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Luminescent and geometric concentrators for building integrated photovoltaics

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Luminescent and geometric concentrators for building integrated photovoltaics

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In developed countries 60% of the electricity consumed is attributable to commercial and public buildings. Even in the UK, the solar energy incident on buildings is more than 7× the electrical energy they consume. This represents a problem (the management of solar heat gain and glare) but also an opportunity that may be taken advantage of using complementary concentrator technologies. We are investigating conventional geometric and luminescent concentrators that may be combined to optimally harvest the direct and diffuse components of sunlight within a double glazed window unit. Initial results suggest that the combined system can achieve power conversion efficiencies approaching 20% under standard AM1.5g illumination at normal incidence.
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LUMINESCENT AND GEOMETRIC CONCENTRATORS FOR
BUILDING INTEGRATED PHOTOVOLTAICS
Amanda J Chatten1, Daniel J Farrell1, Rahul Bose1, Anthony Dixon1, Carl Poelking1, Karl C Gödel1,
Massimo Mazzer2 and Keith W J Barnham1
1Department of Physics, Imperial College London, London SW7 2AZ, UK
2CNR IMEM, Parco Area delle Scienze 37/A, 43100 Parma, Italy
ABSTRACT
In developed countries 60% of the electricity consumed is
attributable to commercial and public buildings. Even in
the UK, the solar energy incident on buildings is more than
7x the electrical energy they consume. This represents a
problem (the management of solar heat gain and glare)
but also an opportunity that may be taken advantage of
using complementary concentrator technologies. We are
investigating conventional geometric and luminescent
concentrators that may be combined to optimally harvest
the direct and diffuse components of sunlight within a
double glazed window unit. Initial results suggest that the
combined system can achieve power conversion
efficiencies approaching 20% under standard AM1.5g
illumination at normal incidence.
INTRODUCTION
Energy Consumption in Commercial Buildings and
Incident Solar Energy
Many estimates have been made of the potential of
building roofs and façades for harvesting the sun’s energy
for heat and light. Muneer [1] estimates that the solar
radiation incident on the surface of UK buildings is more
than 7 times the electrical energy they consume. This
estimate includes the area of all the facades of the
building as well as the roof area. Historically however,
photovoltaic (PV) and solar thermal systems have been
too unwieldy, inefficient, or unattractive for façade
installation, so efforts to harvest solar energy in buildings
have until recently focused mainly on roofs, ignoring the
significant portion of radiation incident on facades [2, 3].
Recent advances in building integrated photovoltaic and
solar thermal technologies have begun to make façade-
integrated systems more feasible, and in particular, the
potential contribution of windows both to energy efficiency
and energy capture and conversion has been the focus of
considerable interest. Hill, Pearsall and others [2, 4, 5]
modelled the usable roof and façade area of the UK
building stock and estimated a potential PV generation
capacity of 63 GW in 1995 and 110 GW in 2020. They
state this implies an energy production capability of 208
TWh in 1995 and 364 TWh in 2020 comparable to the
production of 274 TWh from centralised generators in the
UK in 1989 [2]. Commercial buildings represent about
13% of this or 8 GW and 15 GW in 1995 and 2020
respectively. These estimates take into account the effects
of shading, building orientation, surface suitability and
seasonal variations. Adjustments were made for window
areas, on the assumption that the main role of a window,
to let light into a building, is incompatible with harvesting
the light to generate electricity. With the simplifying
assumption that diffuse solar radiation is isotropic, a
significant conclusion of their work was that a material
percentage of insolation falls on all four walls, not just the
south facing wall, throughout the year. This is illustrated in
Fig. 1 for the case of a building in Plymouth, UK.
Figure 1. All surfaces of this building in Plymouth receive
a significant share of the incident solar radiation at all
times of the year. Based on data in reference [5].
