•Bionic design of latent heat storage unit is provided using fractal tree.•An energy discharging model is developed and experimentally verified.•Transient temperature distribution and solid-liquid ...interface evolution are investigated.•Tree fin induces stronger temperature uniformity and hence faster solidification rate.•Length ratio of 1.3 and width index of 1 are beneficial for superior performance.
The fractal tree-shaped structure has been demonstrated as a promising optimization method to maximize the point-area heat flow access. Aiming to enhance the energy discharging rate of the latent heat storage (LHS) unit, an innovative fractal-tree-shaped structure is introduced to construct the metal fin of a shell-tube LHS unit. An unsteady model of solidification heat transfer in a LHS unit with tree-shaped fins is developed and numerically analyzed using commercial CFD software, in an effort to demonstrate the improvement of the energy discharging performance for a latent heat storage unit using fractal tree-shaped fins. The transient temperature distribution, solid-liquid interface evolution, and dynamic changes of the liquid fraction and the sensible and latent heat in a tree-fin LHS unit are compared with those of a radial-fin LHS unit. The effects of length ratio and width index on energy discharge performance are examined and analyzed. The results indicate that the tree-shaped fin significantly improves the energy discharge performance of a shell-tube LHS unit. The tree-fin LHS unit possesses a faster solidification rate, higher energy discharge rate, and stronger temperature uniformity. The complete solidification time of the tree-fin LHS unit is decreased by 66.2% and its complete melting time is reduced by 4.4% when compared with the radial-fin LHS unit. For superior thermal energy discharge performance, the length of tree-shaped fins in a shell-tube LHS unit should be shorter inward and longer outward; the optimum length ratio is about 1.3 and an appropriate width index is 1.
Latent heat storage (LHS) is considered as the most promising technique for thermal energy storage, due to its high energy storage density and nearly constant working temperature. However, the lower ...thermal conductivity of the phase change material (PCM) used in LHS system seriously weakens thermal energy charging and discharging rates. In order to improve the thermal performance of LHS system, a lot of research on performance enhancement have been carried out. This review paper will concern on the development of PCMs and performance enhancement methods for LHS system in the last decade. The available enhancement methods can be classified into three categories: using high thermal conductivity additives and porous media to enhance PCM thermal conductivity, using finned tubes and encapsulated PCMs to extend heat transfer surface, using multistage or cascaded LHS technique and thermodynamic optimization to improving the heat transfer uniformity. The comparative reviews on PCMs, corresponding performance enhancement methods and their characteristics are presented in present paper. That will help in selecting reliable PCMs and matching suitable performance enhancement method to achieve the best thermal performance for PCM based LHS system. In addition, the research gaps in performance enhancement techniques for LHS systems are also discussed and some recommendations for future research are proposed.
•Reviews on LHS performance enhancement methods and characteristics was performed.•It is helpful for selecting PCMs and matching suitable performance enhancement method.•Research gaps in the performance enhancement techniques for LHS systems were discussed.•Recommendations for the future research were proposed.
This paper is an updated, but totally new, version of “A review on phase change materials (PCMs) integrated in building walls”, an article published in 2011 in Renewable and Sustainable Energy ...Reviews. Both numerical and experimental studies on building walls containing PCMs during the last ten years (2011–2020) are reviewed. The paper also summarizes the main PCMs used in this application and the recent progress in the integration techniques of PCMs in building construction elements. Most of the proposed studies (about 66%) are focused on PCM in walls. In addition, more than half of these studies are numerical where several assumptions are made such as the neglected of both the occupant presence in the building and its behaviour. Based on this review, it is concluded that the used PCMs in building walls showed good potential for reducing both indoor air temperature fluctuations and energy consumption. However, further numerical and experimental studies are needed that take other aspects into consideration, such as the real use of buildings with the occupant behaviour, the economic viability and the environmental impact. This review paper will help scientific researchers and engineers to update the integration techniques of PCM in building walls and to define potential future research works.
•An updated review on PCMs integrated into building walls is presented.•Both experimental and numerical studies over the last ten years are reviewed.•Further studies that take into consideration the real use of buildings are needed.•The economic viability and the environmental impact are not studied sufficiently.
It is well known that there is a need to develop technologies to achieve thermal comfort in buildings lowering the cooling and heating demand. Research has shown that thermal energy storage (TES) is ...a way to do so, but also other purposes can be pursued when using TES in buildings, such as peak shaving or increase of energy efficiency in HVAC systems. This paper reviews TES in buildings using sensible, latent heat and thermochemical energy storage. Sustainable heating and cooling with TES in buildings can be achieved through passive systems in building envelopes, Phase Change Materials (PCM) in active systems, sorption systems, and seasonal storage.
The melting process of phase change material (PCM) in horizontal latent heat thermal energy storage (LHTES) units with longitudinal and annular fins was experimentally studied under the same fin ...volume condition. The PCM melting time of the annular fin unit is reduced by 13.7% compared to that of the longitudinal fin unit. The annular fins are more responsive to the increase in heat transfer fluid (HTF) inlet temperature and the longitudinal fins are more responsive to the increase in HTF flow rate. After that, numerical studies were also performed for the two LHTES units to reveal in-depth the differences in heat storage processes. The results show that the longitudinal fins perform better in the early stage due to their larger contact area with the inner tube and a denser distribution near the inner tube. But it also limits the natural convection in the latter stage. The annular fins have better natural convection performance in the latter stage and therefore have better heat storage performance. Based on the analysis, a composite fin was designed to combine the advantages of longitudinal and annular fins. Numerical results show that the newly designed fin results in a 21.6% melting time reduction compared to the longitudinal fin model and a 9.5% melting time reduction compared to the annular fin model. Finally, two principles to guide the fin design in horizontal shell-and-tube LHTES units were summarized.
