Because fossil-fired closed-cycle gas turbines were so far operated with indirect heat supply via a heat exchanger system, namely an air heater, the heat resistance of the piping material used limited the hot air temperature. Therefore, in order to achieve good levels of efficiency, all installations were equipped with turbine waste heat recovery in a recuperator and intercoolers for the compressor. These components had to be connected to each other with sufficiently large pipes, which meant that the hot air pipes required a great deal of interior insulation. A special compressor set was required for charging the closed working cycle.
This arrangement meant that the turbo set only took up a relatively small portion of the plant costs. Around 40% was accounted for by the gas or air heater alone. Thus, the costs of these power plants were roughly the same of those for steam power plants with the same output. Therefore it is understandable that these plants were mainly installed as heating power plants, where the waste heat that was generated anyway by the cycle cooling system could be used to supply district heating. Unlike the condenser waste heat of a steam power plant, which is practically cold, the coolers of a closed-cycle gas turbine can deliver heating water supplied at 90°C. This heat, roughly the same quantity of energy as the electrical output at the generator terminals, is delivered without the need of extra fuel.
The following section describes the main components of a closed-cycle gas turbine.
