A yield comparison between DC optimised systems and conventional PV systems using Fronius inverters AU
Fronius A yield comparison between DC optimised systems and conventional PV systems using Fronius inverters AU Whitepaper [EN]
A YIELD COMPARISON BETWEEN
DC OPTIMIZED SYSTEMS AND
CONVENTIONAL PV SYSTEMS USING
FRONIUS INVERTERS
White Paper
© Fronius International GmbH, DH, VH
Version 1, 11/2016
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ABSTRACT
In this paper two randomly chosen systems, where DC optimized systems meet conventional string inverters
under non-laboratory comparable conditions (homogeneous shade vs. unshaded, same orientation, same system
size, and comparable string design) have been compared in terms of yield.
The findings show that there is no extra yield to be expected from so called DC optimized system in direct
comparison with conventional string-inverters after a 6 months observation period. There are a few days where
DC optimized systems have shown extra yield which occurs in periods of poor irradiation in winter due to snow
cover. The additionally gained kWhs do not impact the annual yield in a way that it would justify the added effort
in installation, commissioning and added cost of optimizers. A brief cost analysis shows cost advantages of
standard string inverter systems.
Furthermore it was investigated that there is no advantage of DC optimized systems in unshaded or even partly
shaded single-string systems. This is due to the outstanding shade impact mitigation algorithm called Fronius
Dynamic Peak Manager (DPM) that Fronius inverters offer.
INTRODUCTION
Module Level Power Electronics (MLPE) are becoming more and more popular in some regions of the world.
However, traditional string-inverters are still used in the majority of installations worldwide and there are significant
reasons for that.
This paper investigates the yield of two different PV systems where the same part of modules are connected to
Fronius SnapINverter and inverters from an MLPE manufacturer that offers a closed system consisting of
optimizers and inverters. The systems have the same orientation, location and same size in order to determine if
there is a real advantage in terms of extra yield achievable with so called DC optimized systems.
SCOPE AND METHODOLOGY
Based on a theoretical analysis of MPP-tracking and inverter efficiency the two demonstration systems are
introduced by a day-to-day yield comparison between the Fronius standard string inverter and the proprietary DC
optimized system. The data for the investigations comes from the official monitoring platforms Fronius Solar.web
(www.solarweb.com) and the competitor’s.
Yield comparison on a monthly basis and after each observation period will be displayed in this paper.
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EFFICIENCY
FRONIUS
SYMO 15.0-3-M
COMPETITOR 15KVA
+ DUAL MODULE
OPTIMIZERS
COMPETITOR 15KVA
+ SINGLE MODULE
OPTIMIZERS
FRONIUS
PRIMO 6.0-1
COMPETITOR 6KVA
+ SINGLE MODULE
OPTIMIZERS
INVERTER
98.1%
98.0%
98.0%
98.0%
97.6%
INVERTER
97.8%
97.6%
97.6%
97.3%
97.4%
OPTIMISER
99.5%
99.5%
99.5%
OPTIMISER
98.6%
98.8%
98.8%
98.1%
97.5%
97.5%
98.0%
97.1%
97.8%
96.2%
96.4%
97.3%
96.2%
DIFFERENCE
-0.6%
-0.6%
-0.9%
DIFFERENCE
-1.6%
-1.4%
-1.1%
EFFICIENCY COMPARISON
The vendor offering the DC optimized system claims that the DC optimized system generates more energy, so
one might think that the inverters must have a better efficiency than conventional inverters. Especially under the
aspect that the DC-DC conversion stage has been sourced out to the modules, a system’s efficiency is not
represented by the inverters efficiency alone. The optimiser’s efficiency need to be taken into account as well.
The total efficiency in a system consisting of inverter and power optimizer is calculated according to the formula:
The following table shows a comparison between Fronius datasheet and the competitor’s datasheets:
Tabular 1: Comparison of inverter efficiency according to datasheets
As shown in the table above DC optimized system inverters efficiency doesn’t reach the efficiency of the Fronius
inverter. Taking the optimizers efficiency into calculation the weighted efficiency (EU-efficiency plus weighted
optimizer efficiency) DC optimized systems seem to be up to 1.6% less efficient in equal conditions.
Efficiency alone is not a strong argument, mismatch mitigation on the modules side through distributed MPPT as
well as accuracy and speed of MPPT can also have a significant effect in gaining extra yield.
The Fronius Dynamic Peak Manager2 (DPM) not only offers best-in-class inverter integrated shade mitigation
algorithm but also is able to adopt the speed of MPPT according to the environmental conditions. Under bright
sunny conditions the MPPT is working slower than under cloudy conditions. Here the algorithm will automatically
act faster in order to follow the more volatile PV-generator at a maximum dynamic MPPT accuracy and efficiency.
INTRODUCTION OF THE TWO REFERENCE SYSTEMS
Two real-life PV installations (both located in Central Europe) have been used for a yield comparison. There is a
commercial system called “System A” and a residential system called “System B”. All data has been anonymized
to protect the privacy of the owners granting access to their systems.
“System A” – 17.16 kWp Fronius vs. 17.68 kWp optimized system
Commercial “System A” consists of multiple Fronius and competitor’s inverters with a power class of 17kW. For
the comparison two inverters with modules in the same orientation and next to each other on one single roof have
been taken into account. This system consists of 260W modules in multiple strings. The optimized system in this
power range has longer strings than standard inverters. Except for the string lengths the systems are equally
oriented side-by-side on an unshaded south-facing roof.
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