Direct expansion ground source heat pump using R744

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Direct expansion ground source heat pump using R744 ( direct-expansion-ground-source-heat-pump-using-r744 )

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control is not adopted here for the parametric analysis as the CO2 outlet temperature from the gas cooler is kept constant. Superheat controls at the borehole outlet and at the compressor suction are combined here into one control at the compressor suction and the entire CO2 flow is going through the IHE. The overal heat transfer coefficient (U) of the gas cooler varies from case to case and with evaporating temperature due to changes in CO2 mass flow rate and properties (UAGC changes from 0.12 to 0.3 kW/K). Table 5. Different cases for parametric analysis Base Case &U ID cm 0.64 D OD 0.8 Tground Other cases #1 Tout_CO2 #2 Tin_water 25 °C 25 °C 10 °C 1 °C 0.03 kg/s 0.02 kg/s 3578 kPa 10% 0.03 kg/s 25 °C kgrout W/m/K 1.0 Length m 3X30 #3 ΔTsh_comp #4 ΔTsh_comp #5 𝑚𝑚̇ °C 9.0 𝑚𝑚̇ kg/s 0.025 #6 𝑚𝑚̇ 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 Tin_water °C 20 #7 Preceiver Tout_CO2 °C 30 ΔTsh_comp °C 5.0 𝑚𝑚̇ 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 Preceiver kPa 4500 𝜀𝜀𝐼𝐼𝐼𝐼𝐼𝐼 40% #9 Tout_CO2 #8 𝜀𝜀 𝐼𝐼𝐼𝐼𝐼𝐼 Figures 5, 6, 7 and 8 summarize the parametric analysis by demonstrating the effects of several parameters on COP, gas cooler heating capacity, compressor work and discharge pressure over the mean evaporating temperature, respectively. 3.6 3.5 3.4 3.3 3.2 3.1 3 2.9 2.8 2.7 2.6-15 -12 -9 Mean evaporating temperature (C) -6 -3 0 Base case #1 α #2 X#3 ##4 β#5 +φ #6 #7 0#8 α #9 0 +# X# φφ +0# φ φ φ β 0# X 0X#+ β +0 0+ 0 # # α αβ α α β X βX X +0 X## X β φ φ φ 6 5.5 5 4.5 4 3.5 3-15 -12 -9 -6 -3 0 Mean evaporating temperature (C) X #3 βφ #5 #6 + #7 0#8 α#9 # #4 α #φβ φ X# +0# Base case #1 #2 α#φ α +0X φ0 +X# φ+ β φ0X# φβ 0X# X αX φβ α#0 φβ #+ X 0 0β #φ + X Figure 5 Coefficient of performance (COPheating) Figure 6 Gas cooler heating capacity (qh) As shown in Figure 5, case #5 (𝐦𝐦̇ 𝐰𝐰𝐰𝐰𝐰𝐰𝐰𝐰𝐰𝐰=0.03 kg/s) offers the highest COP among cases #1 to #8 for evaporating temperatures from -6 °C to 0 °C (up to 8% at 0 °C). This is due to the fact that the discharge pressure is COP heating Gas cooler heating capacity, qh (kW) Other Gas Boreholes cooler

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