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LUMINESCENT SOLAR CONCENTRATORS: CYLINDRICAL DESIGN

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Luminescent solar concentrators (LSCs) are typically planar low-concentration systems that absorb sunlight over a large area and emit a red-shifted spectrum out of smaller surfaces, where solar cells can be attached. We present the study of a composite system, in which a linear geometrical concentrator is used as primary device to focus sunlight onto a cylindrical LSC encased in a transparent matrix. The idea behind this design is to reduce re-absorption losses, which generally limit the performance of LSCs. Experimental measurements on a cylindrical LSC were compared with a raytrace model, showing a good agreement. Further predictions were made based on the model. It was shown how the reduction of re-absorption losses is achieved by allowing the luminescence from the cylindrical core to travel in the transparent matrix. The proposed design can achieve high optical concentrations with the need for only one-dimensional tracking.
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LUMINESCENT SOLAR CONCENTRATORS: CYLINDRICAL DESIGN
R. Bose1*, D.J. Farrell1, C. Pardo-Sanchez1, M. Pravettoni1,2 , M. Mazzer3, A.J. Chatten1 and K.W.J. Barnham1
1 Department of Physics, Imperial College London, London SW7 2AZ, UK
2 Joint Research Centre of the European Commission, Renewable Energy Unit, 21020 Ispra (VA), Italy
3 CNR IMEM, Parco Area delle Scienze 37/A, 43100 Parma, Italy
* Corresponding author: rahul.bose@imperial.ac.uk
Luminescent solar concentrators (LSCs) are typically planar low-concentration systems that absorb sunlight over a
large area and emit a red-shifted spectrum out of smaller surfaces, where solar cells can be attached. We present the
study of a composite system, in which a linear geometrical concentrator is used as primary device to focus sunlight
onto a cylindrical LSC encased in a transparent matrix. The idea behind this design is to reduce re-absorption losses,
which generally limit the performance of LSCs. Experimental measurements on a cylindrical LSC were compared
with a raytrace model, showing a good agreement. Further predictions were made based on the model. It was shown
how the reduction of re-absorption losses is achieved by allowing the luminescence from the cylindrical core to
travel in the transparent matrix. The proposed design can achieve high optical concentrations with the need for only
one-dimensional tracking.
Keywords: light trapping, concentrator, ray tracing
1 INTRODUCTION
The luminescent solar concentrator (LSC) was
proposed in the 1970s [1, 2] as a way of spectrally
modifying and simultaneously concentrating light for
conversion by photovoltaic (PV) cells. In its conventional
use, the LSC has a planar architecture and collects
sunlight over its entire front surface. Luminescent
materials absorb the incident light and re-emit it at longer
wavelengths. Through total internal reflection a large
fraction of the luminescence is trapped within the plate
and guided to the edges, where it can be coupled into PV
cells. The geometric ratio between the collection area and
the emission area leads to a concentration of the light.
Additionally, LSCs can increase the conversion
efficiency of the attached PV cells through spectral
modification of the light. By replacing the majority of
photovoltaic cells by relatively cheap materials, the costs
of the PV system can be reduced. LSCs can efficiently
collect diffuse irradiation and direct irradiation incident
at oblique angles, eliminating the need for solar tracking.
Not much research has been published on cylindrical
LSCs. Batchelder [3] used this geometry for the study
self-absorption. More recently, a theoretical comparison
of cylindrical and square-planar LSCs was carried out by
McIntosh et al [4], showing that the optical concentration
of the cylindrical LSC can be up to 1.9 times higher than
that of the planar. They also proposed a multi-cylindrical
structure made up of many cylinders aligned side by side
to yield further enhancements.
The design we present in this paper differs from
those aforementioned as it comprises a composite LSC
structure for use as a secondary device in a high
concentration system.
2 CONCEPT
In this composite design, a relatively thin cylindrical
rod containing luminescent material is encased in a
transparent bar, as shown in Figure 1. The refractive
indices of these two components are matched so that
light is not bound by the interface between them and can
travel in the entire composite. Sunlight is focused in one
dimension onto the length of the cylindrical core using a
geometrical concentrator, such as a linear Fresnel lens or
an array of lenses. The PV cells are attached at the two
end surfaces of the bar.
