CONCENTRATING SYSTEMS FOR PV GENERATORS
YIANNIS TRIPANAGNOSTOPOULOS
Physics Department
University of Patras
Physics Department, University of Patras, Patra 26500
GREECE
Tel/Fax: +31 2610 997472 e-mail: yiantrip@physics.upatras.gr
1. Introduction
Several investigations result to lower the cost of photovoltaics increasing also their
electrical efficiency, but their payback time has not been reduced enough to be considered
cost effective. The combination of solar radiation concentration devices with PV modules
is up to now the most viable method to reduce system cost, replacing the expensive cells
with a cheaper solar radiation concentrating system. Besides, concentrating photovoltaics
present higher efficiency than the typical ones, but this can be achieved in an effective
way by keeping PV module temperature as low as possible. For PV cooling, a water or air
circulation mode can be applied to extract the heat from it, avoiding the efficiency
reduction due to the PV module temperature increase. The extracted heat can be rejected
to the ambient or can be used to cover thermal needs of the application and these devices
are the hybrid photovoltaic/thermal (PV/T) solar energy systems.
The concentrating solar systems use reflective (flat or curved mirrors) and refractive
(mainly fresnel lenses) optical devices. Concentration ratio CR is a parameter that
corresponds to the ratio of system available aperture area Aa to the incoming solar
radiation to the receiver (absorber) surface area Ar (CR=Aa/Ar, or X suns). The solar
energy systems are characterized by their concentration ratio and can be combined with
“linear focus” (2D) or “point focus” (3D) absorbers for low (CR<10X), medium
(CR<100X) or high (CR>100X) CR, respectively. Concentrating systems with CR>2.5X
must use a system to track the sun, while for systems with CR<2.5X, stationary
concentrating devices can be used. The low concentrating ratio systems (CR<10X) are of
particular interest for the photovoltaics as they are of linear geometry and thus one
tracking axis is enough for their efficient operation.
Monocrystalline silicon (c-Si) PV cells are the most usual type for the concentrating PV
systems but their electrical output is reduced from the temperature increase and the nonuniform distribution of solar radiation on their surface. Cells of GaAs have higher
conversion efficiency than c-Si cells and can operate more effectively in higher
1
temperatures but are of substantially higher cost. Thin film photovoltaics like CIGS (CuIn-Ga-Se2) are less sensitive to the non-uniform distribution of solar radiation but they are
still of lower efficiency than crystalline silicon cells. Concentrating PV module
efficiencies are in the range of 15% up to about 30%. Plane and mainly curved reflectors
are used often to increase the density of solar radiation on the focal point (3D
concentrators) or on the focal line (2D concentrators), while fresnel lenses of inexpensive
and light in weight plastic material are also developed. The concentrator material should
be inexpensive compared to the cells, as it corresponds to 5, 20, 100 or more times larger
aperture than them and also durable for a long period, aiming to adapt the life of cells
(more than 20 years).
The distribution of the solar radiation on cell surface and the temperature rise of it are two
problems that affect its electrical output. The uniform distribution of the concentrated solar
radiation on cell surface and the application of a suitable cooling mode contribute in all
cases to an effective system operation, considering the achievement of the maximum
electrical output. Non-uniformity is due to mirror shape error, which even if it is small it
has a significant effect on the flux profile. The passive (heat sink) or active (heat
extraction by water or air circulation) cooling of cells are usual modes to keep their
temperature at an acceptance level. As an extension of the simple cooling mode, the
hybrid photovoltaic/thermal (PV/T) solar systems have been investigated to provide
simultaneously electricity and hot fluid, which contributes to a higher conversion rate of
the absorbed solar radiation making the system more practical.
