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This explanation of the control and operational performance is based on a diagram of a helium turbine without inter cooling for the compressor. The recuperator necessary for all gas turbines preheats the helium using turbine exhaust heat. Heat can be supplied to the cycle using fossil, nuclear or solar power, or energy gained using another process. This example considers to a helium turbine cycle integrated into a helium cooled high-temperature reactor. The gas turbine cycle and the entropy diagram can be seen in Fig. 64. Of course, these observations also apply to other working media, such as air.
At a constant turbine inlet temperature and fixed blade geometry, there are two possible control methods for the output and/or rotational speed. These are inventory or pressure level control, which allows the plant to be run at part load with almost unchanged efficiency, and compressor back flow or bypass control, which permits very rapid unloading and reloading of the shaft power, and therefore very precise control of the speed for synchronization and island operation. Figure 65 shows how these two control methods work, with the shading in the two diagrams showing the gas density in the cycle and the accumulator, and arrows indicating the energy flows.
With inventory control, the cycle system is connected to a gas accumulator. At full load, most of the gas inventory is in the gas turbine system, but at part load, some of it is in the accumulator. All the cycle pressures, and therefore the shaft power, behave proportionally to the gas density in the cycle.