AC modules: past, present and future

H. Oldenkamp1, I.J. de Jong2

1
NKF KABEL B.V., Electronics Group, P.O. Box 26, 2600 MC Delft, The Netherlands, Telephone:
+31 15 2605616, Fax: +31 15 2605714, Email: Henk.Oldenkamp@nkf.nl
2
OKE-Services, Nieuwstraat 29, 5611 DA Eindhoven, The Netherlands, Telephone: +31 40 2445262,
Fax: + 31 2464133, Email: oke@euronet.nl

Abstract
AC modules offer a lot of advantages compared to traditional photovoltaic systems with one central
inverter. These do not only relate to safety, but also to economics, especially of smaller systems. AC
modules are especially interesting to use for the integration into buildings, as the independent operation of
AC modules prevents that a whole PV system will function poorly due to shading of only one PV module.

Research and development into AC module inverters started in the early nineties. The development efforts
have resulted in AC module inverters ranging from 100 to 250 Watts. The available inverters are all more
or less mature. Currently we are in the stage of market introduction. Larger projects are being executed to
gain data about the inverters in the field. Besides safety standards are being developed in order to
guarantee safe operation of the inverters.

Future developments of AC modules will focus on lowering the price. At this moment AC modules are
considered price competitive with traditional PV systems up to 700 Watts. Due to economies of scale AC
modules will become competitive for rooftops of about 20 PV modules (2000 Watts). By using new circuit
technologies the price of AC module inverters will drop further, making AC modules even more
competitive. It is expected that with the start of a new century, new inverter technologies will be
introduced.

Why AC modules?
Traditional photovoltaic (PV) systems require
complex DC cabling giving rise to a lot of
problems. In particular grid connected PV
systems with one central inverter have shown
significant problems with respect to:
Ø High DC voltage levels.
Ø Safety.
Ø Cable losses.
Ø Risk of DC arcs.
Ø Fire hazard and protection

Most of these problems can be overcome by

expensive cabling and installation systems, but
such solutions increase the costs at system level.
Components like inverters, switchgear, cables
have to be selected in accordance with system
size. This requires an individual design of each
system and is an obstacle in expanding the
system.

Thus, in general the installation of traditional PV Figure 1. Representation of traditional PV

systems is rather complex, which is reflected by system with one central inverter.
figure 1.

Workshop Installing the solar solution, 22-23 January 1998, Hatfield, UK

 AC modules: past, present and future

AC modules, in which the inverter is integrated The past: development

in the PV module, overcome these problems. The
advantages of AC modules compared to The first idea about AC modules originates from
traditional PV systems are: the seventies. Due to technical limitations the
Ø Each module works independently: if one idea was never put into practice. In the late
fails, the other AC modules will keep on eighties research into AC module inverters was
delivering power to the grid. started by the Institut für Solar Energie-
Ø High modularity allowing easy system versorgungstechnik (ISET) e.V. [1]. Professor
expansion. Kleinkauf promoted the idea of AC modules in
Ø Low minimum system size of one AC several papers. He emphasised the advantages of
module, lowering the threshold for AC modules, by then called module integrated
individuals to start their own PV plant. converters, MICs (see also figure 3).
Ø Use of standard AC installation material,
which reduces costs of installation material
and system design.
Ø Low conduction losses and cable costs.
Ø No mismatch losses at system level as each
AC module operates in its own Maximum
Power Point (MPP).
Ø No need for string diodes.
Ø No need for bypass diodes.
Ø Low lightning induced surge voltages,
because of the compact DC system layout.

The main advantage of the use of AC modules is

reflected by figure 2: simplicity of the total
system. All these features make AC modules an Figure 3. Representation of field operation of AC
interesting option for the integration of (MIC) modules by Kleinkauf in 1992 [2]
photovoltaic systems into buildings. Especially
the independent operation of AC modules, which A couple of years later, in the early nineties, we
prevents that a whole PV system will function see that the promotion by Professor Kleinkauf
poorly due to shading of only one PV module. was not in vane. It was out of the blue adopted
by several companies, both in Europe and the
USA. Research and development into AC
module inverters started in parallel in the
Netherlands, Germany, Switzerland and the
USA.

In the USA we see a strong governmental and

utility push, which has led to AC module
inverters based on traditional techniques. In
Europe developments were rather initiated by
private companies, although in some cases
supported by local governments. We see here
more innovative concepts.

