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Oberpfaffenhofen

Hourly global and direct normal irradiance for Europe, Africa and Asia

Solar Energy Mining (SOLEMI) is a service providing irradiance data. It is mainly based on Meteosat-data with a nominal spatial resolution of 2.5 km and half-hourly temporal resolution. From 2004 the service uses also Meteosat Second Generation imagery with a nominal spatial resolution of 3 km and 15 minutes time resolution.

Fig. 1: Meteosat 7 located at 0° longitude provides Europe and Africa, copyright © 2003 EUMETSAT Fig. 2: Meteosat 5 located at 63°E provides western Asia, copyright © 2003 EUMETSAT

Solar radiation maps and hourly time series are available on request for:

  • global horizontal (GHI) and direct at normal incidence (DNI)
  • temporal coverage: 1991 - today (Europe and Africa), 1999-2006 (Asia)
  • temporal resolution: 1 hour
  • spatial resolution: 2.5 km at sub-satellite
  • spatial coverage: Europe, Africa, Western Asia, parts of Australia and South America

Product Example:

Fig. 3: Monthly sum of the direct normal irradiance (DNI) for August 2000 at the Arabian Peninsula
Fig. 4: Monthly sum of the direct normal irradiance (DNI) for March 2003 in Spain and Northern Africa
Fig. 5: Example of a direct normal irradiance (DNI) time series in hourly resolution for the Gran Canaria Island in the period 1. - 10. January 2003

Methodology:


The transfer of the extraterrestrial solar irradiance to the Earth's surface is influenced by various constituents of the atmosphere. Ozone, aerosol and water vapour are modelled with long time climatological and reanalysis data sets. Clouds which have the largest influence and highest variability are determined from the half hourly Meteosat images. Both visible and infrared channels are used to improve the detection at sunrise and sunset and of high cirrus clouds.

The Heliosat method was originally proposed by Cano et al. (1986) and later modified by Beyer et al. (1996) and Hammer (2000). The basic idea of the Heliosat method is a two-step approach. In the first step, a relative normalised cloud reflectivity - the cloud index - is derived from METEOSAT images. The derived cloud index is correlated to the clear-sky index k, which relates the actual ground irradiance G to the irradiance of the cloud-free case Gclear sky.

Consequently, in addition to the cloud index derived from the satellite signal, a clear-sky model, providing Gclear sky, is necessary for the estimation of the actual ground irradiance. This clear sky model includes knowledge on the sun geometry and atmospheric turbidity to describe aerosol and water extinction. The n-k relation is powerful, validated, and leads to small root mean square deviation (RMSD) between measured and calculated solar irradiance for almost homogenous cloud situations (RRMSD of 13-15% for hourly global irradiance values; Hammer, 2000).

Validation:

The ENVISOLAR science review (A. Zelenka, 2004) states, that the RMSE observed is mainly due to misfit between ground measurement (point) and satellite measurement (pixel average). For global irradiances it is known from variogram analysis, that the accuracy of satellite data is better if the distance between a ground station and the area of interest is larger than 20 to 50 km. For direct irradiance a similar analysis can not be done due to the low density of available ground measurement stations.

There has been a variety of validation studies for the Heliosat algorithm. A synopsis of validation results on different time scales for averaging values is given in table 1.

 
  hourly values
(RMSE)
daily values
(RMSE)
monthly values
(RMSE)
global irradiance up to 15% ~ 10% ~ 5%
direct irradiance ~ 35% ~ 20% ~ 15%
       
Table 1: Relative root mean square error for global and direct irradiance on different averaging time scales (synopsis of several validation studies as e.g. in International Energy Agency benchmarking)

Users and applications using the SOLEMI database:

SOLEMI is aimed at decision makers of solar energy projects and atmospheric researchers, which need solar irradiance data for designing solar energy systems.

Site audit
A site audit is an analysis of the suitability of a single location for solar applications. For this purpose a long-term hourly irradiance time series is extracted from the SOLEMI database for a site specified by its geographical co-ordinates. The site audit results in a quality-proven time-series supplied in digital form and an individual report, which describes the specific solar radiation characteristics of the site and gives visual displays of the gathered information.

Solar irradiance atlas
For mapping of a certain regions we supply solar irradiance atlases. Usually this covers whole countries or large districts of several thousand km2. The results are digital maps suitable for the use in Geographical Information Systems (GIS). The maps display annual sums of the solar irradiance. There will be a map for each analysed year and a map with the average of the analysed years. In its basic form the atlas will be done only for one component of the solar irradiance: Direct Normal Irradiance (DNI) or Global Horizontal Irradiance (GHI). The second component is an additional option. Another additional option is the production of monthly maps. The atlas consists of digital maps and a report describing the method, the quality of the data and the interpretation of the results.
Also as an additional option, the data can be prepared for internet presentation with basic GIS functionality.

Pre-Assessment Study
The pre-assessment study shall give a first insight into the characteristics a specified region. It is based on a irradiance map for one year and a site exclusion map, which takes out sites or pixels unsuitable for solar thermal power generation. The exclusion criteria are land use, geomorphology, land slope, protected areas, water bodies, forests, etc.

Solar resource studies Solar resource studies additionally look at the technical and economical potential of different solar electricity generation technologies. This will be based on additional geographical and country specific data.

Data access:

Data are processed on request (see contact section). Available products are:

  • Digital GIS maps
  • - Digital maps of annual or monthly sums or averages
    - Digital maps of annual or monthly maximum and minimum irradiances
    - Digital maps of irradiance variability

  • Digital maps
  • - Colored jpeg images

  • Time series in ASCII format
  • - Data of solar irradiance in hourly, daily and monthly resolution
    - Additional data on temperature, wind speed, humidity from reanalysis data

Contact:

The SOLEMI database is a joint effort of three units at DLR:

  • DLR - Institute of Technical Thermodynamics
  • DLR - Institute of Atmospheric Physics (PA)
  • DLR - German Remote Sensing Data Center (DFD)

Contact: wdc@dlr.de

Publications:

Beyer, H., Costanzo, C., Heinemann, D. (1996). Modifications of the HELIOSAT procedure for irradiance estimates from satellite images. Solar Energy, 56, 207- 21

Cano, D., Monget, J., Albuisson, M., Guillard, H., Regas, N., & Wald, L.(1986). A method for the determination of the global solar radiation from meteorological satellite data. Solar Energy, 37, 31-39

Hammer, A., 2000, Anwendungsspezifische Solarstrahlungsinformation aus METEOSAT-Daten, Dissertation, Universität Oldenburg, http://oops.ibit.uni-oldenburg.de/volltexte/incoming/2001/347/

Meyer, Richard und Lohmann, Sina und Schillings, Christoph und Hoyer, Carsten, 2006, Climate Statistics for Planning and Siting of Solar Energy Systems: Long-Term Variability of Solar Radiation Derived from Satellite Data. In: Nova Science Publishers Earthlink, pages 55-68. ISBN 1-59454-919-2

Zelenka, A., 2004, ENVISOLAR Project - Environmental Information for Solar Energy Industries, project report no. 6 ‚Science Review'