Fronius Efficient east-west orientated pv systems with one mpp-tracker 2013/08 Technical Article [EN]
EFFICIENT EAST-WEST ORIENTATED PV SYSTEMS WITH ONE MPP TRACKER
ABSTRACT: A willingness to install east-west orientated photovoltaic (PV) systems has lacked in the
past. Nowadays, however, interest in installing PV systems on east-west roofs is steadily increasing.
Although south orientated systems are better, east-west orientated PV systems can also generate
substantial earnings. Moreover, due to the sharp drop in module prices, increased demand for eastwest systems are expected for the future.
From the perspective of grid operators, east-west orientated PV systems are preferable to south
orientated ones, as the energy is fed-in more evenly throughout the day, therefore reducing power
peaks thus relieving the grid. Up to now it was assumed that east-west orientated PV systems require
separate inverters for each orientation, or at least one inverter with multiple MPP Trackers (Maximum
Power Point), to avoid mismatching losses. This paper will show an analysis of east-west orientat ed
PV systems connected to one MPP Tracker and demonstrate the high performance of such systems.
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1 INTRODUCTION
Based on theoretical ana l ys is, the be ha vi our of the MPP of an east-w est or i enta t ed P V system was investigated
and then verified by comparing measurement results. For the practical results two east-west arrays were
installed – on e PV array with th in film modules and o ne PV array with cr ystalline modules. T hese arrays were
then split and put into operation as separate systems – the first with one inverter for the east roof and one
inverter for the west roof, and the second with a single inverter for both roofs.
The thin film modules wer e installed with an azimuth angl e of -67.5° for the eas t generator, 11 2.5° for the wes t
generator, and an inc lination angle of 30°. The crystalline m odules, on the other hand, were mounted with an
exact orientation of -90° for the east generator, 90° for the west generator, and an inclination angle of 15°.
Measurements of the ‘IV’ characteristic were tak en to obtain accurate res ults and possible inverter deviations
were monitored by installing energy meters.
2 MISMATCHING
At first glanc e, the i nstalla tion of a sin gle inv erter i n an eas t-west orientated PV s ystem leads to the expecta tion
of large mismatc hing losse s. D ue to th e dif ferent orien tations in an eas t-west PV system , the solar modules are
exposed to various irradiation levels. For this reason, different module currents occur between the east and
west strings, depending on the time of day. In contrast to large c urrent differences between the east and west
generator, the MPP v oltage s are near ly id entical, as ca n be se en i n Figure 1. Since the total voltag e of the eas t
generator is similar t o the total voltage of the west g enerator, very small mismatching l osses are expected if
these strings are connected in parallel to a single inverter (one MPP Tracker).
The mismatching los ses differ according to the inclination angl e of the instal led solar modules and the m odule
technology used. The greater the inclination angle of the solar modules, the higher the mismatching losses.
Essential for the losses from the module techn ology are the fill f actor and the change of the MPP voltage a s a
function of irradiation.
Figure 1: IV characteristic of a crystalline module at different irradiation levels [1]
The fill factor - which is usually higher for crystalline modules than for thin film modules – is c rucial, since it
determines how steeply the power curve drops before and after the MPP. Figure 2 shows the typical
characteristics of a crystalline module and a thin film module. It can be seen that the power curve of the
crystalline m odule dro ps m ore steep ly arou nd the M PP tha n the po wer c urve of t he th in film module. T heref ore,
it is likely that crystall ine modules lead to higher mismatching losses in east -west orientated PV system s than
thin film modules.
Another important point, howev er, is th e c han ge of the MP P v olt age as a function of irradiat ion [s ee Fi gur e 1]. A
small change of the MP P voltage over a wide irrad iation range causes th e fewest losses. The c hange in MPP
voltage is mainly af fected by th e module temper ature. A lo w temper ature coeff icient and g ood vent ilation of t he
solar modules therefore results in better performance in east-west orientated PV systems. Moreover, a high
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Crystalline module - Fill factor ≈ 80%
0,0
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Voltage [V]
Current [A]
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Power [W]
V/I - Characteristic
MPP V/P - Characteristic V/P - Characteristic
Thin film module - Fill factor ≈ 60%
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Voltage [V]
Current [A]
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Power [W]
V/I - Characteristic MPP V/P - Characteristic V/P - Characteristic
DC Voltage
200
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400
500
04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36
Time
Voltage [V]
DC Voltage: East -Generator
DC Voltage: West-Generat or
DC Voltage: East /West-Generator
Irradiat i on and Temperature
0
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04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36
Time
Irradiation [W/m
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]
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Temperatur e [°C]
Irradiation: East-Generator Irradiati on: West-Genera tor
Temperature: E ast-Generator Temperature: West-Gener ator
low-light perform ance of a solar m odule can also improve the po wer output. Since all these variables differ with
every module, no general conclusion can be drawn about which techno logy is more favourable f or east-west
orientated PV systems.
