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Problems and Disadvantages in
                                                Current Residential & Commercial
                                                On-grid PV Systems


                                                SolarEdge Technologies




Background

Current technology used in On-Grid PV systems                connected in a string of modules so that a voltage
has many drawbacks. The purpose of this paper                high enough for DC/AC inversion (150V to 800V) is
is to discuss and categorize the problems and                achieved. More power can be added to the system
drawbacks inherent in residential and commercial             by adding strings. Since the strings are connected
photovoltaic systems. Typical residential photovoltaic       in parallel, they have to match the other strings
installations suffer from numerous problems that             in all parameters i.e. type of panels, length and
prevent this technology from realizing its full market       orientation. The entire array is connected to a solar
potential. Many of the present problems stem from            inverter which is responsible for harvesting the
power losses – whether due to module mismatch,               electrical energy and inverting it to AC so it can be
orientation mismatch, or partial shading. Other              fed into the grid.
problems stem from system design limitations and
constraints, lack of monitoring and lack of analysis
abilities. In addition, the absence of safety features
poses risks to both workers installing or maintaining
the system, and to firefighters dealing with fires in                                                 Inverter
the vicinity of PV installations.
                                                                                                  MPPT

For an overview of the SolarEdge distributed power
harvesting system, which overcomes the limitations                                                =         ~
of traditional systems, please refer to the SolarEdge
paper “SolarEdge Architecture Overview”.

Typical System Architecture

Current residential PV systems are typically built
from ten to a few hundred PV modules connected in              Solar Strings

a series-parallel connection, as shown in Figure 1.
Several panels (10 to 15 typically) are series-              Figure 1

                                                         1
The inverter handles maximum power point tracking                             4
for the entire array. This is done by finding the DC
working point in which the most power is harvested
                                                                              3
from the array. The harvested power is inverted
                                                                Ii
from DC into AC power that is fed into the grid. The            Ii
inverter is also responsible for conforming to the              Ii
                                                                              2

electrical and safety regulation requirements.                  IMPP

                                                                              1

PV System Drawbacks
                                                                              0
In the following section we will discuss the different                            0    10            20           30
                                                                                                     U1, U2, U3, UMPP
                                                                                                                                   40   50

causes for power losses in typical PV systems, as
well as other key drawbacks of these systems.                Figure 2


Module Mismatch Losses
                                                                               4

The manufacturing process of PV cells produces                       Ii
cells with relatively large tolerances in their power                Ii        3
                                                                     Ii
output capability. To reduce the difference between
                                                                     Ii
cells in the same module, they are sorted during                     IMPP      2
manufacturing to different power categories (bins)
                                                                     IMPP_3
and modules are assembled with cells from the                                  1
                                                                     IMPP3
same bin. This produces panels with smaller
tolerance variances in output power. The panels                      IMPP_3
                                                                               0
themselves are not sorted and panels in the market                                                    Ui               UMPP_3
today have a 3% tolerance in output power.                                             U1i, U2i, U3i, 3 , UMPP, UMPPi, 3



When several modules are connected in series,                                 120
each one has a slightly different MPP current. The
                                                                              100
series connection does not allow the optimal MPP                     P1i
                                                                     P2i
current to be drawn from each module. The inverter                   P3i
                                                                               80
will select the current which gives the average peak                 Pi
                                                                     3
power point of the string or array. This peak power is               PMPP
                                                                               60

always less than the theoretical sum of the individual                         40
                                                                     PMPP1
peak power points of every module. Figure 2 shows
                                                                     PMPP_3
the I-V and power curves of 3 mismatched modules                       3
                                                                               20

(3% tolerance). In Figure 3 the accumulated I-V and                               0
power curves of these modules are shown. It is                                     0    10            20
                                                                                                       Ui
                                                                                                                         30
                                                                                                                          UMPP_3
                                                                                                                                   40   50

clear that the string’s peak power point is different                                   U1i, U2i, U3i, 3    ,
                                                                                                                UMPP UMPPi, 3
                                                                                                                   ,



from the sum of peak power of the three modules.
                                                             Figure 3
          120


          100
                                                             The loss demonstrated here is referred to as
                                                             mismatch loss. In standard residential and
   P1i
           80                                                commercial PV installations, it can be as high as
   P2i
           60
                                                             5%.
   P3i
   PMPP
           40


           20


            0
                0   10    20           30   40   50
                         U1, U2, U3, UMPP




                                                         2
Partial Shading Losses                                         losses are estimated to contribute between 5% to
                                                               25% annual power losses.
Partial shading occurs when part of a panel or panels
are shaded, causing different levels of illumination
on the cells in the panel. This can happen due to
shade from the building itself, light posts, chimneys,
trees, cloud fronts, dirt, snow and other light-blocking
obstacles.