The continuous increase in the level of greenhouse gas emissions and the climb in fuel prices are the main driving forces behind efforts to more effectively utilise various sources of renewable ...energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. However, the large-scale utilisation of this form of energy is possible only if the effective technology for its storage can be developed with acceptable capital and running costs. One of prospective techniques of storing solar energy is the application of phase change materials (PCMs). Unfortunately, prior to the large-scale practical application of this technology, it is necessary to resolve numerous problems at the research and development stage. This paper looks at the current state of research in this particular field, with the main focus being on the assessment of the thermal properties of various PCMs, methods of heat transfer enhancement and design configurations of heat storage facilities to be used as a part of solar passive and active space heating systems, greenhouses and solar cooking.
The development of energy saving technologies is very actual issue of present day. One of perspective directions in developing these technologies is the thermal energy storage in various industry ...branches. The review considers the modern state of art in investigations and developments of high-temperature phase change materials perspective for storage thermal and a solar energy in the range of temperatures from 120 to 1000
°C. The considerable quantity of mixes and compositions on the basis of fluorides, chlorides, hydroxides, nitrates, carbonates, vanadates, molybdates and other salts, and also metal alloys is given. Thermophysical properties of potential heat storage salt compositions and metal alloys are presented. Compatibility of heat storage materials (HSM) and constructional materials have found its reflection in the present work. Data on long-term characteristics of some HSMs in the course of repeated cycles of fusion and solidification are analyzed. Article considers also other problems which should be solved for creation of commercial high-temperature heat storage devices with use of phase change materials.
•Characterization of Phase Change Materials with a phase change between 0 and 250°C.•Review of heating and cooling applications benefiting from a latent heat storage.•Review of indirect latent heat ...storage containers for process heating applications.•Review of heat transfer enhancement techniques to be used within the PCM.
A comprehensive review of phase change materials (PCMs) with phase transition temperatures between 0 and 250°C is presented. From that review, organic compounds and salt hydrates seem more promising below 100°C and eutectic mixtures from 100 to 250°C.
Practical indirect heat exchanger designs for latent heat storage systems were also assessed and feasible heat enhancement mechanisms reviewed. The focus on this temperature range is due to potential CO2 emissions reduction able to be achieved replacing conventional heating and cooling applications in the domestic, commercial and public administration sectors, which represented around a quarter of the UK’s final energy consumption in 2015.
The popular application of latent heat storage (LHS) units using phase change materials is limited in converting and utilizing renewable energy due to their poor thermal efficiency. To address this ...deficiency, a tree-shaped fin inspired by nature is employed for the thermal enhancement of vertical LHS units. A three-dimensional mathematical model using the enthalpy-porosity method is developed to comprehensively evaluate the heat charging/discharging process of the new and traditional LHS units, focusing on the role of heat transfer fluid (HTF) direction. The results indicate that tree-shaped fins improve the temperature uniformity and facilitate the melting/solidification rate, which exhibits a preferable advantage that the full melting/solidification duration shortens by 34.5% and 49.2%, and the time-averaged heat storage/release rate augments by 49.4% and 96.4%. Interestingly, natural convection significantly improves the heat charging performance, while the role of natural convection can be considered negligible during discharging processes. Moreover, the HTF temperature seriously affects the performance of LHS units, while the role of HTF flow rate is less prominent. It is found that the upward flow of HTF is beneficial to the heat charging improvement while a downward flow mode facilitates the discharging performance, providing useful advice for practical applications of vertical LHS units.
In the present study, a numerical model of a vertical shell and tube latent heat storage system validated with the experimental data is presented. The developed model comprises of three blocks of ...phase change materials (PCMs) having melting point temperatures (Tm) 360 °C, 335.8 °C and 305.4 °C, respectively. A non-uniform distribution of fins in three PCM blocks is initially employed to study the performance of the single PCM system (Tm=335.8 °C). The effect of inlet heat transfer fluid temperature on the charging and discharging performances of the single PCM and multiple PCM (m-PCM) systems is analysed by varying a Stefan number (Steref) parameter, calculated based on the single PCM system. The charging and discharging times for the m-PCM system are either similar or lesser than the single PCM system for Steref≥1, however, there is an improvement of 21–25% in the specific power charged and discharged by the m-PCM system for all the Sterefvalues (0.5, 1, 1.5 and 2) considered. By employing a compound enhancement technique which is a combination of non-uniform fin-distribution and PCM blocks length ratio optimization for the m-PCM system, 30% and 9% reduction in the charging and discharging time, respectively, over the single PCM system is achieved.
•Non-uniform fin distribution for heat transfer enhancement in LHS system.•Performance comparison between single and multi-PCM system.•Optimization of PCM blocks length for fixed total storage capacity of m-PCM system.•Compound enhancement using fin distribution and m-PCM length ratio optimization.•Improvements in the melting rate and specific power of 30% and 25% for m-PCM system.
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