Figure 2: Characteristic curves of a crystalline module and a thin film module
3 RESULTS
3.1 Low mismatching losses
As explained in section 2, t he installation of a single inverter in an eas t-west orientat ed PV system necessarily
results in mismatching losses. However, these losses are minimal and are partially compensated by other
positive effects. For example, an east-west orientated PV system with a single inverter operates in a higher
efficiency range for more of the time when compared to an installation with separate inverters. The figures
shown in section 3.1 use data from the east-west orientated PV system with the crystalline modules [see
section 1].
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Energy
201,14
201,34
0
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100
150
200
Sunny day
Energy [kWh]
East/West-Generator
East- + West-Generator
AC power
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4
8
12
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24
04:48
07:12 09:36
12:00 14:24 16:48 19:12 21:36
Time
Power [kW]
East/West-Generator East- + West-G enerator
Figure 3: Comparison of the measured DC voltages with the corresponding irradiation and temperature profile
on a sunny day
Figure 3 shows the D C vo lt age of the eas t/ west ge nerator with a singl e i nv erter compared to the D C v olt age s of
the east/west generator with separate inverters. As can be seen, the voltages of the east and west generat or
are different. In the m orning, the voltage of the west generator is gen erally higher than t he voltage of the eas t
generator, whereas the reverse is true in the afternoon. This is a result of the irradiation and temperature
behaviour of solar cells , since the DC voltage rem ains nearly constant at a global irradiatio n level above ~180
W/m2 and increases/decreases with decreasing/increasing module temperature.
The east/west generat or pr oduc es mismatching losses becaus e th e D C vo ltag e of that g ener a tor is no t id ent i c al
with the DC voltage of the west generator in the morning. The same applies to the DC voltage of the east
generator in the after noon. Although the DC voltage of the east/west generator deviates b y up to 5% from the
voltages of the generator with separ ate inverter s, the energ y losses are ver y small, as can be seen in F igure 4.
This is because the DC voltage of the east/west generator follows the voltage of the east generator in the
morning and the v oltage of the w est gen erator i n the after noon. An a dditiona l poi nt to note is t hat a dev iation of
5% from the optim al MPP voltag e does not le ad to the s ame percenta ge of pow er losses, sinc e a lower/ higher
MPP voltage also causes a higher/lower MPP current.
Figure 4: AC power comparison with the corresponding energy yield on a sunny day. ~ 0.1% energy losses of
the east/west generator with a single inverter compared to the east/west generator with separate inverters
As shown in Figure 4, the AC power curve of the east/west generator with a single inverter overlaps the
combined AC power c urve of the east/west generat or with separate inverters for the whole day. The dif ferent
DC voltages of the generat ors lead to approximatel y 0.5% mismatching losses but the fina l energy losses are
just 0.1% - within the accuracy of measurement of the energy meters of ±1%. As mentioned before, the
mismatching losses are par tiall y com pensated due to t he eas t/west g ener ator wit h a si ngle inv erter o perat ing in
a higher efficiency range.
The energy losses are hig hest on a sunny day because the lower the irr adiation difference between the e ast
and west strings, the lower the deviati on of th e DC voltages. The result is that ener gy losses are ev en lower on
a cloudy day or on days with diffuse irradiation.
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Energy yield - 3 Months
107,96
115,53
137,61
361,10
109,07
117,06
138,47
364,60
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May 09 Jun 09 Jul 09 May 09 - Jul 09
Month
Energy [kWh/ kW p]
East/West-Generator East- + West-Generator
AC power profile
0
0,5
1
1,5
2
05:31 07:55 10:19
12:43 15:07 17:31 19:55
Time
Power [kW]
East- G enerator [1,013 kWp] West-Generator [1,012 kWp]
East/ West-Generator [ 1,711 kWp]
3.2 Energy yield comparison – Part I
The following energy yield comparison shows the result of the east-west orientated PV system with thin film
modules. As can be seen in Figure 5, the energy losses of the east/west generator with a single inverter are
very low over a long period.