The different development efforts (taking 3 to 7

years) have led to AC module inverters ranging
from 100 to 250 Watts. The selection of power
size was based on technical feasibility,
economics and the size of available PV modules.
The switching techniques used range from more
conservative (low frequency, 50-100 kHz) to
more innovative (high frequencies up to 500
kHz). Specifications of available AC module
Figure 2. Representation of PV system inverters are presented in table 1.
with AC modules.

Workshop Installing the solar solution, 22-23 January 1998, Hatfield, UK

 AC modules: past, present and future

Table 1. Specifications of commercially available AC module inverters

ZSW1) Sunmaster Edisun OK4-100 SunSine 300 MI250
-130S E230721G
Panel (# cells) 72 72 144 72 216 144
Rated power (WDC) 100 W 100 240 100 2902) 240
Starting power (W) 0.9 1.5 0.15
Voltage (VDC) 24- 24-40 43-90 24-50 36-752) 52-92
Harmonic distortion 2% < 5% < 3% < 3% < 5% < 3%
Power factor at max. > 0.99 > 0.97 > 0.99 > 0.95 > 99%
Power factor at 10% 0.7 0.6 0.9
Stand by power (W) < 0.8 0.003 < 0.3
Efficiency max. > 90% 92% 91.7% 94% > 90%
MPPT efficiency 99% > 99.5 > 99%
Ambient temp. (o C) -25 – 60 -25 – +60 -40 - +85 -40 – +60
Weight (kg) 1.340 0.6253) 2.6003)
Dimensions (cm) 10x20x5 15x15x5.7 15.7x23x7.2 9.25x12x3 21.6x18.4
x3.2
Relative volume 0.010 0.013 0.011 0.003 0.006
(litres/rated power)
Inverter case Aluminium Plastic Aluminium Aluminium Aluminium Aluminium
Potted No No No Yes No Yes
Communication RS4854) No5) No RS4854) No Power line
Switching frequency < 100 < 100 < 100 Up to 500 100 90
(kHz)
Isolation transformer lf hf hf hf lf hf
Safety approvals KEMA KEMA UL UL
1)
ZSW developed a range of AC module inverters, from 100 up to 400 Watts. Currently only the inverters of
100 and 300 Watts are commercially available. In table 1 only the data of the inverter of 100 Watts are given;
those of the 300-Watts inverter are comparable.
2)
The rated power and voltage range of the SunSine 300 are estimated values
3)
The weight of these inverters is including potting material.
3)
The ZSW and OK4-100 feature extensive monitoring facilities; the OK4-100 also has an internal kWh
counter.
4)
The Sunmaster-130S has an optional RS485 interface with extensive monitoring facilities

All the inverters meet general safety frame. Some of the inverters are fully potted,
requirements and feature island protection. This which has a positive effect on safety and
means that the inverters can only function when lifetime.
connected to the grid. Several inverters include
monitoring facilities for which different
communication techniques are used: commu-
nication by means of a separate two-wire RS485 The present: market introduction
interface or power line communication.
Especially when the AC modules are integrated The first prototype of an AC module inverter
into buildings this is a very handy feature. It was presented in 1992. It was developed by the
enables to check if all individual AC modules are Zentrum für Sonnenenergie und Wasserstoff-
working well. When the monitoring facility Forschung Baden-Württemberg (ZSW),
features an internal kWh counter, monitoring can Germany. It concerned an inverter of 50 Watts,
be limited to checking the output of the inverters using a high frequency concept with a lf
once week or month: the internal kWh counter transformer. In 1994 ZSW produced a first series
shows the output of the individual AC modules of 100 Watts inverters, based on the same
over a period of time and is therefore the technique. Mainly because of economics and
reflection of their performance. electromagnetic interference problems ZSW is
now using low frequency switching concepts for
All the inverters have to be mounted on the their small inverters.
backside of the PV module, while the OK4-100
also offers the possibility to be mounted on the

