11.4 Absorption refrigeration systems
4 min read•july 30, 2024
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
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- Absorption refrigeration systems use a 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 (refrigerant) and (absorbent) or (refrigerant) and water (absorbent)
- The uses heat to separate the refrigerant from the absorbent solution, causing the refrigerant to vaporize and flow to the
- In the , the liquid refrigerant absorbs heat from the cooled space and evaporates, providing the desired cooling effect
- The vaporized refrigerant then enters the , 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 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: , where is the cooling capacity and 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
Key Terms to Review (23)
Absorber: An absorber is a crucial component in absorption refrigeration systems that serves to absorb the refrigerant vapor from the evaporator and convert it back into a liquid. This process involves the use of a suitable absorbent, which interacts with the refrigerant, allowing for heat removal from the refrigeration cycle. The efficiency of an absorber significantly impacts the overall performance and effectiveness of the refrigeration system.
Absorption refrigeration: Absorption refrigeration is a cooling process that utilizes a heat source to drive a refrigeration cycle, where heat is absorbed from a low-temperature environment and rejected to a high-temperature environment. This method typically employs a refrigerant and an absorbent in a system that allows the refrigerant to be absorbed by the absorbent, creating a solution that can be heated to release the refrigerant vapor. Unlike mechanical refrigeration that uses a compressor, absorption systems rely on thermal energy to achieve cooling, which makes them useful in various applications, including those where waste heat is available.
Air conditioning: Air conditioning is a process that cools, dehumidifies, and circulates air in a space to create a comfortable indoor environment. This technology is essential in modern buildings, allowing for climate control and improving the quality of life, especially in areas with extreme temperatures. Air conditioning systems can be classified into various types, including vapor-compression and absorption systems, each utilizing different thermodynamic principles to achieve cooling.
Ammonia: Ammonia is a colorless gas with a distinct odor, consisting of one nitrogen atom and three hydrogen atoms (NH₃). It is commonly used as a refrigerant and is a crucial component in various refrigeration cycles, particularly in gas refrigeration and absorption refrigeration systems. Its properties, including high latent heat of vaporization, make it an effective medium for heat transfer in cooling applications.
Carnot Cycle: The Carnot cycle is an idealized thermodynamic cycle that represents the most efficient possible heat engine operating between two temperature reservoirs. It provides a standard for measuring the performance of real engines and illustrates the principles of energy transfer, work, and heat efficiency in thermodynamic processes.
Coefficient of performance (COP): The coefficient of performance (COP) is a measure of the efficiency of a heating or cooling system, defined as the ratio of useful heating or cooling provided to the work input required to produce that effect. A higher COP indicates a more efficient system, whether it be for refrigeration, heat pumping, or air conditioning applications. Understanding COP is crucial in evaluating the performance of different thermodynamic cycles and systems used for temperature control.
Condenser: A condenser is a heat exchanger used to condense vapor into liquid by removing heat from the vapor, usually through contact with a cooler surface or fluid. This process is crucial in various thermodynamic cycles and systems, enabling efficient heat transfer and phase change essential for energy conversion, refrigeration, and air conditioning.
Evaporator: An evaporator is a heat exchanger that facilitates the phase change of a refrigerant from liquid to vapor, absorbing heat from the surroundings in the process. This essential component plays a critical role in refrigeration and air conditioning systems by removing heat from a space or substance, allowing for effective temperature control and cooling. The evaporator's operation directly influences the efficiency of refrigeration cycles and heat pumps.
Generator: In absorption refrigeration systems, a generator is a crucial component that facilitates the transfer of heat to drive the absorption process. It serves to separate the refrigerant from the absorbent by heating the mixture, allowing the refrigerant vapor to be removed. This process is essential for maintaining the cycle of refrigeration and ensuring efficient cooling in these systems.
Heat Sink: A heat sink is a device or substance that absorbs and dissipates heat from another object, helping to maintain optimal operating temperatures. In systems like absorption refrigeration, heat sinks play a crucial role in absorbing excess heat, allowing the refrigeration cycle to operate efficiently. By transferring heat away from critical components, they help improve the system's overall performance and longevity.
