Reverse Cycle Defrosting Research Articles

Performance of bypass cycle defrosting system using compressor casing thermal storage for air-cooled household refrigerators

Electric heater defrosting (EHD) is extensively used on refrigerator evaporators. However, this method has a high level of energy consumption. Conversely, reverse-cycle defrosting (RCD) cannot be used in refrigerators because frequent reversal of the four-way valve may cause mass leakage of the refrigerant, which is unsafe. Hence, we develop a bypass cycle defrosting system using compressor casing thermal storage (BCD-CCTS). Experiments utilizing the EHD and BCD-CCTS methods are conducted on a refrigerator with a 50 W compressor. Experimental results reveal that in the BCD-CCTS method, defrosting time decreased by 65–77% compared with 180–419 W EHD. Whereas defrost energy consumption decreased by 89–92% compared with 180–419 W EHD method. After 60 h of normal operation, the temperature of the compressor casing exposed to air was maintained at approximately 60 °C, whereas that of the compressor casing exposed to the heat storage phase change material (PCM) was maintained at about 54 °C. The presence of heat storage PCM can effectively reduce the temperature of the compressor casing. It can also reduce the operating noise of the compressor by 18.5% compared with the same type of compressor without heat storage PCM.

Air source heat pump with water heater based on a bypass-cycle defrosting system using compressor casing thermal storage

In this study, the defrosting system of an air source heat pump utilizing compressor casing heat storage combined with a hot gas bypass cycle (ASHP-CCHS-HGBC) was designed. The phase change material for defrosting was selected, the phase change heat storage exchanger was devised, and the ASHP-CCHS-HGBC test system was established. The power consumption, defrosting time, and the influence of the indoor exchanger outlet on the air temperature in the ASHP-CCHS-HGBC method were then compared with those of the reverse-cycle defrosting (RCD) and electric heating defrosting (EHD) methods. Experimental results reveal that the total defrosting time and consumption of the ASHP-CCHS-HGBC method was 100s and 43.6kJ, respectively. These values were lower by 10s (9%) and 12.1kJ (21.7%) relative to those of RCD. Moreover, the compressor suction temperature was increased by 10.1°C during defrosting by ASHP-CCHS-HGBC. Under the normal heating operation for 2.5h, 10L hot water with a temperature of 30°C was obtained, the compressor casing temperature was reduced by 4.6°C. While defrosting, the air temperature of the indoor heat exchanger outlet declined to only 3.3°C and exerted the least influence on the indoor temperature among those of the three defrosting methods.

Energy transfer procession in an air source heat pump unit during defrosting

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.

Simulation analysis of a novel no-frost air-source heat pump with integrated liquid desiccant dehumidification and compression-assisted regeneration

This paper proposed and investigated a new no-frost air source heat pump (ASHP) combined with liquid desiccant dehumidification and compression-assisted regeneration. In the system, an additional vacuum compressor is introduced in the regeneration cycle to recover the regenerated heat, which could improve the performance the hybrid no-frost ASHP system. A mathematical model of the novel system is constructed to further study the characteristics of the system under various operating parameters and climatic conditions. Based on the simulation model, the results show that the COPanverage of the system varies from 2.811 to 3.242 when ambient air temperature rises from −10 to 0°C at a constant relative humidity of 80%. And with the ambient air RH increasing from 70% to 90% the COPanverage decreases slightly from 3.285 to 3.20 when the temperature is at 0°C. In addition, the effects of air-to-solution mass flow rate ratio in the dehumidifier and regeneration temperature on the performance of the system are discussed. Finally, a comparison study between the proposed system and typical reverse-cycle defrosting system is presented, which indicates that the COPanverage of the novel system is at least 36.05–61.19% higher than that of the COPreverse in the variation ranges of the analyzed parameters.

Termination Control Temperature Study for an Air Source Heat Pump Unit During Its Reverse Cycle Defrosting

Abstract For an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil, a reverse cycle defrosting (RCD) operation is always terminated when the tube surface temperature at the exit of the lowest circuit reaches a pre-set value. It is obviously that when the pre-set temperature is higher, the defrosting duration would be prolonged. Not only more energy would be consumed on heating cold ambient air, but also the occupants’ thermal comfort adversely affected. However, if the pre-set temperature is lower, more residual water would be left on the downside surface of fin in an outdoor coil, which degrades the system performance during the next frosting/defrosting cycle. In order to find a suitable DTT or its range, in this paper, an experimental investigation on RCD operation for an ASHP unit with a multi-circuit outdoor coil was conducted and reported. As concluded, DTT is suitable at the range of 20-25 o C, around 22 o C.

