The present study aims to compare the energy and exergy performances of photovoltaic/thermal-thermal wheel (PT) and photovoltaic/thermal-heat pipe-thermal wheel (PHT) systems. These systems are able ...to preheat/precool the outside air in the cold/hot season and supply part of the electricity needed by a building. In the PT system, the ambient air exchanges heat directly with the photovoltaic panels, while in the PHT system, the evaporator section of heat pipes located under the panels transfers the heat received from the panels to the air stream through the condenser section of heat pipes. The annual useful energy and exergy produced by the system were taken as objective functions, the two-objective optimization of the systems was performed, and the performances of the optimal systems were assessed. Overall annual analysis of outcomes showed that the highest useful energy output is achieved by the optimal PT system which outperforms by 2.1% than that of the optimal PHT system, while the maximum useful exergy yield is attained by the optimal PHT system, which is 5.1% higher than that of the optimal PT system. The thermodynamic performance improvement of the PHT system is due to the electricity production increment by recovering the waste heat of photovoltaic panels.
•Comparison of theoretical and experimental conductivity modelsfor mono and hybrid nanofluids.•Theoretical models underestimate heat transfer enhancement.•Theoretical models may results in decrease ...in heat transfer.•Development for prediction of conductivity models of hybrid nanofluids is needed.•Hybridizing the nanoparticles has a good potential for heat transfer enhancement.
The use of hybrid nanofluids has been drawing attention of researchers in order to overcome the drawbacks of mono nanofluid and combine the physical and chemical properties of nanoparticles in a useful way. In the literature, a growing number of researches have been devoted to investigate the thermal performance of hybrid nanofluids. A significant amount of these researches are built on the theoretical correlations for the estimation of thermophysical properties of nanofluids. In the present study, a comparative study is conducted to reveal the influence of theoretical and experimental correlations on the heat transfer performance of hybrid nanofluids. Within this aim, theoretical and experimental based models for predicting thermal conductivity of hybrid nanofluid are evaluated by considering natural convection in a square cavity, which has been studied extensively in the literature. In the study, the natural convection of Al2O3/water, SiO2/water nanofluids and their hybrid combinations are investigated numerically for two different Rayleigh numbers (Ra = 104 and 105) and three different particle volume fractions (ϕ = 1, 2 and 3%). The comparative analysis considering the studied parameters was performed in terms of local and mean Nusselt numbers. The results showed that employing theoretical models for thermal conductivity underestimates the heat transfer performance of both mono and hybrid nanofluids. Furthermore, it is surprising that the theoretically calculated SiO2/water nanofluid deteriorated the heat transfer performance. It was also observed that hybridizing the nanoparticles could perform the same heat transfer enhancement at a lower particle volume fraction compared to mono nanofluid (Al2O3).
Display omitted
•Optimization study based on the maximum activation of latent heat fusion.•Optimization of PCM location, melting temperature (Tm) and thickness (L) for Turkey.•Investigation of energy ...saving, decrement factor and time lag.•Optimum Tm and L vary 6–34 °C and 1–20 mm depending on climatic conditions.•Up to 13.3 h increment in time lag and 18% of energy saving can be obtained.
Integration of the phase change materials (PCM) into the external building walls is an efficient method for reduction of energy consumption and regulation of energy demands due to increasing thermal inertia of the walls. This study aims to reveal the contribution of latent heat to the thermal performance of the wall and to determine the location, thickness and melting temperature of PCM for the maximum exploitation of latent heat for different climatic conditions. A comparative study is carried out for the wall coupled with PCM and the wall with Phase Stabilized PCM (PSM) to reveal the improvement provided by the latent heat. The influence of location, fusion temperature and layer thickness of PCM on energy saving, decrement factor and time lag was examined. The annually optimized PCM fusion temperature and layer thickness which utilizes the latent heat at maximum level considering both heating and cooling loads are determined for three cities of Turkey. The computed results show that the monthly optimized PCM melting temperature and PCM layer thickness vary from 6 to 34 °C, from 1 to 20 mm depending on climatic conditions. It was concluded that an optimization study should be conducted in order to prevent PCM behaves like PSM.
For simulating phase change process of pure solid gallium, previously published literature contains some key problems, including the inappropriate definition & computation for interface errors, and ...the unsatisfactory numerical verifications & validations. Herein, the 2D verifications and 2D validations for the enthalpy-porosity modeling of pure gallium melting starting from a vertical end wall have been reported comprehensively. The well-known and classic experimental data presented in literature are used to verify and validate the model. The groundbreaking contributions in this work are that: i) The definition and computational method of the interface-position errors between the numerical and experimental results are revisited and proven carefully; ii) The numerical schemes as well as major parameters of 2D modeling gallium melting are selected with overall interface error <9%. However, these schemes and parameters are not appropriate for the solidification modeling of gallium due to the irregular and non-reproducible interface shapes caused by highly anisotropic properties of solid crystal gallium as experimentally reported in literature; iii) The 2D validations of enthalpy-porosity modeling of gallium melting are investigated in detail, and the maximum interface error is within the limits of ±12%. Moreover, the simulated liquid volume fractions and dimensionless heat transfer coefficients are well correlated in terms of related criteria parameters, and they are compared with correlations available in literature. It is concluded that to solidly validate the modeling of pure metal melting, both the global parameters (e.g., volume-averaged liquid fraction) and the local parameters (e.g., interface position) need to be considered together.
