DC-DC optimisers, Microinverters, and AC solar panels: Alternative solar system layouts

Microinverters and AC solar modules

by James Martin II on October 16, 2012

This article provides an overview of the different types of solar system layouts and some of the advantages and disadvantages of each, with a focus on microinverters and AC solar panels.

Types of solar PV system arrangements: Centralised inverters, DC-DC optimisers, microinverters, and AC solar panels

The number of solar photovoltaic (PV) systems that use microinverters or ‘AC solar panels’ as opposed to conventional, centralised inverters is still relatively small in the UK–and globally. The number of companies that offer such ‘decentralised’ inverter solutions is growing, however, and microinverters are sure to have a significantly larger share of the market in the future.

In addition to wiring, solar systems are comprised of 2 types of components: solar panels and inverters. The solar panels produce direct current (DC) electricity, which is converted to grid- and appliance-compatible alternating current (AC) electricity via an inverter.

There are essentially 4 different ways that a solar PV system can be arranged. The differences between each arrangement lie in the relationship between the solar panels and the inverter(s).

1. Conventional, centralised inverter layout

Solar systems with centralised inverters make up almost all of systems in the UK. In a centralised inverter setup, DC power from the solar panels is sent to a single inverter–usually placed in a shed or on a wall–where it is converted to usable AC electricity. This is the most ‘tried and true’ type of system arrangement, and the technology can be quite advanced, with high power conversion efficiencies–sometimes higher than 98% in optimal conditions. Because of their history and the degree to which the technologies involved have been vetted by real-world deployment, these systems are generally quite reliable.

However, having a centralised inverter requires panels to be arranged in ‘strings’. Strings of panels are susceptible to the effects of shading and faults, as problems with even one panel can disproportionately affect the the output of the whole string. This is sometimes described as the ‘weakest link’ phenomenon, because the weakest link in the string ‘breaks the chain’.

Another downside of a centralised inverter setup is that it is not possible to monitor the performance of individual panels–only the performance of strings. This makes it difficult to quickly troubleshoot a system where there seems to be a problem with one of the panels, or in decided which if any needs to be replaced or repaired.

Conventional centralised inverter setups

Conventional centralised inverter set-ups. Image via SolarBridge Technologies.

2. DC-to-DC optimiser plus centralised inverter layout

The DC-to-DC optimiser solar system arrangement was developed in an attempt to deal with some of the problems associated with having strings of solar panels, and tend to have greater overall system yields than conventional systems. In this setup, the DC solar power that each panel produces is optimised at the panel level. The ‘optimisers’ are either integrated into the modules themselves, or attached to the mounting. This setup gets around the ‘weakest link’ problem because modules are not arranged in strings. Instead, the optimised power from each module is fed into the wiring and is ultimately converted to DC power at a central inverter.

However, this arrangement can potentially be troublesome because there are a greater number of points (i.e. the optimisers) at which problems could occur in addition to the central inverter. On top of this, the existence of a central inverter leaves the system susceptible to whole-of-system output loss should a problem occur with it. Another consideration is whether the optimisers, if attached to the mounting instead of module-integrated, will add to the installation cost of the system. This will depend on the solar system installer.

SolarEdge is one company that offers a DC-DC optimiser solution.

DC-to-DC optimiser setups

DC-to-DC optimiser set-ups. Image via SolarBridge Technologies.

3. Detached microinverter set-ups

Microinverters take the DC-DC optimiser approach to the next level, converting DC to AC on a per-module basis, eliminating the need for a centralised inverter. One of the popular ways of installing microinverters is on the mounting racks of the system itself–’detached’ microinverters. The other is integration into panels to produce ‘AC solar modules’ (discussed below). A microinverter system is also arguably safer than conventional and DC-DC optimiser systems because not as much cabling is required to carry DC power, which is more dangerous than AC should a problem occur.

Similar to the DC-DC optimiser approach, however, the larger number of components adds complexity to installation and troubleshooting of the system. Additionally, in contrast to solar panels which have standard 25-year warranties, many microinverter manufacturers have only a 5-15 year warranty on their products, meaning that if one of them fails after the warranty has expired, the owner will need to make a decision about whether or not to replace the faulty one or all of them. Centralised inverters have similarly short warranty periods, but the fact that they are easily accessed and replaced means that this is not as much of an issue as with microinverters.

Detached microinverter setups

Detached microinverter set-ups. Image via SolarBridge Technologies.

4. AC solar module set-ups

AC solar modules are a step beyond the detached microinverter arrangement described above. An AC solar module is a solar panel with a microinverter integrated into its frame, forming a single unit. Installation of a solar system that uses AC modules is even simpler than installation of a conventional system (as there is no inverter), and much simpler than DC-DC optimiser or detached microinverter systems.

AC solar modules arrangements are still vulnerable to some of the same problems as detached microinverter arrangements are–most notably, the fact that a greater number of smaller working parts are included in the installation. If one of the module-integrated microinverters fails, an entire panel will need to be replaced. This may actually be less complicated than replacing only a detached microinverter (the panel would need to be removed in order to replace the inverter), however, and–provided the warranty for the modules is 25 years–the owner can rest at east about the system’s performance.

AC solar module setups

AC solar module set-ups. Image via SolarBridge Technologies.

Choosing the solar system type that best suits your needs

As mentioned at the beginning of this article, the vast majority of solar system installations are of the first type–solar panels arranged in ‘strings’ with a centralised inverter. One of the reasons for this is because of their cost-effectiveness and the ample infrastructural knowledge that exists in the UK about how to install them and how they operate. At the time of writing the other 3 types still sit firmly in the ‘niche’ category and tend to be on the higher end of the price scale.

As price point tends to be the primary factor that influences people’s decisions about whether and how to go solar, the latter 3 options described above are ordinarily most popular with those who are seriously concerned with the effects of partial shading on their solar array but still want to enjoy the benefits of installing a system. This is likely to change in the coming years, however, as more and more companies begin manufacturing microinverters and DC-DC optimisers and more installers become familiar with these products.

The solar industry is in a stage of rapid development globally, and few would have believed 4 years ago that solar panels would have hit the low price points that they have hit so quickly. Inverter technologies are also seeing significant price reductions, and fully installed systems are more affordable than ever. The innovative technologies detailed above are likely to follow this curve in the next few years.

© 2012 Solar Selections Ltd

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