Absorption technology

The absorption refrigeration cycle produces cooling from a supply of thermal energy, replacing the mechanical compressor of conventional compression cycles with a thermochemical system made up mainly of a generator, an absorber, a solution pump and heat exchangers.

The thermodynamic principle is based on a refrigerant-absorbent pair. The refrigerant evaporates at low pressure to absorb heat and produce cooling. It is then absorbed by the absorbent, and the resulting solution is pumped to a higher pressure. Heat supplied in the generator separates the refrigerant again and closes the cycle.

This configuration allows thermal energy to be used as the driving energy, including heat sources that cannot be used directly in a conventional compression cycle.

The main working pairs used in absorption refrigeration are:

NH₃ – H₂O cycle (ammonia – water).

H₂O – LiBr cycle (water – lithium bromide).

1.1. NH₃ – H₂O absorption cycle (ammonia – water)

In the NH₃ – H₂O system, ammonia (NH₃) acts as the refrigerant and water (H₂O) as the absorbent.

Because the refrigerant is not limited by the freezing of water, the cycle can produce cooling at both positive and negative temperatures, and can also produce heat in absorption heat pump mode. It operates at pressures above atmospheric and can reject heat directly to the air.

Operating principle

The process takes place in the following stages:

Stage 1. Generation

The ammonia-rich solution receives thermal energy in the generator. The heat supplied separates the ammonia from the water.

Stage 2. Rectification

The ammonia vapour leaving the generator may contain small amounts of water. The rectifier reduces this before the refrigerant reaches the condenser, improving the purity of the ammonia.

Stage 3. Condensation

The high-pressure ammonia gives off heat to the heat rejection medium and condenses into a liquid.

Stage 4. Expansion

The liquid refrigerant passes through the expansion device, which lowers its pressure and temperature.

Stage 5. Evaporation

The low-pressure ammonia absorbs heat from the circuit to be cooled and evaporates. This is where the useful cooling effect is produced.

Stage 6. Absorption

The ammonia vapour from the evaporator enters the absorber and is absorbed by the water. Absorption releases heat, which must be removed to keep the cycle at the right conditions.

Stage 7. Solution pumping

The ammonia-rich solution is pumped back to the generator, closing the circuit.

Stage 8. Solution return

The weak solution in the generator returns to the absorber, giving off heat to the rich solution, and its pressure is reduced by a valve.

Cooling and heat pump

In cooling mode, the cycle extracts heat from a cold source and transfers it to a higher-temperature sink.

In absorption heat pump mode, the heat given off in the absorber and the condenser is the useful heat, delivered at a temperature between that of the cold source and that of the driving heat. This is the principle of gas absorption heat pumps that take energy from the air, water or the ground.

In the H₂O – LiBr system, water acts as the refrigerant and lithium bromide (LiBr) as the absorbent. The circuit operates under vacuum: the water evaporates at very low pressure and temperature and absorbs heat from the chilled water circuit.

The refrigerant evaporates at very low pressure and temperature to absorb heat from the circuit to be cooled. The water vapour produced is then absorbed by a concentrated lithium bromide solution.

Unlike the H₂O-NH₃ cycle, water is the refrigerant, so the operating conditions of the evaporator determine the range of cooling temperatures the system can produce.

Operating principle

Stage 1. Generator

The LiBr solution receives thermal energy. Heating separates the refrigerant water from the absorbent.

Stage 2. Condenser

The water vapour from the generator gives off heat to the heat rejection circuit and condenses.

Stage 3. Expansion

The liquid water’s pressure is reduced before it enters the evaporator.

Stage 4. Evaporator

At low pressure, the water evaporates at a low temperature and absorbs heat from the chilled water circuit, producing the cooling effect.

Stage 5. Absorber

The water vapour from the evaporator is absorbed by the lithium bromide solution. This process releases heat, which must be removed through the heat rejection circuit.

Stage 6. Solution pumping

The dilute solution is pumped back to the generator, raising its pressure to continue the cycle.

Stage 7. Concentrated solution return

The concentrated solution returns from the generator to the absorber, giving off heat to the dilute solution. This closes the cycle.

The main feature of the LiBr-H₂O cycle is that it produces chilled water using thermal energy as the main driving energy of the refrigeration cycle, reducing the need for the electricity associated with mechanical compression.

2.1. Single-effect LiBr-H₂O

In a single-effect configuration, the generator receives the driving thermal energy directly and there is a single main refrigerant generation stage.

This cycle can work with moderate-temperature heat sources, such as hot water (from 70 ºC), superheated water or certain waste heat sources, with a COP of around 0.7 – 0.8.

Its relatively simple configuration makes effective use of available heat sources when temperature conditions do not justify a double-effect cycle.

The double-effect cycle adds a second generation stage that makes more efficient use of the driving thermal energy.

The vapour produced in the first stage supplies thermal energy to a second generation stage, reducing the amount of external energy needed to produce the refrigerant.

This configuration achieves a higher thermal efficiency, with a COP of around 1.4 – 1.5, although it needs a higher-temperature driving source (steam, exhaust gas or direct gas firing).

The choice between single and double effect depends mainly on: