1. No category

HOW TO INCREASE THE EFFICIENCY OF A HIGH CONCENTRATING
PV (HCPV) BY INCREASING THE ACCEPTANCE ANGLE TO ±3.2°
A.Yavrian1, S. Tremblay1, R.Gilbert1 and M. Levesque2
1
Opsun Technologies Inc, Québec, Canada
2
Institut national d’optique (INO), Québec, Canada
e-mail :[email protected]
ABSTRACT
To compare the real efficiency of a HCPV to
any other solar systems, we must evaluate the
energy (Kwh) generated by the HCPV and the
other solar systems located on the same site
over the same period of time (one year) in
order to take into account all the losses
associated to optical, heat, wind, dust,
diffused light, acceptance angle, dispersion
and beam homogenization
Many studies have compared the energy
(Kwh/m2/year) generated by a HCPV and a
silicon PV based system. These studies
conclude that the energy generated by a HCPV
system, in the best conditions, is not higher
than 1.4 times the energy generated by a PV
tracked system even if the efficiency of the
photovoltaic elements of a HCPV is 2.35 times
higher than those of a PV system. How can
such results be explained?
More and more solar system specialists
associate the low performance of the HCPV to
a narrowness of the acceptance angle. The
goal of this Opsun’s R&D project was to find a
way to enlarge the acceptance angle.
Recently, Opsun Technologies Inc. realized a
new type of HCPV at low cost. The most
important feature of this HCPV is its very large
acceptance angle coupled with high optical
efficiency. The outdoor measurements using
the sun as source of light demonstrated more
than ±3.2 degrees of acceptance angle, while
the global optical transmission was at the level
of 87%. The geometrical concentration was
around 380 Suns.
STANDARD PHOTOVOLTAIC PANELS
(PV)
As the price of a Kwh continues to decrease,
solar energy is becoming a more important
source of new electrical generation
installations. In 2011, over 28,000 MW of new
solar systems were installed. The solar panels
using photovoltaic elements can be classified
in two main groups. The first one is standard
PV panels which do not use any concentrating
optical element, while the second one, called
HCPV, is applying extensively concentrating
optics. There are very important differences
between these two categories. The light
power generated by the sun is 1kW per square
meter on earth. The spectrum of sun radiation
is significantly large; it covers from 350 nm up
to 2500 nm. The sun light contains two types
of radiation, direct and diffuse light. The
distribution between the direct DNI and the
diffuse part is strongly dependent on
geographical position and weather conditions.
It is also accepted that direct normal incidence
radiation (DNI) is concentrated within a kind
of cone allowing a ±0.275 degree field of view.
A certain fraction of diffuse radiation is
concentrated within an angular filed of
± 0.275° to ± 3°. This part of the sunlight beam
is called circumsolar.
1
PV panels’ configuration is extremely simple.
They are composed of a glass sheet, serving as
a mechanical support and a protective cover
on which EVA (ethylene-vinyl acetate) is used
to glue the solar cells to the glass. Solar cells
are made of silicon and cover the whole useful
surface of a PV panel; therefore there is no
need to use concentrating optical elements.
The absence of concentrating optics allows
the PV panel to convert direct and diffuse
light. The current commercial efficiency of
PV’s cells is at the level of 17% (1,2). Based on
the fact that silicon has a limited spectral
response (between 450 to 900 nm),
commercial expectation of PV efficiency does
not excess 20%. Based on 2012 second
quarter PV prices, a 16% PV will allow to
generate Kwh at around $0.20US in an area
having high 2,600 Kwh/m2/year solar
exposition. To reach the grid parity, price will
have to continue to decrease by a factor of at
least 50%. Doubling the efficiency of a solar
system is definitively a good way to reach that
goal.
In contrast to PV, HCPV uses optical elements
to concentrate the sun light into solar cells
and also use GaAs type cells instead of
traditional silicon. Multijunction solar cells in
general consist of three different layers having
the capability to extract the energy of
different parts of the sun spectrum. Each layer
is designed for a specific wavelength band.
Hence, the multijunction cells have a much
better spectral response than PV and
therefore are able to convert the sun light into
electricity with an efficiency expected to be at
the level of 50%(3). Actual triple junctions solar
cells lab world record is 43.5%(4,5), while the
efficiency of available commercial triple
junctions cells is in fact at the level of 40%(6).
With such an efficiency, the end user is
expecting a generation of Kwh per square
meter at least twice those generated by PV.
But this is not the case. Why?
HCPV PERFORMANCES’
There are two types of HCPV. The first type
uses refractive type optics to concentrate the
sun light, while the other uses reflective.
