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Takacs M.,ETH Zurich | Ackermann S.,ETH Zurich | Bonk A.,Institute for Geo and Life science | Neises-von Puttkamer M.,Institute of Solar Research | And 3 more authors.
AIChE Journal | Year: 2016

Thermochemical splitting of CO2 via a ceria-based redox cycle was performed in a solar-driven thermogravimetric analyzer. Overall reaction rates, including heat and mass transport, were determined under concentrated irradiation mimicking realistic operation of solar reactors. Reticulated porous ceramic (RPC) structures and fibers made of undoped and Zr4+-doped CeO2, were endothermally reduced under radiative fluxes of 1280 suns in the temperature range 1200-1950 K and subsequently re-oxidized with CO2 at 950-1400 K. Rapid and uniform heating was observed for 8 ppi ceria RPC with mm-sized porosity due to its low optical thickness and volumetric radiative absorption, while ceria fibers with μm-sized porosity performed poorly due to its opacity to incident irradiation. The 10 ppi RPC exhibited higher fuel yield because of its higher sample density. Zr4+-doped ceria showed increasing reduction extents with dopant concentration but decreasing specific CO yield due to unfavorable oxidation thermodynamics and slower kinetics. © 2016 American Institute of Chemical Engineers.

Montecchi M.,ENEA | Delord C.,French National Solar Energy Institute | Raccurt O.,French National Solar Energy Institute | Disdier A.,French National Solar Energy Institute | And 7 more authors.
Energy Procedia | Year: 2015

Mirrors are the first link in the energy-conversion chain from Sun to electricity-delivery in the grid. Shape and solar reflectance are the key-parameters of mirrors, respectively affecting how solar radiation is concentrated around the focus, and how much of the impinging solar power is reflected. In SolarPACES Task III, an expert group is drafting the solar reflectance guidelines; in order to speed up the discussion the SRRR round robin was launched at the beginning of 2013. Identical kits, each one consisting of ten specimens collected from eight cooperating producers, were distributed and measured at six research institutes, acting as evaluators. The kit includes both traditional (glass based) and innovative (first-surface) solar mirrors. The paper only reports on the simplest task among those of SRRR: the solar hemispherical reflectance measurement. Near-specular solar reflectance was also measured and compared but the results are still under investigation and are not part of this paper. The measurements were accomplished according to the guidelines. The differences among the achieved results are within the typical accuracy of spectrophotometers, demonstrating the reliability of the reflectance guidelines. The statistic of the deviations from the true value is analysed separately for each evaluator, and allows us to infer information abut the gauging-status of the adopted reference mirror, as well as the measurement reproducibility. © 2015 The Authors. Published by Elsevier Ltd.

Blanc P.,MINES ParisTech | Espinar B.,MINES ParisTech | Geuder N.,Stuttgart University of Applied Sciences | Gueymard C.,Solar Consulting Services | And 8 more authors.
Solar Energy | Year: 2014

The direct irradiance received on a plane normal to the sun, called direct normal irradiance (DNI), is of particular relevance to concentrated solar technologies, including concentrating solar thermal plants and concentrated photovoltaic systems. Following various standards from the International Organization for Standardization (ISO), the DNI definition is related to the irradiance from a small solid angle of the sky, centered on the position of the sun. Half-angle apertures of pyrheliometers measuring DNI have varied over time, up to ≈10°. The current recommendation of the World Meteorological Organization (WMO) for this half-angle is 2.5°. Solar concentrating collectors have an angular acceptance function that can be significantly narrower, especially for technologies with high concentration ratios. The disagreement between the various interpretations of DNI, from the theoretical definition used in atmospheric physics and radiative transfer modeling to practical definitions corresponding to specific measurements or conversion technologies is significant, especially in the presence of cirrus clouds or large concentration of aerosols. Under such sky conditions, the circumsolar radiation- i.e. the diffuse radiation coming from the vicinity of the sun-contributes significantly to the DNI ground measurement, although some concentrating collectors cannot utilize the bulk of it. These issues have been identified in the EU-funded projects MACC-II (Monitoring Atmospheric Composition and Climate-Interim Implementation) and SFERA (Solar Facilities for the European Research Area), and have been discussed within a panel of international experts in the framework of the Solar Heating and Cooling (SHC) program of the International Energy Agency's (IEA's) Task 46 ". Solar Resource Assessment and Forecasting". In accordance with these discussions, the terms of reference related to DNI are specified here. The important role of circumsolar radiation is evidenced, and its potential contribution is evaluated for typical atmospheric conditions. For thorough analysis of performance of concentrating solar systems, it is recommended that, in addition to the conventional DNI related to 2.5° half-angle of today's pyrheliometers, solar resource data sets also report the sunshape, the circumsolar contribution or the circumsolar ratio (CSR). © 2014.

