1. Introduction
In the past few years, energy consumption in the building sector has exploded as a result of urbanization and economic development. According to the International Energy Agency, global final energy consumption will increase by 60% in the building sector, bringing corresponding environmental challenges, such as carbon emissions [
1]. Air conditioning is an essential addition to most commercial buildings, although it is more widely used in residences. However, such systems consume large amounts of power and generate massive carbon emissions. Air conditioning systems and heat pumps account for about 7.8% of global carbon emissions [
2]. Environmental concerns and the development of low carbon emission buildings have led to restrictions on widely used high GWP refrigerants such as R134a, which pose serious challenges to the transformation and upgrading of the air conditioning industry and buildings. Environmentally friendly refrigerants with high energy efficiency have attracted attentions as a way to reduce carbon emissions [
3]. As alternative refrigerants, R32, R1234yf, R1234ze(E), R513a, R450a, R744a (N
2O) and R744 (CO
2), have been gradually investigated by those with an interest in developing refrigeration systems for air conditioning. The main properties of these refrigerants are listed in
Table 1.
Compared to a R134a system, the study of R32 refrigeration systems showed a 5–7% increase in COP [
5]. The GWP of R32 is relatively high, and so more environmental-friendly refrigerant alternatives are expected to emerge and take its place. R1234yf and R1234ze(E) have been suggested as promising refrigerants for their extremely low GWP and excellent thermal properties, despite their relatively high cost [
6]. Joaquin et al. conducted a theoretical and experimental study of a R1234yf refrigeration system with internal heat exchanger (IHX) [
7]. Although the IHX improved the COP by 2–6%, the COP of the R1234yf system was still 6–13% lower than that of the R134a system in air conditioning conditions. Considering the GWP of the refrigerant and its system COP, both R513a (a mixture of R134a and R1234yf) and R450a (a mixture of R134a and R1234ze(E)) have been considered as alternatives to R134a, as they both have similar thermal properties and relatively low GWP that have been studied. And carbon emissions have, over the life cycle of the system, been increasingly used to assess the environmental impacts [
8]. A substitution study of R450a and R513a showed that R513a had a slightly higher COP than the R450a system for air conditioning [
2,
9]. Compared to the R134a system, their direct carbon emissions were reduced by 50–52% for lower GWP and less refrigerant leakage. The carbon emissions of refrigeration systems during their life cycle need to be considered by referring to several sources, such as refrigerant leakage, electricity consumption and manufacturing processes, etc.
The International Institute of Refrigeration (IIR) developed the life cycle climate performance (LCCP) method to evaluate all the direct and indirect carbon emissions of a refrigeration system during its life cycle [
10], and it is widely used in comparisons between different refrigerants or refrigeration systems [
11,
12]. In the literature [
2,
9], when the carbon emissions generated by energy consumption are taken into account, the carbon emissions of the R513a system are equivalent to the R134a system, while the R450a system is 4.8% higher. In the transformation and upgrading of air conditioning systems, it is essential to not only focus on the use of 0 ODP and low GWP refrigerants, but to also obtain advantages, in terms of life cycle carbon emissions. The above studies imply that refrigerant alternatives are still confronted by challenges, and that the development of new alternative refrigeration will be an essential priority for future research.
Natural refrigerants have received widespread attention in the research of air conditioning systems. Of them, CO
2 (R744) is recognized as a non-flammable and non-toxic attractive option, with superior properties such as extremely low GWP, low production cost, and high thermal conductivity [
13]. Because of a relatively low COP, a basic CO
2 transcritical (BCT) refrigeration cycle has still not been adapted for air conditioning in buildings. Researchers have paid attention to the parameter optimization and cycle innovation of CO
2 transcritical refrigeration systems. Zheng et al. studied the CO
2 mass migration and distribution of an air conditioning system and found that an appropriate charge could improve the COP by 10.1% [
14]. In a subcooler-based CO
2 transcritical system, it was found that optimizing compressor discharge pressure increased the COP by 8.8% [
15]. Moreover, advanced cycles have been found to significantly affect the improvement of COP. A CO
2 ejector refrigeration cycle for air conditioning proposed by Lawrence et al. improved the COP by 20% [
16]. Tashtoush et al. proposed an ejector-cascade refrigeration cycle for air conditioning; when compared to the BCT cycle, this new cycle improved the COP by 30% [
17,
18]. From an environmental perspective, the improvement in COP shows the potential of CO
2 refrigeration in low carbon buildings. An energy and environmental analysis of a CO
2 heat pump system shows that its COP competes with a R134a system, reducing total carbon emissions by 52.05%, while maintaining a lower COP in cooling mode [
19]. A further study of the air conditioning performance of CO
2 refrigeration systems is however necessary.
