Article

An experimental study on defrost heaters applied to frost-free household refrigerators

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Abstract

In this study the performance of defrost systems applied to household refrigerators was experimentally evaluated through a purpose-built testing apparatus. The test bench is comprised of a calorimeter, a refrigerated cabinet and a humidifying system. Three distinct types of electrical heaters (distributed, calrod and glass tube) and three actuation modes (integral power, power steps and pulsating power) were investigated. The experiments were carried out under controlled conditions in order to ensure the same frost accumulation pattern over the evaporator. It was found that the defrost efficiency of the three types of heaters is practically the same for each operating mode. The highest efficiency of approximately 48% was obtained with the glass tube heater operating in power steps. However, this heater reached the highest temperature levels. The calrod heater seems to be the most appropriate not only because of its efficiency, which is compatible with the other options, but also due to its low cost and easy installation. However, standardized energy consumption tests need to be carried out before this finding can be generalized.

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... Despite the abundance of literature on commercial and industrial evaporator defrosting, the situation is reversed when it comes to household applications. Some remarkable studies, however, include those by Kim et al. [114] and Bansal et al. [121] for calrod heaters, Knabben et al. [102] and Özkan et al. [122] for distributed aluminum heaters, Melo et al. [123] and Knabben and Melo [124] for distinct technologies, and Malik et al. [125]. Kim et al. [114] conducted a comparative analysis of distinct defrost calrod heaters applied to a side-by-side refrigerator. ...
... Later, Melo et al. [123] investigated the performance of defrost systems applied to household refrigerators using a purpose-built testing apparatus comprised of a calorimeter, a refrigerated cabinet, and a humidifying system. This approach allowed for an experimental comparison between different defrost systems, maintaining strict control of operating conditions. ...
... As can be seen, similar defrost efficiencies were observed for all technologies assessed at the same operating modes, over a range between 27.5% and 48%. Despite the highest efficiency reached by the glass tube at step power mode (48%), Melo et al. [123] stated that the calrod heater seemed to be the most appropriate technology for general use due to its efficiency, low cost, and easy installation. [124] evaluated a frost-free refrigerator energy consumption using different combinations of heater technologies and control strategies. ...
Article
Full-text available
Modern refrigerators are equipped with fan-supplied evaporators often tailor-made to mitigate the impacts of frost accretion, not only in terms of frost blocking, which depletes the cooling capacity and therefore the refrigerator coefficient of performance (COP), but also to allow optimal defrosting, thereby avoiding the undesired consequences of condensate retention and additional thermal loads. Evaporator design for frosting conditions can be done either empirically through trial-and-error approaches or using simulation models suitable to predict the distribution of the frost mass along the finned coil. Albeit the former is mandatory for robustness verification prior to product approval, it has been advocated that the latter speeds up the design process and reduces the costs of the engineering undertaking. Therefore, this article is aimed at summarizing the required foundations for the design of efficient evaporators and defrosting systems with minimized performance impacts due to frosting. The thermodynamics, and the heat and mass transfer principles involved in the frost nucleation, growth, and densification phenomena are presented. The thermophysical properties of frost, such as density and thermal conductivity, are discussed, and their relationship with refrigeration operating conditions are established. A first-principles model is presented to predict the growth of the frost layer on the evaporator surface as a function of geometric and operating conditions. The relation between the microscopic properties of frost and their macroscopic effects on the evaporator thermo-hydraulic performance is established and confirmed with experimental evidence. Furthermore, different defrost strategies are compared, and the concept of optimal defrost is formulated. Finally, the results are used to analyze the efficiency of the defrost operation based on the net cooling capacity of the refrigeration system for different duty cycles and evaporator geometries.
... The defrosting techniques used in the HVAC&R industry may be adopted in GVR cooling system. Amer and Wang [21] and Hu et al. [22] summarized defrosting methods as follows: (1) on-off cycling [23], (2) hot gas bypass [24e26], (3) reverse cycle [27,28], (4) electric resistive heating [29,30], (5) desiccants as dehumidifiers or (6) ultrasonic wave [31,32] and their combined methods [33e35]. Hot gas bypass (HGB) was employed here as the key candidate defrosting method instead of reverse cycle for simple structure [36]. ...
... Comparing with the defrosting of microchannel heat exchangers [37,38] and fin-tube exchangers [39], the defrosting in low temperature shell-tube heat exchanger shell side was seldom mentioned in previous studies, and was more complex (Fig. 3). In conventional HGB mechanism, the residual refrigerant inside evaporator was removed by evaporation during defrosting [17,30], which consumed the energy of hot gas [23]. Especially in low evaporating temperature conditions, this additional energy consumption will become more and even excessive. ...
... Each curves appeared periodic spike and sustained fluctuation. The pressure spike was synchronizing with the No switching delay, evaporating temperature of 0/-35/-70 C 55% 47% 46.6% Kim et al. [34], Fig. 7 Periodic reverse cycle defrosting e 100% e Kim et al. [34], Fig. 8 Dual hot gas bypass defrosting e 53.8% e Kim et al. [34], Fig. 9 Dual hot gas bypass with inverter heater e 57.1% e Zhang et al. [62], Fig. 12 Reverse cycle defrosting with using heat energy dissipated by the compressor 25% 35.3% e Jang et al. [17], Fig. 9 Periodic reverse cycle defrosting 50% 100% e Jang et al. [17], Fig. 9 Dual spray hot gas defrosting 75% 57.1% e Qu et al. [63], Fig. 11 Cascade air source heat pump, thermal energy storage based reverse cycle defrosting e 73% e Cho et al. [33], Fig. 4 Multi-evaporator refrigeration; on-off cycle 60% 100% e Cho et al. [33], Fig. 7 Multi-evaporator refrigeration; hot gas bypass e 25.5% e Dong [28] Section 4 Reverse cycle defrost experiment, evaporating temperature of 0 C e e 60.1% Melo et al. [30] Table 1 Defrost experiments of electric heaters, freezer temperature of À23 C 48.0% Hoffenbecker [64] Table 3 Coil defrost model of hot gas, freezer temperature of À23 C e e 43.7% Song [65] Table 6 Hot gas defrost experiment, evaporating temperature of 0 C e e 47.1%/58.8% refrigerant switching to the warm evaporator after defrosting (step 2.1 of Section 2.2); while the sustained fluctuation was synchronizing with the hot gas spraying solenoid valve action. ...
Article
Condensation method is comprehensively applied for gasoline vapor recovery (GVR), of which frosts in the heat exchanger is the greatest challenge, especially for the continuous long running cases. A novel dual channel GVR cascade refrigeration system with shell-tube heat exchanger was presented and tested in this paper. With one-work-one-standby evaporator settings, combined with refrigerant evacuation and delay switching strategies, the defrosting of low temperature shell-tube heat exchanger was analyzed and solved. Also multi-stage cycle was introduced to supply three cooling stage, which cooled the gasoline vapor from ordinary temperature to about −70 °C. By the means of industrial application validation and process calculation, the ability of the non-stop cooling during defrosting was verified. The refrigerant evacuation was proposed to prevent high pressure drop caused by frost accumulation, which also improved the cooling capacity by 28.2% and approached the defrost efficiency of 55.4%. In addition, it was found that delay switching can effectively reduce the capacity fluctuation. Based on sensitivity studies, 20 min delay was identified as the best switching timing for this device. The capacity of this system performed lower reduction, higher duty ratio and defrost efficiency.
... Moreover, the energy consumption of the refrigerator and freezer was found to increase by 17.7% due to the automatic defrost. C. Melo et al. [12] compared three types of electrical heaters (distributed, calrod, and glass tube) and three actuation modes (integral power, power steps, and pulsating power). The results showed that the highest efficiency of approximately 48% was obtained with the glass tube heater operating in power steps. ...
... The general expression of exergy destruction for the condenser was calculated using Equations (12) and (13): ...
