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For large scale thermal energy storage at temperatures above 300°C, two-tank molten salt systems mark the current state-of-the-art as they are proven technology in parabolic trough and tower solar thermal power plants. Research is focusing on the utilization of molten salts not only as storage medium but also as heat transferring fluid (HTF) in parabolic trough plants [1]. The current two-tank concept offers serveral cost reduction possibilities. Firstly, instead of storing the hot and cold phase in two separate tanks, the salt could be stored inside a single tank to avoid a large gas volume. The separation of both phases can either be achieved by a floating insulated barrier or simply by the different densities of both phases. Secondly, a high share of the total investment costs of a molten salt storage system is caused by the molten salt itself. For the two-tank system in 50 MWel power plants, this can be as high as half of the total TES costs [2]. In the thermocline with filler concept, a large fraction of the molten salt can be substituted by a cost effective solid material, offering a significant potential for further cost reductions [3]. Finally, gaining operational experience of such systems and the ability to derive optimized operation strategies, promise an additional cost reduction potential. The “test facility for thermal energy storage in molten salts” (TESIS:store) has been set up at DLR in Cologne, Germany. An outside view of the plant can be seen in Fig. 1. The facility operates at temperatures up to 560 °C and a maximum molten salt mass flow of 4 kg/s. The storage volume has a length of 5.4 m with a total tank volume of 22 m³. The plant allows the investigation of the thermocline concept with and without filler and gaining widespread operation experience. Heat tracing along the containment walls and the piping ensures adiabatic conditions.

Résumé. Les observations des changements de masse des glaciers sont essentielles pour comprendre la réponse des glaciers au changement climatique et aux impacts connexes, tels que le ruissellement régional, les changements écosystémiques et l'élévation du niveau de la mer à l'échelle mondiale. Les capteurs optiques et radar spatiaux permettent de quantifier les changements d'élévation des glaciers, et donc les changements de masse pluriannuels, à l'échelle régionale et mondiale. Cependant, les estimations d'un nombre croissant d'études montrent un large éventail de résultats avec des différences souvent au-delà des limites d'incertitude. Ici, nous présentons les résultats d'une expérience intercomparaison communautaire utilisant des données d'interférométrie stéréo optique spatiale (ASTER) et radar à ouverture synthétique (TanDEM-X) pour estimer les changements d'altitude pour des glaciers définis et des périodes cibles qui posent différents défis d'évaluation. En utilisant des modèles d'élévation numériques (DEM) fournis ou autotraités pour cinq sites de test, 12 groupes de recherche ont fourni un total de 97 ensembles de données de changement d'altitude spatiaux en utilisant diverses stratégies de traitement. La validation avec des données aéroportées a montré que l'utilisation d'une estimation d'ensemble promet de réduire les erreurs aléatoires provenant de différents instruments et méthodes de traitement, mais nécessite toujours une enquête et une correction plus complètes des erreurs systématiques. Nous avons constaté que la sélection de la scène, le traitement DEM et le co-enregistrement ont le plus grand impact sur les résultats. D'autres étapes de traitement, telles que le traitement des vides de données spatiales, les différences de périodes d'enquête ou la pénétration radar, peuvent toujours être importantes pour des cas individuels. Les recherches futures devraient se concentrer sur la mise à l'essai de différentes implémentations d'étapes de traitement individuelles (par exemple, le co-enregistrement) et aborder les questions liées aux corrections temporelles, à la pénétration radar, aux changements de zone glaciaire et à la conversion de densité. Enfin, notre communauté a clairement besoin de développer les meilleures pratiques, d'utiliser des logiciels ouverts et reproductibles et d'évaluer l'incertitude globale afin d'améliorer les comparaisons et de renforcer les connaissances sur les processus physiques dans les études de changement d'altitude des glaciers. Resumen. Observar los cambios en la masa de los glaciares es clave para comprender la respuesta de los glaciares al cambio climático y los impactos relacionados, como la escorrentía regional, los cambios en los ecosistemas y el aumento global del nivel del mar. Los sensores ópticos y de radar transportados por el espacio permiten cuantificar los cambios de elevación de los glaciares y, por lo tanto, los cambios de masa plurianuales, a escala regional y global. Sin embargo, las estimaciones de un número creciente de estudios muestran una amplia gama de resultados con diferencias que a menudo van más allá de los límites de incertidumbre. Aquí, presentamos el resultado de un experimento de intercomparación basado en la comunidad que utiliza datos estéreo óptico a bordo del espacio (ASTER) e interferometría de radar de apertura sintética (TanDEM-X) para estimar los cambios de elevación para glaciares