Low temperature air source heat pump technologies

To solve the problems that the heating capacity and COP of air source heat pump decrease with outdoor ambient temperature decreasing, many scholars and engineers have thoroughly studied various schemes and put some of them into practice. Currently, great progress and technical achievements on air source heat pump technology have been made. The heating performance and reliability of air source heat pumps have been greatly improved and their operating temperature range has been extended. At present, low temperature air source heat pump technologies in practice can be mainly classified into the following schemes.

1. Variable speed technology of compressor

From the foregoing reasons of the heating capacity reduction of air source heat pumps, it can be seen that the main reason is the decrease of the volumetric heating capacity of refrigerant. Therefore, the increase of the volume flow rate of the compressor is an effective measure to solve the problem regarding the decrease of the heating capacity of the air source heat pump. The variable speed technology of compressor can increase its volume flow rate by increasing operating speed at fixed working volume of the compressor cylinder; consequently, the reduction of the heating capacity is alleviated effectively. The variable speed technology of the compressor has become one of the important schemes to solve the problem regarding the reduction of the heating capacity of air source heat pumps. In practice, the operating speed of the compressor is controlled based on outdoor ambient temperature and indoor target temperature to alleviate the contradiction between heat supply and heat demand.

In the early 1990s, variable speed technology of compressors was studied in China. Variable speed air conditioners gradually appeared in the market in China around 2005. After 2010, the variable speed technology of compressors had been widely used in the Chinese air conditioner industry, and good economic and social benefits were achieved. According to the data from ChinalOL, variable speed room air conditioners (annual total quantity sold) accounted for 49.2% in 2016 and 55.4% in 2017 of room air conditioners (fixed and variable speed) in the domestic market share of China.

2. Quasi two-stage compression technology

Quasi two-stage compression technology was first applied to screw compressors. А. В. БЫКОВ, a former Soviet Union scholar, first proposed the concept of quasi two-stage compression cycle for screw compressor in 1976. He analyzed the cycle and obtained conservation of energy equations of the economizer and the vapor injection process. In these equations, the vapor injection process was assumed as an isovolumetric mixing at first, then an adiabatic compression. A series of mathematical models reflecting the main characteristics of the cycle were obtained afterwards.

In China, scholars had already studied the quasi two-stage compression system with an economizer in the mid 1980s, and it had been successfully applied to screw units. The research showed that the energy saving effect of this system was remarkable at low outdoor ambient temperature, and it could completely replace the two-stage compression system at an ambient temperature of -30° C. The capacity of screw compressor is generally larger; the advantage of this system decreases gradually with the increase of evaporation temperature compared with a single-stage compression system. Therefore, research of quasi two-stage compression has been limited to heating at a low temperature for a long time, and the feasibility of cooling has not been paid enough attention.

The quasi two-stage compression air source heat pump system with a scroll compressor, which has an vapor injection port, improves the heating capacity and COP at low outdoor ambient temperature. Figure 1.5 shows the schematic diagram of a quasi two-stage compression air source heat pump cycle system using a scroll compressor. The system cycle is as follows: the gas refrigerant of high temperature and high pressure discharged from the compressor discharge port flows into the condenser and condenses into liquid refrigerant of intermediate temperature and high pressure, releasing heat. The liquid refrigerant divides into a main circuit and a branched circuit at the condenser outlet. The refrigerant in the branched circuit flows through the throttling device 2 and turns into two-phase refrigerant mixture of intermediate pressure, and then enters the internal heat exchanger. This part of the refrigerant absorbs heat and evaporates into gas, then enters the vapor injection port of the compressor. At the same time, the main circuit refrigerant cooled down 1 >y the internal heat exchanger flows through throttling device 1 and turns into two-phase refrigerant mixture of low-temperature and low-pressure, and then enters evaporator, evaporates into gas refrigerant after absorbing heat and enters the compressor suction port. Then, the gas refrigerant of low-pressure from the compressor suction port is compressed and then mixes with the gas refrigerant of intermediate pressure from the vapor injection port in the compressor working chamber. After being further compressed, the gas refrigerant of high pressure is discharged out of the compressor, thus forming a complete closed cycle.

FIGURE 1.5

The schematic diagram of a quasi two-stage compression air source heat pump system with scroll compressor

Compared with conventional air source heat pump units, the quasi two- stage compression air source heat pump system units have the following two prominent characteristics:

• 1) The compressor has a vapor injection port, and the gas refrigerant of intermediate temperature and pressure can be injected into the compressor through a branched circuit. Not only the refrigerant mass flow rate of the condenser increases, but also the inlet refrigerant specific enthalpy of the evaporator decreases, thereby the performance of the system is improved at low outdoor ambient temperature.
• 2) The single-stage compression operation mode or the quasi two-stage compression operation mode can be easily chosen by switching off or on the expansion valve in the branched circuit, which cannot only keep the system performance good at normal temperature, but also ensures the system operating safely and reliably at low outdoor ambient temperature.

