Subcooling control to maximize efficiency in reversible A/C-H/P systems

Kimura de Carvalho, Bruno Yuji

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https://hdl.handle.net/2142/117526 Copy

Description

Title

Subcooling control to maximize efficiency in reversible A/C-H/P systems

Author(s)

Kimura de Carvalho, Bruno Yuji

Issue Date

2022-09-07

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

Hrnjak, Predrag S

Doctoral Committee Chair(s)

Hrnjak, Predrag S

Committee Member(s)

• Jacobi, Anthony M
• Miljkovic, Nenad
• Elbel, Stefan
• Zhang, Yuanhui

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)

• Subcooling Control
• Air conditioning
• Heat pump
• Reversible system optimization

Abstract

The increasing demand for higher efficiency in residential air-conditioning and heat pump systems requires the optimization of both operating modes to maximize performance independent of the charge or external conditions. Superheat control of refrigerant flow using thermostatic expansion valves has been widely employed in residential air conditioning and heat pump systems, the refrigerant flow has been controlled with focus on preventing liquid intake at the compressor suction. The use of electronic expansion valves allows new control methods to be employed. Subcooling control is an alternative for refrigerant flow metering, focusing on maximizing COP/HPF or capacity. An electronic expansion valve (EEV) adjusts subcooling while an accumulator prevents liquid return. An ideal cycle and simplified condenser model evaluation of the COP and HPF maximizing subcooling in residential air-conditioning and heat pump systems was developed. It was concluded that the relative potential for performance improvement from subcooling is always greater for cooling over heating under the same operating conditions. Refrigerants with lower ratio of latent heat of vaporization over liquid specific heat show a greater benefit from subcooling, but the shape of the saturation dome and behavior of the isentropic lines also plays a role in defining the improvements. The COP and HPF maximizing subcooling can be defined as a function of the condenser saturation to air inlet temperature difference because this parameter is directly related to the effectiveness of the subcooling region and its potential to subcool the saturated liquid. A decrease in condenser size relative to its load results in a higher efficiency-maximizing subcooling and greater improvements over a system without subcooling. Heating mode shows a deviation in optimal subcooling control when compared to cooling due to its lower isentropic efficiency, condenser air inlet temperature and baseline specific capacity. Heat pump operation performance is less sensitive to subcooling but had similar relative efficiency improvement to air conditioning when the load was equal. Two R410A residential reversible systems were investigated in environmental chambers to determine the real impact of subcooling control over conventional thermostatic valves, piston-orifice tubes and active evaporator superheat control. Both systems were rated at 7 kW, with the High-SEER system using a variable capacity scroll compressor, thermostatic valve for cooling and electronic expansion valve for heating. The Low-SEER system used orifice-piston tubes for both operating modes and a single-speed scroll compressor. The systems charge selection was based on the air-conditioning operation, a common practice due to the decreased usage of heat pump in the current market. The performance was optimal at the AHRI rating condition A with a small deviation for the High-SEER system at off-design conditions, while the Low-SEER system showed a greater penalty in COP as it shifted from the rating condition. In heating mode, both systems operated at suboptimal levels because the charge imbalance prevented them from achieving their respective HPF-maximizing subcooling values. Electronic expansion valves were installed close to the evaporator inlet for both operating modes in the High-SEER and Low-SEER systems. Both systems already had accumulators used to prevent liquid intake during the flow reversal in heat pump defrosting. These accumulators were used as the charge receiver to store the excess refrigerant mass and allow the systems to operate at their respective COP/HPF-maximizing subcooling values at all conditions. Experimental results showed that both systems improved with subcooling control with relative increases in COP from +4.7% to +8.9% for the High-SEER system and from +4.3 to +14.2% in the Low-SEER system. The small-HX system significant showed a greater overall improvement in performance because its baseline used a fixed-expansion device which suffers a decrease in efficiency when operating outside of the rating condition. For heat pump subcooling control, it provided an increase in heating performance factor of +4.1% and +6.2% for the large-HX and small-HX systems at the rating condition, respectively. The control scheme defined with the simplified condenser model and compressor analysis describing the efficiency maximizing subcooling as a linear function of the temperature difference between condenser saturation and air inlet agreed well with the A/C experimental results. The low-SEER H/P results, however, showed no correlation with this control scheme. Evaporator refrigerant maldistribution and varying evaporator outlet quality were the possible causes for the low-SEER H/P’s deviant HPF-maximizing subcooling. In cooling mode, the decrease in latent heat transfer when operating with no dryout in the evaporator reduced the potential improvements in COP from subcooling control. In heating mode, the evaporator outlet quality below 1.0 caused a reduction on the desuperheating heat transfer which decreased the specific heating capacity. A system model was developed and validated. The effect of load on subcooling control was evaluated for both air-conditioning and heat pump. In A/C the relative improvement and COP-maximizing subcooling increases with load and the gains from eliminating dryout in the evaporator are constant for all loads. On the other hand, in H/P operation the load increase coupled with higher pressure ratios and latent heat of vaporization caused a decrease in HPF-maximizing subcooling. The combined effects from both condenser, evaporator and compressor performances make the HPF-maximizing subcooling behavior non-linear decreasing the linear control curve unable to provide good predictions at all conditions. However, the control curve developed could still increase HPF for both systems investigated. The comparison between A/C and H/P experimental and numerical results show that they behave as entirely different system and the subcooling control must be defined separately. The condenser has the strongest effect on the subcooling control, and numerical results show that if a system uses identical indoor and outdoor heat exchangers with the same air flow rates the subcooling control curve will be unified between H/P and A/C operation. An instability in the system was observed when operating at low subcooling values with the accumulator partially filled. The most probable cause was the inability of the accumulator to prevent liquid intake at the compressor suction. The instability caused fluctuations in the discharge and suction pressures, as well as the discharge temperature. The accumulator was modified with a hydrostatic liquid level glass, 8 sight glasses to allow visualization at its inlet, J-tube inlet, and oil bleed-holed at 15 mm and 100 mm. A 1-D model of the accumulator was developed based on pressure drop streams and validated against the experimental data from the liquid level glass measurements. As subcooling was decreased and the accumulator filled up, severe liquid entrainment through the J-tube was observed. Once the liquid level reached the 100 mm oil bleed-hole phase, separation was observed causing the oil to be returned to the compressor through the bleed hole and reducing its concentration inside the accumulator. Sampling of the accumulator contents corroborated these observations, with the oil concentration decreasing from 5% down to 3% over a runtime of 4 hours in cooling operation. This result is important because it indicates that the use of the accumulator as a charge receiver does not trap oil inside which could decrease compressor reliability in the long term.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Bruno Yuji Kimura de Carvalho. All rights reserved.

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