The Experts below are selected from a list of 252 Experts worldwide ranked by ideXlab platform
Shiming Deng - One of the best experts on this subject based on the ideXlab platform.
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A novel characteristic index for frosting suppression based on the configuration and operation of air source heat pumps.
International Journal of Refrigeration, 2020Co-Authors: Shimin Liang, Wei Wang, Yuying Sun, Yao Lin, Li Zhaoyang, Zhao Jihan, Shiming DengAbstract:Abstract It has been known that for space heating ASHPs, their different configurations and operation may influence their frosting performances. However, there has been not a single index that can be used to comprehensively and realistically evaluate the frosting suppression performances for a space heating ASHP unit by considering all the influencing configurational and operational factors. Therefore, in this paper, a novel characteristic index for frosting suppression based on the configuration and operation of ASHPs has been proposed and its development is reported. Firstly, the characteristic index, CICO, was defined, a detailed analysis on the heat transfer taking place in the Outdoor Coil of an ASHP was carried out, and the relationship between CICO and temperature difference in the heat transfer between Outdoor Coil and ambient air (ΔT) was established. Secondly, the description of an experimental setup with four experimental ASHP units is given. The set was used to establish all empirical numerical relationship between CICO and ΔT. Thirdly, using the experimental results, the empirical numerical relationship between CICO and ΔT was established, which suggested that a higher CICO would lead to a smaller ΔT, thus a better frosting suppression performance. The development results suggested that the index of CICO could be used to evaluate the level of frosting suppression performance for ASHPs, and to help improve the frosting suppression performances for an ASHP during its design and operation, to ensure its best possible operation performance.
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A field study on the effects of Outdoor Coil fouling at different ambient air temperatures on the operating performances of a space cooling ASHP unit
Energy and Buildings, 2019Co-Authors: Yao Lin, Jingdong Liu, Wei Wang, Yuying Sun, Shiming DengAbstract:Abstract Air source heat pumps (ASHPs) have been widely used for providing buildings with heating in winter and cooling in summer. In certain locations, with an increased level of air pollutions, fouling of the Outdoor Coil in a space cooling ASHP unit may appear, which affects the operating performances of the ASHP unit. Furthermore, the operating performances of an ASHP unit can also be affected by ambient air temperature (Ta). In this paper, a field study on the effects of Outdoor Coil fouling at different Ta on the operating performances of a field space cooling ASHP unit is presented. The study results demonstrated that the operating performances of the field ASHP unit were significantly affected by the changes in both Ta and percentage of fouled Outdoor Coil surface (PFOCS). For example, at Ta = 30 °C, the variation in PFOCS from 0 to 100% led to a reduction in COP for the field ASHP unit from 2.7 to 2.0. At a fixed PFOCS of 60%, the changes in Ta from 26 to 33 °C resulted in a reduction in COP from 2.6 to 2.1. In order to more conveniently investigate the combined impacts of fouling on the operating performances of an ASHP unit at different Ta, an equivalent ambient air temperature rise (ETR) was proposed. Based on the measured and evaluated operating parameters of the field ASHP unit at different Ta and PFOCS values, ETR values at 0 - 40%, 40 - 60%, 60 - 100% and 0 - 100% PFOCS ranges were evaluated. These ETR values implied that defouling of a fouled Outdoor Coil may be initiated when PFOCS was at 40 - 60%. With the ETR values for the full PFOCS range of 0 - 100%, the impact of full fouling on the operating performances of an ASHP unit may be evaluated as if it was operated with a clean Outdoor Coil but at an equivalent ambient air temperature of Ta + ETR.
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A novel defouling initiation method for a space cooling ASHP unit based on its measurable characteristic ambient and operating temperatures
International Journal of Refrigeration, 2019Co-Authors: Li Zhaoyang, Jingdong Liu, Wei Wang, Yuying Sun, Shiming Deng, Shimin LiangAbstract:Abstract 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.