Workshop Installing the solar solution, 22-23 January 1998, Hatfield, UK

 AC modules: past, present and future

In the period 1994-1996 three other European There is a distinct difference between the
companies introduced AC module inverters: perception of the market in Europe and the USA.
Mastervolt (Netherlands), Alpha Real In Europe the private market is seen as the
(Switzerland) and OKE-Services (Netherlands). largest potential segment. Especially, the
In 1996-1997 the first AC module inverters implementation on rooftops offers opportunities
developed by US companies (Ascension for AC modules. The so-called ‘growth project’
Technology, AES) were launched on the market. that will start in 1998 in the Netherlands may
All these inverters have reached the stage of serve as an example. On 9000 rooftops initially
market introduction on a larger scale; the four AC modules will be installed. The private
products have become more or less mature. owners of the houses will be encouraged to
expand their PV plant with additional AC
Some of the inverters are optimised for specific modules in the years to come. In the USA
PV modules: however, parties expect most of AC module
Ø The Sunmaster-130S for Shell 100 Watts plants of 1 – 10 kWp or even larger. The
module (24 Volts) manufacturers perceive the government and
Ø The Sunsine 300 for the ASE 300 Watts utilities as the main buyers of AC modules.
module (36 Volts)
Ø The MI250 for the Solarex 240 Watts Currently AC modules are being implemented in
module (48 Volts). several larger projects. Most of these projects
In these cases the development was executed in still focus on demonstration and obtaining data
close co-operation with the PV manufacturers. about AC modules in the field. Examples of
Inherently the market introduction focuses on the projects are found all over Europe. In the
AC module as a whole. Netherlands currently 2100 AC modules are
being implemented in a large sound barrier.
The other inverters were developed more
independently, but can be used with several PV With a growing market projects focus on other
modules. For example the Edisun E230721G will aspects of AC modules: safety, lifetime and
work fine with a Solarex 240 Watts module (48 economics.
Volts). The OK4-100 can be combined with any
100-130 Watts PV module (24 Volts). In general In several countries safety standards for AC
all AC module inverters are designed for modules are being developed. In the Netherlands
operation of multiples of 12 Volts modules in this has resulted in the KEMA-approval for AC
series. Currently we see that the companies, who module inverters. Currently all AC module
developed their inverters independently, seek co- inverters used in the Netherlands have to comply
operation with PV manufacturers. This enables with this approval. In the USA the Underwriters
the promotion of AC modules on the market, Laboratories (UL) developed a standard (UL
rather than AC module inverters. Besides, 1741) especially for AC modules. The SunSine
smaller companies are starting co-operation with 300 and the MI250 are both UL listed, while the
bigger companies in order to produce larger OK4-100 probably will be listed in the spring of
series. 1998.

Whether AC modules can compete with In Europe AC modules are mainly being
traditional PV systems does not only depend on implemented on rooftops and buildings.
the price of the inverter itself: the selling price of Therefore costs of reparation will be high.
the whole AC module is decisive. Generally the Moreover, most of the inverters are mounted on
PV manufacturers that sell the AC modules set the PV module. It is therefore important that the
the price. Because of the high additional value, inverters have a lifetime comparable with that of
there is a tendency to increase the selling price of the PV module, which last more than 20 years. In
an AC module compared to a PV module with order to gain more insight into the lifetime of AC
more than the price of the inverter. Prices of AC module inverters, several research projects are
modules therefore differ strongly, ranging from being executed. Amongst others, the Netherlands
US$ 6 to US$ 9 per Watt peak. Depending on Energy Research Foundation ECN is performing
price and country AC modules are price accelerated life tests to obtain data, which can be
competitive with traditional PV systems for used for lifetime prediction calculations.
systems up to about 700 Watts.

Workshop Installing the solar solution, 22-23 January 1998, Hatfield, UK

 AC modules: past, present and future

become available optimised for AC modules,

like custom design chips. Also other production
techniques can be used. Just because of the
economies of scale it is expected, that in the near
future AC modules will be price competitive
with traditional PV systems up to 2000 Watts.
This is about the size of one rooftop.

The size of PV modules also plays an important

role. In Europe PV manufacturers generally
produce 100 Watts modules, which is quite small
compared to the USA where 200 and 300 Watts
modules are more common. The costs of an
inverter do not level with the power. Therefore,
AC modules with higher power (e.g. 300 Watts)
are less expensive in terms of costs per Watt
peak than AC modules with less power (e.g. 100
Watts). This supports the idea to manufacture
larger PV modules. On the other hand, especially
in Europe, the private market for AC modules of
100 Watts is very large.

