3
\$\begingroup\$

According to Wikipedia:

solar panels produce voltages around 30 V. This is too low to be effectively converted into AC to feed to the power grid. To address this, panels are strung together in series to increase the voltage to something more appropriate for the inverter being used, typically about 600 V.

A power optimizer on each panel would then ensure the failure of one panel won't ruin the overall production of the serial circuit of panels, and roughly 600V would be delivered off of the roof to a single inverter.

Microinverters, on the other hand, do not spit out 600V from a serially connected loop. They convert to 120VAC directly at each panel from the ~30VDC output at the panel. These microinverters are more expensive than power optimizers, for obvious reasons, but are touted as being more efficient.

So:

  • The wikipedia article says it's more efficient to convert to residential AC from 600VDC than from 30VDC
  • Industry says the most efficient system is micro inverters, which converts to 120VAC from 30VDC

How can microinverters be more efficient than power optimizers if the most efficient way to convert to residential AC is from 600VDC?

\$\endgroup\$
5
  • 2
    \$\begingroup\$ So.. you're saying that the manufacturers want you to buy the more expensive product? \$\endgroup\$
    – pipe
    Jan 1 '18 at 21:09
  • 1
    \$\begingroup\$ MicroInverters get installed on the back of the panels, hence all the heat of the sun is available to degrade reliability of the microinverter. I've a friend with 20+ roof panels, and after 4 years he's had 3 of 20 inverters need replacement for failure. \$\endgroup\$ Jan 1 '18 at 21:44
  • \$\begingroup\$ @analogsystemsrf, "to degrade reliability of the microinverter"... this simply means that your friend (or his solar provider) choose wrong manufacturing company for the micro-converters. \$\endgroup\$ Jan 1 '18 at 22:22
  • 2
    \$\begingroup\$ @jakewins: By your instant acceptance of an early answer (below) you may discourage others from answering and giving further insights. We generally advise you wait a day or two to give the whole of humanity a chance to chip in. You can unaccept and leave a note to explain ... \$\endgroup\$
    – Transistor
    Jan 1 '18 at 22:27
  • \$\begingroup\$ it depends on what you mean by "efficient": each unit, a residential system, just the converter's output, economically efficient, etc. \$\endgroup\$
    – dandavis
    Jan 2 '18 at 4:32
2
\$\begingroup\$

In a micro-inverter you can use MPPT (Maximum power point tracking) on each panel to ensure you are extracting as much power as you can from each panel in the given sun/shade condition for each one. Further there is more cable loss in a string inverter (600V) system. So I think overall system efficiency is better with mirco-inverters. Here's an article that may help you.

\$\endgroup\$
2
  • 1
    \$\begingroup\$ "Further there is more cable loss in a string inverter (600V) system." - That sounds bogus to me; the string configuration puts ~600V(DC) on the wires; the micro-inverter configuration puts 100-240V(AC) on the wires. Since the string configuration uses a higher voltage on the wires from the panels it puts less current through them, and thus should have less cable loss. \$\endgroup\$
    – marcelm
    Jan 1 '18 at 23:58
  • \$\begingroup\$ I got that part from the source in the article. All the current has to travel in one large loop so perhaps they are referring to there being more loss because more cable is involved in the wiring. Wiring a 3x5 grid you'd have to double back twice from the top left corner to the bottom. \$\endgroup\$ Jan 2 '18 at 0:14
5
\$\begingroup\$

With something like solar power generation efficiency is a tricky thing to pin down.

The micro-invertor itself may indeed be more efficient than the power optimizer, but it does that at a cost of much higher currents in the system. The latter translates into much more expensive wiring, connections, switching systems, and ultimately the micro-invertor itself. All of those can also add significant resistive losses in the system if you do not spend enough money on them that take away from the efficiency before you even reach the invertor.

Further, to complicate matters, when considering the efficiency of solar systems you also need to factor in costs, specifically costs per kW over the lifetime of the system. A system that produces power at less dollars per kW can be considered as a more efficient system even if it is extracting less raw power from the panels. This is especially true if you are trying to redeem your investment costs by feeding back into the grid at a fixed price.

\$\endgroup\$
0
0
\$\begingroup\$

Solar systems use an array of panels. From engineering standpoint the challenge is to sum all power from all panels/elements in most efficient way.

There are three main architectures for solar-to-AC_grid conversion. This short article from EnergySage highlights distinctions of the three:

  1. String inverter. It uses bare solar panels connected in-series, to get about 600 VDC to convert it into AC-grid level. Used where uniform insolation across the array happens.

  2. Micro-inverters, they are attached to each low-voltage panel, and convert each panel directly into AC-grid level. The micro-inverters use MPPT (Maximum power point tracking) technology, which sums up power from individual panels much more efficiently than a single string of panels can do, on a system level. So, even if the efficiency of "string inverter" can be higher than each individual micro-inverter, the overall system output is better;

  3. String inverter with power "optimizers" on each panel. Power optimizers also use DC-DC conversion technology with MPPT method to alleviate panel's uneven output, then the string inverter converts "optimised" string of panels into AC.

Apparently the "power optimizers" either can't optimize uneven outputs from different solar panels to the same level as the micro-inverters can do, or are less efficient than micro-inverters. Obviously the primary currents are the same in both optimizers and micro-inverters, contrary to some other opinions.

So the actual question should be: "Why power optimizers can't do panel equalization as efficiently as micro-inverters can". From higher perspective, the answer is that "power optimizer" DC-DC switchers work with about the same levels (30V) of conversion, while the micro-inverter upconverts 30 V into 120/220 V, or 4X. The 4X conversion to 120 V has much better efficiency than 30-30 conversion, maybe not as good as 600 -> 120 V conversion, but still better.

Than's why micro-inverters outperform "optimizers" with string inverters, and optimizers are considered a compromise between more expensive micro-inverters and the standard string inverter.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.