Shading on any part of the array will reduce its output,
but this reduction will vary in magnitude depending
on the electrical configuration of the array. Clearly,
the output of any shaded cell or module will be
lowered according to the reduction of light intensity
falling on it. However, since this shaded cell or panel
is electrically connected to other unshaded cells and
modules, their performance may also be lessened
since this is essentially a mismatch situation.

For example, if a single module in a series string is
partially shaded, its current output will be reduced
and this may dictate the operating point of the entire
string. Alternatively, the module’s bypass diodes
may conduct, causing this module to stop producing
power1. Partial shading of crystalline solar modules
will result in dramatic reduction of solar module
output. One completely shaded cell can reduce a
solar module’s output by 40% to 95%.
                                                               Figure 4

If several modules are shaded, the string voltage
may be reduced to the point where the open-circuit             MPP Efficiency Losses
voltage of that string is below the operating point
of the rest of the array (under voltage situation),            The MPPT performance is a very significant aspect
resulting in that string not contributing to the array         of the characterization of PV systems, since PV
output.                                                        systems, like other systems based on renewable
                                                               energies, must harvest the maximum available
The reduction of output from an array with partial             energy at every moment from the renewable
shading can be significantly greater than the                  resource. The global efficiency of the MPPT algorithm
reduction in the illuminated area. This is due to the          depends on its ability to make the inverter operate at
loss of output from unshaded cells in the partially            the maximum power point (MPP) at every moment.
shaded module, the loss of power from illuminated              In order to achieve this, the MPPT algorithm has to
modules in any severely shaded string that cannot              accurately track the MPP variations, which can be
maintain operating voltage and the loss of power               caused by factors such as irradiance, temperature
from the remainder of the array because the strings            variations and partial shading. Obviously, this
are not operating at their individual peak power               accuracy will be strongly influenced by both the
points2.                                                       amplitude and the dynamics of the variations of the
                                                               MPP. MPP losses occur due to two main factors:
For residential systems, it is impossible to avoid all
shading without severely restricting the size of the           1. Static losses caused by the inability of the MPP
array and hence losing output at all times (see Figure         algorithm to locate the array’s peak power point. In
4 for examples). In these systems, partial shading             some cases, the MPPT algorithm can lock itself in
                                                               local maxima.


                                                           3
2. Dynamic losses caused by the inability of the                                                                                  process at the end of the day. The second day
MPPT algorithm to track changes in the peak power                                                                                 corresponds to a cloudy day in which atmospheric
point at the right speed.                                                                                                         conditions are varying quickly. The figure shows the
                                                                                                                                  evolution during both days of the maximum power
The main cause of static losses in PV systems is local                                                                            of the generator, obtained from the measured I–V
peaks in the array’s power curve. Shading effects                                                                                 characteristic curves.
cause the array’s power curve to exhibit more than
one maximum power point. More specifically, two or                                                                                                                    Evolution of the maximum power during day 1
more peak power points appear in the curve, where                                                                                                          4500

different modules contribute power to different local                                                                                                      4000

peaks. Most MPPT algorithms are not designed to                                                                                                            3500
handle local peaks efficiently and when the MPPT
                                                                                                                                                           3000
is locked on a local peak, the MPPT efficiency falls
dramatically. Figure 5 shows measured MPPT static                                                                                                          2500




                                                                                                                                      maximum power (W)
efficiency of 4 different inverters in the market3. The                                                                                                    2000

results differ showing losses of 1% to 10%.                                                                                                                1500

                                                                                                                                                           1000

                                                                                                                                                            500
                         1                                                             1

                                                                                                                                                             0
                                                                                                                                                                  8        10         12            14        16    18
                                                                                                                                                                                            time (hours)
                        0.9                                                           0.9
                                                                    MPPT efficiency
    MPPT efficiency




                                                                                                                                                                      Evolution of the maximum power during day 2
                                                                                                                                                           4500
                        0.8                                                           0.8

                                                                                                                                                           4000

                                                                                                                                                           3500
                        0.7                                                           0.7
                              0     20     40       60   80   100                           0    20    40     60   80   100
                                  % of nominal DC power                                         % of nominal DC power                                      3000

                                         a) PCU A                                                  b) PCU B
                                                                                                                                                           2500
                                                                                                                                       maximum power (W)




                                                                                                                                                           2000
                          1                                                            1
                                                                                                                                                           1500

                                                                                                                                                           1000
                        0.9                                                           0.9
      MPPT efficiency




                                                                    MPPT efficiency




                                                                                                                                                            500

                                                                                                                                                              0
                        0.8                                                           0.8                                                                         8        10          12           14        16    18
                                                                                                                                                                                            time (hours)