definidos y períodos objetivo que plantean diferentes desafíos de evaluación. Utilizando modelos digitales de elevación (DEM) proporcionados o autoprocesados para cinco sitios de prueba, 12 grupos de investigación proporcionaron un total de 97 conjuntos de datos de cambio de elevación a bordo del espacio utilizando varias estrategias de procesamiento. La validación con datos aéreos mostró que el uso de una estimación de conjunto es prometedor para reducir los errores aleatorios de diferentes instrumentos y métodos de procesamiento, pero aún requiere una investigación y corrección más exhaustivas de los errores sistemáticos. Descubrimos que la selección de escenas, el procesamiento de DEM y el corregistro tienen el mayor impacto en los resultados. Otros pasos de procesamiento, como el tratamiento de vacíos de datos espaciales, las diferencias en los períodos de encuesta o la penetración del radar, aún pueden ser importantes para casos individuales. La investigación futura debe centrarse en probar diferentes implementaciones de pasos de procesamiento individuales (por ejemplo, registro conjunto) y abordar cuestiones relacionadas con correcciones temporales, penetración de radar, cambios en el área de los glaciares y conversión de densidad. Finalmente, existe una clara necesidad de que nuestra comunidad desarrolle las mejores prácticas, use software abierto y reproducible y evalúe la incertidumbre general para mejorar la intercomparación y potenciar los conocimientos de los procesos físicos en los estudios de cambio de elevación de glaciares. Abstract. Observations of glacier mass changes are key to understanding the response of glaciers to climate change and related impacts, such as regional runoff, ecosystem changes, and global sea-level rise. Spaceborne optical and radar sensors make it possible to quantify glacier elevation changes, and thus multi-annual mass changes, on a regional and global scale. However, estimates from a growing number of studies show a wide range of results with differences often beyond uncertainty bounds. Here, we present the outcome of a community-based inter-comparison experiment using spaceborne optical stereo (ASTER) and synthetic aperture radar interferometry (TanDEM-X) data to estimate elevation changes for defined glaciers and target periods that pose different assessment challenges. Using provided or self-processed digital elevation models (DEMs) for five test sites, 12 research groups provided a total of 97 spaceborne elevation-change datasets using various processing strategies. Validation with airborne data showed that using an ensemble estimate is promising to reduce random errors from different instruments and processing methods, but still requires a more comprehensive investigation and correction of systematic errors. We found that scene selection, DEM processing, and co-registration have the biggest impact on the results. Other processing steps, such as treating spatial data voids, differences in survey periods, or radar penetration, can still be important for individual cases. Future research should focus on testing different implementations of individual processing steps (e.g. co-registration) and addressing issues related to temporal corrections, radar penetration, glacier area changes, and density conversion. Finally, there is a clear need for our community to develop best practices, use open, reproducible software, and assess overall uncertainty in order to enhance inter-comparison and empower physical process insights across glacier elevation-change studies. الخلاصة. تعتبر ملاحظات التغيرات في كتلة الأنهار الجليدية أساسية لفهم استجابة الأنهار الجليدية لتغير المناخ والآثار ذات الصلة، مثل الجريان السطحي الإقليمي وتغيرات النظام الإيكولوجي وارتفاع مستوى سطح البحر العالمي. تتيح أجهزة الاستشعار البصرية والرادارية المحمولة في الفضاء قياس التغيرات في ارتفاع الأنهار الجليدية، وبالتالي التغيرات الكتلية متعددة السنوات، على نطاق إقليمي وعالمي. ومع ذلك، تظهر التقديرات من عدد متزايد من الدراسات مجموعة واسعة من النتائج مع وجود اختلافات غالبًا ما تتجاوز حدود عدم اليقين. هنا، نقدم نتائج تجربة مقارنة مجتمعية باستخدام بيانات الاستريو البصري المحمول في الفضاء (ASTER) وبيانات قياس التداخل بالرادار ذي الفتحة الاصطناعية (TanDEM - X) لتقدير تغيرات الارتفاع للأنهار الجليدية المحددة والفترات المستهدفة التي تشكل تحديات تقييم مختلفة. باستخدام نماذج الارتفاع الرقمية المقدمة أو ذاتية المعالجة (DEMs) لخمسة مواقع اختبار، قدمت 12 مجموعة بحثية ما مجموعه 97 مجموعة بيانات لتغيير الارتفاع المحمول في الفضاء باستخدام استراتيجيات معالجة مختلفة. أظهر التحقق من البيانات المحمولة جواً أن استخدام تقدير المجموعة يعد بتقليل الأخطاء العشوائية من الأدوات وطرق المعالجة المختلفة، ولكنه لا يزال يتطلب تحقيقًا أكثر شمولاً وتصحيحًا للأخطاء المنهجية. وجدنا أن اختيار المشهد ومعالجة DEM والتسجيل المشترك لها أكبر تأثير على النتائج. يمكن أن تظل خطوات المعالجة الأخرى، مثل معالجة فراغات البيانات المكانية أو الاختلافات في فترات المسح أو اختراق الرادار، مهمة للحالات الفردية. يجب أن تركز الأبحاث المستقبلية على اختبار التطبيقات المختلفة لخطوات المعالجة الفردية (مثل التسجيل المشترك) ومعالجة القضايا المتعلقة بالتصحيحات الزمنية واختراق الرادار وتغيرات المنطقة الجليدية وتحويل الكثافة. أخيرًا، هناك حاجة واضحة لمجتمعنا لتطوير أفضل الممارسات، واستخدام برامج مفتوحة وقابلة للتكرار، وتقييم عدم اليقين العام من أجل تعزيز المقارنة البينية وتمكين رؤى العمليات المادية عبر دراسات تغيير ارتفاع الأنهار الجليدية.

