Solar installers and homeowners nowadays have many more options for designing a PV system than in previous years.
One of these options is called Module-Level Power Electronics (MLPE).
This technology is focused on maximising the performance of the PV system by improving the Maximum Power Point Tracking (MPPT) in each module in an independent way.
This is an advantage over the typical string or central inverter configuration in terms of power output, a fact that makes MLPE an important option to take into consideration in the design of PV systems.
The technology is gaining a market share of 55% at a residential level according to the NREL.
The first MLPE that came onto the market was the microinverter from Enphase.
The product consists of a device that performs the same function as a string or central inverter – converting DC to AC. The only difference is that the device performs this function separately in every module. That way, the output power of each module is independent from the performance limitations of the others.
The second MLPE product that came onto the market was the DC power optimizer.
This device is designed as a DC-DC converter that maximises or optimises the DC power output of each module, and then sends optimized DC power to a string or central inverter that performs the DC-AC conversion of the systems.
Just as with the microinverter, the DC-DC converter performs the MPPT of each module, making them independent from the limitations of the other modules.
Now there is a lot of controversy in the definition of the performances, attributes and there are some disadvantages of these MLPE.
So how can we know which one to choose? Let’s analyse them!
Similarities Between Microinverters And DC Optimizers
We have mentioned that MLPE performs the MPPT in such a way that the power output of each module is independent from the others.
That presents a huge advantage! Imagine that you have installed your PV system with the corresponding shading analysis which turned out to be optimum for the conditions.
However, the PV system is expected to have a 25-year lifetime and a lot of things can change during that time. A new building, neighbours house or tree can cause shade on your system.
Under such circumstances, if you install a typical centralised or string inverter topology system, your system would diminish its power production as each string of modules is only as strong as its weakest module - in this case, the shaded one.
However, if you install an MLPE topology, each module that is not shaded will be working at its maximum (according to irradiance levels), while the shaded module will be working below the optimum point, but won’t affect the others.
Both microinverters and DC optimizers perform this function perfectly.
Mismatch Loss Reduction
Sometimes conditions of space are not as ideal for PV panels as they should be.
The ideal landscape for a PV array is a flat, unshaded and vast terrain in the middle of nowhere with a high level of radiation.
However, such an ideal environment is only available at the utility scale. When we supply solar PV at a residential or commercial scale level, the conditions are not always favourable.
Sometimes there are limitations in space which force the installer and homeowner to consider strings in different orientations.
Placing modules in different orientations creates the so-called mismatch losses, referring to a reduction in the total power output of the system due to different maximum power points in each string (each string receives different amounts of solar radiation across the day).
When the central inverter needs to perform the MPPT function, the inverter is unable to find the maximum power output for each string of modules and just sets some point in between. This phenomenon causes losses throughout the year that are undesirable.
Moreover, imagine that you install a set of modules from a particular brand and everything performs just perfectly.
Some years pass, and unfortunately, one of your modules is damaged somehow, and you need to replace it.
The problem is that as the solar market evolves, new technologies become available while old technologies are left behind. Therefore, you might have a situation where the module’s model in your system is no longer available, forcing you to select another power size, module or brand.
Having multiple modules of different sizes, characteristics, models or brands is undesirable in any PV system that works with string or central inverters. The reason is the same as stated before; the string is only as strong as its weakest module.
If you select an oversized replacement module, it will never perform at its maximum, while if you select an undersized module, the whole string will adjust its power output to the new one. This reduction in power is known as a mismatch loss.
Both microinverters and DC power optimizers allow you to completely avoid mismatch losses; just erase them from the map!
The reason is that every module’s power output is independent from the other, meaning that you can set your modules in the orientation that you like!
And you can replace the module with whatever model you like!
Rapid Shutdown Requirements
One of the ongoing trends in the solar industry worldwide is the rapid shutdown requirements of PV systems.
High DC voltage can be deadly for anyone who steps out on the roof without the proper safety precautions.
Grid-tied string and central inverters are designed to shut down when no frequency from the power grid is detected, meaning that all the loads will shut down, but the PV modules will still be generating DC.
Under unusual conditions such as natural disasters - flooding or fire in buildings, firefighters would need to come in and would be at risk of an exposed or damaged wire.
In many cases, people are forced to take shelter on the roof, which could be s full of solar PV. This also exposes unqualified people to the risk of high DC voltage contact.
Taking into account these events, National Electric Codes worldwide (United States, Canada and Europe) have started to include Rapid Shutdown requirements for new PV systems, where a single button or switch should be able to bring all the components of the system (including the module's output) down to a minimum DC voltage. This should be able to activate in the absence of power from the grid, both onsite (roof) and remotely (ground floor for example).
Here in Australia, concern for the introduction of this safety measure is being debated and it is probable that it will eventually be instigated.
Now, how are MLPE and rapid shutdown requirements related?
Unlike central and string inverters, MLPE reduces DC voltage to a minimum in the case of the power grid going out.
Microinverters from Enphase set the voltage to zero at the module level, while DC optimizers from SolarEdge reduce the voltage to 1 V at the module level.
This means your system will remain safe while at the same time keeping firefighters and others safe under unusual conditions.
Unfortunately, these rapid shutdown requirements also increase the cost of the system because a special combiner box is required. However, including microinverters or DC optimizers in your system will save you money on that matter.
However, we must remember that DC power optimizers are DC-DC converters, therefore the process of converting DC to AC still needs to be done by the central inverter the efficiency of which is varied, taking the example of SolarEdge the weighted efficiency is 98.8%.
