Regenerative Brayton Cycle

The general idea of preheating combustion air using exhausted waste heat is that of a regenerator in a Brayton Cycle. The gas turbine exhaust temperature in a Brayton cycle is generally greater than the ambient temperature. A method of recovering this heat, which otherwise would be lost to ambient, is to install a heat exchanger between the pre-combustion air line and the turbine exhaust line. This heat exchanger, called a regenerator, allows preheating of high pressure air before entering the combustor, thereby lowering the amount of fuel required. An ideal regenerative Brayton Cycle and its corresponding T-S diagram are shown in the figures below:

From the figures above, it is clear that the air being heated up in the regenerator will most likely achieve a temperature lower than the temperature of the turbine exhaust. At best, the highest temperature that could be obtained coming out of the regenerator would be the turbine exhaust temperature if the regenerator size is infinitely large. This is a global maximum temperature constraint due to the hot stream inlet temperature driving the heat transfer in the regenerator.