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A Cassegrain Concentrator Photovoltaic System: comparison between
dichroic and multijunction photovoltaic configurations
Conference Paper · November 2014
DOI: 10.1109/IECON.2014.7048761
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A Cassegrain Concentrator Photovoltaic System:
comparison between dichroic and multijunction
photovoltaic configurations
Donato Vincenzi, Stefano Baricordi, Silvia Calabrese
Maura Musio, Alfonso Damiano
Dipartimento di Fisica
Università di Ferrara
Ferrara, Italy
vncdnt@unife.it
Dipartimento di Ingegneria Elettrica ed Elettronica
Università degli Studi di Cagliari
Cagliari, Italy
alfio@diee.unica.it
Abstract—In the present paper a novel configuration of high
concentrator photovoltaic (HCPV) systems based on Cassegrain
optics has been presented. The proposed system has been
designed in order to be suitable for the implementation of both
multi-junction and spectrum splitting configurations. The
Cassegrain optic design, the components and material choices for
the development of HCPV receivers have been described.
Subsequently, the proposed HCPV system has been implemented
considering a multi-junction and dichroic photovoltaic solar cells
configurations. Finally, the outdoor characterization of the two
receiver prototypes has been carried out in order to determine
their energy conversion performances. The obtained outcomes
are then compared and discussed in order to highlight the worth,
and the effectiveness of the proposed HCPV configurations.
Keywords—Concentrator Photovoltaic energy conversion
systems; photovoltaic cells; renewable energy source; dual-axis
tracker; dichroic system.
I.
INTRODUCTION
The Concentration Photovoltaic (CPV) is at the present
time the solar energy conversion technology characterized by
the highest efficiency among sun energy conversion systems
[1]. This is well highlighted by the recent performance
reached by CPV modules that have achieved values of energy
conversion of about 35% [2]-[3]. In particular, the High
Concentration Photovoltaic (HCPV) represents the most
advanced CPV concepts, which has already demonstrated its
reliability and efficiency. The main advantage in the use of
this technology is represented by its potential to increase the
efficiency reducing at the same time the active material used
for the energy conversion process, determining, consequently,
a reduction in the cost of electricity production. In fact, the
HCPV is characterized by an optical system that,
concentrating the sun energy, increases significantly the
incident radiative power on photovoltaic cells surface. As a
consequence, the usage of expensive semiconductor material
is reduced proportionally to the concentration factor.
Moreover, the use of multi junction (MJ) cells or the use of
dichroic spectrum splitting systems, correctly matched with
different energy band gap photovoltaic cells, suitable for high
energy density, allows the extension of the physical energy
conversion limit of a single photovoltaic material [4]-[5].
978-1-4799-4033-2/14/$31.00 ©2014 IEEE
It is worth noting that the approach followed to obtain such
results is different from that fulfilled in the classical crystalline
photovoltaic (PV) devices and requires the development of an
integrated and complex system. In fact, the HCPV system has
to satisfy some strict technical constraints connected to the
necessity of concentrating and focusing continuously the
Direct Normal Irradiation (DNI) on PV cells with an even
spatial distribution. This requires a correct integration among
the concentrator optics, sun tracker, solar cells and the control
systems of tracker and inverter. The complexity connected to
the design, joint to the management and the maintenance of
such systems, is considered, at the present time, the main
drawback for the diffusion of HCPV as distributed generation;
otherwise their performance and their technological
characteristic make it particularly suitable for high power PV
power plant located in sites characterized by high DNI value.
In particular, the main critical aspect of CPV technology is
connected with alignment, and involves both optical elements
and the coupling between the optical system and the solar cell
receiver. In particular, an extremely accurate alignment is
crucial for high HCPV systems: an inaccurate focalization of
the concentrated sunlight on the photoactive area can cause a
reduction of the system efficiency and, in the worst case,
permanent damages to the PV receivers or to the components
inside the module. Another essential issue of this technology
is the correct design of the heat sink. With the increase of the
receiver temperature, the reliability of the module decreases:
the open circuit voltage drops and consequently the system
performance is negatively affected [6]-[8]. Following these
considerations, the choice and the design of the optical system
and heat sink are of particular importance.