Experiments of a Heat Pump Water Heating System Using Stored Solar Energy to Defrost

Abstract A new solar-assisted hot water heat pump system is proposed that uses stored solar energy to defrost the outdoor unit. The system is mainly featured by two water tanks, one of which can be used to collect part of the solar heat during daytime and is used in the defrosting processes during night. Experiments were conducted in lab conditions to test the performance of this novel design, in which the solar heating systems was represented by electrical heating. Results show that the new system had a system COP almost 82% higher than the traditional system where the reverse-cycle defrosting uses the water thank as the evaporator. The advantage of this system is that it can make use of the low grade solar heat, which is difficult to collect or use in normal solar water heating system. This system is particularly suitable for regions with cold and humid winters like the hot summer and colder winter zone in China.

Improving the frosting and defrosting performance of air source heat pump units: review and outlook

ABSTRACTWhen air source heat pump (ASHP) units operate in heating mode at low temperatures in high humidity environments, frost forms and accumulates on the surface of its outdoor coils. This frost layer adversely degrades the operating efficiency of ASHP units rapidly, and can even result in sudden shutdown. Due to this operational problem, many researchers have conducted studies on frost-retardation measures. Since the frost that is present on the ASHP unit after shutdown has to be removed, defrosting becomes necessary. Among several reported defrosting methods, such as compressor shutdown defrosting, electric heating defrosting, hot water spray defrosting and hot gas bypass defrosting, the most popular method is reverse cycle defrosting. In addition, defrosting initiation and termination control play an important role in the frosting and defrosting cycle. Due to a scarcity of reviews of the literature on improving the frosting and defrosting performance of ASHP units, this paper provides a comprehensiv...

Experimental investigations on destroying surface tension of melted frost for defrosting performance improvement of a multi-circuit outdoor coil

When frost forms and accumulates over the outdoor coil’s surface in an air source heat pump (ASHP) unit, system operating performance will be adversely affected. Currently, reverse cycle defrosting is the most widely used standard defrosting method. A previous related study suggested that, for an ASHP unit with a multi-circuit outdoor coil, water collecting trays installed between circuits could eliminate the negative effects of downwards flowing melted frost due to gravity. However, for a vertical multi-circuit outdoor coil with separations, there is also a lot of melted frost remained on the downside surface of each circuit due to surface tension. The remained water would consume energy, and thus deteriorate system defrosting performance. To quantitatively study the negative effects of surface tension, in this paper, an experimental study changing the vertical multi-circuit outdoor coil into horizontally installed in an ASHP unit was conducted. Compared with the surface tension kept, defrosting duration could be shorted from 186s to 167s, or 20s less, and defrosting efficiency improved from 49.4% to 61.4%, or 12.0% more, when the remained water cleaned as the surface tension destroyed.

Performance evaluation of a novel method of frost prevention and retardation for air source heat pumps using the orthogonal experiment design method

In this study, a novel technology of frost prevention and retardation for air source heat pump (ASHP), which reduces thermal discomfort, is introduced. It utilizes auxiliary electric heaters (AEH) on tubes before (B-AEH) and (or) after (A-AEH) the outdoor evaporator to retard frost formation or prevent frost from accumulating on the outdoor heat exchanger. This enables the supply of hot air into the interior space without interruption. This new method differs from the common high pressure hot gas bypass defrosting (HGBD) methods and the reverse cycle defrosting (RCD), both of which could result in significant temperature fluctuations and thermal discomfort. The orthogonal experiment design (OED) was applied in this study to evaluate the performance of the air source heat pump (ASHP) with different magnitudes of AEH power under a range of ambient frosting conditions. The L25(56) orthogonal array was selected for the experiment and data were analyzed by means of the analysis of range (ANORA) and the analysis of variance (ANOVA). The optimum parameter combination affecting the performance of the ASHP was determined and the most significant parameters were identified. It was demonstrated that the proposed ASHP design is effective for frost prevention and retardation in real applications.

