# Transcritical R744 Supermarket Refrigeration System Integrated with a Heat-Driven Ejector Chiller
## 2 Materials and Methods  
### System Description  
The layout of the proposed R744 supermarket refrigeration system coupled to a heat-driven multi-ejector chiller is depicted in Figure 1[16]. The R744 system includes low-temperature (LT) evaporators and medium-temperature (MT) evaporators for use in LT and MT display cabinets. The low-stage compressors (LTC) are used to pressurize the refrigerant from the outlet of the LT evaporators up to the MT level. The superheat at the outlets of the LT and MT is maintained by the expansion valves upstream of the LT and MT evaporators, respectively. The combined flow from the outlet of the LTC and MT evaporators is pressurized by the medium-stage compressor (MTC) up to the gas-cooler/condenser (GC) pressure. The flash vapor accumulated in the receiver tank is handled by the parallel compressor (PC), which pressurizes the vapor up to the GC pressure. However, below 13 \({}^{\circ}\)C outdoor air temperature, the PC was assumed to be switched off [16; 23], as the amount of flash vapor in the receiver is generally low. In that case, the vapor is bypassed down to the MT level using the flash vapor bypass valve (FVBV), which also regulates the receiver pressure. Instead of rejecting the entire heat into the ambient air in the GC, a part of this heat could be utilized to drive a heat-driven ejector chiller system (HEC) for subcooling the R744 at the GC outlet. The HEC unit comprises an evaporator/subcooler, a condenser, a generator, a multi-ejector rack (MEJ), a pump, and an expansion valve. A heat source is required for the generator. When integrated with the R744 refrigeration system, this external heat was supplied by the combined high-temperature R744 stream from the discharge of the medium-temperature (MT) and parallel compressors. The heated fluid at the outlet of the generator was fed to the multi-ejector rack (MEJ) as the motive stream. The MEJ rack consists of five fixed-geometry ejectors having different sizes [16]. The detailed specifications of the ejectors used in the HEC cycle are reported in [16]. The high-temperature, high-pressure vapor from the generator enters the motive nozzle of the ejector. The expansion of the motive flow in the nozzle creates low pressure at the nozzle outlet. This causes entrainment of the low-pressure vapor from the evaporator of the HEC (i.e., subcooler of R744 system). The MEJ lifts the refrigerant from the subcooler pressure up to the heat rejection pressure of the condenser. The refrigerant vapor, while passing through the condenser, rejects the heat into the ambient air. A part of the liquid refrigerant from the condenser outlet is pumped back to the generator. The remaining HEC fluid is throttled down to the pressure of the subcooler, in order to extract the heat from the R744 coming from the GC. This causes the temperature of the R744 at the GC outlet to further reduce below the ambient temperature. The subcooled R744 is then expanded down to the receiver pressure through the high-pressure expansion valve (HPEV). The HPEV is also used to regulate the GC pressure.  
Figure 1: Schematic of the R744 refrigeration unit equipped with parallel compression and HEC [16].