•PCMs integrated building envelope and equipment in 2004∼2017 are reviewed.•Melting temperature range of PCMs used for envelope is 10∼39°C.•Melting temperature range of PCMs used for equipment is ...−15.4∼77°C.•PCMs’ positive effects on energy saving and thermal comfort are demonstrated.•The existing gaps in the research works are identified and classified as 5 aspects.
Confronted with the crises of the growing resource shortages and continued deterioration of the environment, building energy performance improvement using phase change materials has received much attention in recent years. This review work provides an update on recent developments, 2004∼2017, in phase change materials used to optimize building envelope and equipment. Firstly, a review of building envelope optimization methods by integrating surrounding wall, roof, and floor with phase change materials, is given. This is followed by reporting articles on building equipment optimized with phase change materials to reduce regular energy consumption. Series of air cooling, heating, and ventilation systems coupled with thermal energy storage were comparatively investigated. Finally, the existing gaps in the research works on energy performance improvement with phase change materials were identified, and recommendations offered as authors’ viewpoints in 5 aspects. It was also found that the phase change temperature range of PCMs used was changed from 10∼39°C for envelope to −15.4∼77°C for equipment. We believe this comprehensive review might provide an overview of the analytical tools for scholars, engineers, developers, and policy designers, and shed new light on the designing and performance optimization for PCMs used in building envelope and equipment.
•Frost retarding and defrosting studies published in 2000–2017 are reviewed.•Two types of 12 frost retarding measures are classified and analyzed.•5 defrosting methods and 6 improvement methods are ...summarized.•Initiation and termination control strategies of defrosting operation are presented.•The existing gaps in the research works are identified and classified as 5 aspects.
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.
•A numerical study was conducted on a task/ambient air conditioning system.•The effects of supply vane angle on energy utilization was studied.•The effects of supply vane angle on thermal comfort was ...investigated.•TOPSIS method was effectively used for combined evaluation in buildings.•The best supply vane angle of 30° (EUC=1.28, DR=8.89) was obtained.
In subtropical area, air conditioning (A/C) is widely used to provide a suitable thermal indoor environment. During operation, many parameters or configurations may influence the performance of the A/C system. Among them, the supply vane angle is an important factor, which can influence the energy saving and thermal comfort level in the occupied zone, according to previously related studies. However, it was revealed that it was difficult to get a balance between these two aspects. Therefore, based on the previous study, this further study was conducted aiming to determine the suitable angle to achieve the best performance. Hence, the technique for order preferences by similarity to an ideal solution (TOPSIS) method was used to calculate the combined performance considering energy saving and thermal comfort. Finally, the best supply vane angle of 30°, with the energy utilization coefficient (EUC) and draft risk (DR) at the values of 1.28 and 8.89, was obtained, at which the A/C system achieved a medium energy saving performance and a lower draft risk. It’s indicated that using TOPSIS method can help tackle with conflicting effects for A/C systems in buildings.
•Negative effects due to gravity eliminated when outdoor coil horizontally installed.•Defrosting efficiency increased 9.8% after outdoor coil horizontally installed.•Defrosting efficiency decreased ...6.6% when air fan reversed to blow the melted frost.•Total mass of the retained water collected decreased 222g less after wind blowing.•Higher DEV respected better defrosting performance for multi-circuit outdoor coils.
When frost forms and accumulates over the outdoor coil’s surface in an air source heat pump (ASHP) unit, system operating performance will be dramatically deteriorated. Reverse cycle defrosting is the most widely used standard defrosting method. A previous related study reported that downwards flowing of melted frost due to gravity over a vertical multi-circuit outdoor coil would decrease the reverse cycle defrosting performance. If the outdoor coil can be changed to horizontally installed, the flow path of melted frost over coil surface can be shortened, and the flow directions of refrigerant and melted frost changed from opposite to orthogonal. Consequently, a better defrosting performance is expected. In this paper, therefore, an experimental study on defrosting performance for an ASHP unit with a horizontally installed multi-circuit outdoor coil was conducted. Experimental results show that, when a vertical outdoor coil was changed to horizontally installed, the defrosting efficiency increased 9.8%, however, with the same defrosting duration at 186s. Furthermore, when the outdoor air fan was reversed to blowing the melted frost during defrosting, the total mass of the retained water collected decreased 222g. However, the defrosting efficiency was not increased, but decreased 6.6% because of the heat transfer enhanced between hot coil and cold ambient air.
•Energy transfer procession in an ASHP unit during defrosting was explored.•Effect of metal energy storage on defrosting was proposed and calculated.•Metal energy storage effect was changed from ...positive (0.33%) to negative (−2.18%).•Defrosting efficiency was improved about 6.08%, from 42.26% to 48.34%.•Contributions of this study can guide the design optimization of two ASHP’s coils.
Air source heat pump units have found their wide applications in recent decades due to their high efficiency and low environmental pollution. To solve their undesired frosting problem, reverse cycle defrosting is always employed. As a transient and nonlinear heat and mass transfer procession, defrosting performance directly affects the occupants’ thermal comfort. During defrosting, the metal energy storage values of indoor and outdoor coils are varied as their temperature fluctuations. It is therefore necessary to investigate the energy transfer procession in an air source heat pump unit and the effect of metal energy storage during defrosting. However, scarce of attentions were paid to this fundamental problem. In this study, two experimental cases with two-working-circuit and three-working-circuit outdoor coils were conducted basing on frost evenly accumulated on their surfaces. After four types of energy supply and five types of energy consumption during defrosting were calculated, a qualitative and quantitative evaluation on the metal energy storage effect was then given. As concluded, after the outdoor coil enlarged 50%, the metal energy storage effect can be changed from positive (0.33%) to negative (−2.18%). The percentages of energy consumed on melting frost and vaporizing retained water were both increased. Defrosting efficiency was improved about 6.08%, from 42.26% to 48.34%. Contributions of this study can effectively guide the design optimization of indoor and outdoor coils and promote the energy saving for air source heat pump units.
