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This paper aims to find an optimal maximum allowable temperature for a cooling system of a photovoltaic panel. As the operating temperature of a photovoltaic cell increases, its efficiency significantly decreases. Overheating due to excessive solar radiation is one of the main obstacles faced by photovoltaic panels, especially in hot and arid regions. This is why, several cooling systems have recently been integrated into photovoltaic panels. The power consumption of the cooling systems affects the total amount of energy harvested, whereas the goal is to increase it by reducing the operating temperature. It is therefore, necessary to find an optimal value for the maximum allowable temperature which can reduce the cooling system energy consumption while maintaining thermal efficiency at acceptable levels. The maximum allowable temperature is the threshold at which the cooling system activates and reduces the operating temperature to the ambient temperature, where it is deactivated. The activation/deactivation frequency increases when the maximum allowable temperature is low and vice versa. Experimental measurements of the net harvesting energy validated by analytical calculation shows that 45 °C the optimal value of the maximum allowable temperature for the studied system. The procedure proposed in this paper will increase the net efficiency of the photovoltaic panel equipped with such a cooling system. The originality of this paper lies in the fact that it takes into account the negative contribution of the power consumption of a cooling system coupled with a PV panel.

The gradual development of government policies for ecological transition in the modern construction sector leads researchers to explore new alternative and low environmental impact materials with a particular focus on bio-sourced materials. In this perspective, the mechanical, thermal insulation, and the sound absorption performances of a spent coffee grounds/potato starch bio-based composite were analyzed for potential application in buildings. Based on thermal conductivity and diffusivity tests, the coffee grounds waste biocomposite was characterized as an insulating material comparable with conventional thermal insulation materials of plant origin. Acoustical tests revealed absorption coefficients in the same range as other conventional materials used in building acoustical comfort. This bio-sourced material presented a sufficient compressive mechanical behavior for non-load-bearing structures and a sufficient mechanical capacity to be shaped into building bricks. Mechanical, thermal, and acoustic performances depend on the moisture environment. The groundwork was laid for an initial reflection on how this composite would behave in two opposite climates: the continental climate of Reims in France and the tropical climate of Belém in Brazil.

International audience; In the context of global warming associated with the excess of energy consumption, energy saving and environmental sustainability in buildings have become significant issues at both national and European levels. Therefore, the adoption of energy-efficiency measures in new and renovated dwellings will increase the demand for thermal insulation, which is crucial to improve the housing comfort by reducing the heat loss through the building envelope. Over the past two decades, several bio-based materials have been used in building thermal insulation, such as hemp concrete. Promoting the use of hemp concrete in new constructions and restoration of existing buildings first requires a validation of its hygrothermal behavior at both wall and building scales. For this, several models dealing with various physical aspects of hemp concrete were developed to understand their impact on the prediction of changes in temperature and relative humidity. These physical phenomena include hemp concrete hygroscopic hysteresis and thermal dependency of sorption curves. The preliminary results show that hysteresis has a significant influence on the numerical prediction in the short-term, while temperature dependency has less influence at the wall scale. The next part of this article is devoted to the study of these phenomena at the room scale by monitoring the internal conditions and energy consumption of a virtual hemp concrete office. The results highlight the fact that hysteresis and thermal dependency have low impact on indoor conditions, which could probably be related to the other parameters at the room scale.

International audience; The paper aims to present a new insulating material based on cork aggregates and cement designed for building applications. Samples are prepared by mixing natural cork aggregates, sand, cement and water. Four cork volume dosages are considered, 0, 25, 50 and 75% (relative to the sand). Samples are characterised in terms of hardened density, compressive and flexural strength, as well as thermal conductivity and hydric properties (moisture content, vapour permeability, moisture buffering value and absorption by partial immersion). Results show that increasing cork granule amount tends to increase moisture retention and buffering value but decreases density, thermal conductivity and mechanical properties. Depending on the cork content, cork concrete mixture can be used as thermal insulator or as a structural component.

