Microinverters are small grid-connected inverters designed to connect to a single solar module and provide a 240 volt alternating current (AC) output. These devices perform the same functions – and are required to meet the same technical and safety standards – as conventional string inverters, but are designed to be installed on or adjacent to a solar module.

Microinverters – a recent history

Veterans of the photovoltaic (PV) industry may recall the emergence of microinverters in the mid-1990s; it wasn’t long before they were being attached to the back of solar panels to create the first AC modules. History shows that while the concept was sound, the reliability and cost-effectiveness of these early microinverters limited their widespread adoption and within a few years the technology disappeared from the solar landscape.

Despite this disappearance, research into the technology continued, and in 2008 the first of a new generation of microinverters was launched in the United States. This unit, designed by Enphase Energy, heralded the beginning of a new era in module-level electronics.

Since mid-2008, well over 2 million of these next-geneneration microinverters have been put into service around the world, and shipments by manufacturers currently servicing this market exceed 150,000 units per month.

The first of the new generation microinverters to receive certification for use in Australia arrived in the market in October 2011, and 12 months later there is an increasing range of microinverter products available to installers.

Advantages versus potential disadvantages

Microinverters and AC modules have a number of inherent advantages over conventional direct current (DC) string inverters:

• Each solar module is individually maximum power point tracked to maximise the energy produced by the module under all conditions
• Any shading of an array only effects the output of the particular module being shaded instead of affecting the output of the entire string
• DC array cabling is virtually eliminated, significantly reducing fire and safety risks
• Arrays can be easily installed across multiple phases
• Installation time and costs are reduced by not having to install DC cabling and protection, and not physically mounting large string inverters
• Avoidance of a single point of system failure – if a microinverter were to fail it would only affect the output of one module
• Module level monitoring provides oversight of the performance of each individual module in an array, significantly reducing troubleshooting and maintenance times.

On the other side of the equation, microinverters do have some limitations and potential disadvantages:

• Multiple microinverters are currently more expensive than a single string inverter of the same capacity
• Microinverters are physically installed across the extent of the array, so accessing them for maintenance purposes may be more difficult
• Environmental conditions under (particularly roof-mounted) arrays are challenging and microinverters must be designed and tested to operate under these conditions for the life of the device.

The microinverter market

While microinverters entered the market as a niche device particularly well-suited to installations that were partially shaded or had a difficult orientation, their use is rapidly becoming more common. An increasing number of installers are now specialising in and exclusively installing AC solar systems.

The microinverter market both in Australia and internationally is dominated by a small number of manufacturers that specialise in the technology. However, this will change over the next 12 months when most of the major inverter manufacturers around the world are expected to add microinverter products to their range.

While the broad functionality of most microinverters is similar, there are several areas where the available products differ markedly in their specifications. These include:

Communications

Module level monitoring is achieved through communication between each individual microinverter and a central data-aggregating internet connection device. Some manufacturers have chosen power line carrier-based communications for this task, while others have used wireless communication protocols.

While both approaches have proven successful, one may be more suited to a specific installation or application than the other. Most microinverter manufacturers provide installers and end customers with access to system performance data via a web portal.

AC connections

Some manufacturers have adopted a ‘daisy chain’ approach to AC wiring where each microinverter has incoming and outgoing AC cable connections and the units are each connected to (and through) each other. Other manufacturers provide a single AC ‘flying lead’ from the microinverter, requiring the installer to connect each lead to an AC bus that typically runs the length of the array.

Temperature ratings

Most microinverters, including some being released by the major string inverter manufacturers, are designed to operate in ambient temperatures from -400˚C to +650˚C. There are, however, units available with ambient temperature ratings of up to +850˚C which may be preferable when designing arrays in very hot locations.

Due to the harsh environments that microinverters are installed in, reputable manufacturers carry out extensive environmental testing on their products before they are released on the market. These tests include accelerated lifetime testing in environmental chambers, where the units are taken to and beyond the extremes of their operating conditions.

Warranties

Manufacturers’ warranties vary from 15–25 years, and specific terms and conditions vary between manufacturers. Some warranties include provision for small payments to the installer and/or the end user to cover replacement expenses in the event of a confirmed unit failure.

One of the key factors to be taken into consideration when selecting a microinverter for a particular application is the input voltage range of the device. It is critical that the output voltage and current range of the solar module under all operating conditions falls within the input parameters of the microinverter to avoid damage to the device.

The principle standard that applies to the installation of microinverters and AC modules is AS/NZS 3000:2007 Electrical Installations.

As microinverter technology has only relatively recently re-entered solar practice in Australia, there are several areas where existing standards are open to interpretation in relation to microinverter installations. Over time, it is expected that the standards will evolve to effectively cover microinverter installations.

What’s coming up in AC

As with any new technology, it can be expected that rapid and ongoing advances and product improvements will take place in the microinverter sector. One example of this is the recent launch of several new microinverter products designed to connect to two solar modules.

By effectively incorporating the functionality of two microinverters into one package, the overall cost of the AC solar solution is reduced, and further innovations are likely to continue to bring the cost of microinverters closer to that of conventional string inverters.

There is significant ongoing investment in the development and production of microinverter technology worldwide, and market share data suggests that the technology is rapidly coming of age and is likely to play an increasing role in the mainstream deployment of solar PV in coming years.