ISSN 0003-701X, Applied Solar Energy, 2018, Vol. 54, No. 2, pp. 131–133. © Allerton Press, Inc., 2018.
10% Loss of Incident Power through Solar Reactor Window:
Myth or Good Rule of Thumb?
Université de Toulouse, France
Received September 25, 2017
Abstract—It is known that a solar beam crossing a window losses 10% of its incident power. Yet, this affirma-
tion is not supported by many published scientific evidences. In this work, a heat f lux mapping method was
used to determine the heat flux distributions at the focal spot of a solar concentrating device without and with
a window on the incident beams' trajectory. The presence of a window on the beams' trajectory induces a 12%
loss of the total power and a 11% decrease of the peak heat flux density.
Concentrated solar power can be used to supply
heat at high temperature. It features several advantages
compared to conventional fossil fuel burning methods.
Among them is the fact that concentrated solar power
supplies clean heat, i.e., without combustion fumes.
Indeed, those fumes may alter or even destroy the
heated material, e.g. decomposition of limestone .
In addition, whenever the reacting atmosphere needs
to be controlled, it is common to add a windowed
aperture to the reactor design [2–4]. This window
ensures the airtightness of the reactor while allowing
the solar heat flux to enter it.
Nevertheless, adding a window comes with one
main drawback: it lowers the amount of energy enter-
ing the reactor. Indeed, the incident flux crosses the
window and therefore loses part of its power because
of in medium absorption and dioptres reflections. It is,
most of the time, quoted as common knowledge in the
field of solar reactor design that crossing a window
induces a 10% loss of the incident power. Yet, only one
research paper was found to support this claim, in the
very particular case of a dome , and not of a flat
window. Furthermore, this claim is not complete for it
only regards the total power: it does not precise
whether or not the heat flux distribution is modified.
In order to assess for the validity of this claim, the
heat flux distributions at the focal spot of a solar con-
centrating system were mapped with and without a
quartz window on the beams' trajectory. Then, the
total incident power and the shape of the heat flux dis-
tribution were compared.
MATERIALS AND METHODS
Figure 1 shows a schematic of the solar concentrat-
ing device, i.e. an artificial sun, and the heat flux mea-
surement material. To map the incident heat flux, a
screen is set in front of the artificial sun at the focal
spot. Thus, the beams coming out of it are intercepted
by the screen. As beams’ energy is absorbed by the
screen, its temperature rises. The temperature varia-
tions are recorded by IR camera. Then by inverse
methods using the temperature elevation is used to
compute the incident heat flux distribution over the
screen. A 2D model is used to link temperature (T) rise
with incident heat flux (Φ). It accounts for the contri-
bution of the incident radiative heat flux as well as
convective and radiative heat losses:
where ρ, c
, λ, α, ε, e screen density, heat capacity,
conductivity, absorptivity, emissivity and thickness
and h, surrounding temperature and
convective heat flux coefficient.
The model is solved for each pixel of the recorded
images using ordinary least square method. Once
completed, this procedure yields a map of the incident
heat flux. The solar concentrating system and the heat
flux mapping method used in this work have been
extensively described in .
A 3 mm thick flat quartz sheet was used to simulate
a reactor window. The window was set 5 cm above the
screen, parallel to it and perpendicular to the system
revolution axis. The repeatability of the measured heat
flux distributions was assessed by repeating the mea-
The article is published in the original.
ce = e T+
hT T T T