Very high-energy γ-ray observations of the Crab nebula and other potential sources with the GRAAL experiment

The “γ-ray astronomy at Almeria” (GRAAL) experiment uses 63 heliostat-mirrors with a total mirror area of ≈2500 m2 from the CESA-1 field at the “Plataforma Solar de Almeria” to collect Cherenkov light from air showers. The detector is located in a central solar tower and detects photon-induced showers with an energy threshold of 250±110 GeV and an asymptotic effective detection area of about 15 000 m2. A comparison between the results of detailed Monte-Carlo simulations and data is presented. Data sets taken in the period September 1999–September 2000 in the direction of the Crab pulsar, the active galaxy 3C 454.3, the unidentified γ-ray source 3EG J1835+59 and a “pseudosource” were analyzed for high energy γ-ray emission. Evidence for a γ-ray flux from the Crab pulsar with an integral flux of 2.2±0.4 above threshold and a significance of 4.5σ in a total measuring time of 7 h and 10 min on source was found. No evidence for emission from the other sources was found. Some difficulties with the use of heliostat fields for γ-ray astronomy are pointed out. In particular the effect of field-of-view restricted to the central part of a detected air shower on the lateral distribution and timing properties of Cherenkov light are discussed. Upon restriction the spread of the timing front of proton-induced showers sharply decreases and the reconstructed direction becomes biased towards the pointing direction. This is shown to make efficient γ-hadron separation difficult

The GRAAL Project

26th International Cosmic Ray Conference Salt Lake City, Utah August 17-25,1999The GRAAL Project (Gamma Ray Astronomy at ALmeria) makes use of the CESA-1 heliostat field at the “Plataforma Solar de Almeria” (Spain) as a gamma-ray telescope with an energy threshold of about 100 GeV. Cherenkov light generated by EAS is reflected by the heliostats and collected into photomultipliers (PMTs) with nonimaging secondary optics. Each PMT collects the light reflected by 13 - 18 heliostats of 40 m2 using a Winston cone. After successful tests with two collecting cones, a more advanced setup on a dedicated platform, using four collectors and 63 heliostats (total reflecting area of about 2500 m2 ) is being installed. A description of this setup together with Monte Carlo results about its excellent capabilities in the precise determination of pulse arrival times are presented

Medida de alta irradiancia en receptores de plantas de torre

CIES2020 - XVII Congresso Ibérico e XIII Congresso Ibero-americano de Energia SolarRESUMEN: En este trabajo se expone un nuevo método de medida de alta irradiancia solar en centrales de torre. El método hace uso exclusivamente de un radiómetro y una cámara digital. Antes de realizar los ensayos, se ha efectuado un test de homogeneidad y difusividad a la superficie del receptor. Este test se divide en dos partes, la primera en la que se estudia la homogeneidad de la superficie mediante una imagen, y la segunda en la cual se realiza un estudio de la luminancia en función de diferentes ángulos. Se han presentado los resultados de ensayos realizados con un receptor de reducido tamaño en uno de los hornos solares de la Plataforma Solar de Almería, y la comparación con el método del blanco lambertiano. Los resultados muestran un alto grado de concordancia entre ambos métodos. Por último, se ha efectuado el test de difusividad y se han presentado los resultados para un nuevo prototipo de receptor volumétrico de mayor tamaño, fabricado con copas de carburo de silicio, obteniendo que su superficie se comporta como una superficie lambertiana.ABSTRACT: In this work, a new method to measure high solar irradiance in tower plants is presented. The method uses only a radiometer and a digital camera. Before carrying out the tests, a homogeneity and diffusivity test has been carried out on the surface of the receiver. This test is divided in two parts, the first one where the homogeneity of the surface is studied with a picture, and the second one where a study of the luminance is carried out according to different angles. The results of the tests that has been done with a small receiver in one of the solar furnaces of the Plataforma Solar de Almería, and the comparison with the white lambertian method, validate the new method. The results show a high degree of agreement between both methods. Finally, the diffusivity test has been done with a new volumetric receiver prototype, with bigger size, and the results show that the surface of the receiver behaves as a lambertian surface.info:eu-repo/semantics/publishedVersio

