How Can Air Source Heat Pumps Cope With Ultra-Low Temperature Environments?

In recent years, air source heat pumps have received much attention and market popularity. But there are several factors that significantly affect the performance of air source heat pumps: firstly, the performance of air source heat pumps is significantly reduced under low ambient temperature conditions; Secondly, the problem of frosting during the heating process seriously affects energy efficiency and reliability. In recent years, many researchers and engineering technicians have invested a lot of effort in improving air source heat pump technology to solve the above problems.

PART 01 Multi level Compression Technology

Variable frequency compression technology is one of the effective methods to improve the heating capacity of air source heat pumps. Under low-temperature conditions, increasing the compressor speed can significantly increase its displacement, thereby improving the heating capacity of the air source heat pump. However, frequency conversion technology cannot improve system energy efficiency. In order to simultaneously improve heating capacity and energy efficiency at low temperatures, multi-stage compression technology has been developed.

According to the different compression stages and cycle structures, multi-stage compressed air source heat pumps can be divided into cascade cycle and dual/multi-stage compression. In order to improve the heating capacity of a single-stage compression system at low ambient temperatures, refrigerant can be directly injected into the compression chamber during the compression process, which is called quasi two-stage compression. Due to the characteristics of a two-stage compression cycle, this article will include it in the scope of a two-stage compression cycle.

1.1 Stacked air source heat pump

Due to the low ambient temperature, the system pressure ratio is high, the compression power is large, and the throttling loss is large, ultimately leading to low energy efficiency of the heat pump. In order to reduce losses and improve efficiency, the stacked air source heat pump system uses two vapor compression cycles in series instead of a single cycle to reduce the compression ratio of a single-stage cycle.

The cascade system consists of two independent vapor compression cycles, one is the low-temperature stage cycle and the other is the high-temperature stage cycle. These two cycles are connected through a shared intermediate heat exchanger, which operates as both a condenser for the low-temperature cycle and an evaporator for the high-temperature cycle. In winter, the low-temperature cycle absorbs heat from the ambient air through an evaporator and raises the heat to a higher temperature, providing it as a heat source for the high-temperature cycle; In the high-temperature cycle, the heat is once again increased to the temperature required for indoor heating.

By using a stacked air source heat pump, the pressure ratio of the cycle is greatly reduced, reducing the total compression loss and throttling loss, thus improving the energy efficiency of the air source heat pump; Moreover, according to different operating conditions, different refrigerants can be used for the high and low temperature stages of the cascade cycle. Due to the fact that the stacked system can be implemented using two simple single-stage systems, it has been used for heating and hot water supply applications for many years. However, the heat transfer temperature difference of the intermediate heat exchanger in the cascade cycle inevitably leads to a certain efficiency loss; Moreover, the cascade cycle requires the use of two compressors and an additional heat exchanger, which is more expensive compared to single-stage cycles.

1.2 Dual stage compressed air source heat pump

A two-stage compressed air source heat pump connects two refrigeration cycles together, which can be seen as a simplified form of a cascade system. As shown in the figure below, two-stage compressed air source heat pumps can be divided into two categories based on the different economizers used: flash tank (FT) systems and intermediate heat exchanger (IHX) systems.

For the flash tank system, the liquid refrigerant leaving the indoor condenser is throttled into two phases and enters the flash tank. The two-phase refrigerant is separated into saturated gas and saturated liquid in the flash tank; After the saturated gas refrigerant is mixed with the refrigerant exhaust of the low-pressure stage compressor, it is compressed again by the high-pressure stage compressor. The saturated liquid is throttled by the second expansion valve and enters the outdoor evaporator to evaporate into gas. Then it enters the low-pressure stage compressor and mixes with the medium pressure gas in the flash tank.

For the intermediate heat exchanger system, the liquid refrigerant at the condenser outlet is directly divided into two streams: the main stream and the branch stream. The branch refrigerant is throttled to medium pressure and enters the intermediate heat exchanger. The low-temperature refrigerant cools the main refrigerant to a supercooled state. The branch refrigerant absorbs heat and becomes saturated gas or superheated, and mixes with the exhaust of the low-pressure stage compressor before entering the high-pressure stage compressor for further compression. The subcooled refrigerant at the main outlet of the intermediate heat exchanger is throttled, passes through the evaporator, and finally returns to the low-pressure stage compressor, where it is compressed to medium pressure and mixed with the branch refrigerant.

1.3 Quasi dual stage compressed air source heat pump

A quasi dual stage compressed air source heat pump (also known as a supplementary air system) is very similar to a dual stage compression system. The difference is that in the quasi dual stage compression system, a compressor with an intermediate air supply port is used instead of two series connected compressors. In quasi two-stage compression, refrigerant from the flash tank or intermediate heat exchanger is injected into the compression chamber of the compressor, rather than between the two compressors.

Therefore, a quasi dual stage heat pump can be seen as a simplified form of a dual stage heat pump, using a specially designed air supply compressor to replace the two compressors, thereby avoiding the problem of oil equalization between the two compressors and reducing system costs. More importantly, by closing the valve on the air supply branch, the quasi two-stage system can flexibly switch to single-stage circulation mode, thereby optimizing the performance of the quasi two-stage heat pump in winter and summer. Therefore, in recent years, quasi two-stage compression technology has been widely applied in low-temperature heat pumps.

PART 02 Refrigerant Replacement

At present, R22 and R410A are the most commonly used refrigerants in air source heat pump systems, with their main substitutes being R290, R32, R744, R161, and some HFC mixed refrigerants. However, except for R744, all medium and low GWP refrigerants have a certain degree of flammability, and their application must comply with relevant safety standards or regulations considering the charging amount and other special requirements.

As for pure refrigerant substitutes, R290 has similar operating pressure and capacity to R22, and has a higher energy efficiency ratio than R22. However, due to its flammability, it is mainly suitable for small charge systems. Manufacturers have exhibited split air conditioners and air source heat pumps using R290. The operating pressure and capacity of R32 are close to R410A, and the energy efficiency is also comparable to R410A, even slightly higher than R410A.

At present, R32 is suitable for various types of air conditioning equipment in the market, and is used for split units in multiple countries and regions such as Japan, China, South Korea, and Europe. Some manufacturers also use R32 for other types of systems, such as multi split units. The application of R744 in heating and cooling fields is limited due to its energy efficiency ratio in cooling mode, especially when the external temperature is high. However, the R744 transcritical cycle has significant advantages in high-temperature hot water.

In addition to pure refrigerants, there are also many mixed refrigerants specifically designed for air source heat pumps, which are mainly composed of two or more pure refrigerants such as R32, R125, R134a, R152a, R161, R1234yf, R1234ze, R600a, R1270, and R290. Some mixed refrigerants have been named proprietary numbers, such as R444B, R446A, and R447A, while a large number of other mixed refrigerants are still under continuous development. These mixed refrigerants often have operating pressures and capacities similar to R22 or R410A, with GWPs ranging from 150 to 1000, and flammability levels of 1 (high GWP refrigerants) or 2L (medium GWP refrigerants). At present, most of the mixed working fluids have not been mass-produced, and the relevant technical data has not yet been made public.