Input power converted to high temperature heat at the outlet of the gas cooler is one of the limitations leading to low COP, and the high temperature heat might need to be recovered. Efficient absorption refrigeration systems have the advantage of low-grade heat recovery and use natural refrigerants as well. Hybrid refrigeration systems that combine absorption and vapor compression refrigeration systems have the potential to improve overall energy efficiency and reduce carbon emissions. Chan et al. developed a hybrid system for cooling by driving an absorption refrigeration system with condensing heat recovery, resulting in a 53% reduction in carbon emissions [
20]. Similarly, Jain et al. developed a vapor compression-absorption hybrid refrigeration system, with power consumption 170.4% lower than a vapor compression system [
21]. Andrej et al. used the heat from the CO
2 compressor outlet to drive an adsorption chiller [
22]. The hybrid system showed 22% annual energy savings. These studies suggest that coupling a CO
2 transcritical refrigeration system with an absorption refrigeration system may lead to a reduction in overall carbon emissions, and also that the application of CO
2 to air conditioning in low carbon buildings shows promise.
In our previous work, thermodynamic analyses of a two-stage absorption/transcritical hybrid refrigeration system was conducted [
23]. The high-pressure generator of the absorption subsystem is driven by the waste heat from the gas cooler outlet. The hybrid system converts waste heat as low as 45–55 °C into useful cooling of 7 °C, and uses a rational coupling method to recover the gas cooler waste heat with the absorption system. Further research should investigate the potential of the low-pressure generator to recover the waste heat and carbon emission reductions.
In this study, a novel CO
2 transcritical/two-stage absorption (CTTA) hybrid refrigeration system for air conditioning is conducted on the basis of both COP and LCCP value. A simulation model of the CTTA system is established and calculated by EES [
24] and the effects of key parameters on the COP are analyzed. Its carbon emissions are evaluated by using the LCCP method and are then compared with six typical refrigeration systems by using R744 (CO
2), R134a, R32, R450a, R513a, R1234yf, and R1234ze(E) as refrigerants, respectively. Finally, the potential LCCP values of the CTTA system are estimated to demonstrate the prospect it can be applied to low carbon emission buildings in the future.
4. Results and Discussion
The performance of the transcritical refrigeration subsystem is first compared by using CO
2he and N
2O as refrigerants. T
G1 and T
G2 are 55 °C and 52 °C, respectively, and other operating parameters are the same as those listed in
Table 2.
As shown in
Figure 4, both COP
tol and COP
ch of the N
2O subsystem decrease with the increase of discharge pressure (P
c), while the CO
2 subsystem first increases and then decreases because of its thermal property change near the critical point. As the exhaust pressure increases, both the power consumption and the cooling capacity of the transcritical subsystem increase. At the same time, the increase in the cooling capacity of the transcritical subsystem causes the water vapor to be gradually and completely condensed in the condensing subcooler, resulting in an increase in the cooling capacity of the CTTA system. Since the CTTA system consists of a coupled two-stage absorption refrigeration subsystem and a transcritical heat pump subsystem, it is driven by both external low-grade energy and mechanical work. When the Pc is higher than the optimal value for this operating condition, the cooling capacity generated by the input mechanical work is increasing, leading to a decrease in the COP of the CTTA system.