Article
Refrigeration systems can be blocked by frost accumulating on the evaporator, when they operate at a temperature below the dew point of air and the freezing point of water. Therefore, a defrost process that consumes extra energy needs to be started from time to time. Conventional defrosting methods require energy and time to complete, which reduce the refrigeration efficiency and cause a temperature fluctuation in the storage room during the defrosting process. In this study, a new type of defrosting is used in order to reduce the energy needed for defrosting, improve refrigeration efficiency and decrease temperature fluctuations, which utilize the heat from the liquid refrigerant to defrost the evaporator using two evaporators and a four-way valve. When one of the evaporators cools the room, the other is used for sub-cooling the refrigerant before entering the cooler evaporator. The sub-cooling process provides defrosting of the second evaporator. When the cooler evaporator requires defrosting, the four-way valve reverses the cycle, and the second evaporator becomes cooler. The first evaporator then becomes the sub-cooler to be defrosted. A cold room with this concept was built and tested experimentally together with a conventional cooling system. Energy, exergy, economic and environmental (4E) analyses of both systems were made and the results were compared. The results show that the new method can perform the defrosting process using the energy dissipated from the sub-cooling refrigerant without using any external power source and improve refrigeration efficiency by 12%. Moreover, this new system does not interrupt the cooling process during defrosting process as the conventional systems do. Temperature fluctuations also decrease by about 60% following this method.
... According to the references, the anti-frosting technologies include changing the characteristics (temperature, velocity [20], humidity [21], pressure [22] and purified air [23]) of inlet ambient air, changing the cold surface temperature [24], surface treatment [25], changing the structure of air heat exchanger (fin-tube geometry [26][27][28], fin spacing [29,30], fin type [31,32]), changing the interaction between air or frost layer and cold surface (electric field [33], ultrasonic wave [34], magnetic field [35] and low frequency mechanical oscillation [36]). And the defrosting technologies mainly have reverse cycle [37], hot gas bypass [38], electrical heating [19,39,40], warm-air defrosting [41], compressor shutdown [42], hot water spray [43], phase change heat storage, electric field [44], ultrasonic wave [45], magnetic field [46], air-particle jet [47], control strategy [48] and waste heat recovery [49,50]. In the above various technologies, the conventional reverse cycle (in various units) and the hot gas bypass (in industry units) defrosting technologies are the most widely used. ...
... [29,30], fin type [31,32]), changing the interaction between air or frost layer and cold surface (electric field [33], ultrasonic wave [34], magnetic field [35] and low frequency mechanical oscillation [36]). And the defrosting technologies mainly have reverse cycle [37], hot gas bypass [38], electrical heating [19,39,40], warm-air defrosting [41], compressor shutdown [42], hot water spray [43], phase change heat storage, electric field [44], ultrasonic wave [45], magnetic field [46], air-particle jet [47], control strategy [48] and waste heat recovery [49,50]. In the above various technologies, the conventional reverse cycle (in various units) and the hot gas bypass (in industry units) defrosting technologies are the most widely used. ...
Article
Full-text available
Frost layer on the outdoor air heat exchanger surface in an air-source heat pump (ASHP) can decrease the system coefficient of performance (COP). Although the common defrosting and anti-frosting methods can improve the COP, the periodic defrosting not only reduces the system energy efficiency but also deteriorates the indoor environment. To solve these problems, it is necessary to clearly understand the frosting phenomenon and to achieve the system frost-free operation. This paper focused firstly on the analyses of frosting pathways and frosting maps. Followed by summarizing the characteristics of frost-free technologies. And then the performances of two types of frost-free ASHP (FFASHP) systems were reviewed, and the exergy and economic analysis of a FFASHP heating system were carried out. Finally, the existing problems related to the FFASHP technologies were proposed. Results show that the existing frosting maps need to be further improved. The FFASHP systems can not only achieve continuous frost-free operation but reduce operating cost. And the total COP of the FFASHP heating system is approximately 30–64% higher than that of the conventional ASHP system under the same frosting conditions. However, the investment cost of the FFASHP system increases, and its reliability also needs further field test in a wider frosting environment. In the future, combined with a new frosting map, the control strategy for the FFASHP system should be optimized.
... An increase in the amount of frost accumulated on the evaporator increases the power consumption and may even jeopardize the refrigerator functioning with considerable impact on the technical assistance costs. In no-frost appliances, besides increasing the frost formation, the excess of warm and humid outside air infiltration increases the defrost cycle frequency and the compressor on-time after the defrost, what also contributes to an increase in power consumption ( Melo et al., 2012, Borges et al., 2014, Silva and Hermes, 2018. ...
... The infiltration rate stated by the authors -2.1 air changes per hour in a cabinet with new gaskets -implies approximately 190 g of frost formation per day, considering an ambient temperature of 24 °C and humidity of 54%. To reach this amount of frost, Melo et al. (2012) had to place a bowl of water in the refrigerator and heat it up continuously for 9 h. Therefore, the contribution of the article of Afonso and Castro is mainly the use of the tracer gas technique inside a refrigerated compartment. ...
Article
The aim of this study was to develop a methodology to measure the rates of air infiltration into refrigerated compartments based on the tracer gas dilution technique. An experimental apparatus was designed and constructed especially for this purpose. The apparatus includes a means of distributing the tracer gas uniformly into the zone, a means of ensuring a uniform concentration both in the vertical and horizontal directions, a means for obtaining the air samples, and a gas analyzer to measure the gas concentration at known times. Concentration measurements were taken using the infrared photoacoustic spectroscopy technique. Tests were carried out on three distinct refrigerators, with the compressor on and off, and with the concentration decay and constant concentration measurement techniques. It was observed that the tests have a high level of repeatability, with deviations of less than 5% in the air infiltration rate measurements. In addition, adsorption tests were carried out and the results indicated that the adsorption of the tracer gas by the refrigerator materials and gasket is insignificant when compared to the concentration variations caused by air infiltration. To verify that the technique developed is reliable and practical and that it represents a useful tool for the design and analysis of household refrigerators, two new refrigerators were tested. The tests were performed with different gaskets and extra seals in key regions. The results for the variation in the air infiltration were found to be reliable and the technique is practical to apply.
... A large number of studies have been undertaken into defrost systems, both examining how frost forms ( Armengol et al., 2016;Benítez and Sherif, 2017;Brèque and Nemer, 2016;Cui et al., 2011 ;Da Silva et al., 2011;Huang et al., 2008;Özkan and Özil, 2006;Seker et al., 2004;Tso et al., 2006;Yang et al., 2006 ) and how frost is most efficiently removed ( Bansal et al., 2010;Melo et al., 2013;Özkan et al., 2012;Zhang et al., 2016 ). These studies are extremely important as they reveal the mechanisms by which frost forms and the most effective and efficient way of removing frost during the defrosting process. ...
... The process of defrosting in a household refrigerator typically involves stopping the compressor and associated fans, activating a heater embedded in the evaporator to melt frost (defrosting) and then running the compressor to bring the appliance back to normal operating conditions ( Melo et al., 2013 ). During the defrosting process, the temperature in the compartment(s) often increases slightly as the compressor is not operating and therefore there is no cooling for a significant period. ...
Article
Many modern household refrigerating appliances have automatic defrost systems that keep the evaporator clear of frost and ice and obviate any user interaction to remove frost accumulation that occurs during normal use. The energy impact of defrosting has received little attention to date and there is little understanding on how these systems operate in homes. This paper uses laboratory measurements for 110 appliances over a period of weeks and field measurements for 195 appliances over periods of up to one year to observe the energy characteristics of defrosting systems. Field data covering some 55,000 individual defrost events has been analysed to obtain unprecedented insights into defrosting intervals during normal use. This data also provides useful information about the pattern of defrosting and a comparison between laboratory measurements and field measurements. A method to estimate incremental defrost and recovery energy in normal use is proposed.
... As a result of the experiments using outdoor air for the defrost system, it was demonstrated that the power consumption of the system using the outdoor air is 77.6 À 98.2% lower than the electrical heater defrosting system at a similar defrosting time. Melo, Knabben, and Pereira (2013) experimentally evaluated the performance of defrost systems which are three different types of electrical heaters (distributed, calrode and glass tube) and three operating modes (integrated power, power steps and pulsating power). With the glass tube heater, the highest efficiency was achieved with approximately 48%, but this heater reached the highest temperature levels. ...
Article
In this study, open-type refrigeration cabinets were designed and manufactured using hot gas and electrical resistance defrost methods and R290, R449A and R404A refrigerants. The aim of this study is to determine the temperature increase of the products during the process, CO2 emission amount, defrost efficiency, and energy efficiency index (EEI) value with different defrost methods and refrigerants for commercial refrigeration systems. The increase in package temperatures during the defrosting process was determined as 1.33 °C in the electrical method. The results showed that the hot gas method was 7.15% more efficient in terms of defrosting efficiency. The EEI value was calculated as 78.76 with the use of R290 in the hot gas method and the energy class was determined as F and the hot gas emit 5.9% less CO2 than in the resistance experiments. In the hot gas defrost method, it was seen that 20.3% less energy was consumed compared to the total consumption. It was observed that R290 refrigerant and the hot gas defrost method were the most suitable for commercial systems in terms of defrosting process losses, total energy consumption, and temperature rise of the cooled product during defrosting.
