Linke turbidity factors for several sites in Africa

International audienceThe Linke turbidity factor (TL) has been estimated at sixteen locations in Africa (9 stations in Egypt, 2 in Mozambique and Zimbabwe, 1 in Algeria, Tunisia and Zambia). An appropriate processing of time-series of measurements of daily sums of solar global radiation spanning several years provides mean values of TL for each month with a sufficient accuracy. Though limited to the Mediterranean area, Egypt and sub-tropical Southeastern part, this work greatly extends the knowledge on the clearness of the atmosphere in Africa, an area not much addressed in the literature. The relationship between TL and the climate is evidenced. TL is almost constant throughout the year close to the Mediterranean basin with values around 3.5. Stations located in the sub- tropical Southeastern part exhibit large variations of TL

Chain of algorithms to calculate advanced radiation parameters

International audienceAlgorithms linked to form focused chains of algorithms are needed to fill gaps between often measured or available data resources and the parameters requested by many solar applications. This paper describes the formulation of a chain of algorithms for shortwave radiation. The basic inputs into the chain are monthly mean values of the Linke turbidity factor and global radiation. The outputs of the chain are hourly values of global shortwave radiation on inclined planes. This is achieved via stochastic generation of daily and hourly values of global radiation, splitting the global into beam and diffuse radiation and finally calculating the radiation on inclined planes. The cloudless sky chain is based on an improved version of the clear sky model of Rigollier et al. (2000). The stochastic generation process is based on Aguiar et al. (1988, 1992) but important improvements have been made by cross-linking to the clear sky model. The Perez models (1986, 1990) are used for splitting the radiation into its components and for estimating radiation on tilted planes. Datasets produced with the described chains can be accessed via the prototype of the SoDa project at www.soda-is.org

Study of effective distances for interpolation schemes in meteorology

International audienceThis work explores the possibility of integrating the geographical elements such as orography and presence of water bodies as well as the latitudinal effects into an effective distance when interpolating meteorological fields. This effective distance may then be used in any interpolation methods instead of the standard geodetic distance. Several hundreds of sites are used in Europe to assess the benefits of several effective distances. The meteorological parameters under concern are ten-years averages of monthly means of daily sum of horizontal global irradiation, daily sum of sunshine duration, daily extremes of air temperature, atmospheric pressure and water vapor pressure, and of monthly sums of precipitation. This work demonstrates that taking into account the latitudinal effects in the distance increases the accuracy in interpolation. Such effects have been seldom mentioned in previous publications. The orographic effects may be partly corrected by adding the weighted difference in elevation to the geodetic distance. The proposed effective distance between the point P and each of the measuring sites Xi for all parameters, is found to give better results than the others

Worldwide Linke turbidity information

International audienceThis paper describes the algorithms and data used to construct a worldwide Linke turbidity factor (TL, for an air mass equal to 2)database. Two main steps had to be performed to obtain the information: 1. Assembling estimates of TL and 2. fusing different background layers for the construction of the TL maps. The estimates of TL have two forms: either they stand for specific geographical locations and have a high accuracy, or they are available as gridded data averaged over large areas. Point information was gathered from measured time series of hourly beam and daily global radiation, which were transformed to TL. From publications and networks like AERONET other turbidity quantities were obtained and transformed to TL. The basic gridded data are the maps of daily global irradiation supplied by the NASA-Langley Research Center. The included monthly clear sky irradiations were converted to TL with the same method as for the ground sites. Further gridded information was taken from NOAA pathfinder aerosol data and NASA NVAP. An algorithm was devised to fuse these two types of data and to produce gridded maps in a canonical projection and 5' arc angle cells. These final maps should reproduce the values observed at specific locations. The root mean square error of the interpolation is 0.73 TL units

Fusing ground measurements and satellite-derived products for the construction of climatological maps in atmosphere optics

International audienceClimatological maps (gridded data) of optical parameters of the atmosphere often result from application of numerical models or processing of satellite images. Such maps usually exhibit very large cell sizes, of the order of 1 of arc angle. There is a need to increase their spatial resolution to obtain a cell size closer to the spatial representation obtained by standard meteorological instruments at ground level, i.e. 5' of arc angle. It then permits to fuse ground measurements and gridded data, especially to correct for bias ob- served in gridded data. Taking advantage of the availability of other data sets of relevance to the parameter under concern, though different, a method is proposed for the synthesis of the initial gridded data at a higher resolution by the means of a fusion process. This paper describes the method with an application for the con- struction of worldwide maps of the Linke turbidity factor with cells of 5' in size. This factor characterises the atmospheric optical turbidity under clear skies and is a very important parameter in solar radiation studies

