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It is well-known that decision-making processes benefit from the inclusion of uncertainty. Such optimization problems typically extend over a control horizon and could span multiple locations or regions. In addition to uncertainty, these optimization problems require as input a trajectory of scalar values that exhibits the correct spatial and temporal dependencies. Probabilistic forecasts quantify the uncertainty by means of quantiles, predictive distributions or ensembles for a forecast horizon and a site or a region separately, and therefore generally lack spatial and temporal dependencies. One solution is to use a copula to model the spatial or temporal dependencies, which, in combination with the probabilistic forecasts, can be used to issue correlated trajectory forecasts. However, there is currently no benchmark model available to compare multivariate probabilistic solar forecasts with. This paper proposes a multivariate probabilistic ensemble (MuPEn) benchmark model and shows that it generalizes the complete-history persistence ensemble (CH-PeEn) to the multivariate case. The proposed benchmark model requires a forecast issue time and a forecast horizon to construct a multivariate empirical distribution of historical clear-sky index measurements from which a multivariate ensemble forecast can be sampled. Similar to CH-PeEn, the proposed benchmark model generates forecasts that are generally calibrated and consistent in terms of energy score and variogram score.

International audience; This study proposes a novel sepiolite-based Aquivion electrolyte membrane which could be operated at low relative humidity. In the study, it was discovered that the functionalization of sepiolite with fluorinated groups (i.e., -C7F15), named SEP-F, helps its dispersion in the composite membrane, compared with the use of natural sepiolite. The Aquivion/SEP-F composite membrane showed increased water uptake, thermo-mechanical and chemical stability as well as proton conductivity and decreased swelling compared with commercially available Nafion HP and pristine Aquivion. Their behavior in single cell MEA testing conditions was also assessed. Aquivion/SEP-F composite membrane can be an interesting alternative for low relative humidity operation of PEMFC (proton exchange membrane fuel cell).

International audience; Dynamic Line Rating is a technology devised to modify an overhead line's current-carrying capacity based on weather observation. The benefits of this modification may include reduced congestion costs, an increased renewable energy penetration rate, and improved network reliability. DLR is already well developed, but few papers in the literature investigate DLR day-ahead forecasting. The latter is central to DLR development since many of the decisions related to grid management are taken at least on a day-ahead basis. In this paper, two problems related to DLR forecasts are dealt with: how to achieve precise, reliable calculations of day-ahead forecasts of overhead line ampacity and how to define a methodology to calculate safe rating values using these forecasts. On the first point, four machine-learning algorithms were evaluated, identifying the best approach for this problem and quantifying the potential performance. On the second point, the developed methodology was tested and compared to the current static line rating approach.

The building sector is responsible for a significant part of global energy consumption and greenhouse gas emission. Nowadays, there is a growing interest in the so-called super-insulating materials, particularly Aerogels. The present work examines the performance of novel aerogel blankets obtained thanks to an innovative ambient drying process. Based on these aerogel blankets, two thermal insulation systems are developed: one as external thermal insulation composite system (ETICS) and another one as internal thermal insulation multi-layer system (ITI). The objectives of the current work are to assess the performance of the core material (aerogel blanket) as well as the two developed systems. At a material (core layer) scale, thermal, hydric, and mechanical performance characterizations are carried out. At a system level, in-situ hygrothermal, acoustic, and fire performance characterizations are carried out in real scale test cells. Hygrothermal characterization revealed that the thermal conductivity of the studied blanket is about 0.0165 W/(m.K) in ambient and dry conditions but can increase by up to 40% in a very humid environment (RH > 90%). The integration of the aerogel blanket panels into an interior insulating system, tested in a test-cell located in south of France, show great thermal performance. The U value of the wall decreased from 0.63 W/(m2.K) to 0.33 W/(m2.K)after retrofitting. The in-situ hygrothermal results showed that the external insulation system protects the wall against the moisture risks. For the internally insulated wall, the relative humidity remains lower than 85% even when high moisture generation is present inside the test cell. Concerning the thermal performance, the envelope's thermal transmittance is reduced by more than 80% after applying the insulation systems. The ITI system was tested in-situ to airborne sound insulation (facade), reporting significant acoustic improvement in acoustic insulation: +7 dB, equal to a noise reduction to one quarter compared to the base wall without insulation. The fire test of the ETICS proceeded without any material ignition, while the fire behavior of the internal insulation system depends significantly on the adhesion stability between all single components.

