The efficiency of a Rankine cycle can be improved by _______.

Concept:

Lowering the condenser pressure

• The temperature of heat rejection for cycle 1–2–3–4–1 condensing at atmospheric pressure is 100°C.  The temperature of heat rejection for the lower pressure cycle 1–2”–3”–4”–1 is correspondingly lower, so this cycle has the greater thermal efficiency.
• It follows that decreasing the condenser pressure tends to increase the thermal efficiency.

Superheating the steam to high temperatures

• The cycle with superheat (1’–2’–3–4–1’) has a higher average temperature of heat addition than the cycle without superheating (1–2–3–4–1), so the thermal efficiency is increased.

Increasing the boiler pressure

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• The average temperature of heat addition is seen to be greater for the higher-pressure cycle 1’–2’–3–4’–1’ than for cycle 1–2–3–4–1.
• It follows that increasing the boiler pressure of the ideal Rankine cycle tends to increase thermal efficiency.

Regenerative feed water heating​

• With regenerative feed water heating, the working fluid would enter the boiler at a state between 4 and 7. As a result, the average temperature of heat addition would be increased, thereby tending to increase the thermal efficiency.