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A deep-black Cu, Mn, Fe- pigment with a spinel structure was employed to coat standard proppants in order to improve long term solar absorptance. The coating process was performed by high-energy, high-speed mixing of proppants and small quantities of spinel powders in a resonant acoustic mixer. A continuous powder coating is achieved by electrostatic attraction between the proppant surface and the coating particles. Consolidation and strong attachment of the coating is achieved by the subsequent sintering beyond the spinel melting temperature. Chemical reaction and bonding between spinel coating and proppant lead to the incorporation of Al, Mg and Ti into the spinel structure. Coated bauxite proppants exhibit a significantly improved, long-term stable solar absorption accompanied by a promising abrasion resistance. The presented coating methodology is considered to be scalable to industrial production.

Minerals 12(2), 121 (2022). doi:10.3390/min12020121, Published by MDPI, Basel

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2018; Aachen 1 Online-Ressource (xii, 181 Seiten) : Illustrationen (2018). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2018, Two linear focus solar simulator test benches were built for the calorimetric measurement of the optical efficiency of parabolic trough receivers. The test benches, called ElliRec and OptiRec, consist of mirrors of elliptical cylinder geometry, flat end mirrors, and metal-Halide lamps. Water at ambient temperature is directed on a spiralling path at the inner surface of the absorber. Absorbed power of the receiver is calculated from temperature increase and flow rate of the water and amounts to 6.5 kW or 4.8 kW and is compared to that by a reference to yield the relative optical efficiency. Expanded uncertainty at k = 2 of optical efficiency in the solar simulator of a receiver relative to that of a reference receiver is 0.6 % for the ElliRec and 0.34 % for the OptiRec; expanded uncertainty at k = 2 of the absolute power measurement is 1.2 % for ElliRec and 1.6 % for OptiRec. Several aspects of the transfer of laboratory measurement to the application in the field are investigated. Assuming idealized spectra for absorber and glass systematic deviations of optical efficiency between solar simulator and field of typically up to 4 % were calculated. For comparison of relative measurements, as typically performed, deviations between solar simulator and field are < 0.2 % for measured spectra for both test benches. An experimental comparison of the ElliRec solar simulator with two spectrophotometric test benches showed a disagreement of < 0.6 % standard deviation for five out of six compared receivers. Differences between the ElliRec and OptiRec for seven receivers were measured to < 1.1 %. A 1-d model is used to calculate changes from room temperature to operating temperature yielding corrections for the hot receiver of up to 0.7 % depending on bellow design. Detailed 3-d-ray-tracing models yield a spatial non-uniformity in longitudinal direction calculated for - 2.03 m to + 2.03 m is 1.6 % for ElliRec and 2.3 % for OptiRec. Neglecting source volume and slope deviations of the mirror it is shown, that in contrast to the field, in the solar simulator angle and position of incidence on the absorber are linked. The investigation of the impact of focus deviation based on the Fresnel Equations shows a decreasing effective absorptance with increasing collector angle of incidence and mean focus deviation. For example, assuming Gaussian radiation distribution of focus deviation of half of the absorber radius and a collector angle of incidence of 0° results in an additional loss in effective absorptance of 1 % due to reduced absorptance. The optical efficiency measurement in the linear focus solar simulator is a fast measurement with high repeatability. The influence of the effects in transfer to the field are small enough to consider the measurement a meaningful assessment of the quality of a receiver and to resolve differences between products. Due to the simple optical geometry the method has the potential to be easily reproduced by other laboratories., Published by Aachen

Für leistungsfähige solarthermische Kraftwerke sind Entwickler, Komponentenhersteller sowie Erbauer und Betreiber immer stärker auf eine objektive Vergleichbarkeit der Leistungsfähigkeit von Komponenten, aber auch auf Abnahmeprozeduren für ganze Solarfelder angewiesen. Zu Beginn des Projektes existierten noch nicht die benötigten einheitlichen Prüf- und Abnahmes-tandards, um die Qualität der CSP-spezifischen Bauteile vergleichend zu beurteilen, die Leis-tungsfähigkeit in ein Verhältnis zu den Angebotspreisen zu setzen und damit eine nachhaltige Kostensenkung zu erreichen. Diese Lücke zu schließen führt unmittelbar zu mehr Transparenz und Konkurrenz, verbesserter Bauteilqualität und Kraftwerkseffizienz und somit kurzfristig zu geringeren Stromgestehungskosten. Weitere Innovationen werden zielgerichteter und neue Komponenten können schneller in den Markt eingeführt werden.

Parabolrinnenreceiver zur Umwandlung von konzentrierter Solarstrahlung in Nutzwärme sind Schlüsselkomponenten solarthermischer Kraftwerke. Zur Charakterisierung ihrer typischen thermischen und optischen Eigenschaften wurden im DLR zahlreiche Labor und Feldmessmethoden und Simulationswerkzeuge entwickelt. Anhand einer Ringversuchs.Kampagne mit einem ausgewählten Satz von Receivern werden diese nun erstmals systematisch verglichen.l

The perfomance of a parabolic torugh reciever is best described by optical efficiency and heat loss power. Their constancy over the lifetime are tested with accelerated aging at DLR. Tests are performed on entire receivers and on small glass samples, where aging of the AR-coating is investigated. Before and after the aging the samples are tested for performance

Das Projekt Pareso hat zum Ziel, die thermische Qualität von in einem Solarfeld eingebauten Parabolrinnen-Receivern mit hoher Genauigkeit zu bestimmen