An experimental study on defrosting performance for an air source heat pump unit with a horizontally installed multi-circuit outdoor coil

When frost forms and accumulates over the outdoor coil’s surface in an air source heat pump (ASHP) unit, system operating performance will be dramatically deteriorated. Reverse cycle defrosting is the most widely used standard defrosting method. A previous related study reported that downwards flowing of melted frost due to gravity over a vertical multi-circuit outdoor coil would decrease the reverse cycle defrosting performance. If the outdoor coil can be changed to horizontally installed, the flow path of melted frost over coil surface can be shortened, and the flow directions of refrigerant and melted frost changed from opposite to orthogonal. Consequently, a better defrosting performance is expected. In this paper, therefore, an experimental study on defrosting performance for an ASHP unit with a horizontally installed multi-circuit outdoor coil was conducted. Experimental results show that, when a vertical outdoor coil was changed to horizontally installed, the defrosting efficiency increased 9.8%, however, with the same defrosting duration at 186s. Furthermore, when the outdoor air fan was reversed to blowing the melted frost during defrosting, the total mass of the retained water collected decreased 222g. However, the defrosting efficiency was not increased, but decreased 6.6% because of the heat transfer enhanced between hot coil and cold ambient air.

A modeling study on alleviating uneven defrosting for a vertical three-circuit outdoor coil in an air source heat pump unit during reverse cycle defrosting

Reverse cycle defrosting is the most widely used standard defrosting method for air source heat pump (ASHP) units. It was suggested in previous experimental studies that downwards flowing of the melted frost over a vertical multi-circuit outdoor coil in an ASHP unit has negative effects on reverse cycle defrosting performance. To quantitatively study the negative effects, an experimental study and a modeling study on draining away locally the melted frost for an experimental ASHP unit with a three-circuit outdoor coil were carried out and separately reported. However, for exiting ASHP units, it is hardly possible to install water collecting trays between circuits. To alleviate uneven defrosting for a vertical multi-circuit outdoor coil in an existing ASHP unit, an effective alternative is to vary the heat supply to each refrigerant circuit by varying the opening values of modulating valves installed at an inlet pipe to each circuit. In this paper, a modeling study on varying heat (via refrigerant) supply to each refrigerant circuit in a three-circuit outdoor coil to alleviate uneven defrosting is reported. Finally, in the designed three study cases, defrosting energy use could be decreased to 94.6%, as well as a reduction of 7s in defrosting duration by fully closing the modulating valve on the top circuit when its defrosting terminated.

Defrosting performances of a multi-split air source heat pump with phase change thermal storage

Recently, multi-split air source heat pumps (ASHPs) have been used increasingly for space heating in cold winter. When it operates in frosting environment, periodic defrosting is necessary to maintain a high system performance. However, researches on its defrosting were few due to its high capital and complicated controls. To solve its fundamental problem of insufficient heat available during defrosting, a novel reverse-cycle defrosting (NRCD) method based on the phase change thermal storage has been developed. In this paper, comparative experiments using both standard reverse-cycle defrosting (SRCD) method and NRCD method were carried out on a multi-split ASHP unit with a phase change material heat exchanger (PCM-HE) acting as energy accumulator during heating operation and heat source during defrosting operation. Experimental results suggested that when using the NRCD method, the system performances, such as suction pressure and temperature, defrosting and heat-resumption durations, COP during defrosting operation can be effectively improved.

A combined Dual Hot-Gas Bypass Defrosting method with accumulator heater for an air-to-air heat pump in cold region

Abstract The Dual Hot-Gas Bypass Defrosting (DHBD) cycle is an effective defrosting method compared to a Reverse Cycle Defrosting (RCD) method to remove frost from the outdoor heat exchanger (HEX) of an air-to-air heat pump, especially the outdoor temperature is above 0 °C. However, the DHBD method has a drawback when the heat pump operates in cold outdoor environment, below 0 °C due to rapid decrease in refrigerant temperature followed by lower hot-gas bypass temperature. In order to overcome lower discharge temperature of compressor, a combined defrosting cycle with DHBD and the accumulator heating method is developed. An induction heater (IH) is adopted as the accumulator heater. The dynamic performance and defrosting time are compared between the conventional RCD method and the combined DHBD–IH method using a medium size air-to-air heat pump of 16 kW under the condition of −5 °C outdoor temperature. Due to the additional heater, the combined DHBD–IH method sustained higher discharge temperature of the compressor and reduced 15% of the defrosting time than that of the RCD method with nonstop indoor heating operation. The overall heating capacity of the DHBD–IH cycle including defrosting mode was 2.5 kW higher than that of the RCD cycle.