International audience; Construction is considered as one of the most relevant sectors in terms of environmental impacts, due to the significant use of raw materials, fossil energy consumption and the consequent Greenhouse Gases emissions. The use of unconventional and environmentally-friendly materials and technologies is worldwide recognised as a key factor to enable the decrease of material and energy consumption in buildings. Between natural/sustainable materials, those using hemp products and by-products (fibres and hurds) have rapidly widened their field of application in the building industry, mainly because of their good hygrothermal and acoustic insulation properties. Moreover, the usage of these materials allows high carbon storage due to the CO2 sequestration during the agricultural phase.This study represents an energy and environmental assessment of hemp crop cultivation in France, carried out through a Life Cycle Assessment approach, showing positive and negative contribution related to the different life cycle phases. The total CF evaluated through the IPCC, 2013 GWP 100 method (IPCC, 2013) is equal to 0.975 kgCO2eq, in view of a CO2 uptake of −1.29 kgCO2eq. So, it is understood that the total CF results therefore lower than the CO2 uptake due to the biogenic carbon captured and stored during hemp growth. The total Energy Footprint, instead, was calculated in 17.945 MJ. The Upstream phase came out as the main contributor to the impacts. A sensitivity analysis was performed to explore changes in results related to main inputs assumptions and, in particular, the environmental benefits associated with the replacement of conventional fertilisers (ammonium sulphate) with organic matter were highlighted.

International audience; Global environmental issues are arising over the developed and developing world. Improving energy efficiency and reducing Green Gases emissions become the key issues in all the economic sectors and, in particular, the building sector that is one of the most energy-consuming. It is, therefore, important to find alternative building materials with low-environmental impact contributing to global sustainability. In that context, this study reports on the performance of sustainable materials produced from natural resources as hemp-concrete or from recycled-waste nonbiodegradable materials including recycled polyethylene terephthalate. Three façades employing three different materials (hemp-concrete, hemp-concrete with brick and recycled polyethylene terephthalate) were investigated in three cities in France (Nancy and Carpentras) and Italy (Perugia) with different climate. The energy performance of each façade was assessed in terms of cooling and heating demands, electrical consumption for a constant flow rate ventilation mode, considering different orientations. The study also shows the effect of window size (10%, 25%, and 40%) on the annual energy consumption.

International audience; Hemp shives is a lightweight material known for its insulating properties. Associating these bio-fibers with plant-based matrix instead of a mineral binder in insulation applications is of ecological interest. This article describes the study of a low environmental impact 100% plant-based material made solely from wheat starch and hemp shives. The hemp/starch ratio (H/S) influence and hemp shive size are studied. Samples are evaluated in terms of physical properties, mechanical behaviour and hygrothermal properties through an experimental approach. Results show that increasing 0–5 mm hemp shive proportion from 15% to 30% leads to a significant enhancement of the mechanical and hygric characteristics due to the load transfer and porosity. However, when the H/S ratio increases, mechanical and hygrothermal characteristics decrease slightly. Finally, the hemp-starch agro-material with H/S equal to 8 and 30% of 0–5 mm hemp shive seems to be the optimal composition between the studied samples.

International audience

International audience; In a global warming context associated to the abuse of energy consumption, actual researches focus more and more on reducing energy costs in the building sector. This target could be achieved by using innovative building materials, such as hemp concrete, due to its positive impacts on thermal and environmental levels. The aim of this work is to carry out a numerical study of a hemp concrete wall subjected to several indoor and outdoor conditions of temperature and relative humidity using the program object oriented SPARK. The hygrothermal behaviour of the wall is investigated taking into account heat and moisture transfer within the wall as well as hysteresis phenomenon between the sorption and desorption curves and their temperature dependency.

International audience; Refreshing air remains a crucial problem in warm climates where electricity consumption for air conditioning has become excessive and irrational for several years, notably in Algeria. Research in this field is increasingly oriented towards new techniques that can reduce costs and environmental impacts. Among these techniques, the evaporative dew point cooling technology is the most promising as it can cool outdoor air to temperatures below its wet bulb temperature. The aim of this work is to model and design a dew point cooler for french and algerian climates. This model is used to study the effect of the cooler parameters such as its length, water temperature and working air ratio on its cooling effectiveness and supply temperature.

