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Energies 2020, 13, 5654 10 of 18 temperature of the storage tank is 67.5 ◦C with an upper dead band of 1.5 ◦C, so the heat pump is shut down when the temperature of the storage tank is higher than 69 ◦C. As can be seen in the chart during this day, the system works continuously, the heat pump starts for 29 times, and each on and off cycle lasts about 50 min. The pressure of the R744 transcritical cycle is shown in Figure 6b. The high pressure (HP—Point 3 of the scheme in Figure 2) of the cycle at the gas cooler varies between 95 and 80 bar while the low pressure (LP—Point 9 of the scheme in Figure 2) and suction pressure (SP—Point 6 of the scheme in Figure 2) at the compressor vary between 40 and 50 bar. A zoom of the on and off cycle of the heat pump is reported in Figure 7. In the charts, temperatures and pressures between 6:28 a.m. and 7:26 a.m. are reported. At the beginning of the on and off cycle, the high pressure rises to the maximum value, decreasing gradually until the stop of the heat pump when the setpoint temperature of the storage tank was reached. As can be seen in the chart in Figure 7a, before and during the first part of the cycle, the values of Tinwg_gc + cd and Toutwg_gc + cd were the same. This happens because the heat pump compressor was off until 6:37 a.m., but the circulator of the hydronic module started about two minutes before the compressor. Indeed, the compressor has a time delay of switch-on after the start of the circulator for safety and protection of the components of the heat pump. The details Energies 2020, 13, x FOR PEER REVIEW 11 of 17 Energies 2020, 13, x FOR PEER REVIEW 11 of 17 of the pressures of the R744 cycle are reported in Figure 7b. As can be seen in the chart, the SP and evaporator pressure (LP) are the same during the operation of the heat pump, as expected, while the heated up to 67/71 °C, maintaining an average storage tank temperature of 67.5 °C. At the source side heated up to 67/71 °C, maintaining an average storage tank temperature of 67.5 °C. At the source side SP is equal to the HP when the compressor is off because the two pressures are in equilibrium through of the heat pump, the heat carrier fluid from the BHE field loop has a monthly average temperature of the heat pump, the heat carrier fluid from the BHE field loop has a monthly average temperature the compressor. The LP is less than the other pressures because of the expansion valves present in the of 13.5 °C in November and 8.4 °C in December. The fluid is then cooled inside the evaporator to 9 of 13.5 °C in November and 8.4 °C in December. The fluid is then cooled inside the evaporator to 9 refrigerant loop. °C and 6 °C in November and December, respectively. °C and 6 °C in November and December, respectively. (a) (b) (a) (b) Figure 6. System’s temperatures (a) and pressures (b)—18 November 2018. Figure 6. System’s temperatures (a) and pressures (b)—18 November 2018.. (a) (b) (a) (b) Figure 7. System’s temperatures (a) and pressures (b)—18 November 2018—6:28/7:26. Figure 7. System’s temperatures (a) and pressures (b)—18 November 2018—6:28/7:26. Figure 7. System’s temperatures (a) and pressures (b)—18 November 2018—6:28/7:26. In Figures 8–11, similar charts are presented for two representative days in December. As for 4 December 2018, the plant was working for 14.9 h, providing water to the storage tank at 67.8 ◦C on average. From the graph in Figure 8a, it can be noticed that the switching-on cycles were lessPDF Image | Novel Ground-Source Heat Pump with R744
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