Absorption refrigeration systems use heat to power cooling, unlike traditional systems that rely on mechanical compressors. They work with a binary solution, separating and recombining refrigerant and absorbent to create a cooling effect. This unique approach offers advantages in certain situations.

While less energy-efficient than vapor-compression systems, absorption refrigeration shines when waste heat is available. It's quieter, needs less maintenance, and can use eco-friendly refrigerants. These systems are great for large-scale cooling in industries or places with abundant heat sources.

Absorption Refrigeration Systems

Working Principle

• Absorption refrigeration systems use a heat source to provide the energy needed for the cooling process, unlike vapor-compression systems that rely on a mechanical compressor
• The working fluid in an absorption system is a binary solution, typically consisting of water (refrigerant) and lithium bromide (absorbent) or ammonia (refrigerant) and water (absorbent)
• The generator uses heat to separate the refrigerant from the absorbent solution, causing the refrigerant to vaporize and flow to the condenser
• In the evaporator, the liquid refrigerant absorbs heat from the cooled space and evaporates, providing the desired cooling effect
• The vaporized refrigerant then enters the absorber, where it is absorbed back into the weak absorbent solution, forming a strong solution

System Components

• The main components of an absorption refrigeration system include a generator, absorber, condenser, evaporator, and solution heat exchanger
• The condenser liquefies the refrigerant vapor by rejecting heat to the surroundings, and the liquid refrigerant then flows to the evaporator
• The strong solution is pumped to the generator, where the cycle repeats
• The solution heat exchanger is used to transfer heat between the strong and weak absorbent solutions, improving the system's efficiency

Absorption vs Vapor-Compression Systems

Performance Comparison

• Absorption refrigeration systems typically have lower coefficients of performance (COP) compared to vapor-compression systems, indicating lower energy efficiency
• The COP of absorption systems is usually in the range of 0.5 to 1.5, while vapor-compression systems can achieve COPs of 2 to 4 or higher
• Absorption systems require a heat source, such as natural gas, steam, or waste heat, to drive the refrigeration process, while vapor-compression systems primarily use electrical energy to power the compressor
• Vapor-compression systems typically have faster cooling rates and can achieve lower temperatures compared to absorption systems

System Characteristics

• Absorption systems have fewer moving parts compared to vapor-compression systems, resulting in lower maintenance requirements and longer lifespans
• Absorption systems are quieter in operation due to the absence of a mechanical compressor, making them suitable for applications where noise is a concern
• Absorption systems are more suitable for large-scale applications, such as industrial processes or district cooling, where waste heat is readily available

COP and Cooling Capacity

Coefficient of Performance (COP)

• The coefficient of performance (COP) is a measure of the efficiency of a refrigeration system, defined as the ratio of the cooling capacity to the heat input required to drive the system
• The COP of an absorption refrigeration system can be calculated using the formula: $COP = Q_c / Q_g$, where $Q_c$ is the cooling capacity and $Q_g$ is the heat input to the generator
• Factors that influence the COP of an absorption system include the operating temperatures of the generator, condenser, absorber, and evaporator, as well as the properties of the working fluid
• Higher generator temperatures generally lead to higher COPs, as more refrigerant can be separated from the absorbent solution
• Lower condenser and absorber temperatures improve the COP by increasing the efficiency of heat rejection and absorption processes

Cooling Capacity

• The cooling capacity of an absorption system is determined by the mass flow rate of the refrigerant and the enthalpy difference between the refrigerant entering and leaving the evaporator
• Increasing the mass flow rate of the refrigerant or the enthalpy difference across the evaporator can enhance the cooling capacity of the system

Advantages and Disadvantages of Absorption Systems

Advantages

• Can utilize waste heat or renewable energy sources, such as solar thermal or geothermal energy, reducing the reliance on electricity
• Lower operating costs when waste heat is readily available, as the primary energy input is heat rather than electricity
• Quieter operation compared to vapor-compression systems due to the absence of a mechanical compressor
• Longer lifespan and lower maintenance requirements due to fewer moving parts
• Environmentally friendly refrigerants, such as water and lithium bromide, have low global warming potential (GWP) and ozone depletion potential (ODP)

Disadvantages

• Lower energy efficiency compared to vapor-compression systems, resulting in higher primary energy consumption when waste heat is not available
• Larger system size and higher initial costs compared to vapor-compression systems of similar cooling capacity
• Slower response to cooling demand changes due to the thermal inertia of the system
• Limited temperature range and cooling capacity compared to vapor-compression systems
• Requires a consistent heat source at a sufficiently high temperature to drive the refrigeration process effectively

Applications

• Industrial processes with abundant waste heat (power plants, refineries, chemical plants)
• Solar cooling systems in regions with high solar radiation, using solar thermal collectors to drive the absorption process
• District cooling systems that can utilize waste heat from nearby industrial processes or power plants
• Refrigeration in remote or off-grid locations where electricity is scarce or unreliable, but heat sources are available
• Tri-generation systems that combine cooling, heating, and power generation to maximize overall energy efficiency