COP using CO
2 is higher than that using N
2O when the discharge pressure is higher than 8.3 MPa and the GWP of N
2O is higher than CO
2. So the novel hybrid system uses CO
2 as the refrigerant and the simulation is carried out in different working conditions, and the other designing parameters are the same as those listed in
Table 2.
In order to analyze the COP improvement of the hybrid system, a BCT refrigeration system with IHX previously examined by Zhang et al. is used as a comparison [
40]. The maximum COP obtained for this BCT system is 2.75 when the T
E is 10 °C, the gas cooler outlet temperature is 20 °C, and the P
c is 10 MPa; in this study, it is named COP
t.
CS is a key component of the CTTA system that was introduced in our previous study [
23]. The effect of T
m on COP
tol is first investigated, and the results are shown in
Figure 5. The other parameters are the same as those of the design conditions.
Figure 5 shows that there is a maximum COP
tol , which occurs with the increase of T
m. The optimal value of T
m is 19 °C and the maximum COP
tol is 2.38, which is 4.4% higher than the minimum COP
tol. The absorption pressure of AB1 decreases with the increase of T
m. The concentration of the dilute solution at the outlet of AB1 decreases when the absorption temperature is constant, so it can be driven by a lower temperature at the same generation pressure. The concentration of dilute solution entering G2 remains unchanged but the generation pressure decreases, making both T
G2 and T
G1 increase. CS is the evaporator for the CO
2 transcritical subsystem. With the increase of T
m, the power consumption of COMP is gradually reduced, and the performance of the CO
2 transcritical subsystem is then improved. Adjusting T
m is beneficial for reducing the input temperature of G1 and G2, which means more discharge heat can be recovered by a two-stage absorption refrigeration system. In a word, T
m has a significant impact on the performance of the CTTA system, and so it is necessary to match T
m reasonably.
Figure 6 shows the effects of T
G2 on COP
tol and COP
ch, while the other parameters are the same as in the design conditions mentioned above. It can be seen that the CTTA system has an optimal value of T
G2 in the obtaining of high COP. The maximum value of COP
tol and COP
ch is 3.70 and 2.90, respectively, when T
G2 is 58 °C. The mass flow rate of water vapor generated at G2 increases with the increase of T
G2, so the refrigeration capacity increases, which leads to an increase in COP
ch and COP
tol. With the further increase of T
G2, the water vapor cannot be totally condensed at CS. Thus, the outlet quality in CS gradually increases after the mixture process, which leads to the increase of throttling loss at TV3. COP
tol and COP
ch then decline. Meanwhile, the traditional absorption refrigeration system cannot work normally when the generation temperature is below 58 °C. The refrigeration capacity is entirely generated by the input mechanical work, and so COP
ch increases. When T
G2 is higher than 58 °C, the refrigerating capacity generated by the absorption system gradually increases, leading to the rapid decline of COP
ch. The maximum COP
tol is 27.6% higher than the maximum COP
ch, and 34.5% higher than COP
t, which shows the advantage of utilizing the exhaust heat of GC.
The effects of T
E on COP
ch and COP
tol are depicted in
Figure 7, while the other parameters are the same as design conditions. COP
ch and COP
tol keep rising as T
E increases. The maximum COP
tol is 3.84 which is 21.5% higher than COP
ch, and 39.6% higher than COP
t. T
E causes the increase in the concentration difference of solutions in AB2, which leads to the increase of the mass flow rate of refrigerant in G2 and the refrigeration capacity in E. Therefore, the performance of the absorption refrigeration subsystem rises first, and COP
tol increases as well. However, when T
E is higher than 6 °C, more water vapor is generated; this vapor cannot be totally condensed in CS, so the quality of CS at the outlet gradually increases after the mixture process, which leads to the increase of throttling loss at TV3. Thus, the performance of the absorption refrigeration subsystem decreases and the slope of the COP
tol curve goes down.
