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Abstract: When an air source heat pump (ASHP) operates in heating mode, frost can be accumulated on the surface of its finned outdoor coil. Frosting deteriorates the operation and energy efficiency of the ASHP and periodic defrosting becomes necessary. Currently the most widely used standard defrosting method for ASHPs is reverse cycle defrost. On the other hand, electronic expansion valves (EEVs) are commonly used in heat pump/refrigeration systems, including ASHP units, as throttle regulators of refrigerant flow. This paper reports on an experimental investigation of the performance of a reverse cycle defrost operation for a 6.5kW experimental heating capacity residential ASHP whose outdoor coil had four-parallel refrigerant circuits, and with an EEV as the refrigerant flow throttle regulator. The detailed descriptions of the experimental ASHP unit and the experimental procedures of ASHP are firstly presented. This is followed by reporting the comparative experiments of two control strategies for the EEV to investigate their effects on defrosting performance: the EEV being fully open and the EEV being regulated by a degree of refrigerant superheat (DS) controller. 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. [Copyright &y& Elsevier]

Abstract: The experimental part of the study of the reverse cycle defrosting performance on a four-circuit outdoor coil unit in an experimental 6.5kW heating capacity residential ASHP has been reported in Part I of a two-part series. The experimental results indicated that downwards flowing of the melted frost along a multi-circuit outdoor coil surface would have negative effects on defrosting performance. In order to quantitatively analyze the effects, this paper, the second part of the two-part series, reports on a modeling analysis where a semi-empirical model of the airside defrosting in the four-circuit outdoor coil unit was developed. The model was validated by comparing the predicted defrosting duration and temperature variation of collected melted frost with the corresponding experimental data presented in Part I of the two-part series. Using the validated model, the frost-melting rate and the temperature of melted frost on the surface of each circuit were predicted. The effects of the downwards flowing melted frost along the entire outdoor coil surface on defrosting performance were quantitatively analyzed. [Copyright &y& Elsevier]

Abstract: When an air source heat pump (ASHP) unit operates in heating mode, frost can be accumulated on the surface of its finned outdoor coil which normally has multiple parallel circuits on its refrigerant side 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. Frosting deteriorates the operation and energy efficiency of the ASHP unit and periodic defrosting becomes necessary. Currently the most widely used standard defrosting method for ASHPs is reverse cycle defrost. This paper, the first part of a two-part series, reports on the experimental part of a study of the reverse cycle defrosting performance on a multi-circuit outdoor coil unit in an experimental 6.5kW heating capacity residential ASHP unit. Firstly the experimental ASHP unit is described and experimental procedures detailed. Secondly, the experimental results are reported. This is followed by the discussion on the effects of downwards flowing of the melted frost along a multi-circuit outdoor coil surface on defrosting performance. Finally, the evaluation of the defrosting efficiency for the experimental ASHP unit is provided. In the second part of the series, a modeling analysis on the effects of downwards flowing of the melted frost along the multi-circuit outdoor coil surface on defrosting performance of the experimental ASHP unit will be presented. [Copyright &y& Elsevier]

• A novel bed-based air source heat pump (B-ASHP) system was proposed. • A prototype experimental B-ASHP system was established and tested. • The B-ASHP system can be operated at both nighttime and daytime modes. • Better thermal comfort and bed surface temperature distribution can be achieved. • A number of issues requiring further considerations were identified. Air source heat pumps (ASHPs) have been increasingly used for space heating due to their high energy efficiency and installation flexibility. On the other hand, in many rural areas in northern Chinese provinces, Kangs, a kind of traditional heating systems based on burning biofuels, are still being used for space heating in winter. Based on the pros and cons of both ASHPs and Kang-based heating systems, a novel bed-based ASHP (B-ASHP) system has been proposed to combine the advantages from both an ASHP and a Kang heating system. In this paper, a prototype experimental B-ASHP system was designed and established, and its dynamic and steady-state operating performances tested in four test cases. The test results from the four test cases demonstrated that the prototype experimental B-ASHP system can achieve quicker increases in indoor air temperatures and bed-surface temperatures than Kang-based heating systems. The use of experimental B-ASHP system can lead to a more uniformed vertical indoor air temperature gradient, thus a better occupants' thermal comfort, than the use of a conventional convection-based ASHP system. Furthermore, compared to using a Kang-based heating system, a more uniformed bed surface temperature may be obtained when using a B-ASHP system. Nonetheless, although currently the prototype experimental B-ASHP system can be satisfactorily operated, a number of issues related to the designs of a B-ASHP system such as choices of refrigerant, refrigerant tube layout optimizations, and the use of variable speed compressors for capacity control, should be carefully considered to further improve its operational safety and energy efficiency. [ABSTRACT FROM AUTHOR]