In linear concentrating PV systems the longitudinal radiation flux profile along the string
of cells is affected by the shape of mirror, shading due to receiver supports and gaps in the
illumination. The flux profile is not same for all cells in series and this limits the current
and the performance of them. Maximum deviation from the ideal shape of the system is
less than 1 mm and even small deviations from the perfect shape can cause significant
non-uniformities in the flux at the focal line. In these cases a reduction in short circuit
current and open circuit voltage are observed reducing finally the electrical output of the
used photovoltaics. Another effect is that the temperature in locations of high illuminance
can be 10-15 oC higher than elsewhere in the cell, reducing the open circuit voltage. In the
absence of a flux modifier the electrical losses are in the order of 5-15 %. In addition,
other optical losses due to tracking process, wind and high ratio of diffuse solar radiation
cause a further reduction of the final system electrical output.
2. Main improvements in concentrating photovoltaics.
Several research centers, universities and companies have improved systems regarding the
concentrating photovoltaics. Among them we can refer the following:
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1. Alpha Solarco, which has develped a 14kW point focus fresnel lens system at
Pahrump, Nevada, USA.
2. Amonix Inc and SunPower Corp. have developed a 20 kW point-focus fresnel lens
array, installed at Arizona, USA.
3. Australian National University has developed a linear trough concentrator system of
30X and 15% overall efficiency at Spring Valley, Australia.
4. Ben-Gurion University has developed a very large dish PV system at Negev, Israel.
5. BP solar and the Polytechnical University of Madrid have installed a 480 kW parabolic
trough 38X concentrating PV system at Tenerife with 13% overall efficiency
(EUCLIDES project).
6. Entech Inc has improved fresnel lenses for 20X concentrating PV systems of 100 kW
and 300 kW at Texas, USA.
7. Fraunhofer-Institute for Solar Energiesystems in Freiburg, Germany has developed
300X fresnel lens concentrators with GaAs cells to achieve 24% efficiency.
8. National Renewable Energy Laboratory (NREL) of USA has developed high efficiency
multi-junction GaInP/GaAs cells to achieve efficiency of 30% for a variety of 150X
concentrators.
9. Polytechnical University of Madrid, apart of EUCLIDES project has developed static
concentrators and also 100X very thin concentrators combined with 1 mm GaAs cells.
10.Photovoltaics International has developed acrlic 10X single-axis fresnel lenses.
11.Solar Research Corporation has developed a 85 W reflective dish concentrators of
239X and 22% efficiency with GaAs modules at Australia.
12.SunPower Corporation manufactures point-focus fresnel lens-type concentrators of
250X to 400X with peak efficiency 27%, in collaboration with Stanford University.
13.University of New South Wales, Australia, has developed an innovative static 4X
concentrator.
14.University of Reading is researching a variety of point focus fresnel modules and
reflective trough modules.
15.Tokyo ART University has been involved in 2D (1.65X0 and 3D (2X) refractive static
concentrators.
Concentrators definitely have the potential to be comparative on cost but they must be
effectively designed to take this benefit. Considering the basic categories of concentrating
photovoltaics, low (CR<10X) medium (CR<100X) and high concentration (CR>100X) we
can refer the more characteristic works that have been published in journals and presented
in conferences. The solar radiation concentration devices are the reflectors (flat, V-trough,
CPC, cylindrical parabolic, dishes etc) and the lenses (linear fresnel lenses, point focus
fresnel lenses, dielectric type lenses, etc).
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3. High concentration photovoltaics
Regarding high concentration PV systems, we can refer the 240X fresnel lens (AMONIX)
with achieved efficiency 20.3 % by Garboushian et al (1994) and the 100X fresnel lens
with CPC refractive secondary concentrator (SunPower) achieving efficiency 26.8% by
Verlinden et al (1995). An analysis of a paraboloidal dish reflector for a high flux (500X)
concentrating photovoltaics was presented by Ries et al (1979), a fresnel lens system 120X
by Rumyantsev et al (2000), a 3D reflector with concentration 1000X was developed by
Feueremann and Gordon (2001) and an optical two stage concentrator with 1000X (RXI,
HERCULES) on GaAs cells and efficiency 25% was suggested by Algora et al (2000).
