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• The interconnections distribution in fibers determines the heat transfer model. • The development of a thermal conductivity equation considers the radiative heat transfer. • The bale's thermal conductivity varies according to the fiber orientation. • Thermal conductivity depends on the chemical composition of the straw. Straw fibers are natural fibers that have a strong insulating property and a minimal environmental effect, making them suitable construction materials. In the construction sector, to predict the building energy consumption, the thermal conductivity of the materials composing the envelopes must be precisely known, which is not the case for the straw material. The properties of a straw bale are variable and mainly depend on its density, fibers orientation, chemical composition, temperature and relative humidity. Consequently, this study consists of determining a mathematical equation to predict the effective thermal conductivity of straw bales that can be applied to all straw types, in different circumstances. For this purpose, the size, distribution, and morphology of straw fibers are first determined from microscopic images. Second, a mathematical model for estimating the thermal conductivity is suggested using the heat transfer models of porous and fibrous materials. The numerical model is validated by an experimental study that measures the thermal conductivity of straw bales by altering their density between 80 and 120 kg/m3, their relative humidity between 15 % and 95 %, and their temperature between 15 ˚C and 55 ˚C. The experimental results show that the thermal conductivity increases from a minimum of 0.047 W/(m∙K) to a maximum of 0.09 W/(m∙K) when increasing the three altered factors. For a straw fiber having 40% cellulose content, the thermal conductivity increased from a minimum of 0.05 W/(m∙K) to a maximum of 0.0832 W/(m∙K). This behavior is found the same for the numerical findings. The comparison of numerical and experimental values shows a good agreement, with a root mean square error of 0.005 W/(m∙K) and a scatter index of 5.5 %. [ABSTRACT FROM AUTHOR]

This paper investigates the thermal performance of a translucent solar wall providing, concurrently, storage and restitution of heat, super thermal-acoustic insulation and daylighting to the interior environment. The wall is composed of glazing, silica aerogel used as a transparent insulation material (TIM) and glass bricks filled with fatty acid, an eutectic phase change material (PCM). To assess the TIM–PCM wall thermal behavior, experimentations were conducted in-situ in a full-sized test cell located in Sophia Antipolis, southern France. Experimental data shows that the tested wall is more effective in winter and might cause overheating during the summer mainly due to solar gains and un-cycling behavior of PCM which remains in liquid state. To enhance the energy performance of the wall in summertime, a numerical model describing the heat transfer mechanisms occurring in the PCM layer in combination with the other transparent wall layers is developed. Then, the model of the wall is linked to TRNSYS software to assess the thermal performance of the whole building. The numerical model is validated experimentally and a good agreement is shown comparing the simulated values with the measured data for seven consecutive days in summer and winter. The importance of considering the natural convection effect in the liquid PCM is also demonstrated. Moreover, it was shown that shading devices can effectively reduce overheating while natural night ventilation decreases the indoor temperature without affecting the PCM performance since the outdoor temperature is always higher than the phase change temperature. The use of a glass with selective solar reflection properties depending on the season instead of the ordinary glazing is shown also to be very effective way to overcome the overheating problem. Finally, the TIM-PCM wall is tested under different climate conditions and passive solutions are given to ensure thermal comfort in summer season. [ABSTRACT FROM AUTHOR]

Highlights: [•] Co-simulation is used; coupling a 2D heat transfer code developed in MATLAB with EnergyPlus. [•] Windows offset thermal bridges energy load constitutes around 2-8% of the total house load. [•] Applying the coating reduces the windows offset thermal bridge load by about 24% to 50%. [ABSTRACT FROM AUTHOR]

In this paper, melting of a phase change material (PCM) inside a rectangular enclosure, possibly finned and inclined, is studied numerically. The application of this work is related to the temperature control of a finned PV panel filled with PCM and installed at different tilt angles. The studied system is modeled as a 2D rectangular enclosure filled with PCM (RT25) and packed between two aluminum plates, where the front side is exposed to a constant heat flux of 1000 W/m2 for 2 h. Four geometries were considered including a non-finned PCM enclosure, a PCM enclosure with one centered full-width fin, one half-width fin attached to the front plate, and one half-width fin attached to the back plate. Results have shown that the most efficient thermal management of the PV-PCM panel is obtained when the PCM enclosure is equipped with a full-width fin simultaneously attached to the front and back plates. With such a PV panel design, the PCM melting is dominated by natural convection heat transfer from both sides of the PCM enclosure at an early stage, with added heat losses from the back plate to the external environment. Accordingly, low values of the front and back plates temperatures can be maintained during a stabilization time of 80 min as long as the tilt angle is varied from 0° to 75° from the vertical. The efficient temperature control resulting from the full-width fin geometry is mainly related to the high overall heat transfer coefficient obtained during the whole melting process. [ABSTRACT FROM AUTHOR]

