Download SIMPLE SYSTEM PLANNING WITH REFUsol

SIMPLE SYSTEM PLANNING
WITH REFUsol INVERTERS
Connecting module strings of different
lengths to an inverter
REFUsol GMBH 02/2013
2
REFUsol TECHNOLOGY
CONNECTING MODULE
STRINGS OF DIFFERENT
LENGTHS
Introduction
Making the best
use of a roof
surface with
solar modules
requires good
planning.
REFUsol has always attached great importance to maximizing
energy yields while keeping the complexity of the inverters to
a minimum. This is demonstrated, for example, by the standardized housing geometry in the 8 to 23 kW power class. Only
REFUsol is able to feed 23 kW of power into the grid with a device weighing just 41.5 kg – with a record inverter power density
of 555 Wp per kilogram. This goal can only be achieved by focusing consistently on necessary functionalities. Not least, this is
demonstrated by the technological development of the precise,
highly efficient MPP tracker installed in every REFUsol device.
This article explains how your solar system can realize very
high energy yields with a REFUsol inverter, even if installation
conditions are less than ideal.
The problem
With rooftop systems, the maximum number of modules is usually determined on the basis of the roof surface area. This might
be an even or odd number. Even distribution of modules in 2, 3
or 4 strings is therefore not always possible.
Suggested solution
To make the best use of the roof surface, it is permissible, under
certain circumstances, to connect module strings of different
lengths to one inverter.
Thomas Wurster and Markus B. Schubert from the Institute
for Photovoltaics at the University of Stuttgart have simulated
various combinations and obtained some surprising results.
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Examples
Case A)
Solar generator with two
strings, one string with one
fewer module
Case B)
Solar generator with four
strings, one string with one
fewer module
AC
Case B)
With four strings, the power loss of the three long strings is
very small, while that of the short string is relatively high.
AC
DC
MPP
inverter voltage
string power Ps [kW]
DC
5,7
5,6
23 modules
Vmpp
22 modules
MPP
5,5
ipv
670
680
690
700
710
string voltage Vs [V]
Effect
In these cases, the inverter’s MPP tracker attempts to set a DC
voltage that achieves the highest power for the solar generator in the current situation. This voltage will always lie between
the MPP voltage of the shorter and the longer module strings
and be close to the optimum for the individual string by around
half the MPP module voltage in each case. As the power curve
of the solar module becomes relatively flat around the maximum power point with varying voltage, the power losses are
negligible in practice.
This is shown in the following graph. The power loss in each
case is shown in red for the characteristic curves of the two different strings.
Case A)
For two strings of different lengths, the power loss is small
for the short string and higher for the long string.
Fortunately, the effect on the annual yield remains less than
half a percent for the cases calculated above.
Summary
The yield lost by leaving off one module on long module
strings of around 20 modules is negligible. When connecting
the strings, therefore, one module can be omitted if necessary.
General rules
QQ The longer the module string, the smaller the yield deviation.
QQ The fewer strings out of the total number of strings that are
reduced by one module, the lower the loss of yield.
who do not want to give away one single kilowatt
hour can of course also use the REFUsol 020K-SCI. With its
higher efficiency of up to 98.7%, it can compensate for the
effects just described.
QQ Those
MPP
5,7
5,6
5,5
23 modules
22 modules
Vmpp
MPP
ipv
670 680 690 700
string voltage Vs [V]
710
Source: T Wurster, MB Schubert, University of Stuttgart,
Institute for Photovoltaics, Stuttgart, Germany 2013
REFUsol GmbH | Uracher Straße 91 | 72555 Metzingen | Germany | Тel. +49 7123 969-0 | Fax +49 7123 969-165 | [email protected] | www.refusol.com
REFUsol_WP-04_20130219
string power Ps [kW]
inverter voltage