                        0.7                                                           0.7                                         Figure 6
                              0     20     40       60   80   100                           0     20    40    60   80   100
                                  % of nominal DC power                                         % of nominal DC power
                                           c) PCU C                                                    d) PCU D
                                                                                                                                  With this information, it is possible to calculate
                                                                                                                                  the real maximum energy that could have been
Figure 5                                                                                                                          obtained with optimal MPPT during these days. This
                                                                                                                                  value was then compared with the energy effectively
Dynamic losses are directly related to the speed and                                                                              harvested by the leading PV inverters in the market.
accuracy of the MPPT algorithm. The peak power                                                                                    The comparison of the available energy to that
point shifts mainly due to changes in the irradiance.                                                                             actually harvested in the first (sunny) day is shown
While these shifts are slow and often monotonous                                                                                  in Figure 7.
under clear, cloudless atmospheric conditions, they
can also be relatively quick during some atmospheric                                                                              The graph shows how, during most of the day, the
conditions. Figure 6 shows measurements from two                                                                                  MPPT algorithm tracks the maximum power point
consecutive days in Spain4. The first day corresponds                                                                             quite well. However, during the final part of the day
to a sunny day that is affected by a partial shading                                                                              (circled in red), due to the process of partial shading,

                                                                                                                              4
the MPPT algorithm fails to track the evolution of                                                                                 System Design and Roof Utilization
the generator maximum power point. During these
hours, the performance of the MPPT algorithm in                                                                                    In addition to the power loss contributors previously
terms of energy harvested from the generator (MPPT                                                                                 discussed, current PV system architecture also
efficiency) is 96%, that is, 4% of the energy has been                                                                             presents great challenges to installers and system
lost due to the fact that the MPPT algorithm is not                                                                                designers. The following factors result in a system
able to handle this real life scenario.                                                                                            which is less optimal and, in many cases, smaller
                                                                                                                                   than desired:
                                                              Test and characterization of PV commercial inverter (day 1)
                                                   4500

                                                   4000
                                                                 Maximum power point
                                                                 Inverter operating point                                             The string’s voltage is bound by the specific
       inverter and MPP (W) and energy (kWh/200)




                                                                                                                                   inverter’s minimum and maximum permissible
                                                   3500
                                                                power                                                              voltages
                                                   3000

                                                   2500                                                                              The strings are placed in parallel and must be
                                                   2000
                                                                                                                                   identical in length, combined with the specific
                                                                                                                                   geographical and physical constraints of the roof
                                                   1500

                                                                                                       energy
                                                   1000
                                                                                                                                   When designing the system, the installer has to take
                                                    500                                                                            into account the inverter’s maximal input voltage
                                                      0
                                                                                                                                   and make sure that in the most extreme open-
                                                          8             10         12           14
                                                                                        time (hours)
                                                                                                                16      18
                                                                                                                                   circuit conditions, the string’s voltage won’t reach
                                                                                                                                   this maximal voltage. This is difficult because many
Figure 7                                                                                                                           parameters like voltage tolerance, temperature
                                                                                                                                   variations and sunlight have to be taken into
The results for the second day pattern are shown                                                                                   account. On the other hand, the minimal inverter
in Figure 8. The day is cloudy with quick variations                                                                               input voltage must be maintained at all times in
in irradiance due to atmospheric conditions. The                                                                                   order to keep harvesting power. Taking into account
MPPT efficiency of the inverter in terms of energy                                                                                 the same parameter changes and the possibility of
extracted from the PV generator is reduced to 95%                                                                                  partial shading in the string make things even more
during the entire day.                                                                                                             complicated. In some cases, these constraints lead
                                                                                                                                   to low roof utilization. There are many documented
                                                                                                                                   events5 where installers elected to use a shorter
                                                              Test and characterization of PV commercial inverter (day 2)
                                                   4500                                                                            string in order to increase roof utilization, causing
                                                                Maximum power point
                                                                                                                                   system under-voltage during the summer.
   inverter and MPP (W) and energy (kWh/400)




                                                   4000         Inverter operating point

                                                   3500
                                                                                                                                   The roof utilization issue is even more dramatic on
                                                   3000                                                               energy       commercial roofs, where the roofs’ irregular shapes
                                                   2500                                                                            and additional obstructions (chimneys, vents, air-
                                                   2000       power                                                                conditioning units, etc.) increase the difficulty in
                                                                                                                                   selecting just one string length. The average roof
                                                   1500
                                                                                                                                   utilization in commercial roofs is 65% to 70% due
                                                   1000
                                                                                                                                   to roof-related problems.
                                                    500

                                                      0                                                                            Multi-faceted roofs represent another set of
                                                          8           10          12            14              16     18
                                                                                        time (hours)
                                                                                                                                   challenges related to PV Systems. In these cases the
                                                                                                                                   use of multiple inverters enables better utilization
Figure 8                                                                                                                           of the roof surface. For example an installation
                                                                                                                                   on a roof with 2 facets (south-east and south-west
                                                                                                                                   facets) will require two inverters or a multi-input
                                                                                                                                   inverter. Both solutions are less cost effective.




