With the lower refrigeration temperature fixed, which action improves the coefficient of performance (COP) of a refrigeration cycle?

Introduction / Context:
The coefficient of performance (COP) for a refrigerator is Q_L/W, where Q_L is heat removed at the low temperature and W is compressor work input. The temperature lift between evaporator and condenser largely determines the work required. Reducing this lift generally improves efficiency.

Given Data / Assumptions:

• Evaporator (low) temperature is fixed by process/comfort requirement.
• Condenser rejects heat to ambient; its temperature depends on heat exchanger effectiveness and ambient conditions.

Concept / Approach:
For an idealized reversed Carnot refrigerator, COP = T_L / (T_H − T_L). With T_L fixed, decreasing T_H increases COP. Practical vapor-compression systems follow the same trend: better heat rejection (lower condensing temperature via larger condenser, improved airflow, cooler ambient) reduces compression ratio and work input.

Step-by-Step Solution:

Verification / Alternative check:
Performance maps of compressors show lower power at lower condensing temperatures for the same evaporating condition, confirming the theoretical trend.

Why Other Options Are Wrong:

• Changing speed alone does not guarantee higher COP; it affects capacity more than fundamental cycle efficiency.
• Raising T_H increases lift and reduces COP.

Common Pitfalls:
Confusing capacity changes (kW of cooling) with efficiency; a system can deliver more capacity at higher speed but worse COP.

Final Answer:
Lower the higher (condensing) temperature