Solar-driven absorption refrigeration – Working pair For absorption refrigerators using heat from solar collectors, water is used with which absorbent as the standard working pair?

Introduction / Context:
Absorption refrigeration systems use a refrigerant–absorbent working pair instead of a mechanical compressor. Solar thermal heat is well suited to drive generators in absorption chillers. Knowing the correct pair is critical to understanding cycle design and operating envelopes.

Given Data / Assumptions:

• Low to medium temperature solar heat available (e.g., 80–100 °C).
• Cooling for air-conditioning, not sub-zero freezing.
• Common commercial absorption pairs considered.

Concept / Approach:
The water–lithium bromide (H₂O–LiBr) pair is widely used for comfort cooling. In this pair, water is the refrigerant and lithium bromide is the absorbent. It operates under vacuum with evaporator temperatures typically above 0 °C, making it ideal for chilled water air-conditioning.

Step-by-Step Solution:
Identify target application: solar-driven building cooling (above freezing).Select pair compatible with above-zero evaporator temperature: H₂O–LiBr.Confirm role: water flashes in the evaporator; LiBr solution absorbs water vapor in the absorber; generator heat (from solar) drives water out of the solution.

Verification / Alternative check:
Compare with NH₃–H₂O (ammonia–water) systems: those use ammonia as refrigerant and can reach below 0 °C but require higher generator temperatures; H₂O–LiBr is the norm for solar-assisted comfort cooling.

Why Other Options Are Wrong:

• Sulphur dioxide and R-12 are outdated mechanical compression refrigerants.
• Carbon dioxide is not used as an absorbent with water in standard absorption cycles.
• “Ammonia as absorbent” is incorrect; in NH₃–H₂O, water is the absorbent and ammonia is the refrigerant.

Common Pitfalls:
Mixing up which component is refrigerant versus absorbent; assuming LiBr systems can provide sub-zero refrigeration—they cannot due to water’s freezing point and vacuum constraints.

Final Answer:
Lithium bromide