posted on July 14, 2013 |
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Solar panels produce DC electricity which, in most cases, must be converted to AC before it can be used. That’s the job of the inverter. A photovoltaic system typically includes several panels wired together in a series/parallel configuration, with their total DC output going to a central “string” inverter.
This design has a few significant flaws. First, it represents a single point of failure. If the inverter goes down, the whole system is down. Second, at any given moment, each solar panel may be producing different amounts of power depending on shading, age, wear, and other factors. In the central configuration, the overall system performance is dragged down by the weakest link. Inverters use maximum power point tracking (MPPT) to constantly adjust the voltage and current outputs so the array delivers maximum power for a given load and lighting condition, but a string inverter has to go with the average of the array rather than optimizing the outputs of each panel. In this configuration, shading just 10% of the array could result in a 50% decrease in total power output.
Microinverters were supposed to solve these problems. As you can probably guess, a microinverter is a small inverter that’s dedicated to a single panel. Instead of wiring all the panels together and sending a large DC output to a string inverter, we connect the microinverter outputs together to create a large AC signal. A microinverter will establish MPPT for its own panel, optimizing the overall system output regardless of minor shading issues and other variations among individual panels.
But no solution comes without trade-offs. Because each microinverter produces AC, we need a way to synchronize them so all of the signals are in phase. This is done by synchronizing the array output with grid voltage. So a microinverter system works well, but only in a grid-tied system. If the grid goes down, your solar power shuts down also. (Many string inverters, on the other hand, are multimode. They can operate grid-tied or they can go stand-alone if the grid power is down.) Also, adding a microinverter to each panel increases the cost of the system, and places the microinverters in the harsh outdoor weather (mounted directly under the PV panels where it gets very hot) instead of a basement or utility room.
CyboEnergy recently introduced a patent-pending technology: the CyboInverter, a mini-inverter that combines the best features of string inverters and microinverters. Like a string inverter, we connect several panels (up to four) to one inverter, reducing cost and wiring complexity. However, each panel has its own dedicated input, so the CyboInverter performs MPPT on individual panels, improving overall system performance. CyboEnergy offers two models: a grid-tied version and a stand-alone (off-grid) model.
I see the benefits of this, but I also notice that the CyboInverter doesn’t resolve two issues with microinverters: they’re still installed with the panels (harsh environment) and in the case of the grid-tied model, if the grid fails the whole system shuts down. Since they have both grid-tied and stand-alone models, I wonder why they don’t marry the two and create a multimode inverter - one that’s grid-tied but can disconnect from the grid and operate stand-alone in the case of a grid failure. (CyboEnergy executives: if you adopt that idea, you may send royalty checks to Tom Lombardo via Engineering.com.) By itself, I don’t see the CyboInverter as a game-changing technology, but it could be a nice intermediate step towards a better photovoltaic system.