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Heating and energy storage characteristics of multi-split air source heat pump based on energy storage defrosting
Applied Energy, 2019Co-Authors: Bowen Yang, Mengjie Song, Yiqiang Jiang, Jiankai Dong, Long Zhang, Shiming DengAbstract:Abstract In recent decades, multi-split air source heat pump (M-ASHP) unit has been widely used for space heating. Similar to the split-ASHP unit, frost would accumulate on its Outdoor Coil surface under frosting condition, which significantly deteriorates its heating performance. In the previous study, a novel reverse-cycle defrosting method based on phase change energy storage was developed and investigated, and the study demonstrated that the novel defrosting method could significantly improve defrosting performance. To provide guidelines for designing the M-ASHP units with phase change energy storage, it is necessary to investigate the characteristics of heating and energy storage for M-ASHP units, which is presented in this paper. Experimental results showed that there was no frost on Outdoor Coil when only the indoor unit with heating capacity of 7.1 kW was turned on under frosting condition. In addition, the energy storage period increased as the number of operating indoor unit increased, namely the load rate. Furthermore, the optimal starting time of the energy storage operation was around 10 min after the start-up of M-ASHP unit. Even though, compared with the conventional heating mode with three turned-on indoor units, the energy storage mode shortened the heating period by 8 min and decreased the average heating capacity and COP by 4.4% and 1.9%, respectively. The above negative impacts of energy storage operation on the heating performance of the M-ASHP unit could be neglected by considering its positive impacts on defrosting performance, for a periodic heating-defrosting condition.
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techno economic analysis on frosting and defrosting operations of an air source heat pump unit applied in a typical cold city
Energy and Buildings, 2018Co-Authors: Kai Wang, Mengjie Song, Shiming DengAbstract:Abstract In recent years, air source heat pump (ASHP) units have found applications worldwide due to their advantages. For an ASHP unit with a multi-circuit Outdoor Coil, when the refrigerant distribution adjusted with the valves, system defrosting efficiency could be optimized. Meanwhile, adjusting the refrigerant distribution by using valves located at each circuit could improve the frosting evenness value, and system coefficient of performance and the defrosting efficiency are thereby both optimized. However, in open literature, no tech-economic analysis work on frosting/defrosting performances of ASHP units is reported, which limits the development of innovation technologies in this field. Therefore, a techno-economic analysis on frosting/defrosting operations for an ASHP unit used in typical cold regions is given, basing on previous experimental work and series of assumptions. As concluded, the total running costs of the new ASHP unit could decrease as much as 5,327.99 CNY ($ 798.87), or 7.67%, and the total cost about 5,177.99 CNY ($ 776.36), or 6.68%, in 15 years’ service life, compared with a traditional one. The payback period of additional first cost of valves is less than 1 year. Conclusions of this study might provide a new analytical tool for scholars, researchers, product developers, and policy designers, and shed new light on the designing and performance optimization of ASHP units.
Mengjie Song - One of the best experts on this subject based on the ideXlab platform.
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Experimental study on the melted frost influence on the metal energy storage during an air source heat pump defrosting
Energy and Buildings, 2020Co-Authors: Mengjie Song, Ning MaoAbstract:Abstract When an air source heat pump works at reverse cycle defrosting mode, the metal temperature of its indoor Coil decreases and that of Outdoor Coil increases. The energy storage of two Coils’ metal at heating mode would influence the system defrosting efficiency. For a vertically installed Outdoor Coil having multiple circuits, meanwhile, the melted frost downwards flowing from upside circuit(s) also delay the defrosting process of downside circuit(s). To quantitatively evaluate the influence from two parameters, metal energy storage and melted frost, four experimental cases were designed and carried out in this study, based on a specially made three-circuit Outdoor Coil. Three circuits are working at the frosting mode, while two and three are used at the defrosting mode in four cases. As concluded, the melted frost influence on defrosting efficiency and metal energy storage effect would be increased from 4.87% to 10.45% and from 0.77% to 1.49%, respectively, after the Outdoor Coil changed from two- to three-working circuit. Outcomes of this study are meaningful for the structure optimization and energy saving of air source heat pump units at defrosting mode.