... The temperature in the refrigerator drops from 11°C to 5°C at the time of defrosting and improves the defrosting efficiency by 15%. Some researchers such as Melo et al. [24] used three different types of electrical heaters, distributed, chlorine and glass. They discovered that all three types of heaters had the same defrosting efficiency. ...
Article
Full-text available
The paper presents a literature review regarding defrosting methods for household refrigeration and heat pumps. The literature review covers the period between 2008 and 2020. A number of 61 papers have been studied. The literature review was conducted by dividing the defrost methods into passive and active. Passive methods involve surface coating and off-cycle. The active defrost methods involve reverse cycle, ultrasonic vibration, hot–gas defrost, electric heating and hot fluid. Surface coating and ultrasonic vibration are recent defrost methods proposed in the literature. Also, recently, phase change materials have been used in household refrigerators to improve their efficiency. This is a topic that needs further investigation and is a subject of interest as it can lead to lower energy consumption during defrost cycles. From the present study, the research direction regarding the use of PCMs in defrost cycles can be highlighted.
... [11][12][13][14][15][16][17] Using electric heating to inhibit frost formation or to melt the as-formed frost is probably the most widely used approach, but it is energy consuming and costly. 18,19 Although the application of external ultrasonic vibration provides potential in defrosting via the resonance effect, the necessity of additional oscillation facilities limits its broad applications. [20][21][22][23] Moreover, the use of antifreeze chemical substances, such as propylene glycol and ethylene glycol, would generate chemical hazards to the environment. ...
Article
Full-text available
As exemplified by the extreme cold weather hitting central China and causing great economic losses in 2018, as well as the 2021 Texas power crisis involving more than 4.5 million homes and businesses in the United States owing to frosting/icing, anti-frosting/anti-icing has been a long-standing challenge in modern society. Over the past decade, various approaches have been used to achieve anti- frosting, including active approaches and passive approaches. Given that active anti-frosting strategies are inefficient, energy consuming, and ponderous, tremendous progress has been made for passive anti-frosting strategies, owing to the booming of mate- rial engineering and the advanced measuring approaches. This re- view focuses on the process of condensation frosting, progress on passive anti-frosting, and prospects for this field. We aim to bring a comprehensive framework on passive anti-frosting in the broad sense: from inhibition of ice nucleation to reduction of frost/ice adhesion, from nanoscale to millimeter scale, and from surface design to material selection.
... In below figure we can see the heaters mounted on the evaporator coil for defrosting. [10] conducted an experiment with different types of heaters and found that all are having same defrosting efficiency. ...
Conference Paper
Full-text available
When outdoor coil of heat pump is subjected to low ambient temperature and high relative humidity, surface temperature of outdoor coil falls below dew point temperature and freezing temperature then frost starts accumulating on the outdoor coil surface. This accumulated frost reduces the heat transfer between coil and surrounding hence degrades system performance, so this frost needs to be removed to avoid this. Currently many methods are available for defrosting and lot of literature is available for each method. In this work detailed literature survey for various methods is carried out. Drawbacks of each method and modification to be done is also discussed here and finally most suitable method of defrosting for air source water heat pump is suggested.
... Ordinarily, the refrigerator compartment has high humidity because of the moisture gained from both the stored food and the air entered during door opening (Melo, Knabben, & Pereira, 2013). The evaporator gets frosted due to the moisture deposition on the evaporator surface. ...
Article
This paper reviews and discusses various studies on the application of phase change materials (PCMs) with domestic refrigerators and freezers regarding thermal management and performance enhancement. Covered previous studies in this review indicate the potential of PCMs in improving COP and exergy efficiency of refrigeration systems, in addition to reducing energy consumption, CO2 emissions and compartment temperature fluctuation. The paper also presents the effect of PCM performance under different thermal loads including phase change temperature, ambient temperature, evaporation temperature, door opening, defrosting and power failure. The effect of PCM's configuration, geometry, orientation and location on the performance of refrigeration systems, are also discussed. The literature review concludes that many researches have concentrated on the incorporation of PCM at the evaporator and compartment, while few studies have been carried out at the condenser section. The most common PCMs used in domestic refrigeration systems are water and eutectic PCMs solutions. We conclude that eutectic PCMs demonstrate better system performance than water.
... In below figure we can see the heaters mounted on the evaporator coil for defrosting. [10] conducted an experiment with different types of heaters and found that all are having same defrosting efficiency. ...
Experiment Findings
Full-text available
When outdoor coil of heat pump is subjected to low ambient temperature and high relative humidity, surface temperature of outdoor coil falls below dew point temperature and freezing temperature then frost starts accumulating on the outdoor coil surface. This accumulated frost reduces the heat transfer between coil and surrounding hence degrades system performance, so this frost needs to be removed to avoid this. Currently many methods are available for defrosting and lot of literature is available for each method. In this work detailed literature survey for various methods is carried out. Drawbacks of each method and modification to be done is also discussed here and finally most suitable method of defrosting for air source water heat pump is suggested.
... In addition, they [24] designed and carried out four experimental cases based on a specially made threecircuit outdoor coil to quantitatively evaluate the influence from metal energy storage and melted frost. Many scholars paid more attention to the air side characteristics of heat exchangers [25][26][27]. Especially, Du et al. [28] found that the R290 heat pump is more likely to have throttle valve failure than other heat pumps during heating-defrosting operation. However, little attention has been focused on internal thermodynamics parameters of R290 ASHPs during the dynamic process. ...
Article
As a viable alternative for HCFCs and HFCs in the refrigeration and air conditioning industry, R290 has obvious advantages in aspects of environmental protection and thermal properties. At present, the application of R290 in air source heat pumps (ASHPs) is still in the stage of exploration and promotion. In this paper, the dynamic behaviors of an R290 ASHP were experimentally investigated during defrosting process, and an experimental platform of an R290 ASHP system was built to conduct defrosting experiments in a multifunctional enthalpy-difference laboratory. The variation of pressure and temperature of main components and the refrigerant status in the accumulator during defrosting process are shown in this paper. Experimental data was analyzed by combining the physical characteristics of R290, and the dynamic characteristics under three different working conditions were compared. The defrosting time of the three working conditions was 420s, 435s and 865s respectively. In addition, there was a point M that the temperature difference between the inlet and outlet of outdoor heat exchanger (ODHE) approached zero at 370s of condition II. The results and analysis of this research can provide effective guidance for improving the reliability of R290 ASHPs.
... To alleviate the negative effect of frosting, many defrosting methods for ASHP units are carried out, including compressor shut-down defrosting [4], hot gas bypass defrosting [5,6], electrical heating defrosting [7], and hot water spraying defrosting [8], etc. Among all the defrosting methods, reverse cycle defrosting becomes the most widely used one [9]. ...
Article
Frosting is widely seen in the surface of an evaporator extracting heat from air in heat pump or liquid natural gas fields. The solidification of a water droplet on a cold plate surface, a phase change from liquid status to solid status as reaching its freezing point, is the key process at the early frosting stage. To accurately control the frosting/icing process, it is necessary to well understand the heat and mass transfer mechanism during the water solidification. As a fundamental problem, the characteristics of water droplet's solidification on a cold horizontal flat plate surface is reviewed. The objectives and scopes of this paper are firstly introduced. This is followed by the detailed tips before and during freezing of droplet, including the temperature variation, nucleation rate, opaque phenomenon, singular shape, evolution of freezing front, trapped air bubbles, etc. The literatures mentioned take a long time span, from 1950s to 2020. To analyze some seemingly contradictory conclusions or ideas, some of them are experimentally investigated by authors. The current research obstacles are also discussed, with the meaning of this topic re-emphasized by its large time and space scales. This comprehensive and systematic review might provide an overview of the frontiers around water droplet's solidification, and shed new light on the following experimental design and numerical validation.
... Results showed that the outdoor air or phase-change-material thermal storage serving as defrosting energy could reduce the defrosting power consumption by more than 70%. Melo et al. [28] evaluated the defrosting performance for three kinds of electric heaters, of which the highest defrost efficiency was 48% while the highest temperature rise was 12.4 °C. They suggested the calrod type to be the best choice when considering both its compatible defrosting performance and low cost. ...