Performance investigation of a novel frost-free air-source heat pump water heater combined with energy storage and dehumidification

Air-source heat pump (ASHP) often operates with substantial frost formation on the outdoor heat exchanger at low ambient temperature in winter, it insulates the finned surface and also reduces heat transfer rate, leading to performance degradation or even shutdown of ASHP systems. Although several defrosting methods have been reported, the frosting and defrosting processes reduced energy efficiency and resulted in, in some cases, heat pump breakdown. To solve this problem, a novel frost-free air-source heat pump water heater (ASHPWH) system has been developed, which coupled with an extra heat exchanger coated by a solid desiccant (EHECSD) with an energy storage device (ESD). Based on the previous studies, a further analysis and comprehensive research on the novel frost-free ASHPWH system is presented in this paper. The dynamic characteristics of the novel system are investigated experimentally in different ambient conditions. An experimental setup and experimental procedures are described in detail. Thereafter, the dehumidification efficiency and regeneration efficiency of EHECSD, suction and discharge pressures of the compressor, the temperature of PCM are evaluated during the heating and regeneration modes respectively. Results indicate that the system can keep the evaporator frost-free for 32, 34, 36min during heating mode at the ambient temperatures of −3°C, 0°C and 3°C and 85% RH. Compared with the reverse-cycle defrosting (RCD), COP of the frost-free ASHPWH are 17.9% and 3.4% higher at the ambient temperature of −3°C and 3°C respectively. With this innovative technology, it has been proved that the new system can realize continuous heating and excellent performance at a low ambient temperature.

A semi-empirical modeling study on the defrosting performance for an air source heat pump unit with local drainage of melted frost from its three-circuit outdoor coil

Reverse cycle defrosting is the most widely used standard defrosting method for air source heat pump (ASHP) units. It was suggested in previous experimental studies that during reverse cycle defrosting, downwards flowing of the melted frost due to gravity over a vertically installed multi-circuit outdoor coil in an ASHP unit has negative effects on defrosting performance. Therefore, an experimental study on draining away locally the melted frost for an experimental ASHP unit having three refrigerant circuits using water collecting trays was carried out and the study results were separately reported. To enable further quantitative analysis on the effects of local draining away the melted frost on reverse cycle defrosting performance of the ASHP unit, a modeling study on the defrosting process, at the two experimental settings of with and without the use of water collecting trays between circuits was carried out and is reported in this paper. Two semi-empirical mathematical models, corresponding to the two settings, were developed. In this paper, firstly the detailed development of the two semi-empirical models is presented. This is followed by reporting the validations of the two models using the experimental data previously reported. Finally, detailed discussions on the potential uses of the two models developed and the limitations of the modeling work reported are included.

An experimental study on the negative effects of downwards flow of the melted frost over a multi-circuit outdoor coil in an air source heat pump during reverse cycle defrosting

When the surface temperature of the outdoor coil in an air source heat pump (ASHP) unit is lower than both freezing point of water and the air dew point, frost can be formed and accumulated over outdoor coil surface. Frosting affects the energy efficiency, and periodic defrosting therefore is necessary. Reverse cycle defrosting is currently the most widely used defrosting method. A previous related study has indicated that during reverse cycle defrosting, downwards flow of the melted frost over a multi-circuit outdoor coil could affect the defrosting performance, without however giving detailed quantitative analysis of the effects. Therefore an experimental study on the effects has been carried out and a quantitative analysis conducted using the experimental data. In this paper, the detailed description of an experimental ASHP unit which was specifically built up is firstly reported. This is followed by presenting experimental results. Result analysis and conclusions are finally given.

A novel defrosting method using heat energy dissipated by the compressor of an air source heat pump