A major loss mechanism in LSCs is the re-absorption
of luminescence, which can result in escape cone and
non-radiative losses. The motivation behind the
composite design is to achieve a reduced probability of
re-absorption. This is possible because the cross-section
of the core is small compared to cross-section of the
transparent bar. Therefore, luminescence travelling in the
composite has a smaller chance of being in the core
where it could be absorbed.
Compared to a conventional planar LSC that would
cover the entire area of light collection, i.e. the area of
the Fresnel lens, the composite design requires a smaller
area to be covered with PV cells. This cost reduction,
however needs to be weighed up against the additional
cost of the primary concentrator, which would also
require one-dimensional solar tracking. A significantly
larger optical efficiency could make this design viable
for application.
Figure 1: Schematic of the composite cylindrical LSC
comprising a cylindrical core with the active material
encased in a transparent cuboidal matrix, designed for
use with a primary geometrical concentrator focussing
light in one dimension onto the core.
3 EXPERIMENTAL CHARACTERISATION
The cylinder shown in Figure 2, made of a polymer
doped with the dye Fluor Yellow, with a length of about
40cm and a radius of 2mm was used for the experimental
measurement. The dye emits in the green with a
luminescence quantum efficiency (LQE) of 95%. It has a
relatively small Stokes shift and a narrow absorption
band, which means that this concentrator is not optimised
for a high yield under a broad spectrum.
Figure 2: Cylindrical core containing the luminescent
material (Fluor Yellow dye).
In order to investigate how well this cylinder guided
incident light to the end surfaces, a distance dependent
optical respones measurement was carried out. Figure 3
shows the setup, in which a laser was used to illuminate
spots along the length of the cylinder while the
photocurrent out of one end was measured with a
photodetector.
Figure 3: Experimental setup using a red (404nm) laser
and a silicon photodetector. The photocurrent out of one
end surface was measured as the distance of the
illumination position was varied along the length of the
cylinder.
We have developed and tested a raytrace model [5]
that uses a Monte Carlo method and can model a variety
of planar LSCs. It has recently been extended to describe
cylindrical LSCs and the composite structure described
in this paper. A comparison between experimental and
modelling results for the cylinder is shown in Figure 4.
There is a good overall agreement between model
and experiment. Minor discrepancies at small distances
can be attributed to a larger effect of errors in the
experimental measurement when the illumination spot is
close to the photodetector.
Figure 4 shows a steep drop of the LSC output with
distance of illumination spot. This is due to multiple re-
absorption losses and means that the cylinder has a poor
optical efficiency as only light incident close to the end
contributes significantly to the photocurrent.
Figure 4: Response of the output from the luminescent
cylinder as a function of illumination position, measured
experimentally and predicted with the raytrace model.
4 COMPUTATIONAL STUDY
4.1 Effect of the transparent bar
Using the raytrace model the experiment described in
the previous section was repeated for the composite
structure with the transparent material around the
cylinder. A comparison between composites of three
different cross-section widths and the cylinder alone is
shown in Figure 5.
The presence of the transparent bar drastically
improves the perfomance of the LSC, and even
luminescence originating on the far end of the rod is
guided to the detection end quite efficiently. Re-
absorptions are effectively reduced as the light travels in
the composite, proving the concept of this design. As
expected, the output increases with bar width, but the
loss in geometrical concentration ratio due to a larger PV
cell area needs to be considered. In the given case, a
width of 1cm appears to be a suitible choice.
Figure 5: Raytrace simulation of the effect of the
transparent bar on the luminescent output. The
experiemtn was simulated for three different cross-
section widths of the bar.
4.2 Optical efficiencies and concentrations
For a concentrator system, the optical efficiency and
concentration, presented in Table I for the three
composite structures of different widths, are important
measures. The optical efficiency is conventionally the
fraction of incident light that is coupled into the PV cells.
In this calculation however, the optical efficiency is
expressed as a fraction of incident light absorbed by the
luminescent material. The geomatrical concentration
ratio is the ratio between light collection area and output
area. In this case the collection area is the product of
diameter of the cylinder and length of the composite, and
the output area is twice the sqare of the bar width. The
optical concentration, defined as the ratio of photon
output to input, is given by the product of geometrical
ratio and optical efficiency.