Metodologías para la determinación de la extinción atmosférica de la radiación solar para distintos usos y casos de aplicación

CIES2020 - XVII Congresso Ibérico e XIII Congresso Ibero-americano de Energia SolarRESUMEN: La extinción atmosférica de la radiación solar es un fenómeno de gran importancia en las plantas solares de concentración de torre. Es necesario medir y controlar este parámetro para el correcto funcionamiento y rendimiento de este tipo de plantas. En este trabajo se desarrollan metodologías para determinar la extinción para distintos usos y casos de aplicación. Por un lado, es necesario conocer los niveles de extinción medios, máximos y mínimos anuales en la búsqueda de localizaciones óptimas para nuevas plantas. Para ello se ha desarrollado la metodología AOT que determina los niveles de extinción característicos de cualquier emplazamiento. Por otro lado, es necesario monitorizar los niveles de extinción en tiempo real para la operación diaria de una planta, junto con el resto de parámetros meteorológicos críticos. Con ese fin se ha desarrollado el sistema de medida CIEMAT que mide la extinción instantánea heliostato-torre con un error del 2% absoluto. Las metodologías han sido validadas en la PSA y pueden ser aplicadas en cualquier localización de interés para la energía solar de concentración.ABSTRACT: The atmospheric extinction of solar radiation is a phenomenon of great importance in concentrating solar tower plants. It is necessary to measure and control this parameter for the correct operation and performance of this type of plants. In this work, some methodologies are developed to determine the extinction for different uses and cases of application. On the one hand, it is necessary to know the average, maximum and minimum annual extinction levels in the search for optimal locations for new plants. For this purpose, the AOT methodology has been developed to determine the characteristic extinction levels of any site. On the other hand, it is necessary to monitor the extinction levels in real time for the daily operation of a plant, together with the rest of the critical meteorological parameters. For this purpose, the CIEMAT measurement system has been developed, which measures instantaneous heliostat-tower extinction with an error of 2% absolute. The methodologies have been validated at the PSA and can be applied at any location of interest for CSP.info:eu-repo/semantics/publishedVersio

Estimation of visibility from spectral irradiance using artificial neural networks

Visibility has become a key input to determine the transmission losses of solar radiation propagating between heliostats and the receiver of solar tower power (STP) plants. Recent studies suggest that haze can reduce visibility and increase these losses up to 25% compared to clear conditions. Monitoring visibility would thus be needed for proper design and operation of STPs, but this is usually not done at all potential sites. In this work, the dependence of visibility's magnitude on relative humidity (RH) and aerosol optical depth (AOD) at three different wavelengths is analyzed. To that effect, 1-min observations from a visibilimeter located in Huelva (southwestern Spain) are analyzed during the winter season. RH is linearly correlated with visibility and explains 46% of its variability. A complex non-linear relationship between visibility and AOD is observed with also dependence on RH. Artificial neural networks (ANN) are thus investigated here for mapping the complex and non-linear relationships between visibility, RH and AOD at multiple wavelengths. This improves results significantly, increasing the explained visibility variability up to 68% and reducing RMSD from 30% to 22% with almost zero bias. The ANN analysis shows that the visibility-AOD relationship is not sensitive to the specific wavelength at which AOD is measured. These findings show that, using ANN, visibility can be estimated from local observations of RH and AOD at only a single wavelength.The authors are grateful for the financial support provided by Spanish Project PRESOL (Forecast of Solar Radiation at the Receiver of a Solar Power Tower) with references ENE2014-59454-C3-2-R, ENE2014-59454-C3-1R1 and ENE2014-59454-C3-3-R which is funded by the Ministerio de Economıa y Competitividad and co-financed by the European Regional Development Fund (FEDER). The authors thank INTA-ESAt team for their assistance and for allowing the equipment to be installed in their facilities. The authors also thank the principal investigators of the El Arenosillo AERONET site: Dr. Victoria E. Cachorro and Dr. Margarita Yela.Departamento de Ciencias Agroforestale