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A modeling study on the revere cycle defrosting of an air source heat pump with the melted frost downwards flowing away and local drainage
Energy and Buildings, 2020Co-Authors: Mengjie Song, Gongnan Xie, Libor Pekař, Ning MaoAbstract:Abstract Reverse cycle defrosting is widely used for air source heat pumps. When a multi-circuit heat exchanger is vertically installed in a heat pump as an Outdoor Coil, the melted frost could be kept downwards flowing or locally drained during defrosting by using water collecting trays. To analyze the performance differences of melted frost, two defrosting models were developed and previously reported by authors. In this study, the defrosting performance of an air source heat pump was numerically investigated based on the two models, with the melted frost downwards flowing away or local drainage considered. The following physical parameters are predicted and analyzed, including the thermal resistance of refrigerant, temperature of melted frost on tube and fin’s surface, mass of melted frost and energy consumption from refrigerant during defrosting. As calculated, after the melted frost locally drained, the predicted total energy consumption could be decreased from 898.1 kJ to 727.5 kJ, and defrosting efficiency increased from 47.5% to 57.6%. This work is helpful to optimizing the intelligent control strategy of an air source heat pump unit, as well as saving energy for buildings.
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Defrosting start control strategy optimization for an air source heat pump unit with the frost accumulation and melted frost downwards flowing considered
Sustainable Cities and Society, 2019Co-Authors: Mengjie Song, Ning Mao, Zhihua Wang, Jiankai Dong, Haoran ZhangAbstract:Abstract When an air source heat pump (ASHP) unit operates at reverse cycle defrosting mode, it is always started with a pre-set timer, corresponding to a fixed mass of frost accumulation on the surface of a multi-circuit Outdoor Coil. It is difficult to quantitatively give the exact duration to start a defrosting operation due to the complicated and dynamic frosting conditions, resulting in a series of mal-defrosting phenomenon. The uneven frost distribution condition also affects the aforementioned time-based defrosting start control strategy. In this paper, defrosting performance of a specially made experimental ASHP unit with different frost accumulations evenly distributed were comparatively investigated. Basing on the fluctuations of system operation parameters, as well as its energy performance, a suitable frost accumulation to start defrosting was finally suggested. The defrosting efficiency could reach its peak at 46.05% when the frost accumulation is at 930 g. Meanwhile, the time-based defrosting start method was demonstrated to be optimized, with the frost accumulation and melted frost downwards flowing considered. Contributions of this study are expected to be used for adjusting the control strategy of an ASHP unit, and thus promote energy saving in sustainable cities and society.
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Heating and energy storage characteristics of multi-split air source heat pump based on energy storage defrosting
Applied Energy, 2019Co-Authors: Bowen Yang, Mengjie Song, Yiqiang Jiang, Jiankai Dong, Long Zhang, Shiming DengAbstract:Abstract In recent decades, multi-split air source heat pump (M-ASHP) unit has been widely used for space heating. Similar to the split-ASHP unit, frost would accumulate on its Outdoor Coil surface under frosting condition, which significantly deteriorates its heating performance. In the previous study, a novel reverse-cycle defrosting method based on phase change energy storage was developed and investigated, and the study demonstrated that the novel defrosting method could significantly improve defrosting performance. To provide guidelines for designing the M-ASHP units with phase change energy storage, it is necessary to investigate the characteristics of heating and energy storage for M-ASHP units, which is presented in this paper. Experimental results showed that there was no frost on Outdoor Coil when only the indoor unit with heating capacity of 7.1 kW was turned on under frosting condition. In addition, the energy storage period increased as the number of operating indoor unit increased, namely the load rate. Furthermore, the optimal starting time of the energy storage operation was around 10 min after the start-up of M-ASHP unit. Even though, compared with the conventional heating mode with three turned-on indoor units, the energy storage mode shortened the heating period by 8 min and decreased the average heating capacity and COP by 4.4% and 1.9%, respectively. The above negative impacts of energy storage operation on the heating performance of the M-ASHP unit could be neglected by considering its positive impacts on defrosting performance, for a periodic heating-defrosting condition.