This article deals with the influence of both morphology and amount of Typha on hygrothermal behavior of a Typha–clay composite for building application. An agromaterial containing the fiber mix of Typha Australis and clay was made in three samples: three fiber mixtures were prepared with different amounts Typha and cut type (transversal or longitudinal). The physical properties of these materials were studied in terms of porosity, apparent and absolute density, thermal conductivity, and hygric properties. Results show a real impact of the Typha fraction type and its volume content on hygrothermal properties of the studied material due to the porosity. The transversal fraction of Typha (80% in volume weight) seems to be the optimal composition for a better hygrothermal behavior.

International audience; Hemp-concrete is a green material which has become nowadays highly recommended in the construction field. It finds applications as internal or external thermal insulator in wooden frame walls. At wall scale, studies proved that using Hemp-concrete in building envelope can improve indoor hygrothermal comfort. However, at building scale, hemp-concrete is scarcely studied. In that context, a French building in Grand-Est region, Champagne-Ardennes, employing hemp-concrete as external insulator is selected and studied. An apartment is monitored for several months. Indoor temperatures, relative humidities, thermal heat flux as well as external weather conditions are measured using sensors installed inside the apartment and a weather station at the building roof. Measurements underline the hemp-concrete ability to dampen external weather conditions by showing good results for both indoor temperature and relative humidity. Experimental approach is then coupled with a numerical validation at the wall and room scales using SPARK simulation tool. Investigations are conducted on thermal heat flux through the wall, indoor office air temperature, and relative humidity. Results show a good agreement between numerical values and experimental measurements.

International audience; Following the environnement grenelle, reaching 10% of bio-resourced materials in building sector except wood is one of goals until 2020. Among these agro-materials, one can distinguish the hemp concrete. Based on hemp hurds and mineral binders including lime or polymer as starch, the hemp composites are appreciated for their hydrothermal and acoustic properties. In this study, a vegetal binder based on starch and extracted from wheat is used with treated and non-treated hemp hurds to form an ecological composite. The studied hempstarch is prepared with a ratio (H/S = 3,3). Hygrothermal properties such as moisture buffering value, sorption isotherm, water vapor permeability, thermal conductivity, and heat capacity are experimentally studied. The pore size distribution is performed using mercury porosimetry. Results reveal that the hemp-starch composite may be a promising solution in the insulation field due to its relatively low thermal conductivity and capacity to regulate the indoor relative humidity.

Refreshing air remains a crucial problem in warm climates where electricity consumption for air conditioning has become excessive and irrational for several years, notably in Algeria. Research in this field is increasingly oriented towards new techniques that can reduce costs and environmental impacts. Among these techniques, the evaporative dew point cooling technology is the most promising as it can cool outdoor air to temperatures below its wet bulb temperature. The aim of this work is to model and design a dew point cooler for french and algerian climates. This model is used to study the effect of the cooler parameters such as its length, water temperature and working air ratio on its cooling effectiveness and supply temperature.

International audience; This study deals with the impact of the amount and granulates type of Typha Australis on the hygric performance of building material based on clay. Three compositions are tested: Two of them have the same proportion of granulate but differ by their morphology (transversal or longitudinal cut of the plant). The third composition contains less Typha granulates resulting from the transversal cut. Hygric properties are experimentally, such as permeability, sorption isotherms, and moisture buffering value. Results show an influence of the Typha granulate and its volume content on the hygrothermal properties of the material due to the porosity.

International audience; This article describes the results of a research project carried out at the University of Reims Champagne-Ardenne. The aim of this article is to develop a new insulating material produced by bonding hemp shives with wheat starch as a binder and to characterize its physical properties such as sorption isotherm, water vapour permeability, thermal conductivity, heat capacity and porosity. The equations of coupled heat and moisture transfer within the panels are introduced. These governing equations are applied on the room level in order to assess the hygrothermal behaviour of the panels and its impact on energy consumption and indoor comfort. Simulations are performed for Nancy (France) winter conditions with the simulation environment Simulation Problem Analysis and Research Kernel (SPARK) suited for complex problems.

