ACTIVE VS PASSIVE COOLING
OF POWER ELECTRONICS
Why active cooling is the better technology for power electronics.
© Fronius International GmbH
Version 1.0, 11/2019 , Jürgen Wolfahrt, Franz Breitwieser, Volker Haider, Jasmin Gross
Solar Energy
Fronius reserves all rights, in particular rights of reproduction, distribution and translation. No part of this document may be reproduced,
in any form whatsoever, or stored, processed, duplicated or disseminated with the aid of electronic systems, without the written consent
of Fronius. You are hereby reminded that the information published in this document, despite the greatest care being exercised in its
preparation, is subject to change and that neither the author nor Fronius can accept any legal liability. Gender-specific wording refers
equally to the male and female form.
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TABLE OF CONTENTS
1 Introduction .....................................................................................................................................5
1.1 Definition of passive cooling .............................................................................................................5
1.2 Definition of active cooling ................................................................................................................5
2 PLANNING & SYSTEM DESIGN .....................................................................................................6
2.1 Active cooling means greater flexibility .............................................................................................6
2.1.1 Flexibility in system design ...............................................................................................................6
2.1.2 Flexibility of installation .....................................................................................................................7
3 MAINTENANCE ...............................................................................................................................9
3.1 Active cooling reduces costs ......................................................................................................... 10
4 SERVICE LIFE .............................................................................................................................. 11
4.1 A longer service life thanks to active cooling ................................................................................. 12
4.1.1 Service life of a fans ....................................................................................................................... 12
5 YIELD............................................................................................................................................. 14
5.1 Active cooling means higher yields ................................................................................................ 14
5.1.1 Repercussions, using a reference system as an example ............................................................ 15
6 Thermal comparison.................................................................................................................... 17
6.1 Behaviour of passively cooled devices .......................................................................................... 17
6.2 Behaviour of actively cooled devices ............................................................................................. 19
7 ASPECTS OF THE TWO COOLING SYSTEMS .......................................................................... 20
7.1 Aspects of active cooling technology (Active Cooling) .................................................................. 20
7.1.1 Noise levels .................................................................................................................................... 20
7.1.2 Lower costs over the whole product lifetime .................................................................................. 20
7.1.3 Lower weight means increased convenience ................................................................................ 20
7.1.4 Longer service life .......................................................................................................................... 20
7.1.5 Greater flexibility in system design ................................................................................................ 20
7.1.6 Higher yield .................................................................................................................................... 21
7.2 Aspects of passive cooling technology .......................................................................................... 21
7.2.1 Noise levels .................................................................................................................................... 21
7.2.2 Efficiency ........................................................................................................................................ 21
8 FURTHER DEVELOPMENTS IN ACTIVE COOLING .................................................................. 22
8.1 Fronius GEN24 Plus ...................................................................................................................... 22
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8.2 Flow behaviour in the heat sink of the Fronius GEN24 Plus ......................................................... 23
8.2.1 Contributing to effective energy production ................................................................................... 24
9 SUMMARY .................................................................................................................................... 25
10 BIBLIOGRAPHY ........................................................................................................................... 26
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1 INTRODUCTION
High ambient temperatures not only affect the yield of an entire PV system, they also have a considerable
effect on the service life of the inverters. It is not just the inverters themselves that are negatively affected by
high outside temperatures, but also the performance and the service life of the electronic components inside
the device.
So the question is, how to stop electronic components overheating, without investing vast sums of money in
an air-conditioned inverter environment, for example.
This white paper examines and compares in some detail two standard inverter cooling technologies found on
the market. The comparative tests are designed to highlight the differences and beneficial features of passive and active cooling technology.
1.1 Definition of passive cooling
Passive cooling technology relies on natural convection. Large heat sinks are used to keep the internal temperature low, which tends to make the device heavy.
1.2 Definition of active cooling
The aim of active cooling technology is to proactively avoid heat fields by using interior fans and to remove
warm air in a controlled manner.
At Fronius, Active Cooling is a technology standard in all devices. In addition to a small heat sink, there is a
fan to ensure that the air inside the inverters is circulating and so-called hot spots are avoided. A further fan
is responsible for keeping the cooling fins of the power electronics at a low temperature. The fan speed varies, subject to the temperature inside the device.
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2 PLANNING & SYSTEM DESIGN
System data
AC output
Overdimensioning
Module output
Intelligent system design is a particularly important issue when it comes to family homes, as firstly, the roofs
of the houses are often irregularly shaped and oriented towards different points of the compass and secondly, the roof area is limited.
