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Fouling and frosting of the outdoor coil in an air source heat pump (ASHP) unit are common but undesirable operating problems. At an increased level of air pollution, fouling of an outdoor coil becomes increasingly severe during its space cooling operation in summer and may potentially impact its frosting operation in winter. This paper reports on a field study on the operating performances of a space heating ASHP unit with a foul outdoor coil as a result of a prolonged cooling operation. Firstly, a detailed description of the field ASHP unit with the foul outdoor coil and test cases is given. Secondly, field study results for the test cases and their analyses are presented. The results demonstrate that fouling of an outdoor coil during its space cooling operation in summer can significantly impact its frosting performance during its space heating operation in winter. [ABSTRACT FROM AUTHOR]

Defrosting under unnecessary conditions is a typical and undesirable operating phenomenon for air source heat pump (ASHP). This paper aims to evaluate the ASHP performance under unnecessary defrosting phenomena in different regions of China. Firstly, energy loss for two kinds of unnecessary defrosting phenomena was quantified by 60 days field test. Secondly, a method was developed to predict unnecessary defrosting times. The prediction accuracy can be controlled within 10%. Furthermore, the ASHP performance of unnecessary defrosting was evaluated for nine typical cities in China. It was found that the average unnecessary defrosting rate was 53% for northern cities, 38% for mid-land cities, and 15% for southern cities during a heating season. Correspondingly, the total energy loss was 233 MJkW −1 , 161 MJkW −1 , and 44 MJkW −1 . These results indicate that unnecessary defrosting processes should not have been ignored. It is a valuable work to improve the defrosting control method. [ABSTRACT FROM AUTHOR]

• A novel method for defouling fouled outdoor coils in ASHPs has been developed. • The method is based on three directly measurable operating temperatures. • Further field studies to verify the method's applicability were carried out. • The method provided an accurate and convenient way for defouling initiation. Air source heat pumps (ASHPs) have been widely used for space cooling in recent years. When a space cooling ASHP unit is operated in an ambience with poor air quality, fouling of its outdoor coil can be easily built up, leading to a significant drop in the operating efficiency and output cooling capacity of the ASHP unit. Therefore, as a follow-up to a previous related study, a study to develop a novel defouling initiation method for the fouled outdoor coil in a space cooling ASHP unit based on its measurable characteristic ambient and operating temperatures has been carried out and the study results are reported in this paper. Firstly, a characteristic parameter of TTT, which can be evaluated based on ambient air Temperature, outdoor coil metal surface Temperature and outdoor coil outlet air Temperature, is defined. Secondly, the development of the novel defouling initiation method is detailed. Finally, further field comparative studies using the developed novel defouling initiation method have been carried out, and the study results demonstrated that the novel defouling initiation method can provide an accurate and convenient way to initiate defouling of the fouled outdoor coil in an ASHP unit at a reasonable defouling frequency to ensure its best possible operating performances. [ABSTRACT FROM AUTHOR]

To improve the defrosting accuracy of air source heat pumps (ASHPs), this paper proposes a novel defrosting control method by applying tube encircled photoelectric sensors (TEPSs). A field test was conducted for two heating seasons in Beijing, China, to verify the feasibility and practicality of the novel TEPS method. The test results revealed that irrespective of the environmental conditions, the TEPS method can initiate defrosting in similar situations: most of the heat exchanger surface had been covered by frost; the compressor suction temperature decreased by ~9 °C; the compressor discharge temperature increased by ~16 °C; and the heating capacity decreased by ~30%. Furthermore, the TEPS method was verified to make more accurate and more reasonable defrosting decisions than the traditional T-T method under both frosting and non-frosting conditions. The results indicate that the TEPS method is a competitive defrosting control method that can be used for ASHPs. [ABSTRACT FROM AUTHOR]

To improve the defrosting accuracy and the energy efficiency of the air-source heat pump (ASHP) under frosting and defrosting conditions, a novel Temperature–Humidity–Time (T–H–T) defrosting control method, based on a frosting map for the ASHP unit, is proposed in this paper. A field test was conducted for two heating seasons, to verify the feasibility and applicability of the T–H–T method. The advantages of the T–H–T method, compared to the conventional Temperature–Time (T–T) defrosting control method, are presented. In total, eight cases are shown in this paper. Cases 1 to 4 were chosen to reveal the T–H–T performance under different frosting conditions. It was found that no matter what kind of frosting conditions, defrosting was always initiated in a similar situation: ∼90% of the outdoor coil surface was covered by frost; the temperature difference between the compressor suction and discharge increased by ∼20%; and the heating capacity decreased by ∼30%. These results indicate that the T–H–T method can make an accurate decision under different frosting conditions. Cases 5a, 5b and Cases 6a, 6b were two groups of cases to compare the advantages of the T–H–T method against the conventional T–T method. Cases 5a and 5b were chosen for the non-frosting condition. It was found that the T–T method initiated the defrosting operation 31 times within 24 h. However, none of the defrosting operations was conducted for the T–H–T method. Cases 6a and 6b were chosen to compare these two methods under consecutive and variable frosting conditions. For the T–T method, 63% of the defrosting processes were found to be executed under conditions where defrosting was not necessary. However, for the T–H–T method, all the defrosting controls were found to be accurate and reasonable. These results indicated that the novel T–H–T method is suitable for the defrosting control of the ASHP, and has a more competitive performance than the conventional T–T method. [ABSTRACT FROM AUTHOR]