When an air source heat pump (ASHP) unit is used for space heating at low ambient temperatures in winter, frost may form on its outdoor coil surface. Since the accumulated frost adversely affects its performance and energy efficiency, periodic defrosting of the outdoor coil is necessary. Currently, the reverse-cycle defrosting (RCD) method is widely used for the defrosting of ASHP. However, this operation interrupts space heating during the defrosting process. A time lag occurs to resume heating at end of the defrosting cycle. Moreover, frequent reversing of the 4-way valve may cause mass leakage of the refrigerant, even make the system unsafe. Furthermore, some amount of heat is dissipated to the atmosphere through the compressor casing. To improve the defrosting process and use this waste heat, a novel ASHP unit is developed. The space is heated during the defrosting process using the heat dissipated by the compressor. Experiments using both the RCD method and the novel reverse cycle defrosting (NRCD) method developed in this study are conducted on an ASHP unit of 8.9kW nominal heating capacity. The experimental results indicated that in the NRCD method, the discharge and suction pressures are increased by 0.33MPa and 0.14MPa, respectively, the defrosting time is shortened by 65% while the resuming heating period vanished with the NRCD method, and that the total energy consumption in comparison to RCD method is reduced by 27.9% during the period which is composed of defrosting period and resuming heating period. Moreover, the NRCD method ensured continuous heating during defrosting. The mean temperature difference between the air entering and leaving the indoor coil reaches 4.1°C during defrosting. Over a test period of 125min, compared to RCD method, the total heating capacity and input power are increased by 14.2% and 12.6%, respectively. The increase in the system COP is 1.4%.

An Experimental Study on Performance During Reverse Cycle Defrosting of an Air Source Heat Pump with a Horizontal Three-circuit Outdoor Coil

When the surface temperature of the outdoor coil in an air source heat pump (ASHP) unit is below both the air dew point and freezing point of water, frost can be formed and accumulated over the surface of the outdoor coil, which is usually of multi-circuit structure in order to minimize its refrigerant pressure loss and enhance the heat transfer between refrigerant and ambient air. However, frosting adversely affects the operational performance and hence the energy efficiency of the ASHP unit, therefore periodic defrosting is necessary. Currently, the most widely standard defrosting method used for ASHP units is reverse cycle defrosting. A previous related study has suggested that during reverse cycle defrosting, downwards flowing of melted frost due to gravity over a vertical multi-circuit outdoor coil in an ASHP unit could deteriorate defrosting performance, by using more energy for defrosting and prolonging a defrosting process. If however an outdoor coil can be horizontally installed, the flow path for melted frost over coil surface can be shortened, and a better defrosting performance expected. In this paper, therefore, an experimental study on performance during reverse cycle defrosting of an ASHP unit having a horizontal three-circuit outdoor coil has been carried out. Firstly, a detailed description of the ASHP unit is presented. This is followed by reporting the experimental results. Finally, a comparative and quantitative analysis is given.

Influence of Outdoor Air Conditions on the Air Source Heat Pumps Performance

The purpose of the present work is the investigation of the effect of the outdoor air temperature and relative humidity on the performance of an air heat pump, when the reverse-cycle defrosting is considered. The frost formation process has been analyzed by developing a simplified model which relates the number of defrost cycles to the outdoor air conditions. Moreover the energy consumption due to the defrosting has been taken into account in the evaluation of the heat pump performance. The results, carried out for many Italian sites, point out that the outdoor air conditions play an important role in determining the amount of defrost cycles; however the frost formation is mainly affected by the relative humidity. The analysis highlights also that the defrosting contribution on the heat pump performance is not negligible when the heat pump that operates in wet weather, although cold; in these conditions the hourly COP may be reduced by up to 20%. However, this effect becomes less relevant, but not negligible, when the seasonal heat pump performance is evaluated; the maximum decrease of SCOP, observed for the all analyzed cases, is less than 13%.

Continuous heating of an air-source heat pump during defrosting and improvement of energy efficiency

During winter operation, an air-source heat pump extracts heat from the cold outside air and releases the heat into the living space. At certain outside air conditions, when it operates in heating mode, frost can form on the air-cooled heat exchanger, decreasing the heating performance. Conventionally, reverse-cycle defrosting (RCD) has been the common method of frost removal. But this method requires the interruption of heating during defrosting, as well as a period of time to reheat the cooled pipes of the indoor units after defrosting. In this study, a new technology called continuous heating was developed, which utilize only a hot gas bypass valve to remove the frost from the outdoor heat exchanger and thus enabling the supply of hot air to indoors without any interruption. For this, a new high temperature and low pressure gas bypass method was designed, which is differentiated from the common high pressure hot gas bypass methods by its use of low pressure. Various refrigerant mass flow distributions were examined, and the most effective defrosting mass flow was 50% in this case. Heating capacity was increased by 17% because of continuous heating, and the cumulated energy efficiency was increased by 8% compared to the traditional reverse cycle defrosting over 4h including two defrost operations. Also, cumulated energy efficiency was increased by 27% compared to electronic heaters that supply the same heating capacity during defrosting. By this new technology, it has been proved that continuous heating and energy savings could be achieved without adopting expensive technologies.