An experimental and numerical study on improving defrosting performances for an air source heat pump unit having a multi-circuit outdoor coil

Abstract

Air source heat pump (ASHP) units have found wide applications worldwide in recent decades as an energy efficient and environmental friendly means for indoor environmental control. However, when an ASHP unit operates in heating mode, frost might be formed and accumulated on the surface of its outdoor coil. Frosting deteriorates its operational performance and energy efficiency, and therefore periodic defrosting is necessary. Currently, reverse cycle defrosting is the most widely used standard defrosting method for ASHP units. For an outdoor coil in an ASHP unit, multiple parallel refrigerant circuits are commonly seen for minimized refrigerant pressure loss and enhanced heat transfer efficiency. However, uneven defrosting over the entire airside surface of a vertically installed multi-circuit outdoor coil was observed during reverse cycle defrosting for ASHP units. Uneven defrosting could lead to a prolonged defrosting duration and a lower defrosting efficiency. One of the reasons for uneven defrosting was downwards flowing of melted frost over the surface of a vertically installed multi-circuit outdoor coil due to gravity. Therefore, it is necessary to experimentally study the negative impacts of downwards flowing of melted frost on defrosting performance for ASHP units, by comparing the measured defrosting performances with and without the melted frost downwards flowing from up circuit(s) to down circuit(s) in an experimental multi-circuit outdoor coil. On the other hand, to enable further quantitatively analyzing the effects of locally draining away the melted frost on reverse cycle defrosting performance of an ASHP unit, and to develop methods to alleviate the negative effects of uneven defrosting on defrosting performance of an existing ASHP unit, modeling studies should also be carried out. This thesis begins with reporting an experimental study on the negative effects of downwards flowing of melted frost over a vertically installed experimental three-circuit outdoor coil in an experimental ASHP unit on defrosting performance during reverse cycle defrosting. Three experimental study cases, with different arrangements of water collecting trays placed between or under circuit(s), were designed and carried out. The temperatures of tube surface at the exits of each refrigerant circuit, coil fin surface at the center of each circuit, and the melted frost collected were measured. The experimental results and corresponding quantitative analysis revealed the negative effects of allowing melted frost to freely flow downwards due to gravity over the airside surface of the vertically installed experimental three-circuit outdoor coil in the experimental ASHP unit on defrosting performance during reverse cycle defrosting: a longer defrosting duration and more energy consumption. In addition, the study results also suggested that the use of water collecting trays between circuits for locally draining away the melted frost before flowing into down circuit(s) was effective in mitigating these negative effects. Secondly, the thesis presents a mathematical modeling study on the defrosting performance for the experimental ASHP unit with local drainage of the melted frost from its vertically installed three-circuit outdoor coil. Two semi-empirical mathematical models, corresponding to two settings of with and without the use of water collecting trays between circuits, were developed. In this modeling study, a defrosting process on the airside of an outdoor coil was divided into four stages: (1) preheating, (2) frost melting without water flowing away from a circuit, (3) frost melting with water flowing away from a circuit, and (4) water layer vaporizing. The two semi-empirical models were validated by comparing the experimental data obtained in the experimental study and the predicted data using the models for the key operating parameters of the experimental ASHP unit, with good agreements. The validated models could adequately describe the defrosting operation for the experimental ASHP unit with local drainage of the melted frost from its outdoor coil. Finally, to explore the potential methods of alleviating the negative effects of downwards flowing of melted frost and thus improving the defrosting performance of an existing ASHP unit, a modeling study on alleviating uneven defrosting for the experimental vertical three-circuit outdoor coil in the experimental ASHP unit during reverse cycle defrosting was carried out and the study results are reported. To alleviate uneven defrosting for an existing ASHP unit, it can be effective to vary the heat supply (via refrigerant flow) to each refrigerant circuit by varying the openings of modulating valves installed at an inlet pipe to each circuit. Three study cases, with different mechanisms of both varying the openings of modulating valves and introducing other operational changes, were designed and corresponding modeling studies carried out using the validated semi-empirical model developed at the setting of without the use of water collecting trays between circuits. Modeling results suggested that the best defrosting performances in terms of shortening defrosting durations and reducing defrosting energy use were achieved in the study case of fully closing the modulating valve on the top circuit when its tube surface temperature at the exit of the circuit reached defrosting termination point. While further experimental studies to validate the modeling results are to be carried out, it is however expected that with more refrigerant circuits in an outdoor coil in an ASHP unit, the method of fully closing the modulating valves on top circuit(s) would yield better defrosting performance for the ASHP unit.