
2017

Two themes are considered
this year:

Enhancing the accuracy of the SIMv.1
clear sky solar irradiance model by better accounting for the aerosol
influence on the atmospheric transmittance

Modeling the diffuse fraction

20152016

The relationship
between solar irradiance and sunshine duration was investigated from the
very beginning of solar radiation measurements. Many studies were devoted
to this topic aiming to include the complex influence of clouds on solar irradiation
into equations. The main topic of the seminar was focused on the famous ÅngströmPrescott equation, which was investigated
from different perspectives: (1) physical basis (mathematical derivation,
physical meaning), (2) accuracy of the empirical models and (3) sensitivity
to various astronomical and geographical factors; (4) Intrinsic limitations
and the possibility to find a formula with general applicability.
Paulescu M, Stefu
N, Calinoiu D, Paulescu
E, Pop N, Boata R, Mares O (2016) Ångström – Prescott equation: Physical basis,
empirical models and sensitivity analysis. Renewable and Sustainable
Energy Reviews 62: 495506.
In this paper the ÅngströmPrescott equation is reviewed from three
different perspectives: (1) the physical basis, (2) the accuracy of the
empirical models and (3) the sensitivity to various astronomical and
geographical factors. A mathematical derivation of the ÅngströmPrescott equation is performed, showing the
approximations behind it and the physical meaning of the coefficients. A
number of 33 empirical ÅngströmPrescott
equations of different degrees of complexity and originated from
different location around the world are being analyzed and tested against
data recorded at 59 European stations. No model is ranked as the best,
but the specific situations when a model performs better than others are
discussed. A comparative study on the influence of different parameters
(latitude, altitude, season, local climatology) on the performance of the
ÅngströmPrescott equations is presented. It is
shown that an ÅngströmPrescott equation having
relative sunshine, altitude and the month index as input parameters can
explain roughly 90% of the variability in the data from the entire
database considered in this paper.
Stefu N, Paulescu
M, Blaga R, Calinoiu
D, Pop N, Boata R, Paulescu
E (2016) A theoretical framework for Ångström
equation. Its virtues and liabilities in solar energy estimation. Energy
Conversion and Management, 112, 236245
This study is focused on
the linear relationship between the clear sky index and the relative
sunshine proposed by the pioneering work of Ångström.
A full semiempirical derivation of the equation, highlighting its
virtues and liabilities, is presented. Specific Ångström
– type equations for beam and diffuse solar irradiation were derived
separately. The sum of the two components recovers the traditional form
of the Ångström equation. The physical meaning of the Ångström parameter, as the average of the clouds
transmittance, emerges naturally. The theoretical results on the Ångström equation performance are well supported by
the tests against measured data. Using longterm records of global solar
irradiation and sunshine duration from thirteen European radiometric
stations, the influence of the Ångström
constraint (slope equals one minus intercept) on the accuracy of the
estimates is analyzed. Another focus is on the assessment of the
degradation of the equation calibration. The temporal variability in
cloud transmittance (both longterm trend and fluctuations) is a major
source of uncertainty for Ångström equation
estimates.