Regardless of concentrator type, HCPV
contains primary and secondary optical
elements. The main concentration is
performed by primary optical element. In
addition, secondary optics are needed to
homogenize the profile of the beam. Whether
HCPV is a reflective type or refractive, the
HCPV can only concentrate the direct light
(DNI).
In refractive type concentrator, Fresnel lenses
are used as primary optics, mainly because of
their low manufacturing costs. However,
prismatic structure of Fresnel lenses causes
relatively high optical losses. Actual
commercial Fresnel lenses have between 75%82% of optical transmission. Based on these
elements, a HCPV using Fresnel lenses and
triple junctions solar cells having an efficiency
of 40% and primary optical efficiency of 80% is
expected to generate a global efficiency of
32% compare to PV which is at 16%. When we
were expecting that a HCPV would generate
twice the energy than a PV during the same
period, it was demonstrated that it is not the
case. Some HCPV generate a PV equivalent
efficiency as low as 19%. That means that
there is an additional 60% loss of energy with
HCPV. What is the main source of such
additional losses?
ACCEPTANCE ANGLE
HCPV modules, in contrast to PV, need to be
constantly aligned with respect to DNI sun
beam. This is why HCPV are mounted on
sophisticated trackers which follow the sun in
order to guarantee normal incidence of DNI
beam. If a concentrator is not perfectly
aligned with the sun beam, it will lose part of
the available energy. This misalignment angle
2
at which the performance of concentrator is
reduced by more than 10% is called
acceptance angle. To this date, the race to
obtain the most efficient HCPV module has
been mostly governed by the desire to
increase the optical transmission of used
optical elements. During this race, another
very important parameter has been neglected,
the acceptance angle of the complete
assembly.
If the impact of optical transmission and
geometrical concentration ratio on the cost
and on annual energy production are
relatively easy to predict, the role of the
acceptance angle is more complicated to
evaluate and understand. Intuitively, the
acceptance angle must be limited by the direct
beam DNI angular divergence and the tracking
system accuracy. Considering a ±0.275 degree
of field of view’s DNI, and the fact that
nowadays tracker precision is around ±0.1° ±0.2°, the acceptance angle ±0.5° should allow
to collect 100% of the available energy.
Actual HCPV’s modules demonstrate various
concentration ratios, but the acceptance angle
of all these HCPVs are within ±0.5° - ±1°.
During the fall of 2011, GreenMountain
Engineering (GreenMountain) and Institute de
Sistemas Fotovoltaics de Concentracion
(ISFOC, Spain) published a quite important
study of HCPV performances(7). Namely,
during eight weeks, the performances of HCPV
systems, installed in the field, were collected
and analysed. A particular attention was put
on the relation between the generated energy
and the acceptance angle. These studies
provide an answer to the question of where
additional losses of HCPV come from. ISFOC
and GreenMountain demonstrated the
importance of the acceptance angle.
According to this study, a module having ±0.5
degree of acceptance angle would generate
an additional Kwh loss of 60%, and as HCPV
has an acceptance angle of ±1 degree, energy
losses will be at the level of 25 %. This is
obtained with tracking systems having
demonstrated ±0.1° - ±0.2° degree of angular
precision.
In the past, similar studies were conducted
with less devastating results. These
experiments were performed using single
module mounted on a laboratory type wellcontrolled tracker. The results change when a
study is conducted with a real HCPV system
installed in a field and exposed to the sun light
and to the outdoor conditions as in the case of
ISFOC and GreenMountain studies.
Real HCPV systems use several modules
assembled side by side (forming a platform)
and can reach several square meters of
surface area. Such systems become more
sensitive to mechanical deformations and
manufacturing
errors.
Mechanical
deformations can have various origins. They
can be caused by wind, gravitational forces as
well as by thermal expansions. For example,
gravitational forces lead to deformation of
extremities of the platform or deformation of
central pedestals, on which the platform is
mounted (axes of rotations). These
deformations are amplified by wind load
which, in addition, can result in pedestal
torque and shakiness (or back and forth) of
whole tracking system.
Taking all the above-mentioned factors into
account, it becomes easier to explain the
results
obtained
by
ISFOC
and
GreenMountain.
Thus,
HCPV
with
low-acceptance angle will often be misaligned,
hence generating significantly less energy than
the predicted value.
It is important to mention that the geographic
location also contributes to the problem of
low-acceptance angle. This is due to the
presence of some energy in circumsolar
3
The high-acceptance angle of a HCPV module
not only increases annual energy production,
but also leads to significant decrease of the
cost of HCPV’s systems; lower acceptance
angle requires more sophisticated complex
tracking systems. In fact, the trackers used for
HCPV modules are more expensive than those
used for the flat top PV modules; this is the
direct results of low-acceptance angle. By
increasing the acceptance angle, the trackers
having lower precision can be used; hence a
significant cost reduction in the final system
will be obtained.
achievable acceptance angle. This angle is
determined by the following equation:
sin sin Where α is the acceptance angle, ns is the
refractive index of the optical material
deposed on triple junctions solar cells surface,
θcel is the incident angle on solar cell. Figure
1presents the theoretical curve representing
the variation of acceptance angle with respect
to concentration ratio.