4. Medium concentration photovoltaics
In the range of medium concentration PV systems there is a variety of devices, based on
2D and 3D reflectors and fresnel lenses. Beckman et al (1966) presented the design of a
high flux PV concentrating system based on small flat mirrors, which was combined with
a cooling system. A similar system was studied by Ittner (1980), where images generated
by an array of tracking mirrors for PV concentrating systems were calculated and a system
based on fresnel lenses including study of the chromatic dispersion was presented by Sassi
(1980). The same period Cluff and Call (1980) give experimental results for 2X – 10X
concentrating photovoltaics with active cooling for hot water and space heating. O’Leary
and Clements (1980) studied a parabolic trough PV concentrating system using 2.4 cm
cells and Gibart (1981) introduced the hybrid photovoltaic/thermal collectors with
cylindroparabolic reflectors of 10X, 20X and 40X on 2 cm and 1 cm cell width. Later,
Gordon (1966) presented the optical design for 50X – 100X PV concentrators with
homogeneous irradiance on the absorber.
Last ten years some new investigations on PV concentrating systems were developed. The
single-mirror-two stage-concentrator (SMTS) of 60X or 30X by Benitez et al (1995) and
the Dielectric SMTS (DSMTS) type of 100X or 30X (using a dielectric secondary
concentrator) by Benitez et al (1997), were two new concentrating PV systems. Blakers
and Smeltink (1998) worked on parabolic trough reflector system of 28.1X (actual 20.1X)
with cell efficiency 20% and total system efficiency up to 15%. A big project on medium
concentrating photovoltaics, the EUCLIDES project, was presented Luque et al (1997) and
by Sala et al (2000) with the 480 kW demonstration plant consisting of 140 linear
parabolic mirrors and 138 molules of 250 m2 aperture area and 38.2X. Recently, Coventry
(2005) studied CHAPS (Combined Heat and Power Solar Collector), a hybrid PV/T
system with parabolic trough 37X and peak flux intensity ~100 suns, achieving electrical
efficiency 11% and thermal efficiency 58%. Apart of the reflectors, fresnel lenses were
used for medium concentrating photovoltaics, with James and Williams (1978) studying
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fresnel optics for concentrating photovoltaics, Lorenzo and Sala (1979) developing glass
type fresnel lenses, O’Neill et al (1990) presenting details from the fabrication, installation
and operation of a linear fresnel lens with photovoltaics and Bottenberg et al (2000)
suggesting SunFocus, a linear fresnel type concentrator combined with linear cells.
5. Low concentration photovoltaics
In the category of low concentration photovoltaics, flat and curved reflectors, fresnel
lenses and dielectric lens type concentrators have been studied. Among the first works are
the publications of Ralph (1966) and Roy (1978), with Luque (1981) studying the
advantage of static concentrators and Stacey and McGormick (1984) giving performance
results for low concentration photovoltaics, reporting measurements of the effect of Vtrough low concentrators on the performance of conventional passively cooled PV
modules. We can also refer the works on low concentration photovoltaic/thermal systems
for electricity and heat generation by Mbewe (1985), Sharan et al (1986), Al Baali (1986)
and Garg et al (1991) and some other studies on low concentration photovoltaics, as of
Chanumba et al (1990) and Parretta et al (2002). Many works have been performed in the
field of low concentration photovoltaics, which can be grouped regarding the used mode
of concentration in systems with V-trough reflectors, with CPC (Compound Parabolic
Concentrators) type reflectors, with refractive concentrators and linear fresnel lenses, other
concentrating systems and also systems with bifacial PV modules.
