The refrigeration cycle is a thermodynamic process that enables heat to be transferred from a lower-temperature environment to a higher-temperature one, using a refrigerant and specific equipment. This system forms the basis for the operation of devices such as refrigerators, air conditioners, and heat pumps.
Through a sequence of changes in the refrigerant’s state and pressure, heat is absorbed and released in a controlled manner.
The refrigeration cycle consists of four main phases: compression, condensation, expansion, and evaporation. The absorption refrigeration cycle differs significantly from the compression cycle. In fact, traditional heat pumps use an electric or internal combustion compressor to compress the refrigerant, whereas in the absorption cycle this phase is replaced by a chemical process of generation and absorption. Let’s take a closer look at each phase.
In compression systems, the refrigerant in the form of low-pressure gas is compressed in a compressor, increasing both its pressure and temperature. This process is essential to prepare the refrigerant for condensation. In the absorption cycle, this phase is absent because it is replaced by a supply of thermal energy in the generator.
The refrigerant, either compressed (in compression systems) or heated in the generator (in absorption systems), enters the condenser. Here, it releases the accumulated heat to the surrounding environment or to a heat transfer fluid. During this phase, the refrigerant changes state from gas to liquid.
After condensation, the liquid refrigerant passes through an expansion valve or throttling device. In this process, the refrigerant’s pressure and temperature drop significantly, preparing it for evaporation.
Finally, the refrigerant enters the evaporator, where it absorbs heat from the surrounding environment. This causes it to evaporate, turning it back into gas and thus closing the cycle. In the case of absorption heat pumps, the energy for evaporation is primarily provided by renewable sources such as air, water, or the ground.
The GAHP absorption heat pump is a thermodynamic device designed to transfer heat from low-temperature thermal sources to heating subsystems, raising the thermal level of the extracted energy.
The refrigeration circuit of GAHPs is based on the GAX (Gas Absorption heat eXchanger) cycle. Compared to the classic refrigeration cycle of electric systems derived from the theoretical Carnot cycle, the absorption system differs by replacing compression with the phases of generation and absorption.
Following the generation phase are condensation and evaporation, ending with the absorption of the refrigerant into the absorbent fluid, accompanied by significant heat release. The generation phase essentially consists of the separation of ammonia from water through evaporation, using the thermal input of a flame. This phase is preceded by a series of heat exchanges for preheating the solution entering the generator.
The absorption phase is an exothermic chemical reaction resulting from the physicochemical properties of the two compounds used and the characteristics of the mixing process that governs them.
The distinctive feature of the cycle used in absorption machines is its ability to generate significant thermal energy within the cycle itself thanks to the absorption reaction between the refrigerant and the absorbent. This feature allows for a reduction in the machine's energy demand, lowering fuel consumption and making the machine's efficiency less affected by the temperature of the renewable energy source (air, water, or ground).
For a detailed description of the thermodynamic cycle, refer to the hermetic circuit of an actual machine shown in the diagram, representing a reversible air-source GAHP-AR absorption heat pump.
The multi-gas burner (D) is used to heat the absorbent-refrigerant solution, causing the separation of the two components through evaporation of the refrigerant in the distillation column (C). The burner-distillation column unit is called the generator and, in absorption machines, replaces the compressor found in traditional vapor compression systems.
The hot refrigerant vapor leaving the generator passes through the rectifier (B), where it is separated from the remaining water content, and enters the shell-and-tube heat exchanger (L), which in winter functions as the condenser-absorber of the system.
In this part of the circuit, the heat exchanger serves as the condenser for the refrigerant, which releases its latent heat of condensation to the water in the heating system.
This phase change of the refrigerant thus represents the first useful effect of the machine. The refrigerant exiting the condensation section passes through a first throttling stage, a “tube-in-tube” heat exchanger (G), and a second throttling stage, where it undergoes progressive pressure and temperature drops until reaching the ideal conditions to evaporate again into the gaseous phase.
In the finned coil (A), the refrigerant evaporates by absorbing heat from the outdoor air. In this part of the circuit, the heat pump brings renewable aerothermal energy into the cycle.
It is worth noting that the refrigerant used by GAHP heat pumps can evaporate at atmospheric pressure even at -33°C.
This thermodynamic property of the refrigerant allows renewable energy to be extracted from the air even when outdoor temperatures are extremely low, eliminating the need for backup boilers.
The ammonia evaporated in the finned coil, after being superheated in the “tube-in-tube” heat exchanger (G), enters the pre-absorber (F), where it meets the sprayed absorbent (water), triggering the actual absorption reaction.
Absorption is an exothermic chemical reaction that requires the removal of the heat released to proceed. In the pre-absorber, part of this energy is used to preheat the water-ammonia solution about to re-enter the generator. To complete the absorption reaction, the solution is sent again to the shell-and-tube heat exchanger (L).
In this phase of the cycle, the heat exchanger acts as an absorber and allows a significant amount of thermal energy to be transferred to the heating system’s heat transfer fluid, representing the second useful effect of the machine.
The water-ammonia solution exiting the heat exchanger (L) is pumped by the solution pump (E) back to the generator, passing again through the pre-absorber (F) and the rectifier (B), where it is preheated by recovering heat from the cycle itself.
At this point, the refrigeration cycle restarts in the generator. Position (H) in the diagram represents the heat pump’s reversing valve, a mechanical component that redirects the refrigerant flow within the circuit. This allows the operating mode to be seasonally switched, producing hot water in winter and chilled water in summer.
Position (K) indicates the defrosting valve, which, when necessary, enables quick defrosting of the finned coil without reversing the refrigeration cycle or activating electric auxiliaries.
As shown in the diagram, only one of the two energy inputs to the evaporator is diverted toward the coil, specifically hot ammonia vapor. This ensures rapid ice removal (within approximately 180 seconds) while maintaining 50% heating power in the circuit, without significantly affecting the machine’s efficiency.
NOTES:
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