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Absorption Heat Pumps: In-Depth Examination

The absorption heat pump transfers heat from a cold source through a secondary fluid. Discover how it works and what changes compared to traditional heat pumps

The heat pump is a thermal system that allows heating and cooling of any structure, as well as the production of hot water needed for daily needs, through the transfer of energy from a low-temperature environment to a higher-temperature system.

Compared to many other technological systems, it helps reduce energy consumption and environmental sustainability, resulting in economic savings on bills, also thanks to numerous state incentives and benefits.

Based on the type of operation and the type of power supply, there are different types of heat pumps on the market:

  • electric compression heat pumps;
  • endothermic compression heat pumps;
  • absorption heat pumps;

In this article, we will delve into the operation of absorption heat pumps in detail, to understand their technical characteristics and how they differ from others.

How does an absorption heat pump work?

Absorption heat pumps are devices that exploit the thermodynamic principle of absorption to transfer heat from a low-temperature source to a higher-temperature source. Unlike traditional heat pumps that use mechanical compressors, absorption heat pumps rely on chemical or physical reactions to achieve their heating or cooling effect.

The operation of an Absorption Heat Pump (AHP), which can be powered by any thermal source, is based on two substances, a refrigerant and an absorbent.

These two substances, combined, create a cycle in which the mechanical work is approximately 1% of the thermal energy introduced into the generator.

Let’s analyze in detail all the phases of the absorption heat pump cycle:

  1. It all starts with a generator, the primary source of energy that increases the temperature of the refrigerant-absorbent solution. As the temperature of the solution increases, the two components separate, and the refrigerant evaporates along the distillation column;
  2. The refrigerant vapor then passes through the rectifier, where it separates from any water residues and enters the heat exchanger, usually a tubular bundle;
  3. In this tubular bundle heat exchanger, the condensation of the refrigerant fluid occurs, transferring heat to the system’s water (secondary fluid). Here, the first useful effect of the machine occurs, heating a water flow;
  4. Subsequently, the refrigerant, after leaving the condensation section, passes through a series of laminations and a concentric tube heat exchanger, where pressure and temperature gradually decrease, allowing it to reach the ideal conditions to absorb heat from the external air;
  5. The refrigerant cools the external air, absorbing heat, and then evaporates;
  6. Next, the refrigerant is superheated first in the heat exchanger and then in the pre-absorber, where it reacts with water, giving rise to the actual absorption;
  7. Absorption is an exothermic reaction (energy is removed), so if part of this energy is used to heat the water-refrigerant solution, the remaining energy is transferred to the tubular bundle heat exchanger;
  8. In this phase of the cycle, the heat exchanger acts as an absorber and allows a considerable amount of thermal energy to be transferred to the thermal fluid of the thermal system, constituting the machine’s second useful effect;
  9. The water-refrigerant solution, exiting the heat exchanger, is returned from the solution pump to the generator, passing again through the pre-absorber and the rectifier, where a pre-heating phase occurs through heat recovery from the cycle itself. The refrigeration cycle described above then restarts in the generator.

Absorption heat pump scheme

Absorption heat pump scheme

Difference between an absorption heat pump and a compression one?

The main difference between an absorption heat pump and a compression one (electric or gas) lies in the working phases: an absorption heat pump is without the compression phase, which is entirely replaced by two distinct phases: generation and absorption.

As seen earlier, the first phase occurs thanks to a burner, used as the primary source of energy, while the second phase occurs through a specific liquid, usually water, called an absorbent.

Furthermore, the efficiency of an absorption heat pump, measured in GUE, Gas Utilization Efficiency, is higher than that of a traditional heat pump. Let’s see the two main reasons together:

  1. The GUE, resulting from the ratio between the energy transferred to the environment and the energy supplied to the burner, is much lower than the COP of traditional heat pumps. A GUE typically stands around 1.5, compared to a COP value around 4;
  2. When the external temperature is very cold, a traditional heat pump does not work well because more work will be required to transfer heat from the outside to the inside, and also, the humidity of the cold air tends to freeze the fins of the outdoor unit. This does not happen in an absorption heat pump: the thermodynamic cycle it operates on does not create functionality limits for the machine even at low external temperatures.

Overall, the high efficiency of an absorption heat pump results in a higher price at the time of purchase; however, it is worth noting that there is undoubtedly a return on investment thanks to energy savings within 4/5 years.

Types of absorption heat pumps

In the air conditioning systems market, two common types of absorption heat pumps can be found, direct flame or indirect feed.

A direct flame heat pump is powered by the flame generated by a fuel such as natural gas, LPG, biomass, etc., and is mainly used in environments where continuous service is required even in the absence of electricity.

An indirect feed heat pump instead has a thermal source such as a heat transfer fluid (hot or superheated water, vapors, combustion products of endothermic engines) that is constantly reused in the machine’s cycle.

It is important to remember that there are many European Directives promoting the use of energy-efficient air conditioning and hot water production systems, with verifications resulting from the development of ecological projects.

Absorption heat pumps represent a promising solution for heating and cooling buildings, offering significant advantages in terms of energy efficiency, emission reduction, and flexibility in the use of renewable energy sources.