Microinverters, on the other hand, perform the AC conversion directly, therefore the rated efficiency represents the overall efficiency of the power output. Taking the case of the IQ7 Series from Enphase the weighted efficiency is close to 97%.
We can also see a typical difference of nearly 2%, such value can vary depending on brand and model but in most cases the difference is minimum.
One of the greatest advantages of MLPE is the ability to monitor the rated performance of each module through an Energy Management System like Envoy from Enphase. This allows evaluation of the estimated energy output and peak power from every module.
How is this useful? When you think about the normal operation you can evaluate whether or not the system is behaving as predicted by the design.
Moreover, detection of failures is easier with these systems.
Differences Between Microinverters and DC Optimizers
The first important difference that might come to mind is the system’s expansion restriction.
Microinverters are excellent for expansion of the system, as every module-microinverter kit works independently. Therefore, if you want to expand your system, just work out how many panels you need to add and simply add the same number of microinverters.
On the other hand, DC power optimizers and modules need to be sized according to the maximum DC voltage input of the central or string inverter of the system. Therefore, expanding the system could involve changing the inverter, which would be more expensive.
As mentioned above, DC optimizers and microinverters shut down if the grid fails or if it’s turned off.
However, the difference lies in the normal operation of the system. Under the DC optimizers configuration, modules connected in series add more voltage, resulting in an elevated DC voltage.
High DC voltage is considered dangerous in any case, as can be seen in the video below:
On the other hand, microinverters self-extinguish any arc faults between conductors, which increases the safety of the overall system.
Potential Induced Degradation (PID)
This phenomenon is associated with another electrical-physical phenomenon known as leakage currents.
In order to present it in a simple way, leakage currents are undesired current flows between the surface of the crystalline silicon and the metallic frame of the modules, which cause the module’s output power to decline.
Follow this link to study more about the PID.
There are several causes for PID, but one of them is a high DC voltage of PV systems - 1000 V and above.
When considering these levels of voltage, the phenomenon can become a PV system problem of medium to long-term, being a cause of failure that is very hard to detect.
As mentioned earlier, DC optimizers only perform the MPPT tracking at the module level, but the output voltage of the entire system remains high, therefore, DC optimizers do not have any favourable effect against PID.
On the other hand, microinverters maintain voltage and power conversion at the module level, therefore, they greatly reduce the chance of failure due to PID in a PV system.
One of the greatest disadvantages of DC optimizers not often mentioned is that if any individual optimizer, communication system (between optimizer and inverter) or the inverter fails, then the complete string and even the PV array, fails too.
But microinverters have the highest reliability levels of all PV systems because the AC power output has multiple “small sources” of power, so is not concentrated on a single component.
Cost per Watt
Cost is the main downside for microinverters as the system involves more electronic devices that need to be purchased for every module, so the overall cost tends to be higher than all the other configurations of PV systems.
Just for a quick comparison, according to SolarEdge, the cost per Watt of DC power optimizers is close to 0.4 - 0.55 USD per Watt, while for microinverters, price tends to vary between 0.56 and 0.66 USD per Watt.
Another great downside of microinverters not often mentioned is the power clipping.
SolarEdge DC power optimizers are allowed to work with modules of 420 Wp and 125 Vdc, allowing the maximum DC power output according to radiation levels at the site, which is then converted to AC power from the inverter.
However, Enphase microinverters have limited module power DC input (depending on the microinverter) and what is more important, a huge AC output limit.
For instance, if you take a look at the Enphase IQ7 Plus (latest microinverter on the market) datasheet, you will find that the highest DC power module that can be connected to the microinverter is 440 Wp, but the maximum power output that can be obtained in AC is 295 VA. Assuming a power factor of 1, the active power would be 295 W, nearly 70% of the maximum peak power of the module!
This is a game-changing aspect, as the option to apply storage expands the possibilities of different configurations of the PV system (off-grid, backup, grid-tied with battery back-up).
Here, DC optimizers can interact with batteries as they have a DC power output that can be connected to charge controllers to charge the batteries and then supply power to DC loads or to the inverter.
It’s important to mention that in order to obtain backup features with the DC optimizer, it is important to select the StorEdge inverter, otherwise when the grid goes off so will the system, as a safety measure.
Microinverters have a great downside here.
As the output of the microinverter is AC, no charge controller can be connected to charge batteries, therefore a battery-based inverter needs to be installed (the one that uses AC to charge the batteries).
Enphase offers a solution for this problem with an IQ Storage, which basically consists of an integrated system of inverter-battery of 1.2kW standard, meaning that if you want to add more batteries, they must all be 1.2 kW too.
The issue with this storage system is that it does not work for back-up and off-grid purposes.
The system only allows the user to store solar energy and use it when the tariff of the electricity from the grid is higher.
As an overall conclusion, the winner of the battle between microinverters and DC optimizers depends on the topology of the system that is going to be implemented.
If the topology is for off-grid or backup purposes the answer is DC optimizers. Moreover, if the modules selected are over 300 Wp then probably, DC optimizers would present a better power performance. Finally, the cost issue is always a determining fact.
For grid-tied systems with modules below 300Wp and where expansion and reliability are important for the project, the winner could be microinverters.
As an unofficial advance, this conclusion could change greatly in the next few months, because Enphase is about to release the IQ8 model, a system under which off-grid and backup features may be possible!
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Photo credit: Depositphotos