Regarding the V-trough reflectors, many works are referred to one or two tracking axis
using flat mirrors. The planar reflectors are used to increase solar radiation on PV module
surface, with sun tracking to result in a uniform distribution of solar radiation on it. They
are simple devices, achieving concentration ratios up to about two with east-west or northsouth orientated reflectors. Freilich and Gordon (1991) presented a case study on installed
photovoltaics with V-trough tracking concentrators and Gordon et al (1991) calculated
yearly collectible energy from this type systems. Klotz et al (1995) showed the technical
feasibility and operational effectiveness of PV V-trough system with passive tracking and
Fraidenraich (1998) presented V-trough cavities achieving uniform illumination of the
absorber. Klotz et al (2000) presented the ARCHIMEDES project, which was developed
on the idea of V-trough concentrators and passive tracking. Poulek and Libra (2000) and
Dobon et al (2001) presented simple tracking systems, combining commercial PV
modules with flat reflectors.
CPC type concentrators is another category of devices that are coupled with PV modules.
Most of them are static concentrators (no movements to track the sun), but the nonuniform distribution of solar radiation on the surface of cells affects their electrical
efficiency decreasing it. Among the works of this category we can refer the papers of
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Almonacid et al (1987), Goetzberger (1988), Zanesco and Lorenzo (2002), Mohedano et al
(1998) and Uematsu et al (1998). In the same category there are also some other works
based on stationary CPC reflectors, as of Garg and Adhikari (1999), Brogren et al (2000,
2002), Helgesson et al (2004) and Nilsson et al (2004). Bifacial cells are used in order to
adapt concentrating system geometry, reducing in this way cell material. Ortabasi (1997),
Bowden et al (1993), Mayregger et al (1995), Hernandez et al (2000), Libra (2004). In the
same group we could also consider the work of Moehlecke et al (2001) with the bifacial
cells combined with colored diffuse reflectors.
Finally, among the first studies on dielectric lens-type concentrators, Edmonds et al (1987)
presented the design and performance of stationary concentrators, considering of Vtroughs filled with oil or water. Yoshioka et al (1997, 1999) presented optical results for
3D static acrylic lens concentrators, achieving a reduction of 62% in cell surface. Show
and Wenham (2000) give ray-tracing results for a 3D lens with 2X utilizing refracting and
total internal reflection system optics and Zacharopoulos et al (2000) present a 3D analysis
on dielectric non-imaging concentrating PV systems of asymmetric geometry and
regarding fresnel lenses, Kaminar et al (1991) presented a linear-focus 10X frenel lens
concentrator.
Comparison results give an idea about the benefits of concentrating photovoltaics.
Wheldon et al (2000) present results from a study of several types of PV modules
(concentrating and flat type) showing that concentrators can reduce the environmental
impact of PV. Whitfield et al (1998) compared several types of concentrating PV systems
and Verlinden et al (2000) compared a point-focus concentrating system with a fixed flat
plate PV module and showed that the concentrating system produces 37% greater
electrical energy than the flat PV module.
6. The diffuse reflector concept
The increase of the total energy output of photovoltaics can be achieved by using diffuse
reflectors of aluminium sheet, as boosters, which give an almost uniform distribution of
the reflected solar radiation on PV surface (Tripanagnostopoulos et al, 2001, 2002). In
horizontal roof photovoltaic stationary installations, the PV modules are usually placed in
parallel rows, with a distance between them to avoid shading. A fraction of the incoming
solar radiation on the horizontal PV installation is not used by PV modules from Spring to
Autumn, as solar rays are striking the free horizontal surface between the parallel PV rows
(because of the higher sun altitude). This fraction of radiation could be partially used by
the PV modules if booster diffuse reflectors are placed between the parallel PV rows and
increase the solar radiation on PV surface. The diffuse booster reflectors achieve a
smoother distribution of the additional solar radiation on the PV surface, which can be
6
almost uniform for some reflector-PV module geometries (Tripanagnostopoulos et al,
2002, Hall et al, 2005). The additional solar input on the surface of PV modules is lower
than that of specular reflectors, but diffuse reflectors are cheaper and can be combined
easily with typical size PV modules.