The optical effiency decreases with the cross-section
width, wile the optical concentration increases. An
appropriate width would strike the balance between
achieving a high concentration to reduce area of PV cells
required and yet maintaining a large enough efficiency to
generate a reasonable amount of power for the given
concentrator size.
Table I: Optical efficiencies and concentrations
Bar cross-section width 10mm 15mm 20mm
Geometrical ratio 16.0 7.1 4.0
Optical efficiency 0.30 0.34 0.37
Optical concentration 4.8 2.4 1.5
4.3 Performance projections
Assuming partially optimised conditions, the
perfomance of the concentrator was modelled under an
AM1.5 direct solar spectrum, concentrated 50 times by
the primary optics. The dimensions of the bar were
modelled to be 1m in length with a cross-section of
8x8 mm2 and a core diameter of 4mm. A background
absorption of 0.3m-1, typical for high quality polymers,
was assumed for both the core and the bar. Futhermore,
mirrors were placed on the two long sides of the bar and
a diffuse reflector on the bottom. This allowed for
incident light outside the absorption range of the dye to
be partially reflected towards the PV cells. The selection
of a more suitible luminescent centre, which is a crucial
optimisation, was not carried out at this stage.
The projections for two different solar cells are
presented here: a silicon cell and an InGaP cell. In the
case of the Si cell, the overall system efficiency was
2.1%, yielding a power of 8.3W. For comparison, the
cells alone, placed directly under the light, generate only
a sixth of this power. Figure 6 shows the spectrum of a
silicon PV cell and the spectra of the dye and the indident
sunlight. One can see that the dye can absorb short
wavelenghts, where the cell quantum efficiency is low,
and emit a red-shifted spectrum that can be converted
better by the cell, thus improving the efficiency.
As the light concentration can reach several hundred
suns, it is reasonable to use high-efficiency solar cells. In
the case of the InGaP cell, a system efficiency of 4.6%
and a power of 18.6W was predicted. However, it was
found that the dye did not significantly contribute to any
improvement. It can be seen in Figure 7 that the IQE of
the InGaP cell is already high in the absorption region of
the dye. In fact, the efficiency achieved with the mirrors
alone and without the dye was only 1% (relative) smaller
than the the efficiency with the dye.
It is clear that the dye was not ideal for this
application. A broad absorption is required to achieve
larger gains. A collection of dyes could achieve this. Our
group has been investigating the use of quantum
structures, such as quantum dots (QDs) and nanorods
(NRs) as luminescent species [6]. These inorganic
structures have are a broad absorption spectrum,
extending far into the UV, and a narrow emission peak.
They are also inherently more photo-stable than the
organic dyes.
Figure 6: Spectra of the dye, AM1.5d and a silicon cell
internal quantum efficiency (IQE).
Figure 7: Spectra of the dye, AM1.5d and an InGaP cell
internal quantum efficiency (IQE).
5 CONCLUSION
A composite cylindrical luminescent concentrator has
been examined for use as a secondary device for
concentrated photovoltaics. Good agreement was found
between experimental measurements and raytrace
simulations of the cylinder. It has been shown, using the
raytrace model, how luminescence can travel effectively
inside the transparent bar, reducing re-absorption losses.
Performance projections showed that the system has the
potential to boost the power output of solar cells by
several factors. However, it was found that a broad
absorption range is required to make it feasible.
Moreover, dyes may degrade at high concentrations
while inorganic luminescent centres are expected to be
more stable. By choosing suitable luminescent materials,
this design may help reduce the cost of photovoltaic
energy.
6 ACKNOWLEDGEMENTS
The authors would like to acknowledge EPSRC for a
studentship.
7 REFERENCES
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[2] A. Goetzberger and W. Greubel, Appl. Phys. 14, 123
(1977)
[3] J.S. Batchelder, PhD thesis, California Institute of
Technology (1982)
[4] K.R. McIntosh, N. Yamada and B.S. Richards, Appl.