                                                                                                                               5
It is clear that the serial-parallel architecture puts         Retrofit and Long Term Fault Tolerance
many constraints on the PV system, preventing the
full utilization of the roof and reducing the actual
                                                               Another drawback of PV systems is the difficulty of
annual energy harvested from the system.
                                                               retrofitting and replacing faulty modules throughout
                                                               their life cycle. If a PV module breaks in an installation
                                                               (due to extreme weather conditions or accidents),
System Feedback and Troubleshooting                            it has to be replaced with a module with similar
                                                               electrical characteristics, and since PV modules are
Another inherent problem of the serial-parallel                improving all the time, it is not possible to use a new
connection is the inability to verify the proper               PV module in an old installation. For this reason, PV
installation and operation of every element in the             module companies keep an inventory of cells and
system. The only data points available are measured            modules for 25 years as a supply for old systems.
at the inverter (input voltage, input power) and               The same reason prevents the gradual retrofit of
can only partially assist in evaluating the system             old systems—you are essentially stuck with old
performance.                                                   technology.

Since usually only overall power output is monitored,
significant problems with an installation can go               System Safety
undetected yet cost the owners 10% or more of
their energy payback. Without actionable diagnostic            The safety of PV systems has always been an
information, the only alternative when operating               issue of concern. Over the years, many codes and
problems are suspected is to deploy technicians to             standards were devised and discussed to improve
the site to search for the problem with little direction       the safety of these systems. Two main safety issues
or guidance. This search becomes more difficult                remain:
as the installation size increases. The average
commercial solar site installed in California in 2004          1. The risk of electrocution while installing the
had 1,000 solar modules, making this difficulty very           system. Even the series connection of two PV
real.                                                          modules exposed to sunlight generates an unsafe
                                                               voltage with enough power to kill whoever touches
The parallel connection of strings prevents the                the exposed contacts. Many work procedures and
detection of defective modules and in some cases               safety precautions employed during the installation
the detection of a disconnected string. For example,           processes slow the work but maintain employee
in a system with 8 strings, the detection of 12.5%             safety.
power loss, which is equivalent to one string, is
not trivial, especially since the system’s expected            2. The risk to firemen during a fire. The first thing
power output is dependant on all the other criteria            firemen do in a building fire is cut the power. This
mentioned above.                                               enables them to spray water and use axes to cut
                                                               holes in the roof to let smoke out. These actions
Even when a fault is suspected and technicians                 are dangerous when PV systems are installed on
are sent to the site, maintenance is expensive                 the roof. Firemen are trained to cut the power to
because it requires the deployment of skilled                  the building. After doing so, they believe that there
technicians to the solar site. Once on-site,                   is no power source that can endanger them. In
their only recourse is to dismantle the system                 PV installed buildings cutting the power does not
and search for the cause of the failure.                       eliminate the dangerous voltages present at the
The same lack of feedback also increases the initial           string ends. Firemen preparing to chop a hole in
installation time due to lengthy verification and              the roof or to use water could be electrocuted6.
debugging.                                                     Regulatory bodies and firefighting officials are
                                                               pushing for new regulations that will ensure the
                                                               shutdown of PV modules in case of fire.




                                                           6
Conclusion

We described the structure of PV systems and several
drawbacks and disadvantages in the way these
systems are designed and built. Though previous
works focused on improving different aspects of
photovoltaic systems, no single holistic approach
was presented that could solve many of the current
problems present in PV installations. SolarEdge
Technologies distributed power harvesting system is
the only power harvesting system and architecture
that offers a full, robust solution that harvests up to
25% more energy and resolves the issues described
in this paper.