Heat source: A heat source is a system or component that provides thermal energy to another system, typically by raising its temperature. In absorption refrigeration systems, the heat source plays a vital role in supplying energy that drives the refrigeration cycle, allowing the system to remove heat from a designated area. Understanding the characteristics and function of heat sources is crucial for grasping how these systems operate effectively.
Industrial cooling: Industrial cooling refers to the processes and systems used to remove heat from industrial equipment, buildings, or processes to maintain optimal operating conditions. This is essential for preventing overheating, ensuring efficiency, and extending the life of machinery and products. Effective industrial cooling systems often involve specialized refrigeration techniques and technology to control temperature in large-scale operations.
Lithium bromide: Lithium bromide is a chemical compound commonly used as an absorbent in absorption refrigeration systems. It plays a crucial role in these systems by absorbing water vapor, enabling the cooling process to occur efficiently. Its unique properties make it suitable for use in large-scale cooling applications, particularly in chillers that rely on waste heat or low-grade energy sources.
Phase Change: Phase change refers to the transition of a substance from one state of matter to another, such as from solid to liquid or from liquid to gas. This process involves energy transfer, either absorbed or released, and is crucial in understanding how systems interact with energy in various forms. Recognizing phase changes helps in analyzing energy efficiency and conservation in different applications, including refrigeration and heating processes.
Plate heat exchanger: A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. These plates are arranged in a way that creates multiple channels for the fluids to flow, allowing for efficient heat transfer due to the large surface area provided by the plates. This design is particularly useful in absorption refrigeration systems, where effective heat exchange is critical for maintaining performance and energy efficiency.
Rankine cycle: The Rankine cycle is a thermodynamic cycle that converts heat into work through a series of processes involving a working fluid, typically water or steam. It consists of four main processes: isentropic compression, isobaric heat addition, isentropic expansion, and isobaric heat rejection, making it a foundational concept in the study of heat engines and energy conversion systems.
Refrigerated transport: Refrigerated transport refers to the process of moving goods at controlled low temperatures to preserve their quality and extend shelf life. This method is crucial for perishable items such as food, pharmaceuticals, and chemicals, ensuring that they remain safe and effective during transit. Maintaining optimal temperature control is vital, especially in the context of various refrigeration systems used to achieve efficient cooling, including absorption refrigeration systems.
Second Law of Thermodynamics: The Second Law of Thermodynamics states that the total entropy of an isolated system can never decrease over time, and it tends to increase, leading to the concept that energy transformations are not 100% efficient. This law establishes the directionality of processes, implying that certain processes are irreversible and energy has a quality that degrades over time, connecting tightly to concepts of heat transfer, work, and system analysis.
Shell-and-tube heat exchanger: A shell-and-tube heat exchanger is a type of heat exchanger that consists of a series of tubes, one set carrying the hot fluid and another set carrying the cold fluid, all enclosed within a shell. This design allows for efficient heat transfer between the fluids as they flow in separate paths, with the heat being exchanged through the walls of the tubes. It's commonly used in absorption refrigeration systems to transfer heat between refrigerants and absorbents, facilitating the cooling process.
Solution concentration: Solution concentration refers to the amount of solute that is dissolved in a given quantity of solvent or solution. It is a critical parameter in various applications, as it influences the behavior of solutions in different systems, such as absorption refrigeration. Understanding how concentration affects thermodynamic properties helps in optimizing processes and ensuring efficient operation.
Thermal efficiency: Thermal efficiency is a measure of how well an energy conversion system, such as a heat engine, converts heat energy into useful work. It is defined as the ratio of the useful work output to the heat input, typically expressed as a percentage. This concept is crucial for evaluating and optimizing the performance of various thermodynamic cycles and systems.
Vapor-compression refrigeration: Vapor-compression refrigeration is a widely used cooling process that involves the phase change of a refrigerant, moving it through a cycle of compression, condensation, expansion, and evaporation to remove heat from a designated area. This system relies on the properties of refrigerants to absorb heat during evaporation and release it during condensation, making it essential for various applications like air conditioning and refrigeration systems.
Water: Water is a vital substance that acts as a working fluid in various thermal systems, serving as the medium for heat transfer and energy conversion. Its unique properties, such as high specific heat capacity, high latent heat of vaporization, and ability to exist in three phases (solid, liquid, gas) under standard conditions, make it an ideal choice for applications in power cycles and refrigeration systems.