Feldhoff J.F.,German Aerospace Center | Eickhoff M.,Institute of Solar Research | Keller L.,Institute of Solar Research | Alonso J.L.,CIEMAT | And 4 more authors.
Energy Procedia | Year: 2013

One option to improve the cost-effectiveness and environmental friendliness of parabolic trough power plants is the utilization of water/steam in the solar field, the so-called direct steam generation (DSG). First commercial stand-alone plants using DSG are now in operation in Thailand (parabolic trough with superheating by Solarlite) and in Spain (linear Fresnel for saturated steam by Novatec Solar). To further bring down the costs of a DSG solar field, the research project DUKE aims at the development and demonstration of a commercially applicable once-through mode design. The demonstration will be done at the DISS test facility at the Plataforma Solar de Almería (PSA), Spain, by the DLR-Institute of Solar Research in close collaboration with the Spanish CIEMAT. For this purpose, the DISS test facility was upgraded to a length of 1000 m by three new collectors. It now has commercial-scale size and, in addition, is able to stand 500 °C and 110 bar at the outlet [1]. The construction and commissioning of the new plant has been completed (see Fig. 1) and the first test period has started in May 2013. The paper examines two aspects: illustrate the design changes of the solar field and show the performance of the two new main components: the DSG receivers as well as the new collectors. © 2013 The Authors.

Polo J.,CIEMAT | Wilbert S.,Institute of Solar Research | Ruiz-Arias J.A.,University of Jaén | Meyer R.,Suntrace GmbH | And 16 more authors.
Solar Energy | Year: 2016

At any site, the bankability of a projected solar power plant largely depends on the accuracy and general quality of the solar radiation data generated during the solar resource assessment phase. The term "site adaptation" has recently started to be used in the framework of solar energy projects to refer to the improvement that can be achieved in satellite-derived solar irradiance and model data when short-term local ground measurements are used to correct systematic errors and bias in the original dataset. This contribution presents a preliminary survey of different possible techniques that can improve long-term satellite-derived and model-derived solar radiation data through the use of short-term on-site ground measurements. The possible approaches that are reported here may be applied in different ways, depending on the origin and characteristics of the uncertainties in the modeled data. This work, which is the first step of a forthcoming in-depth assessment of methodologies for site adaptation, has been done within the framework of the International Energy Agency Solar Heating and Cooling Programme Task 46 "Solar Resource Assessment and Forecasting". © 2016 Elsevier Ltd.

Reinhardt B.,German Aerospace Center | Buras R.,Ludwig Maximilians University of Munich | Bugliaro L.,German Aerospace Center | Wilbert S.,Institute of Solar Research | Mayer B.,Ludwig Maximilians University of Munich
Atmospheric Measurement Techniques | Year: 2014

Reliable data on circumsolar radiation, which is caused by scattering of sunlight by cloud or aerosol particles, is becoming more and more important for the resource assessment and design of concentrating solar technologies (CSTs). However, measuring circumsolar radiation is demanding and only very limited data sets are available. As a step to bridge this gap, a method was developed which allows for determination of circumsolar radiation from cirrus cloud properties retrieved by the geostationary satellites of the Meteosat Second Generation (MSG) family. The method takes output from the COCS algorithm to generate a cirrus mask from MSG data and then uses the retrieval algorithm APICS to obtain the optical thickness and the effective radius of the detected cirrus, which in turn are used to determine the circumsolar radiation from a pre-calculated look-up table. The look-up table was generated from extensive calculations using a specifically adjusted version of the Monte Carlo radiative transfer model MYSTIC and by developing a fast yet precise parameterization. APICS was also improved such that it determines the surface albedo, which is needed for the cloud property retrieval, in a self-consistent way instead of using external data. Furthermore, it was extended to consider new ice particle shapes to allow for an uncertainty analysis concerning this parameter. We found that the nescience of the ice particle shape leads to an uncertainty of up to 50%. A validation with 1 yr of ground-based measurements shows, however, that the frequency distribution of the circumsolar radiation can be well characterized with typical ice particle shape mixtures, which feature either smooth or severely roughened particle surfaces. However, when comparing instantaneous values, timing and amplitude errors become evident. For the circumsolar ratio (CSR) this is reflected in a mean absolute deviation (MAD) of 0.11 for both employed particle shape mixtures, and a bias of 4 and 11%, for the mixture with smooth and roughend particles, respectively. If measurements with sub-scale cumulus clouds within the relevant satellite pixels are manually excluded, the instantaneous agreement between satellite and ground measurements improves. For a 2-monthly time series, for which a manual screening of all-sky images was performed, MAD values of 0.08 and 0.07 were obtained for the two employed ice particle mixtures, respectively. © 2014 Author(s).

