What primarily affects the thermal efficiency of a steam power plant?

• A. Boiler feedwater flow rate
• B. Condenser vacuum level
• C. Temperature difference between heat source and sink
• D. Pressure ratio across turbine

Answer: (C)

The main idea is that a steam power plant’s efficiency is governed by the temperature difference between the heat source (the boiler) and the heat sink (the condenser). By the Carnot-like limit, the largest possible efficiency depends on 1 minus the cold-side temperature divided by the hot-side temperature. In the Rankine cycle, heat is added at high temperature and rejected at low temperature, so widening that temperature gap increases the potential efficiency.

What you do with feedwater flow mainly affects how much steam you can generate (capacity) and the plant’s heat rate, not the fundamental efficiency ratio. The condenser vacuum level and the turbine’s pressure ratio influence how much work you can extract and how much heat you reject, but they operate within the efficiency set by the source-sink temperatures. Hence, the temperature difference between the heat source and sink primarily controls the thermal efficiency.