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Evaluation of subcooling control in residential heat pumps through experimental and model analysis.

Number: 2283

Author(s) : CARVALHO B.Y. K. de, HRNJAK P.

Summary

Reversible systems present an issue in properly optimizing the charge for both cooling and heating operation to achieve maximum efficiency. Subcooling control can prevent this problem by regulating the expansion device and provide the best performance in both cooling and heating. This paper focuses on subcooling control in residential heat pumps with both large and small heat exchangers (HX) and evaluating the potential improvement from this expansion control strategy as a function of the heat exchanger sizes. The work is divided into an experimental investigation and a model analysis of the effect of both indoor and outdoor heat exchanger’s UA values on the performance increase obtained through subcooling control. Two 2-Ton (7 kW) off-the-shelf residential systems were evaluated: a high-efficiency large-HX system with variable compressor speed and superheat-controlled electronic expansion valve; and a small-HX system with fixed compressor speed and orifice piston tube. The results showed 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. Although subcooling control allows the evaporator to operate with no superheat increase its saturation pressure and resulting in lower compressor power, there’s a reduction on the desuperheating heat transfer which leads to decreased heating capacity, as observed with the small-HX system. A system model was validated using the experimental data, and its results show that a decrease in condenser size is not strongly correlated to an increase in the potential improvement in performance from subcooling control. But the improvement is substantial regardless of heat exchanger size when considering that most systems’ charge is not optimized for heating operation.
The control scheme for heat pumps also does not match the predicted curve use in air conditioning which is based on the difference between condenser saturation temperature and condenser air inlet temperature. The deviation in subcooling control behavior for heat pumps is possible due to its large variation in pressure ratios or maldistribution introduced by the installation of the EXV. A linear correlation for the pressure ratio does provide better agreement, especially for the small-HX system, but further investigation is required to develop a simple control strategy to maximize efficiency for heating without the need for pressure measurement.

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  • Original title: Evaluation of subcooling control in residential heat pumps through experimental and model analysis.
  • Record ID : 30030645
  • Languages: English
  • Subject: Technology
  • Source: 2022 Purdue Conferences. 19th International Refrigeration and Air-Conditioning Conference at Purdue.
  • Publication date: 2022

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