Currently, the main part of the CPV modules in the market
employs refractive optics (mainly Fresnel lenses) because of
their relatively low cost and of a reduction of system
complexity. In fact, Fresnel lenses are usually made with low
cost Poly-methyl methacrylate (PMMA) lenses characterized
by a typical thickness of 4-5 mm. However, the Fresnel lenses
introduce chromatic aberration phenomena which contribute
to the degradation of the overall system performance [9].
Moreover, the spatial distribution of concentrated radiation is
a critical issue and can cause an incorrect heating, leading to
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hot spot effects that increase the series resistance and the solar
cells degradation. To overcome this problem, a secondary
optical element, represented by a homogenizer, is usually
implemented [10]. This device is typically constituted by light
guide of customized geometry and material. The
implementation of this solution allows at the same time to
increase the acceptance angle of HCPV receivers and panel
and to reduce the effect of optic misalignment and of the
position accuracy requirement of the sun tracking system [4].
An alternative configuration of HCPV system is based on
reflective optics. In some cases parabolic mirrors are used but
more often it is preferable the use of a Cassegrain optics
configuration. Cassegrainian type concentrators are
characterized by a double reflection system typically
composed of a main collector and a secondary reflector that
constitute together the primary optical element. In order to
make uniform the concentrated radiation pattern and increase
the acceptance angle, an optical homogenizer is often used as
secondary optics. In this case the optic design inputs are less
challenging permitting the implementation of simplified non
imaging optics. In fact, these systems are not affected by
chromatic aberrations and hence a correct optical alignment is
only required in the developing of the design process [11].
In the present paper a novel configuration of a Cassegraintype concentrator photovoltaic module, characterized by
geometric concentration factor of 400x, has been presented.
The designs process, the development of the system and the
outdoor characterization of two prototypes of HCPV receivers
(implementing high performance MJ photovoltaic solar cells
and spectrum splitting photovoltaic system, respectively) are
described, discussed and compared in the following.
II.
DESIGN OF THE CASSEGRAIN-HPVC RECEIVER
In this section the general design and assembly of both
prototypes are presented. The prototypes structure is based on
a new concentrating photovoltaic system which relies on a
Cassegrain architecture. It is characterized by short focal
length that determines a reduction in the height dimension of
the HCPV receiver permitting the accomplishment of a thin
panel structure. To achieve this purpose the optical
components have been designed to obtain the optical focus on
the back plane of the primary collector. In order to minimize
loss of efficiency due to an imperfect alignment between the
primary collector and secondary reflector and increase the
acceptance angle, the concentrator optic is equipped with a
secondary optic based on a non-imaging reflective system that
develops the homogenizer function, as well. Defined the
geometry, the main dimension has been fixed referring to the
irradiation required by the high performance solar cell and to
its dimension. Considering the standard dimension of HCPV
cells, the focusing surface has been imposed to 1 cm2 and the
geometric concentrator factor has been set to 400x.
A. Optic design of Cassegrain Optics
The design of the Cassegrain optics has been developed
considering collection surface of 400 cm2 and its geometry can
be observed in Fig. 1. It is characterized by a parabolic mirror
as main reflector that collects the radiant energy on a square
area of 20x20 cm2 in size.
Fig. 1. Novel Cassegrain-type optic proposed for HPVC
The geometry of the first reflector has been designed in
order to optimize the active reflective surface. The second
reflector has an aspheric shape and focuses the sun radiative
energy in the central position where is located the secondary
optic element (SOE). As a consequence, the active collection
surface is reduced to 375 cm2. A square hole of 2x2 cm2 in
size has been located in the center of the main reflector in
order to allow the positioning of the second optic. This fulfills,
in the case of the first prototype based on MJ cells, the
homogenizing function and it is characterized by a truncated
pyramid shape with a square base and reflective surface,
allowing the uniform irradiation of photoactive area of the
solar cell.
The optical concentrator has been designed using the
software Zemax and it has been tested carrying out a nonsequential simulation of each single component and the whole
assembly. The optical configuration for different misalignment
of the solar concentrator is showed in Fig. 2. Particular
attention has been devoted to the homogenizer. A non-imaging
optic structure based on the use of reflective material has been
used. The effect of the light guide is to ensure a uniform
spatial distribution of radiant energy even in case of
misalignment between the optical axis of the concentrator and
the sun direction.
Fig. 2. Zemax design of the Cassegrain-type concentrator system covered with
a protective layer of silicon oxide.