Article
Full-text available
The three-temperature frost-free refrigerator is popular in China currently, which adopts a single evaporator and separate air ducts to distribute cold air into all compartments, i.e., the fresh food compartment (FFC), the variable temperature compartment (VTC), and the freezer compartment (FZC). Two identical refrigerators with cyclopentane insulation are comparatively studied in this article, one with vacuum insulation panels (VIPs) in two sidewalls and the other without. Results show that VIPs reduce the steady-state energy consumption of the refrigerator by 12.4% due to smaller average power input and longer compressor-off duration. The decrease in average power input originates from a shorter FFC-damper-on duration since VIPs reduce its thermal load. However, the FFC average temperature rises unexpectedly with shorter damper-on and longer damper-off durations, and induces thermal load transfer from FFC to FZC through the mixing of return air in the evaporator chamber. Consequently, FZC-only duration increases and compensates for the decrease in FFC-damper-on duration, resulting in an overall similar compressor-on duration. Simulation results of the heat transfer process through the evaporator are also provided in both cases to obtain a full view of the thermal load transfer process.
... Energies 2019, 12, 3117 2 of 18 defrosting [12,13], (4) fin surface treatment [14,15], and (5) reverse-cycle defrosting (RCD) [6,16,17]. Although these defrosting methods effectively delay or remove the frost accumulated on the surface of outdoor coils, they exhibit limitations and problems. ...
Article
Full-text available
Previous studies have indicated that a basic frost layer negatively affects the heat-transfer efficiency and is difficult to remove using a single ultrasonic transducer. Herein, an ultrasonic phased array technology is proposed for evaporator coil defrosting. First, the dispersion curve of the guided wave in the vibration transfer plate and frosting fin is calculated, and the advance time of each ultrasonic vibrator and the ultrasonic near-field pressures under different velocities are determined through numerical calculations using the MATLAB software. Next, according to the advance time, ultrasonic array focusing is performed to remove the basic frost layer. Finally, the power consumption, heat-supply enthalpy difference, and coefficient of performance (COP) of the air-source heat pump (ASHP) unit are analysed. The theoretical analysis, numerical calculations, and experimental results consistently revealed that ultrasonic array focusing compensates for the energy dissipation and expends the effective defrosting area. Additionally, the perpendicular stress elicited by the Lamb wave and the differential transverse shear stress generated by the SH wave exceed the tensile strength and adhesion stress of the basic frost layer. The basic frost layer cracks and falls away, owing to the combination of the ultrasonic stress effect and the cavitation effect. The defrosting power consumption of the ASHP unit under ultrasonic array excitation decreases from −3.27% to 0.12%, whereas the heat-supply enthalpy difference increases from 4.47% to 10.86%. Therefore, the percentage increment of the COP is between 7.16% and 11.12%, and the power consumption of the reverse-cycle defrosting is 3–12 times that of ultrasonic array defrosting.
... Because the frosting duration accounts more than 80% of operation time in a frosting-defrosting cycle, frost retarding measure exploration first attracts more attentions [13]. For example, adjusting the refrigeration could make the frost evenly distributed [14], and increasing the temperature [15,16,17], decreasing the relative humidity [18] and increasing the flowrate [19,20] of inlet air could decrease the frosting rate [21]. However, frost retarding measures is always consume additional equipment and energy, and frost that is presented after delaying would have to be removed finally [22,23]. ...
Article
Full-text available
For an air source heat pump (ASHP) unit, uneven frost distribution would result in mal-defrosting, or adversely degrade system defrosting efficiency. Previous investigations demonstrated that, for an ASHP unit with a multi-circuit outdoor coil, when the frosting evenness value (FEV) on its surface of outdoor coil is higher, the defrosting efficiency would also be higher. On the other hand, as reported, the downwards flowing melted frost during defrosting would also play negative effects on defrosting performance. Although the comprehensive influence of the two factors, uneven frost distribution and the downwards flowing melted frost, had been experimentally investigated, the influence of uneven frost distribution or the FEV on negative effect of downwards flowing melted frost was still unknown. As a fundamental and interesting problem, it is meaningful to be explored. It was therefore quantitatively analyzed here basing on six experimental cases. Water collecting trays were used to take away the melted frost during defrosting in three cases, and the other three ones without trays installed stood for the traditional status. Meanwhile, their FEVs of two groups were separately adjusted to be at three different levels. Results clearly showed the positive effects of increasing FEV and taking away the melted frost on system defrosting efficiency. After the melted frost was taken away, the FEV effects was increased. The increase of defrosting efficiency seems unchanged, but the reduction of defrosting duration was obviously increased from 17 s to 22 s. However, the differences of defrosting duration and efficiency changed were too small when melted frost was locally drained or not. More experimental cases with lower FEVs should be carried out.
... Thus, non-frosting ASHP unit was widely explored, such as by adding extra heat exchanger coated with a solid desiccant [12,13] , or a solution spray subsystem [14] in a conventional ASHP system. However, frost that is present after delaying would have to be re- moved, which resulting in series of defrosting methods explored, including compressor shutdown [15] , electric heat [16] , hot water spraying [17] , ultrasonic vibration [18] , air-particle jet [19] , and hot gas bypass [20] , etc. Finally, reverse cycle defrosting (RCD) becomes the most widely used method for ASHP units due to its advantages of simple structure, convenient application, high efficiency, system safety, and few modifications [21] . During RCD, when the working mode of an ASHP unit turns from heating to defrosting, the roles of indoor and outdoor coils changed. ...
Article
Air source heat pump (ASHP) units are widely used in recent years, and reverse cycle defrosting becomes the most popular method to solve their undesired frosting problem. During defrosting, a transient and nonlinear heat and mass transfer procession, the metal energy stored in the indoor and outdoor coils are varying as their temperature fluctuations. On the other hand, authors have previously confirmed the effects of melted frost and metal energy storage on system defrosting performance. However, detailed energy transfer procession without melted frost influence is still not identified. This fundamental problem directly affects the development and modification of two coils in a novel ASHP unit or an existing one. Consequently, basing on frost evenly accumulated on each circuit's surface, two cases were thereby designed in this study. Experimental results show that, the heating supply of indoor air thermal energy contributed about 80% of the total energy usage for defrosting, nearly 90% of energy consumed on frost melting and ambient air heating, respectively. After the total area of outdoor coil was enlarged by 50%, the metal energy storage effect was changed from −0.44% to −3.67%. Meanwhile, defrosting efficiency was improved by 11.66%, from 47.13% to 58.79%. Contributions of this study can effectively guide the design optimization of an ASHP unit, improve occupant's thermal comfort and promote the energy saving in buildings and industry.
... Kim et al. [92], Bansal et al. [93] and Ozkan et al. [94] conducted comparative studies using different types of defrosting heaters, but no quantitative defrosting results or frosting conditions were provided. In 2013, Melo et al. [95] carried out a series of experiments through a purposely-built testing apparatus. In three types of heaters, the highest efficiency of approximately 48% was obtained with a glass tube heater. ...
Article
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.
... Electric heating defrosting usually involves electrically heating up the surface of an outdoor coil to melt off the frost, which is similar to the frost-retardation method of preheating the inlet air of the outdoor coil. Although many scholars have tried different methods of using this defrosting method at a high efficiency [28], additional electrical energy is required to melt the frost that is high-quality energy. The ASHP unit is also out of operation during this defrosting process, which results in the interruption of indoor air heating. ...
Article
Full-text available
When 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 comprehensive review of experimental and modelling studies which explore this topic. This review can be used for guiding the future design and control strategy optimisation for ASHP units.
... Currently, there are many defrosting methods, such as 1) compressor shut-down defrosting [2], 2) electric heating defrosting [3,4], 3) hot water spray defrosting [5], 4) hot gas by-pass defrosting [6,7], 5) compressed air blowing defrosting [8,9], 6) ultrasonic vibration defrosting [10,11], etc. However, among these used methods, the most widely used standard defrosting method is reverse cycle defrosting (RCD). ...
Article
Full-text available
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 oC, around 22 oC.
... A few methods and correlation equations were proposed in order to predict the frost property [6,7]. Defrosting methods had been investigated, including: (1) hot water spray [8], (2) electric heating [9,10], (3) compressor shut-down [11], (4) hot gas by-pass [12][13][14], (5) reverse cycle [15][16][17], and (6) using an air-particle jet [18]. ...