International audience; In the prospect of a large scale deployment of Renewable Energy for electricity production, plasmas will definitively be a major option to get tuneable, high temperature enthalpy sources without direct CO2 emissions. This paper focuses on the direct decomposition of methane for the simultaneous synthesis of hydrogen and high value-added carbon black. After a review of gas phase carbon particle nucleation and growth physico-chemical phenomena, a new original model for the plasma decomposition of methane is presented. The model solves a reactive turbulent flow in a 3D geometry. The nucleation is based on a detailed reaction mechanism and the particle growth is handled by a sectional method. This model opens the way towards a better understanding of carbon particles gas phase nucleation and growth and consequently to a fine control of high value-added carbon black grades.

International audience; New hybrid organic-inorganic benzoxazine aerogel blankets for space applications have been synthesized and studied. Aerogel blankets were produced using a one-pot synthesis method with a PET unwoven fibrous network core, resorcinol, formaldehyde and a silica source (APTES with MTES or MTMS as silica co-precursors). Modifying the composition of the sol significantly impacts the physical characteristics of the resulting material such as texture (density and porosity), hydrophilicity, and thermal conductivity. The apparent density of the materials decreases when the percentage of solid in the sol (%solid) decreases and the molar ratio nMTES/nAPTES increases. Within the density range studied, apparent density and effective thermal conductivities are inversely proportional. By replacing a part of APTES by MTES or MTMS the density of the aerogels decreases notably whilst maintaining the level of thermal conductivity. This replacement is detrimental to water adsorption which increases for most conditions studied. When MTMS is used as silica co-precursor instead of MTES no significant differences in water adsorption and thermal conductivity were found. Using the RF-APTES-MTES chemical system we were able to obtain a 3%solid flexible hybrid blanket with both low apparent density (0.04 g cm−3) and relatively low effective thermal conductivity (0.027 W m−1 K−1 in room conditions). A proof of concept with large panels of this material was done by evaluating them as prototype materials composed of encapsulated aerogel blanket in vacuum and low pressure atmosphere. The blankets show a combination of key characteristics representing the right mix for thermal insulation in space applications operating in non-vacuum environments.

International audience; In the life cycle assessment (LCA) context, global sensitivity analysis (GSA) has been identified by several authors as a relevant practice to enhance the understanding of the model's structure and ensure reliability and credibility of the LCA results. GSA allows establishing a ranking among the input parameters, according to their influence on the variability of the output. Such feature is of high interest in particular when aiming at defining parameterized LCA models. When performing a GSA, the description of the variability of each input parameter may affect the results. This aspect is critical when studying new products or emerging technologies, where data regarding the model inputs are very uncertain and may cause misleading GSA outcomes, such as inappropriate input rankings. A systematic assessment of this sensitivity issue is now proposed. We develop a methodology to analyze the sensitivity of the GSA results (i.e. the stability of the ranking of the inputs) with respect to the description of such inputs of the model (i.e. the definition of their inherent variability). With this research, we aim at enriching the debate on the application of GSA to LCAs affected by high uncertainties. We illustrate its application with a case study, aiming at the elaboration of a simple model expressing the life cycle greenhouse gas emissions of enhanced geothermal systems (EGS) as a function of few key parameters. Our methodology allows identifying the key inputs of the LCA model, taking into account the uncertainty related to their description.

