This article discusses various features of supercharged gas turbine and supercharged analysis. One 400 MW supercharged gas turbine power plant variant analysed by Wettstein yielded a predicted thermal efficiency of 60 percent, rivaling current combined cycle values. The supercharged gas turbine power plant proposed by Wettstein is a semi-closed (SC) cycle. The SC cycle is an amalgamation of closed and open cycles. It consists of a gas turbine having an internal combustor for energy input to the cycle. With a SC cycle, a designer now has some of the best features of both open and closed to move SC power plant operation in different directions. With internal combustion, the SC cycle is not constrained by the temperature limitations of the closed cycle. The supercharged gas turbine power plant looks very promising. In another ASME paper, Wettstein shows how gas turbine supercharging could benefit marine propulsion.
At our Turbo Expo ’14 last June in Düsseldorf, IGTI author Hans Wettstein presented a paper proposing a thermodynamic cycle with a supercharged gas turbine . Along with the promise of higher efficiencies over a wide range of loads, supercharging would provide more power to a gas turbine of a given size, just as it does for Diesel and Otto engines. One 400 MW supercharged gas turbine power plant variant analyzed by Wettstein yielded a predicted thermal efficiency of 60%, rivaling current combined cycle values.
To understand Wettstein's supercharge analysis, we first consider a closed cycle gas turbine where a working fluid (air, but it could be helium, etc.) is recirculating without any internal combustion. (Closed cycle electric power plants, twenty-four of which were installed from 1940 into the 1980s are treated and reviewed by Frutschi ).
Approximating a thermodynamic Brayton cycle, the closed cycle gas turbine has two heat exchangers, one for rejecting heat from working fluid entering the compressor and one for energy addition to flow entering the turbine. The latter is heated by an external energy source such as combustion (no problem here with burning “dirty” fuels), a solar collector or a nuclear reactor.
Closed Cycle Handicap
The chief disadvantage of the closed cycle gas turbine is that the allowable working temperatures of the heated heat exchanger surfaces impose a fairly low upper limit on maximum temperatures in the cycle, keeping achievable cycle thermal efficiencies reported on by Frutschi  in the 20-30% range (compared to a modern gas turbine combined cycle plant at about 60%). Also, the heated heat exchanger has to be very large compared to other components, amounting to as much as 40% of the plant capital cost.
Consequently, open cycle gas turbines operating at much higher maximum temperatures can have thermal efficiencies in the 40-50% range, and dominate in gas turbine power plants. (As Haywood  points out, open cycle (where atmospheric air is drawn into a compressor and the turbine exhausts to the atmosphere) is a misleading and confusing term, since the unit is not cyclic.) The higher open cycle temperatures are realized by using combustors directly and by using compressor air for cooling hot section parts. Thus we have seen little of closed cycle gas turbine plants, except those that have been proposed recently for use with high temperature gas cooled nuclear reactors .
Closed Cycle Advantage
The part load characteristics of a closed cycle gas turbine are remarkably better than those of open cycle operation. In closed cycle operation, load reduction is achieved by bleeding the working fluid from the closed loop. This reduces the mass flow rate, reducing system pressures, lowering gas density, and lowering power output, but maintaining constant gas velocities at constant rpm. In gas turbine designer terminology the turbomachinery velocity triangles remain the same, so that closed cycle design efficiency will remain the same over a wide range of load operations.
The Semi-Closed Cycle
The supercharged gas turbine power plant proposed by Wettstein , shown in Figure 1, is a semi-closed (SC) cycle. The SC cycle is an amalgamation of closed and open cycles. It consists of a gas turbine having an internal combustor for energy input to the cycle. Part of the turbine exhaust is rejected to the atmosphere, with the remainder recirculated into the compressor. Atmospheric air is also introduced into the compressor, to maintain a desired mass flow rate and provide makeup oxygen to sustain combustion.
Horlock  goes through a simple illustration to show that given similar conditions for both open and closed cycles, a semi-closed cycle should have approximately the same efficiency as either. However, with a SC cycle, a designer now has some of the best features of both open and closed to move SC power plant operation in different directions.
With internal combustion, the SC cycle is not constrained by the temperature limitations of the closed cycle. Also, the SC cycle will have the favorable part load efficiencies of the closed cycle if pressure levels can be varied.
This is where Wettstein introduces a supercharger (labeled as the charger group in Fig. 1). This allows the pressure levels in the SC cycle to be varied, which provides the opportunity to control part load output, independent of turbine inlet temperature.
As shown in Fig. 1, the unit has a recuperator and an intercooler. For the configuration shown, with an output of 417 MW, the calculated thermal efficiency is 60%. This is about the efficiency of the newest combined cycle plants but without the need for steam condenser coolant and Rankine cycle hardware.