Quality of HelioClim DNI values
Dear Customers, this page of information is an old composite information between HelioClim1 (2001 - 2005) and HelioClim3 DNI (2006) data. We remind that the highest temporal resolution of the HelioClim1 database is the day. The data are only over the horizontal plane. Sub-daily DNI values are obtained using profiles, and the yearly DNI values result from the agregation of these data. Our current database HelioClim3 offers a higher temporal resolution which enables a more accurate DNI estimation.
Unfortunately, only a few recent ground stations measure the DNI components. Moreover, these data are most of the time under a non disclosure agreement. Indeed, we generally access these data through our Customers who charge us to calibrate the HelioClim3 data using their own ground station data, and ask for confidentiality for the commited data. We hope being able soon disseminating more statistical information concerning DNI HelioClim3 values soon.
Methodology
For assessing the quality of the retrieval of the annual sum of DNI, we have assembled series of measurements of DNI for several sites of the BSRN network. BSRN (Baseline surface radiation network) is the reference in measuring solar radiation. It has several sites in the world.
Our first task was to compute annual DNI from the time-series of observations. It was not an easy task because time-series are incomplete. It may happen that a large portion of a month is missing. The partial lack of data has a strong impact on the quality of the annual DNI. We made several trials that show that the relative error in assessing the assual DNI from ground observations amounts to 10 %.
Statistics and Graphs
Taken this into account, we have compared the annual DNI from the ground measurements and that delivered by the SoDa Service exploiting the HelioClim databases for three stations: Tamanrasset in Algeria, Odeillo in France and Payerne in Switzerland.
The site of Tamarasset exhibits large values in irradiation. The period of comparison is 2001 to 2006. The data of Odeillo cover the period [2001; 2005]. Payerne was selected because this is where are located the headquarters of the BSRN and that the quality of the measurements should be the best. The annual DNI is low in this site, and it covers the same period as Odeillo.
NB: The site Sede Boqer in Israel would also offer a nice opportunity to check the quality of the HelioClim assessments but it suffers from long missing gaps in data that render the computation of the annual DNI inaccurate.
| Lat (°) | Lon (°) | Height (m) | Mean DNI (kWh/m²) | Bias (kWh/m²) | RMSD (kWh/m²) | |
| Tamanrasset | 22.783 | 5.514 | 1385 | 2 634 | 291 (11%) | 302 (11%) |
| Odeillo | 42.48 | 2.12 | 1590 | 1 906 | 6 (0%) | 47 (2%) |
| Payerne | 46.8167 | 6.95 | 491 | 1 309 | -103 (-8%) | 184 (14%) |
Last update: 2/23/12 5:20 PM