1
  “Effects of Reverse-Biased PV Cell in Panels”,
SolarEdge Technologies (2006)
2
  “Photovoltaic Modules, Systems and Applications”,
Nicolla M. Pearsall & Robert Hill (2001)
3
  “Performance Assessment of the Inverter based
Grid Connection of Photovoltaic Systems”,
Gianfranco Chicco, Roberto Napoli, Filippo Spertino
(2004)
4
  “On the Testing, Characterization, and Evaluation
of PV Inverters and Dynamic MPPT Performance
Under Real varying OperatingConditions”, Pablo
Sanchis, Jesu´s Lo´pez, Alfredo Ursu´a, Eugenio
Gubı´a and Luis Marroyo (2007)
5
  “PV Installations, A Progress Report”, John C.
Wiles, Bill Brooks, Bob-O Schultze (2002)
6
  “Commercial Building Product Magazine”, (2007)




                                                          7
About SolarEdge

SolarEdge provides next generation power conversion electronics that
effectively remove all known system constraints across the photovoltaic
energy space. Our Smart DC technology enables increased production
of clean, grid-ready energy at a lower cost per watt than any other
competitive offering.

SolarEdge technology marries traditional photovoltaic workflows and
installation methods with a groundbreaking holistic system approach. It
is a quiet revolution that is at once disruptive because of its profound
benefits in changing the manner in which energy is harvested, deployed,
managed and delivered and complementary because it fits into the
current photovoltaic workflow.

At SolarEdge we believe the PV delivery chain is ultimately only as
strong as its weakest link. By adopting a “system first” philosophy that
identifies and eliminates the Achilles heel in each step in the process,
we enable a constraint-free delivery of sun harvested energy.




USA              2225 East Bayshore Rd., Suite 200, Palo Alto CA 94303, USA
Japan            B-9 Ariake Frontier Building, 3-7-26 Ariake, Koto-Ku, Tokyo 135-0063, Japan
Israel           6 HaHarash St. P .O.Box 7349, Neve Neeman, Hod Hasharon 45240, Israel
Germany          Königstr. 5, 01097 Dresden, Germany

www.solaredge.com
© Copyright SolarEdge. 2009. All rights reserved. SolarEdge, the SolarEdge logo, PowerBox and
architects of energy are trademarks or registered trademarks of SolarEdge, Ltd. All other trademarks
mentioned herein are believed to be trademarks of their respective owners.
Date: 01/2010

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Problems and Disadvantages in Current Residential & Commercial On-grid PV Systems