We consider a concentration factor C that corresponds to a homogeneous increase of the
incoming solar radiation on PV module surface, with value C =1 for net solar input
(without additional solar radiation from the diffuse reflector) and C =1.1 for an effective
10% additional solar input, etc, up to C =1.5 for 50 % additional solar input on PV
module. Higher values of factor C are not usually achieved in practice, because of the use
of diffuse reflector in the collector - reflector geometry. To have comparative results, the
systems are tested under almost same incident net solar radiation G and concentration
factor C , in a small range of ambient temperatures T and wind speed Vw . In practice the
additional solar radiation is not uniformly distributed on PV module surface and we
consider an effective concentration factor Ct that corresponds to solar radiation profile
from bottom to top of PV module surface.
To measure the effective concentration factor Ct of the diffuse reflector, aluminium sheet
with angles  of 90° and 120° between it and the plane of the PV module can be selected,
in order to get the distribution of solar radiation on the PV surface as a function of the
angle of incidence  of it on the diffuse reflector. The angle  =120° is almost an upper
limit of effective angles. A photosensor of small c-Si PV cell (1 cm2) can be used, to
measure the net ( I net ) and the total ( I tot ) incident solar radiation on the plane that
corresponds to the PV module surface.
With the collected data we have the distribution of the total incident solar radiation on PV
module expressed by the effective concentration factor Ct . In Fig. 1 we show the device
for the measurement of the concentration factor Ct for variable values of the ratio L / R ,
where L and R are the PV module width OL and R the reflector width OR
respectively. We consider that L / R  1 in practical applications of booster diffuse
reflectors. In the experiments flat aluminium sheet as diffuse reflector was used as a
widely available low cost material with satisfactory reflectance.
Simulating the plane of PV module we can move the photosensor (PS1) along the line L
of the rod [OL], recording the variation of the total solar radiation (net plus reflected,
I tot  I net  I ref , in mA) for variable incidence angles  of solar rays on the diffuse
reflector. In every circle of these measurements, the value of the net solar radiation
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parallel of the PV plane ( I net in mA, PS2) must be also measured. The concentration
factor Ct is determined by Ct  I tot / I net and is calculated along [OL] for various angles  .
Fig. 1 Measurement of Ct by using photosensors PS1 ( I tot ) and PS2 ( I net ), for variable
angle  of the incident solar radiation on the diffuse reflector, which forms angle 
with PV module surface.
The distribution of the reflected solar radiation from the diffuse reflector on PV module
surface depends on sun altitude and azimuth, on sky clearness (level of diffuse solar
radiation), on angle between reflector and PV module, on PV module slope and on the
optical properties of the reflector material. The tests are performed during noon with
diffuse solar radiation  25% of total and the reflector must be properly adjusted to PV
plane slope to achieve the desired angles  and  (Fig. 1), for the position of sun during
the experiments. In Fig. 2 we give the results in diagrams for angle  =90° between
reflector and PV plane and also in Fig. 3 the results in diagrams for angle  =120°,
regarding angle  of 0°, 15°, 30°, 45°, 60° and 75° in both figures. The results show that
the use of an angle  >90° i is less effective than an angle  = 90° between PV module
and diffuse reflector.
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1.7
angle φ = 90°
1.6
Concentration factor C
t
45°
1.5
15°
1.4
1.3
30°
1.2
0°
1.1
60°
1.0
0.9
0.00
75°
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
L / R ratio
Fig. 2 Variation of the concentration factor Ct for various L / R ratio values, regarding the
angles  =0° to  =75° of the incident solar radiation on the diffuse reflector, which
forms an angle  =90° with PV module surface.
1.7
Concentration factor C
t
1.6
angle φ = 120°
1.5
1.4
60°
1.3
75°
1.2
45°
0°
1.1
30°
1.0
0.9
0.00
15°
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
L / R ratio
Fig. 3 Variation of the concentration factor Ct for various L / R ratio values, regarding the
angles  =0° to  =75° of the incident solar radiation on the diffuse reflector, which
forms an angle  =120° with PV module surface.