Figure 5: Energy yield comparison of the east-west orientated PV system with thin film modules over a period
of 3 months. ~1% energy losses of the east/west generator with a single inverter compared to the east/west
generator with separate inverters
Based on the results from May to July, it can be expected that the annual energy losses of the east/west
generator with a single inverter will be less than 1%. The east/west generator with a single inverter therefore
has a clear advantage compared to an installation with separate inverters. It is also superior to an installation of
a single inverter with two MPP Trackers. In fact, the east/west generator with a single inverter is the cheaper
solution whilst generating almost the same energy yield, as only one inverter is needed. In addition, the single
inverter can have a lower nominal power than the sum of the nominal power of the separate inverters. This is
because the power peaks of the east and west generator are time-shifted, as shown in Figure 6. The nominal
power reduction depends on the inclination angle of the solar modules – the higher the inclination angle, the
lower the nominal power of the single inverter. As explained in section 1, the thin film modules were installed
with an inclination angle of 30°, allow ing the nominal power of the single inverter to be reduced by
approximately 15%.
Figure 6: Example of the AC power profile of the east-west orientated PV system with thin film modules. A
nominal power of ~85% of the sum of the nominal power of the separate inverters is sufficient for the single
inverter
From these results it can be concluded that the cost savings are greater than the energy losses. This means
that the payback time of the east-west PV system with a single inverter is shorter.
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Energy yield - 3 Months
121,10
127,51
149,48
398,09
121,15
127,49
149,54
398,18
0
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400
May 10 Jun 10 Jul 10 May 10 - Jul 10
Month
Energy [kWh/ kW p]
East/West-Generator
East- + West-Generator
AC power profile
0
5
10
15
20
25
04:48 07:26 10:04 12:43 15:21 18:00 20:38
Time
Power [kW]
East- G enerator [11,35 kWp] West-Generator [11,5 kWp]
East/ West-Generator [ 21.8 kWp]
3.3 Energy yield comparison – Part II
The results in section 3.3 are the energy yield comparison of the east-west orientated PV system with
crystalline modules. Since the inclination angle of the solar modules is just 15°, there is very little energy loss,
as can be seen in Figure 7. The mismatching losses are between 0.3% and 0.5%, but they are compensated
due to the higher efficiency operation of the single inverter.
Figure 7: Energy yield comparison of the east-west orientated PV system with crystalline modules over a
period of 3 months. The energy yield of the east/west generator with a single inverter is nearly the same as that
of the east/west generator with separate inverters
In this case as well, the e as t/west generator with a s ingle inverter is the lower-cost option. T he cost s avin gs are
obvious and roughly the same as set out in section 3.2. Firstly, onl y one inverter is required. Secondly, the
nominal power of th e single inverter can be reduc ed by approx imately 5%, as s hown in Figure 8. The nomina l
power reduction of 5% results from the 15 ° inclination angle of the crystalli ne solar modules, as exp lained in
section 3.2. At this point, it should be mentioned that a single inverter with twice the nominal power of one
separate inverter is always cheaper than two smaller inverters.
Figure 8: Example of the AC power profile of the
east-west orientated PV system with crystalline modules. A nominal power of ~95% of the sum of the nominal
power of the separate inverters is sufficient for the single inverter
Consequently, the payback time of the east/west generator with a single inverter is shorter than that of the
east/west generator with separate in verters.
4 BASIC INSTALLATION RULES
The following rules must be observed in order to ens ure that an east-west orientated P V system with a s ingle inverter operates optimally:
/ Shading must be avoided
/ The number of solar modules must be identical in all strings
/ Within a single string, the inclination angle and orientation of the solar modules must be identical
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5 CONCLUSION
The investigations o n both PV systems have dem onstrated that in an east-west orientated PV s ystem, with a
single inverter for the eas t and west generator, m ismatching losses occur. As expected, these losses are very
small and are parti ally com pensated by the f act that th e single i nverter o perates i n a higher eff iciency range . In
contrast to minim al yield losses, the following cos ts can be reduced sign ificantly: firs tly the number of inverters
can be reduced and secondly the nominal power of the single inverter can be reduced by up to 35% depending on the inclination angle of the installed solar modules. Furthermore, installation costs can be
minimised.
If one considers basic installation rules, the inclination angle of the solar modules and module technology,
installing a single inverter in an east-west orientated system can be cheaper than installing a system with
separate inverters. Finally, installing a single inverter has no disadvantages compared to instal ling an inverter
with two MPP-Trackers.
6 REFERENCES
Fraunhofer Institute for Solar Energy Systems ISE, How fast does an MPP Tracker really need to be, 24th European Photovoltaic Solar Energy Conference, Germany, 2009
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