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techno economic analysis on frosting defrosting operations for an air source heat pump unit with an optimized multi circuit Outdoor Coil
Energy and Buildings, 2018Co-Authors: Mengjie Song, Christopher Yu Hang Chao, Chili WuAbstract:Abstract Air source heat pump (ASHP) units are used in applications around the world. After optimizing the multi-circuit Outdoor Coil by installing water collecting trays between circuits and adjusting the refrigerant distribution by using valves located at each circuit, system frosting/defrosting operation performances could be effectively improved. Before practical industry-scale application of trays and valves, their economic performances should be evaluated. However, in current literature, techno-economic analysis on operation performance of ASHP units is rare, which limits the development of innovative technologies. Therefore, a techno-economic analysis on frosting/defrosting operations is carried out in this study. Firstly, the frosting/defrosting experiments are introduced, followed by a series of assumptions and calculations. Then, the economic analysis results are provided in detail. Compared with a traditional ASHP unit, the total running costs of the modified unit in the heating season could save as much as 3681.75 CNY, or 10.33%, and the total cost decreased by 3516.75 CNY, or 4.67%, over 15 years of service life. The payback period of the additional initial cost is less than 8 months. Contribution of this work plays an important role in the evaluation and application of new technologies in the HVAC field.
Yiqiang Jiang - One of the best experts on this subject based on the ideXlab platform.
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Heating and energy storage characteristics of multi-split air source heat pump based on energy storage defrosting
Applied Energy, 2019Co-Authors: Bowen Yang, Mengjie Song, Yiqiang Jiang, Jiankai Dong, Long Zhang, Shiming DengAbstract:Abstract In recent decades, multi-split air source heat pump (M-ASHP) unit has been widely used for space heating. Similar to the split-ASHP unit, frost would accumulate on its Outdoor Coil surface under frosting condition, which significantly deteriorates its heating performance. In the previous study, a novel reverse-cycle defrosting method based on phase change energy storage was developed and investigated, and the study demonstrated that the novel defrosting method could significantly improve defrosting performance. To provide guidelines for designing the M-ASHP units with phase change energy storage, it is necessary to investigate the characteristics of heating and energy storage for M-ASHP units, which is presented in this paper. Experimental results showed that there was no frost on Outdoor Coil when only the indoor unit with heating capacity of 7.1 kW was turned on under frosting condition. In addition, the energy storage period increased as the number of operating indoor unit increased, namely the load rate. Furthermore, the optimal starting time of the energy storage operation was around 10 min after the start-up of M-ASHP unit. Even though, compared with the conventional heating mode with three turned-on indoor units, the energy storage mode shortened the heating period by 8 min and decreased the average heating capacity and COP by 4.4% and 1.9%, respectively. The above negative impacts of energy storage operation on the heating performance of the M-ASHP unit could be neglected by considering its positive impacts on defrosting performance, for a periodic heating-defrosting condition.
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An experimental study on frosting and defrosting performances of a novel air source heat pump unit with a radiant-convective heating terminal
Energy and Buildings, 2018Co-Authors: Long Zhang, Shiming Deng, Yiqiang Jiang, Jiankai Dong, Shun HuangAbstract:Abstract When the Outdoor air temperature is low and relative humidity is high, frost will occur and accumulate on the Outdoor Coil of an ASHP unit, which may decrease the heating performances of the ASHP unit to a large degree. In addition, the defrosting performances of the commonly used defrosting method (reverse cycle defrosting) are poor due to the closed indoor air fan and insufficient energy stored in a conventional indoor Coil. On the other hand, the conventional indoor Coil based on forced convective heat transfer may cause strong draught sensation and dry eye problem and make users feel less comfortable than radiant heating terminals. Therefore, to improve the system defrosting performances and combine the merits of the conventional indoor Coil and those of radiant heating terminals, a novel radiant-convective heating terminal was developed and integrated with an ASHP unit. The frosting and defrosting performances of the novel system were experimentally investigated, and the results showed that the average values of radiant panel surface temperature, outlet air temperature for the novel heating terminal, and system COP were 36.6 °C, 29.4 °C, and 2.78, respectively, during a 66-min frosting period (air temperature is 2.0 °C and relative humidity 84.0%). In addition, the ratio of the radiant heating capacity to natural convective heating capacity to forced convective heating capacity remained at around 1.5:1.0:7.4 during the whole frosting period. Furthermore, the experimental results demonstrated that the novel heating terminal could provide sufficient energy for the defrosting, and some energy could be provided for space heating through radiant and natural convective heat transfer during defrosting. The defrosting and resuming heating periods were 105 and 65 s, respectively.