8
7
Acceptance angle (deg.)
radiation. As mentioned above, sun radiation
is composed of DNI and diffuses radiations.
However, there is still energy within angles
±0.27° to ±3°. Depending on geographic
location as well as weather conditions, the
energy confined between theses angles can be
quite significant. Thus, a low-acceptance angle
will not allow effective capture of this energy
neither.
6
5
4
3
2
1
0
0
200
400
600
800
1000
1200
1400
1600
Geometrical concentration ration
As mentioned above, actual HCPV modules
require the use of tracking systems whose
precision is within ±0.1° - ±0.2°. Such high
precision has to be maintained during 20 -25
years, which is a quite difficult task. However,
modules showing a higher acceptance angle
will be able to use lower precision trackers.
This will reduce maintenance costs, leading to
increase of customer confidence in long-term
reliability of HCPV systems.
OPSUN’S APPROACH OF HCPV
DESIGN
Opsun has concentrated its attention on the
acceptance angle issue. Opsun wanted to
achieve the highest possible acceptance angle
without sacrifying optical efficiency. It is
known that the maximum theoretical
acceptance angle is limited by the
concentration ratio. For a given geometrical
concentration ratio there is a maximal
Figure 1.
The efficiency of solar cells is changing when
the incident angle θcell is increased. However,
for a wide range of incidence angles, this
variation is still negligible. Thus, the
theoretical curve presented in figure 1 was
obtained with the values of ns and θcel giving
the minimal optical losses.
According to ISFOC and GreenMountain
studies, for HCPV modules, whose acceptance
angles are higher than ±1.2° - ±1.4°, there is
no loss related to acceptance angle
As one can see from Figure 1, the acceptance
angles increase with the decrease of
geometrical concentration ratio. However, the
reduction of geometrical concentration ratio
increases solar cells cost. Thus, the final choice
of acceptance angle should take into account
4
a careful analysis of tracker system, solar cells
and annual energy gain.
which has a worldwide recognized expertise in
optical design(8).
After choosing an optimal concentration ratio,
it is important to design HCPV whose
acceptance angle is as much as possible close
to corresponding theoretical value. At Opsun,
we have developed a new type of HCPV
concept which makes it possible to obtain
acceptance angles close to theoretical values
for very wide range of geometrical
concentrations (300 – 2000 Suns). Opsun is
confident to reach an acceptance angle of
±1.9° at concentration ratio of 1000 Suns.
To measure the acceptance angle, Opsun
HCPV system was mounted on a tracking
system. The measurements were conducted in
the region of Quebec City (42), between
March and May 2012. The sun radiation
power was constantly detected by two
identical pyrometers, one to measure DNI +
circumsolar and the other isotropic diffuse
radiation. As mentioned above, in order to
characterize the optical performances of the
HCPV, short current generated by a
multijunction cell was used.
EXPERIMENTAL REALIZATION AND
The general design of HCPV consists of two
main optical elements. Namely, it contains
primary concentrating element and secondary
one. The role of the first element is to do the
main concentration, while the second optical
component is applied to increase the
acceptance angle as well as to homogenize
the intensity profile (flat top beam) of the
concentrated beam on the solar cell.
Some HCPV producers are avoiding the use of
secondary optical component in order to
reduce the cost of HCPV module. However,
their acceptance angle is extremely low
(±0.5°). In order to reach high-acceptance
angle values, a secondary optical element has
to be used. Therefore, Opsun’s module has
been designed with both components:
primary refracting focusing element and
secondary one. Nevertheless, no commercially
available optical elements allow for an
acceptance angle of ±3°. Therefore,
completely new types of optical components
were designed and fabricated by Opsun. The
Opsun’s concentrator is a result of fruitful
collaboration between Opsun Technologies
Inc. and INO (Institut national d’optique)
In Figure 2, the variation of Opsun’s HCPV
output with respect to the incident angle is
presented. As shown, the generated energy
was almost constant until the concentrator
was misaligned with respect to its initial
position by more than 3.2° (half-angle). The
transmission of the HCPV is practically
unchanged within the incident angle ±3°.
However, when the module was misaligned by
more than ±3°, transmission dramatically
decreased. Such behaviour indicates a
perfectly designed concentrator, otherwise
more bell-like transmission curve would have
been observed.
Reciver photocurrent a.u.