The quasi two-stage compression technology with a scroll compressor has been successfully applied to low temperature air source heat pump systems. The heating performance of the systems was obviously improved at low outdoor ambient temperature, compared with the single-stage compression system. and the heating capacity was increased more than 20% at outdoor ambient temperature of -15°C.

Besides in the scroll compressor and the screw compressor, the quasi two- stage compression can also be realized in a single-stage rolling piston compressor, which is called quasi two-stage rolling piston compressor. A vapor injection port is arranged on the cylinder of a single-stage rolling piston compressor, through which the gas refrigerant of intermediate pressure is injected into the working chamber. The gas refrigerant injection is controlled by a vapor injection valve.

When the intermediate pressure is higher than that of the cylinder working chamber, the vapor injection valve opens and the gas refrigerant of intermediate pressure injects into it, thus the compressor operates with vapor injection process. When the pressure of the cylinder working chamber is higher than the intermediate pressure, the vapor injection valve shuts off and the vapor injection process pauses, but the compressor continues to complete the compression and discharge processes. Figure 1.6 shows the schematic diagram of part of the structure of the quasi two-stage rolling piston compressor.

FIGURE 1.6

The schematic diagram of the compression structure of the quasi two-stage rolling piston compressor

3. Single-compressor two-stage compression technology

At present, most of the small single-compressor two-stage compression air source heat pump systems use rolling piston compressors. The vapor compression mechanism of a double-cylinder two-stage rolling piston compressor is composed of a low-pressure stage cylinder and a high-pressure stage cylinder in series. There is an mixing chamber on the gas passage between the two cylinders, connected with the vapor injection pipe. The gas refrigerant of intermediate pressure injected through the vapor injection pipe mixes with the gas refrigerant discharged from the low-pressure stage cylinder in the interstage mixing chamber, and then enters the high-pressure stage cylinder.

There are several system structures of the two-stage compression heat pump cycle; one of them is called the two-stage compression two-step throttling interstage incomplete cooling cycle. Figure 1.7 shows the schematic diagram of this cycle. It consists of a two-stage compressor, a condenser, a first-step throttling device, a flash tank, a second-step throttling device and an evaporator.

FIGURE 1.7

The schematic diagram of the two-stage compression two-step throttling interstage incomplete cooling cycle

Compared with the air source heat pump system with a quasi two-stage scroll compressor or a quasi two-stage rolling piston compressor, the air source heat pump system with a double-cylinder two-stage rolling piston compressor has the following advantages:

• 1) The vapor injection mass flow rate is larger than that of the quasi two- stage system, which is beneficial to improving the heating capacity and reducing the discharge temperature. The tested results show that when the outdoor ambient temperature is -15°C, the heating capacity of the well-designed two- stage compression air source heat pump system is 40% higher than that of the conventional single-stage compression air source heat pump system, which is higher than that of the quasi two-stage compression air source heat pump system.
• 2) The total pressure ratio of the compressor is shared by the low and high pressure cylinders, so that the pressure ratio of each cylinder significantly reduces and the volumetric efficiency and isentropic efficiency of the compressor increase, which are beneficial to improving the heating capacity and COP of the heat pump.
• 4. Double-compressor two-stage compression technology

In order to meet the heating demand of air source heat pumps in winter in cold climate regions without an auxiliary electric heater, a double-compressor two-stage compression air source heat pump system with two compressors in series was designed.

Figure 1.8 shows the schematic diagram of the double-compressor two- stage compression variable speed air source heat pump system. Two variable speed compressors are connected in series to form a two-stage compression incomplete cooling cycle with internal heat exchanger, which can change operation modes according to the operating conditions. The operating principle is as follows.

FIGURE 1.8

The schematic diagram of double-compressor two-stage compression variable speed air source heat pump system

When the outdoor ambient temperature is higher, the refrigerant discharged from the low-pressure stage compressor enters the condenser directly through the high-pressure stage four-way valve, and the high-pressure stage compressor unloads. In this operation mode, the system is a common single- stage compression air source heat pump system.

