Research and Development of a Concentrated Photovoltaic Module of Very High Efficiency
P. Antonini1,2, S. Centro1,2, S. Golfetto3, A. Saccà2,4
1
Centro Fermi, Roma, Italy. 2Dipartimento di Fisica e Astonomia dell'Università di Padova, Padova, Italy.
3
INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy. 4Dipartimento di Ingegneria
Industriale dell'Università di Padova, Padova, Italy.
INTRODUCTION
Spurred by the 1973 oil crisis, research on concentrating
PV systems began in earnest in 1975 [1].
The main idea is simple: to concentrate solar light using
cheap optical elements such as mirrors or lenses over much
smaller (down to 1/1000), and that to reduce the costs of
installed power (and thus the costs of produced energy).
This cost reduction can be met using cheap materials for
the optics and modules, e.g. molded plastic, glass or
aluminum [2].
Notwithstanding this apparent simplicity, the expected
high efficiency can be reached only mastering the several
scientific and technological fields involved. Thus, the
study of concentrated photovoltaic has as a result not only
the prototype of an innovative module, but also an
interdisciplinary knowledge useful in other applications.
fact). The structure of the 3J cells is shown in Figure 1.
This structure allows to the cells to convert a larger part of
the solar spectrum, relative to the span possible to silicon
or thin film.
The high concentration allows the use of very small
cells: in our case a concentrator has a projected area of
360 cm2, and with a concentration factor of 600 times it
feeds two 0.3 cm2 3J solar cells.
Goal of this research are the design and prototyping of a
CPV system, using concentrators made of molded plastic
mirrors.
The first three years of the project resulted in several
prototypes, the last one used in four demonstrators
(18 kWp) already working.
The scientific work done so far resulted in solar modules
with in-field efficiency of 23.6% average, higher that
several competitors. Moreover, several prototypes with
efficiency larger than 25% (state of the art result) have
already been made.
PRINCIPAL GOALS ACHIEVED
Figure 1. The structure of the triple junction solar cells
The interdisciplinary competencies maturated within this
project have a natural application into lighting techniques,
LED, thermal solar (CSP) and hybrid automotive. Since
the efficiency of CPV concentrator depends on the solar
spectrum, the precise measurement of the spectrum and its
dependency on pollution is of great importance for CPV,
and can have as result also the prototyping of optical
sensors for polluting agents.
The CPV technology is mainly based on triple junction
solar cells, which have very high efficiency (more than
40%, still increasing) [3]. Differently from silicon solar
cells, on 3J the Sun’s light can be concentrated up to 2000
times, without loss of efficiency (efficiency increases in
During the last year it has been possible to develop and
test several configurations of concentrating photovoltaic
modules. They are innovative, with high concentration
factor (600X) and high efficiency (up to 25%), patented.
Different optical configurations have been developed
and tested, each time with an increase in efficiency. The
final configuration of the optics, used to build the larger
modules is shown in Figure 2.
It is a non-imaging optical system (the shape of the solar
disc is not conserved, in order to ensure a larger power
homogeneity over the solar cell)[4]. Figure 3 shows the
uniformity of the power over the 3J cell. A very
homogeneous power profile over the cell is very important
for the efficiency of the module, and its reliability.
Figure 2.. The optics developed has a symmetric structure. Each
of the two mirrors is divided into four sectors to increase the
homogeneity of the illumination.
The concentrator is made of two symmetric mirrors,
each concentrating the light on a 3J solar cell, placed
opposite. Glued to each of the cells is a quartz
homogenizer (called the secondary optics), that is needed
to increase the acceptance angle (the maximum
Figure 5. A4.5 kWp demonstrator, fully working.
misalignment angle still giving 90% of the maximum
power) and is needed to have a more homogeneous power
profile over the cell.
Figure 3 shows one of these secondary optics elements,
glued to the 3J solar cell, illuminated by the concentrated
Sun’s light.
Such a high concentration (600X) can result in a too
high temperature of the cells: a very accurate study of the
heat sink is necessary [5], otherwise the efficiency of the
cell (which decreases linearly with increase of temperature,
at a rate of -0.06%/K) will be too low.
