Ever since solar microinverters were first conceived of
thirty years ago, the development of solar modules that directly produce grid-compatible AC power has been a long-sought goal of
the PV industry.
As a step in that direction, module-level microinverters
were first introduced to the PV market in the 1990s. Only recently, however, have they progressed to the point where they provide a
compelling alternative to the central inverters most commonly used today in PV systems. Additional forms of module-level
electronics promising gains in energy harvest among other benefits have also emerged recently, and many industry analysts expect that
within a few years most PV modules will feature some form of module-level power electronics.
This article summarizes the various forms of power
electronics available today for PV modules, ranging from central inverters used in traditional DC-based systems to newly-developed AC
modules. The primary characteristics examined lie in the areas of energy harvest, installation costs, communications,
and ongoing service requirements over the operational life of a PV system, typically 25 years.
Central Inverters
Central inverters are the most common form of power
electronics used in PV systems today. In this model a single, large inverter is connected to many PV modules wired in series
to form strings with up to 600V of open-circuit voltage (1,000V in Europe). Multiple strings within the array may also be
wired together in parallel before converging at the inverter, yielding some added flexibility in system design and performance.
Capitalizing on many years of development, the DC-to-AC
conversion efficiency of many central inverters is 95% or higher, and they feature a relatively low unit cost per watt.
However, central inverters have multiple drawbacks. They
perform maximum power point tracking (MPPT) on the combined DC voltage and current produced by the series-connected
modules, resulting in lost energy harvest due to module mismatch and varying shading conditions across the array. The use of
high-voltage DC wiring raises some safety concerns, including a higher risk of arc faults, a primary cause of PV-related fires.
Central inverters cannot monitor the performance of individual PV modules, so damaged or otherwise compromised modules often go
undetected.
Central inverters also necessitate additional
installation and system design costs, and a failure of the inverter results in
a complete loss of production from the entire array. As most central
inverters carry five- or ten-year warrantees, such a system-level outage can occur several times over the operating life of a PV
system, and leads to the costly purchase and installation of a replacement inverter each time.
Finally, central inverters limit the design and site
selection of PV systems, particularly in residential applications. They require co-planar module layouts and a lack of partial shading
from chimneys, trees, vent pipes, etc. PV installers may opt out of half ormore of potential sites due to these restrictions.
DC-DC Optimizers
DC-DC optimizers supplement a central inverter with
individual DC-DC converters installed for each PV module. There are several types of DC-DC optimizers – some are wired serially
in strings, while others produce high voltage and are wired in parallel.
Some step each module’s DC output voltage either up or
down, while others can do both. optimizers perform MPPT at the module level. This allows
each module to produce its full output without being held back by any under-performing modules in the array. DC-DC optimizers
also permit module-level communications and performance monitoring.
However, DC-DC optimizers retain a key disadvantage of
central inverters – a failure of the central inverter still results in a completeloss of system output. Furthermore, some DC-DC optimizer
systems also require a separate command-and-control device to operate, creating one more point of potential system
failure in addition to the central inverter.
With additional equipment to purchase and install, DC-DC
optimizers add to the initial cost of a PV system. The added module- level hardware also imposes a penalty on overall
system-level efficiency by introducing an additional stage of lossy power conversion.
Detached Microinverters
Detached microinverters are installed on the racking
system beneath each PV module (Fig 4). By performing MPPT at the module level, detached microinverters offer enhanced energy
harvest relative to central inverters, and have developed to the point where some achieve power conversion efficiencies close to that
of central inverters. Module-level communication and monitoring is possible, typically via power line communications, and
system design is simplified by eliminating the need to account for varying levels of performance across modules in the array.
However, the additional labor required to install
detached microinverters adds substantially to the initial cost of a PV system.
Furthermore, as most detached microinverters offer only
five- to fifteen-year warrantees, it is almost certainly necessary to replace all of them – at varying times – during the operational life
of a PV system, which requires the costly de-installation and re-installation of one or more modules each time. As such, ongoing
operating costs for detached microinverter systems can be significant. Enhanced safety by eliminating all exposed DC wiring from
the system altogether.
To capture the full advantage of AC modules, however, the
integrated microinverter must achieve a high level of reliability and longevity, enabling it to support a module-compatible
25-year warranty and obviating the need for the system owner to buy and install multiple replacement inverters. For example,
SolarBridge Pantheon microinverters feature an advanced design that eliminates failure-prone components such as electrolytic
capacitors and opto-isolators, replacing them with highly-ruggedized components with no near-term wear-out mechanisms.
Conclusion
This article summarizes solar power electronics
architectures, including several that have emerged in recent years as
alternatives to traditional PV systems in which series-wired DC
modules are connected as a group to a central inverter. Of these architectures, AC modules with integrated, long-life microinverters such
as the SolarBridge Pantheon™ achieve the dual advantages of both enhanced energy harvest and significantly reduced
installation and operating costs, yielding the lowest levelized cost of energy and hastening the arrival of grid parity for solar
energy.
BY Patrick Chapman, PhD – CTO, SolarBridge Technologies
Greg Madianos – Product Line Director, SolarBridge
Technologies