Have a read of this research paper from Fronius... it doesnt copy and paste well but the words give you enough.
EFFICIENT EAST-WEST ORIENTED 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 oriented PV systems can also generate substantial earnings. Moreover, due to the sharp drop in module
prices, increased demand for east-west systems are expected for the future. From the perspective of grid operators, eastwest
oriented 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 orientated
PV systems connected to one MPP Tracker and demonstrate the high performance of such systems.
1 INTRODUCTION
Based on theoretical analysis, the behaviour of the
MPP of an east-west orientated PV system was
investigated and then verified by comparing results. For
the practical results two east-west arrays were installed –
one PV array with thin film modules and one PV array
with crystalline modules. These 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 were installed with an azimuth
angle of -67.5° for the east generator, 112.5° for the west
generator, and an inclination angle of 30°. The crystalline
modules, 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 taken to
obtain accurate results and possible inverter deviations
were monitored by installing energy meters.
2 MISMATCHING
At first glance, the installation of a single inverter in
an east-west oriented PV system leads to the expectation
of large mismatching losses. Due to the different
orientations in an east-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 current differences between the east and west
generator, the MPP voltages are nearly identical, as can
be seen in Figure 1. Since the total voltage of the east
generator is similar to the total voltage of the west
generator, very small mismatching losses are expected if
these strings are connected in parallel to a single inverter
(one MPP Tracker).
The mismatching losses differ according to the
inclination angle of the installed solar modules and the
module technology used. The greater the inclination
angle of the solar modules, the higher the mismatching
losses. Essential to understand the losses from the
module technology are the fill factor and the change of
the MPP voltage as 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 crucial, 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 module drops
more steeply around the MPP than the power curve of the
thin film module. Therefore, it is likely that crystalline
modules lead to higher mismatching losses in east-west
orientated PV systems than thin film modules.
Another important point, however, is the change of
the MPP voltage as a function of irradiation [see Figure
1]. A small change of the MPP voltage over a wide
irradiation range causes the fewest losses. The change in
MPP voltage is mainly affected by the module
temperature. A low temperature coefficient and good
ventilation of the solar modules therefore results in better
performance in east-west orientated PV systems.
Moreover, a high low-light performance of a solar
module can also improve the power output. Since all
these variables differ with every module, no general
conclusion can be drawn about which technology is more
favourable for east-west oriented PV systems.
Crystalline module - Fill factor ≈ 80%
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Thin film module - Fill factor ≈ 60%
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V/I - Characteristic MPP V/P - Characteristic V/P - Characteristic
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, the installation of a single
inverter in an east-west orientated 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 effiency 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].
DC Voltage
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Time
Voltage [V]
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DC Voltage: West-Generator
DC Voltage: East/West-Generator
Irradiation and Temperature
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Irradiation [W/m2]
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Irradiation: East-Generator Irradiation: West-Generator
Temperature: East-Generator Temperature: West-Generator
Figure 3: Comparison of the measured DC voltages with
the corresponding irradiation and temperature profile on
a sunny day
Figure 3 shows the DC voltage of the east/west
generator with a single inverter compared to the DC
voltages of the east/west generator with separate
inverters. As can be seen, the voltages of the east and
west generator are different. In the morning, the voltage
of the west generator is generally higher than the voltage
of the east 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 remains nearly constant at a global irradiation
level above ~180 W/m2 and increases/decreases with
decreasing/increasing module temperature.
The east/west generator produces mismatching losses
because the DC voltage of that generator is not identical
with the DC voltage of the west generator in the morning.
The same applies to the DC voltage of the east generator
in the afternoon. Although the DC voltage of the
east/west generator deviates by up to 5% from the
voltages of the generator with separate inverters, the
energy losses are very small, as can be seen in Figure 4.
This is because the DC voltage of the east/west generator
follows the voltage of the east generator in the morning
and the voltage of the west generator in the afternoon. An
additional point to note is that a deviation of 5% from the
optimal MPP voltage does not lead to the same
percentage of power losses, since a lower/higher MPP
voltage also causes a higher/lower MPP current.
Energy
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Energy [kWh]
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AC power
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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 curve of the east/west generator
with separate inverters for the whole day. The different
DC voltages of the generators lead to approximately
0.5% mismatching losses but the final energy losses are
just 0.1% - within the accuracy of measurement of the
energy meters of ±1%. As mentioned before, the
mismatching losses are partially compensated due to the
east/west generator with a single inverter operating in a
higher efficiency range. The energy losses are highest on
a sunny day because the lower the irradiation difference
between the east and west strings, the lower the deviation
of the DC voltages. The result is that energy losses are
even lower on a cloudy day or on days with diffuse
irradiation.
