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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focused on the numerical modeling of CO2-filled U-tube vertical boreholes under forced circulation. Very few works have looked at the whole CO2 GSHP cycle. Austin and Sumathy (2011) simulated a simple CO2 transcritical cycle. However, they did not account for dynamic characteristics of the system. A recent study by Eslami Nejad et al. (2015) developed a quasi-transient CO2 transcritical ground source heat pump model along with numerical and experimental validation of the borehole portion. In the present study, the model is validated using a set of experiments performed at CanmetENERGY Research Laboratory and then, it is modified and used to perform a parametric analysis on several system parameters. SYSTEM DESCRIPTION Figure 1 shows the schematic presentation of a single-stage transcritical CO2 DX-GSHP system with hot gas bypass working in heating mode. The system consists of eight main system components including compressor (1-2), gas cooler (2-3), internal heat exchanger (3-4), two different expansion valves (4-5 and 6-7), pressure regulating valve (9-10), receiver (5-6) and boreholes (7-8). As shown in Figure 1, CO2 (Refrigerant) is flowing through a complete cycle by going directly down to the borehole, changing direction at the bottom (U connection) and coming up to extract heat (qb) from the ground by evaporation. Then it enters the internal heat exchanger (IHE) to exchange heat with the gas at the gas cooler exit in order to be superheated to a certain degree. The gas is then compressed by the compressor to supercritical pressure with a corresponding temperature rise. The high pressure/high temperature vapor enters the gas cooler to heat the water (qh). After the IHE, low temperature/high pressure CO2 gas is throttled to the intermediate pressure level of the cycle. Two-phase CO2 (vapor and liquid) enters the separator and the vapor part is bypassed around the boreholes. Both liquid and vapor parts are throttled to the low pressure level of the cycle. Finally, CO2 with very small vapour quality enters the boreholes and mixes with bypassed vapor at the borehole outlet. (a) Figure 1 (a) System schematic and (b) qualitative temperature-enthalpy curve (b)

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