The performance of a reversible R744 vapor compression system with an internal heat exchanger in continuous frosting/ defrosting cycles

Zhang, Wenying

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https://hdl.handle.net/2142/115866 Copy

Description

Title

The performance of a reversible R744 vapor compression system with an internal heat exchanger in continuous frosting/ defrosting cycles

Author(s)

Zhang, Wenying

Issue Date

2022-06-15

Director of Research (if dissertation) or Advisor (if thesis)

Hrnjak, Pega S

Doctoral Committee Chair(s)

Hrnjak, Pega S

Committee Member(s)

• Jacobi, Anthony M
• Elbel, Stefan
• Kozlowski, Tomasz

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• HVAC
• automobile heat pump
• R744
• CO2
• heat pump
• frosting
• defrosting
• microchannel heat exchanger
• defrosting energy

Abstract

To study the performance of the reversible R744 heat pump (HP) system with components designed and optimized for air-conditioning (AC) mode only, a comprehensive study was conducted to examine the component- and system-level performance using a full reversible R744 heat pump system in a wide range of practical operating conditions including frosting conditions. The performance of an integrated internal heat exchanger (IHX) and accumulator (Acc) has been studied numerically and experimentally. First, the potential benefits of introducing an IHX into reversible R744 systems are discussed and analyzed. Overall, the results of cycle analysis underestimate the efficiency improvement of introducing an IHX into an R744 vapor compression system, and the deviation between the theoretical and experimental results increases with the refrigerant temperature at the gas cooler exit. Experimental data show a more than doubled efficiency improvement due to IHX compared to the cycle analysis results when the refrigerant temperature at the gas cooler exit for both basic and IHX systems is 41.2℃. Because simplified theoretical analysis neglects the change of the compressor efficiencies and the refrigerant distribution in the evaporator. Also, a 1-D EES model of the integrated IHX/Acc was developed and it predicts the heat transfer rate and pressure drop of the low-pressure side within ±10%, by considering the effects of oil circulating rate on heat transfer and pressure drop. To provide more detailed information on the behavior of the accumulator, a 3-D model of the accumulator was developed. The flow characteristics of the two-phase CO2 and oil mixture in the accumulator in a typical HP operating condition were studied, considering the effects of different sizes of the oil bleed hole. For reversible air-source AC/HP systems, one of the major concerns in a cold climate is the frosting issue, so the performance of the reversible R744 HP system in frosting conditions has been studied, considering the environmental factors, the defrost-initiation criteria, and the orientation of the outdoor coil. The heating capacity, efficiency, frost accumulation in HP operation, as well as the defrosting capacity, efficiency, energy flow, and water drainage were studied comprehensively. The results show that with a fixed relative humidity of ambient air, the outdoor air temperature affects the capacity and efficiency of the HP system significantly, as well as the frost accumulating speed and the frosting time. When the outdoor air temperature drops from 0 to -10˚C, the peak value of heating capacity reduces from 4.85 to 3.54 kW and HPF reduces from 1.61 to 1.47 in the first cycle. In the meanwhile, the frost accumulating speed decreases from 0.013 to 0.005 kg/min, and the frosting time increases from 32 to 108 minutes. However, the relative humidity of the outdoor air has a negligible impact on the capacities and efficiency of the HP system. Though it directly affects the humidity ratio and thus the frost accumulating speed and the frosting time, which increases significantly from 32 to 84 minutes when the outdoor air temperature is 0˚C and the relative humidity decreases from 90% to 80%. The heating capacity and the efficiency of the HP system drop only by less than 9% during the frosting cycles with 5 times the initial air-side pressure drop as the defrost-initiation criterion in all experimental conditions. But heating capacity and HPF drop by about 30% and 15% during the frosting cycles with 10 times the initial air-side pressure drop as the defrost-initiation criterion in all experimental conditions. Considering the operating-time-averaged performance, the 30% and 15% performance reduction can be considered mild, which indicates a longer operating time in HP mode, a lower defrosting frequency, fewer impacts on passengers’ comfort, and potentially a higher periodic performance. Although the optimal defrost-initiation and termination criteria are still open to discussion and require further investigation. In addition, the results of the defrosting energy flow study show that it is more time- and energy-efficiently to apply 10 times the initial outdoor DPea as the defrost-initiation criterion than 5 times at a cost of more temperature fluctuations of the indoor air during the defrosting cycles. However, it is possible to defrost the outdoor coil when the vehicle is parking in the parking lot or charging center and minimize the negative effects on passengers’ comfort, considering the 68-minute-long operating time. Also, the vertical orientation of the outdoor coil reduces the defrosting time and energy by increasing the surface temperature of the outdoor evaporator in HP mode and improving the water drainage during the defrosting. The conclusion of this dissertation can be applied by the researchers and engineers working on designing and optimizing automotive heat pumps.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Wenying Zhang

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