Potential of controlling subcooling in residential air conditioning system

Xu, Lihan

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https://hdl.handle.net/2142/72818 Copy

Description

Title

Potential of controlling subcooling in residential air conditioning system

Author(s)

Xu, Lihan

Issue Date

2015-01-21

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

Hrnjak, Predrag S.

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Subcooling control
• Residential air-conditioning (RAC) system
• System performance improvement
• Control strategy

Abstract

This work presents an experimental and simulation study of one way of improving residential air-conditioning (RAC) system performance by controlling subcooling. Instead of conventional superheat control at evaporator outlet, condenser subcooling is monitored and controlled and the corresponding effects on vapor-compression system performance as well as the sensitivity of subcooling control to different operating conditions and different condenser sizes are discussed. Both experimental and simulation study indicate that there is COP maximizing subcooling due to trade-off of increasing cooling effect and increasing specific compression work as condenser subcooling increases. In the experimental investigation, the potential of performance improvement (COP and cooling capacity Q) by controlling subcooling using EXV is quantified and compared to superheat controlled TXV system. The maximum of 33.0% COP improvement and 14.7% capacity gain is achieved at the same optimum subcooling, benefited from both subcooling control and improved evaporator effectiveness. It is also found that COP maximizing subcooling is a function of ambient temperature T_cai: COP maximizing subcooling temperature increases with increasing ambient temperature. In the simulation investigation, the potential of performance improvement by controlling subcooling is also identified. Both COP and Q maximizing subcooling increase with increasing T_cai. In addition, the effect of condenser size on subcooling controlled system performance is evaluated and the results indicated that smaller size of condenser is more sensitive to change of condenser subcooling. COP or Q maximizing subcooling decreases with increasing condenser size. Combining the simulation and experimental results, subcooling effect and condenser size effect are both interpreted as effects of condenser air refrigerant temperature difference ∆Tin and attempts are made to quantify the linear relationship between ∆Tin and COP (or Q) maximizing subcooling ∆Tsub. With the correlations of COP and Q maximizing subcooling, a control strategy using EXV (electronic expansion valve) with ∆Tin as input signal for controlling subcooling (adjusting the EXV opening) is proposed to provide COP or cooling capacity Q maximizing subcooling for the RAC system as conditions change.

Graduation Semester

2014-12

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http://hdl.handle.net/2142/72818

Copyright and License Information

Copyright 2014 Lihan Xu

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