Synthetic hourly irradiance |
  
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Synthetic hourly irradiance |
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Global Horizontal
·Version
5:
For the irradiance, the generation of GHI hourly values from
monthly averages is performed by using stochastic models, due to
the
Collares-Pereira team in the 1980's. This model
generates a sequence of days, and then a sequence of hours in the
day, using Markov transition matrices. These matrices have been
established in order to produce an hourly sequence, with
distributions and statistical properties analogous to real hourly
meteo data measured on more than 30 sites all around the world.
·Version 6
:
From version 6, the generation of GHI hourly values from monthly
averages is performed by the Meteonorm 7.2 algorithm
(included in PVsyst (7.1 up to PVsyst 6.73)). The algorithm is an
improvement of the original method, it adapts the Markov matrices
to take into account local conditions such as the turbidity, it
also uses modern knowledge about clear sky models. The generation
of hourly value from daily values is based on the
Collares-Pereira algorithmv (this model consists of two
parts: the first part calculates an average daily profile, the
second part simulates the intermittent hourly variations).
From PVSyst
6.60, the randomizer may be initialised through the
first random
seed parameter in the
synthetic hourly data generation section of the
software. 10 different first random numbers of the generation
algorithm of hourly radiation can be chosen. By changing this
number, different time series of all meteorological parameters are
generated due to a different initialization of the stochastic
process. The monthly means remain the same. When performing
simulations of grid-connected systems, this may produce variations
of 0.5 to 1% in the yearly result.
Diffuse Horizontal
·Version 5:
When the beam
horizontal or the normal beam is available in the original data,
they are used to calculate the diffuse part. Otherwise, the program
uses the
Erbs correlation. At the end of each month, the diffuse
values are renormalized in
order to match the specified monthly diffuse (scaling of the Kt
value): this was not the case in the version 4, and in middle
Europe the Erbs correlation had a tendancy to over-estimate the
diffuse. Now in PVSyst when the
monthly diffuse is specified, with usually lower values
compared to the Erbs one, this increases the beam component and
therefore the transposition results. This renormalization procedure
can be skipped but it is not recommended to do so.
·Version
6:
For Synthetic generation, the Meteonorm 7.2 algorithm
uses the
DirInt model to separate global into diffuse and beam.
The
DirInt global to direct model is based on a
quasi-physical model, the
DISC model, which has the form of a clear sky irradiance
based on a Linke turbidity factor TL equal to 2.2, attenuated by a
function of the clearness index Kt. This beam component is then
corrected by a function of the modified clearness index Kt as
defined in Perez et al. (Perez
1990), the solar zenith angle, and a stability index
that accounts for the dynamics of the time series. The
corresponding coefficients are obtained from a four-dimensional
lookup table consisting of a 6 6 5 7 matrix. The
DirInt model was developed with a hypothesis of an
average Linke turbidity factor of TL = 3. For an evaluation of the
DirInt model, please download
Ineichen 2008.
At the end of each month, the diffuse values are renormalized in
order to match the monthly diffuse, if specified. This is done
through a correction which is not Kt dependant: each hourly value
is corrected by the ratio between the specified monthly value and
the as-generated-hourly-cumulated monthly diffuse.
Remember that with hourly measured data,
PVSyst uses the Erbs correlation
to generate diffuse from global, as in version 5.