Article
Many researchers have focused on the investigation into frost formation mechanism and tried to find various effective defrosting methods in recent years. Study on frost is divided into three stages: the frost formation process and mechanism, defrosting methods, and restraint frost methods. The three stages are carried out sequentially or in parallel. Compared to defrost, restraint frost is becoming more welcomed by peoples because of no energy or smaller consume. The affecting factors on frosting directly induce frost formation on cold surface, which leads to blockage, performance decrease, and even malfunction of the low temperature heat exchanger. This paper divides the factors into three categories: the characteristics (temperature, humidity and velocity) of the moist air, the features (temperature, structure and position, treatment)of the cold surfaces, the interaction between the air or the formed frost and the cold surface (electric field, ultrasonic wave, magnetic field, oscillation effects). One or several of the factors must be changed in order to restrain frost formation. Relative humidity is considered as that it has a larger effect on the frost formation, compared to the air temperature and air velocity. However, only a few researches pay attention to the restraint frost by controlling the characteristics of moist air. The structural parameters such as the fin spaces have the most important effect on the heat transfer performance of a heat exchanger under frosting conditions. Therefore, an optimization of the heat exchanger design should be considered for restraint frost. Hydrophobic surface is recognized as an effective way to improve the energy efficiency of a refrigeration system under frosting conditions. However, the research results are not as satisfactory as expected because of the difficult fabrication of scalable hydrophobic properties.
... This porous medium is commonly formed on evaporator fins, food humidity, air leakage by door openings, air circulation around evaporator, and door gaskets [1]. Cooling capacity and, in turn, coefficient of performance of a refrigerator is negatively affected by frost formation [2]. It's worth noting that at early stages, frost formation increases the efficiency of the evaporator because of incremental evaporator surface friction, but as the time passes and frost gets thicker, higher evaporator pressure drop occurs and thermal resistance increases in the airside, which in turn decreases the efficiency [3]. ...
Article
With the development of human modern society at the turn of the new century, more and more people tend to use new technologies. To keep in tandem with modern technologies, food storage became easier with domestic refrigerators. In such systems, frost formation can be a fundamental problem especially in tropical countries with hot and humid ambient conditions. An adaptive method for defrosting the evap-orator of a domestic refrigerator system is presented in this paper. In this method, parameters namely open door time, compressor ON time, previous defrost duration, mode of compressor, fans, and heater action before, during, and after defrost have been considered effective on defrost operation. Experiments have been carried out on two different case study top mounted freezers. Experimental results for case study 1 showed that the adaptive defrost in comparison to the fixed defrost cycles could reduce the energy consumption and Total Equivalent Warming Impact (TEWI) about 13% and 12.5%, respectively. The same parameters for case study 2 were decreased 5.5% and 5.2%, respectively. Furthermore, temperature variation of the compartments for both case studies decreased in comparison to that of their corresponding base case, which prevents quality loss of the stored food during defrost. Briefly, the results showed lower energy consumption, more environmentally friendly effects, and lower compartment temperature change.
... This porous medium is commonly formed on evaporator fins from food humidity, air leakage by door openings, air circulation around evaporator, and door gaskets [1]. Cooling capacity and, in turn, coefficient of performance of a refrigerator is negatively affected by the frost formation [2]. It's worth noting that at early stages, frost formation increases the efficiency of the evaporator because of incremental evaporator surface friction, but as the time passes and frost gets thicker, higher evaporator pressure drop occurs and thermal resistance increases at the airside , which in turn decreases the efficiency [3]. ...
Article
In refrigerator, the frost accumulated at the evaporator surface and melt with the defrost heater over the operation cycles. While the undesirable airflow from a drain hole in the refrigerator fan (R-fan) module can deteriorate the frost layer formation and defrost process. The thermal resistance increased by the frost layer can significantly affect the cooling capacity and power consumption of refrigerators. Different from the research on frost generation mechanisms, this study focused on product performance related to frost accumulation and flow features. The relationship between frost accumulation and airflow from drain hole was investigated by combining the experimental method and computational fluid dynamics (CFD) analysis without phase change. A new drain hole design with a circular shape was selected considering Laplace pressure for stable drainage. In CFD results, proposed model shows significant reduction of disorderly airflow around the evaporator and improvement of the performance including the flow rate delivered to the refrigerator and temperature distribution around the evaporator. The validation test of the proposed model fabricated by 3D printer was also conducted. The results corresponded well with the simulation result. The in-situ experimental results showed that the flow loss from the R-fan module drain hole and the average defrost cycle time were improved with a reduction of 28.7% and 8.0% compared with the original model, respectively. Although the internal structure of different products may be slightly different, the research results can be applied to products with similar defects for product development and improvement.
Article
Ice cream quality is influenced by temperature fluctuations during storage which cause ice crystal growth or recrystallization. Minimum temperature fluctuations occur during normal freezer on-off operation. However, large temperature variation may occur during the freezer defrost process which aims to melt accumulated ice on the freezer cooling coil. In this work, an experimental study was conducted to measure the temperature fluctuations inside two walk-in freezers of different sizes during normal and defrost operating conditions. Numerical simulations were carried out to examine the ice cream response to the temperature fluctuations inside the freezers using transient heat transfer model with variable boundary conditions. The data showed that the defrost timing and the maximum elevated temperature during the defrost process are strongly related to the freezer size and refrigeration unit specifications. Based on the ice cream response to the freezer temperature fluctuation and times of exposure, ice recrystallization rate and frozen water content are discussed and linked to the deterioration of the ice cream quality for two different ice cream formulations.
Article
This paper proposes a novel method to estimate the amount of accumulated frost using acoustic signals and artificial intelligence. The objective of this method is to estimate the amount of accumulated frost on the surface of the evaporator inside a domestic refrigerator. This methodology generates a set of acoustic signals with different frequency values. These signals interact inside the evaporator cavity. Each signal with a different frequency is absorbed and reflected depending on the amount of accumulated frost. This method uses an active speaker and a simple microphone to estimate the accumulation of frost on the evaporator surface. Additionally, we propose the use of sound pressure levels to reduce the data dimensionality. These sound pressure level values are used to create a training set and a validation dataset. In this work, we propose the use of two intelligent models to analyze the datasets. These models are based on artificial neural networks and probabilistic neural networks. To collect the data, several tests were performed in a domestic refrigerator following the typical usage habits. Each test ran for 24 h to measure sound pressure levels at different frequencies and different periods. The main contribution of this work is a low-cost method to estimate the amount of frost accumulated in the evaporator surface. One of the main advantages of our method is that it can be easily incorporated into a domestic refrigerator. The results indicate that our method is capable of estimating with accuracy four different levels of frost accumulation on the evaporator surface of a domestic refrigerator.
Article
The air flow mode is forced convection in frost-free refrigerator with high velocity and water evaporation rate. Larger water evaporation rate has a negative impact on food preservation. This paper proposed a correlation to forecast water evaporation rate of frost-free refrigerator based on the classic Dalton water evaporation model. Dalton model is suitable for natural convection on water surface of open space. But the water evaporation in frost-free refrigerator is driven by forced convection in enclosed space. The water evaporation rate correlation is modified by airflow of upper and lower layers. The correlation is verified by experimental results of another frost-free refrigerator. The results show that the influence of air exchange rate and vapor partial pressure difference on water evaporation rate is independent. The water evaporation rate is linearly correlated with the above two influencing factors, respectively. The error of water evaporation rate between the experimental and the correlation predicted values is 2.36%. The correlation can be used to forecast the water evaporation rate of frost-free refrigerator. It also provides a reference for the derivation and modification of water evaporation rate in other enclosed spaces.
Article
The infiltration of warm and humid air from the outside into a freezer results in frost accumulation around the evaporator in household refrigerators. This can significantly reduce the cooling capacity of refrigerators. In this study, the pressure, temperature, and frost around a proposed drain tube were monitored through in-situ measurements until the first defrost process at about 54 h. The proposed drain tube has 220 times higher hydraulic resistance and shows 55 times lower inflow in computational fluid dynamics (CFD) analysis. Once the door of the freezer compartment is opened and closed, external warm air replaces the cold air in the freezer. Negative pressure is produced by the shrinkage of this external air under low-temperature conditions. The other role of the drain tube is to vent air between the outside and inside of the refrigerator under negative pressure. To release the negative pressure, the proposed drain tube has a movable shutter. In the CFD results, the secondary inlet increases the inflow by 3.2 times compared to the inflow in an I-type tube. To determine the design, the weight and inner diameter of the drain tube were changed. The performance of the proposed tube was validated by the lack of blockage of discharge until the first defrost process. The movable shutter did not allow the negative pressure to exceed -100 Pa by facilitating air venting after opening the door for 10 seconds and then closing it.
