Solar Energy Perspectives - IEA

Chapter 2: The solar resource and its possible uses
shadow band pyranometer is used to determine the direct normal irradiance without
using a pyrheliometer, by measuring the difference between global and diffuse
horizontal irradiances and the sun’s elevation angle.
A traditional measurement is also often reported in meteorological observations. This
is the "duration of sunshine." The standard instrument used to measure this parameter
is the Campbell-Stokes sunshine recorder, which consists of a glass sphere that
focuses the direct solar radiation and burns a trace on a special pasteboard card.
These recorders have been replaced in most installations by photo detector activated
“sunshine switches”. The data produced by these instruments are of minimal use to
engineers because there is no measure of intensity other than threshold intensity.
However, attempts have been made to correlate these data with daily or monthly
solar radiation levels.
Periodic ground observations of cloud cover also provide useful information. These
are made at least hourly at weather observation stations around the world. Cloudcover
data along with other weather data have been used to predict solar irradiance
levels for locations without solar irradiance measurement capabilities.
Meteorological satellites in space can help fill in the resource knowledge gaps. The
considerable advantage they offer is a complete coverage of the inhabited regions of the
world, as well as the time depth for those that have been in service for years. Ground
stations are scarce and cannot rival the resolution of satellites, often of 10-km scale.
Interpolations can be simply wrong as weather patterns change on relatively small scales.
However, the information delivered by satellites is not reliable enough, especially with
respect to DNI, which is the relevant information for any concentrating system. Existing
geographic information systems focused on solar and other renewable data show
differences in estimates. For example, eight satellite data sets from different providers for
one specific location in Spain were analysed and compared to ground measurements.
DNI values ranged from 1 800 kWh/m 2 /y up to 2 400 kWh/m 2 /y (Figure 2.12). Other
examples, easily found, for example, on the Solar and Wind Energy Resource Assessment
(SWERA) website, would reveal even larger differences. Similar tests on GHI data – the
relevant information for non-concentrating devices – would show significantly smaller
discrepancies.
Therefore, in many cases ground measurements are critically necessary for a reliable
assessment of the solar energy possibilities of sites, especially if the technology envisioned
depends on concentrating the sun’s rays. Nevertheless, satellite data can be used in this
case to complement short ground measurement periods of one or two years with a longer
term perspective. Ten years is the minimum necessary to have a real perspective on annual
variability, and to get a sense of the actual average potential and the possible natural
deviations from year to year. Satellite data should be used only when they have been
benchmarked by ground measurements, at least as far as DNI and concentrating devices
are concerned.
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© OECD/IEA, 2011