• Energy Research
• Open Access close
• Open Source close
• Embargo close
• DE close
• German Aerospace Center close

Abstract Secondary concentrators are used in solar concentrating systems to redirect solar beams reflected by the primary concentrators to the focal point or line. These components allow to increase the concentrated solar flux density and hence to lower thermal radiation losses. Solar reflectors for secondary concentrators are permanently exposed to environmental conditions, high radiation fluxes and elevated temperatures that potentially cause stress and degradation throughout the time. Therefore, analyzing solar reflectors of secondary concentrators by simulating these conditions is crucial. No previous research works about the durability of solar reflector materials for secondary concentrators have been reported. The present work is focused on studying the degradation of the reflector materials by simulating accelerated aging, caused by several ambient parameters and the effect of concentrated radiation. Both cooled and uncooled systems for secondary concentrators are included in this study. According to results obtained, aluminum reflectors and thin silvered-glass reflectors glued to an aluminum structure showed minimum reflectance losses and structural degradation under the operation conditions of cooled 3D secondary concentrators (tower systems). Following critical aspects to avoid reflector degradation were identified: to select a suitable adhesive material to glue the thin silvered-glass reflector to the support aluminum structure, to properly protect reflectors edges, to design a suitable cooling system and to avoid the combination of high radiation fluxes with mechanical stress. In addition, laminated silvered-glass reflectors have shown to be suitable for uncooled 2D secondary concentrators (Fresnel collectors). Furthermore, a comparison with naturally aged secondary concentrators using silvered-glass reflectors glued to an aluminum structure revealed that the simulated degradation under accelerated conditions performed in this work did reproduce the most frequent degradation patterns suffered in real operating conditions.

The successful electricity grid integration of solar energy into day-ahead markets requires at least hourly resolved 48 h forecasts. Technologies as photovoltaics and non-concentrating solar thermal technologies make use of global horizontal irradiance (GHI) forecasts, while all concentrating technologies both from the photovoltaic and the thermal sector require direct normal irradiances (DNI). The European Centre for Medium-Range Weather Forecasts (ECMWF) has recently changed towards providing direct as well as global irradiances. Additionally, the MACC (Monitoring Atmospheric Composition & Climate) near-real time services provide daily analysis and forecasts of aerosol properties in preparation of the upcoming European Copernicus programme. The operational ECMWF/IFS (Integrated Forecast System) forecast system will in the medium term profit from the Copernicus service aerosol forecasts. Therefore, within the MACC‑II project specific experiment runs were performed allowing for the assessment of the performance gain of these potential future capabilities. Also the potential impact of providing forecasts with hourly output resolution compared to three-hourly resolved forecasts is investigated. The inclusion of the new aerosol climatology in October 2003 improved both the GHI and DNI forecasts remarkably, while the change towards a new radiation scheme in 2007 only had minor and partly even unfavourable impacts on the performance indicators. For GHI, larger RMSE (root mean square error) values are found for broken/overcast conditions than for scattered cloud fields. For DNI, the findings are opposite with larger RMSE values for scattered clouds compared to overcast/broken cloud situations. The introduction of direct irradiances as an output parameter in the operational IFS version has not resulted in a general performance improvement with respect to biases and RMSE compared to the widely used Skartveit et al. (1998) global to direct irradiance conversion scheme. Cloudy situations and especially thin ice cloud cases are forecasted much better with respect to biases and RMSE, but large biases are introduced in clear sky cases. When applying the MACC aerosol scheme to include aerosol direct effects, an improvement especially in DNI biases is found for cloud free cases as expected. However, a performance decrease is found for water cloud cases. It is assumed that this is caused by the lack of an explicit modelling of cloud-aerosol interactions, while other meteorological forcings for cloud processes like the temperature field are modified by the aerosols.

This dataset contains 365 hyperspectral albedo maps of Earth, one for each day of the year. The hyperspectral albedo maps are built from a 10-year average of the MODIS Surface Reflectance dataset (MCD43D 42-48, version 6.1). Using a Principal Component Analysis (PCA) regression algorithm we combine different hyperspectral laboratory and in-situ measurements spectra of various dry soils, vegetation surfaces and mixure of both to reconstruct the albedo maps in the entire wavelength range from 400 to 2500 nm, with a spectral resolution of 10 nm. The hyperspectral albedo maps have a spatial resolution of 0.05° in latitude and longitude. Additional hyperspectral albedo maps with a coarser spatial or spectral resolution are available upon request. Access to dataData files are available for download at: https://opendata.physik.lmu.de/kXZksootPPjK6XC

