Types and Characteristics of Low-Carbon Technology Air Source Heat Pumps

Abstract: Air source heat pumps have good application prospects and have developed various types due to their easy access to low-temperature temperatures. Different types of air source heat pumps have their own characteristics and applicability, and suitable products can be selected according to the application scenario. 

The low-temperature heat of air source heat pumps comes from the ubiquitous air, so they are widely used in projects with scarce waste heat resources and have broad application prospects. Air source heat pumps have also experienced rapid development, with various types of products appearing in the market.

Different types of air source heat pumps have their own characteristics, and this article introduces some common air source heat pumps.

Electric compression air source heat pump

Electric compression air source heat pump is the most common type and the main technology for many regions to convert coal to electricity.

Electric compression air source heat pump uses an electric motor driven by electricity to drive the compressor to rotate, pressurizing low-pressure refrigerant vapor to high pressure and increasing the condensation temperature; In the condenser, the refrigerant vapor condenses and releases heat, achieving external heating, while the refrigerant vapor becomes liquid refrigerant; After the liquid refrigerant is depressurized by the throttle valve, the evaporation temperature decreases. The evaporator absorbs external air heat, and the liquid refrigerant evaporates into low-pressure refrigerant vapor, which then enters the compressor for the next cycle.

The electric compression air source heat pump uses a compressor and a throttle valve to divide the refrigerant into high and low pressure parts, achieving the function of heating from low-temperature air. During this process, a small amount of electricity is consumed, and this energy is also added to the heating system. The electric compression air source heat pump has a small unit capacity and is suitable for household heating. If used for centralized heating (providing heating to residential areas, etc.), there are many installations and a large footprint.

The main problem with electric compression air source heat pumps is frosting. When the evaporation temperature of the refrigerant is lower than the dew point temperature of the air, frost will form on the surface of the air heater, affecting the heat transfer efficiency. The defrosting process will increase energy consumption and affect comfort. Although various manufacturers have developed technologies such as energy islands and multi zone heat exchangers for processing, there is no way to fundamentally solve this problem.

The temperature rise amplitude of electric compression air source heat pumps is relatively high, and according to different compressor forms, they can be divided into various forms such as single-stage compression, multi-stage compression, and intermediate air supply. Especially with the application of CO2 working fluid, there are more options for electric high-temperature air source heat pumps.

Gas compression air source heat pump

Gas compression air source heat pump uses a gas engine to provide kinetic energy, driving the compressor to rotate. The operation of other parts is the same as that of an electric compression air source heat pump. The compressed air source heat pump unit is relatively small, and the power of the gas engine is not high. It can be powered by a car engine.

Gas compression air source heat pumps also have the problem of frosting. Due to the use of gas as the driving energy, it can be defrosted with flue gas, resulting in higher thermal comfort compared to electric compression air source heat pumps. However, the cost of gas compression air source heat pumps is relatively high.

Ammonia absorption air source heat pump

The ammonia water absorption air source heat pump adopts the absorption cycle principle, and the working fluid is an aqueous solution of ammonia. Heating concentrated ammonia solution with a high-temperature heat source, evaporating ammonia gas and forming dilute ammonia solution; High pressure ammonia gas condensation provides external heating while producing liquid ammonia; Liquid ammonia undergoes low-temperature evaporation after throttling, absorbing heat from external air to form ammonia gas; Ammonia is absorbed by dilute ammonia solution, releasing heat to the outside, and the resulting concentrated ammonia solution enters the next cycle.

This process uses ammonia as the refrigerant and water as the absorbent. The evaporation temperature of liquid ammonia is very low, which can absorb the heat of low-temperature air. However, during the operation of the unit, ammonia undergoes gas-liquid two-phase conversion, resulting in high internal pressure and easy leakage; The leaked ammonia gas poses a certain level of danger and requires proper protection and ventilation.

The single unit capacity of ammonia water absorption air source heat pump is also relatively small. If the heat load is large, multiple units need to be operated in parallel.

The device also has frost problems. However, due to the fact that the unit itself is driven by high-temperature heat (such as gas, steam, etc.), defrosting is relatively simple and has little impact on heating comfort.

Lithium bromide absorption air source heat pump

The above-mentioned air source heat pumps all have problems with frosting and defrosting. Although they have their own solutions, they also affect the comfort of heating, and more importantly, affect the COP of the heat pump and increase energy consumption. The lithium bromide absorption air source heat pump completely solves the problem of frosting.

Lithium bromide absorption air source heat pump adopts absorption cycle for waste heat recovery. In the conventional lithium bromide absorption cycle, water is used as the refrigerant and lithium bromide solution is used as the absorbent. Water is the refrigerant, and the refrigeration temperature must be higher than 0 ℃, otherwise the water will freeze and cause the unit to fail to operate. This is also the reason why conventional lithium bromide absorption heat pumps cannot absorb heat below 0 ℃.

To solve this problem, the lithium bromide absorption air source uses a solution spray method to extract heat from the air, avoiding the problem of water freezing at 0 ℃ and achieving low-temperature waste heat recovery.

The lithium bromide absorption air source heat pump has a large unit capacity and is not suitable for household heating. It is more suitable for centralized heating, especially in energy-saving renovation of boiler rooms. The lithium bromide absorption air source heat pump adopts a spray heat exchange method, which uses a solution to absorb heat from the air. Not only does it not have the problem of frosting, but it can also absorb the latent heat of water vapor in high humidity air, improving the heat extraction efficiency.

Lithium bromide absorption air source heat pumps are more suitable for application in centralized heating projects in the south. Even if the air has high humidity, it will not frost and instead promotes waste heat recovery.

Summary

The market for air source heat pumps is vast and diverse, suitable for various energy-saving heating scenarios. Choosing the appropriate type of air source heat pump based on the characteristics of the application scenario can achieve better heating effects.