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experimental study on concentration change of spray solution used for a novel non frosting air source heat pump system
Energy and Buildings, 2014Co-Authors: Yiqiang Jiang, Huiying FuAbstract:Abstract Frost formation on the Outdoor Coil surface of an air source heat pump (ASHP) unit may occur when it is used for space heating at a low ambient temperature in winter. To reduce the impact of frosting on the operational performance and energy efficiency of ASHPs, a novel non-frosting air-source heat pump (NASHP) system was proposed. The detailed introduction of NASHP prototype was given, and its test rig was developed, as well as its operating principles and modes. Then the characteristics of some conventional dehumidification solutions were compared; the glycerol was selected as spray solution for NASHP system, due to its great influence on the operating performance of the system. The glycerol solutions with different concentration and different mass flow were investigated experimentally, and the best practice by which NASHP system has the most remarkable efficiency was found finally. Such results will provide helpful reference for the potential application of NASHP system and future research work.
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an experimental investigation on reverse cycle defrosting performance for an air source heat pump using an electronic expansion valve
Applied Energy, 2012Co-Authors: Minglu Qu, Shiming Deng, Yiqiang JiangAbstract: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.
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an experimental study on defrosting heat supplies and energy consumptions during a reverse cycle defrost operation for an air source heat pump
Applied Thermal Engineering, 2012Co-Authors: Shiming Deng, Jiankai Dong, Yiqiang JiangAbstract:Abstract For a space heating air source heat pump (ASHP) unit, when its Outdoor Coil surface temperature is below both the air dew point temperature and the freezing point of water, frost will form on its Outdoor Coil surface. Frosting affects its operational performance and energy efficiency. Therefore, periodic defrosting is necessary. Currently, the most widely used standard defrosting method for ASHP units is reverse cycle defrost. The energy that should have been used for space heating is used to melt frost, vaporize the melted frost off Outdoor Coil surface and heat ambient air during defrosting. It is therefore necessary to study the sources of heat supplies and the end-uses of the heat supplied during a reverse cycle defrost operation. In this paper, firstly, an experimental setup is described and experimental procedures are detailed. This is followed by reporting the experimental results and the evaluation of defrosting efficiency for the experimental ASHP unit. Finally, an evaluation of defrosting heat supplies and energy consumptions during a revere cycle defrost operation for the experimental ASHP unit is presented. The experimental and evaluation results indicated that the heat supply from indoor air contributed to 71.8% of the total heat supplied for defrosting and 59.4% of the supplied energy was used for melting frost. The maximum defrosting efficiency could be up to 60.1%.
Ning Mao - One of the best experts on this subject based on the ideXlab platform.
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A modeling study on the revere cycle defrosting of an air source heat pump with the melted frost downwards flowing away and local drainage
Energy and Buildings, 2020Co-Authors: Mengjie Song, Gongnan Xie, Libor Pekař, Ning MaoAbstract:Abstract Reverse cycle defrosting is widely used for air source heat pumps. When a multi-circuit heat exchanger is vertically installed in a heat pump as an Outdoor Coil, the melted frost could be kept downwards flowing or locally drained during defrosting by using water collecting trays. To analyze the performance differences of melted frost, two defrosting models were developed and previously reported by authors. In this study, the defrosting performance of an air source heat pump was numerically investigated based on the two models, with the melted frost downwards flowing away or local drainage considered. The following physical parameters are predicted and analyzed, including the thermal resistance of refrigerant, temperature of melted frost on tube and fin’s surface, mass of melted frost and energy consumption from refrigerant during defrosting. As calculated, after the melted frost locally drained, the predicted total energy consumption could be decreased from 898.1 kJ to 727.5 kJ, and defrosting efficiency increased from 47.5% to 57.6%. This work is helpful to optimizing the intelligent control strategy of an air source heat pump unit, as well as saving energy for buildings.