International audience

Energy efficiency and the reduction of greenhouse gas emissions are actual key issues in all the economic sectors and, in particular, in buildings which is acknowledged worldwide as one of the most energy-consuming. In this context, it would be desirable to duly address those issues by searching for and assessing proper solutions and strategies: the usage of eco-friendly construction materials can be considered as one of those. This paper reports upon the performance of three facades containing sustainable products that are manufactured using natural resources and, alternatively, post-consumer waste based materials; those are: hemp-concrete; and Recycled PolyEthylene Terephthalate (R-PET), respectively. The energy performance of each facade was assessed in terms of cooling and heating demands, electrical consumptions and indoor thermal comfort including indoor temperature and relative humidity. Additionally, a Carbon Footprint (CF) assessment was carried out considering both the estimated energy demands and the life-cycle emission factors associated with the energy mix of the countries where the facades were located, i.e. France and Italy. Based upon the findings of the study, the R-PET facade represented the most performing solution between the three facades in all the scenarios considered and, moreover, the humidity-sensitive flow rate ventilation system came out as a solution able to reduce the electricity consumptions. Finally, considering the Carbon Footprint results, the energy country mix emerged as a key issue, making the Italian case study the worst one, though the total electrical energy consumption were comparable with those of the other case studies.

In grinding, the high frictional energy is converted into heat, which may cause thermal damage and degradation of the wheel and the workpiece. Unwanted thermal effects must thus be reduced, often by external cooling using a curved-duct coolant distributor to match the wheel geometry. The performance of such a system depends strongly on the impinging jet flow properties to ensure efficient sprinkling of the hot spots. The fluid distributor, placed above the workpiece, is pierced with a certain number of identical nozzle fittings, providing multiple jets at the outlet of the nozzles. These jets sprinkle the solids over a given zone and remove the heat by convective transfer. The cooling is hence dependent on the flow structure, meaning the jet diameters, trajectories and velocities, determined up-flow by the distributor design. The present study is devoted to the hydrodynamics aspects of the fluid distributor, aiming to determine the flow-rate distribution at the different orifices and the flow-rate–pressure relationship, for a variety of nozzle diameters and feeding flow rates, under isothermal conditions. A simple hydraulic balance in the device was not able to predict with sufficient accuracy the actual measurements, even when the Venturi effect was accounted for. This discrepancy is due to the curvature of the distributor, inducing secondary flows in interaction with the nozzle outlets, which leads to a rather complex flow pattern. To overcome this issue, a computational fluid dynamics (CFD) tool was used and compared with in situ experiments – global flow rate and pressure measurements were additionally taken with particle image velocimetry (PIV) to gain insight into the local structure. Simulations were performed with a 3D turbulence model for Reynolds numbers up to 100,000. This model provides an efficient tool for coupling with the thermal study at a later step, allowing global sizing and energetic optimization of the grinding process.

Flat glass temperature at the vicinity of the grinding wheel during grinding can become very high and reach that of the glass transition (typically around 550-600 degrees C). In such cases, the mechanical strength of glass is greatly affected and the grinding process cannot be carried out properly. Hence, thermal phenomena must be managed by adjusting the machining parameters to avoid overheating. For this purpose, it is very important to be able to measure the glass temperature, especially at the grinding interface. However, measuring the interfacial glass temperature is difficult and none of the existing methods for metal grinding is adequate for glass grinding. This work shows a novel temperature method that uses constantan and copper strips on both sides of the glass plates; thermoelectric contact being provided by the metallic binder of diamond particles in the grinding wheel. This new technique allows the measurement of the glass edge temperature during the wheel displacement around the glass plate. The experimental results show an average glass edge temperature between 300 and 600 degrees C depending on the value of the machining parameters such as work speed, wheel speed, depth of cut and water coolant flow rate. As this new thermal instrumentation is rather intrusive, glass temperature biases were analysed using a 3D heat transfer model with a moving source. Model computations performed using finite elements show that the temperature biases are less than 70 degrees C, which is smaller than the standard deviation of the glass edge temperatures measured during grinding.