International audience; A facile method to synthesize light weight thermally superinsulating composite aerogel is being presented here. A polymethylsilsesquioxane (PMSQ) – cellulose composite aerogel starting from a trifunctional precursor viz. methyltrimethoxysilane (MTMS) and kapok fibers have been synthesized. Kapok fibers have been employed as they have a homogeneous hollow structure and also possess intrinsic low density. A PMSQ-Kapok composite aerogel with a density as low as 0.053 gcm−3 with a thermal conductivity of 0.018 W m−1 K−1 in room conditions has been synthesized. Besides, a flexural strength (at maximum stress) of 108 kPa ± 21 has been obtained through three points bending test. All the composite aerogels are mesoporous as characterized by N2 sorption isotherms and hydrophobic as shown by sessile drop experiments. On comparison with the earlier reported works and with some of the commercially available aerogel composites, the current results seem promising. For demonstrating the real application purpose, thin composite aerogel sheets have also been synthesized which could be easily rolled/bended.

International audience; Cooling demand in the building sector is growing rapidly; thermal energy storage systems using phase change materials (PCM) can be a very useful way to improve the building thermal performance. The right use of PCM in the envelope can minimize peak cooling loads, allow the use of smaller HVAC technical equipment for cooling, and has the capability to keep the indoor temperature within the comfort range due to smaller indoor temperature fluctuations. This article presents an overview of different PCM applications in buildings for reducing cooling loads under different climate conditions, and the factors affecting the successful and the effective use of the PCM. Many drawbacks have been found in PCM applications, mainly the intense impact of summer weather conditions over the PCM performance, which prohibits its complete solidification during night, and thus, limiting its effectiveness during the day. Proposed solutions are reviewed in this article. Finally, a topology diagram is presented to summarize the steps leading to an effective use of PCM in building applications.

International audience; SnO2 nanoparticles were synthetized in three different ways (solvothermal, hydrothermal, sol-gel) and heat-treated under argon at 600 °C to obtain different physico-chemical characteristics (texture, structure and surface chemistry) determined by X-ray powder diffraction (XRD), infrared spectroscopy (FTIR), 119Sn solid state Nuclear Magnetic Resonance (NMR), Electron Spin Resonance (ESR), scanning electron microscopy (SEM) and 77 K nitrogen sorption. When used as electrode material in a lithium ion battery, their electrochemical properties were evaluated by galvanostatic measurements. Among crystallinity, particle size, specific surface area and associated porosity, hydroxyl groups and paramagnetic centers, only the last two parameters appear as determinants of electrochemical performance. Solvothermal and hydrothermal syntheses lead to the presence of certain hydroxyl groups in the oxide whereas sol-gel one prevents their formation but forms paramagnetic species. The hydroxyl groups favour a good coulombic efficiency and an interesting reversibility of the conversion process. Paramagnetic species limit the electrochemical process. Their elimination by a heat-treatment at 1000 °C under argon improves the electrochemical properties. Understanding the key factors to favour SnO2-based materials allows to obtain capacities of about 900 mAh.g−1 over 5 cycles.

International audience; Electrochemical and physicochemical characteristics of three tin-based materials with various oxygen amount used as negative electrode in lithium-ion battery have been evaluated and compared. Nanosizing structuration impact in case of SnO2 is also discussed. Depending on the oxygen amount in those materials used as electrode, energy and power density of lithium-ion battery can be modulated up to 2000 Wh/kg and 8000 W/kg for a maximum sustainable 3C rate. Fading depends mainly on the oxygen amount and, to a lesser extent, on nanostructuration. SnO appears as the best candidate in this material family concerning irreversible capacities and coulombic efficiency. Post-mortem characterizations after 100 cycles by XRD, SEM and EDX highlight a lower aggregation of tin particles for oxygen-based materials upon cycles and formation of α- crystallographic tin phase which can be supposed as beneficial for coulombic efficiency.

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.

