PowerSource

Power-generating installations of solar photovoltaic panels are changing. Rather than the massive solar farms of the past10-20 years, smaller residential and industrial roof-top installations are gaining favor. These small-scale installations have different characteristics from the solar farms with their acres of arrays of solar panels that all face in the same direction and receive the same amount of sunlight. Solar farms have no obstacles such as neighbors’ trees or utility poles to shade a panel, causing a panel-to-panel variation in power output. On the other hand, residential and industrial installations have some features in common with solar farms: They are subject to dirt and require washing at regular intervals, and have panels that age at slightly different rates, causing variations in panel outputs.

Solar panels, or modules, consist of arrays of individual solar cells and typically produce about 200W at around 25-30Vdc. These panels are themselves cascaded in series, forming solar panel strings with a string output voltage of 300-350Vdc. These strings in turn can be paralleled into large solar arrays. The output of these strings or an array feed into a central inverter (UPDATE: pictured to the left) that transforms the dc voltage to ac and syncs the ac voltage to the grid. Efficiencies for central inverters are as high as 98.5%.

However, because of variations due to shading, dirt, and aging of solar panels, individual panel voltages will vary, causing the output voltages of strings of panels to vary. To deal with these variations, the central inverter constantly performs maximum power point tracking (MPPT) to find the sweet spot of power generation from the panel string or array. A common technique is for the inverter’s power transformation circuitry to attempt to draw a little more current and see what the response is: Does the voltage drop or not? The algorithm searches for the point where it gets the maximum power from the string or array.

Because the central inverter is dealing with strings or an array of these varying panels, the chances are high that rather than finding a absolute maximum for the array, the inverter will find a local maximum: The algorithm will be dominated by the performance of one poorly performing panel. If all the panels are well-matched, the difference between the true maximum power point and the local won’t be significant – but you can’t count on well-matched panels. Thus, one poorly performing module can dictate the power that the other modules in series with it deliver.

I attended Solar Power International last week in Anaheim, CA, where my quest was to learn more about micro-inverters. Micro-inverters are pretty new on the solar scene – at least this time around. Rather than having an inverter that tries to find the MPPT for a string or array of panels, you have individual inverters capable of about 200W, called micro-inverters, dedicated to an individual solar panel. Each panel now produces at its maximum power point.

European solar companies tried this approach about 10 years ago, but the cost at that time of multiple individual micro-inverters far exceeded the cost of a single central inverter, and the concept died. However, over the years things have changed. A major difference now is the above-mentioned trend towards smaller installations. Within these mini-solar farms, architectural limitations may dictate different rooftop orientations. Utility poles and neighbors trees aren’t so easily removed. It’s more important now to optimize each panel.

In addition, central inverters are physically large and heavy, calling for cement pads and single, centralized connections to the grid, making the installation capital expensive and labor costs high. A 200W micro-inverter that only has to deal with 30Vdc input and no special installation investment becomes very attractive for small installations. Safety is also an issue: A solar panel string or array’s input to a central inverter can be as high as 600Vdc in the US and 1000Vdc in Europe -- hazardous voltage levels for installers, maintenance personnel, and emergency responders. Output from modules attached to a micro-inverter scheme will be at much lower levels between 200 -- 300Vac. 

Ok, this is where the spooky fore-shadowing music comes in: Microinverters have to deal with quite a bit of ripple current and operate in elevated temperatures, constraints well-known for eating capacitors. At the same time, they are part of solar installations that typically require equipment guarantees of 20-25 years (typical for solar panels). How do micro-inverters meet the reliability challenge? Next up: Solar micro-inverter addresses capacitor reliability issues.