Understand optimizers for photovoltaic systems

Have you ever encountered losses in a photovoltaic system due to shading? Or cases in which the photovoltaic modules would have to be located at different inclinations or orientations?

These are typical cases in which the photovoltaic system designer has the difficult task of proposing alternatives to reduce losses during the energy generation process.

It is on these occasions that modular photovoltaic architectures with MLPE devices (module-level power electronics). This type of architecture includes microinverters, already widespread on the market, and power optimizers.

In this article, we will talk about the advantages of power optimizers in photovoltaic systems. In summary, they offer the following possibilities in a photovoltaic installation:

• solar panels with different angles and orientations on the same string;
• panels of different powers in the same string;
• strings with different numbers of panels on the same inverter;
• reduction of the effect of shadows on photovoltaic modules;
• MPPT per module (maximum power point tracking – individualized maximum power tracking);
• greater safety by reducing open circuit voltage when the system is off;
• total elimination of the risk of electric arcs and fires.

What is a power optimizer?

The power optimizer for photovoltaic systems is a device whose main function is to reduce losses in a photovoltaic system [2], increasing the system's efficiency [3]. The generic architecture of a photovoltaic system with power optimizers is shown in the following figure.

The central idea of the system with optimizer is that the solar panels are not connected directly to the DC-AC inverter. Instead, the panels are connected to DC-DC converters that pre-process the energy before delivering it to the inverter.

Direct current (DC-DC) converters can be interconnected in series or parallel, supplying power to a conventional inverter or to a specific inverter for use with power optimizers.

The choice regarding the connection method or type of inverter changes according to the manufacturer and power optimizer model, as there are different ways to carry out the optimization process. The architecture that employs series optimizers has been more common on the market, as we will see below.

In general, power optimizers perform optimization by locating the maximum power point [4] for each photovoltaic module.

In this way, unlike the inverter string or central, which seeks the point of maximum power considering the output of the set of photovoltaic modules, the optimizer operates individually on each photovoltaic module, increasing the precision in locating the point of maximum power.

Thus, in situations where a photovoltaic module limits others by being shaded (changing the curve IV global system), the existence of the optimizer will prevent this limitation from happening to a considerable extent.

Although the idea of the optimizer is to act individually on each solar module, there are optimizer options on the market that work with up to two modules.

Advantages of optimizers

Photovoltaic systems with power optimizers have advantages, including some that we already mentioned at the beginning of this article:

• Increased efficiency (greater energy generation) provided by the location of the maximum power point per module;
• Ease of maintenance due to the possibility of isolating the defective photovoltaic module from the system;
• Reduction of energy losses due to mismatch (losses caused by power differences between the modules of a string);
• Possibility of using modules of different brands and characteristics;
• Possibility of installing modules in different angles, orientations and shade conditions;
• Greater safety in electrical installation, reducing the open circuit voltage of the strings;
• Option to individually monitor the energy produced by photovoltaic modules.

Regarding the advantages mentioned in the previous paragraph, it is worth highlighting an important characteristic of photovoltaic systems with optimizers: the elimination of the risk of electric arc, which is a very common problem in conventional photovoltaic systems.

The optimizers have an automatic shutdown system when an electric arc is detected, making photovoltaic systems completely immune to fires.

Architecture with SolarEdge optimizers

The figure below shows how the power optimizers work considering the architecture of the SolarEdge. Figure A shows the system working without shading and Figure B with shading.

In the system without shading, the output voltage of the optimizers is divided equally, bringing the value closer to the inverter's DC link.

When a photovoltaic module is shaded, the optimizer output voltage for the shaded module is reduced and the other voltages are increased in order to maintain the same current at the output of each optimizer and the voltage on the DC link.

Optimizer x microinverter

Now that you know what a power optimizer is, you may be wondering “What is the difference between a power optimizer and a microinverter?”.

Well, the main difference is in the losses during the DC-AC conversion process (from direct to alternating current).

In systems with optimizers, the DC-AC conversion is done in the inverter, while in microinverters the conversion is done in each of them. In the latter case, there is an increase in conversion losses.

It is worth noting that, even so, the efficiencies of photovoltaic systems with power optimizers and microinverters are really close [6].

The border between microinverters and optimizers is very thin, but optimizers can be more advantageous. The economic viability of using optimizers, compared to the use of microinverters, increases as the power of the photovoltaic system increases.

In short, in very large systems the optimizer ends up being cheaper than the microinverter.

Considerations

Optimizers are very interesting and have many advantages, as we mentioned previously. If the solar system is subject to shadows, optimizers are undoubtedly an excellent option.

However, in any type of system (even without shadows), optimizers increase energy generation, as even in homogeneous solar irradiation conditions there are always modules with different characteristics in the strings.

The presence of individualized MPPT systems for each photovoltaic module is a factor that increases the efficiency of photovoltaic systems in any case. This is more true when there are modules installed at different angles and inclinations, conditions in which traditional systems with inverters strings do not perform well.

If the safety of photovoltaic installations is an important aspect for the designer or owner of the photovoltaic system, the use of optimizers is very attractive.

It is worth highlighting that the best solution for reducing losses is a well-designed photovoltaic system, even in the face of problematic scenarios. Therefore, before opting for systems with MLPE (optimizers and microinverters), the designer must check the positioning of the photovoltaic modules in the best possible way.

That is why it is very important to use photovoltaic system simulation software such as PVsyst, Helioscope or Designer from SolarEdge.

There are advantages and disadvantages to all photovoltaic system architectures.

The objective of this article was to present the advantages of optimizers, not only in relation to optimizing energy generation in systems with shadows and modules in different operating conditions, but also in terms of safety.

If we mainly analyze the safety aspect of photovoltaic electrical installations, optimizers become a good choice for any type of photovoltaic system, as they eliminate the risks of electric arcs and fires.

References

• [1] P. Trapp. What is a Power Optimizer?
• [2] R. Orduz, J. Solorzano del Moral, MA Egido, and E. Roman, “Analytical study and evaluation results of power optimizers for distributed power conditioning in PV arrays,” Program. Photovoltaics, 2011
• [3] João Lucas de Souza Silva, HS Moreira, DB Mesquita, Marcelo Gradella Villalva, “Analysis of Power Optimizers in Photovoltaic Power Plant,” 13th IEEE/IAS International Conference on Industry Applications, 2018
• [4] Marcelo Gradella Villalva. Understanding the IV and PV curves of photovoltaic modules
• [5] T. Solaredge, “Technical Note SolarEdge Fixed String Voltage, Concept of Operation” pp. 7–9, 2012
• [6] EnergySage. Comparing microinverters vs. microinverters power optimizers