International audience; The manual shift of residential wet appliances (dishwashers and washing machines) in French households that have subscribed to a Time of Use (ToU) tariff represents a flexibility potential available already today and at zero-cost. In this paper we quantify, for the first time, this potential by building a national representative synthetic population for France and using a novel bottom-up model. This includes a stochastic occupant behaviour model for predicting the use of the wet appliances and an overlying agent-based model for predicting their manual shifting under a ToU tariff system. The two models are calibrated and validated using empirical data collected over two recent monitoring campaigns, each carried out in more than 100 French households. By using the new model we observe that a 100% penetration rate of the ToU tariff together with an improved (but practically feasible) households’ flexibility could lead, between 19:00 and 20:00, to a mean hourly load reduction of 0.48 GW, which represents around 0.5% of the typical French annual peak load. This flexibility potential, which is today only partially exploited, could be achieved with zero additional costs for new infrastructures, by encouraging eco practices and/or making the ToU tariff more economically attractive for the households.

International audience; Reduction of energy consumption in the building sector has been identified as a major instrument to tackle global climate change and improve sustainability. In this paper, we propose a methodology to address a retrofit planning problem at a community level, with a building resolution. The resulting tool helps local decision-makers identify pertinent actions to improve the environmental behavior of their territories. Two building retrofit levers are considered, namely envelope insulation and heating systems replacement. Retrofit planning is treated here as a single-objective optimization problem aimed at reducing the total costs of retrofit actions by minimizing their net present value. A multidimensional multiple-choice knapsack problem formulation is proposed through the adoption of adequate decision variables. It suitably balances the complexity induced by the large number of potential retrofit action combinations and the number of variables in the problem and permits a linear formulation. An optimization of virtual building stocks is performed to highlight the developed model's capacity to tackle large problems (5,000 buildings) in a few minutes. Finally, three analyses finally are led on a real case-study territory, featuring both appropriate retrofit solutions and building stock information. Long-term evaluation of retrofit strategies over the short-term results in an additional 10% reduction of energy consumption and greenhouse gases emissions and encourages thermal insulation. When targeting a 40% reduction in energy demand, retrofit costs ranging from 20 to 800e/m 2 are observed. Finally, the developed method was used to draw a CO 2 abatement cost curve at territory level. A 70% reduction of emissions can be achieved with costs under 50 e/tCO 2 e.

International audience; Convex relaxations of the optimal power flow (OPF) problem have received a lot of attention in the recent past. In this work, we focus on a second-order cone (SOC) relaxation applied to an OPF based on a branch flow model of a radial and balanced distribution system. We start by examining various sets of conditions ensuring the exactitude of such a relaxation, which is the main focus of the existing literature. In particular, we observe that these sets always include a requirement on the objective to be a minimization of a function increasing with the branch flow apparent powers. We consider this hypothesis to be at odds with what is to be expected of an active distribution system and demonstrate in specific case studies its counterproductive impact. We continue by introducing an objective function allowing distributed generations and storages (DGS) to take advantage of the benefits they bring to the power system as a whole. As this entails the possibility for the relaxation not to be exact, we describe and prove the theoretical convergence to optimality of an algorithm consisting in adding an increasingly tight linear cut to the SOC relaxation. In order to allow the attainment of a solution satisfying the network constraints in a finite number of steps, we continue by introducing a tailored termination criterion. Afterwards, we investigate the ability of our algorithm to obtain a satisfactory solution on several case studies, spanning various network sizes, number of nodes equipped with DGs and their level of penetration. We then conclude on the benefits brought about by this approach and reflect on its limits and the opportunities for further improvements.