Figure 4. AC modules in the future: a reliable Further improvements of AC module inverters

and proven product with a long lifetime. are expected. In Australia a lot of effort is being
put into the development of a new AC module
inverter. Also in Europe several development
projects started aiming primarily at lowering the
The future: a growing market and price of AC module inverters.
improving designs
In the Netherlands for example two AC module
The market of AC modules will strongly grow inverter projects have started, both funded by
especially when private persons will enter the Novem, the Netherlands agency for energy and
market. It is expected that before the next the environment. One of these projects aims at a
century 50.000-100.000 AC modules will be cost reduction of even 50% next to an increase of
installed. With these numbers the reliability of the efficiency to 95%. Detailed calculations on
AC modules will be proven by the year 2000. new innovative topologies indicate that this
target seems feasible. Thus, we expect that with
Customers will separate the wheat from the chaff the start of the next century, new inverter
and only the best inverters will survive in the technologies will be introduced.
market place. A shake up of the market can be
expected within three years. Moreover, potential Innovative technologies, larger PV modules and
buyers and users of AC modules will set higher high volume production can make the
demands, especially relating to installation and expectations of Professor Kleinkauf in 1992 [2]
monitoring. Low cost standard AC cabling come true: a drop of the price of AC module
assemblies will be developed, enabling easy inverters to below US$ 0.5 per Watt peak.
system installation. Extensive monitoring
facilities will be included in all inverters. These
will also become more intelligent and will
include e.g. automatic signalling when AC Acknowledgements
modules do not function properly. The software
will also become more enhanced and user We would like to thank the following companies
friendlier and will eventually be replaced by low and persons for supplying us with information
cost monitoring displays with fault indication about their AC module inverters:
and diagnostics. Ø ZSW, Dr. Werner Knaupp.
Ø Alpha Real ag, Mr. Markus Real.
With a growing market the production series will Ø Ascension Technology, Mr. Greg Kern.
become larger and mass production will start. Ø Mastervolt, Mr. Roel J. ter Heide.
Because of the higher volumes components will

Workshop Installing the solar solution, 22-23 January 1998, Hatfield, UK

 AC modules: past, present and future

References Ø Oldenkamp, H.; Haan, S.W.H. de; Jong, I.J.

de; Baltus, C.W.A.; Frumau, C.F.A.;
Ø Kleinkauf, W.: Photovoltaic Power Verhoeven, S.A.M.: Competitive
Conditioning; 10th EPVSEC, Lisbon, implementation of multi-kilowatts grid
Portugal, 1991 connected PV-systems with OKE4 AC
Ø Kleinkauf, W.; Sachau, J.; Hempel, H.: modules, 13th EPSEC, Nice, France, 1996
Developments in inverters for photovoltaic Ø Knaupp, W.; Schekulin, D.; Volgtlander, I.;
systems; 11th E.C. Photovoltaic Solar Energy Bleil, A.; Binder, C.: Operation of a 10 kW
Conference, Montreux, Switzerland, 1992 PV façade with 100W AC photovoltaic
Ø Haan, S.W.H. de; Oldenkamp, H.; Frumau, modules, 25th PVSC, Washington D.C.,
C.F.A.; Bonin, W.: Development of a 100W 1996
resonant inverter for AC modules, 12th Ø Oldenkamp, H.; Jong, I.J. de; Baltus,
EPSEC, Amsterdam, The Netherlands, 1994 C.W.A.; Verhoeven, S.A.M.; Elstgeest, S.:
Ø Haan, S.W.H. de; Oldenkamp, H.; Reliability and accelerated life tests of the
Wildenbeest, E.J.: Test results of a 130W AC module mounted OKE4 inverter, 25th
AC module; a modular solar AC power PVSC, Washington D.C., 1996
station, 24th PVSC, Hawaii, 1994 Ø Verhoeve, C.W.G.; Frumau, C.F.A.; Held,
Ø Schekulin, D.; Schumm, G.: AC modules – E. de; Sinke, W.C.: Recent test results of AC
Technology, characteristics and operational module inverters, 14th EPSEC, Barcelona,
experience; 13th EPSEC, Nice, France, 1995 Spain, 1997
Ø Schekulin, D.; Bleil, A.; Binder, C.: Ø Oldenkamp, H.; Jong, I.J. de; Baltus,
Schumm, G.: Module-integratable inverters C.W.A.; Verhoeven, S.A.M.: Advanced high
in the power range of 100-400 Watts, 13th frequency switching technology of OK4 AC
EPSEC, Nice, France, 1995 module inverters break the 1 US$ per Watt
price barrier, 14th EPSEC, Barcelona, Spain,
1997

Workshop Installing the solar solution, 22-23 January 1998, Hatfield, UK