Absorption refrigeration cycle was used before vapor-compression refrigeration became popular, especially in factories for cooling and making ice. When electric compressors came into the picture, vapor-compression systems took over because they could cool more effectively. They also used cheaper refrigerants and were safer.
Nevertheless, absorption refrigeration still has its place in situations when low noise levels is important or when there’s a cheaper energy source available.
Here’s how absorption refrigeration works:
When you add heat to a liquid, its temperature gradually rises until it hits a certain point. Once it exceeds that point, the liquid changes into a gas through evaporation, absorbing heat as it does so. Remove that heat, and the gas turns back into a liquid through condensation.
This principle lets us move heat from the area we want to cool to the outside using a special refrigerant. It’s all part of a cycle that keeps repeating until we reach the temperature we want.
A basic absorption refrigeration cycle has a few key parts: evaporator, absorbent, generator, expansion valve, pump, and refrigerant (usually a mix of water and ammonia or water and lithium bromide).
How absorption refrigeration cycle works using water and ammonia:
- Ammonia at low pressure comes out of the expansion valve and goes into the evaporator.
- The evaporator absorbs heat from the cooling area, making the ammonia turn into a vapor.
- The vapor goes to the absorber.
- In the absorber, the vapor mixes with water.
- The mix gets pumped to the generator.
- The generator uses heat from outside to raise the pressure of the mix.
- Water separates from the ammonia gas and goes back to the absorber.
- The ammonia gas goes to the condenser.
- Heat gets released outside, turning the ammonia into a liquid.
- The liquid ammonia goes through the expansion valve, entering the evaporator at low pressure again.
- The cycle starts over, moving heat from the cooling area to the outside until we reach the right temperature.
Comparing the operation of the absorption cycle with the compression cycle:
- The absorption cycle relies on thermal, physical, and chemical processes of the refrigerant, while the compression cycle depends on adding energy through the electrical compressor.
- The refrigerant consists of two substances, for example, (water-ammonia) or (water-lithium bromide), while in the compression cycle, there is only one refrigerant.
- The initial cost of an absorption device is high relative to the generated capacity, but the operating cost may be lower if we rely on a cheap or free heat source such as solar energy.
- The absorption cycle system is quieter than the compression cycle due to the absence of moving parts.
- Maintenance costs are lower than the compression cycle because there are no moving parts that can cause mechanical and electrical failures, oil changes, and so on.
Conclusion
We use this method for cooling in small office refrigerators, in some air conditioning chillers, and when we can utilize solar heaters.