The thermal conductivity of commonly used phase change materials (PCM) for thermal energy storage (TES), such as, fatty acids, paraffin etc., is relatively poor, which is one of the main drawbacks for limiting their utility. In the recent past, few attempts have been made to enhance the thermal conductivity of PCM by mixing different additives in the appropriate amount. Graphene nanoparticles, having higher thermal conductivity may be a potential candidate for the same, when mixed appropriately with different PCM. In present study authors have carried out the numerical investigation for the melting of graphene nano-particles dispersed PCM filled in an aluminum square cavity heated from one side. In this work, the graphene nanoparticles are mixed in three different volumetric ratios (1%, 3%, and 5%), with three different commonly used categories of organic, inorganic and paraffin PCM (namely, Capric Acid, CaCl 2 ·6H 2 O, and n -octadecane) to see the effect on melting of composite PCM developed. The resulting transient isotherms, velocity fields, and melting front and melt fractions thus have been deliberated in detail. These results clearly indicate that the addition of graphene nanoparticles increases melting rate but can also hamper the convection heat transfer within large cavities. The study also shows that such enhanced PCM can be effectively used for different TES applications in different fields. The prediction of temperature variation and rate of melting or solidification may be found useful especially for designing such TES devices. [ABSTRACT FROM AUTHOR]

The higher operating temperature of photovoltaic panels (above the standard operating temperature, usually 25 °C) adversely affects the panel’s efficiency. PV panel coupled with phase change materials (PCM) could be a feasible solution due to the higher energy storage density of such materials. However, heat transfer studies of PCM combined with PV panel are quite intricate due to variable ambient conditions and changing thermo-physical property of the material with phase change. In the present study, detailed heat transfer study of the PV panel coupled with PCM has been performed. The computational fluid dynamic study of the PV module coupled with PCM is carried out in which essential heat transfer mechanisms between PV module to PCM and environment have been accounted for. Additionally, its effect on power output has been investigated to see the variation of PV module operating temperature. This study clearly suggests that for realistic simulation of heat and mass transfer studies of PV panel attached with PCM, it is very important to consider following effects into account: convection effect within melted PCM, the velocity of wind, and angle of inclination of PV panel. The results from the study demonstrate the ability and importance of such model to reasonably simulate the thermal characteristics of PV panels coupled with PCM. The maximum panel operating temperature with conduction and convection effect is found to be 54.90 °C and 58.5 °C when convection mode in melted PCM (only conduction mode) is not considered. It has also been shown that the higher wind velocity and tilt angle leads to lower operating temperature of PV panels. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) panels, depending on the PV cell technology used, convert only a small amount of incident energy into electricity (about 5–25% for commercial systems), and the rest is converted into heat. The produced heat is partly transferred back to the environment while the remaining part causes the enhancement of the PV panel temperature itself. This increase in the PV panel temperature further affects power production adversely, if the PV panel temperature rises above the standard operating temperature (usually 25 °C). The present study deals with the thermal analysis of PV panel involving (i) numerical study based on finite element heat transfer, (ii) the outdoor experimental validation of the thermal model developed. The thermal model is based on the energy balance of the PV module in which all essential heat transfer mechanisms between the module to the environment and related power output are modeled to observe the net change in PV module temperature. The results clearly demonstrate the need and ability of the model to realistically simulate the thermal behavior of PV panels. Additionally, using the same model, numerical simulations have been also carried out for two different cities, namely Allahabad and Jodhpur, for Indian climate, to predict the hourly average module temperature of PV panels for different months. The results show that normal module temperature is raised much above the standard test conditions for the months of April, May and June. [ABSTRACT FROM AUTHOR]