A solar micro-inverter, or simply microinverter, is a device used in photovoltaics that converts direct current (DC) generated by a single solar module to alternating current (AC). The output from several microinverters is combined and often fed to the electrical grid. Microinverters contrast with conventional string and central solar inverters, which are connected to multiple solar modules or panels of the PV system.

 

Microinverters have several advantages over conventional inverters. The main advantage is that small amounts of shading, debris or snow lines on any one solar module, or even a complete module failure, do not disproportionately reduce the

 

output of the entire array. Each microinverter harvests optimum power by performing maximum power point tracking for its connected module.[1] Simplicity in system design, simplified stock management, and added safety are other factors introduced with the microinverter solution.

 

Solar panels produce direct current at a voltage that depends on module design and lighting conditions. Modern modules using 6-inch cells typically contain 60 cells and produce a nominal 30 volts.[2] For conversion into AC, panels are connected in series to produce an array that is effectively a single large panel with a nominal rating of 300 to 600 VDC.[N 1] The power then runs to an inverter, which converts it into standard AC voltage, typically 230VAC/50Hz for South Afrca, or 110VAC/60 Hz for the North American market.[3]

 

The main problem with the "string inverter" approach is the string of panels acts as if it were a single larger panel with a max current rating equivalent to the poorest performer in the string. For example, if one panel in a string has 5% higher resistance due to a minor manufacturing defect, the entire string suffers a 5% performance loss. This situation is dynamic. If a panel is shaded its output drops dramatically, affecting the output of the string, even if the other panels are not shaded. Even slight changes in orientation can cause output loss in this fashion. In the industry, this is known as the "Christmas-lights effect", referring to the way an entire string of series-strung Christmas tree lights will fail if a single bulb fails.[4]

 

Additionally, the efficiency of a panel's output is strongly affected by the load the inverter places on it. To maximize production, inverters use a technique called maximum power point tracking (MPPT) to ensure optimal energy harvest by adjusting the applied load. However, the same issues that cause output to vary from panel to panel, affect the proper load that the MPPT system should apply. If a single panel operates at a different point, a string inverter can only see the overall change, and moves the MPPT point to match. This results in not just losses from the shadowed panel, but the other panels too. Shading of as little as 9% of the surface of an array can, in some circumstances, reduce system-wide power as much as 54%.[5][6]

 

Another issue, though minor, is that string inverters are available in a limited selection of power ratings. This means that a given array normally up-sizes the inverter to the next-largest model over the rating of the panel array. For instance, a 10-panel array of 2300 W might have to use a 2500 or even 3000 W inverter, paying for conversion capability it cannot use. This same issue makes it difficult to change array size over time, adding power when funds are available. If the customer originally purchased a 2500 W inverter for their 2300 W of panels, they cannot add even a single panel without over-driving the inverter.

Other challenges associated with centralized inverters include the space required to locate the device, as well as heat dissipation requirements. Large central inverters are typically actively cooled. Cooling fans make noise, so location of the inverter relative to offices and occupied areas must be considered.

 

Citations

• [1] Where Microinverter and Panel Manufacturer Meet Up Zipp, Kathleen “Solar Power World”, US, 24 October 2011.

• [2] SolarWorld's SW 245 is a typical modern module, using 6" cells in a 6 by 10 arrangement and a  of 30.8 V

• [3] SMA's SunnyBoy series is available in US and European versions, and the recommended input range is 500 to 600 VDC.

• [4] "Productive", Enphase

• [5] Muenster, R. 2009-02-02 “Shade Happens” Renewable Energy World.com. Retrieved on 2009-03-09.

• [6] "Increase Power Production", eIQ Energy