Article
Power optimization was conducted for the defrosting heaters (conductive and radiant heaters) in a freezer to enhance their performance. The distribution of frost in the evaporator was measured experimentally, and the required defrosting energy corresponding to frost accumulation was calculated. The influence of defrosting heaters on frost was evaluated by measuring the heat quantity of the evaporator. The optimization was carried out such that the two heaters distribute the valid energy to the evaporator minimizing the excessive waste heat. At the optimum heater power, the maximum surface temperature of the evaporator was reduced, and the temperature distribution of the evaporator became uniform. The heater power and defrosting time were reduced through power optimization, thereby increasing the defrosting efficiency up to 6.7%. The effective heater power distribution could be further promoted to other heat exchangers with various heat transfer technologies.
Article
In winter, the heat transfer characteristics and energy efficiency of air source heat pumps (ASHP) will deteriorate due to the frost forming on the surface of the evaporator. In order to recover the thermal performance of the heat exchanger, defrosting is necessary. The optimal defrosting condition is one of the crucial factors to achieve efficient operation of ASHP. However, most previous control strategies are only devoted to the change of experimental parameters after frosting and influence on the heat transfer performance of the heat exchanger, and pay no attention to the influence of the frosting process on the energy transfer between air and refrigerant. These methods cannot achieve the high efficiency of system energy utilization. Thus, this study investigated the feasibility of using the entropy analysis control method to seek the optimal defrost conditions. Firstly, the trend of the temperature difference and the frictional pressure drop (PTP) as the key parameters of entropy generation analysis is developed. Secondly, based on the minimum principle of entropy generation and entropy resistance, the optimal defrost conditions of the evaporator under frosting conditions are studied. Finally, experiments are carried out to verify the optimal defrost parameters for the entropy analysis control method using the proportion of the temperature difference and the drop (PTP. Results indicate that due to the formation of frost, the entropy flow and entropy production caused by the temperature difference and pressure drop show a negative correlation trend, which makes the entropy generation and entropy resistance show decreasing trend initially and then increasing. Simultaneously, the irreversibility distribution ratio is approximately 1 and the minimum of the entropy generation and entropy resistance was achieved. Hence, the minimum irreversible degree and the peak of the energy efficiency can be obtained. It can be determined that the proportion of the temperature difference and the PTP can be used as irrational for defrosting control parameters. When the proportion of the temperature difference and the PTP is approximately 0.8, the system entropy generation and entropy resistance are the smallest. It is verified that the defrosting strategy of evaporator based on entropy analysis can realize the optimal heat transfer performance, the highest heat transfer efficiency, and the high-efficiency operation in the low-temperature environment of the ASHP system.
Article
It is universally acknowledged that the growth of frost layer will deteriorate the performance of refrigeration system in cold storage. Seeking efficient and rapid defrosting methods is a common concern in academic. This study presents a new type of defrosting method named as Hot Liquid Defrosting System with Multiple Evaporators (HLME) for cold storage. Multiple evaporators are installed in different refrigeration rooms in HLME system. When one of them needs to be defrosted, hot liquid refrigerant out of condenser is first used to remove the frost layer of the fins with achieving liquid refrigerant sub-cooled at the same time. The system can continuously refrigerate during defrosting without increasing additional power consumption. To further study the refrigeration system working performance, an experimental apparatus of this system was developed. A tested room with artificial load was constructed and the refrigeration performance of the system has been experimentally investigated in the process of frosting and defrosting, which mainly included the variation of the vapour suction, discharge pressure and temperature of the compressor, the liquid refrigerant temperature, the sub-cooler heat exchange, refrigeration capacity and refrigeration COPc .
Article
The applicability of common defrost technologies depends on how easy it is for air source heat pumps to frost under local climate conditions. At the same time, the degree of frost formation of the air source heat pumps outdoor units are mainly affected by local climatic conditions. Therefore, this study proposes the concept of frosting degree hours, which is used to specifically characterize the impact of climatic conditions on air source heat pumps frost. In order to make the analysis and calculation of the frosting degree hours more accurate, this study carried out the collection of the actual operation data of the air source heat pumps in winter in many cities in China. Through the selection and analysis of the actual operating data, the data fitting method was used to analyze the different variables and the characteristic temperature difference between the evaporation temperature and the outdoor dry bulb temperature of systems with different power types was obtained. Finally, according to the analysis of the frosting degree hours under different characteristic temperature differences, combined with the frost characteristics of air source heat pumps of different power types in 275 cities in China, the climatic division was performed. This can provide a reference for the climate-adaptive design of air source heat pumps and the development of defrost control strategies.
Article
Frost dramatically deteriorates the operating performance of an air source heat pump unit and leads to additional energy consumption. Results of our previous study show that intermittent ultrasonic vibration can greatly improve the defrosting efficiency and operation performance. To meet the ultrasonic defrosting demands of large-scale evaporators and to eliminate the standing wave effect of ultrasonic defrosting, an optimised ultrasonic array method for defrosting is proposed based on synergism of standing wave intermittent phase-stagger and multi-frequency excitation. Firstly, the surface vibration characteristics and defrosting effect difference of copper plate with standing wave intermittent phase-stagger in random mode and regular mode are investigated using a plate model. On this basis, the surface vibration characteristics and defrosting effect of a finned-tube evaporator are analysed in both phase-stagger modes. Finally, to optimise further the defrosting effect of the ultrasonic array, the standing wave intermittent phase-stagger and multi-frequency excitation are integrated to realise chaotic motion of the standing wave on the evaporator surface. Finite element analysis results and experimental results indicate that the standing wave effect of ultrasonic array defrosting can be eliminated using random intermittent phase-stagger, and the defrosting efficiency under random intermittent phase-stagger is superior to the regular method. Moreover, when arrayed ultrasonic transducers are controlled by synergistic random intermittent phase-stagger and multi-frequency excitation, there is no dead defrosting angle on the evaporator surface, the average frost layer thickness is approximately 0.282 mm, and it is 81.1% of mode I (intermittent and multi-frequency excitation).
Article
The frost-free refrigerator typically uses a bottom-placed calrod heater to remove frost on evaporator surface. However, the mismatch between full coverage of frost and bottom-up heat transfer extends defrost duration, and warm air intrusion into freezing cabinets (FCs) during defrosting leads to increased FC-temperature rise and overall energy consumption. Therefore, comprehensive measures were applied in this article to enhance defrost performance of the frost-free refrigerator. A special fan cover was adopted which opened during cooling cycles to supply cold air into cabinets normally, but closed during defrosting to block warm air intrusion passage. Moreover, an extra heater (60 W) was inserted in the middle height of the evaporator besides the original bottom one (180 W) to form better frost-heat match. Results show the fan cover and dual heaters altogether reduced defrost duration by 4.5 min and FC-temperature rise by 2.7 °C. Lower FC-temperature after defrost further declined energy consumption for the subsequent recovery cycle, which compensated for its increase for the defrost cycle and dominated the 1.2% reduction in overall energy consumption of the refrigerator. Both measures proved effective in enhancing defrost performance. However, the middle heater may affect cooling cycle performance of the refrigerator, necessitating further investigation.
Article
This paper proposes an adaptive defrost control method for single compartment refrigerators which adapts the prediction model used to estimate the frost accumulation on the evaporator according to changes in the ambient without the need for actually measuring the ambient variables. The proposed method does not require any additional sensor, since it makes use of the already available power, door opening, and temperature transducers. The experimental evaluation considered both tests in steady state and with door opening and showed that the algorithm was able to compensate for changes in the ambient condition. Comparative tests between the proposed method and the original defrost logic of the refrigerator considered for analysis showed that the proposed method significantly reduced the number of defrost cycles, from 14 to 2 in steady-state tests, and from 8 to 5 in the test with door opening without any impact on the food preservation characteristics. In terms of power consumption, the improvement of the defrost logic represented an average overall reduction of 0.8% in steady state and 2.6% in the case with door opening, thus increasing the energy efficiency of the refrigerator in all the evaluated situations.
