Given that solar-photovoltaic (PV) generating technology has been around for decades, one might think its general application in commercial and consumer circles might be a little more pervasive. After all, it is clean, safe, and relatively simple. Ask any expert and it’s likely you’ll get a litany of reasons why it has not gained more general acceptance — implementation costs, tricky installation aspects, long ROI, performance, and reliability are just a few. Fortunately, there is a game-changing technology — microinverters — that is ready to push solar PV’s acceptance and application past many of the current barriers.

Inversion Technologies 101

A microinverter manages the energy of its own individual panel and allows each module to contribute power independently, so if one panel is in shade, its performance won't degrade the overall output of the array.
Solar-PV modules produce DC electricity, which typically needs to be inverted to AC before use. The traditional method to invert this power involves string inverter systems, which combine the energy from the various modules in an array on the DC side of the inverter, and invert them to DC all at once.

Microinverter systems for solar-PV inversion move the inversion capacity from a single inverter to multiple smaller inverters, immediately located to each PV module. These are connected in parallel on the AC side of the inverter, and connected to the utility system to create a grid tied system. The technological benefits and applications for this product make it an attractive choice for PV inversion.

As required by electrical code, microinverter systems on the market today include utility interactive phasing and anti-islanding technologies. Utility interactive phasing allows inverters to properly sync with the utility power that is present. This is accomplished by “pinging” the utility power with a waveform slightly different from the utility waveform and measuring the “push” that is received back from utility power. If no utility power is detected, these systems will automatically disconnect AC power production, a process known as anti-islanding. This is to protect utility workers who would be under the impression that no power is present when the utility system is down.

Microinverter installations offer users many benefits, including increased energy output, improved electrical safety, ease of system design and installation, and more precise monitoring capabilities. This is not the case with string inverted systems. With a typical string inversion system, every module in a string is limited by the weakest-performing module. For example, if a single module is partially shaded and loses 50% of its output, every module on that string becomes limited to the same 50% output.

More Power, More Safety

In a microinversion system, each module becomes an independent power-producing unit, and if one module is reduced to 50%, the other modules are not affected. Because the balance of the string is still producing at full capacity, more energy is harvested from the same modules when using microinverters.

Microinverters offer improved safety as a result of the reduction of the scale in the DC side of the system. Most installers are more familiar with AC wiring and AC arcing and grounding conditions, which are less dangerous and easier to isolate and control than DC arcing and grounding conditions. In a microinverter installation, the amount of DC wiring in the system is reduced to virtually zero.

Because there is no interaction between modules, installers are not required to balance systems. That means system designers and installers can use whatever roofs face the Sun at any given time (or Sun position) and be less concerned with the angles, and do not have to design around shading issues. While any of these will affect the performance of an individual module, a microinverter system as a whole can create usable power in layouts that a string inverted system would struggle to match in size and complexity.

Users are also getting more sophisticated and are more frequently requiring the ability to monitor the energy production of the their solar-PV installations. With a string inversion system, most users are only able to measure total system production, as produced by the single inverter. With a microinversion system, each module can be monitored independently, making maintenance and upkeep simpler and more straightforward.

To eliminate any communications wiring needed to accomplish module-level monitoring, most microinversion systems utilize a power line carrier (PLC) signal. That allows a communications signal to be sent out over the electrical conductors used to carry the power that is produced. This information is then aggregated to give system-level performance information, if so desired.

Applications

Microinverters, like the one shown here, bring new reliability, configurability, and better overall economics to solar energy-generating systems.
There are several major applications in which microinverters will significantly outperform their string inverted counterparts.

For shade-prone jobsites, microinverter systems give installers much more flexibility in installation location and usable roof area. On the roof of a typical factory, for example, there are rooftop AC units, ventilation systems, and other obstructions like smokestacks, trees, and other tall buildings nearby. Such obstacles present a major problem for string inverted systems. On the other hand, microinverted systems can take full advantage of the space, because the system is not universally affected by efficiency loss, as detailed previously.

For many installations with non-traditional layouts or rooflines, the varying angles present a problem for string inverter systems. For those systems to function at their peak, all the modules need to have relatively the same intensity of sunlight. This forces all modules to be mounted at the same angle of incidence of sunlight, and facing the same direction.

Microinverters will draw maximum power available from the module it is attached to, regardless of its orientation and relative to the installation angle of the other modules in the array. The classic example of this issue is a building with a pitched roof. Regardless of which direction the building faces or the angle of sunlight at a given time, microinverters are able to capture maximum available power from each module.

Microinverter-based installations are also inherently scalable. If a facility owner plans on growing, or wants to increase the capacity of the solar-PV system at a later point, adding modules incrementally is as simple as extending the AC cabling to the next array or set of modules. This is critically important for those enterprises that are in high-growth industries, plan on adding capacity at a later point, or merely want options in spite of not having an immediate need or enough money to implement more power.

Keeping an Eye on Things

Microinverters also capitalize on advanced monitoring capabilities. Arrays or individual modules across multiple installations can be merged into a single monitoring package. This makes monitoring campus or distributed systems much simpler. Each array can be grid tied independently, then monitored as a bundled, single installation. For example, at a college or university, there are dozens of buildings that can each receive their own installation, and can be combined to provide campus-level monitoring.

It’s apparent that recent advances in microinverter technology will only bring good things to the solar-PV universe. Any technology that introduces efficiency, simplicity, performance, and reliability, while driving down cost and complexity, is really going to go places — and not just on Planet Earth.

This article was written by James Washburn, Product Manager, AFCI & GFCI Renewable Energy Products, at Siemens Industry, Inc., Alpharetta, GA. For more information, Click Here .


NASA Tech Briefs Magazine

This article first appeared in the May, 2011 issue of NASA Tech Briefs Magazine.

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