The absorption heat pump principle is a thermodynamic process that uses thermal energy to transfer heat, unlike traditional heat pumps that rely on mechanical energy. This system employs a refrigerant-absorbent pair, such as ammonia-water or water-lithium bromide, to achieve heating and cooling effects. The process involves key components like the generator, condenser, evaporator, and absorber, working together in a cycle to efficiently move heat from one place to another.
What is the Basic Principle of an Absorption Heat Pump?
The absorption heat pump principle revolves around the use of thermal energy to drive a refrigeration cycle. Unlike conventional heat pumps that use mechanical compressors, absorption heat pumps utilize the ability of certain substances to absorb and release a refrigerant. This process allows for the transfer of heat from a low-temperature source to a high-temperature sink.
Key components in this system include:
- Generator
- Condenser
- Evaporator
- Absorber
- Expansion device
The cycle begins in the generator, where heat is applied to a strong solution of refrigerant and absorbent. This causes the refrigerant to vaporize and separate from the absorbent. The high-pressure refrigerant vapor then moves to the condenser, where it releases heat and condenses into a liquid. The liquid refrigerant passes through an expansion device, lowering its pressure and temperature before entering the evaporator. In the evaporator, the refrigerant absorbs heat from the surroundings and vaporizes. Finally, in the absorber, the refrigerant vapor is reabsorbed into the weak solution, releasing heat in the process.
How Does the Refrigerant-Absorbent Pair Function?
The choice of refrigerant-absorbent pair is crucial for the efficient operation of an absorption heat pump. Two common pairs are:
- Ammonia (NH3) and Water
- Water and Lithium Bromide (LiBr)
In the ammonia-water system:
– Ammonia serves as the refrigerant
– Water acts as the absorbent
For the water-lithium bromide system:
– Water is the refrigerant
– Lithium bromide is the absorbent
These pairs are selected based on their ability to efficiently separate and recombine under different temperature and pressure conditions. The refrigerant-absorbent interaction is at the heart of the absorption heat pump principle, enabling the system to move heat without the need for a mechanical compressor.
What Are the Efficiency Metrics for Absorption Heat Pumps?
Efficiency in absorption heat pumps is typically measured by the Coefficient of Performance (COP). The COP values vary depending on whether the system is used for heating or cooling:
| Mode | COP Range |
|---|---|
| Heating | 1.2 – 2.5 |
| Cooling | 0.8 – 1.6 |
These values indicate that for every unit of thermal energy input:
– In heating mode, the system can produce 1.2 to 2.5 units of useful heat
– In cooling mode, it can remove 0.8 to 1.6 units of heat
Compared to traditional heat pumps, absorption systems can achieve higher efficiencies when waste heat or low-grade thermal energy is available. For instance, using natural gas as a heat source, an absorption heat pump can reach an efficiency of approximately 150%, meaning 1 kWh of natural gas can generate 1.5 kWh of heat.
What Are the Key Components and Their Specifications?
The main components of an absorption heat pump system include:
- Generator: Requires a heat source (e.g., natural gas, steam) to evaporate the refrigerant from the strong solution.
- Condenser: Typically air-cooled or water-cooled, it condenses the refrigerant vapor.
- Absorber: Where the gaseous refrigerant is absorbed into the weak solution, releasing heat.
- Evaporator: Operates at low pressure and temperature to absorb heat from the surroundings.
- Expansion Device: Reduces the pressure of the refrigerant, causing a temperature drop.
- Internal Heat Exchanger: Used to preheat the cold mixture with the hot mixture, increasing efficiency.
Operational limits and specifications:
– Maximum output temperature typically does not exceed 150°C
– Temperature rise (ΔT) usually ranges from 30–50°C
– System sizes can vary from small (70-500 kW) for cooling to larger industrial systems
How Are Absorption Heat Pumps Applied in Residential and Industrial Settings?
Absorption heat pumps find applications in both residential and industrial environments:
Residential Applications:
- Provide simultaneous heating and cooling
- Offer hot water for heating and chilled water for cooling
- Higher installation costs compared to traditional heat pumps
- COP values of 1.4 or higher for heating, and 0.8 to 1.6 for cooling
Industrial Applications:
- Used for large-scale heating and cooling
- Integrated into geothermal systems for efficient operation
- Utilize waste heat or low-grade thermal energy effectively
- Achieve higher COP values due to the availability of waste heat
In industrial settings, absorption heat pumps are particularly advantageous where both heating and cooling are required, and where waste heat can be efficiently utilized. The ability to use low-grade thermal energy makes them more efficient in these applications compared to residential use.