The effects of T
C are shown in
Figure 8, while the other parameters are the same as those of the design conditions. With the increase of T
C, the heat transfer efficiency in C decreases, which causes an increase in refrigerant quality at C outlet and CS. Then, the refrigeration capacity declines and the performance of the absorption subsystem decreases. Meanwhile, the conventional two-stage absorption refrigeration system cannot be driven when T
G2 is lower than 60 °C. The increase of T
C will reduce the performance of the CO
2 transcritical subsystem, and the COP
ch shows a similar trend with COP
tol. The maximum COP
tol is 4.01 and COP
ch is 3.01 in the T
C of 28 °C. The maximum COP
tol is 33.2% higher than COP
ch, and 45.8% higher than COP
t.
The effects of T
AB on COP
ch and COP
tol are illustrated in
Figure 9, and the other parameters are the same as the design conditions. The heat load on the refrigeration water side of the absorber decreases due to the increase of T
AB, which causes the vapor flow from G1 and G2 to decrease. Therefore, the refrigeration capacity of the CTTA system is reduced and COP decreases with the increase of T
AB. But because the vapor generated by G1 and G2 can be completely condensed in CS, the throttling loss of the system is reduced. In addition, the decrease of the vapor flow rate will also enhance the heat transfer efficiency of heat exchangers. Therefore, when T
AB is below 28 °C, the COP
tol is still high; when T
AB is greater than 28 °C, the system performance starts to decrease; and when T
AB is below 28 °C, the generation temperature is low, so the COP
a is 0 and the COP
ch decreases with the increase of T
AB. The maximum COP
tol of 3.78 and the maximum COP
ch of 3.02 are obtained at T
AB of 28 °C.
The effects of T
air on COP
ch and COP
tol are shown in
Figure 10, while the other parameters are consistent with the design conditions. As T
air increases, an optimal value of T
air exists, that can be used to obtain the maximum COP. The temperature of CO
2 at the GC outlet increases as T
air rises, resulting in increases of T
G1 and T
G2. So the performance of the absorption subsystem is increased first. In the CO
2 transcritical subsystem, the increase in T
air causes the temperature of CO
2 at the outlet of GC to increase, resulting in a decrease in CO
2 flow rate, refrigeration capacity, and COP of the CO
2 transcritical subsystem. There is therefore an optimal T
air to maximize the COP
tol. For COP
ch, an increase in T
air leads to a temperature increase in CS; an optimum value of T
m also exists, so the COP
ch increases first and then decreases. The optimal value of T
air is 58 °C, when the maximum COP
ch is 3.32 and the maximum COP
tol is 4.32, which is 30.1% higher than COP
ch.
The effects of P
c on COP
ch and COP
tol are displayed in
Figure 11, while the other parameters are the same as the design conditions. There is also an optimal value for P
c to maximize system performance. Both the power consumption and the refrigeration capacity of the CO
2 transcritical subsystem increase as P
c increases. The increase in refrigeration capacity of the CO
2 transcritical subsystem allows the water vapor to be completely condensed in CS, resulting in an increase in the refrigeration capacity of the absorption subsystem. When P
c is lower than the optimal value, the refrigeration capacity contributed by the absorption subsystem is relatively high, so COP
tol gradually increases. And when P
c is higher than the optimal value, the refrigeration capacity generated by the CO
2 transcritical subsystem input mechanical work increasingly becomes larger, leading to a reduction in the performance of the CTTA system. The optimal value of P
c is 9.1 kPa when the maximum COP
tol is 4.18 and the corresponding COP
ch is 3.32.
The determination of the optimal parameters is essential to improve the performance of the CTTA system and reduce its carbon emissions. In the three optimal operating conditions of
Figure 6,
Figure 10 and
Figure 11, COP
tol improved by 34.5–57.1% over COP
t. The coupling method to GC is the major difference between the CTTA system and the conventional hybrid refrigeration system. Optimizing T
air can improve 27.6% of the COP
tol, compared to COP
ch, while optimizing T
G2 can improve 13.2%. The highest COP
tol of 4.32 indicates that the proposed coupling method provides a significant improvement when compared to conventional hybrid systems, and accordingly will result in a lower carbon emission of the refrigeration system, making the application of CO
2 to air conditioning in low carbon emission buildings more competitive.
