Cleantech Market Intelligence
Microinverters Brighten Future of Distributed Solar
The Achilles’ heel of solar PV has been its low efficiency. Even the most efficient commercially available monocrystalline silicon PV panel is only 20% efficient; the majority of modules installed today range from 12% to 18%. This compares with wind at 30% and fossil fuel generators at 80% to 95%. However, this is only part of what determines the solar PV system’s overall output. Shading, dirt, cabling, voltage drop, inverter efficiency, and heat also affect the overall energy harvest. Breakthroughs in microinverters (which convert DC electricity to AC power) and DC optimizers (which provide a steady and “optimum” voltage level for the current fed to the inverter) are two of the most disruptive technologies in the solar PV sector today, as we describe in Pike Research’s recently published report Inverters for Renewable Energy Applications. The main benefit of microinverters is an overall higher energy yield because they prevent one panel’s failure from affecting the overall system’s energy harvest (unlike solar PV installations that use string or central inverters). At the installation site, microinverters are easily installed on the back of each panel, matching the rated capacity.
With microinverters, each panel is individually monitored, thus removing the need for DC cabling. This architecture distributes the overall risk of failure among all the panels in the installation, and relies on information technology to identify and isolate failed panels. By contrast, central inverters have a single point of failure, which can lead to longer periods of downtime if that inverter fails. With maximum power point tracking (MPPT) at the panel level instead of the string level, microinverters are ideally suited for residential and small commercial systems that are likely to be shaded more often that centralized solar power plants in the desert. Microinverter companies claim their technologies reduce the overall levelized cost of energy (LCOE) by 15% to 20% as compared to string inverters. Individual modules can also be shut down remotely and, with AC electricity, safety is increased for installers. Microinverter manufacturer Enphase Energy claimed to have 34% of the California residential market in 2011, based on wattage.
The downside of microinverters is that they are typically two to three times as expensive as string inverters and have lower efficiencies (although they do typically result in a better overall energy harvest). While the distributed architecture removes the risk of a single point of failure, many more electronics are being introduced to the PV system. Startup microinverter companies claim higher reliability than string inverters, but microinverters have not been used long enough to test this claim. Furthermore, placing microinverters on the back of a panel and on a roof could increase the risk of heat damage to the inverter due to higher rooftop temperatures, compared to string and central inverters, which are normally housed elsewhere.
The future of microinverters could very possibly end up being with fully integrated AC panels, of the type that SolarBridge Technologies and SunPower are experimenting with. Under this architecture, the microinverter is pre-installed on the module at the factory. The main value-add is the reduction in installation time from not having to separately install the microinverter or AC bus cabling at the job site.
Despite the doom and gloom in the headlines, and as we describe in the recently published Renewable Distributed Energy Generation report, the future of distributed solar PV is bright. Microinverters will continue to play an increasingly important role in this future.