Call F.,Institute of Solar Research | Roeb M.,Institute of Solar Research | Schmucker M.,German Aerospace Center | Sattler C.,Institute of Solar Research | Pitz Paal R.,Institute of Solar Research
Journal of Physical Chemistry C | Year: 2015

Developing an efficient redox material is crucial for thermochemical cycles that produce solar fuels (e.g., H2 and CO), enabling a sustainable energy supply. In this study, the effects of varying the rare earth content y in Ce0.85 yZr0.15REyO2 0.5y with RE = Y, La, Sm and Gd on the fuel productivity and long term stability were investigated. Compared to the none RE doped reference material, Ce0.85Zr0.15O2, none of the compositions exhibits higher performances. However, long term cycling of more than 80 cycles reveals enhanced performance due to rare earth doping. Ce0.85Zr0.15O2 suffers from linear degradation of the yields and of the CO:O2 ratio r, which is attributed to declining oxidation kinetics, whereas for instance Ce0.82Zr0.15Sm0.03O1.99 features stable yields and kinetics. The suggested rationale behind is found in a vacancy depleted region that occurs in the grains of Ce0.85Zr0.15O2. While cycling, the specific surface decreases and the impact of these regions on the reaction rate increases which leads to declining oxidation kinetics. In contrast, Ce0.82Zr0.15Sm0.03O1.99 displays structural vacancies corresponding to the Sm3+, which remain during oxidation. Because of these structural vacancies the oxygen bulk transport is enhanced resulting in a constant reaction rate. Because of the long term cycling, rare earth doping is in particular beneficial for the oxidation kinetics and hence, important for the technical realization of the process. © 2015 American Chemical Society.

Wilbert S.,German Aerospace Center | Reinhardt B.,German Aerospace Center | DeVore J.,Visidyne, Inc. | Roger M.,Institute of Solar Research | And 3 more authors.
Journal of Solar Energy Engineering, Transactions of the ASME | Year: 2013

Due to forward scattering of direct sunlight in the atmosphere, the circumsolar region closely surrounding the solar disk looks very bright. The radiation coming from this region, the circumsolar radiation, is in large part included in common direct normal irradiance (DNI) measurements, but only partially intercepted by the receivers of focusing collectors. This effect has to be considered in the performance analysis of concentrating collectors in order to avoid overestimation of the intercepted irradiance. At times, the overestimation reaches more than 10% for highly concentrating systems even for sky conditions with relevant DNI above 200 W/m2. The amount of circumsolar radiation varies strongly with time, location and sky conditions. However, no representative sunshape measurements exist for locations that are now of particular interest for concentrating solar power (CSP) or concentrating photovoltaics (CPV). A new sunshape measurement system is developed and analyzed in this study. The system consists of the sun and aureole measurement instrument (SAM), an AERONET sun photometer and postprocessing software. A measurement network is being created with the presented system. The uncertainty of this system is significantly lower than what was obtained with previous devices. In addition, the spectral optical properties of circumsolar radiation are determined. As a result, the necessary information for CSP and CPV systems, and a basis for the development of modeling methods for circumsolar radiation, can now be achieved. Copyright © 2013 by ASME.

Schenk H.,German Aerospace Center | Hirsch T.,German Aerospace Center | Wittmann M.,German Aerospace Center | Wilbert S.,Institute of Solar Research | And 2 more authors.
Energy Procedia | Year: 2015

The area covered by large-scale solar thermal power plants extends out to one up to several square kilometers. During the passage of clouds, the solar fields of these plants get partially shaded while other areas remain irradiated. In the case of line-focus plants these transient effects lead to inhomogeneous outlet temperatures of collector loops, while in point-focus plants the flux density at the receiver is influenced. Nowadays, yield prognoses based on measured irradiation data at one specific site, neglecting the effect of inhomogeneous irradiation. One of the aims of the Turikon project is to provide temporally and spatially resolved maps of direct normal irradiance (DNI). On this account, a distributed irradiance measurement network has been installed at the Plataforma Solar in Spain within the research activities of the Turikon project. The network,which consists of 20 silicon pyranometers and a data acquisition system, is in operation since 2013, delivering distributed values for the global horizontal irradiation with a temporal resolution of one Hertz.In the present work a methodology is presented, that makes use of measurements from the network and additionally a vector of cloud movement, derived from shadow pictures. In that manner, data points can be shifted in the direction of cloud movement in order to obtain a temporally resolved DNI map that overlaps the circumference of the measurement network. In this publication, an example is presented that already shows the good applicability of the methodology. Subsequently, the methodology will be tested and validated with a broader data-base. © 2015 The Authors. Published by Elsevier Ltd.

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