Impressive as these figures are they nevertheless
understate the case since the following assumptions have
been made: (i) window surface area (assumed to be 33%
in reference [2]) is ignored in the estimates of available
façade area, since solar panels are not typically
transparent; (ii) the portion of the incident spectrum which
is not convertible to electricity was ignored; and (iii)
demand-side impacts were not considered - the ways in
which incident solar energy influences the energy
consumption of the building, including its effect on heating,
cooling and lighting demand. None of these is insignificant
and each points to important emerging opportunities for a
greater contribution from BIPV to the new energy
landscape in the near future. However, we are concerned
with the first, so we now turn to the role of windows.
Windows and Energy in Buildings
Given that the average lifetime of a window is 20-50 years,
the design and performance of windows can and does
have a material impact on future energy consumption of
buildings. Arasteh et al. [6] estimate that windows are
responsible for 35% of the heating and 28% of the cooling
energy use in US commercial buildings, representing an
annual impact of 1.3 EJ (361 TWh) of primary energy. Of
this, they estimate 0.7 EJ could be saved with more
energy efficient window technologies including passive
blinds, highly insulating low emissivity windows, and
dynamic tintable windows whose transmittance changes in
response to conditions.
While the foregoing approaches reduce cooling, heating
and lighting energy consumption, they do not provide
energy capture or conversion capability. Technologies
which can do this in a window include semi-transparent
dye sensitized solar cells such as the 30 cm x 30 cm
glass-integrated module developed by Hinsch et al. [7].
However, these have achieved efficiencies of only 4.2% in
outdoor conditions and their lifetime is significantly shorter
than silicon cells.
Low conversion efficiencies (leading to a requirement for
large façade or roof areas), the high cost of traditional
materials, and the loss of most of the incident energy in
the form of heat are among the reasons for the limited
adoption of building integrated PV. Most approaches to
addressing these limitations have attempted to incorporate
concentrating devices in windows or window blinds to
enhance conversion efficiency and reduce cost, and
thermal systems to capture and utilise the heat energy.
The systems described in the literature however do not
incorporate tracking so concentration is limited to < 10x.
Indeed, it is generally assumed that cost effective
deployment of 3rd generation [8] conventional high solar
concentration systems based on large parabolic mirrors or
arrays of Fresnel lenses to focus solar irradiation onto high
efficiency multi-junction or quantum well solar cells for
conversion to electricity can only be achieved in open
areas with a high degree of direct insolation. However, as
a result of the recent advances in the manufacturing of low
cost lenses, such solar concentrators have become a
viable option for small and light modules that can be
incorporated into buildings. These building integrated solar
concentrators (BISCs) can take advantage of the
transparency of small plastic concentrators and be
incorporated into double glazed window elements in order
to provide environmentally friendly electricity. Furthermore
these solar tracking transparent BISCs can be considered
as highly effective “solar blinds” since, like the alternative
technologies discussed previously, they also shield the
interior from direct sunlight, which is converted to
electricity, thereby protecting the building from excessive
heating and reducing the need for expensive air-
conditioning.
THE BISC VENETIAN BLIND
The modules resemble Venetian blinds as shown in Fig. 2,
which eliminate the direct component of sunlight, but also
provide a view through the window. In addition they
transmit a significant fraction of the diffuse sunlight, which
has been scattered in the atmosphere and is incident over
a wide range of angles, for glare-free natural interior
illumination, even when the sun faces the window. This
eliminates the need for interior lights when the blind is
working (see Fig.2a).
Figure 2. (a) Impression of a BISC Venetian blind. (b)
Schematic of a possible design for the BISC blind.
A schematic of a possible design for the BISC blind is
shown in Fig. 2b. It consists of an array of plastic, linear
Fresnel lenses which focus stripes of light onto light-bars
(discussed below) which serve as waveguiding secondary
concentrators that couple the light to high efficiency solar
cells at the ends of the light-bars. A 1.5 axis tracking
system rotates the Fresnel lenses to follow the elevation of
the sun and also moves the lenses towards and away
from the light-bars so as to keep the sunlight focused on
them as the angle of incidence of the direct beam varies
throughout the day. The high efficiency solar cells can be
water cooled to also supply a component of the hot water
requirement of the building.