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B. Optic Prototype development
Once the optimization of the optical components and of the
assembly has been carried out, the design process has been
focused on the realization of a number of prototypes in order
to study the behavior of different PV receivers with the same
concentrating optics.
The main collector is made out of polycarbonate realized
by injection molding and subsequently coated by PVD
metallization in aluminum. The developed main collector
prototype is reported in Fig. 3-a. The secondary reflector is a
convex mirror made out of BK-7 optical glass, with a diameter
of 50 mm and a thickness of 10 mm, and coated with an
aluminum layer and silicon oxide protection. The system is
such optimized that the acceptance angle is at least ± 0.6° for
90% of the nominal power. In order to obtain this acceptance
angle the MJ-concentrators are equipped with rectangular light
pipe made with Alanod MIRO high reflective 95, reported in
Fig. 3-b. Four receivers have been arranged to be closely
packed in order to achieve the largest collecting area of the
module. The concentrator prototype is composed by a 2x2
receivers array, characterized by a collecting area of 42x42
cm2. The final configuration of the prototype is reported in
Fig. 4.
III.
THE HCPV PHOTOVOLTAIC SYSTEM
Typical HCPV are equipped with III-V MJ solar cells
which total spectral response covers a wavelength ranging
from about 300 nm to 1800 nm, as can be observed in Fig. 5.
In the present paper the array of solar concentrators has been
equipped by both MJ solar cells and a spectrum splitting PV
receiver, both using the same concentration optic and both
tested under the same outdoor conditions.
In order to compare the performance of PV receivers based
on MJ solar cells and of a spectrum splitting receivers, and to
demonstrate the flexibility of the proposed Cassegrain
concentrator, two different prototypes of high concentrator
photovoltaic system have been developed. The first one is
based on a CESI Ge/InGaAs/InGaP solar cell featuring an
efficiency of 30% and the second is a spectrum splitting
receiver using a dichroic filter that splits the concentrated
sunlight in two different wavebands, eventually diverted
towards two different single junction PV cells. The first cell
used in the spectrum splitting receiver is a mono-crystalline
silicon device manufactured by Narec, while the second is a
GaAs device manufactured by CESI. The cumulative
efficiency of the spectrum splitting receiver referred to PV
solar cells test condition is 27%.
Fig. 4. Proposed HCPV Cassegrain prototype
Considering that the maximum operation temperature of
MJ cells is of 70°C, particular attention has been dedicated to
the thermal management of the photovoltaic cells. In fact, the
photoactive area is subjected to an intensive heat generation
due to both the high concentration level of sunlight and to the
cells losses. To match the temperature constraints of the PV
devices, the PV receivers have been equipped with heat sinks
engineered to be thermally coupled with the back-panel of the
HCPV module, made out of aluminum. The contact thermal
resistance between solar cell and heat sink has been reduced
using high thermal conductivity compound. These features
make the HCPV back-panel an integrated passive cooling
system which serves also as mechanical reference for all the
optical components and the solar PV receivers. The solar cells
have been soldered onto a high thermal conductivity substrate
(Metal Core PBC) with a proprietary reflow technique. This
allows the achievement of a value of conductivity of 1.4
W/m∙K between the heat sink and the solar cell. The
assessment of the void percentage in the soldering joint
between solar cell and metal-core PCB has been carried out by
means of RX analysis at 60 keV (W anode/Mo filtering). The
test reported in Fig. 6 reveals a void percentage lower than
4%, which is compatible with the target thermal resistance of
the PV receiver.
Top cell
Middle cell
Bottom cell
1
Spectral Response
0.8
0.6
0.4
0.2
0
a
b
Fig. 3. Proposed HCPV receiver Cassegrain: a- primary optics main collector
prototype; b- secondary optics prototype
0
200
400
600
800
1000
1200
Wavelenght [nm]
1400
1600
1800
2000
Fig. 5. Spectral responses of top, middle and bottom junctions in a typical
triple-junction cell.
