By Blair Reynolds, Product Manager, Energy Storage, SMA America
Understanding Traditional DC Optimizer Technology
To understand how a power optimizer operates, it is first necessary to understand that a power optimizer is actually just a buck-boost converter with built-in communications. A buck–boost converter is a type of DC-to-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It operates by simply adjusting DC voltage either up or down. Modern string inverters also have this component built-in, but it is commonly referred to as a Maximum Power Point Tracking (MPPT) input.
A power optimizer-based system architecture is actually built upon a string inverter topology. Groups of PV modules are connected together in series strings. The primary difference is that the DC:DC stage of the inverter is not built-in to the inverter, but instead it is distributed across the PV array. What the power optimizers are actually doing is “optimizing” the string voltage to match the design input voltage of the inverter (typ. 380 or 400 Vdc for single phase systems).
Myth: Power Optimizers generate more energy for shaded PV arrays
This myth is based on the marketing claims that power optimizers produce more power when one or more modules are shaded. While the basis of this claim is true, more power does not always result in more energy yield. Allow me to explain: As I mentioned above, the power optimizer serves the same function as the DC:DC stage on a modern string inverter — the primary difference being that they are distributed across the array rather than built-in to the inverter. Similar to the maximum power point tracker of a modern string inverter, the power optimizers adjust the operating voltage of the PV modules to match the DC string voltage required to operate the inverter. When one or more PV modules in the string become heavily shaded (relative to the other modules in the string) the function of the power optimizer behaves a lot like MPPT because the current path through those shaded portions of the PV modules have essentially been bypassed (not by the optimizer, but rather by the PV module bypass diodes). The power optimizer will therefore operate (1/3) or (2/3) of the module at a maximum power point based on the reduced voltage output of the shaded module.
Now, consider that the time of day when shadows are the longest are in the early morning and in the late afternoon. During these times of day, the irradiance is also the lowest. Therefore a 4-5% power gain in the mornings does not translate into very much additional energy yield. In fact, a number of studies suggest that the internal power consumption of the optimizers bucking and boosting voltage all-day everyday outweighs the additional energy harvest early and late in the day. Power optimizers would only contribute a meaningful benefit in terms of energy yield if one or more modules were heavily shaded during the middle of the day. In this scenario, I would question the value of placing the module(s) in that location in the first place. If the entire array is shaded evenly (like from a cloud for example) the power optimizers do not contribute any meaningful additional power or energy.
Myth: Traditional Power Optimizers generate more additional energy than they consume
Power optimizers are subject to tare losses associated with running the onboard power electronics and powerline communications. These devices consume processing power and they suck that power from the PV modules anytime the PV modules are in operation. Furthermore, power optimizers are almost constantly bucking or boosting voltage all-day, every day. Remember: unlike a microinverter which is truly operating independently, the job of a power optimizer is to operate the module at a voltage which allows the aggregate group in a string to match the DC bus voltage (380 or 400V) required to operate the inverter. Under sub-optimal operating conditions (for example, when there is module mismatch or shading) the optimizers are forced to adjust their operating voltage such that the optimizer efficiency is reduced. The worse the operating conditions become, the optimizer efficiency declines accordingly, because the devices are forced to work harder to adjust the voltage.
And, then there is the increased voltage drop (which is not accounted for on the power optimizer datasheet). A power optimizer adds 8.84 feet of extra 12 AWG cable (including both input and output cables) to every module it is connected to. That extra cable alone creates a voltage drop of about 0.27 Volts lost per optimizer in full sun. That’s over 145 Watts of lost power in a 12 kWp system which correlates to more than 265 kWh of lost solar production per year, due to extra cabling alone. Note: this calculation does not factor in the added resistive losses associated with the additional four connectors per power optimizer.
At the end of the day, plant owners care most about savings, which is directly related to energy yield.
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