An absorption chiller is a refrigeration system that uses a heat source (e.g., steam, hot water, or gas) to drive the cooling process, unlike vapor-compression chillers that rely on mechanical compression. It operates on the absorption refrigeration cycle, leveraging the ability of certain fluids to absorb and release refrigerants when heated or cooled. Below is a detailed explanation of its working principle.
Working Principle of an Absorption Chiller
The absorption chiller cycle typically uses a lithium bromide-water (LiBr-H₂O) or ammonia-water (NH₃-H₂O) solution, where water or ammonia acts as the refrigerant, and lithium bromide or water acts as the absorbent. The process involves four main stages: generation, condensation, evaporation, and absorption. Here’s how it works:
- Generation (Generator):
- A heat source (e.g., steam, hot water, natural gas, or waste heat) is applied to a solution of refrigerant and absorbent (e.g., LiBr-H₂O) in the generator.
- The heat causes the refrigerant (water in LiBr systems, ammonia in NH₃ systems) to evaporate, separating it from the absorbent.
- The refrigerant vapor moves to the condenser, while the concentrated absorbent solution (e.g., strong LiBr solution) is pumped or flows to the absorber.
- Condensation (Condenser):
- The refrigerant vapor enters the condenser, where it releases heat to a cooling medium (typically water from a cooling tower or air).
- As the vapor loses heat, it condenses into a liquid refrigerant at high pressure.
- The liquid refrigerant then flows to the expansion valve.
- Evaporation (Evaporator):
- The liquid refrigerant passes through an expansion valve, reducing its pressure and temperature.
- In the evaporator, the low-pressure refrigerant absorbs heat from the fluid to be cooled (e.g., water for air conditioning or industrial processes).
- The refrigerant evaporates into a low-pressure vapor, cooling the fluid, which is then circulated to the target application (e.g., HVAC system).
- Absorption (Absorber):
- The refrigerant vapor from the evaporator enters the absorber, where it is absorbed by the concentrated absorbent solution (e.g., LiBr) from the generator.
- This absorption process releases heat, which is removed by a cooling medium (often water from a cooling tower).
- The diluted solution (refrigerant + absorbent) is pumped back to the generator, completing the cycle.
Key Components of an Absorption Chiller
- Generator: Heats the refrigerant-absorbent solution to separate the refrigerant vapor.
- Condenser: Cools and condenses the refrigerant vapor into a liquid.
- Evaporator: Allows the refrigerant to absorb heat from the fluid being cooled, producing the chilling effect.
- Absorber: Combines the refrigerant vapor with the absorbent, forming a diluted solution.
- Heat Exchanger: Improves efficiency by preheating the diluted solution (from the absorber to the generator) using the hot concentrated solution (from the generator to the absorber).
- Pump: Circulates the solution between the absorber and generator.
- Expansion Valve: Reduces the pressure of the liquid refrigerant before it enters the evaporator.
- Cooling Medium: Typically water from a cooling tower, used in the condenser and absorber to remove heat.
Types of Absorption Chillers
- Single-Effect Absorption Chiller:
- Uses one generator and a single heat source.
- Typical COP (Coefficient of Performance): 0.6–0.8.
- Suitable for low-temperature heat sources (e.g., 80–100°C hot water).
- Double-Effect Absorption Chiller:
- Uses two generators to improve efficiency by utilizing higher-temperature heat sources (e.g., 150–180°C steam).
- Typical COP: 1.0–1.2.
- More complex but more efficient.
- Direct-Fired Absorption Chiller:
- Uses a gas burner or other direct heat source instead of steam or hot water.
- Common in regions with high electricity costs or limited power availability.
Refrigerant-Absorbent Pairs
- Lithium Bromide-Water (LiBr-H₂O):
- Water is the refrigerant, LiBr is the absorbent.
- Used in air conditioning and large-scale industrial cooling (above 0°C).
- Cannot operate at sub-zero temperatures due to water freezing.
- Ammonia-Water (NH₃-H₂O):
- Ammonia is the refrigerant, water is the absorbent.
- Suitable for industrial refrigeration requiring lower temperatures (below 0°C).
- Common in food processing or cold storage.
Advantages of Absorption Chillers
- Uses waste heat or low-cost heat sources, reducing electricity consumption.
- Environmentally friendly due to minimal use of mechanical components and eco-friendly refrigerants (e.g., water).
- Quiet operation with fewer moving parts compared to vapor-compression chillers.
- Ideal for facilities with access to steam, hot water, or waste heat (e.g., cogeneration plants).
Disadvantages of Absorption Chillers
- Lower COP (0.6–1.2) compared to vapor-compression chillers (COP 3.0–6.0).
- Larger footprint and higher initial cost.
- Requires a cooling tower for heat rejection, increasing maintenance.
- Performance depends on the quality and availability of the heat source.
Applications
- HVAC Systems: Cooling large buildings, hospitals, or data centers where waste heat is available.
- Industrial Processes: Cooling in chemical plants, food processing, or manufacturing.
- Cogeneration Systems: Utilizing waste heat from power generation.
- Solar Cooling: Paired with solar thermal systems for sustainable cooling.
Example of Operation (LiBr-H₂O System)
- Heat Input: Steam at 120°C heats a LiBr-water solution in the generator, vaporizing water (refrigerant).
- Condensation: Water vapor condenses in the condenser at ~40°C, rejecting heat to cooling tower water.
- Evaporation: Liquid water expands to a low pressure (~6–8 mmHg) in the evaporator, absorbing heat from the chilled water (e.g., cooling it from 12°C to 7°C).
- Absorption: Water vapor is absorbed by a concentrated LiBr solution in the absorber, forming a dilute solution, which is pumped back to the generator.
- Cycle Efficiency: A single-effect chiller might produce 100 tons of cooling with ~150 kW of heat input, achieving a COP of ~0.7.
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