  • 1. Problems and Disadvantages in Current Residential & Commercial On-grid PV Systems SolarEdge Technologies Background Current technology used in On-Grid PV systems connected in a string of modules so that a voltage has many drawbacks. The purpose of this paper high enough for DC/AC inversion (150V to 800V) is is to discuss and categorize the problems and achieved. More power can be added to the system drawbacks inherent in residential and commercial by adding strings. Since the strings are connected photovoltaic systems. Typical residential photovoltaic in parallel, they have to match the other strings installations suffer from numerous problems that in all parameters i.e. type of panels, length and prevent this technology from realizing its full market orientation. The entire array is connected to a solar potential. Many of the present problems stem from inverter which is responsible for harvesting the power losses – whether due to module mismatch, electrical energy and inverting it to AC so it can be orientation mismatch, or partial shading. Other fed into the grid. problems stem from system design limitations and constraints, lack of monitoring and lack of analysis abilities. In addition, the absence of safety features poses risks to both workers installing or maintaining the system, and to firefighters dealing with fires in Inverter the vicinity of PV installations. MPPT For an overview of the SolarEdge distributed power harvesting system, which overcomes the limitations = ~ of traditional systems, please refer to the SolarEdge paper “SolarEdge Architecture Overview”. Typical System Architecture Current residential PV systems are typically built from ten to a few hundred PV modules connected in Solar Strings a series-parallel connection, as shown in Figure 1. Several panels (10 to 15 typically) are series- Figure 1 1
  • 2. The inverter handles maximum power point tracking 4 for the entire array. This is done by finding the DC working point in which the most power is harvested 3 from the array. The harvested power is inverted Ii from DC into AC power that is fed into the grid. The Ii inverter is also responsible for conforming to the Ii 2 electrical and safety regulation requirements. IMPP 1 PV System Drawbacks 0 In the following section we will discuss the different 0 10 20 30 U1, U2, U3, UMPP 40 50 causes for power losses in typical PV systems, as well as other key drawbacks of these systems. Figure 2 Module Mismatch Losses 4 The manufacturing process of PV cells produces Ii cells with relatively large tolerances in their power Ii 3 Ii output capability. To reduce the difference between Ii cells in the same module, they are sorted during IMPP 2 manufacturing to different power categories (bins) IMPP_3 and modules are assembled with cells from the 1 IMPP3 same bin. This produces panels with smaller tolerance variances in output power. The panels IMPP_3 0 themselves are not sorted and panels in the market Ui UMPP_3 today have a 3% tolerance in output power. U1i, U2i, U3i, 3 , UMPP, UMPPi, 3 When several modules are connected in series, 120 each one has a slightly different MPP current. The 100 series connection does not allow the optimal MPP P1i P2i current to be drawn from each module. The inverter P3i 80 will select the current which gives the average peak Pi 3 power point of the string or array. This peak power is PMPP 60 always less than the theoretical sum of the individual 40 PMPP1 peak power points of every module. Figure 2 shows PMPP_3 the I-V and power curves of 3 mismatched modules 3 20 (3% tolerance). In Figure 3 the accumulated I-V and 0 power curves of these modules are shown. It is 0 10 20 Ui 30 UMPP_3 40 50 clear that the string’s peak power point is different U1i, U2i, U3i, 3 , UMPP UMPPi, 3 , from the sum of peak power of the three modules. Figure 3 120 100 The loss demonstrated here is referred to as mismatch loss. In standard residential and P1i 80 commercial PV installations, it can be as high as P2i 60 5%. P3i PMPP 40 20 0 0 10 20 30 40 50 U1, U2, U3, UMPP 2
  • 3. Partial Shading Losses losses are estimated to contribute between 5% to 25% annual power losses. Partial shading occurs when part of a panel or panels are shaded, causing different levels of illumination on the cells in the panel. This can happen due to shade from the building itself, light posts, chimneys, trees, cloud fronts, dirt, snow and other light-blocking obstacles. Shading on any part of the array will reduce its output, but this reduction will vary in magnitude depending on the electrical configuration of the array. Clearly, the output of any shaded cell or module will be lowered according to the reduction of light intensity falling on it. However, since this shaded cell or panel is electrically connected to other unshaded cells and modules, their performance may also be lessened since this is essentially a mismatch situation. For example, if a single module in a series string is partially shaded, its current output will be reduced and this may dictate the operating point of the entire string. Alternatively, the module’s bypass diodes may conduct, causing this module to stop producing power1. Partial shading of crystalline solar modules will result in dramatic reduction of solar module output. One completely shaded cell can reduce a solar module’s output by 40% to 95%. Figure 4 If several modules are shaded, the string voltage may be reduced to the point where the open-circuit MPP Efficiency Losses voltage of that string is below the operating point of the rest of the array (under voltage situation), The MPPT performance is a very significant aspect resulting in that string not contributing to the array of the characterization of PV systems, since PV output. systems, like other systems based on renewable energies, must harvest the maximum available The reduction of output from an array with partial energy at every moment from the renewable shading can be significantly greater than the resource. The global efficiency of the MPPT algorithm reduction in the illuminated area. This is due to the depends on its ability to make the inverter operate at loss of output from unshaded cells in the partially the maximum power point (MPP) at every moment. shaded module, the loss of power from illuminated In order to achieve this, the MPPT algorithm has to modules in any severely shaded string that cannot accurately track the MPP variations, which can be maintain operating voltage and the loss of power caused by factors such as irradiance, temperature from the remainder of the array because the strings variations and partial shading. Obviously, this are not operating at their individual peak power accuracy will be strongly influenced by both the points2. amplitude and the dynamics of the variations of the MPP. MPP losses occur due to two main factors: For residential systems, it is impossible to avoid all shading without severely restricting the size of the 1. Static losses caused by the inability of the MPP array and hence losing output at all times (see Figure algorithm to locate the array’s peak power point. In 4 for examples). In these systems, partial shading some cases, the MPPT algorithm can lock itself in local maxima. 3