Several heat sinks have been numerically simulated and
then validated with experiments.
The lessons learned from these modules were
implemented in newer modules, built at the end of 2011.
(Figure 5).
Some of these newer modules were tested. A
measurement is shown in Figure 6.
Efficiency reached 23%, blue line. Black line is the
measurement of the Direct Normal Irradiation (DNI), in
W/m2.
Figure 6. Measurement of efficiency, December 12, 2011.
Figure 4. Fused silica secondary optics is used to increase
acceptance angle and homogeneity over the solar cell
A first complete module was made in August 2010. The
measurements made on this module resulted in several.
modifications, implemented in further modules installed in
October 2010.
During 2012 year such trackers were monitored and
many lessons about CPV technology have been learnt.
Starting from these observations, the project of a second
TwinFocus release began in order to create a product that
is more reliable and cost effective.
The next Figure 7 shows a comparison between the
TwinFocus CPV module (blue line), a standard silicon
module (240 Wp) installed on a dual-axis sun tracker
(green line) and a standard silicon module fixed, oriented
south (red line). The loss of power on the last part of the
green line is due to shadowing from a wall, but
nevertheless the plot shows how to rate the different power
yield among the three systems. Powers are normalized to
equal surface for the systems.
m2)
Figure 7. Three photovoltaic system, with the same area (16
are compared. The blue line represents the power from the
TwinFocus CPV modules mounted on a high precision dual-axis
sun tracker. The green line represents the power from 16 m2 of
240 Wp st
The loss of power on the last part of the curve is due to
shadowing form a wall and is not relevant. The red line
represents the power output from fixed, south-oriented
silicon modules, same type of green line ones. Weak and
strong points of this technology were considered and some
choices were faced for the design of a new concentrator.
Many issues were considered, in particular those about
dependency of the efficiency on irradiance flux,
concentration factor, irradiance spectrum, temperature, and
illumination profile. These aspects affect the final
concentrator efficiency at different importance levels, and
each of these was studied to find out a balanced solution.
The new module, called TwinFocus2, started from the
experience accumulated in the implementation of the first
set and uses optical schemes more efficient, and a general
structure of the module that integrates mechanical
functions and cooling. Figures 8 and 9 show some details
of the new module.
Figure 8. The new layout of the concentrator allows a more
practical and cheaper construction. Moreover it allows for more
efficient optics and heat sinking.
Figure 9. The new layout of the concentrator allows a more
practical and cheaper construction. Moreover it has a more
efficient optics and heat sinking.
The innovative details are a larger projected area, which
is 420 cm2, an aluminum heat sink that is produced through
an extrusion process and that has also a structural function,
and a simpler and cheaper cable management between the
solar receivers.
CONCLUSION
During the first part of the project the main task was to
realize a CPV demonstrator and to test it on field.
Four trackers have been installed in some industrial
partner sites. More than 2000 concentrators were
assembled, so we got enough experience to be used on the
design of the second concentrator release. The goal of the
second release is to produce a marketable product. The
group acknowledges all the staff of the Laboratori
Nazionali di Legnaro (INFN) that helped in the realization
of the project, the Polo Fotovoltaico Veneto and the Centro
Studi e Ricerche Enrico Fermi for their support.
[1] R. M. Swanson: The promise of concentrators. Progress in
Photovoltaics: Research and Applications, 8, 93-111 (2000).
[2] P. Antonini: Concentrated Photovoltaics: is it a real
opportunity? Lecture Notes of the International School on
Energy 2012.
[3] RR King, et al, 40% efficient metamorphic
GaInP/GaInAs/Ge multijunction solar cells, Appl. Phys.
Lett. 90 (18) (2007) 183516:1–3
[4] R. Winston, J. C. Miñano, P. Benítez: Nonimaging Optics,
Elsevier (2005).
[5] A. Royne, Ch. J. Dey, D. R.Mills: Cooling of photovoltaic
cells under concentrated illumination: a critical review.
Solar Energy Materials and Solar Cells 86, 451-483 (2005).