•Correlations of frosting characteristic parameters are summarized and analysed.•Effects of frosting conditions on average frost layer characteristics are reviewed.•Frosting distribution ...characteristic for different cold surfaces are reviewed.•Eight research gaps relating to frosting mechanism and behaviors are identified.
Frosting causes negative impacts in many technical and engineering fields, such as refrigeration, air source heat pump, aircraft, wind power generation, and power grids. To alleviate the negative impacts of frosting, studies on frosting characteristics, avoiding and delaying frosting, and defrosting mechanisms and methods have been carried out for decades. Among these studies, frosting characteristics, which are the fundamental basis for defrosting, avoiding, and delaying frosting are consistently the hot research spots. Considering that complex structures can be usually regarded as the combination of simple geometries, in this paper, a comprehensive review of the literature related to the frosting mechanism and behaviors on surfaces with simple geometries is presented. The former covers frost formation process and morphology, frost layer structure model, and frosting characteristic parameters, such as frost thickness and effective frost thermal conductivity. The latter is further divided into six parts based on the configurations of cold surfaces, including flat plates with a fixed surface temperature, flat plates with a fixed base temperature, cylinders, parallel plates, set of fins and semipermeable membranes. Based on the review, the research gaps in the related study fields are identified, and recommendations for further research are offered. The literature review presented in this paper can provide a comprehensive and systematic reference for the people who are working in the study fields where frosting is commonly encountered.
•We proposed a TES based defrosting method for cascade air source heat pump.•The method can shorten defrosting duration and reduce defrosting energy consumption.•The thermal energy stored can also ...become a source for indoor space heating.
To encourage a wider application of air source heat pumps (ASHPs) to colder areas due to the advantage of a higher energy efficiency, adopting cascade air source heat pumps (CASHPs) is a promising option. When CASHPs are operated in heating mode, frosting/defrosting has been problematic. However, the defrosting methods commonly used by conventional ASHPs cannot be directly applied to CASHPs. Therefore, a thermal energy storage (TES) based reverse cycle defrosting method for CASHPs has been proposed and an experimental study on its operating performances was carried out. Comparative tests when using both the standard hot gas by-pass defrosting method and the TES based reverse cycle defrosting method were carried out. The results suggested that when using the TES based reverse cycle defrosting method, defrosting duration was shortened by 71.4–80.5%, and defrosting energy consumption reduced by 65.1–85.2%, as compared to those when using the standard hot gas by-pass defrosting method. In addition, the thermal energy stored can also become a source for indoor space heating, and 37.7% of the normal heating capacity can be provided to a heated indoor environment when using the TES based reverse cycle defrosting method.
•Energy transfer process in ASHP during defrosting was quantitatively explored.•Effect of the melted frost taken away during defrosting was fully considered.•Metal energy storage effect on defrosting ...in two-circuit case was calculated at −0.44%.•Defrosting efficiency could be increased by 11.66%, from 47.13% to 58.79%.•The metal energy storage effect on defrosting is calculated at −3.67% for three-circuit case.
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.
•An A/C system at five settings with different heights of supply outlet.•Its ventilation performance was numerically evaluated.•Air flow field, distributions of MAA and CO2 concentrations were ...studied.•Air change efficiency was studied.•Better performance was achieved with a lower supply outlet.
It was revealed in a previous related study that for an air conditioning (A/C) system used in a sleeping environment, the height of its supply air outlet would significantly affect its ventilation performance in terms of effectiveness. To further examine the underlying reasons of the differences in ventilation performance, the air flow field, distributions of mean age of air, air change efficiency and distributions of CO2 concentrations inside an experimental bedroom were numerically studied using CFD method under five different settings, where its supply air outlet was positioned at 5 different heights. The study results suggested that positing a supply air outlet at a lower level was effective in both saving energy and removing the exhaled CO2 in a breathing zone, and when a supply outlet was positioned at a higher level, less CO2 was removed from the region near the mouth of a sleeper.
•A numerical study was conducted on convective heat transfer in sleeping environments.•Regression equations on convective heat transfer coefficients (hconv) were proposed.•Individual convective heat ...transfer coefficients for each body segments were studied.•Influences of coverage and manikin surface temperature on hconv were studied.
Task/ambient air conditioning (TAC) systems have been introduced to solve the problem of thermal comfort in bedrooms during night. The key point of this problem is the thermal environment or heat transfer between a sleeping human body and the surrounding environment. Therefore, a numerical study on the convective heat transfer between a thermal manikin and the surrounding environment was carried out in a bedroom equipped with a TAC system. Firstly, the influence of supply conditions on indoor environment was investigated. Based on this section and previous studies on the convective heat transfer coefficients (hconv) between the human body and surrounding environment, equations depending on temperature difference between the thermal manikin and the environment or the air velocity in the surrounding environment were respectively established. Considering the effects of temperature difference and air velocity on the hconv, the equations were modified to be a function of both temperature difference and air velocity. The further analysis of the new equation indicated that this modified equation can tackle with variations of both temperature difference and air velocity, and can also give a good prediction. Besides, the hconv distributions on the 16 body segments were presented. Higher hconv values can be obviously found at body segments of heat, neck, hand and trunk. Due to the importance of clothes to thermal comfort level, the influence of coverage (blanket) was studied. When the thermal manikin was covered with blanket, the hconv values were about 0.5–1.0W/(m2K) higher than the naked ones and the temperature differences were much lower than the naked ones.
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