In a combined cycle plant, how do gas turbine exhaust and steam turbine cycles interact for overall efficiency?

• A. Exhaust is vented to atmosphere.
• B. Steam is produced directly from gas turbine exhaust without HRSG.
• C. The gas turbine exhaust feeds a heat recovery steam generator (HRSG) to produce steam that drives the steam turbine.
• D. The cycles operate independently.

Answer: (C)

The main idea is that combining the cycles gains efficiency by reclaiming energy from the gas turbine exhaust. Even after the gas turbine exhaust leaves, it’s still very hot and carries useful heat. A heat recovery steam generator (HRSG) uses that heat to convert feedwater into steam. This steam then drives a steam turbine, producing additional electrical power without needing extra fuel. So the two cycles are tied together: the gas turbine provides high-temperature heat while the HRSG converts that heat into steam for the steam turbine, boosting overall plant efficiency. If the exhaust were simply vented, that energy would be wasted and efficiency would drop. Producing steam directly from exhaust without an HRSG isn’t practical, because you still need a proper heat exchanger and steam at the right pressure and temperature. The cycles are not independent; they operate as an integrated system where waste heat from one cycle becomes the energy source for the other.