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Combined Cycle Plant

A combined-cycle plant uses a gas turbine's hot exhaust to raise steam for a bottoming Rankine cycle; combined efficiency η_CC ≈ η_GT + η_ST − η_GT·η_ST reaches ~55–60 %, per P.K. Nag.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Bottoming steam cycle utilises GT exhaust (400–600°C)
  • Single shaft vs multi-shaft configurations
  • Fast start GT + efficient steam cycle = best of both

Topic details

Introduction

The combined-cycle gas turbine (CCGT) plant is the most efficient fossil generation technology, widely built in India. It couples a Brayton (gas) topping cycle with a Rankine (steam) bottoming cycle via a heat-recovery steam generator (HRSG).

Scope in B.Tech and GATE syllabus

The gas turbine generates power and exhausts hot gas; the HRSG recovers that heat to raise steam for a steam turbine, so one fuel input drives two cycles. This cascading of heat lifts overall efficiency well above either cycle alone.

Why this topic matters in practice

The combined efficiency formula captures how the bottoming cycle recovers the topping cycle's rejected heat. Computing combined efficiency and understanding the HRSG link are the exam tasks.

Key relations & formulas

ηCC=ηGT+ηST−ηGT⋅ηST\eta_{CC} = \eta_{GT} + \eta_{ST} - \eta_{GT}\cdot \eta_{ST}
(approximate combined)

Formulas (Indian textbook notation)

  • HRSG:exhaustgasheatsfeedwater→steamforbottomingcycleHRSG: exhaust gas heats feedwater → steam for bottoming cycle
ηCC≈55−60\eta_{CC} \approx 55-60%
(modern plants)

Formulas (Indian textbook notation)

  • Powersplit:GT 60Power split: GT ~60%, ST ~40% of total output typical

Notation and sign conventions

Relation 1 —
ηCC=ηGT+ηST−ηGT⋅ηST\eta_{CC} = \eta_{GT} + \eta_{ST} - \eta_{GT}\cdot \eta_{ST}
ηCC=ηGT+ηST−ηGT⋅ηST\eta_{CC} = \eta_{GT} + \eta_{ST} - \eta_{GT}\cdot \eta_{ST}
(approximate combined)
Write this relation with symbols exactly as in Power Plant Engineering — P.K. Nag before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
HRSG:exhaustgasheatsfeedwater→steamforbottomingcycleHRSG: exhaust gas heats feedwater → steam for bottoming cycle

Formulas (Indian textbook notation)

  • HRSG:exhaustgasheatsfeedwater→steamforbottomingcycleHRSG: exhaust gas heats feedwater → steam for bottoming cycle
Write this relation with symbols exactly as in Power Plant Engineering — P.K. Nag before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
ηCC≈55−60\eta_{CC} \approx 55-60%
ηCC≈55−60\eta_{CC} \approx 55-60%
(modern plants)
Write this relation with symbols exactly as in Power Plant Engineering — P.K. Nag before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 4 —
Powersplit:GT 60Power split: GT ~60%, ST ~40% of total output typical

Formulas (Indian textbook notation)

  • Powersplit:GT 60Power split: GT ~60%, ST ~40% of total output typical
Write this relation with symbols exactly as in Power Plant Engineering — P.K. Nag before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

In a combined cycle the gas turbine (high-temperature topping cycle) does work and rejects heat in its exhaust at several hundred degrees. Rather than wasting it, the heat-recovery steam generator uses this exhaust to produce steam for a steam-turbine (bottoming) cycle.

Governing relations in practice

Because the bottoming cycle converts part of the topping cycle's rejected heat into extra work, the overall efficiency exceeds either alone: η_CC = η_GT + η_ST − η_GT·η_ST, derived from cascading the heat streams. Modern CCGTs reach 55–60 %.

Design and analysis considerations

The HRSG is the key link; multi-pressure HRSGs extract more energy from the exhaust temperature profile. No extra fuel is burned in the bottoming cycle (unless supplementary firing is used).

Advanced theory and extensions

Advantages are high efficiency, moderate emissions per kWh, and operational flexibility; the main requirement is gas/liquid fuel and cooling for the steam condenser. Computing η_CC and appreciating the topping-bottoming cascade are the core competencies.

