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Reported Problems and Solutions in HelioClim |
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- The Snow
- The Glitter => solved!
- The Effect of the Low Resolution of Heights on irradiation maps => solved!
The Snow
 click on the image to magnify |
The HelioSat-2 method used to update the HelioClim3 database has the default of not being able to distinguish the snow from the clouds. Reminder of the HelioSat-2 principle: each HC3 radiation pixel value is computed using the "proportion of cloud" or "proportion of white" observed in this pixel in a combination of two channels located in the visible part of the spectra. This "proportion" is computed as an attenuation coefficient of the clear sky value. When the snow covers the ground and there is a sunny day, HelioSat-2 is though confusing the sunny weather with a large cloud coverage. The HelioClim3 radiation is thus drastically underestimated in snowy areas. As a consequence, the electrical production of a solar power plant covered with the snow and the corresponding HelioClim3 values are coherent. But the counterpart is that you will not be able to rely on the HelioClim3 to know if the panels need to be cleaned of from snow. The left illustration is a product from the National Snow and Ice Data Center (NSIDC). It represents the snow cover over Europe for the day of January 24th 2011. This image is released every day, but the availability or timely delivery is not garanteed. A first option for our Customers would be to get at least one image per day on this website, as a kind of warning that the HelioClim data could be drastically underestimated. Reference of the illustration: National Ice Center. 2008, updated daily. IMS daily Northern Hemisphere snow and ice analysis at 4 km and 24 km resolution. Boulder, CO: National Snow and Ice Data Center. Digital media. |
TESTIMONIES
T.1_ I contemplate purchasing an annual subscription to one of our services. However, I compared monthly irradiation values over an inclined (48°) plane from PVGIS (Classic PVGIS or Climate-SAF PVGIS) and from SoDa (HelioClim-3) for the year 2005 for a site in southern Germany (48°, 12°), the irradiation values are strongly lower in your data (half of what is retrieved in PVGIS whatever the database). What is the origin of this discrepancy? Alexey Mineev at REN Solar AS, Norway
A. The PVGIS irradiation values are monthly irradiation values averaged over the years available: over Europe, from 1981 to 1990 for Classic-PVGIS and from 1985 to 2005 and from June 2006 to May 2010 for Climate-CMSAF PVGIS. The data he used is from a specific year 2005 in the database HC3, thus the comparison between averaged values and not-averaged ones should differ but be of the same magnitude. However the values were (rounded):
- monthly HC3 (2005, in kWh/m2): 21, 21, 73, 135, 156, 164, 147, 131, 124, 100, 20, 11
- Climate-CMSAF PVGIS (average): 42, 65, 114, 152, 153, 161, 156, 149, 120, 88, 49, 37
The values are close for almost half a year, but the values are strongly underestimated in HelioClim-3 during the winter period. After a verification, in 2005 the snow cover has been reported by Meteorological Ground stations which confirmed our idea. The answer of this Customer was: The difference during the winter time looks discouraging to us and this is the area of our further interest to look into.
T.2_ I am a student of renewable energies and currently I write my bachelor thesis. In the bachelor thesis I evaluate yield prognosis for pv systems and compare predicted yield values with those measured during the operation of the pv systems. For this purpose I need irradiation data for the last years. Would your data suit my needs? J. W., Berlin
A.The document in pdf format accessible here describes exactly the project of this student. More precisely, the influence of snow shading of pv systems is often underestimated. His purpose was to investigate the yield losses through snow shading of pv systems. After the explanations relative to the limits of the HelioSat-2 model, here was my response: As a consequence, for you to be able to carry out your research in good conditions and even if it means that we lose a potential customer, I do not recommend you to use the HelioClim data in order to determine the yield losses through snow shading.
The Glitter => solved!
Date of this correction: March 2011. Definition: a glitter is defined as a sparkling or glistening light, or as well as small pieces of light-reflecting decorative material. We define the "glitter" as the reflexion of the solar radiation of the ocean. To avoid the bright halo over the earth, all the pixels of HC3 corresponding to the following regions have been set to 0:- longitude: [-66°; +35°] - latitude: [-35°; -16°] and [+16°; +35°] The consequence is that if you visualize a raw image of radiation values, this appears under the form of a black window, in which only the clouds are different from "0". That is why the clouds appear in gray in the left illustration. The period when this artefact is visible is from January 2009 until February 2011 and will not be corrected immediately. This bug has been corrected to offer a full cover of irradiation values and should be available from April 2011.
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 a_ Before correction |
 b_ After correction |
The Effect of the Low Resolution of Heights on irradiation maps => solved!
Date of this correction: end of March 2011 illustration a: Yearly average irradiation map of GHI over Togo with TB5 as default heightillustration b: Yearly average irradiation map of GHI over Togo with SRTM as default height Since October 2009, we provide a service of map creation to our Customers. This service is on request only. Before the date of correction, large squarres of tints areas might appear on the irradiation maps we provided, such as on the illustration a, which does not appear anymore after the correction of this bug. More information on this correction |
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Last update: 2/23/12 5:20 PM