  • 4. 2. Dynamic losses caused by the inability of the process at the end of the day. The second day MPPT algorithm to track changes in the peak power corresponds to a cloudy day in which atmospheric point at the right speed. conditions are varying quickly. The figure shows the evolution during both days of the maximum power The main cause of static losses in PV systems is local of the generator, obtained from the measured I–V peaks in the array’s power curve. Shading effects characteristic curves. cause the array’s power curve to exhibit more than one maximum power point. More specifically, two or Evolution of the maximum power during day 1 more peak power points appear in the curve, where 4500 different modules contribute power to different local 4000 peaks. Most MPPT algorithms are not designed to 3500 handle local peaks efficiently and when the MPPT 3000 is locked on a local peak, the MPPT efficiency falls dramatically. Figure 5 shows measured MPPT static 2500 maximum power (W) efficiency of 4 different inverters in the market3. The 2000 results differ showing losses of 1% to 10%. 1500 1000 500 1 1 0 8 10 12 14 16 18 time (hours) 0.9 0.9 MPPT efficiency MPPT efficiency Evolution of the maximum power during day 2 4500 0.8 0.8 4000 3500 0.7 0.7 0 20 40 60 80 100 0 20 40 60 80 100 % of nominal DC power % of nominal DC power 3000 a) PCU A b) PCU B 2500 maximum power (W) 2000 1 1 1500 1000 0.9 0.9 MPPT efficiency MPPT efficiency 500 0 0.8 0.8 8 10 12 14 16 18 time (hours) 0.7 0.7 Figure 6 0 20 40 60 80 100 0 20 40 60 80 100 % of nominal DC power % of nominal DC power c) PCU C d) PCU D With this information, it is possible to calculate the real maximum energy that could have been Figure 5 obtained with optimal MPPT during these days. This value was then compared with the energy effectively Dynamic losses are directly related to the speed and harvested by the leading PV inverters in the market. accuracy of the MPPT algorithm. The peak power The comparison of the available energy to that point shifts mainly due to changes in the irradiance. actually harvested in the first (sunny) day is shown While these shifts are slow and often monotonous in Figure 7. under clear, cloudless atmospheric conditions, they can also be relatively quick during some atmospheric The graph shows how, during most of the day, the conditions. Figure 6 shows measurements from two MPPT algorithm tracks the maximum power point consecutive days in Spain4. The first day corresponds quite well. However, during the final part of the day to a sunny day that is affected by a partial shading (circled in red), due to the process of partial shading, 4
  • 5. the MPPT algorithm fails to track the evolution of System Design and Roof Utilization the generator maximum power point. During these hours, the performance of the MPPT algorithm in In addition to the power loss contributors previously terms of energy harvested from the generator (MPPT discussed, current PV system architecture also efficiency) is 96%, that is, 4% of the energy has been presents great challenges to installers and system lost due to the fact that the MPPT algorithm is not designers. The following factors result in a system able to handle this real life scenario. which is less optimal and, in many cases, smaller than desired: Test and characterization of PV commercial inverter (day 1) 4500 4000 Maximum power point Inverter operating point The string’s voltage is bound by the specific inverter and MPP (W) and energy (kWh/200) inverter’s minimum and maximum permissible 3500 power voltages 3000 2500 The strings are placed in parallel and must be 2000 identical in length, combined with the specific geographical and physical constraints of the roof 1500 energy 1000 When designing the system, the installer has to take 500 into account the inverter’s maximal input voltage 0 and make sure that in the most extreme open- 8 10 12 14 time (hours) 16 18 circuit conditions, the string’s voltage won’t reach this maximal voltage. This is difficult because many Figure 7 parameters like voltage tolerance, temperature variations and sunlight have to be taken into The results for the second day pattern are shown account. On the other hand, the minimal inverter in Figure 8. The day is cloudy with quick variations input voltage must be maintained at all times in in irradiance due to atmospheric conditions. The order to keep harvesting power. Taking into account MPPT efficiency of the inverter in terms of energy the same parameter changes and the possibility of extracted from the PV generator is reduced to 95% partial shading in the string make things even more during the entire day. complicated. In some cases, these constraints lead to low roof utilization. There are many documented events5 where installers elected to use a shorter Test and characterization of PV commercial inverter (day 2) 4500 string in order to increase roof utilization, causing Maximum power point system under-voltage during the summer. inverter and MPP (W) and energy (kWh/400) 4000 Inverter operating point 3500 The roof utilization issue is even more dramatic on 3000 energy commercial roofs, where the roofs’ irregular shapes 2500 and additional obstructions (chimneys, vents, air- 2000 power conditioning units, etc.) increase the difficulty in selecting just one string length. The average roof 1500 utilization in commercial roofs is 65% to 70% due 1000 to roof-related problems. 500 0 Multi-faceted roofs represent another set of 8 10 12 14 16 18 time (hours) challenges related to PV Systems. In these cases the use of multiple inverters enables better utilization Figure 8 of the roof surface. For example an installation on a roof with 2 facets (south-east and south-west facets) will require two inverters or a multi-input inverter. Both solutions are less cost effective. 5