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Based on these results we can test the PV modules regarding the “effective” electrical
efficiency  el of it as a function of concentration ratio C. Actually the obtained value of
 el is rather the system performance than the system efficiency. In the steady state tests of
PV modules with booster diffuse reflector we can use in all experiments the proper
module - reflector geometry to achieve the same effective concentration factor Ct with
homogeneous illuminance of PV module surface (we take C  Ct ).
We tested a commercial pc-Si PV module and during tests the PV electrical was connected
to a load, simulating real system operation. With the tested PV module at thermal
equilibrium under ambient conditions, we determine the values of current I m (in A) and
voltage Vm (in V) at maximum power point of PV module operation from the collected
I (in A), V (in V) data. The values of I m and Vm and the net incoming solar radiation G
are used to find the PV module electrical efficiency el for system aperture area Aa using
the relation:
el  I mVm / A G
Studying the effect of booster diffuse reflectors we calculated the electrical efficiency
from the net incoming solar radiation G on the PV module surface (without the additional
solar input from the diffuse reflector).
The study of the experimental PV modules with diffuse reflector includes tests with
variable percentage of the additional solar radiation from the diffuse reflector, with respect
to its electrical efficiency as a function of the pc-Si PV module temperature. We consider
a concentration factor C that corresponds to a homogeneous increase of the incoming
solar radiation on PV module surface, with value C =1 for net solar input (without
additional solar radiation from the diffuse reflector) and C =1.1 for an effective 10%
additional solar input, etc, up to C =1.5 for 50 % additional solar input on PV module, as
higher values of factor C are not usually achieved in practice, because of the use of
diffuse reflector in the collector - reflector geometry of Fig.1.
We calculated el for C  1.0 to C =1.5 (with step 0.1) adjusting properly the experimental
device for the achievement of the corresponding values of total solar radiation on PV
module surface during noon and circulating water through the heat exchanger of the
thermal unit, which was mounted on PV rear surface to achieve operation of the system at
several values of PV temperature T pv . In Fig. 4 the results from these tests, using fitting
lines for the values of el of the corresponding concentration factor C are presented.
These results correspond to PV module of pc-Si type. The solar radiation from the diffuse
reflector affects the PV module electrical performance but in calculations we use the net
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solar radiation on PV module (“effective” nel ) to get the electrical output from the
proposed combination in comparison with that without using diffuse reflector (results for
C  1). The results of Fig. 4 show that an electrical efficiency increase of about 25% for
PV operating temperature T pv  40 C to about 35% for T pv  70 °C is achieved, comparing
the effect of using stationary booster diffuse reflector with its practical possible maximum
value of concentration factor ( C  1.5) to the plain PV system ( C  1.0).
0.18
Electrical efficiency η
el
0.17
0.16
C=1.0
C=1.1
C=1.2
C=1.3
C=1.4
C=1.5
0.15
0.14
0.13
0.12
0.11
0.10
0.09
35
40
45
50
55
60
65
70
PV Temperature ( °C )
Fig. 4 Results of electrical efficiency el for the diffuse reflector concentration factors
C  1 to C  1.5 (step 0.1) and for variable pc-PV module operating temperature.
From the results of Fig. 4 we also calculate a mean electrical efficiency drop of 0.08 %
per K of PV temperature increase. The use of stationary booster diffuse reflectors with PV
modules on horizontal building roof installations is an effective application. This
combination increases system cost by 4%-5%, but results to an improvement of the
electrical output (for C  1.35) by 15%-16%. The results depend on the location of the
installation and the achieved concentration factor during winter (for Patras) is low
( C  1.35), while the weather conditions are not favorable (cloudy sky) for the operation
of systems. In Spring, Summer and Autumn the corresponding concentration factor is
higher ( C  1.5), resulting to an efficient operation of the system. The system with booster
diffuse reflectors of C =1.35 can achieve a percentage increase of electrical efficiency up
to 19.2 %, considering total additional cost of about 14 % to the use of plain PV modules.
This is a significant improvement considering the use of stationary reflectors and the
additional thermal output of the system.
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