Wärmeverluste, welche an Receiverneines solarthermischen Parabolrinnen-Kraftwerks auftreten, haben signifikanten Einfluss auf den Kraftwersertrag. Bislang konnten Receiver nur in ausgebautem Zustand im Labor vermessen werden. Das Ziel des Projekts ist, die Wärmeverluste von in einem Solarfeld eingebauten Receivern mit hoher Genauigkeit quantitativ bestimmen zu können. Hierzu ist ein Messverfahren entwickelt worden, dass auf transienter Infrarot-Thermographie beruht. Mit dem entwickelten Verfahren können thermische Verluste im Feld quantitative mit einer Unischerheit kleiner als als 20 W/m im eingebauten Zustand bestimmt werden. Zusätzlich können die Verlustmechanismen (d.h. die Qualität der selektiven Beschichtung und des Ringspaltvakuums) eingegrenzt werden. Das Prinzip des Messverfahrens ist im Labor und im Feld validiert. Das Verfahren ergänzt durch seine Genauigkeit und Detailtiefe komplementär qualitative Screening-Methoden mit Infrarotkameras.

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.

The performance of a parabolic trough receiver can be described by two parameters, the specific heat loss at operating temperature and the optical efficiency. DLR operates two test benches for the measurement of these parameters with high accuracy in its QUARZ Center. In the heat loss test bench THERMOREC the receiver is electrically heated to operating temperatures. In a steady state of constant heating power and absorber temperature an energy balance equation yields the thermal losses. Recent work was dedicated to the introduction of end heaters which lead to a significantly more even absorber temperature profile. Furthermore brass is now used for the homogenization tube with far reduced oxidation at the tube surface compared to the formerly used copper. In the ELLIREC, the linear focus test bench for the measurement of optical efficiency, the receiver is irradiated with solar simulator lamps. The enthalpy increase of cooling water flowing through the receiver at ambient temperature indicates the optical efficiency. The test bench has been characterized for reproducibility to 1 σ = 0.25% and homogeneity of the incident radiation in the linear focus to ± 5 % of the average. By comparison of the enthalpy increase to that of the defined reference receiver DLR 70-1 the relative optical efficiency is determined.

Vorgestellt wird eine auf transiente Thermografie basierende Feldmesstechnik zur Bestimmung der Wärmeverluste von Parabolrinnen-Receivern. Durch thermische Anregung des innen liegenden Absorberrohres und Messung von Absorberrohr-, Glashüllrohr- und Umgebungstemperaturen lässt sich unter Verwendung eines numerischen Receivermodells auf die Wärmeverluste schließen. Zusätzlich ableitbar sind Emissivität der selektiven Beschichtung und die Güte des Vakuums, so dass Aussagen über einen potenziellen Schadensmechanismus ermöglicht werden. Die transiente Methode ergänzt bestehende stationäre Messtechniken. Sie ist, verglichen mit einer reinen Messung der Glashüllrohrtemperatur, aufwändiger in der Durchführung, bietet aber eine kleinere Messunsicherheit und einen höheren Informationsgehalt.

In order to evaluate the quality and suitability of reflector materials for concentrating solar power (CSP) applications a measuring procedure has been developed for the determination of optical key values. These were defined for their relevance in CSP. After measuring those parameters also on artificially degraded samples, an extensive analysis of the optical material properties and comparative lifetime estimation are provided with the intention to lead to a standardized test method. The high relevance of spectral properties as well as high mechanical resistance is emphasized.

In order to evaluate the quality and suitability of reflector materials for concentrating solar power (CSP) applications a measuring procedure has been developed for the determination of optical key values. These were defined for their relevance in CSP as the solar weighted hemispherical reflectance SWH, the solar weighted direct reflectance SWD within an acceptance angle of interest (25 mrad) and the specular beam diversion spec. After measuring those parameters also on artificially degraded samples, an extensive analysis of the optical material properties and comparative lifetime estimation are provided with the intention to lead to a standardized test method. Evaluation of various reflector material samples demonstrates the superiority of glass mirrors over alternative materials (polymer film, aluminium) in all concerned respects with hemispherical and direct reflectances of almost 0.94 and minimal losses after degradation. The high relevance of spectral properties as well as high mechanical resistance is emphasized.

Eine wichtige Komponente in der konzentrierenden Solartechnologie ist der Kollektorspiegel. Neben Formstabilität und Nachführungs-Genauigkeit muss das Spiegelmaterial selbst gute Reflektivitäts-Eigenschaften und Witterungs-Beständigkeit besitzen. Mit der Markteinführung solarthermischer Kraftwerke besteht nun ein dringender Bedarf für genormte Qualitätsgarantien und Zertifizierungen der Materialien. Für die Qualifizierung der optischen Spiegelmaterial-Eigenschaften werden die signifikanten Parameter definiert und die geeigneten Instrumente und Messverfahren zur Bestimmung dieser vorgestellt.

Two new test benches have been developed for the combined measurement of the thermal properties and overall optical efficiencies of receivers for the use in parabolic trough collectors. In the first test bench two receivers are simultaneously irradiated by natural sunlight via parabolic troughs that are mounted on a two axis tracking stage. Shutters enable the variation of the aperture for quasi constant solar input power over the test period. The second, the solar simulator test bench consists of an elliptical cylinder with flat end reflectors. Metal halide lamps (HMI) and the receiver are positioned in the two respective focal lines. The test benches are operated in two modes: Cold performance tests with water flow through the receiver at near ambient temperature in order to measure the optical efficiency in the stationary enthalpy increase, secondly hot testing with the empty receiver at typical operating temperature in order to compare irradiant power and steady state thermal loss power and calculate the efficiency. The test benches are operated in combination with classic heat loss measurement, together the three test benches form a complementary set. First comparative measurements with cold testing on the solar simulator test bench show high reproducibility and a measurement precision relevant for qualification and comparison of different receiver types at industrial and research level.