Trends in global radiation between 1950 and 2100

Abstract This analysis is based on long time series of global radiation with a duration of at least 40 years and the forecasts of global radiation till 2100, based on results of IPCC The future changes are relatively small. On an average the global radiation will decrease slightly. However, in the Mediterranean region the trend is positive (+ 2 -3 % till 2100)

First Steps in the Cross-Comparison of Solar Resource Spatial Products in Europe

International audienceYearly sum of global irradiation is compared from six spatial (map) databases: ESRA, PVGIS, Meteonorm, Satel-Light, HelioClim-2, and NASA SSE. This study does not identify the best database, but in a relative cross-comparison it points out to the areas of higher variability of outputs. Two maps are calculated to show an average of the yearly irradiation for horizontal surface together with the standard deviation that illustrates the combined effect of differences between the databases at the regional level. Differences at the local level are analysed on a set of 37 randomly selected points: global irradiation is calculated from subset of databases for southwards inclined (at 34°) and 2-axis tracking surfaces. Differences at the regional level indicate that within 90% of the study area the uncertainty of yearly global irradiation estimates (expressed by standard deviation) does not exceed 7% for horizontal surface, 8.3% for surface inclined at 34°, and 10% for 2-axis tracking surface. Higher differences in the outputs from the studied databases are found in complex climate conditions of mountains, along some coastal zones and in areas where solar radiation modelling cannot rely on sufficient density and quality of input dat

Benchmarking of different approaches to forecast solar irradiance

Ponencia presentada en: 24th European Photovoltaic Solar Energy Conference and Exhibition celebrada del 21-25 de septiembre de 2009 en Hamburgo.Power generation from photovoltaic systems is highly variable due to its dependence on meteorological conditions. An efficient use of this fluctuating energy source requires reliable forecast information for management and operation strategies. Due to the strong increase of solar power generation the prediction of solar yields becomes more and more important. As a consequence, in the last years various research organisations and companies have developed different methods to forecast irradiance as a basis for respective power forecasts. For the end-users of these forecasts it is important that standardized methodology is used when presenting results on the accuracy of a prediction model in order to get a clear idea on the advantages of a specific approach. In this paper we introduce a benchmarking procedure to asses the accuracy of irradiance forecasts and compare different approaches of forecasting. The evaluation shows a strong dependence of the forecast accuracy on the climatic conditions. For Central European stations the relative rmse ranges from 40 % to 60 %, for Spanish stations relative rmse values are in the range of 20 % to 35 %

Exploitation of distributed solar radiation databases through a smart network: the project SoDa

The project SoDa answers the needs of industry and research for information on solar radiation parameters with a satisfactory quality. The methodology is user-driven with a large involvement of users in the project, who will gauge the progresses and achievements. A prototype service will be developed, using Internet technology, that will integrate and efficiently exploit diverse networked information sources to supply value-added information. A multi-disciplinary consortium has been assembled, which gathers companies and researchers with the necessary expertise in solar radiation and information and communication technologies. Customers and potential users are also represented as partners in the consortium via the involvement of commercial private vendors of solar radiation databases and of representatives of large research and development programs. A call is launched to recruit customers to assess the prototype. The project SoDa builds on the expertise gained in previous projects, such as the digital atlases MeteoNorm and European Solar Radiation Atlas, the Web servers Satel-Light and Helioserve, and the Guide of the Chartered Institute of Building Services Engineers of United Kingdom. Access to data and applications will be improved; efforts will be made on interpolation methods and satellite data processing to achieve better quality; emphasis will be put on applications to supply information actually needed by customers, instead of raw data

MESoR - Management and exploitation of solar resource knowledge

CD-ROMKnowledge of the solar resource is essential for the planning and operation of solar energy systems. A number of data bases giving information on solar resources have been developed over the past years. The result is a fragmentation of services each having each own mechanism of access and all are giving different results due to different methods, input data and base years. The project MESoR, co-funded by the European Commission, reduces the associated uncertainty by setting up standard benchmarking rules and measures for comparing the data bases, user guidance to the application of resource data and unifying access to various data bases