Grinding is a machining process which may encounter excessive heat generated by the friction between the wheel and the material, and therefore degrade the tool as well as the material. The heat has hence, to be removed as efficiently as possible, most often by external cooling. The fluid is projected on the hot interface between the tool and the material through a curved duct coolant distributor. The performance of such a system is strongly dependent on the fluid flow in the curved duct and on the impinging jet flow properties. To optimize cooling setup, CFD simulations and in-situ experiments using particle image velocimetry (PIV) have been made, as well as global flow rate and pressure measurements in the curved duct. A three-dimensional model of a curved duct with 25 outlet nozzles has been studied for duct Reynolds number up to 100,000. Different geometrical configurations for various nozzle diameters have been studied. Due to the complexity of the distributor geometry, it is shown that the global hydraulic balance is not appropriate for sizing the industrial process. On the contrary, satisfactory trend matching in fluid flow streamline behavior is between numerical and experimental results, and an accurate prediction of the pressure drop both show that the numerical simulation is reliable to capture the flow pattern within the curved channel distributor.Copyright © 2013 by ASME

A prelude to the thermal management of grinding processes is measurement of the thermal properties of working materials. Indeed, tool materials must be chosen not only for their mechanical properties (abrasion performance, lifetime...) but also for thermal concerns (thermal conductivity) for efficient cooling that avoids excessive temperatures in the tool and workpiece. Sintered diamond is currently used for grinding tools since it yields higher performances and longer lifetimes than conventional materials (mineral or silicon carbide abrasives), but its thermal properties are not yet well known. Here the thermal conductivity, heat capacity and density of sintered diamond are measured as functions of the diamond content in composites and for two types of metallic binders: hard tungsten-based and soft cobalt-based binders. The measurement technique for thermal conductivity is derived from the flash method. After pulse heating, the temperature of the rear of the sample is measured with a noncontact method (infrared camera). A parameter estimation method associated with a three-layer nonstationary thermal model is used to obtain sample thermal conductivity, heat transfer coefficient and absorbed energy. With the hard metallic binder, the thermal conductivity of sintered diamond increased by up to 64% for a diamond content increasing from 0 to 25%. The increase is much less for the soft binder: 35% for diamond volumes up to 25%. In addition, experimental data were found that were far below the value predicted by conventional analytical models for effective thermal conductivity. A possible explanation is poor heat transfer at the diamond-binder interface. Indeed, better agreement between measurements and model was found by taking into account a thermal contact resistance between matrix and diamond particles equal to 0.75 and 1.25 x 10 (6) m(2) KW (1) for respectively the cobalt- and tungsten-based composites. (C) 2012 Elsevier B.V. All rights reserved.

Sintered diamonds are used in grinding because they offer better mechanical properties than conventional materials (mineral or silicon carbide abrasives) and yield high grinding speed and long life. In addition, because of their thermal performance, they contribute to cooling the workpiece, avoiding excessive temperatures. Thus in order to choose the best material for the worktool, one often must know the thermal conductivity of sintered diamond. In this work, the thermal conductivity of sintered diamond is evaluated as a function of the volume fraction of diamond in the composite and for two types of metallic binders: hard and soft. The measurement technique is based on the flash method that associates heating and measurement devices without sample contact and on parameter estimation using a three-layer thermal model. With a hard metallic binder, the thermal conductivity of sintered diamond was found to increase up to 64% for diamond volume fraction increasing from 0 to 25%. The increase is much smaller for the soft binder: 35% for diamond volume reaching 25%. In addition, experimental data were found far below the value predicted by conventional analytical models for effective thermal conductivity. A possible explanation is that the thermal conductivity of such composites is affected by poor heat transfer at the diamond/binder interface, the thermal contact resistance between matrix and diamond particles being estimated at between 0.75 and 1.25 10−6 m2K.W−1.Copyright © 2012 by ASME