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significantly lower (Figure 8) and thus the compressor work (Figure 7). However, Case #5 demonstrates slightly lower gas cooler heating capacity compared to the base case. Lowering the gas cooler outlet temperature (case #1) also presents a positive effect on COP, particularly at low evaporating temperature (up to 6% at -12 °C). The gas cooler heating capacity also improves despite the increase in the discharge pressure (Figure 8) and compressor work (Figure 7). This is mainly due to the superior gas cooler performance at specific pressure and temperature conditions. On the contrary, by increasing the water temperature entering the gas cooler the COP reduces by up to 6% mainly due to the increase in the discharge pressure. Changing the IHE size and the intermediate pressure (#8 and #7 respectively) does not change the COP and qh. Under given conditions, more superheat at the compressor suction improves marginally the COP, while less superheat (#4) decreases slightly the COP. Lowering the water mass flow rate in #6 decreases significantly the COP by 15% at 0 °C. In order to satisfy the given Tout_CO2 in #6, discharge pressure increases (Figure 8) and ultimately does the compressor work (Figure 7). 14000 13000 12000 11000 10000 9000 8000 7000-15 -12 -9 -6 Mean evaporating temperature (C) -3 0 X #3 # #4 β #5 φ#6 + #7 0#8 α#9 φ φ φ#+0 φφ #+X Base case #1 #2 φ φ #0αα #0+X β φ #+Xαβ φ #0+Xα β β #0+X #X+Xβ 0+ φ φ 2 1.8 1.6 1.4 1.2 1-15 -12 -9 -6 -3 0 Mean evaporating temperature (C) Base case #1 #2 X #3 ##4 β #5 φ#6 + #7 0#8 α #9 φ φ φ φ# +0 φ#X φ #0Xα + φ #+X0 0+# X 0 φ #0Xαβ #+X β φ0αβ φ +0 #X β β # Xα Figure 7 Compressor work (Wcomp) Figure 8 Discharge pressure (Pdischarge_comp) A combination of #5 and #1 (best cases) is also presented (#9) at the end of this study to show how proper design and control can promote a good system performance. In this case Tout_CO2 and ṁ are changed compared to the base case (Table 5). Results show a COP improvement by up to 25% compared to #6 and 10% compared to the base case. Gas cooler capacity is also improved by 7.5% compared to the base case CONCLUSION In this study, an experimental test bench of a transcritical CO2 DX-GSHP that was built at CanmetENERGY research laboratory was described. Then, a previously developed theoretical model of the system is modified and validated against a set of experimental results. Finally, a parametric analysis was performed using the theoretical model for understanding the system and at exploring the performance improvement actions. Under using identical components and given conditions, the effect of various operating parameters on COP, gas cooler heating capacity, compressor work and discharge pressure was investigated. Results showed that improper control of some parameters such as gas cooler CO2 outlet temperature and discharge compressor pressure can water Compressor work, Wcomp (kW) Discharge pressure, Pdischarge_comp (kPa)PDF Image | Direct expansion ground source heat pump using R744
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