Conference Agenda

A dynamic simulation model was established for a commercial refrigeration system with both medium temperature display cases and low temperature display cases connected to a CO₂ based booster rack that has vapor injection scroll MT compressors and digital scroll LT compressors installed. System performance under different environment conditions is studied with gas cooler pressure controller, flash tank pressure controller and economizer superheat controller as well as display case superheat controller and suction pressure controller being implemented through either a software based PID controller approach or a hardware in the loop approach. A new control strategy of such system is proposed based on simulation results.

By connecting a hardware controller with a software dynamic simulation model through MOD bus communication protocol, a CIL (controller in the loop) simulation study on a CO₂ booster rack commercial refrigeration system is presented. The implementation of the CIL simulation model is built on GT-SUITE platform with a python module to send the simulated system operation parameters as sensor readings to the hardware controller as inputs and to read the controller actuation command outputs to control simulated actuators action inside model. The CIL simulation model provides a much less expensive implementation compared to HIL (hardware in the loop) approach but still enable engineers to investigate dynamic characteristic of target system as demonstrated with examples in this study.

To assist building heating electrification, this paper numerically investigates a load flexible heat pump system for commercial buildings. The system consists of a CO₂ vapor compression cycle, a sensible thermal storage tank, and an air handling unit. The thermal storage medium is inexpensive, non-toxic and stable anti-freeze solution (30% potassium acetate). The air handing unit has an indoor coil and a ventilation coil. The system can be used to manage building electric load. During peak hours, the heat pump is off and the hot solution water is discharged from the tank to heat up the indoor air and ventilation air. During the hour of charge, the heat pump delivers hot solution water to the tank and to the air. The tank can also stand by while the heat pump provides space heating directly.

We selected a medium sized office building located in Minnesota as the representative building and used EnergyPlus to obtain its 24 hour load data. We designed three storage tank volumes assuming 50 °C, 65 °C and 80 °C tank temperatures to independently provide the building load for 4 hours in the morning. The higher the tank temperature, the smaller the required volume, and thus higher energy density. The effective energy density is 78 with an 80 °C tank, and 40 kWh_th/m³ with 50 °C.

We simulated the tank integrated heat pump performance subjected to the 24-hour building load profile and ambient data. The baseline is the same system without storage tank. There was a trade-off between the storage energy density and the charging COP. The charge hour COP was 2.77 to charge the tank to 80 °C, and 3.01 to 50 °C. The proposed system could shift building load from the peak hours (8:00 – 12:00) to off-business hour (23:00 – 7:00⁺¹). It eliminated 100% compressor electricity use during the peak hours, and avoided a peak electric power of 34 kW. The 65 °C tank saved 9.5 kWh_e (4%) considering all day operation, which was the best balance between energy density and the system operation efficiency among the three options.

During the vapor-compression refrigeration system operation, some amount of the compressor lubricating oil is inevitably discharged from the compressor, and this oil gradually accumulates in other system components. Oil retention in the system becomes crucial if this deposited oil does not eventually return to the compressor. The heat exchangers’ effectiveness and the compressor reliability are reduced due to the larger oil retention volume in the heat exchangers. A potential solution to this oil retention problem could be utilizing a two-phase ejector in an HVAC&R system to pump the accumulated compressor oil from the system components, for example, the evaporator, and ensure sufficient oil return to the compressor. So far, refrigerant ejectors are used in vapor-compression systems mainly for expansion work recovery and evaporator liquid overfeed. In this paper, a novel method aimed at pumping the accumulated oil using an ejector is introduced, and the performance of a vapor-liquid ejector for the oil pumping application is experimentally investigated. An experimental test facility consisting of an oil separator and a dedicated ejector for oil pumping is built and integrated with a standard two-phase ejector system using R744 (CO₂) as the refrigerant with PAG 46 oil. A portion of the high-pressure R744 vapor stream at the compressor outlet is bypassed and used as the motive fluid for the ejector, and the PAG 46 oil is entrained through the ejector suction chamber. For the initial experimental study, a standard two-phase R744 ejector geometry is used to gain insights into the oil entrainment capability and other ejector performance metrics. Experiments are conducted for a wide range of motive inlet pressures and flow rates. The vapor-liquid ejector performance is characterized by different parameters such as bypass ratio, entrainment ratio, pressure lift, and compression efficiency. Experimental results reveal valuable insights regarding the oil entrainment capability through the vapor-liquid ejector for the considered test conditions and ejector geometries. A numerical model for the oil entrainment ejector system is developed and validated using the experimental results.

Bladeless turbines are a promising application for small scale power extraction and would be an ideal candidate for power extraction in refrigeration cycles. This paper presents the numerical analysis of a bladeless turbine by use of Computational Fluid Dynamics (CFD). An unstructured meshing approach with detailed refinement around the nozzle is presented as well as the numerical framework to analyze such turbines. Performance parameters include torque, power, and coefficient of discharge and are calculated for various operating conditions (reduced mass flows and reduced speeds). Additionally, two different nozzles are compared to each other to dissect the loss and performance. Finally, the experimental counterpart and preliminary data is presented and compared to the numerical simulation.