling impossible. Class F means
machines with film cooling, TBC and directionally solidified blades. These allow
TIT in the order of 1350 °C and reach above 55 % combined-cycle efficiency, typically around 57–58 %. Class H meant up to some years ago machines with closed
loop steam cooling both in stationary and rotating parts, advanced TBC and singlecrystal blades in the first stage of the turbine part. These machines allow TIT in
the order of 1450 °C, even 1500 °C, and reach combined-cycle efficiency above
60 %. The most recent turbine with closed-loop steam cooling is the GE 9HA from
General Electric. The version 02 reaches 470 MW and 41.5 % efficiency in simplecycle mode and 710 MW and 61.4 % efficiency in combined-cycle operation. Class
G machines have closed-loop steam cooling only in stationary parts, which means
steam cooling of the liners with can-annular combustion chambers, the transition
pieces and the vanes of the first stages of the turbine part. These machines have a
TIT up to 1500 °C, similarly to class H machines, and reach 58–59 % combinedcycle efficiency. Steam cooling is still used by the manufacturers General Electric
and Mitsubishi, but has been abandoned by Siemens. The reason is that the role of
combined gas and steam turbine power plants has moved somewhat from base-load
plants to mid-load plants. By mid load we mean plants which follow the variation
of the electricity consumption on the time scale of the day. The need for more mid
load comes from the growing fraction of electricity produced by wind turbines and
solar cells. Combined steam and gas turbines with steam cooling of the gas turbine
have a large thermal inertia. This is avoided with pure air cooling. In the past years,
much effort has been made by manufacturers to decrease the start-up time of gas
turbines. With current F-class machines, the time needed for reaching full power of
the gas turbine (not including the steam turbine) starting from standstill has been
reduced to about 10 min, which was about 30 min up to 10 years ago. This is not
possible with big thermal inertia. It is possible to obtain above 60 % combined-cycle
efficiency with air-cooled machines. This is proved by the SGT5-8000H machine
by Siemens. Siemens describes the machine as H-class, although there is no steam
cooling, to express the advanced technology level. The improvement with respect
to the F-class comes from improved component design. Compressor efficiency is
better due to improved blade design. Turbine efficiency is better, mainly due to a
clearance control system (displacement of the shaft in axial direction). This improves the simple-cycle efficiency from 39.5 % of the SGT5-4000F to 40 % with