2013  2014

The main topic of the
seminar was focused on the evaluation of the aerosol influence on the
solar energy collected at the ground level. Within these studies a
parametric model for solar irradiance components (SIMv.1) was developed.
Also a part of the seminaries was devoted to forecasting solar
irradiance. Significant results:
Calinoiu D, Paulescu
M, Ionel I, Stefu N,
Pop N, Boata R, Pacurar
A, Gravila P, Paulescu
E, TrifTordai G. (2013) Influence of aerosols
pollution on the amount of collectable solar energy. Energy Conversion
and Management 70, 7682.
The
solar energy loss due to the atmospheric pollution with aerosols is
assessed in this paper. For this, a parametric clear sky solar irradiance
model has been built and validated. For applying the model, a set of four
meteorological parameters (surface air pressure, ozone column content,
nitrogen dioxide column content and the Ångström
turbidity coefficient) are required at input.
Eight episodes of
pollution in Timisoara, Romania during 2011 have been identified. The
energy loss in a pollution episode was evaluated as the difference
between the clear sky solar irradiation computed in two situations: (1)
using measured values for all inputs and (2) using measured values for
all inputs excepting the Ångström turbidity
coefficient, for which a climatological reference value has been assumed.
The calculations show that the aerosol pollution can lead to a
significant loss of collectable solar energy, of over 20%. Such very high
values of losses are irregular in time and difficult to anticipate.
However, such a reduction of collected energy should be taken into
account when calculating the solar resource for sizing or operating a photovoltaic
system. A practical simple equation which connects the collectable energy
losses due to aerosols pollution to the Ångström
turbidity coefficient has been established.
Calinoiu D, Stefu N, Paulescu M, TrifTordai G, Mares O, Paulescu
E, Boata R, Pop N, Pacurar
A. (2014) Evaluation of errors made in solar irradiance estimation due to
averaging the Angstrom turbidity coefficient. Atmospheric Research, 150.
6978.
Even though the
monitoring of solar radiation experienced a vast progress in the recent years
both in terms of expanding the measurement networks and increasing the
data quality, the number of stations is still too small to achieve
accurate global coverage. Alternatively, various models for estimating
solar radiation are exploited in many applications. Choosing a model is
often limited by the availability of the meteorological parameters
required for its running. In many cases the current values of the
parameters are replaced with daily, monthly or even yearly average
values. This paper deals with the evaluation of the error made in
estimating global solar irradiance by using an average value of the Ångström turbidity coefficient instead of its current
value. A simple equation relating the relative variation of the global
solar irradiance and the relative variation of the Ångström
turbidity coefficient is established. The theoretical result is
complemented by a quantitative assessment of the errors made when hourly,
daily, monthly or yearly average values of the Ångström
turbidity coefficient are used at the entry of a parametric solar
irradiance model. The study was conducted with data recorded in 2012 at
two AERONET stations in Romania. It is shown that the relative errors in
estimating global solar irradiance (GHI) due to inadequate consideration
of Ångström turbidity coefficient may be very
high, even exceeding 20%. However, when instead of the current value of
the Ångström turbidity coefficient an hourly or
a daily average value is used, the relative errors are acceptably small,
in general less than 5%. All results prove that in order to correctly
reproduce GHI for various particular aerosol loadings of the atmosphere,
the parametric models should rely on hourly or daily Ångström
turbidity coefficient values rather than on the more usual monthly or
yearly average data, if currently measured data is not available.

2010 2012

The topic of the seminar
was focused on modeling the UV solar radiation.
The main results were
reported in two papers:
Paulescu M, Stefu
N, TulcanPaulescu E, Caliniu
D, Neculae A, Gravila
P (2010) UV solar irradiance from broadband radiation and other
meteorological data. Atmospheric Research 96, 141148.
Two original models
for estimating UV solar irradiance are reported in this paper. The first,
UV4PM, evaluates UV solar irradiance using four surface parameters: air
pressure, the ozone column content, the nitrogen dioxide column content
and the Ångström turbidity coefficient. As
demonstrated in the paper, UV4PM is able to trace exactly the atmospheric
transmittances calculated by integrating the original spectral
transmittances. The second model, UV2G, correlates the UV solar
irradiance components with the broadband ones. UV2G is derived from UV4PM
aiming to remove the ozone and nitrogen dioxide column content from the
list of the input parameters. The model can be used in any sky conditions
since broadband solar irradiance measurements carry within the
information concerning the actual state of the sky. Comparison with other
models and measurements shows a fair level of accuracy of UV4PM and UV2G
models.
The models presented
in this study can be regarded as starting points to develop other
parametric models able to evaluate the biological effects of UV
radiation. This goal can be achieved by just adding appropriate spectral
weighting functions in the first step of the parameterization algorithm.
Paulescu E, Stefu
N, Gravila P, Boata
RS, Pop N, Paulescu M (2012) Procedure of
embedding biological action functions into the atmospheric transmittance.
Theoretical and Applied Climatology 109, 323332.
The method may be adapted
to construct other parametric models able to evaluate the required
biological effective irradiance. This goal can be achieved by simply
putting in the appropriate spectral weighting function in the first step
of the parameterization algorithm. The user should then verify if using
the weighted average in the parametric model is fit enough to approximate
the original spectral one, otherwise we indicate a solution based on the
generalized mean. To our knowledge, this is the first study using
generalized instead of weighted average to enhance the accuracy of
effective parametric transmittances. In order to guide potential users to
derive models according to their own requirements, the procedures for
elaborating an effective atmospheric transmittance parametric model has
been described in detail. For users interested in speedintensive
computation of the effective solar irradiance, a PC program based on the
parametric equations above along with a user guide is posted online at (http://solar.physics.uvt.ro/srms,
section download_software). The application
runs with celerity and is very easy to be exploited.

