Questions: Combined Cycle Systems and Cogeneration

Question 1

A standalone gas turbine plant achieves 40% thermal efficiency. A standalone steam plant achieves 35%. An engineer proposes combining them into a combined cycle where gas turbine exhaust drives a steam generator. Which efficiency outcome best describes the combined plant?

• A. Approximately 75% — the combined cycle adds both efficiencies since two independent cycles are running
• B. Approximately 40% — efficiency is limited by the Brayton topping cycle, and the bottoming cycle cannot exceed it
• C. Approximately 60–65% — the steam turbine generates additional electricity from exhaust heat that would otherwise be wasted, without burning more fuel
• D. Approximately 35% — the combined system is limited by the Rankine bottoming cycle since that is the final energy converter

Answer: (C)

Combined cycle efficiency is not the sum of the two individual efficiencies — that would violate the first law. The key insight is that the steam turbine is powered by exhaust heat that would otherwise be vented to atmosphere. The fuel energy produces Brayton cycle work (~40% of input) plus exhaust heat (~50% of input at 500–600°C). Capturing that exhaust heat to run a steam cycle produces additional work without burning any more fuel. The combined efficiency ends up around 60–65%, not because both cycles magically add up, but because less energy is wasted as hot exhaust.

Question 2

In a combined cycle plant, reducing the HRSG 'pinch point' temperature difference from 20°C to 5°C would have what effect?

• A. No effect on plant efficiency, since the pinch point only affects condenser sizing at the cold end
• B. Decrease efficiency because a smaller temperature difference reduces the thermodynamic driving force for heat transfer, slowing steam generation
• C. Increase steam generation from the exhaust heat and improve bottoming cycle output, but require a larger and more expensive heat exchanger
• D. Allow the gas turbine to operate at higher turbine inlet temperatures, increasing topping cycle efficiency

Answer: (C)

The pinch point is the minimum temperature difference between the flue gas and the steam at any cross-section of the HRSG. A tighter pinch means the gas can be cooled closer to the steam temperature — extracting more heat and generating more steam. This directly increases bottoming cycle power output. But the rate of heat transfer is Q = U·A·ΔT_lm, so for the same heat duty, a smaller ΔT requires a larger heat exchanger area (and cost). This is a classic tradeoff: tighter pinch = higher efficiency + higher capital cost. Typical designs use 10–15°C pinch as a balance point.

Question 3

In a combined cycle plant, the gas turbine exhaust is used to generate steam rather than being vented to atmosphere, and roughly 30–40% of the plant's total electricity comes from what would otherwise be wasted heat.

• T. True
• F. False

Answer: True

A gas turbine producing 40% efficiency has about 60% of the fuel energy leaving as hot exhaust. The steam cycle captures enough of this exhaust heat to generate an additional 20–25 percentage points of electrical efficiency — bringing the total to 60–65%. Relative to the gas turbine alone, roughly one-third of total output comes from steam. This is why combined cycle plants dominate new gas-fired power generation: the 'free' steam electricity dramatically improves fuel economics with only modest additional capital cost (the HRSG and steam turbine).

Question 4

Cogeneration achieves higher electrical efficiency than a standard combined cycle plant because it extracts more turbine work from the steam before condensing it.

• T. True
• F. False

Answer: False

Cogeneration sacrifices electrical efficiency in exchange for total energy utilization. Instead of expanding steam fully through the turbine (maximizing electricity), cogeneration extracts steam at an intermediate pressure and supplies it as process heat or district heating — consuming the steam's latent heat usefully rather than rejecting it in the condenser. This reduces electricity output but raises total energy utilization from ~60% to over 80%, because the latent heat that a pure power plant discards in the condenser now does useful thermal work. Cogeneration optimizes total energy value, not electricity output.

Question 5

Why can a combined cycle plant achieve 60–65% thermal efficiency when neither the gas turbine alone nor the steam cycle alone can exceed about 42%, and what role does the HRSG play in this?

Model answer: A gas turbine exhausts large amounts of heat at 500–600°C — far too hot to simply discard but too cool for further gas turbine work. The HRSG is a heat exchanger that captures this exhaust heat and uses it to generate and superheat steam, which then drives a steam turbine to produce additional electricity. No extra fuel is burned for the steam cycle — it runs entirely on heat that would otherwise be wasted. The combined plant extracts work from a wider temperature range than either cycle alone can span, approaching the limit set by the highest gas turbine temperature and the cold condenser. The HRSG pinch point constrains how efficiently the handoff happens.

The thermodynamic principle is simple: efficiency rises when you operate over a wider temperature range. The Brayton cycle tops out at combustion temperature (~1400°C) but rejects heat at 500–600°C instead of at ambient. The Rankine cycle accepts heat at 500–600°C and rejects at ~40°C (condenser). Together they span from ~1400°C to ~40°C — a far wider range than either alone. The Carnot efficiency for this range is approximately 1 − (313 K / 1673 K) ≈ 81%, so the 60–65% achievable in real systems represents capturing much of the theoretical maximum beyond what single-cycle designs can reach.