  • 6. It is clear that the serial-parallel architecture puts Retrofit and Long Term Fault Tolerance many constraints on the PV system, preventing the full utilization of the roof and reducing the actual Another drawback of PV systems is the difficulty of annual energy harvested from the system. retrofitting and replacing faulty modules throughout their life cycle. If a PV module breaks in an installation (due to extreme weather conditions or accidents), System Feedback and Troubleshooting it has to be replaced with a module with similar electrical characteristics, and since PV modules are Another inherent problem of the serial-parallel improving all the time, it is not possible to use a new connection is the inability to verify the proper PV module in an old installation. For this reason, PV installation and operation of every element in the module companies keep an inventory of cells and system. The only data points available are measured modules for 25 years as a supply for old systems. at the inverter (input voltage, input power) and The same reason prevents the gradual retrofit of can only partially assist in evaluating the system old systems—you are essentially stuck with old performance. technology. Since usually only overall power output is monitored, significant problems with an installation can go System Safety undetected yet cost the owners 10% or more of their energy payback. Without actionable diagnostic The safety of PV systems has always been an information, the only alternative when operating issue of concern. Over the years, many codes and problems are suspected is to deploy technicians to standards were devised and discussed to improve the site to search for the problem with little direction the safety of these systems. Two main safety issues or guidance. This search becomes more difficult remain: as the installation size increases. The average commercial solar site installed in California in 2004 1. The risk of electrocution while installing the had 1,000 solar modules, making this difficulty very system. Even the series connection of two PV real. modules exposed to sunlight generates an unsafe voltage with enough power to kill whoever touches The parallel connection of strings prevents the the exposed contacts. Many work procedures and detection of defective modules and in some cases safety precautions employed during the installation the detection of a disconnected string. For example, processes slow the work but maintain employee in a system with 8 strings, the detection of 12.5% safety. power loss, which is equivalent to one string, is not trivial, especially since the system’s expected 2. The risk to firemen during a fire. The first thing power output is dependant on all the other criteria firemen do in a building fire is cut the power. This mentioned above. enables them to spray water and use axes to cut holes in the roof to let smoke out. These actions Even when a fault is suspected and technicians are dangerous when PV systems are installed on are sent to the site, maintenance is expensive the roof. Firemen are trained to cut the power to because it requires the deployment of skilled the building. After doing so, they believe that there technicians to the solar site. Once on-site, is no power source that can endanger them. In their only recourse is to dismantle the system PV installed buildings cutting the power does not and search for the cause of the failure. eliminate the dangerous voltages present at the The same lack of feedback also increases the initial string ends. Firemen preparing to chop a hole in installation time due to lengthy verification and the roof or to use water could be electrocuted6. debugging. Regulatory bodies and firefighting officials are pushing for new regulations that will ensure the shutdown of PV modules in case of fire. 6
  • 7. Conclusion We described the structure of PV systems and several drawbacks and disadvantages in the way these systems are designed and built. Though previous works focused on improving different aspects of photovoltaic systems, no single holistic approach was presented that could solve many of the current problems present in PV installations. SolarEdge Technologies distributed power harvesting system is the only power harvesting system and architecture that offers a full, robust solution that harvests up to 25% more energy and resolves the issues described in this paper. 1 “Effects of Reverse-Biased PV Cell in Panels”, SolarEdge Technologies (2006) 2 “Photovoltaic Modules, Systems and Applications”, Nicolla M. Pearsall & Robert Hill (2001) 3 “Performance Assessment of the Inverter based Grid Connection of Photovoltaic Systems”, Gianfranco Chicco, Roberto Napoli, Filippo Spertino (2004) 4 “On the Testing, Characterization, and Evaluation of PV Inverters and Dynamic MPPT Performance Under Real varying OperatingConditions”, Pablo Sanchis, Jesu´s Lo´pez, Alfredo Ursu´a, Eugenio Gubı´a and Luis Marroyo (2007) 5 “PV Installations, A Progress Report”, John C. Wiles, Bill Brooks, Bob-O Schultze (2002) 6 “Commercial Building Product Magazine”, (2007) 7
  • 8. About SolarEdge SolarEdge provides next generation power conversion electronics that effectively remove all known system constraints across the photovoltaic energy space. Our Smart DC technology enables increased production of clean, grid-ready energy at a lower cost per watt than any other competitive offering. SolarEdge technology marries traditional photovoltaic workflows and installation methods with a groundbreaking holistic system approach. It is a quiet revolution that is at once disruptive because of its profound benefits in changing the manner in which energy is harvested, deployed, managed and delivered and complementary because it fits into the current photovoltaic workflow. At SolarEdge we believe the PV delivery chain is ultimately only as strong as its weakest link. By adopting a “system first” philosophy that identifies and eliminates the Achilles heel in each step in the process, we enable a constraint-free delivery of sun harvested energy. USA 2225 East Bayshore Rd., Suite 200, Palo Alto CA 94303, USA Japan B-9 Ariake Frontier Building, 3-7-26 Ariake, Koto-Ku, Tokyo 135-0063, Japan Israel 6 HaHarash St. P .O.Box 7349, Neve Neeman, Hod Hasharon 45240, Israel Germany Königstr. 5, 01097 Dresden, Germany www.solaredge.com © Copyright SolarEdge. 2009. All rights reserved. SolarEdge, the SolarEdge logo, PowerBox and architects of energy are trademarks or registered trademarks of SolarEdge, Ltd. All other trademarks mentioned herein are believed to be trademarks of their respective owners. Date: 01/2010