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HKIE Professional Assessment - Energy Discipline study materials
HKIE Professional Assessment - Energy Discipline study resources

HKIE Professional Assessment - Energy Discipline: Energy Conversion Systems and Power Plant Engineering

HKIE Professional Assessment - Energy Discipline · Work through your exam, one topic at a time.

All guide sections

Energy Conversion Systems and Power Plant Engineering

Domain 2 tests how well you can convert thermodynamic principles into plant performance: cycle selection, heat rate, boiler and turbine efficiency, condenser vacuum and auxiliary power. Questions often ask you to compare a proposed plant with an existing one, or to explain why sent-out efficiency differs from the design figure.

Core rules

  • The Rankine cycle is the reference for steam plant. Reheat and regenerative feed heating raise the mean temperature of heat addition and reduce irreversibility; they do not beat the Carnot limit.
  • The Brayton cycle governs gas turbines. A combined cycle uses the gas-turbine exhaust in a heat-recovery steam generator (HRSG) to drive a steam turbine; modern CCGTs reach about 57–62% LHV efficiency, while open-cycle gas turbines are typically 35–42%.
  • Thermal efficiency = 3,600/heat rate, where heat rate is in kJ/kWh. State whether the heat rate is gross or sent-out. Auxiliary power of 5–8% is normal for steam plant; sent-out output is gross output minus auxiliary load.
  • Boiler efficiency can be direct (heat output/fuel energy input) or indirect (100% minus stack, radiation and unburned losses). Turbine performance is expressed by isentropic or polytropic efficiency; condenser vacuum matters because it sets the low-temperature sink.

Worked example

A unit has a sent-out heat rate of 7,200 kJ/kWh. Its sent-out efficiency is 3,600/7,200 = 50%. For 500 MW sent out over 24 hours, energy = 500 × 24 = 12,000 MWh = 12,000,000 kWh. Fuel energy = 12,000,000 × 7,200 kJ = 86.4 × 10^9 kJ = 86.4 TJ. If the fuel has an LHV of 50 MJ/kg, fuel mass = 86.4 × 10^9/50 × 10^6 = 1,728 tonnes. If the stated heat rate were gross, you would first subtract auxiliary power before calculating sent-out efficiency.

Traps

Mixing HHV and LHV; quoting gross efficiency as sent-out; applying design heat rate at part load; ignoring condenser back-pressure degradation; treating reheat as a way to exceed Carnot; and forgetting that CCGT output falls as ambient temperature rises.