DX ground-coupled HPs use refrigerant in DX, flooded, or recirculation evaporator circuits for the ground pipe coils. In systems with refrigerant-to-water heat exchangers, a water or antifreeze solution is pumped through horizontal, vertical, or coiled pipes embedded in the ground. An HP may have a refrigerant-to-water heat exchanger or may be direct-expansion (DX). Ground-coupled HPs use the ground as a heat source and sink. Ioan Sarbu, Calin Sebarchievici, in Solar Heating and Cooling Systems, 2017 9.3.4.5 Ground-Coupled Heat Pumps Hence, adjusting airflow and/or refrigerant distribution could improve defrosting performance. Based on these conclusions, they utilized the model to further evaluate the effects of uneven air and refrigerant flow distributions and the benefits of upstream hybrid control during defrosting for an ASHP unit. The results showed that the upstream refrigerant flow control consistently outperformed the downstream refrigerant flow control, and recovered most of the loss in cooling capacity and COP due to nonuniform airflow distribution. experimentally and numerically investigated a hybrid-individual degree of superheat control method for refrigerant flow balancing in a multicircuit evaporator: upstream vs downstream flow control.
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With refrigerant flow controlled by a TEV, the worst-performing circuit affected the performance of the entire outdoor coil by as much as 35%. compared the predicted and experimental heat transfer performances for a finned tube outdoor coil, and found that airflow mal-distribution reduced the performances of an evaporator circuit.
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Mal-distribution of refrigerant or airflow might result in uneven defrosting, thus degrading the defrosting performance. Song Mengjie, Deng Shiming, in Defrosting for Air Source Heat Pump, 2019 2.4.1.3 Airflow and refrigerant distribution adjustment