In an Electrolux (three-fluid absorption) refrigerator fired by a small gas burner, how is the coefficient of performance (COP) defined?

Introduction / Context:
Absorption refrigeration systems are thermally driven; instead of mechanical compressor work, they use heat input to drive the cycle. The Electrolux refrigerator uses a gas flame with ammonia–water–hydrogen to achieve cooling. The coefficient of performance for such systems compares useful cooling to the thermal energy supplied.

Given Data / Assumptions:

• Evaporator provides useful refrigeration effect Q_evap.
• Gas burner supplies heat Q_in to the generator.
• Neglect minor auxiliary work (no mechanical compressor).

Concept / Approach:
By definition for a heat-driven refrigerator, COP = desired effect / required input. Here, desired effect is heat absorbed in the evaporator (cooling), and required input is heat added by the burner to the generator. Therefore, COP = Q_evap / Q_in.

Step-by-Step Solution:

Verification / Alternative check:
Textbook definitions for absorption COP consistently use Q_evap divided by total heat supplied to the generator/absorber as the performance index, aligning with the chosen option.

Why Other Options Are Wrong:

• Inverted or difference-based ratios do not represent COP and can give values < 0 or > 1 inappropriately.
• Pump work term is negligible and not the defining energy input for Electrolux systems.

Common Pitfalls:
Confusing mechanical COP definitions (based on compressor work) with thermally driven systems; mixing up which energy term belongs in the denominator.

Final Answer:
COP = (Heat absorbed by the evaporator) / (Heat supplied by the burner)