Assumptions and validity limits

State assumptions explicitly before using any relation for combined cycle plant — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Power Plant Engineering viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Power Plant Engineering papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to combined cycle plant.
4. Use equation 1:
ηCC=ηGT+ηST−ηGT⋅ηST\eta_{CC} = \eta_{GT} + \eta_{ST} - \eta_{GT}\cdot \eta_{ST}
.
5. Use equation 2:
HRSG:exhaustgasheatsfeedwater→steamforbottomingcycleHRSG: exhaust gas heats feedwater → steam for bottoming cycle
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Combined Cycle Plant appears in thermal and combined-cycle plants. In Indian mechanical curricula this topic is tested because it connects theory to steam and gas-based power generation.
GATE and semester exams often combine combined cycle plant with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use combined cycle plant?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Adding η_GT and η_ST directly without the −η_GT·η_ST correction
• Forgetting the HRSG is what links the two cycles
• Assuming the bottoming cycle needs its own major fuel input
• Confusing combined-cycle with cogeneration (heat + power)

Quick revision checklist

Before attempting combined cycle plant problems, confirm you can:
1. Bottoming steam cycle utilises GT exhaust (400–600°C)
2. Single shaft vs multi-shaft configurations
3. Fast start GT + efficient steam cycle = best of both
Revise the solved examples in Power Plant Engineering — P.K. Nag and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Combined-cycle efficiency

Problem

A gas turbine has efficiency η_GT = 0.38 and the bottoming steam cycle η_ST = 0.30. Find the combined-cycle efficiency.

Solution

η_CC = η_GT + η_ST − η_GT·η_ST = 0.38 + 0.30 − (0.38 × 0.30) = 0.68 − 0.114 = 0.566, i.e. 56.6 %.

Conceptual check — Combined Cycle Plant

Problem

In a Power Plant Engineering semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of combined cycle plant." What should a complete answer include?

Practice questions

Most-asked interview and GATE questions for this topic — expand any item for a model answer.

  1. 1
    What is Combined Cycle Plant, and why does it appear in B.Tech / GATE syllabi?

    Model answer

    A combined-cycle plant uses a gas turbine's hot exhaust to raise steam for a bottoming Rankine cycle; combined efficiency η_CC ≈ η_GT + η_ST − η_GT·η_ST reaches ~55–60 %, per P.K. Nag.
  2. 2
    State the relation η_CC = η_GT + η_ST − η_GT·η_ST and name each symbol.

    Model answer

    The governing relation is ηCC=ηGT+ηST−ηGT⋅ηST\eta_{CC} = \eta_{GT} + \eta_{ST} - \eta_{GT}\cdot \eta_{ST}. Write every symbol with SI units before substituting numbers.
  3. 3
    State the relation HRSG: exhaust gas heats feedwater → steam for bottoming cycle and name each symbol.

    Model answer

    The governing relation is HRSG:exhaustgasheatsfeedwater→steamforbottomingcycleHRSG: exhaust gas heats feedwater → steam for bottoming cycle. Write every symbol with SI units before substituting numbers.
  4. 4
    State the relation η_CC ≈ 55–60% and name each symbol.

    Model answer

    The governing relation is ηCC≈55−60\eta_{CC} \approx 55-60%. Write every symbol with SI units before substituting numbers.
  5. 5
    State the relation Power split: GT ~60%, ST ~40% of total output typical and name each symbol.

    Model answer

    The governing relation is Powersplit:GT 60Power split: GT ~60%, ST ~40% of total output typical. Write every symbol with SI units before substituting numbers.
  6. 6
    Explain: Bottoming steam cycle utilises GT exhaust (400–600°C)

    Model answer

    Bottoming steam cycle utilises GT exhaust (400–600°C) — state the assumption range and one exam trap linked to this point.
  7. 7
    Explain: Single shaft vs multi-shaft configurations

    Model answer

    Single shaft vs multi-shaft configurations — state the assumption range and one exam trap linked to this point.
  8. 8
    Explain: Fast start GT + efficient steam cycle = best of both

    Model answer

    Fast start GT + efficient steam cycle = best of both — state the assumption range and one exam trap linked to this point.
  9. 9
    How would you correct this error in a viva: Adding η_GT and η_ST directly without the −η_GT·η_ST correction?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  10. 10
    How would you correct this error in a viva: Forgetting the HRSG is what links the two cycles?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  11. 11
    How would you correct this error in a viva: Assuming the bottoming cycle needs its own major fuel input?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  12. 12
    How would you correct this error in a viva: Confusing combined-cycle with cogeneration (heat + power)?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.

Exams & GATE

  • 1
    P.K. Nag — sketch combined cycle layout with HRSG linking cycles.
  • 2
    Avoid: Adding η_GT and η_ST directly without the −η_GT·η_ST correction
  • 3
    Avoid: Forgetting the HRSG is what links the two cycles
  • 4
    Avoid: Assuming the bottoming cycle needs its own major fuel input

📖 Standard books (India)

  • Power Plant Engineering — P.K. Nag

    Read: Syllabus unit

    Steam, gas turbine, and plant economics

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