RESULTS
1,3
1,2
1,1
1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
Opsun HCPV
Commercial HCPV
-5
-4
-3
-2
-1
0
1
2
3
4
5
DNI incident angle (deg)
Figure 2.
Based on the above-mentioned definition of
the acceptance angle, we can certainly claim
5
that Opsun’s HCPV demonstrates a ±3.2°
acceptance angle, which is very close to
theoretical value of ±3.6° (see fig. 3). Slight
difference between the theory and
experiment is explained by the presence of a
small angular divergence (angular field of
view) of DNI sun beam (theoretical curve
assumed collimated light).
8
Acceptance angle (deg.)
7
Theoretical value of Opsun HCPV
6
5
4
Analysis of experimental results.
Obtaining such a high-acceptance angle
means the possibility to generate much more
annual energy than current commercial HCPV
modules. Based on ISFOC and GreenMountain
studies, we can make some preliminary
estimations
about
Opsun’s
module
performance installed in a region where
available solar energy is 2600 kWh/m2/y and
average ambient temperature is 20°C (see
table 1). Using the data from PV6 simulator
and from NREL(9), approximate performances
of 2-axis tracked PV module, a commercial
HCPV, and Opsun’s HCPV modules were
considered.
3
2
PV
comercial
HCPV
comercial
HCPV
comercial
HCPV
Opsun
1X
380X
380X
380X
Acceptance Angle
±90 °
±0,5°
±1 °
±3,2 °
Available
Kwh/m2/Year
2600
2600
2600
2600
Efficiency of
photovoltaic elements
17%
40%
40%
40%
Diffuse light
15%
15%
15%
15%
Production of
Kwh/m2/Year without
losses
442
884
884
884
Optical losses
6%
20%
20%
13%
Kwh/m2/Year after
optical losses
415
707
707
769
Thermic losses
10%
3%
3%
3%
Acceptance Angle
losses
0%
60%
25%
0%
Annual real energie
production
Kwh/m2/Year
374
274
514
746
1
0,73
1,38
2,00
Type of modules
1
Experimental value of Opsun HCPV
Concentration Ratio
0
0
200
400
600
800
1000
1200
1400
1600
Geometrical concentration ration
Figure 3.
It is worth to note that Quebec region is not
the ideal place for HCPV’s test since the
energy present in circumsolar radiation could
be significantly high. We expect that in the
regions well-suitable for HCPV operations, a
higher acceptance angle will be detected.
Note that the same performance is observed
with respect to elevation angle variation, since
the concentrator was designed to perform in
the same manner for all directions.
In parallel to acceptance angle, other optical
parameters play a very important role in the
performance of a HCPV module. One of them
is the homogeneity of the sun beam incident
on the solar cells. Due to Opsun’s HCPV
careful optical design, output beam is quite
homogeneous for almost the whole sun light
spectrum and, nearly for all incidence angles
confined within the acceptance angle.
Ratio of Kwh
generated by
HCPV/PV
Table 1.
The difference between a ±0.5° and ±1°
acceptance angle module and Opsun HCPV is
mostly explained by the curve supplied by
GreenMountain and ISFOC(6). From this
review, one can determine that the losses
related to an acceptance angle of ±0.5° will be
6
at the level of 60% and of 25% with a ±1°
acceptance angle, while losses will be at 0%
with Opsun’s HCPV.
CONCLUSION
A new type of concentrator having very
high-acceptance angle was designed and
successfully tested. The demonstrated
acceptance angle was ±3.2 degrees for a
geometric concentration ratio of 380 Suns. To
our best knowledge, this is the highest
acceptance
angle
which
has
been
demonstrated up to now for HCPV. We expect
that, due to such high-value of acceptance
angle, the annual energy production will
increase significantly while tracking system,
manufacturing, maintenance and installation
costs will decrease.
References:
1. Canadian Solar,
http://www.canadiansolar.com
2. JA Solar, http://www.jasolar.com
3. A. Luque, Will we exceed 50% efficiency in
photovoltaics? J. Appl. Phys. 2011; 110;
031301-031301-19
4. Solar Junction, http://www.sj-solar.com/
5. Sharp, http://sharp-world.com/
6. Spectrolab, http://www.spectrolab.com/
7. B. Stafford1, M. Davis1, J. Chambers1, M.
Martinez1, D. Sanchez2, Tracker accuracy:
field experience, analysis, and correlation
with meteorological conditions.
1
GreenMountain Engineering, LLC, SanFrancisco, CA, and Somerville, MA, USA
2
Instituto de Sistemas Fotovoltaicos de
Concentracion S.A., Puertollano, SPAIN
8. INO (Institut National d Optique),
www.ino.ca
9. http://rredc.nrel.gov/solar/pubs/redbook/
7