When the outdoor ambient temperature is relatively lower, the discharge gas of the high-pressure stage compressor passes through the high-pressure stage four-way valve and enters the condenser for condensation and liquefaction. After flowing through the check valve 2 and the high-pressure accumulator, the refrigerant divides into the main circuit and the branched circuit. The refrigerant in the branched circuit passes through the solenoid valve and the throttling device 3 for throttling and then enters the internal heat exchanger for evaporation. At the same time, the refrigerant in the main circuit is further subcooled in the internal heat exchanger and flows through the throttling device 1 for throttling, then enters the evaporator for evaporation. After that, it passes the low-pressure stage four-way valve and flows into the low-pressure stage compressor. After being compressed, it passes the low-pressure stage and the high-pressure stage four-way valve in turn, and then mixes with the branched circuit gas refrigerant from the internal heat exchanger. The refrigerant mixture enters the high-pressure stage compressor and is further compressed then discharges.

The air source heat pump with double-compressor two-stage compression can be switched between single-stage compression mode and two-stage compression mode, which cannot only meet the requirements of heating operation in normal temperature condition, but also operate steadily and reliably for long time at the low temperature environment of — 18°C. The discharge temperature of the compressor is always lower than 130°C, which can meet the heating demand in winter in the cold climate regions without auxiliary electric heater, and the COP is relatively higher.

5. Two-stage coupled heat pump technology

The two-stage coupled heat pump system consists of an air-to-water heat pump system and a water-to-water heat pump system. The principle of this system is shown in Figure 1.9. The air source heat pump system and the water-to-water heat pump system are the first-stage and the second-stage, respectively.

FIGURE 1.9

The schematic diagram of two-stage coupled heat pump system

When the outdoor ambient temperature is higher, the second-stage (water- to-water) heat pump system does not operate, and the first-stage (air-towater) heat pump system operates. The hot water is pumped to the terminal (fan coil or floor heating) by pump 1, and then returns to the condenser of the first-stage heat pump system after releasing heat at the terminal.

When the outdoor ambient temperature is relatively lower, the first-stage (air-to-water) heat pump system and the second-stage (water-to-water) heat pump system both operate and the two three-way valves reverse. The relative^ low temperature hot water of 10-20°C produced by the first-stage heat pump system is pumped by the pump 1 to the evaporator of the second-stage heat pump system. The second-stage heat pump system absorbs heat from the relatively low temperature hot water to produce the high temperature hot water, which is then transported to the terminal by the pump 2 and returns to the condenser of the second-stage heat pump system after releasing heat at the terminal.

As is shown in Figure 1.9, under low and ultra-low temperature conditions, the two-stage coupled heat pump system reduces the pressure ratio of each stage of the compressor. Compared with the conventional single-stage compression heat pump system, the two-stage coupled heat pump system has the advantages of higher heating capacity, higher COP and lower discharge temperature. Compared with the double-compressor two-stage compression heat pump system shown in Figure 1.8, the heat loss increases due to the extra heat exchange process. Without the vapor injection process, the heating capacity and COP of the two-stage coupled heat pump system will be relatively lower, and the discharge temperature will be relatively higher, but the operation control of the system is simpler comparatively.

6. Cascade heat pump technology

The cascade heat pump system is composed of two relatively independent single-stage compression heat pump sub-cycles coupled by a condensation- evaporator, and the two sub-cycles are a high temperature cycle and a low temperature cycle, respectively. The system cycle is shown in Figure 1.10. Generally speaking, the high temperature cycle uses intermediate temperature refrigerant, and the low temperature cycle uses low temperature refrigerant.

In the low temperature cycle, the refrigerant enters the evaporator to evaporate after being throttled and depressurized by throttling device 2. Then the refrigerant is compressed by the compressor 2 and flows into the condensation- evaporator to condense and liquefy. Finally, it is throttled by throttling device 2, and the whole cycle finishes. While in the high-temperature cycle, after being throttled in the throttling device 1, the refrigerant enters the condensation- evaporator to evaporate and then enters compressor 1 for compression. After that, it flows into the condenser to condense, and enters throttling device 1 to throttle at last, which means the whole cycle finishes.

In the cascade heat pump system, the condensation-evaporator is the key component of the two coupled sub-cycles, the low temperature cycle and the high temperature cycle. It is the condenser of the low temperature cycle as well as the evaporator of the high temperature cycle, whose function is to transfer heat from the low temperature cycle to the high temperature cycle.

Compared with the conventional single-stage compression heat pump system. the pressure ratio of the low temperature cycle and the high temperature cycle of the cascade heat pump system decreases significantly, and the refrigerant temperature at the condenser outlet of the low temperature cycle decreases obviously. So the cascade heat pump has a higher heating capacity and COP, as well as lower discharge temperature when heating at low temperature. However, compared with the quasi two-stage or the two-stage compression heat pump system, the irreversible loss increases due to the addition of heat transfer process of the condensation-evaporator, so the heating capacity and COP are relatively lower, and the discharge temperature is relatively higher.

FIGURE 1.10

The schematic diagram of cascade heat pump system