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[5] R. Bose et al, Proc. PVSAT-4 Bath (2008)
[6] K.W.J. Barnham et al, Appl. Phys. Lett. 76, 1197
(2000)
... Experiences with LSC-PV in a product context are limited and design issues with this PV technology may be different than with conventional solar cells in PIPV (Apostolou and Reinders, 2014). In particular, geometric modifications and efficiency (Bose et al., 2009; Chatten et al., 2011; Corrado et al., 2013; Inman et al., 2011; McIntosh et al., 2007; Pravettoni et al., 2009) play a role with regards to product-integrated LSC-PV. To evaluate these aspects (Viswanathan et al., 2012), this paper explores the effect of integration on the energy performance of LSC-PV in a light pole for outdoor lighting, like the one shown inFig. 1 , using both experiments and modeling. ...
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To employ new solar photovoltaic technologies in products and buildings, many systems need to be adapted. Inspired by the cylindrical shape, in this work we have evaluated the performance of luminescent solar concentrator photovoltaic (LSC-PV) elements with narrow PV cell strips that could be integrated in an outdoor lighting pole. Silicon photovoltaic (PV) cells were attached to the back of both flat and cylindrically bent PMMA lightguide sheets containing the dye Lumogen Red 305, and mirrors to non-covered edges of the light guides. The energy performance of these two elements was measured. The flat and bent LSC-PV elements were also simulated using optical modeling and the resulting performance parameters from simulations and experiments were compared. From simulations for a flat LSC-PV, the optical collection efficiency, concentration and electrical conversion efficiencies were found to be 18%, 1.8% and 2.8%, respectively, for a geometric gain of 10. For a bent LSC-PV shape, the respective values are 21%, 1.4% and 3.4% for a geometric gain of 6.7. Due to reduced sensitivity to the angular dependence of incoming irradiance it is expected that these bent LSC-PV elements would perform well on both sunny and cloudy days.
... Optical performance of all LSC generations depends on their configuration parameters such as the device shape, geometric gain, host material, luminescent species, and PV solar cell spectral response [41,48,[85][86][87]. In an ideal LSC, the host material must provide long term photo-stability, compatibility with dispersing luminescent materials, be low cost, highly transparent, and also exhibit low attenuation and scattering coefficients [88][89][90]. ...
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A luminescent solar concentrator consists of a waveguide collector and solar cells connected to the waveguide sides or bottom. Luminescent centers, such as organic dye molecules and nanoparticles absorb incoming photons and emit red-shifted photons that are mostly trapped inside the waveguide due to total internal reflection and finally coupled out into the solar cells, where they are converted into electricity. Typical designs lead to concentration of both direct and diffuse light. This chapter describes the theoretical principles of the luminescent solar concentrator and discusses factors that determine the overall device efficiency. Also, various choices for luminescent materials are detailed as well as various designs and experimental results. The future application area can be wide, ranging from greenhouses to building integrated photovoltaics, in which transparent energy-generating windows could contribute to energy neutrality of buildings.
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The present invention relates to a luminescent solar concentrator for a solar cell, comprising a collector with a luminescent substrate, and a wavelength selective filter, wherein the wavelength selective filter is arranged above the surface of the collector, wherein the luminescent substrate has an absorption edge which corresponds to a wavelength λex and emits radiation around a wavelength λem, wherein the selective filter h a refractive-index contrast Δn with a negative or zero dispersion, and wherein the wavelength selective filter is designed to keep the emitted radiation inside the collector while shifting the reflection band of the incident radiation to angles ≧25° and/or to narrow the reflection band to a range of ≦10°.
  • K W J Barnham
K.W.J. Barnham et al, Appl. Phys. Lett. 76, 1197 (2000)
  • A Goetzberger
  • W Greubel
A. Goetzberger and W. Greubel, Appl. Phys. 14, 123 (1977)
  • J S Batchelder
J.S. Batchelder, PhD thesis, California Institute of Technology (1982)
  • W H Weber
  • J Lambe
W.H. Weber and J. Lambe, Appl. Opt. 15, 2299 (1976)
  • K R Mcintosh
  • N Yamada
  • B S Richards
K.R. McIntosh, N. Yamada and B.S. Richards, Appl. Phys. B 88, 285 (2007)