Article
To preserve food nutrients, texture, and taste, as well as to prevent its putrefaction, food is frozen and kept at around −20 °C. Refrigerators and freezers are highly energy demand systems which can suffer a considerable decrease in operational efficiency due to frost growth on the evaporator. Defrost processes are launched periodically to avoid the frost built-up, consuming a relevant part of the total energy demand. To control the defrost launching and to improve the energy performance of the refrigeration system an accurate measurement of the frost level is required. Many frost detecting methods are expensive, not feasible due to their size, or simply they cannot measure the frost stacking precisely enough to swerve mal-defrost phenomena. This study provides an accurate parameter to indirectly estimate the frost layer built-up on the evaporator. The new parameter called thermal variation easiness (TVE) was experimentally tested and validated by comparison with another frost leveling method, ΔT method, on a walk-in freezer unit. Then the TVE was successfully tested on a multi-cold room refrigeration system, proving its applicability on both walk-in freezers run by remote condensing units and multiple cold rooms fed by a rack of compressors. The novelty of this parameter lays on its capacity to work on refrigeration facilities which are used to feed several walk-in fridges and refrigerated displays in big installations, such as supermarkets.
Article
Electric heater defrosting (EHD) is effective in removing frost on the evaporator and recovering capacity/COP for the frost-free refrigerator. The objective of this study is to propose a method of reducing temperature rise in the freezing cabinet (FC) during EHD process. Based on the idea of blocking the infiltration passage of the heater-generated warm air, a special cover was designed for the axial fan of the evaporator which was kept open during cooling cycles to supply cold air into cabinets normally, but was forced closed during EHD cycles so that defrost heat was prevented from infiltrating into the FC. Results show that defrost duration is shortened by 3.4%, FC-temperature rise within the defrost cycle is declined by 1.6 °C and the overall energy consumption for defrost and recovery cycles altogether is reduced by 1.9% with the fan cover. Further optimization would be made by proper delay of fan starting after defrost.
Article
To improve the defrosting accuracy and the energy efficiency of frost-free household refrigerators, a novel Time-Temperature difference (t-dT) defrosting control method, based on experimental and theoretical studies, is proposed in this paper. Experimental frosting and defrosting results verify the feasibility and applicability of the t-dT defrosting control method. The experimental data comparing two refrigerators at three different ambient temperatures show that the refrigerator applying the t-dT method had a shorter defrosting period and a shorter interval than those of the Time-Temperature (t-T) defrosting control method. Furthermore, the experimental results simulated real-life processes prove that the t-dT defrosting method could save more energy. Under the standard testing condition, the average total energy consumption including defrosting in the refrigerator applying the t-dT defrosting method is from 1.04 kW⋅h to 1.06 kW⋅hwithin 24 hours, and the ratio of defrosting energy consumption to the whole energy consumption is under 5%. All the results indicate that the t-dT method being suitable for the frost-free refrigerator has a more competitive performance than the t-T method.
Article
Heat removal is the most extended method for food preservation in food manufacturing industry by lowering food temperatures to stop microorganisms growing, which might spoil the product and could cause toxicity. Therefore, walk-in freezers are used for that purpose consuming a relevant part of the energy on service sector. The compression refrigeration system of the walk-in freezers can be blocked by the frost accumulated on the evaporator. For that reason a defrost process, which requires an important part of the energy consumption, has to be launched from time to time. In this paper, the schedule which manages the defrost process is investigated to limit its activation only when it is necessary. Moreover, different fan operation strategies were tested regarding the energy efficiency of the whole refrigeration system. This study has provided a system control strategy both for defrost and fans operation, depending on the frost built up on the evaporator. The control improves the energy performance of the whole refrigeration system.
Article
The defrosting method of the conventional household refrigerator increases freezer temperature during the defrosting operation, and defrosting efficiency reduces because the heater consumes more power than the amount of frost on the surface of the heat exchanger. To solve this problem, three defrost heater control methods, applicable to refrigerators equipped with conduction and radiation heaters, are being proposed. The control methods were classified as a method of simultaneously pulsating two heaters, method of individually pulsating two heaters, and method of step-by-step reduction of radiation heater power. The operation effect of each heater on freezer temperature was analyzed. For the three methods, a heater control optimization process was performed to reduce the temperature increase in the freezer. The power consumed by the heater was minimized and defrosting efficiency improved. The Best performance was observed when two heaters pulsated individually. For this method, the variation in freezer temperature, between before and after the defrost process, was reduced from approximately 11 °C to 5 °C. Additionally, the defrosting efficiency improved by 15%.
Article
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.
Article
The three-temperature frost-free refrigerator has a variable-temperature (5 °C to −18 °C) compartment (VC) besides regular refrigerating compartment (RC) and freezing compartment (FC). The air cooled by the evaporator always flows into FC during compressor-on period, but into RC and VC through their respective air-damper controlled by their temperature sensors. The effect of RC and VC air-damper control strategy on the refrigerator performance was studied experimentally in this paper. Experimental results show that the FC average temperature could not decrease during air-damper opening period. The main reason was that the evaporator supply-air temperature increased due to higher RC and VC return-air temperature, and up to higher than the FC average temperature. Then, the compressor-on time ratio during on/off cycle was increased. Meanwhile, the compressor input power would be also increased with higher evaporating temperature. The independent and synchronous opening control strategies for RC and VC air damper were compared experimentally. The synchronous opening control shortened the overall time of air-damper opening by 16.7% compared with the independent control since the RC and VC opening time overlapped with each other. Finally, the refrigerator energy consumption decreased by 3.1% while average temperatures of RC, VC and FC stayed almost unchanged, attributed to the decrease in the compressor-on time ratio and the average compressor input power of the refrigerator.
Article
It is universally acknowledged that the accumulation of frost on the evaporator surface will reduce the cooling capacity and performance of the cold storage. Therefore defrosting must be periodically and effectively carried out to avoid the evaporator blockage. The electric heater defrosting (EHD) and reversed cycle defrosting (RCD) methods are usually used in cold storages. However, no matter what method is adopted, there will be extra thermal load distributed to the compartments which lead to an unfavorable storage temperature fluctuation and then cause quality loss of the stored food. In order to solve the above-mentioned problems, a novel cold storage defrosting device based on EHD and RCD was proposed in this paper. Experiment was performed on the device to investigate its effect on several aspects including energy saving, reduction of cold storage temperature fluctuation and improvement of compressor operational stability etc. The results showed that this novel device could remarkably increase energy utilization and reduce temperature fluctuation of the cold storage, which was beneficial for long-term storage of products in the cold storage. Moreover, it was able to increase the operational stability of the compressor.
Article
A novel frost-free air source heat pump (ASHP) combined with membrane-based liquid desiccant dehumidification and humidification is proposed and investigated in this paper. Liquid-to-air membrane dehumidifier (LAMD) is employed to dehumidify the air before entering the outdoor coils in the proposed system so that frosting can be prevented. Diluted solution is regenerated to humidify the supply air via the liquid-to-air membrane regenerator(LAMR),which can overcome the droplets carry-over problem in the liquid-to-air energy exchanger. Model simulations are carried out to study the system performance under various operating parameters and climatic conditions,meanwhile the comparison between the proposed system and conventional defrosting system is performed. Results demonstrate that the COPsen and the COPtot of the novel system are at least 37.7% and 64.3% higher than that of the COPreverse of conventional reverse-cycle defrosting ASHP system in the variation ranges of the analyzed parameters,respectively.Humidification effects on the supply air depend on the ambient air properties.And air humidification by the regenerator can meet the needs of the air-conditioning room comfort when the ambient air is above 0 °C and 70% RH.
Article
In the defrosting process of most of existing frost-free refrigerators, major part of the heat generated by electric heater cannot be absorbed by the frost, resulting in the raise of the energy consumption. The objective of this study is to present a new method to improve the energy efficiency in the defrosting process. Based on the idea of making the frost absorb most of the heat generated by electric heater, a new method of defrosting is proposed, and its principle is to make the defrosting process at different locations complete simultaneously by optimizing the frost mass distribution on the evaporator. In this method, the defrosting heat distribution on evaporator surface, the required frost mass distribution, the required air flow distribution and the structure of return air duct are determined, respectively. Application of this method in an actual household refrigerator shows that the defrosting efficiency is improved by 29.8%.
