Short Answer:

Absorption refrigeration systems are driven by heat instead of mechanical work. In these systems, a heat source such as steam, hot water, gas flame, or solar energy provides the energy required to separate the refrigerant from the absorbent in the generator. This heat causes the refrigerant to evaporate, creating vapor that later produces cooling in the evaporator.

The system works efficiently in places where waste heat or low-grade heat is available. By using thermal energy instead of electricity to drive the cycle, absorption systems reduce electrical energy consumption and are suitable for industrial processes and large air-conditioning applications.

Detailed Explanation :

Heat-Driven Absorption Systems

Absorption refrigeration systems are unique because they rely on heat to operate rather than mechanical compressors. The basic principle is that heat is applied to the generator, which separates the refrigerant from the absorbent. This process creates high-pressure refrigerant vapor that moves through the system to provide cooling. Heat can come from various sources, such as natural gas, steam, hot water, solar energy, or industrial waste heat.

Working Process

Generator:
The generator is where the heat is applied. In an ammonia–water system, for example, heat causes the ammonia (refrigerant) to vaporize from the water (absorbent). In a water–lithium bromide system, heat causes water to evaporate from lithium bromide. This step is critical because the supplied heat provides the energy required to separate the refrigerant without using a mechanical compressor.
Condenser:
The high-pressure refrigerant vapor flows from the generator to the condenser, where it releases heat to the surrounding environment and condenses into a liquid. This step is similar to compression-based refrigeration but is driven indirectly by heat instead of mechanical energy.
Expansion and Evaporation:
The condensed refrigerant passes through an expansion device, which lowers its pressure. Then it enters the evaporator, where it absorbs heat from the space or fluid that needs cooling. The absorption of this heat produces the refrigeration effect.
Absorber:
After evaporation, the refrigerant vapor enters the absorber, where it is absorbed by the absorbent, creating a solution. The heat of absorption released during this process helps maintain the circulation of the solution and supports continuous operation.
Circulation:
The absorbent–refrigerant solution is pumped back to the generator, and the cycle repeats. The only external energy needed is heat for the generator and minimal pumping energy for the solution circulation.

Sources of Heat

Solar Energy: Solar collectors can provide heat to drive absorption systems in buildings or remote areas.
Waste Heat: Industrial processes often produce low-grade heat that can be reused to operate absorption chillers.
Fuel Combustion: Natural gas, LPG, or steam boilers are commonly used to supply heat for generators in commercial systems.

Advantages of Heat-Driven Systems

Reduced electrical consumption compared to compression systems.
Can utilize waste heat or renewable energy sources.
Suitable for large-scale air conditioning, industrial cooling, and district cooling systems.
Quiet operation due to lack of mechanical compressors.

Limitations

Generally lower coefficient of performance (COP) compared to mechanical systems.
Large system size is often required for the same cooling capacity.
Proper heat management is essential to maintain efficient operation.

Conclusion

Absorption systems are driven by heat supplied to the generator, which separates the refrigerant from the absorbent. This heat-driven principle enables the system to provide refrigeration without relying heavily on electrical energy. By using solar energy, waste heat, or other thermal sources, these systems are efficient, environmentally friendly, and suitable for industrial and commercial cooling applications. Proper design and heat management are critical for reliable and effective operation.