•Revisit definition and computational method of local interface error.•Define evaluation indexes of global interface-position errors synthetically.•Clarify numerical schemes and key parameters of 2D modeling gallium melting.•Validate 2D enthalpy-porosity modeling melting by literature's experimental data.
Literature survey found that three important scientific problems including: i) The feasibility of 2D modeling was not be further validated by compared the 3D & 2D simulations with the famous classic ...experiments of literature; In literature's work, ii) the plus exponent of the aspect ratio in liquid-fraction correlation is questionable, and iii) how to calculate the Mean Liquid Layer Thickness (lc) was not mentioned and/or discussed that would lead to get the value of lc-based Nusselt number difficultly. Therefore, in this work, the 3D validation and 2D feasibility are roundly reported as compared by all the experimental data of literature. The novel results and core findings in the paper are that: i) It is confirmed that the present 3D simulation on pure gallium melting by enthalpy-porosity technique is completely validated, and 3D simulation can be replaced by 2D one, and it also should be replaced by 2D due to high running time ratio nearly from 17 to 27 times depended powerfully on the aspect ratio of computational domain; Thereinto, ii) the sign of the aspect-ratio exponent shown in the correlation equation of Liquid fraction vs. Stefan number of literature is clarified and corrected, and iii) the correlation of lc is now well-proposed to calculate the lc-based Nusselt number directly and quickly, and then the developed lc's correlation is commented from multi-points of views. Finally, the discrepancy sources between current 3D & 2D modeling and literature's experimental measuring on the pure solid‑gallium phase change are discussed and summarized in detail. It should be mentioned that the currently obtained new results and novel findings will promote the new understandings and high efficiency applications in related fields, e.g., transient cooling of high heat-flux electronic devices in aerospace and navigation that would be researched in the future.
•Validate 3D enthalpy-porosity modeling pure gallium melting completely.•Illuminate feasibility on 3D simulation replaced by 2D simplification strictly.•Correct aspect-ratio exponent in literature's liquid-fraction correlation as negative.•Develop correlation of mean liquid layer thickness to calculate Nusselt number.
•Combined effect of fin length, fin position, heated wall orientation and nanoparticles•For side-heating, assembling fin near the bottom wall (l/W = 0.25) is more effective•Position of fin may ...promote melting rate from 13 to 40% for bottom-heating•Shorter fins could be as effective as longer ones by introducing nanoparticles•Combined utilization of fin and nanoparticles augments melting rate up at least 54%
A comprehensive numerical study is presented to investigate the melting process in a square enclosure whose two facing walls are maintained at different constant temperatures while the other facing walls are thermally insulated. Thermophysical properties of the utilized PCM depend on both temperature and phase. The influence of two different orientations of heated and cooled walls, various lengths and positions of the fin that is attached to the hot wall on phase change process is studied. Besides, analyses are performed considering the contribution of CuO nanoparticles and the contribution of each investigated parameter on the enhancement of the melting process is identified. The obtained results show that all the studied parameters have a remarkable influence on melting rate and energy storage, besides, natural convection has an incontrovertible role in melting process. Assembling the fin in the enclosure shortens the melting time between by 27 to 52% depending on its length and 13 to 68% depending on its position compared to the case without fin. It was also found that assembling fin promotes the melting rate up to 52% depending on the heated wall orientation. A further enhancement is attained with the addition of nanoparticles particularly for shorter fin lengths and bottom-heating case. The computed results also reveal that a combined utilization of a shorter fin together with nanoparticles can provide a thermal performance as high as that of a long fin, thus the utilization of nanoparticles with shorter fins may be an alternative to assembling of long fins without nanoparticles.
Display omitted
The main objective of this review paper is to present the historical development, current utilization, practices and opportunities of geothermal energy in Algeria. Algeria has relatively abundant ...geothermal resources of low-enthalpy type that could form part of the total energy network of the country. Algeria is the leading country of the direct use of geothermal energy in Africa with a total amount reaching 54.64 MWt installed thermal power, and also among the first five countries in the world in air conditioning application. The main utilization field of geothermal energy in Algeria is balneology, which accounts for about 82% of the total geothermal power utilization, the remaining 18% of the energy is consumed for other purposes such as space heating, heat pumps and fish farming. In this study, the existing literature and current practices of the Algeria’s geothermal resources are reviewed and it is revealed that geothermal energy development remained almost stagnant over the past two decades in Algeria although there is a large potential and resources available in the country. Furthermore, geothermal energy use and latest technologies around the world have been summarized. This study elucidates the influencing factors in the development of geothermal energy in Algeria and suggests the development roadmap and a number of measures that can enhance the development of this significant energy source.