Ca-Mn-based perovskites doped in their A- and B-site were synthesized and comparatively tested versus the Co3O4/CoO and (Mn,Fe)2O3/(Mn,Fe)3O4 redox pairs with respect to thermochemical storage and oxygen pumping capability, as a function of the kind and extent of dopant. The perovskites' induced heat effects measured via differential scanning calorimetry are substantially lower: the highest reaction enthalpy recorded by the CaMnO3–δ composition was only 14.84 kJ/kg compared to 461.1 kJ/kg for Co3O4/CoO and 161.0 kJ/kg for (Mn,Fe)2O3/(Mn,Fe)3O4. Doping of Ca with increasing content of Sr decreased these heat effects; more than 20 at % Sr eventually eliminated them. Perovskites with Sr instead of Ca in the A-site exhibited also negligible heat effects, irrespective of the kind of B site cation. On the contrary, perovskite compositions characterized by high oxygen release/uptake can operate as thermochemical oxygen pumps enhancing the performance of water/carbon dioxide splitting materials. Oxygen pumping via Ca0.9Sr0.1MnO3–δ and SrFeO3–δ doubled and tripled, respectively, the total oxygen absorbed by ceria during its re-oxidation versus that absorbed without their presence. Such effective pumping compositions exhibited practically no shrinkage during one heat-up/cool-down cycle. However, they demonstrated an increase of the coefficient of linear expansion due to the superposition of “chemical expansion” to thermal-only one, the effect of which on the long-term dimensional stability has to be further quantified through extended cyclic operation.

The use of liquid metals in solar power systems is not new. The receiver tests with liquid sodium in the 1980s at the Plataforma Solar de Almer a (PSA) already proved the feasibility of liquid metals as heat transfer fluid. Despite the high efficiency achieved with that receiver, further investigation of liquid metals in solar power systems was stopped due to a sodium spray fire. Recently, the topic has become interesting again and the gained experience during the last 30 years of liquid metals handling is applied to the concentrated solar power community. In this paper, recent activities of the Helmholtz Alliance LIMTECH concerning liquid metals for solar power systems are presented. In addition to the components and system simulations also the experimental setup and results are included.

Abstract The market diffusion of plug-in electric vehicles (PEVs) is a research topic which is often addressed, yet PEV market diffusion models differ in their approaches, the factors they include and results. Here, 40 market diffusion models for PEVs are compared in their scope, approach and findings to point out similarities or differences and make recommendations for future improvements in modeling in this field. Important input factors for the US are the purchase price and operating costs, while for Germany energy prices and the charging infrastructure are mentioned more often. Furthermore, larger sales shares of plug-in hybrid electric vehicles than battery electric vehicles are often found in the short term results (until 2030) while the picture is not so clear for the medium- to long-term. Future market penetration models should include specific PEV features like the limited range of battery electric vehicles or access to charging infrastructure, which are currently not covered by many models. Also, the integration of current policy regulations and, if possible, indirect policy incentives would enhance research in this field.

Stretchable electronic devices are known as elastic electronics or circuits. They are typically built by deposition or embedding of devices and circuits onto stretchable substrates, which can sustain large strains without failure. The research on this emerging technology has focused on the development of components based on insulators, conductors, or semiconductors to overcome inherent difficulties of generating mechanically stable layers and interconnections. In addition to the mechanical challenges, the heat-dissipation problem, remains yet to be solved, since stretchable electronics are generally fabricated using materials with poor thermal conductivities. To simultaneously address stretchability and heat dissipation, novel cooling platforms are required. As one promising pathway, this paper presents the fabrication and performance optimization of a thermoelectric device that is stretchable and effectively dissipates heat. The device is easy to fabricate and has a relatively simple structure, consisting of a silicone elastomer (Ecoflex), liquid metal, and thermoelectric legs. It is mechanically stable over 1000 stretching-and-releasing cycles with 30% strain. A thermal analysis reveals that the Ecoflex substrates impose significant thermal resistances to the entire device, degrading its cooling performance. To address this issue, alumina powder is added into the Ecoflex substrates. For the case of Ecoflex/alumina composite substrates with 60 wt % alumina, the maximum temperature drop is enhanced by 80 and 50% at 0 and 30% strains, respectively, under no heat load conditions. The devices with alumina addition also show a highly improved cooling per unit electrical power input, suggesting their potential merit in cost and energy saving. The cooling capability of the stretchable thermoelectric devices is further confirmed under both conductive and convective heat load conditions.

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 approaches. 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 to enhance inter-comparison and empower physical process insights across glacier elevation-change studies.