## 4 Conclusions  
In this study, energy and exergy performances and the environmental impacts of three different system configurations using low GWP hydrocarbon refrigerants were investigated. R290, R600a, and R1270 were the proposed hydrocarbon refrigerants, and R134a was selected as a baseline. The proposed configurations included a single-stage cycle, a cycle with an internal heat exchanger, and a two-stage cycle with vapor injection.  
It was found that the utilization of the IHX and vapor injection had a significant influence on the system's performance. The COP of the system increased by approximately 3% with the IHX and about 9.9% with vapor injection compared to a single-stage cycle using hydrocarbon refrigerants. The heating capacity decreased when using the IHX and vapor injection compared with the single-stage cycle. It decreased by 1.4% and 4% when utilizing the IHX and vapor injection, respectively. The discharge temperature increased dramatically when using the IHX, while it decreased with vapor injection. Considering the performance of the hydrocarbon refrigerants, R290 and R1270 showed higher heating capacities than R134a but lower COPs. R600a had a higher COP but lower heating capacity than R134a. In this case, a relatively larger compressor size is required to meet the baseline performance.  
The compressor had the highest exergy destruction in all cycles to be the main system performance improvement source. The vapor injection utilization improved the exergy efficiency of the system by approximately 8% compared with the single-stage cycle, while the IHX improved the exergy efficiency by between 2% and 3%. The use of hydrocarbons mitigated the CO\({}_{2}\)-eq emissions of the system compared with R134a remarkably. The CO\({}_{2}\)-eq emissions decreased by between 1% and 8% when using an internal heat exchanger and vapor injection compared with a single-stage cycle.  
The paper proves the energy and exergy benefit caused by advanced configurations (IHX and economizer); however, it also shows the other side of the coin: a slight decrease in heating capacity. Therefore, the trade-off solution between R134a and different refrigerants must be considered for each particular application (e.g., COP versus heating capacity). Exergy analysis shows that particular attention must be devoted to the compressor and improved with configurations. Benefits are extended to the carbon footprint. In light of these results, hydrocarbons are a promising alternative in heat pumps if the refrigerant charge does not limit the implementation. Because of the flammability of HCs, these refrigerants are subject to international safety guidelines and legislation, limiting their use to a per-system charge of no more than 150 g. Therefore, it is highly recommended to investigate the performance of refrigeration systems with a charge amount below the limit with enhanced heat enhancement or new hydrocarbon mixtures, or blended with HFCs/HFOs but with a higher content of hydrocarbons.  
Conceptualization, M.G. and A.M.-B.; methodology, M.G. and A.M.-B.; formal analysis, M.G. and B.E.B.; investigation, M.G. and A.M.-B.; resources, M.G. and A.M.-B.; data curation, M.G.; writ-ing--original draft preparation, M.G.; writing--review and editing, A.M.-B. and B.E.B.; visualizing-tion, M.G.; supervision, R.K.; funding acquisition, R.K. All authors have read and agreed to the published version of the manuscript.  
The research was funded by the Swedish Refrigeration Cooperation Foundation, KYS, and the Swedish Energy Agency.  
Not applicable.  
[PERSON] acknowledges the financial support of the Valencian Government by means of the postdoctoral contract APOSTD/2020/032.  
The authors declare no conflict of interest.