Mazzer et al. [9] predicted the average electricity
generated by 1 m2 of the solar blind over a year as a
function of building shape and orientation for London and
San Francisco. With two walls entirely clad with solar
blinds it is estimated that the blind could reduce the
electricity consumed for artificial lighting and cooling in a
typical commercial building in the US by 50%, and provide
enough on-site generation for 50% of the remaining needs
of the building for example, to run office equipment and
refrigeration. A PV cell efficiency of 30% and an optical
efficiency of 80% for the light-bar were assumed.
The electricity demand over 24-hour periods at Imperial
College London was monitored and found be almost
identical in winter, spring and summer [10]. The demand
profile, with a peak approximately double the night-time
base load, is typical of public, educational and commercial
buildings. Solar PV systems are ideally suited to meet
these requirements as they provide power at the time of
peak demand that occurs between 11am and 5pm [10].
HARVESTING DIFFUSE INSOLATION
a) b)
It has long been recognized that in much of Europe over
half the insolation is diffuse [11]. Recent calculations [12],
using standard NREL insolation software BIRD to provide
the daily and yearly solar insolation with appropriate
parameters for aerosol absorption and humidity taken from
a NASA database, suggest that the figure is closer to 60%
in, for example, London (see Fig. 3).
0.0
0.5
1.0
1.5
2.0
2.5
0 28 56 84 112 140 168 196 224 252 280 308 336 364
day
Energy (kWh/m2/day)
Diffuse
Energy/(kWh/m2/day)
Direct
Energy/(kWh/m2/day)
Figure 3. Direct and diffuse insolation on a vertical south
facing façade in London [12]
The highest building integrated photovoltaic power
generation density will therefore be achieved by making
optimal use of both the diffuse and direct components of
sunlight.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
300 400 500 600 700 800 900 1000 1100
Wavelength / nm
Power density / W m-2 nm-1
0
0.2
0.4
0.6
0.8
1
1.2
Relative eye sensitivity / a.u.
AM1.5 diffuse
eye sensitivity
Figure 4. Relative sensitivity of the human eye and diffuse
solar spectrum estimated by the difference between
AM1.5g and AM1.5d.
The conventional BISCs described above only harvest the
direct sunlight and we have therefore studied a
“transparent” luminescent solar concentrator (LSC) which
can be added to the window glass to harvest the
significant short wavelength fraction (< 450nm) of the
diffuse sunlight to which the eye is insensitive. This is
illustrated in Fig. 4 in which the diffuse spectrum is
estimated by the difference between the standard global
(AM1.5g) and direct (AM1.5d) spectra. It can be seen that
this estimate of the diffuse spectrum peaks below 500 nm
rendering the sky blue.
THE “TRANSPARENT” LSC
The LSC, as illustrated in Fig. 5 generally consists of a
transparent plate doped with luminescent centers that first
absorb the incident sunlight and then re-emit it, usually
isotropically, at longer wavelengths such that a significant
fraction is trapped by total internal reflection and
waveguided to the edges where it can be converted by
solar cells with system power conversion efficiencies in
the range of 5-10% [13-15].
Figure 5. Schematic of a conventional LSC
LSCs are particularly suited to diffuse radiation because
the luminescent efficiency of most luminescent materials is
highest at short wavelengths, and, in addition they accept
light at all angles of incidence with the absorption
increasing with angle. The diffuse sunlight will see a
significantly thicker absorber as it is incident over a very
wide angular range. The direct sunlight impinging on the
LSC within its absorption range (e.g. < 450nm) will see the
minimum thickness of absorber and a significant fraction
will be transmitted to the conventional BISC if the LSC is
integrated within the front pane of a double glazing unit.
The “transparent” LSC could also be integrated into the
back pane where the solar irradiation will be lower due to
multiple Fresnel reflections at all the interfaces. But, since
this arrangement has the advantage of not absorbing any
of the direct light that could be utilized in the tracking BISC
solar blind, which operates at a higher system efficiency of
around 20%, this configuration is preferred here.
SIMULATION TECHNIQUES
The Imperial group has developed three independent
models to analyze LSCs. The first of these is the
Thermodynamic Model, see e.g. references [16, 17],
which is based on a detailed-balance approach, relating
absorption and luminescence via the brightness theorem.
