
2018

Two themes are considered
for 2018:

Aerosol influence on diffuse fraction

Satellitebased models for solar
radiation at the Earth surface

2017

In
this year, the seminar was focused on enhancing the accuracy of clearsky
solar irradiance models, by a better capturing the aerosol influence on
the atmospheric transmittance.
A
key result is SIMv.2 model, an upgraded version of our parametric
clearsky solar irradiance model (Energy Conversion and Management 70
(2013) 7682), aiming to improve the estimates accuracy in arid
environment. The new elements of SIMv.2, such as new equations for
aerosol absorption and downward fraction, are introduced targeting a better
capture of the peculiarities of the solar radiation extinction by
aerosols. On the whole, the results of testing SIMv.2 at twelve stations
located in regions with temperate and arid climate show that SIMv.2
performs much better than SIMv.1, an improvement in nRMSE of 37.1% for global solar irradiance and of 24.7% for
the diffuse component being noticed.
A report entitled SIMv.2: A simple and versatile route
to solar irradiance was prepared and submitted to Energy Conversion
and Management,

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.