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950
0
900
-0.02
850
-0.04
800
-0.06
750
-0.08
Error x
-0.1
10:34
Error y
11:03
11:32
700
DNI
12:01
DNI [W/m2 ]
Error [degrees]
Fig. 6. RX analysis of the soldering process for a MJ CPV solar Cell: 1Receiver with integrated temperature sensor and homogenizer; 2-RX image at
60 keV; 3- Contrast enhanced image revealing 4% voids
0.02
12:30
650
12:59
Fig. 9. Tracker performance as measured in a sunny morning
+0.4°
+0.3°
+0.2°
Elevation Error [degrees]
Fig. 7. Dichroic beam splitter mounted on the filter holder (left) and heat sink
equipped with GaAs solar cell (right)
+0.1°
+0.1° + 0.2° +0.3° +0.4°
-0.1°
-0.2°
-0.3°
-0.4°
Azimuth Error [degrees]
Fig. 10. Polar plot of tracking error in a partially sunny day
Fig. 8. Schematic representation of the spectrum splitting PV receiver,
coupled with the concentrating optics.
A. The dichroic filter and the beam splitting configuration
The spectrum splitting function is accomplished by a
dichroic short-pass beam splitter characterized by a cutoff
wavelength of 785 nm. The mirror is held at 45° with respect
to the optical axis of the concentrator by an aluminum holder
and eventually glued with high temperature epoxy resin (Fig.
7). The short wavelength portion of the spectrum is
transmitted by the filter towards the GaAs solar cell, whereas
the long wavelength portion of the spectrum is reflected at 90°
with respect to the optical axis of the concentrator towards the
Si solar cell. A schematic representation of the proposed beam
splitting photovoltaic energy conversion system is reported in
Fig. 8.
B. Solar Tracker
On the basis of the optical design, the angular acceptance of
the module is ±0.6°. In order to ensure the highest conversion
efficiency, the pointing error of the tracking systems should be
smaller than the CPV module’s acceptance angle. To
accomplish this task, a dual-axis solar tracker, controlled by
means of a four-quadrant sensor and characterized by a
tracking accuracy of 0.01° in tilt and 0.15° in azimuth, has
been utilised. The sun tracking accuracy has been firstly
monitored by AKKUtrack instrument [13]. This tool measures
in real time the tracking error in pointing the solar source and
consequently its degree of accuracy. Fig. 9 shows the tracker’s
performance in a cloudless morning. This data gives some
information about the tracker pointing errors, revealing an
azimuth error (black line) within ±0.05° and a maximum
elevation error (red line) of about 0.09°. The same results are
resumed in polar form in Fig. 10. The results highlight that the
tracking errors during sunny day are mainly within the range
of ±0.3° making it suitable for the proposed HCPV system.
IV.
CARACTERIZATION OF HCPV CASSEGRAIN PROTOTYPE
The HCPV Cassegrain prototype reported in Fig.4 has been
outdoor characterized in a site located in Sardinia (Italy) and
suitable for this kind of tests (39° 14’ 52.933” N, 8°58’17.306”
E). The I-V curves, DNI, Global Normal Irradiance (GNI) and
ambient temperature measurements have been carried out,
monitored and recorded. The I-V curve tracer has been
developed using a Keithley 2651A high power source meter
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Fig. 12. Comparison between the efficiency of the CPV module based on MJ
solar cell (Red), Spectrum Splitting PV receiver (Blue) and a Reference
Silicon solar cell (Green)
Fig. 11. I-V curve tracer and environmental parameters monitoring system
Labview GUI interfaces
which is controlled by a LabView based graphical user
interface (GUI) program running on a laptop (Fig. 11-top). In
order to acquire a complete I-V curve, the device has been
programmed to generate variable voltage steps and collet
current and voltage quantities at the same time, according to
the user settings. In the GUI, I-V and P-V curves, obtained
processing the measured data, are shown in a X-Y graph
together with Isc, Voc, Pmax, Impp, Vmpp and Fill Factor (FF)
values. The external atmospheric conditions (DNI, GNI,
ambient temperature, wind speed and direction, relative
humidity) and the module temperature have been measured by
several sensors, signals of which have been acquired by a
National Instruments data acquisition platform. This device is
directly connected to the personal computer in order to
elaborate the signals and show and store the measurements
(Fig. 11-bottom).
To measure the CPV receivers’ temperatures, four PT100
probes have been placed on the back of the module as near as
possible to the photovoltaic cells. This equipment allows the
atmospheric variables to be acquired at the same interval as
the concentrating photovoltaic system electrical and thermal
parameters. Therefore, thanks to the outdoor characterization,
the performance of the module can be studied and analyzed at
different and real atmospheric conditions, evaluating the
weather patterns on the CPV panel power and efficiency [14].