The resulting differential equations are solved numerically
according to the boundary conditions while allowing the
photon chemical potential and incident and absorbed
fluxes to vary in three dimensions.
The main advantages of the Thermodynamic Model are that
it is an absolute calculation, can be used to predict
luminescence profiles and Stokes’ shifts and to determine
the luminescence QY of the dopant species. Its main
disadvantage is lack of flexibility, being restricted to
rectangular, homogeneous samples and it cannot be
adapted for the thin-film devices studied in this project. It is
also difficult to avoid computational problems when
modeling large area devices. The thermodynamic model has
however been extremely useful in providing an absolute
standard for calculation of homogeneous concentrators for
our two Raytrace Monte Carlo programs developed by
Rahul Bose and Daniel Farrell (pvtrace [18]).
Figure 6. Comparison of predictions of the three models
for the total photon fluxes exiting the various faces of a
homogeneously doped luminescent plate.
Fig. 6 compares the predictions for all three models for the
fluxes exiting the faces of a homogeneous LSC with the
same absolute absorption and emission spectra for the
dopant species. Very good agreement is observed from all
faces and for all three models. The Raytrace models track
individual photon paths through the LSC and use Monte
Carlo methods to determine the outcome of events such as
reflection at surfaces, absorption depth, whether an
absorbed photon is emitted, and if so in what direction and
at what wavelength it is emitted. Input to the model consists
of the absorption and emission spectra of the materials
comprising the device together with its configuration,
dimensions and illumination source.
A variant of the LSC is the thin film concentrator. It
consists of a thin layer of heavily doped material (typically
a polymer) on top of a transparent substrate, ideally with
matching refractive indices, such that trapped emission
travels within the entire substrate without refraction or
reflection at the interface. Our previous experimental
measurements and computational simulations have shown
that for the same dopant absorptivity there no noticeable
difference in performance between conventional,
homogeneously doped LSCs and thin film LSCs [19]. The
increased re-absorption losses in the optically dense
active layer of the thin film LSC are balanced by the
reduced losses in the transparent substrate. However, for
a “transparent” LSC on window glass a thin film LSC offers
distinct advantages as dopant stability is not in general
compatible with the high temperatures involved in
dispersal in glass and coating is a considerably more
convenient manufacturing technique.
RESULTS
Simulation of “transparent” LSCs
Simulations have been performed for a window on a
vertical, south-facing, wall in London by weighting the
diffuse and direct spectra (AM1.5g-AM1.5d and AM1.5d
respectively) by the data in Fig. 3. The thickness of the
LSC was assumed to be 3mm. The absorption and
luminescence data for the UV absorbing dye, Lumogen F
Violet 570 was taken from experimental measurements
and the dye has a luminescence QY of 95%. A system
efficiency of 2% and an energy harvesting yield around 30
kWh m-2 year-1 in London are predicted for 25cm x 25cm x
0.3cm LSC modules with optimal dye concentration and
GaInP cells (bandgap 1.8eV) attached to all four edges.
This cell was chosen as we have both external quantum
efficiency (EQE) and dark current data. We use the known
wavelength dependence of the EQE to determine the
short circuit current (JSC) from the photon flux coupled into
the cell. We then assume the light IV curve is given by the
difference of the JSC and the dark current. A higher
bandgap cell would be a better match than GaInP not only
in terms of higher EQE and hence higher JSC but also less
thermalisation loss and a higher open circuit voltage, VOC.
In the III-V system a GaP cell with a bandgap of 2.3eV
would be appropriate and could also be grown on Si which
would dramatically reduce costs. We estimate that the
system efficiency in London would be increased to 3%
with a 20% higher EQE and increase in VOC estimated by
the ratio of bandgaps.
The Lumogen F Violet 570 dye only absorbs 1.5% of the
direct and 8.5% of the diffuse photon fluxes (out to
4000nm). We predict that the power conversion
efficiencies of the simulated module with GaP cells under
AM1.5 direct and diffuse illumination individually with
attached GaP cells would be about 0.8% and 4.5%
respectively. The power conversion efficiency under
AM1.5g with only around 15% diffuse photons would be
1.32%. A dye with similar properties to Violet 570 but that
absorbs out to 490nm would absorb 6% and 25% of the
AM1.5 direct and diffuse spectra respectively resulting in
power conversion efficiencies of around 3% and 13% for
the direct and diffuse components individually and 4.5%
for AM1.5g. However, these increases would come at the
expense of a window that is more strongly tinted.
