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Title:Intermediate vapor bypass: A novel design for mobile heat pump at low ambient temperature
Author(s):Feng, Lili
Director of Research:Hrnjak, Pega S.
Doctoral Committee Chair(s):Hrnjak, Pega S.
Doctoral Committee Member(s):Jacobi, Anthony M.; Zhang, Yuanhui; Elbel, Stefan
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Heat pump
Electric vehicles
Range extension
Intermediate vapor bypass
Reversible condenser/evaporator
Heat exchanger design
Refrigerant charge.
Abstract:The electric vehicle market is currently in exponential growth, and drive range on a single charge is one of the most highly valued specs. Passenger thermal comfort, especially heating in cold weather, is usually the largest auxiliary onboard energy user, and can significantly reduce the EV drive range. Heat pumps dramatically reduce the energy consumption for the same heating capacity, hence improves drive range largely. More and more EV manufacturers are adopting heat pump systems for cabin comfort heating. A mobile heat pump system using the most widely adopted architecture and the same heat exchangers from the first commercially available production EV heat pump was built in the lab. Performance characteristics of this system was experimentally studied using low pressure refrigerants R134a and R1234yf. Severe heating capacity drop was noticed at extremely low ambient temperature, mainly due to low operating pressure in the outdoor evaporator. Decreasing suction density along with dropping evaporating temperature and suction pressure led to much lower refrigerant mass flow rate with maximum compressor speed. At extremely low ambient temperature, as suction pressure approached atmospheric pressure, the compressor speed had to be lowered to prevent vacuum inside the system, and heating capacity was further limited when it was most needed. Based on the experimental data, a system model was developed and validated. Refrigerant maldistribution in the outdoor heat exchanger in heating mode was modeled with a given liquid refrigerant flow rate distribution profile. Without considering maldistribution, the outdoor heat exchanger capacity in heating mode was overestimated by an average of 17%. By including maldistribution degradation, the model agreed with measurements within ±20% on component level, and when integrated into the system model, both heating capacity and HPF matched experimental measurement within ±10%. Through analysis using the system model, the increasingly high sensitivity of saturation temperature to pressure at lower evaporating pressure caused large temperature glide in the outdoor heat exchanger, and significantly reduced the air-to-refrigerant temperature difference that drove heat transfer. While by assuming uniform refrigerant two phase distribution rather than maldistribution in the outdoor heat exchanger, higher compressor speed could be allowed, and heating capacity at -20 ̊C can be increased by 19%. The necessary refrigerant charge amount in both cooling and heating modes was experimentally investigated, and refrigerant and oil mass distribution among the components were measured using quick close valve method in both modes. A large refrigerant charge imbalance was found when switching between heating and cooling modes, with cooling mode needing more refrigerant for the heat exchangers. The refrigerant charge imbalance from all the three heat exchangers was about the same amount as the outdoor heat exchanger charge retention change when it’s switched from a condenser to an evaporator. Large charge imbalance not only required an accumulator that’s large enough for storing the excess refrigerant mass, but also resulted in oil trapping in the accumulator. Storing excess refrigerant at a lower vapor quality location can dramatically reduce oil trapping. Refrigerant mass distribution inside the heat exchangers were studied by combining the system performance model and a void fraction correlation. A reversible outdoor heat exchanger with integrated receiver/separator design was examined using the model. By bringing part of the outdoor heat exchanger into low vapor quality, and reversing its flow direction when switching modes, refrigerant mass retention change was largely reduced. Moreover, the integrated separator/receiver served as a low vapor quality charge storage device that won’t result in oil trapping. In order to increase heating capacity at low ambient temperature while still using current low pressure refrigerants, the intermediate vapor bypass concept was proposed to reduce pressure drop and to improve refrigerant two phase distribution in the downstream pass of the outdoor heat exchanger. The proof-of-concept modified outdoor heat exchanger helped increase the heating capacity at -20 ̊C from 2.48 kW to 3.36 kW using R1234yf. A prototype of a fully reversible outdoor heat exchanger, which worked as an intermediate vapor bypass evaporator in heating mode, and as an integrated receiver subcooler condenser in cooling mode, was made, and demonstrated working as intended in both modes. Besides dramatically improving the extremely low ambient temperature heating capacity, using the intermediate separator/receiver as a charge storage device allowed excess refrigerant charge storage without sacrificing system performance or trapping large quantity of oil in the refrigerant liquid. The outdoor heat exchanger for current mobile heat pump systems work as a condenser in cooling mode, and as an evaporator in heating mode. Heat transfer coefficient and pressure gradient increase with refrigerant mass flux during both evaporation and condensation. Pressure drop affects the heat exchanger heat transfer capacity through change of refrigerant saturation temperature, and the temperature difference between refrigerant and air. Because of the higher sensitivity to pressure drop, working as an evaporator favored a much larger channel diameter than that of the same heat exchanger working as a condenser. When designing the heat exchanger for one mode, significant sacrifice of performance in the other mode was inevitable. Size of microchannel diameter, as well as pass circuitry of the outdoor heat exchanger to minimize performance sacrifice was investigated using the heat exchanger model.
Issue Date:2019-02-04
Rights Information:Copyright 2019 Lili Feng
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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