Article
Among household refrigerators, air cooled frost-free household refrigerators have become popular in the market because of the advantages of large storage volume and frost-free compartment(s). However, the large power consumed by the process of defrosting the evaporator restricts the widespread application of such refrigerators. In this study, a new type of thermal storage defrosting system combined with bypass cycle is presented, the feasibility of the thermal storage defrosting system is analyzed, four kinds of different PCMs are tested, the operation modes of the overall system are proposed, and the structure of the heat storage exchangers is designed and optimized. Next, the experimental prototype is set up, and the experimental study is carried out for different defrosting modes. The results show that the best defrosting mode enhances the defrosting speed by about 50% more than the original electric heating model as well as reduces defrosting electric power consumption by about 71%.
Article
Periodical defrost actions are essential for the frost-free refrigerator-freezer to remove frost on the evaporator and recover its heat transfer performance. However, defrost heat could leak into the freezer and raise its temperature during defrost cycles. In this paper, effect of defrost heat leakage on freezer temperature rise of a frost-free refrigerator-freezer using an electric defrost heater was experimentally investigated. The defrost cycle was subdivided into three stages: heating, melting and draining. Results show that in the heating stage, little warm air leaked into the freezer due to the obstruction of the frosted evaporator, so freezer temperature hardly rose (by less than 0.1 °C/min). In the melting stage, flow area between adjacent fins gradually enlarged with frost melting from the bottom to the top of the evaporator; thus, warm air leaked into the freezer, raising its temperature by 0.6 °C/min in average. In the draining stage, the adjacent-fin flow area reached maximum as nearly all the frost had been removed throughout the evaporator; hence, large amount of warm air entered the freezer and raised its temperature by 1.0 °C/min. To our knowledge, the stage-based acceleration in freezer temperature rise during defrost cycles was covered for the first time.
Article
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%.
Article
Frost formation is an important problem for household refrigerator and air conditioning equipment manufacturers. When frost accumulates on the evaporator surface, it acts as a thermal insulator and reduces heat flow. Therefore, frost negatively affects evaporator performance. The purpose of this study is to decrease energy consumption and increase the efficiency of the defrosting process. In the first part of the experiment, frost formation on a no-frost refrigerator evaporator at real operating conditions was investigated. The ambient temperature was maintained at a constant 23°C. It was observed that when the evaporation temperature reaches –35°C, the frost formation on the evaporator exhibits a rather dense structure that is unlike the needle-type structure observed at higher temperatures. In the second part of the experiment, the defrosting process was observed with an endoscopic camera, and the initial melting points were investigated. The experiment revealed that although the heater density is higher on the lower rows, the frost on the higher rows of the evaporator melts faster. On the theoretical side, we prepared an analytical model that calculates the melting time of the frost on the fin. The experimental and theoretical results are within 5%.
Article
This paper advances an investigation on the frost nucleation on flat surfaces. The study focuses on the relevant parameters affecting the frost formation process, i.e., the surrounding air temperature and humidity, and the surface conditions (temperature, roughness and contact angle). The process of ice crystal nucleation was investigated both theoretically and experimentally in order to provide the physical basis for a criterion for predicting the occurrence of frost nucleation as a function of the operating conditions and surface characteristics. A mathematical model for the heterogeneous frost nucleation on smooth surfaces was put forward based on the classical nucleation theory. Experiments were carried out using a purpose-built apparatus to acquire data to validate the model. When compared to in-house as well as independent experimental data, the model predictions showed a good level of agreement.
Article
The present study advances a theoretical and experimental investigation of the frost growth and densification on flat surfaces. This study focuses on the most important factors affecting the frost formation process, i.e. the surrounding air temperature, humidity and velocity, and the surface temperature. The processes of frost growth and densification were investigated experimentally in order to provide a physical basis for the development of a theoretical model to predict the variation of the frost layer thickness and mass with time. The mathematical model was based on mass and energy balances within the frost layer, assuming the frost as a porous medium and accounting for the supersaturation of the moist air on the frost surface. The governing equations for mass and heat diffusion were integrated analytically, giving rise to a semi-algebraic formulation which requires numerical integration of only one time dependent ordinary differential equation. When compared with experimental data, the model predictions of the frost thickness as a function of time agreed to within ±10% error bands. The experimentally-validated model was then used to predict the frost layer growth and densification with respect to the operation conditions such as plate surface temperature, air stream temperature, humidity and velocity.
Article
The selection of correct pipe size for carrying hot gas to defrost evaporators is an important factor in conserving energy by such practices as maintaining low condensing pressures. The correct size will ensure that the resultant increase in specific volume of the hot gas that occurs at low pressures does not cause prohibitive pressure drops in the pipes. The way in which hot gas flow rates during defrost affects the pipe size selection is discussed together with the influence of saturation temperature.
Article
During the defrosting process, the temperature in the cabinet of a showcase becomes higher than the set point. This phenomenon is undesirable for foods or products stored. It is essential to develop an efficient defrosting method to prevent large temperature fluctuations. In the present study, the performance of the showcase refrigeration system with three evaporators was measured during on–off cycling and hot-gas bypass defrost. Based on the test results, the effects of off-period in the on–off cycling and EEV opening in the hot-gas bypass defrosting cycle on the performance of showcase system were analyzed. In addition, the operating characteristics of the hot-gas bypass defrosting cycle were compared with those of the on–off cycling. The hot-gas bypass defrosting method showed higher refrigerating capacity and less temperature fluctuations than the on–off cycling under frosting/defrosting conditions even though it required more compressor power.
Article
This study is concerned with a hot gas (refrigerant) bypass method to retard the formation and propagation of frost in an air-source heat pump. The feasibility of the hot gas bypass method was investigated experimentally and the method's performance is compared with that of a normal, 1.12 kW capacity air-source heat pump system with no defrost equipment such as an electric resistance heater. Results indicate that the hot gas bypass method is useful for retarding the formation and growth of frost at the outdoor coil. The best performance is shown under a bypass refrigerant flow rate of 0.2 kg/min (20% of the whole system refrigerant flow rate). During 210 min of heat pump operation, the hot gas bypass method improved COP and heating capacity at an average of 8.5% and 5.7%, respectively, relative to the normal system.
Article
When an air-source heat pump (ASHP) unit is used for space heating at a low ambient temperature in winter, frost may be formed on its outdoor coil surface. Frosting affects its operational performance and energy efficiency, and therefore periodic defrosting is necessary. Currently, the most widely used standard defrosting method for ASHP units is reverse-cycle defrost. During a standard reverse-cycle defrosting process, the indoor coil in an ASHP unit actually acts as an evaporator, therefore, no heating is provided and hence indoor air temperature in a heated space can drop. Furthermore, a longer period of time is needed before space heating can become available immediately after the completion of defrosting. Consequently, occupants’ thermal comfort may be adversely affected. To improve the indoor thermal comfort for occupants during reverse-cycle defrosting, a novel thermal energy storage (TES) based reverse-cycle defrosting method has been developed and the improvement to occupants’ thermal comfort experimentally evaluated and is reported in this paper. Comparative experiments using both the novel TES based reverse-cycle defrosting method and the standard reverse-cycle defrosting method were carried out. Experimental results and the evaluated indoor thermal comfort indexes clearly suggested that when compared to the use of standard reverse-cycle defrost, the use of the novel reverse-cycle defrosting method can help achieve improved indoor thermal comfort, with a shorter defrosting period and a higher indoor supply air temperature during defrosting.
Article
This paper presents a thermal analysis of a defrost cycle in order to design more efficient defrosting mechanisms in household refrigerators and freezers. A simple heat transfer model has been developed to determine energy flows from a defrost heater across various components of a refrigerator/freezer. The study measures power consumption and temperatures of a single temperature vertical empty freezer (in normal operation with and without defrosts) to determine the heat distribution from the radiant type electric defrost heater and its effect on power consumption. The surface temperature of the defrost heater was measured to be 520 °C for minimal frost and 560 °C for heavy frost. The efficiency of a defrost heater was measured to be 30.3%, while power consumption of the freezer was found to increase by 17.7% due to automatic defrost.
Numerical and experimental investiga-tion of the frosting and defrosting processes in no-frost evaporators, XI Congress of Refrigeration and Air Conditioning e CIAR
  • F T Knabben
  • C J L Hermes
  • C Melo
F.T. Knabben, C.J.L. Hermes, C. Melo, Numerical and experimental investiga-tion of the frosting and defrosting processes in no-frost evaporators, XI Congress of Refrigeration and Air Conditioning e CIAR, Mexico, 2011.