Prototype “transparent” LSC
We fabricated a prototype “transparent” LSC by coating a
plate of fused quartz glass with a thin film of the Lumogen
F Violet 570 fluorescent dye (see Fig. 7). Silicon solar cells
(125mm x 3.3mm) with no front contacts from Solaronix
were bonded to the four edges of the LSC using
Krystalflex PE399 transparent thermosoftening film.
Figure 7. The prototype “transparent” LSC.
With the four cells connected in parallel the power
conversion efficiency of this prototype illuminated by a
class B solar simulator was found to be only 0.31% but
does not represent the ultimate efficiency achievable for
this concept. The reasons for the low power conversion
efficiency are that the thin-film was not optimally doped,
transmitting more than 10% of the incident light even at
the absorbance peak, and that the bandgap of the Si solar
cells at the edges was not matched to the wavelength of
the luminescent light emitted from the edges of the device.
Our simulations (above) suggest that with matched solar
cells and optimal doping a power conversion efficiency of
over 10% is possible under diffuse light. This figure could
be further increased (at least doubled) with an optimal
luminescent material that reduces self-absorption losses
through a larger Stokes shift or that provides directional
emission. A further strategy to increase the efficiency
would be to utilize wavelength selective mirrors on the top
and bottom surfaces, similar to those we tested in
reference [17], that transmit incident light over the
wavelength range the luminescent material can absorb but
reflect the longer wavelength emitted light.
Developing the light-bar
The light-bars are a key element of the BISC Venetian
blind. Their function is to act as secondary concentrators
to waveguide the direct light focused by the linear Fresnel
lenses to the ends, where it may be converted by high
efficiency solar cells. These could be either multi-junction
or strain-balanced quantum well solar cells depending on
whether a geometric or luminescent design is preferred.
a) b)
Figure 8. Luminescent a) and geometric b) designs for the
light-bar.
We have investigated both approaches using the
methodology of testing simple prototypes, to verify the
accuracy of the simulation techniques. This then enables
optimized designs, which are more difficult to fabricate, to
be simulated with confidence. Fig. 8 illustrates the best
luminescent and geometric designs to date.
In order to simulate these complex designs the Raytrace
model developed by Daniel Farrell was extended to
include many different cross sections e.g. rectangular,
circular and triangular. The model was then further
extended by Carl Poelking to allow addition, subtraction
and intersection of the 3D objects via constructive solid
geometry. Further development by Karl Gödel added 2D
polygons and arbitrary convex objects to the model.
Cylindrical luminescent light-bars
For the light-bar, a cylindrical LSC is advantageous owing
to improved waveguiding properties and, in order to
reduce the re-absorption losses, a composite LSC with a
thin cylindrical luminescent core was suggested [20]. The
sunlight is focused on the luminescent core which absorbs
and re-emits the light, whereas the outer transparent bar
functions as a waveguide with a greatly reduced re-
absorption probability. Modelling results showed that if a
ray gets absorbed by the dye, the probability that it
reaches the edges can be quite high: For the composite
cylindrical light-bar depicted in Fig. 8a) the probability of
an absorbed photon reaching the ends can be as high as
55%. However, highly luminescent materials can generally
only absorb the fraction of the solar spectrum below
650nm thereby limiting optical efficiencies to around
13.5%. This figure could be further dramatically improved
with the advent of highly luminescent IR absorbers or
broad-band efficient up-conversion.
Figure 9. Cross section of the luminescent light-bar.
We have fabricated a prototype composite luminescent
light-bar as shown in Fig. 9. It has a length of 47cm and
diameter of 2cm with a core doped with Lumogen F Red
305 and was optimized for the 15cm wide linear Fresnel
lens we have available that provides a 1cm wide focus.
