The semi-closed gas turbine discussed in chapter 9, see Figs. 146 to 148, is the logical link between open cycle and closed-cycle gas turbines. It would be able to unite the benefits of both systems, namely direct heat supply to a combustion chamber, charging the cycle to increase the power density, as well as high efficiency at part load. Besides the great advantage of dispensing with the air heater system required by closed-cycle gas turbines, which as mentioned swallows up around 40% of the investment costs, a pre cooler would be needed as a heat sink, unless a combined cycle configuration were designed as in Fig. 148 so that the condenser in front of the steam process is the only heat sink. Nevertheless, turbo charging the gas turbine cycle would allow the pre cooler in front of the compressor and any intercooler for the compressor to be of a much more compact size.
As mentioned before, the nitrogen turbine in Fig. 134 shows the way forward in compact turbo set design. However, it is also possible to build a semi-closed gas turbine system based on turbochargers, as used for large diesel engines. Thus, a turbocharger between the fan and turbine would be supplemented by a combustion chamber. By selecting a suitable material for the turbine blades, a hot gas temperature of around 750°C might be possible. Together with an efficient recuperator, which is easy feasible with turbocharged processes, and a pre cooler for the compressor, efficiencies in the region of 35% could be obtained. This cycle system would need a second turbocharger of roughly equal size to charge the cycle. Starting at ambient pressure, this would supply around a quarter of the cycle mass flow - in other words the necessary combustion air. A pressure ratio of 4 would be suitable for both of these. In this case, the outlet of the charging blower would end directly in front of the pre cooler. If there were inter cooled compressors, the outlet might end in the recuperator, as mentioned with Fig. 146.
An electrical synchronous machine connected to the grid via frequency converters (possibly with a permanent magnetic rotor) could be used on small plants both as a generator and as a starter motor. Figure 150 shows these gas turbine systems in both single shaft and split shaft arrangements. The charger could be started either by slightly throttling the low pressure flow path in the recuperator between the inlet and outlet of the turbo set, or by installing a small starter motor.