V.
COMPARISON BETWEEN DIFFERENT CPV RECEIVERS
The comparison between CPV modules based on MJ solar
cells and on spectrum splitting PV receivers has been carried
out calculating the overall conversion efficiency for different
time of the day. As the atmospheric condition change during
the day, in terms of humidity and air mass, the spectrum of the
solar radiation at the ground undergoes to daily variation.
Seasonal variations are also affecting the spectrum of the solar
radiation at the ground, but at this stage we focused mainly on
the daily variation. In particular, the blue portion of the solar
spectrum is progressively absorbed or scattered as the air mass
increases. In fact, in the late afternoon, the blue portion of the
direct radiation at the ground is substantially reduced. In MJ
devices, the junctions are connected in series and the current
flowing in the device is determined by the junction that
generates the lowest photocurrent, which is usually the top
one, more sensitive to the blue radiation. In late afternoon, as
the blue portion of the solar spectrum is progressively
absorbed or scattered, the top junction generates a lower
current and thus chokes the current flowing in the whole
device. As can be seen from Fig. 12, the efficiency of the CPV
receiver based on MJ solar cell decreases progressively during
the day. On the contrary, the efficiency of the CPV splitting
receiver remains quite constant during the day because the two
solar cells (Si and GaAs) are not bounded to have the same
current. As a reference, in Fig. 12 we also reported the
efficiency measurements of a standard Silicon cell. The
corresponding DNI for every time record is represented at the
top horizontal axis. It hasn’t been possible to collect data at
DNI lower than 430 W/m2 because the tracking systems was
not reliable enough when the elevation of the sun was lower
than 30° or when the sky was partially cloudy.
The overall efficiency of the spectrum splitting receiver
turns out to be lower because of the narrower energy band
converted (400-1200 nm instead of 400-1600 nm for the MJ
device) and because of the optical losses in the dichroic beam
splitter. With an optimization of the optical components of the
beam splitter we expect a raise in the efficiency of the
spectrum splitting receiver up to 23%, using the same solar
cells. The overall efficiency can also be raised by using double
anti-reflection coated front glass and a more reflective coating
of both the primary collector and of the secondary mirror.
Still the energy production of a PV system based on MJ
solar cells appear to be higher than the one based on spectrum
splitting PV receiver, but it’s interesting to note the
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differences in the efficiency trend of these two different
receivers. The efficiency trend of the reference silicon cell is
basically constant which reveals that the spectral mismatch for
single-junction solar cells is negligible and the temperature
variations are not large enough to justify the performance drop
registered in MJ solar cells. All the receivers have been
mounted on the same base plate and they were at the same
temperature throughout the outdoor tests. Regardless this test,
we expect that in operative conditions MJ solar cells would
undergo higher temperature because of the higher power
density impinging on the receivers with respect to spectrum
splitting ones.
VI.
[11] S. Horne, G. Conley, J. Gordon, D. Fork, P. Meada, E. Schrader, and T.
Zimmermann, “A Solid 500 Sun Compound Concentrator PV Design,”
in Proc. of IEEE 4th World Conference on Photovoltaic Energy
Conference, 2006, vol. 1, pp. 694–697.
[12] C. Dominguez, I. Anton, G. Sala, “Solar simulator for indoor
characterization of large area high-concentration PV modules”, in Proc.
of 33th IEEE Photovoltaic Specialists Conference, 2008, pp.1-5
[13] www.akkutrack.com
[14] A. Damiano, I. Marongiu, C. Musio and M. Musio, “Outdoor
characterization of a Cassegrain-type concentrator photovoltaic
receiver,” in Proc. of 39th Annual Conference of the IEEE Industrial
Electronics Society (IECON), pp. 8110 – 8115, Nov. 2013.
CONCLUSION
In the present paper the design and the prototype
characteristics of high concentrator photovoltaic systems
based on Cassegrain optics has been presented. The flexibility
of the proposed structure has been demonstrated by means the
implementation of two different photovoltaic configurations
based on multi-junction solar cells and on a beam splitting
configuration, respectively. Finally, the characterization of the
developed prototypes and the comparison between their
energy performances have been reported and discussed.
ACKNOWLEDGMENT
The present work has been supported by Regione
Autonoma della Sardegna (LR7/2007). The authors
acknowledge Sardegna Ricerche for technical support and
assistance.
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