The simulations for the bare light bar and for the diffuse
PTFE mirror coating are well confirmed by our
experiments. For the bare light-bar the simulated and
experimental optical efficiencies were 2.43±0.24% and
3.48±0.32%. With the PTFE reflector these figures
increase to 8.52±0.12% and 8.30±0.97%. From these
studies we conclude that with the materials currently
available the ambitious target of 80% for the optical
efficiency of the light-bar (used in reference [9]) cannot be
achieved with a luminescent design at present.
Geometric light-bar
We therefore turned to geometric concentrator designs for
the light-bar The rationale behind our geometric design
(see Fig. 6b) is to reflect the focused light off mirrored
facets such that it is trapped within totally internally
reflected modes on the other surfaces. Given the difficulty
of finding luminescent materials with ideal properties we
have found this approach has significant advantages. The
dimensions chosen for this study were a light-bar length of
20cm, width 2.5cm and height 2cm. The PMMA
(absorption coefficient 0.3m- 1) light-bar had air-gap
specular reflectors (R=97%) covering the vertical surfaces
and the remainder of the bottom surface not occupied by
the deflection mirrors (for which R=97% was also
assumed). The linear Fresnel lens available for this study
provided a concentration of 15x but a lens with a tighter
focus could be produced providing a concentration of 25x.
We therefore studied the effect of focal width and the
optical efficiency of the light-bar as a function of the
azimuthal angle,
φ
, of incidence (which will vary
throughout the day) of the direct beam, for two different
focal widths, are compared in Fig. 8. We have also studied
the effects of varying the light-bar geometry and the
detailed parameters of the deflection mirrors but there is
insufficient space to discuss these here.
Figure 8. Comparison of the optical efficiency of the light-
bar with different focal widths.
CONCLUSIONS
With a module size of 25cm x 25cm incorporating a tinted
LSC window glass with GaP cells on all edges, Fresnel
lenses providing a concentration of 25x and an optimized
light-bar (with a peak optical efficiency of 60% when
φ
=0,
at noon for a south facing wall) with water cooled triple
junction cells operating at a power conversion efficiency of
36% [21] optically coupled to the ends of the light-bar in
the frame, it is expected that the full integrated system
would operate at a peak efficiency of 19% under AM1.5g
at normal incidence. This is 2 to 3 times the efficiency of
current building integrated PV systems based on 2nd
generation thin or semitransparent a-Si or CIS cells and
this system makes more effective use of the solar
spectrum. We predict that in the standard AM1.5g
spectrum a tinted LSC window could achieve power
conversion efficiencies of 4.5%. However, in the UK or
other regions of high diffuse insolation, both the efficiency
and the boost to a BISC Venetian blind provided by a
tinted LSC window will be higher and it may also find
application as a stand-alone component.
REFERENCES
[1] T Muneer et al., Windows in Buildings: Thermal,
Acoustic, Visual and Solar Performance, (Oxford,
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[3] IEA Photovoltaic Power Systems Program, IEA. Report
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[5] N M Pearsall et al., Renew. Energ., 5, 348-355, (1994).
[6] D Arasteh et al. Proc. of the 2006 ACEEE Summer
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[8] M A Green, Electrochemical Society Proc. Vol. 2001-10,
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[9] M Mazzer et al., Solar Cities Conference, (Oxford, UK,
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[10] K W J Barnham et al., Nature Mats., 5, 161, (2006).
[11] A Goetzberger, Appl. Phys. 16, 399, (1978).
[12] M Mazzer, IMEM CNR, private communication.
[13] E E Bende et al., Proc. of the 23rd EUPVSEC,
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[14] L H Slooff et al., Phys. Stat. Sol. (RRL), 2, 257,
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[15] M J Currie et al., Science, 321, 226, (2008).
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[17] W G J H M Van Sark et al., Opt. Expr. 16, 21773,
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[18] http://github.com/danieljfarrell/pvtrace
[19] R Bose et al., Proc. 22nd EUPVSEC, 210 (Milan, Italy,
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ACKNOWLEDGEMENTS
The authors would like to thank the UK EPSRC, the EC,
Saint-Gobain and ONRG for financial support.