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Experimental study on the melted frost influence on the metal energy storage during an air source heat pump defrosting
Energy and Buildings, 2020Co-Authors: Mengjie Song, Ning MaoAbstract:Abstract When an air source heat pump works at reverse cycle defrosting mode, the metal temperature of its indoor Coil decreases and that of Outdoor Coil increases. The energy storage of two Coils’ metal at heating mode would influence the system defrosting efficiency. For a vertically installed Outdoor Coil having multiple circuits, meanwhile, the melted frost downwards flowing from upside circuit(s) also delay the defrosting process of downside circuit(s). To quantitatively evaluate the influence from two parameters, metal energy storage and melted frost, four experimental cases were designed and carried out in this study, based on a specially made three-circuit Outdoor Coil. Three circuits are working at the frosting mode, while two and three are used at the defrosting mode in four cases. As concluded, the melted frost influence on defrosting efficiency and metal energy storage effect would be increased from 4.87% to 10.45% and from 0.77% to 1.49%, respectively, after the Outdoor Coil changed from two- to three-working circuit. Outcomes of this study are meaningful for the structure optimization and energy saving of air source heat pump units at defrosting mode.
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Defrosting start control strategy optimization for an air source heat pump unit with the frost accumulation and melted frost downwards flowing considered
Sustainable Cities and Society, 2019Co-Authors: Mengjie Song, Ning Mao, Zhihua Wang, Jiankai Dong, Haoran ZhangAbstract:Abstract When an air source heat pump (ASHP) unit operates at reverse cycle defrosting mode, it is always started with a pre-set timer, corresponding to a fixed mass of frost accumulation on the surface of a multi-circuit Outdoor Coil. It is difficult to quantitatively give the exact duration to start a defrosting operation due to the complicated and dynamic frosting conditions, resulting in a series of mal-defrosting phenomenon. The uneven frost distribution condition also affects the aforementioned time-based defrosting start control strategy. In this paper, defrosting performance of a specially made experimental ASHP unit with different frost accumulations evenly distributed were comparatively investigated. Basing on the fluctuations of system operation parameters, as well as its energy performance, a suitable frost accumulation to start defrosting was finally suggested. The defrosting efficiency could reach its peak at 46.05% when the frost accumulation is at 930 g. Meanwhile, the time-based defrosting start method was demonstrated to be optimized, with the frost accumulation and melted frost downwards flowing considered. Contributions of this study are expected to be used for adjusting the control strategy of an ASHP unit, and thus promote energy saving in sustainable cities and society.
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An experimental study on time-based start defrosting control strategy optimization for an air source heat pump unit with frost evenly distributed and melted frost locally drained
Energy and Buildings, 2018Co-Authors: Mengjie Song, Cheng Fan, Ning MaoAbstract:Abstract Defrosting operation of an air source heat pump unit is always started with a pre-set time, corresponding to a fixed mass of frost accumulation. The exact defrosting initiation time is hard to be quantitatively given due to dynamic and uneven frosting conditions, and thus resulting in series of mal-defrosting phenomena. For an air source heat pump unit with a multi-circuit Outdoor Coil, when the melted frost locally drained during defrosting was considered, the optimization of start defrosting control strategy becomes more complicated. Here, this fundamental problem was experimentally investigated, with frost nearly evenly distributed on the surface of the Outdoor Coil at the start of defrosting. Defrosting performance of the experimental air source heat pump unit at different frost accumulations with the melted frost local drainage were then comparatively analyzed. These measured and calculated physical parameters include the temperature of tube surface and melted frost, compressor suction and discharge pressures and their difference, thermal energy taken from indoor air and electricity inputs on compressor and fans during defrosting, etc. Results showed that, the defrosting efficiency reached its peak at 51.80% when frost accumulation was at 933 g. Thus, the time-based start defrosting control strategy was demonstrated to be optimized with this method. Contributions of this study could be used for adjusting the control strategy of air source heat pump units, which are also benefit for energy saving in their industry and residential applications.