International audience; This paper presents the first passive solar wall providing simultaneously super insulation, heat storage and daylighting to the inner space. The wall’s external layer is composed of a silica aerogels bed for high insulation and solar radiation transmission. The second layer, internal, is composed of glass bricks filled with a eutectic phase change material (PCM) for heat storage and restitution. The whole wall is translucent. The experimentations carried out to characterize thermal and optical properties of the materials used are described. Then results are given for a full scale comparative experimentation on a twin zones building located in the south of France for the solar wall and a standard opaque high thermal capacity wall. Results show the heat losses through the wall are very low while the heat and light gains are high: The U value of the solar wall is 0.59 W m−1 K−1 and 0.72 W m−1 K−1 respectively when the PCM is in liquid and solid state. With the test building in free floating condition, the temperature difference between the outdoor and indoor air provided by the wall is about 9 °C in winter. The wall can provide up to 500 lux to the inner environment, which is sufficient for conference rooms. The tested wall has proven more effective in winter and shoulder season, particularly for cold sunny climates, but may cause overheating in summer.

International audience; This study aims at examining the energy behaviour of the buildings' multi-layer exterior wall structures. Most of the studies dealing with this issue have considered a continuous operation of the cooling/heating systems. Our objective here is to find the best wall structure and the number and position of insulation layers within exterior walls for continuous heating, intermittent heating, and no heating operation modes for some commonly used construction materials. Furthermore, among the different materials used, a recently patented insulating coating based on the (super)-insulating materials "Silica Aerogels" is being assessed. Results from a one-dimensional heat transfer numerical model in a multi-layer wall structure are compared to on-site measurements of an experimental set-up, having the new aerogel-based coating, under real weather conditions. The numerical model is then used to carry out all the simulations. Several assessment parameters are used: time lag, decrement factor, energy consumption, and thermal comfort index. Results show that for continuous and no heating cases, the best wall from maximum time lag and minimum decrement factor perspective is dividing the insulation layer into two and placing one at the middle of the wall and one at the exterior surface. For intermittently heated spaces, placing the insulation material as one layer at the interior wall surface is the most efficient from an energy consumption perspective. Also, when using the thermal comfort index for the no heating operation mode, the best performance is achieved when placing the insulation at the interior wall surface. For most of the cases studied, the aerogel-based insulating coating shows better performance than other insulating materials.

International audience; Thermal bridges are weak areas of the building envelope in which they can significantly increase the energy load of houses. In this study, we tackle the thermal bridges resulting from windows offset from exterior walls. First, we present an innovative insulating coating which can be used to limit thermal bridge effects. Second, we compute the cooling/heating load coming from the windows offset thermal bridges of a typical French house before and after adding the insulating coating. Third, we compare the time lag and decrement factor when the 2D heat transfer effects of the thermal bridge are taken into consideration. The methodology is to incorporate 2D heat transfer into a whole building energy simulation program. This is done through co-simulation between a 2D heat transfer model developed in MATLAB and the building energy simulation software EnergyPlus using the software BCVTB. This latter enables us to link the two programs and allow them to exchange data at each simulation time step. Results showed that the windows offset thermal bridges energy load percentage of the total house load constitutes around 2-8% depending whether exterior walls have interior insulation or not. Applying 1 cm and 2 cm of the coating on these thermal bridges reduces the windows offset energy load by about 24-50%. Concerning time lag and decrement factor, we obtain high values for decrement factor and low values for the time lag for wall positions near the thermal bridge. Applying the coating decreases, significantly, the decrement factor and increases the time lag.

International audience; Distributed energy resources (DER) and new types of consumer equipment create many challenges for distribution system operators (DSOs). Power congestions that can potentially be created during normal or contingency situations will lead to increased investments into grid reinforcement. An alternative solution is to use the flexibility provided by the local resources in the grid. In this paper value of flexibility (VoF) is used as an indicator that can be utilized by the DSO to compare it against costs of the different active elements (AEs) providing flexibility services (FSs). The paper proposes flexibility characterization framework that allows to generalize the process of the cost estimations of any AE by using combinations of cost functions. A case study based on an actual distribution grid is provided to demonstrate the potential application of the framework. Results show that by comparing VoF and total cost of the flexibility the most cost-efficient solution could be found.