... Reducing spectral losses and enhancing efficiencies in Photovoltaic (PV) cells can be implemented by non-imaging optics based solar concentrating technology, such as Luminescent Solar Concentrator (LSCs). LSCs have been introduced as an efficient concentrator [1][2][3][4][5] whose static nature makes them an attractive choice for Building Integration Photovoltaic (BIPV) systems and façades of buildings which brings us closer to the goal of constructing buildings with zero carbon energy consumption as mandated by EU by 2020. [6] In LSC, light irradiated on the top surface, is absorbed by luminescent species and emitted at higher wavelengths; then, through total internal reflection it is waveguided to one of the LSC edges where the PV cell is located. ...
... Luminescent Solar Concentrator (LSCs). LSCs have been introduced as an efficient concentrator [1][2][3][4][5] whose static nature makes them an attractive choice for Building Integration Photovoltaic (BIPV) systems and façades of buildings which brings us closer to the goal of constructing buildings with zero carbon energy consumption as mandated by EU by 2020. [6] In LSC, light irradiated on the top surface, is absorbed by luminescent species and emitted at higher wavelengths; then, through total internal reflection it is waveguided to one of the LSC edges where the PV cell is located. ...
... A variation of this idea has been to use luminescent solar collectors (LSC) for concentrating solar radiation onto PV cells situated at the edges of windows. However, despite increased research into LSC's, their performance requires further optimisation, e.g., photostability of organic dye based concentrators over prolonged ultraviolet (UV) exposure periods or the relatively low luminescent quantum efficiency exhibited by Quantum dot based luminescent concentrators [8][9][10]. ...
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... 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 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 in Fig. 1, using both experiments and modeling. ...
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Chapter
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The rational design of organic semiconductors for optoelectronic devices relies on a detailed understanding of how their molecular and morphological structure condition the energetics and dynamics of charged and excitonic states. Investigating the role of molecular architecture, conformation, orientation and packing, this work reveals mechanisms that shape the spatially resolved densities of states in organic, small-molecular and polymeric heterostructures and mesophases. The underlying computational framework combines multiscale simulations of the material morphology at atomistic and coarse-grained resolution with a long-range-polarized embedding technique to resolve the electronic structure of the molecular solid. We show that long-range electrostatic interactions tie the energetics of microscopic states to the mesoscopic structure, with a qualitative and quantitative impact on charge-carrier level profiles across organic interfaces. The computational approach provides quantitative access to the charge-density-dependent open-circuit voltage of planar heterojunctions. The derived and experimentally verified relationships between molecular orientation, architecture, level profiles and open-circuit voltage rationalize the acceptor-donor-acceptor pattern for donor materials in high-performing solar cells. Proposing a pathway for barrier-less dissociation of charge transfer states, we highlight how mesoscale fields generate charge splitting and detrapping forces in systems with finite interface roughness. The associated design rules reflect the dominant role played by lowest-energy configurations at the interface.
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The luminescent properties of core-shell quantum dots are being exploited in an unconventional solar concentrator, which promises to reduce the cost of photovoltaic electricity. Luminescent solar collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited by the performance of luminescent dyes. We present experimental and theoretical results with a novel concentrator in which the dyes are replaced by quantum dots. We have developed a self-consistent thermodynamic model for planar concentrators and find that this three-dimensional flux model shows excellent agreement with experiment.
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The fluorescent energy conversion principle using several sheets of transparent material doped with fluorescent molecules to concentrate radiation is extended to include diffuse radiation. Two cases are treated here: diffuse radiation only and a composite spectrum consisting of 40% direct and 60% diffuse radiation simulating the average illumination of a flat exposure in central Europe. In both cases photovoltaic conversion efficiency is significantly higher than with the AM1 spectrum. This is due to the blue shift and narrow shape of the diffuse spectral distributions. With realistic boundary conditions the theoretical conversion efficiency is 1.56 times higher than for the AM1 case. The highest theoretical conversion efficiency is now 38%.
Windows in Buildings: Thermal, Acoustic, Visual and Solar Performance
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