1 3
Topics in Current Chemistry (2018) 376:45
organic photochemistry” only briefly cited sunlight alongside with several
remarks on its shortcomings [6]. During the second half of the twentieth century,
the use of sunlight in organic photochemistry has mostly remained neglected,
with the sole exception of wastewater treatment applications, which are out of the
scope of this chapter [7].
More recently, the interest in solar photochemistry has resurged thanks to the
increased interest towards greener chemical processes [8]. After the seminal work
conducted in the 1990s at the Plataforma Solar de Almería (PSA) and at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) near
Cologne [9], in the 2000s several examples of photochemical reactions powered
by natural sunlight started to appear in the literature. In most of the cases, though,
the use of sunlight did not constitute the main object of research but served merely
as a proof over the mildness condition required for reaction activation (i.e., visible
light as opposed to UV photochemistry [10]). Among the photoreactors specifically
designed for solar applications (i.e., SOLFIN, SOLARIS, PROPHIS, MPI linefocusing, sunflow, and LSC-PM), each of them features a continuous-flow design.
The wide application of flow in solar photochemistry is not fortuitous but constitutes the deliberate choice of maximizing the photon flux received by the reaction
mixture. In fact, given the relatively low intensity of the solar irradiance, the efficient use of solar photons is of paramount importance. For this reason, a description
of the main parameters affecting the solar radiation at ground is provided in the next
section.
3 Solar Radiation
3.1 Solar Constant
The extraterrestrial solar spectral irradiance has been intensely studied since the
1960s, mostly because of its importance in satellite-mounted photovoltaics [11]. The
standard intensity of the solar extraterrestrial radiation on a unit area exposed normally to the sun rays at one astronomical unit is called the “solar constant”. The
value of the solar constant has been a subject of debate in radiometry over the twentieth century, mainly due to the low precision of ground-based instruments. The
value of the solar constant as measured in space is about 1366 Wm
−2
[12]. Despite
its name, this value is not constant [13] but slightly fluctuates, due to the variation
in solar activity, on every timescale at which it has been measured (from minutes
to decades). Moreover, since the Earth’s orbit around the Sun is elliptical, yearly
variations in the Sun–Earth distance (about 3%) are also affecting the total solar
irradiance.
3.2 Spectral Distribution of the Extraterrestrial Solar Irradiance
While nowadays the absolute value of the solar constant is known with high accuracy and precision, larger uncertainties are associated with its spectral distribution.
3
Reprinted from the journal
Topics in Current Chemistry (2018) 376:45
organic photochemistry” only briefly cited sunlight alongside with several
remarks on its shortcomings [6]. During the second half of the twentieth century,
the use of sunlight in organic photochemistry has mostly remained neglected,
with the sole exception of wastewater treatment applications, which are out of the
scope of this chapter [7].
More recently, the interest in solar photochemistry has resurged thanks to the
increased interest towards greener chemical processes [8]. After the seminal work
conducted in the 1990s at the Plataforma Solar de Almería (PSA) and at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) near
Cologne [9], in the 2000s several examples of photochemical reactions powered
by natural sunlight started to appear in the literature. In most of the cases, though,
the use of sunlight did not constitute the main object of research but served merely
as a proof over the mildness condition required for reaction activation (i.e., visible
light as opposed to UV photochemistry [10]). Among the photoreactors specifically
designed for solar applications (i.e., SOLFIN, SOLARIS, PROPHIS, MPI linefocusing, sunflow, and LSC-PM), each of them features a continuous-flow design.
The wide application of flow in solar photochemistry is not fortuitous but constitutes the deliberate choice of maximizing the photon flux received by the reaction
mixture. In fact, given the relatively low intensity of the solar irradiance, the efficient use of solar photons is of paramount importance. For this reason, a description
of the main parameters affecting the solar radiation at ground is provided in the next
section.
3 Solar Radiation
3.1 Solar Constant
The extraterrestrial solar spectral irradiance has been intensely studied since the
1960s, mostly because of its importance in satellite-mounted photovoltaics [11]. The
standard intensity of the solar extraterrestrial radiation on a unit area exposed normally to the sun rays at one astronomical unit is called the “solar constant”. The
value of the solar constant has been a subject of debate in radiometry over the twentieth century, mainly due to the low precision of ground-based instruments. The
value of the solar constant as measured in space is about 1366 Wm
−2
[12]. Despite
its name, this value is not constant [13] but slightly fluctuates, due to the variation
in solar activity, on every timescale at which it has been measured (from minutes
to decades). Moreover, since the Earth’s orbit around the Sun is elliptical, yearly
variations in the Sun–Earth distance (about 3%) are also affecting the total solar
irradiance.
3.2 Spectral Distribution of the Extraterrestrial Solar Irradiance
While nowadays the absolute value of the solar constant is known with high accuracy and precision, larger uncertainties are associated with its spectral distribution.
3
Reprinted from the journal