A novel thermal storage based reverse cycle defrosting method and the operating performance evaluations for an air source heat pump

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A novel thermal storage based reverse cycle defrosting method and the operating performance evaluations for an air source heat pump

 

Author: Qu, Minglu
Title: A novel thermal storage based reverse cycle defrosting method and the operating performance evaluations for an air source heat pump
Degree: Ph.D.
Year: 2012
Subject: Hong Kong Polytechnic University -- Dissertations
Heat pumps.
Heat storage.
Department: Dept. of Building Services Engineering
Pages: xxvi, 214 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2530117
URI: http://theses.lib.polyu.edu.hk/handle/200/6729
Abstract: Air source heat pumps (ASHPs) have been widely used worldwide as an energy efficient and environmental friendly means for indoor environmental control. However, when an ASHP unit operates in heating mode, frost may be accumulated on the surface of its outdoor coil. Frosting deteriorates its operational performance and energy efficiency, and therefore periodic defrosting becomes necessary. Currently, the most widely used standard defrosting method for ASHPs is reverse cycle defrost. However, a fundamental problem for such a method is that there is insufficient heat available for defrosting, which leads to a number of associated operational problems including low-pressure cut-off or wet compression, a prolonged defrosting duration, the risk of having a lower air temperature inside a heated space without heating being provided during defrosting and longer transition from defrosting to heating-resumption, etc. On the other hand, for an outdoor coil used in an ASHP, on its refrigerant side, multiple parallel circuits are commonly used for minimized refrigerant pressure loss and enhanced heat transfer efficiency. On its airside, however, there is usually no segmentation corresponding to the number of refrigerant circuit. Consequently, uneven defrosting over the entire airside surface of the multi-circuit outdoor coil possibly caused by the downwards flowing of melted frost due to gravity is likely to occur. Therefore, it is necessary both to develop a novel thermal energy storage (TES) based reverse cycle defrosting method for ASHPs so as to address the fundamental problem of inadequate heat available for defrosting through TES technology, and to study the defrosting performance on the airside surface of a multi-circuit outdoor coil. This thesis begins with reporting the development of a novel TES-based reverse cycle defrosting method using phase change material (PCM). The operating performances of an experimental ASHP unit using the novel TES-based reverse cycle defrosting method have been experimentally studied. The measured operating performances on the refrigerant side of the experimental ASHP unit are firstly reported. Three operating modes at which extensive experiments were carried out for the experimental ASHP unit, namely, standard heating and reverse cycle defrosting, parallel TES-based heating and reverse cycle defrosting, and finally serial TES-based heating and reverse cycle defrosting, are detailed. The experimental results on the refrigerant side performances of the experimental ASHP unit suggested that the use of the novel TES-based reverse cycle defrosting method would result in a number of advantages, including a smoother transition from defrosting to space heating resumption, an enhanced operational reliability and reduced energy consumption.
Secondly, the thesis presents the operational performances on the airside of the experimental ASHP unit during the three operating modes, as well as the indoor thermal comfort characteristics during the use of the TES-based reverse cycle defrosting method. The well-known Fanger's thermal comfort model is briefly introduced, with appropriate assumptions and simplifications. This is followed by reporting the measured indoor thermal parameters that were necessary for evaluating indoor thermal comfort under the standard reverse cycle defrosting method and the TES-based reverse cycle defrosting method. The experimental results on the airside performances of the ASHP unit and the evaluated indoor thermal comfort indexes (PMV and PPD) clearly suggested that the use of the novel TES-based reverse cycle defrosting method would lead to a shorter defrosting duration, a higher indoor air temperature and consequently, occupants' indoor thermal comfort can be significantly improved during a reverse cycle defrosting operation. Thirdly, the thesis reports on a study on the airside defrosting performance of an on outdoor coil having four parallel circuits in the experimental ASHP unit, with a particular focus on studying the impact of allowing melted frost to flow downwards freely due to gravity along the coil surface on defrosting performance, using both experimental and modeling analysis approaches. The experimental part of the study is firstly reported. It was observed that defrosting was quicker on the airside of the upper circuits than that on the lower circuits in the four-circuit outdoor coil. The effects of downwards flowing of the melted frost along a multi-circuit outdoor coil surface in an ASHP unit on defrosting performance were discussed. The defrosting efficiency for the experimental ASHP unit was evaluated. Then the modeling analysis part of the study is presented. A semi-empirical defrosting model for the four-circuit outdoor coil in the experimental ASHP unit, the first of its kind, was developed based on the fundamentals of mass and energy conservation, and using the experimental data. The model was validated by comparing the predicted defrosting duration and the temperature variation of the collected melted frost with the corresponding experimental results. Using the validated model, the negative effects of downwards flowing of the melted frost along the surface of a multi-circuit outdoor coil on defrosting performance were quantitatively analyzed and are reported. The model developed provided a useful tool for studying and understanding the effects of downwards flowing melted frost on the defrosting performance in the multi-circuit outdoor coil of an ASHP unit. Finally, an experimental investigation on reverse cycle defrosting operation for the experimental ASHP unit when using an EEV as a refrigerant flow throttle regulator is reported. Comparative experiments under two control strategies for the EEV, i.e., the EEV being fully open and the EEV being regulated by a degree of refrigerant superheat (DS) controller during defrosting, were conducted. The experimental results revealed that when the EEV was regulated by the DS controller during defrosting, a higher defrosting efficiency and less heat wastage would be resulted in.

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