1. Introduction
The energy consumed in the building sector is about 20% of the world’s delivered energy consumption, and it increased by 1.3% per year from 2018 to 2050 [
1]. Meanwhile, about half of the total building energy consumption is used for space heating and hot water supply [
2]. Heat pumps are mechanical devices to transfer heat energy from a lower to higher temperature level mainly for space and hot water supply, which mainly includes a mechanical vapor compression cycle, an absorption cycle and an adsorption cycle [
3]. The heat-operated absorption and adsorption cycles are suitable for application in a place where inexpensive heat source or waste heat is available because the compressor is not needed in these heat pump cycles. However, their achievable coefficient of performance (COPs) are lower than that of the mechanical vapor compression heat pump cycle, which is widely used nowadays [
4]. Therefore, a vapor compression heat pump system has high efficiency and is considered to be competitive to decrease energy consumption and thus reduce CO
2 emission [
5,
6]. Continuous efforts with regard to research on more efficient systems have mainly focused on viewpoints using alterative cycles [
7,
8,
9,
10] and the use of new refrigerants [
7,
11].
Refrigerant injection technique has been utilized as one of the technologies to improve the efficiency of heat pumps or widen the operation range of heat pumps by decreasing the discharge temperature. This can be classified into two types: liquid refrigerant injection and vapor refrigerant injection [
12]. The vapor refrigerant injection technique has been well justified to enhance the performance of a heat pump system, especially at severe operating conditions [
13]. Cho et al. [
14] stated that the heat pump COP was 2.6–7.0% higher than those without vapor injection. The vapor injection cycle injects refrigerant vapor into the injection port of a two-stage compressor, which can be categorized into flash tank vapor injection (FTVI) cycle and internal heat exchanger vapor injection (IHXVI) cycle, according to the methods of acquiring the saturated or super-heated vapor to be injected [
15,
16,
17]. The FTVI cycle uses a flash tank to separate two-phase refrigerant into saturated vapor and liquid, and the saturated vapor is injected into the suction port of the compressor. Conversely, the IHXVI cycle adopts internal heat exchanger to vaporize two-phase refrigerant, and super-heated refrigerant is produced and then injected into the suction port of compressor [
12,
13]. The use of vapor injection technology application in the heat pump system has attracted much attention in recent years, mainly with a special compressor with an injection port.
Ma and Chai [
18] studied the heating performance of an air source heat pump employing the IHXVI cycle. Their results showed that both the heating capacity and heating COP increased remarkably. Ma and Zhao [
19] compared the performance of heat pump system installed a flash tank with that of an air source heat pump with a sub-cooler experimentally. The heat pump system with a flash tank showed better efficiency than of the sub-cooler heat pump system at a low ambient temperature of −25 °C. Wang et al. [
15,
16] investigated the performance of an 11 kW air source heat pump system with vapor-injected two-stage scroll compressor using R410A as refrigerant. The FTVI cycle and the IHXVI cycle showed comparable performance improvement experimentally, and a maximum COP improvement of 2–4% could be obtained by adopting the vapor injection technique. Heo et al. [
20] examined the effect of FTVI on the heating performance of a heat pump installed in an inverter-driven compressor. The heating capacity and heating COP were enhanced by 25% and 10%, respectively at the temperature of −15 °C due to increased total mass flow rate of refrigerant. Their results showed that the heating and cooling performances could be improved effectively by using vapor injection techniques. Yan et al. [
21] tested the heating performance of a R410A heat pump with the FTVI cycle using an inverter-driven twin-rotary compressor. Zhang et al. [
22] examined the heating performance of an economized vapor injection air-source heat pump system in a cold climate, and a 4–6% improvement was obtained. The vapor injection cycle was suggested to be used at low ambient temperatures to get higher heating capacity and heating COP compared with conventional heat pump cycle without vapor injection. In addition, the compressor discharge temperature can be decreased by vapor injection techniques.
Some heat pump configurations with novel vapor injection cycles have been proposed and investigated in order to improve the performance of heat pumps or enlarge the heat pump’s operation range. An air source heat pump with an IHXVI cycle was modified by the authors of [
23] by adding an additional expansion valve at the outlet of condenser. A high intermediate pressure was likely to obtain a high heating capacity and heating COP, however the vapor injection range was limited. Heo et al. [
24] evaluated the heating performance of two air source heat pumps using novel vapor injection cycles: one was the combined flash tank and sub-cooler cycle and the other was the double expansion sub-cooler cycle. The average COPs of the two cycles were similar to those of the FTVI and IHXVI cycles. Xu et al. [
25] proposed a vapor injection with sub-cooling cycle, which can further decrease the discharge temperature of the compressor to assure a stable operation when the evaporating temperature is lower than −20 °C.
However, compared to a conventional single-stage heat pump system, the above-described vapor injection heat pumps are more complex and expensive since additional components, such as a special compressor with a supplementary port, are needed. In recent years, heat pump systems with vapor injection to the suction port of a single-stage compressor have been recommended as a relatively inexpensive solution for vapor injection heat pump system. Lai et al. [
10] developed a single-stage air source heat pump with indirect vapor injection piping connecting a flash tank at the refrigerant outlet of the condenser and the accumulator of a scroll compressor. They concluded that the COP increments of 5–15% had been obtained when the heat pump was operated under ambient temperatures ranging from 5 °C to 35 °C. Roh and Kim [
13] proposed a novel vapor injection cycle by injecting vapor refrigerant into the accumulator instead of the injection port of compressor. The system was tested when the compressor frequency was in the range of 60–100 Hz. Their results showed that this cycle can increase both the heating capacity and heating COP slightly at a high compressor frequency of 80 Hz when the outdoor air temperature was set at −8.3 °C of the dry-bulb temperature. Lee et al. [
26] tested the cooling performance of an R22 refrigeration system of 9.6 kW by injecting vapor and liquid refrigerant to a single-stage compressor inlet through accumulator at high compression ratio. Their experimental results showed that for the vapor injection system, the COP decreased with an increasing injection ratio, while in the liquid injection system, a maximum injection ratio of 10% was recommended to minimize the performance decrease and to decrease the discharge temperature of the compressor, and thus extent the operating range of a refrigeration system.
The above-mentioned single-stage refrigerant injection systems mostly concern the heating or cooling performance of a vapor or liquid injection system with a flash tank and accumulator together. Fewer studies have investigated the performance of a single-stage vapor injection heat pump with only a gas–liquid separator, especially when the system is operated under different primary temperatures and compressor frequencies. In this study, a single-stage heat pump with vapor injection has been developed by installing a small sized surface tension type gas–liquid separator instead of a flash tank to separate the vapor and liquid refrigerants. The vapor separated by the gas–liquid separator is injected to the inlet of the single-stage compressor. The performance of the developed vapor injection heat pump was investigated in order to understand the heating performance of the vapor injection heat pump under different compressor frequencies and primary temperatures.