In practice, there is also only a limited amount of free choice about where to install the inverter; you have to
adapt to the given conditions. However, some inverters have strict regulations regarding the type, position
and site of installation. These restrictions are usually caused by the cooling system used in the inverter.
2.1 Active cooling means greater flexibility
2.1.1 Flexibility in system design
If you look more closely at the maximum input current (I
) of an MPP tracker for passively cooled de-
DCmax
vices, you will discover that the flexibility of these is restricted. Due to the often limited amperage of MPP
trackers for passively cooled devices, only one module string can usually be connected to a tracker. This is
because higher amperages also cause higher component temperatures (P = I2 x R).
Passively cooled devices tend to use evenly distributed MPPT inputs. Due to the limited heat dissipation, the
amperages of the MPPT inputs are limited. This results in limited design flexibility, as comparatively fewer
DC module strings can be connected per MPPT input. This is also the reason why asymmetric distribution is
only possible to a limited extent.
Actively cooled devices, on the other hand, can dissipate more heat, allowing higher amperages. In turn, the
higher amperages per MPP tracker mean greater flexibility in system design, as more parallel strings can be
connected.
Actively cooled devices from Fronius allow especially high currents (e.g. 25 A for a 5 kW Symo GEN24 Plus)
with at least one MPPT input. It is then possible to connect two or more DC strings at this MPPT input. In addition to this, actively cooled Fronius inverters have at least one more MPPT input. This makes it possible to
have a broad asymmetric distribution to the two MPPT inputs. This, together with the possible overdimensioning of the DC output by up to 150%, allows greater flexibility in system design, which is why it is also
called SuperFlex Design.
This can be illustrated by a simple example.
Comparative example:
With an actively cooled Fronius inverter, there is no problem connecting both the DC strings in our example
system to one MPPT input. It would also be possible to connect additional solar modules from a roof with a
different orientation to the second MPPT input.
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5 kW 130% (6.5 kWp) 285 watt (8.97 A)
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By combining the SuperFlex Design with an active cooling system, Fronius inverters make it possible to produce the perfect design for roofs of different complexity. With passively cooled devices on the other hand, a
relatively symmetrical PV generator distribution is often allocated, as can be seen on the following graphic.
Figure 1: Comparison of MPP tracker power distribution for passively and actively cooled 5 kW inverters
It is apparent that the power distribution of the MPP trackers for actively cooled devices is more generously
proportioned than for passively cooled devices. The reason for this is that inverters with passive cooling of
the 5 kW power category, for example, usually only allow maximum currents of 10 A to 15 A.
In relation to our example system, this means that with passively cooled devices, the maximum asymmetric
distribution is limited to 4.5 kW (MPPT1) and 2 kW (MPPT2), whereas with an actively cooled Fronius inverter, asymmetric distribution can be implemented with 5.7 kW (MPPT1) and 0.8 kW (MPPT2).
In a system with a standard 285 watt solar module (Trina TSM-285), which has a current of 8.97 A, Fronius
inverters allow string lengths of 3 to 22 solar modules to be connected at tracker 1 and 3 to 20 solar modules
at tracker 2. With a passively cooled device, on the other hand, only 7 to 15 solar modules can be connected
at both trackers.
2.1.2 Flexibility of installation
With passively cooled inverters, the air must be allowed to flow in and out as freely as possible, which requires specific positioning of the inverter.
Inverters with a passive cooling system can only be mounted vertically (90°). There are also restrictions on
side-by-side or one on top of the other installation, as otherwise the air heats up from device to device,
greatly reducing the cooling effect. It is also inadvisable to position the inverter in direct sunlight, as the device with the passive cooling system is warmed by the radiant heat in addition to the ambient temperature.
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There are no installation restrictions for devices with an active cooling system. The inverters can be mounted
vertically or horizontally (0° - 90°) and even flat (on a roof).
Figure 2: Flexible mounting options for actively cooled Fronius inverters [source: Fronius]
With an intelligent, active air guide, it is possible to mount inverters side-by-side. The cool air is drawn in
from the side, for example, and the heated air is dissipated upwards, as shown in the diagram below:
Figure 3: Flexible installation - an intelligent, active air guide allows installation side-by-side or one on top of the other
[source: Fronius]
Controlled convection allows heat dissipation to increase approximately five-fold compared to passive convection, which means that the inverters can also be placed in locations with high levels of insolation.
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