In contrast to stiff backing materials (e.g. carbon paper) softer ones like carbon cloth are compressed over the ribs of the gas distributors or impressed into the channels when the PEFC is assembled. During fuel cell optimisation the interactions between the gas diffusion layer and the flow field are frequently neglected; hence flow fields as well as gas diffusion layers are commonly optimized independently. The DLR has investigated these interactions with a two-stage approach: At first the basic influence of the mechanical compression was analysed by measuring the generated current and the local current density distribution in the potentiostatic mode as a function of the contact pressure. These investigations were carried out with a segmented laboratory cell with an active area of 25 cm2. The overall performance as well as the local current density distribution shows a strong dependency on the compression state of the cell. Thereby, an optimal contact pressure exists, where the influences of the contact resistance and the mass transfer are minimized. The impression depth of the gas diffusion layer into the flow field channels was determined as a function of contact pressure by two different methods: optically and electronically. For this purpose an appropriate device was developed that consists of a copper plate with spark-eroded channels, ridges of acrylic glass therein and the gas diffusion layer on the ribs. While the outer appearance of this device is equal to the gas distributor of the fuel cell, this set-up acts as a parallel connection of different capacities. Accordingly, the depth of impression in the gas diffusion layer can be determined by measuring the change of the electrical capacity if the material is pressed in the channel structures.

Remote sensing is increasingly used for managing urban environment. In this context, the H2020 project URBANFLUXES aims to improve our knowledge on urban anthropogenic heat fluxes, with the specific study of three cities: London, Basel and Heraklion. Usually, one expects to derive directly 2 major urban parameters from remote sensing: the albedo and thermal irradiance. However, the determination of these two parameters is seriously hampered by complexity of urban architecture. For example, urban reflectance and brightness temperature are far from isotropic and are spatially heterogeneous. Hence, radiative transfer models that consider the complexity of urban architecture when simulating remote sensing signals are essential tools. Even for these sophisticated models, there is a major constraint for an operational use of remote sensing: the complex 3D distribution of optical properties and temperatures in urban environments. Here, the work is conducted with the DART (Discrete Anisotropic Radiative Transfer) model. It is a comprehensive physically based 3D radiative transfer model that simulates optical signals at the entrance of imaging spectro-radiometers and LiDAR scanners on board of satellites and airplanes, as well as the 3D radiative budget, of urban and natural landscapes for any experimental (atmosphere, topography,…) and instrumental (sensor altitude, spatial resolution, UV to thermal infrared,…) configuration. Paul Sabatier University distributes free licenses for research activities. This paper presents the calibration of DART model with high spatial resolution satellite images (Landsat 8, Sentinel 2, etc.) that are acquired in the visible (VIS) / near infrared (NIR) domain and in the thermal infrared (TIR) domain. Here, the work is conducted with an atmospherically corrected Landsat 8 image and Bale city, with its urban database. The calibration approach in the VIS/IR domain encompasses 5 steps for computing the 2D distribution (image) of urban albedo at satellite spatial resolution. (1) DART simulation of satellite image at very high spatial resolution (e.g., 50cm) per satellite spectral band. Atmosphere conditions are specific to the satellite image acquisition. (2) Spatial resampling of DART image at the coarser spatial resolution of the available satellite image, per spectral band. (3) Iterative derivation of the urban surfaces (roofs, walls, streets, vegetation,…) optical properties as derived from pixel-wise comparison of DART and satellite images, independently per spectral band. (4) Computation of the band albedo image of the city, per spectral band. (5) Computation of the image of the city albedo and VIS/NIR exitance, as an integral over all satellite spectral bands. In order to get a time series of albedo and VIS/NIR exitance, even in the absence of satellite images, ECMWF information about local irradiance and atmosphere conditions are used. A similar approach is used for calculating the city thermal exitance using satellite images acquired in the thermal infrared domain. Finally, DART simulations that are conducted with the optical properties derived from remote sensing images give also the 3D radiative budget of the city at any date including the date of the satellite image acquisition.

International audience We test two cloud products: Optimal Cloud Analysis (OCA) of EUMETSAT, and APOLLO from the German Aerospace Center (DLR), for the assessment of surface downwelling solar irradiance (SSI). Each product is input to the Heliosat-4 method, and the SSI estimates are compared to accurate measurements performed in the Baseline Radiation Network (BSRN). The performances obtained by the two products are compared. The overall performance of Heliosat-4 method by using different cloud products is given and conclusions on the benefit of each product for an operational Heliosat-4 are drawn.