•Previous researches on geothermal energy in Algeria are reviewed.•Geothermal energy utilization and advanced technologies around the globe are summarized.•Current geothermal energy utilization in Algeria is discussed.•Exploitation opportunities of geothermal energy in Algeria are addressed.•Countermeasures to promote geothermal energy development in Algeria are proposed.
Literature survey showed that three important and key scientific problems including: i) the mechanisms or underlying causes of the discrepant sources, especially at the top phase interfaces, are yet ...to be investigated, which are crucial to recognize the validation of the numerical technology for metal isothermal phase-change applications. Moreover, revisiting the well-known experiments in literature, they did ii) not display the interface shape under the conditions of aspect ratio = 0.714 & Stefan number = 0.0391 at the dimensionless time of 0.0184, but also iii) not discuss the equilibrium state of pure solid‑gallium phase change. Therefore, it is a fascinating and interesting concern that what happened for these two kinds of important problems or key questions ii) and iii). Here, all above-mentioned three problems i) to iii) are explored and addressed by finite-volume-based enthalpy-porosity methodology. The core findings in this work are that: i) The novel mechanisms of convective false diffusion are revealed and clarified for enthalpy-porosity modeling solid-liquid interface of pure solid‑gallium melting; ii) The novel phenomena of asymmetrical solid-liquid interface are found and explained for a large aspect ratio of 0.714; iii) The innovative findings and fitting correlations are discussed by extended literature’s study to phase-change equilibrium state, and it is comprehensively demonstrated that the dimensionless correlation of mean liquid layer thickness is equivalent to that of global liquid volume fraction.
•Clarify novel mechanisms of convective false diffusion in modeling phase interface.•Explain novel phenomena of asymmetrical interface even if geometry & boundary conditions are symmetric.•Extend literature's study with innovative findings of phase-change equilibrium state.•Find correlation of mean liquid layer thickness closes to that of liquid fraction.
In phase-change applications, once the melting process is completed, the natural convective heat transfer of liquid material may be promoted or degraded by the fins, which is the main motivation of ...this research. For this reason, the natural convection heat transfer inside a PCM container that can be a representative model for PV/PCM systems is numerically investigated in this study by considering three aspect ratios (AR = 1, 2 and 4), three Rayleigh numbers (Ra = 104, 105 and 106), two types of fins as rectangular and tree-like branching fin, and three different length-to-height ratio of rectangular fin (w/H = 0.3, 0.4 and 0.5). The rates of increment and decrement are presented taking the finless enclosure as the reference case. The computed results revealed that the natural convection is promoted up to 20%, depending on Ra and fin length by the inclusion of fins when the AR = 1, while it is degraded down by 5.5% for AR = 4. Interestingly, at AR = 2, the percentage increase and decrease of mean Nu numbers are slighter compared to other aspect ratios. Besides, it is also noticed in this study that utilizing a tree-like branching fin is not as effective as a rectangular fin with the same mass.
Display omitted
•Natural convection of molten PCM in a finned PV/PCM system investigated.•The impacts of rectangular and three-like branching fins compared.•Fin attachment can improve or impair heat transfer.•Balance exists between the strength of natural convection and blockage effect of fins.•There is a critical AR and Ra where the fin attachment becomes disadvantageous.
•This review stated research framework of glazing units containing PCM in building.•Detailed derivation of PCM in glazing units and their physical properties was given.•Developments on performance ...modeling for glazing units containing PCM were highlighted.•Possible further developments of glazing units containing PCM were outlined.•Suggestions for future research and application guidance were also illustrated.
Glazing units are important for providing passive solar gain and air ventilation in buildings, but their thermal performance is very poor compared to other building components, which results that energy loss from building envelope becomes much more drastic when the glazing area is large. Incorporating phase change material (PCM) in the glazing unit is an effective approach to increase its thermal performance. The glazing unit containing PCM can absorb part of the solar radiation for thermal energy storage while letting the visible radiation enter the indoor ambient for daylighting, which results in reduction of the temperature fluctuations and improvement of the thermal comfort of indoor occupants. A lot of researchers investigated optical and thermal performance of window systems containing PCM in buildings. However, there is a lack of published research review including numerical methods of optical and thermal performance and physical parameters of PCM used in the glazing units, especially for the optical performance of glazing units containing PCM. The present work reviews the experimental and simulation researches on the optical and thermal performance of glazing units containing PCM and discusses the employed research methods, mathematical models and important conclusions drawn. Finally, the challenges and future works of glazing units containing PCM are addressed.