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Energy transfer procession in an air source heat pump unit during defrosting
Applied Energy, 2017Co-Authors: Mengjie Song, Ning Mao, Shiming DengAbstract:Abstract Air source heat pump units have found their wide applications in recent decades due to their high efficiency and low environmental pollution. To solve their undesired frosting problem, reverse cycle defrosting is always employed. As a transient and nonlinear heat and mass transfer procession, defrosting performance directly affects the occupants’ thermal comfort. During defrosting, the metal energy storage values of indoor and Outdoor Coils are varied as their temperature fluctuations. It is therefore necessary to investigate the energy transfer procession in an air source heat pump unit and the effect of metal energy storage during defrosting. However, scarce of attentions were paid to this fundamental problem. In this study, two experimental cases with two-working-circuit and three-working-circuit Outdoor Coils were conducted basing on frost evenly accumulated on their surfaces. After four types of energy supply and five types of energy consumption during defrosting were calculated, a qualitative and quantitative evaluation on the metal energy storage effect was then given. As concluded, after the Outdoor Coil enlarged 50%, the metal energy storage effect can be changed from positive (0.33%) to negative (−2.18%). The percentages of energy consumed on melting frost and vaporizing retained water were both increased. Defrosting efficiency was improved about 6.08%, from 42.26% to 48.34%. Contributions of this study can effectively guide the design optimization of indoor and Outdoor Coils and promote the energy saving for air source heat pump units.
Zhihua Wang - One of the best experts on this subject based on the ideXlab platform.
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Defrosting start control strategy optimization for an air source heat pump unit with the frost accumulation and melted frost downwards flowing considered
Sustainable Cities and Society, 2019Co-Authors: Mengjie Song, Ning Mao, Zhihua Wang, Jiankai Dong, Haoran ZhangAbstract:Abstract When an air source heat pump (ASHP) unit operates at reverse cycle defrosting mode, it is always started with a pre-set timer, corresponding to a fixed mass of frost accumulation on the surface of a multi-circuit Outdoor Coil. It is difficult to quantitatively give the exact duration to start a defrosting operation due to the complicated and dynamic frosting conditions, resulting in a series of mal-defrosting phenomenon. The uneven frost distribution condition also affects the aforementioned time-based defrosting start control strategy. In this paper, defrosting performance of a specially made experimental ASHP unit with different frost accumulations evenly distributed were comparatively investigated. Basing on the fluctuations of system operation parameters, as well as its energy performance, a suitable frost accumulation to start defrosting was finally suggested. The defrosting efficiency could reach its peak at 46.05% when the frost accumulation is at 930 g. Meanwhile, the time-based defrosting start method was demonstrated to be optimized, with the frost accumulation and melted frost downwards flowing considered. Contributions of this study are expected to be used for adjusting the control strategy of an ASHP unit, and thus promote energy saving in sustainable cities and society.
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review on improvement for air source heat pump units during frosting and defrosting
Applied Energy, 2018Co-Authors: Mengjie Song, Shiming Deng, Chaobin Dang, Zhihua WangAbstract:Abstract Air source heat pump (ASHP) units have found worldwide applications due to their advantages of high energy efficient and environmental friendly. Frost deposition and accumulation on the surface of the Outdoor Coil in an ASHP unit is inevitable and always play significant negative effects. To accurately predict and control a frosting-defrosting cycle, the interrelated heat, mass, and momentum transport phenomena within frost, melted frost and at the air-frost interface, a moving boundary condition, should be clearly understood. This review paper focuses on the developments in frost retarding and defrosting investigations for ASHP units from 2000 to 2017. 12 frost retarding measures and 5 defrosting methods are firstly introduced, followed by 6 typical system optimization methods during reverse cycle defrosting. Alternative control strategies to start and end a defrosting operation are thereby presented. Basing on previous analysis, the existing gaps in the research works on frost retarding and defrosting are identified, and recommendations are finally offered as per the viewpoint of the present authors. This comprehensive and systematic review around an entire frosting-defrosting cycle might provide an overview of the analytical tools for scholars, researchers, product developers, and policy makers, and shed new light on the designing and performance optimization of ASHP units.
