In absorption refrigeration systems, calcium bromide (CaBr₂) typically serves as an absorbent, paired with a refrigerant (such as water) to form a CaBr₂-H₂O working pair. Its function is based on the absorption-desorption cycle of the solution, with the following key processes:
1. Core Working Pair: CaBr₂-H₂O
- Refrigerant: Water (H₂O), which evaporates under low pressure, absorbing heat to provide cooling.
- Absorbent: Calcium bromide (CaBr₂), which strongly absorbs water vapor to maintain low pressure in the system.
2. Cycle Steps and the Role of CaBr₂
(1) Evaporation
- Liquid water evaporates in the evaporator (low-pressure zone), absorbing heat from the surroundings (cooling effect).
- The resulting water vapor must be quickly removed to sustain low pressure, which is where CaBr₂ comes into play.
(2) Absorption
- A concentrated CaBr₂ solution in the absorber absorbs the water vapor, forming a dilute solution and releasing heat (which must be dissipated via cooling).
- Due to its high hygroscopicity (strong solubility), CaBr₂ efficiently captures water vapor, maintaining the evaporator’s low pressure.
(3) Regeneration
- The diluted CaBr₂ solution is pumped into the generator, where an external heat source (e.g., waste heat or solar energy) evaporates the water from the solution.
- The water vapor moves to the condenser to liquefy, while the reconcentrated CaBr₂ solution returns to the absorber, completing the cycle.
3. Advantages of CaBr₂
- High solubility: Can absorb large amounts of water vapor even at low temperatures, improving system efficiency.
- Stability: Chemically stable and resistant to decomposition, suitable for long-term cycling.
- Eco-friendliness: Non-toxic and non-explosive compared to ammonia (NH₃)-based systems.
- Applications: Ideal for small-scale refrigeration systems driven by low-grade heat (e.g., industrial waste heat or solar energy).
4. Comparison with Other Absorbents
| Absorbent | Refrigerant | Pros | Cons |
|---|---|---|---|
| CaBr₂ | H₂O | Non-toxic, low-pressure operation | Limited cooling temperature (≥0°C) |
| LiBr | H₂O | High efficiency, widely commercialized | Prone to crystallization, expensive |
| NH₃ | H₂O | Suitable for low temps | Toxic, requires high pressure |
5. Practical Applications
- Solar cooling: Uses solar energy to regenerate the CaBr₂ solution, driving the refrigeration cycle.
- Waste heat recovery: Industrial waste heat powers the generator, enabling energy reuse.
6. Challenges
- Corrosiveness: CaBr₂ solutions may corrode metal components, requiring corrosion inhibitors or resistant materials (e.g., stainless steel).
- Lower COP (Coefficient of Performance): Efficiency depends on heat source temperature, necessitating system optimization.