To demonstrate the potential of the CTTA system to contribute to air conditioning in low carbon emission buildings, the LCCP method is used to compare the CTTA system with a BCT system and seven conventional refrigeration systems that use CO
2, R134a, R32, R450a, R513a, R1234yf, and R1234ze(E), respectively. The coefficients required in the calculation of the LCCP values are obtained from the IIR guidelines [
10], and the COP reference values for each system are obtained from the literature [
40,
41,
42,
43,
44]. The main data required for the calculations are listed in
Table 4. And the calculation results are shown in
Table 4 and
Figure 12.
As seen in
Figure 12, systems with low GWP refrigerants all show a considerable reduction in direct carbon emissions, when compared to R134a systems. Direct carbon emission of the BCT system, for example, is comparable to the R1234yf and R1234ze(E) systems, and is seen to be significantly reduced when compared to the R134a and R32 systems. It shows that CO
2 used for air conditioning has a huge advantage in direct carbon emissions, compared to other low GWP refrigerants. Indirect carbon emissions account for the largest share of the LCCP value, of which carbon emissions generated from power generation are the main cause.
The BCT system has the highest LCCP value of 5788 kg CO2e/kW, as its COP is much lower than the other systems. It also shows that the BCT system for air conditioning is still confronted by challenges. The R134a system already has a high COP at present, so its indirect emissions are 40% lower than the BCT system. But the higher GWP of the refrigerant also makes its LCCP value higher than the other systems. With a higher COP, the CTTA system has an LCCP value of 3768 kg CO2e/kW, which is not only 53.6% lower than the BCT system, but also lower than the R134a system, which shows the potential of the CTTA system for air conditioning. R450a and R513a still have higher GWP, and their direct carbon emissions are 400% higher than that of the CTTA system. The R1234yf and R1234ze(E) systems, which also have extremely low GWP, have the lowest LCCP values , 18% and 20% lower than the CTTA system, respectively, which is because their COP is also higher. The LCCP values of the R32, R450a and R513a systems are reduced compared to the R134a system, but are slightly higher than the R1234yf and R1234ze(E) systems. From the perspective of the LCCP value, the CTTA system achieves ultra-low carbon emissions (compared to conventional BCT systems), greatly improves competitiveness when using CO2 for air conditioning, and also has the advantages of natural refrigerants. In order to underscore the potential for CTTA system applications, comparisons should be made with the same models and operational conditions. Unfortunately, few studies of the life cycle analysis for related systems can be found. The lack of comparative data obtained in the same operational conditions clearly challenges efforts to accurately evaluate CTTA systems. However, there is a growing interest in a trend that seeks to demonstrate application potential by undertaking life cycle analysis.
It is worth mentioning that the CTTA system has huge potential to reduce carbon emissions, as the efficiency of the CO
2 transcritical subsystem could be further improved. In the Introduction, performance enhancement methods for CO
2 transcritical refrigeration systems were discussed [
14,
15,
16,
17]. The COP of a BCT subsystem could be enhanced by the above methods. The potential of the CTTA system to reduce the LCCP value could also be estimated by calculating the improvement in COP by using the above methods. The COP of the CTTA system is evaluated in
Table 5. The LCCP values are also compared with other conventional systems in
Figure 13, and are summarized in
Table 5.
For example, it is only after the key parameters of the CO
2 transcritical cycle are optimized that the potential carbon emissions of the CTTA system could be reduced to 3433 kg CO
2e/kW, which is 9.1% lower than the R134a system and equivalent to the R32 system [
14]. When the CO
2 subsystem uses ejector to lift COP, the carbon emissions of the CTTA system might be reduced to 3160 kg CO
2e/kW [
16], which is comparable to the carbon emissions of the R1234yf system. Further, with the use of the ejector-cascade cycle [
17], the potential carbon emissions of the CTTA system are reduced to 2926 kg CO
2e/kW. This carbon emission is 3.4% lower than the R1234ze(E) system, demonstrating the prospect that the CTTA system could positively contribute to air conditioning applications in ultra-low carbon emission buildings.