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Experimental performance analysis and evaluation of a novel frost-free air source heat pump system
Energy and Buildings, 2018Co-Authors: Zhihua Wang, Mengjie Song, Fenghao Wang, Wenke FanAbstract:Abstract To tackle the problem of frosting occurred on the Outdoor Coil surface when an air source heat pump (ASHP) is operated in winter, a novel frost-free ASHP system has been developed. However, the dehumidification performance and the frost-free time of the system operated are highly dependent on the electronic expansion valve (EEV) opening of the 1st stage throttle. In addition, the coefficient of performance (COP) of the systems is controlled by the EEV opening of the 2nd stage throttle. Therefore, the objective of this study is to investigate the effect of the EEV opening on the system thermodynamic performance in terms of dehumidification efficiency, regeneration efficiency, compression ratio, heating capacity and COP. It was observed that the optimal performance was achieved when the EEV openings of the 1st stage throttle and 2nd stage throttle were 75% and 40% respectively. In addition, a correlation of COP with the EEV openings of the 1st stage throttle and 2nd stage throttle was developed to optimize the system performance. The results showed that the system thermodynamic characteristics were highly sensitive to the variation in the EEV openings of both the1st stage throttle and 2nd stage throttle.
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Experimental investigation on an air source heat pump unit with a three-circuit Outdoor Coil for its reverse cycle defrosting termination temperature
Applied Energy, 2017Co-Authors: Mengjie Song, Shiming Deng, Guangcai Gong, Zhihua WangAbstract:Abstract Air source heat pump units could efficiently recover low grade waste heat from ambient air for indoor air heating or hot water supplying, which makes them widely applied in recent decades. For a vertically installed multi-circuit Outdoor Coil, a reverse cycle defrosting operation is always used to solve its frosting problem at high humidity and cold climate. Reverse cycle defrosting operation is terminated when the tube surface temperature at exit of the lowermost circuit reaching a pre-set value. It is obviously that when the pre-set temperature is higher or lower, the defrosting duration would be prolonged or more residual water left, respectively. Both of them result in potential energy waste for an air source heat pump unit, or even adversely degrade the indoor thermal comfort. However, as reported, a wide range of 10–35 °C was used as the pre-set defrosting termination temperature, without a fixed value or range given. To save energy for an air source heat pump unit, in this paper, an experimental methodology was firstly presented. Then, an air source heat pump unit with three-circuit Outdoor Coil was specially selected, and experimental investigation conducted. Finally, the defrosting termination temperature was concluded suitable at 20–25 °C, around 22 °C for this study. This methodology makes contributions to the control strategy optimization and energy saving for air source heat pump units.
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An experimental study on the uneven refrigerant distribution over a vertically installed multi-circuit Outdoor Coil in an air source heat pump unit during reverse cycle defrosting
Applied Thermal Engineering, 2015Co-Authors: Mengjie Song, Ning Mao, Zhihua Wang, Ying ChenAbstract:Abstract For an air source heat pump (ASHP) unit with a vertically installed multi-circuit Outdoor Coil, many parameters would affect system defrosting performance, especially the uneven distribution of refrigerant for each circuit and downwards flowing of melted frost due to gravity from up circuit(s) to down circuit(s). Currently, the negative effects of melted frost have been demonstrated and quantitatively studied, without however giving a fundamental study on the effects of uneven refrigerant distribution due to gravity and tube internal resistance for an ASHP unit with a vertically installed multi-circuit Outdoor Coil. Therefore, an experimental study on system defrosting performance when refrigerant was evenly or unevenly distributed into each circuit has been carried out and a comparative and quantitative analysis conducted using the experimental data. In this paper, the negative effects of refrigerant uneven distribution on system defrosting performance were demonstrated and an increase of 6.9% in defrosting efficiency when refrigerant evenly distributed, compared with the case of refrigerant unevenly distributed, was reported.
