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Energies 2021, 14, 4103 6 of 28 The CO2 compressors remove vapor from the evaporator pressure level and discharge it to the gas coolers at high pressure. The compressor rack consists of four parallel piston compressors, in which compressor 1 in Figure 1 is equipped with a variable speed drive (VSD). Compressors 2 to 4 operate at a fixed speed and are controlled by ON/OFF, while the VSD compressor continuously adjusts the compressor section capacity according to demand. Thus, a broad range of capacities can be achieved by employing the VSD compressor alongside various combinations of the fixed compressors. The number of active compressors is determined based on the magnitude and combination of thermal demands; typically, high SH and DHW demands at low ambient temperatures and high AC cooling demands at high ambient temperatures. The CO2 gas cooler section is applied for heat recovery to SH and DHW, heat rejection to the ambient, or as a combination of the aforementioned. When heat rejection to the ambient is required, the directional valve, DV1, directs the flow towards the air-cooled gas coolers, GCair. The gas cooler section producing SH, GCSH, are in these instances bypassed. The number of gas cooler units employed is determined based on the AC cooling and the DHW demand. The main CO2 gas cooler section consists of three plate heat exchangers that supply heat for DHW and SH. The temperature span of each heat exchanger is arranged according to the transcritical temperature glide of CO2 [20]. Preheating of the DHW takes place at the lower end of the CO2 temperature glide, in GCDHW,1, as cold city water enters the heat exchanger. The DHW is further reheated to its setpoint temperature in GCDHW,2. The modulating valve, MV1, continuously controls the flow of CO2 through GCDHW,2 to reach the DHW setpoint temperature. Thus, the load distribution between GCDHW,1 and GCDHW,2 automatically adjust according to the load and temperature profile of the mid heat exchanger, GCSH. For instance, the majority of the DHW load is rejected through GCDHW,1 when the SH demand and setpoint temperature are high. During operations with low SH demand, most of the DHW load is rejected through GCDHW,2. This configura- tion enables continuous low load production of DHW, which in turn reduces gas cooler outlet temperature and enhances overall system COP [30]. The requested DHW heating load is determined based on the energy reserve in the DHW storage, constituted by the temperature and volume in the storage tanks. The storage, which is shown in Figure 1, has a water volume of 6 m3 and is comprised of hot water tanks connected in series. The energy reserve is calculated based on the temperature boundary across the storage. At times when the temperature in the storage is low, a signal is sent to the DHW pump, P1, to increase the mass flow and thus DHW charging load. DHW enters the first tank in the series, T1, and the hot water boundary gradually moves across the storage from right to left during charging. Cold water is supplied from city water or drawn from the last tank, Tn, and is directed towards preheating. The storage is fully charged when the last tank in the series, Tn, reaches a high and uniform temperature. The control of the high pressure is achieved with the high-pressure valve (HPV) in Figure 1. The integrated unit typically operates in the transcritical region (above 73.8 bar) to ensure the DHW setpoint temperature is reached. The maximum operating pressure of the system is 105 bar. After expansion, liquid enters the receiver, which holds a pressure between 36 to 50 bar. The receiver pressure is controlled by the flash gas valve, FGV, and is regulated according to evaporating pressure. During operations with low ambient temperature and high heating loads, receiver pressure is reduced to limit the vapor fraction at the inlet of the evaporators. 2.2. Alternative System Configurations Two alternative system designs are presented in Figure 2. Both configurations, (a) par- allel compression (PC) and (b) ejector-supported parallel compression (EJ), introduce compression from the liquid receiver at INT pressure level. Compressors 2 and 3 are equipped with pivoting suction ports, which directions are controlled by DV3 and DV4,PDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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