How Absorption Refrigeration Systems Work: A Complete Guide

Absorption refrigeration is a thermodynamic process that provides cooling using thermal energy rather than mechanical work. This makes it particularly useful in scenarios where waste heat or renewable heat sources (like solar energy) are available.

Industrial absorption refrigeration system with generator, absorber, and heat exchanger mounted on a steel frame, designed for efficient cooling using thermal energy.

Unlike conventional vapour compression systems that rely on mechanical compressors, absorption systems utilise a chemical absorption process to achieve the same cooling effect.

🔁 The Basic Principle

At its core, an absorption refrigeration system follows the same vapour compression cycle thermodynamically, but replaces the compressor with an absorber, pump, generator, and solution heat exchanger.

The main working fluids include:

Refrigerant: Typically ammonia (NH₃) or water
Absorbent: A substance that absorbs the refrigerant (e.g. water in NH₃–H₂O systems or lithium bromide in H₂O–LiBr systems)

🔧 Key Components and Their Functions

Evaporator
The refrigerant evaporates by absorbing heat from the environment, producing the cooling effect.
Absorber
The low-pressure refrigerant vapour is absorbed into the absorbent, forming a solution.
Pump
This pump raises the pressure of the liquid solution, requiring much less work than compressing vapour.
Generator (Desorber)
Heat is applied to the pressurised solution, causing the refrigerant to vaporise and separate from the absorbent.
Condenser
The refrigerant vapour condenses, releasing heat to the surroundings.
Expansion Valve
The high-pressure liquid refrigerant is expanded to low pressure before entering the evaporator again.

🔄 Cycle Description

The cycle can be divided into two loops:

1. Refrigerant Loop

The refrigerant evaporates in the evaporator, producing a cooling effect:

$Q_{\text{evap}} = m_r \cdot h_{\text{evap}}$

Where:

$Q_{\text{evap}}$ = heat absorbed
$m_r$ = mass flow rate of refrigerant
$h_{\text{evap}}$ = latent heat of vaporisation

The vapour is then absorbed in the absorber.

After regeneration in the generator, it flows to the condenser:

$Q_{\text{cond}} = m_r \cdot h_{\text{cond}}$

2. Solution Loop

The absorbent solution flows from the absorber to the generator, driven by the pump.

Absorption refrigeration cycle diagram illustrating heat and refrigerant movement between generator, absorber, condenser, and evaporator using thermal energy rather than mechanical work.

The required pump work is:

$W_{\text{pump}} = \frac{m_s \cdot \Delta P}{\rho}$

Where:

$m_s$ = solution mass flow rate
$\Delta P$ = pressure increase
$\rho$ = solution density

In the generator, heat is applied to release the refrigerant:

$Q_{\text{gen}} = m_s \cdot (h_{\text{out}} - h_{\text{in}})$

The refrigerant and absorbent separate: refrigerant moves to the condenser, and the lean solution returns to the absorber.

⚙️ Coefficient of Performance (COP)

The efficiency of an absorption refrigeration system is measured using the Coefficient of Performance (COP):

$COP_{abs} = \frac{Q_{evap}}{Q_{gen} + W_{pump}}$

Since $W_{\text{pump}} \ll Q_{\text{gen}}$ , it’s often approximated as:

$COP_{abs} \approx \frac{Q_{evap}}{Q_{gen}}$

Typical values of COP for ammonia–water and water–lithium bromide systems range from 0.6 to 1.2.

🔬 Working Fluids

1. Ammonia–Water (NH₃–H₂O)

Refrigerant: Ammonia
Absorbent: Water
Operates at higher pressures
Suitable for sub-zero refrigeration

2. Water–Lithium Bromide (H₂O–LiBr)

Refrigerant: Water
Absorbent: Lithium bromide
Operates under vacuum
Cannot go below 0 °C (water freezes)

🔥 Applications

Solar-powered air conditioning
Industrial waste heat recovery
Gas-fired chillers
Off-grid refrigeration
Mobile (RV and caravan) cooling

✅ Advantages

Very low electrical demand
Can operate with low-grade or waste heat
Quiet operation (few moving parts)
Low maintenance
Compatible with Combined Heat & Power (CHP) systems

⚠️ Limitations

Lower COP than mechanical systems
Bulky components
Crystallisation risk (LiBr systems)
Heat source must be stable

🧠 Summary

Absorption refrigeration offers a heat-driven alternative to compressor-based systems. Its use of waste or renewable thermal energy makes it ideal for sustainable cooling, despite a lower COP.

$\boxed{COP_{abs} \approx \frac{Q_{cooling}}{Q_{heat_input}}}$

This method forms a key part of modern low-carbon refrigeration strategies.

Product Development Engineers Ltd

Absorption refrigeration explained

🔁 The Basic Principle

🔧 Key Components and Their Functions