By Jeremy Cook
Harvesting solar energy "out of thin air" once felt as futuristic as human flight did in previous centuries.Today, solar power is a commonplace technology, but there's still the potential for vast growth in efficiency and application.
One materials technology poised to transform solar power management is silicon carbide (SiC).Solar manufacturers use this wonder material to build highly efficient and robust solar inverter systems that turn DC power from photovoltaic (PV) cells into household and business AC power.There are three primary inverter architectures: micro PV inverter, PV string inverter and PV central inverter.
This article will look at these architectures and how SiC fits into the picture.
Silicon carbide technology: a long history, new for today
Scientists first synthesized SiC in 1891. SiC occurs naturally, though it is rare on Earth.Initially, SiC was used as an abrasive, and has been employed in a variety of structural and electrical applications.Today, however, it is implemented as a cutting-edge semiconductor material, with properties that far outclass its traditional silicon counterparts.
The challenge with SiC is that even though it was synthesized over a century ago, making it in sufficient purity for use as a semiconductor is still very difficult.Unlike producing transistor-grade silicon (Si) (which can be processed from a liquid state into a to-be-cut crystal), growing SiC crystals involves a sublimation/deposition process where gas is transformed directly into a solid.
Manufacturing SiC transistors are therefore more complicated (and expensive) than producing Si devices.However, SiC's enhanced properties mean this cost is not always an apples-to-apples comparison.SiC has a wide bandgap, meaning that transistors made with SiC can operate at higher voltage, temperature and frequency ranges than Si devices.
SiC is also a superior thermal conductor to Si and is far better at conducting electricity at higher temperatures.One of the most important uses of silicon carbide is for solar inverter systems.
How solar inverter systems use SiC
Using SiC for solar inverters presents a vast array of benefits, including:
- Higher efficiency
- Lower overall system volume and weight
- Better thermal management
- Higher reliability
Since SiC devices conduct and endure heat better than Si, there is typically less design and component expense for cooling in the overall inverter implementation.Inverters can also be smaller, meaning lower material costs.Add on the other performance benefits outlined above — plus the idea that SiC manufacturing costs will likely come down with future development — and the value proposition for SiC looks very attractive.
Once manufactured, the solar inverter (or an array of inverters) must be installed in a photovoltaic (PV) system to use it.Solar installers have three primary methods/topologies for setting up the system.
The three common solar PV inverter topologies
An inverter — which inverts DC power into AC power — is a general-use technology.One might argue that a solar inverter is used to convert DC power from a PV array to AC power .There are three primary types of PV inverter topology: micro inverter, string inverter and central inverter.Each is appropriate for different situations and scales.
Application guide for solar inverters
Micro inverter
The plural "micro inverters" is most accurate here, as an array of small (micro) inverters transform DC power from solar panels into AC to supply a home or other small facility.Each solar section produces between 40 and 80VDC, with a typical output of either 110 or 230VAC.
The plural "micro inverters" is most accurate here, as an array of small (micro) inverters transform DC power from solar panels into AC to supply a home or other small facility.Each solar section produces between 40 and 80VDC, with a typical output of either 110 or 230VAC.
These setups are highly flexible, with an overall power range of 200W to 1.5kW and a moderate efficiency of around 96%.The size reduction allowed by SiC technology is especially attractive in this scenario since multiple inverters typically need to be transported to and integrated with the panels.However, micro inverters are also the most expensive type of solar inverter system.
String inverter
Solar panels link together (in a string), sending overall DC power to a single inverter.This topology is appropriate for power ranges from 1kW to 200kW, powering everything from a single home to buildings and industrial complexes.Input voltages can be up to 1500VDC, with output up to 800VAC 3-phase.This topology is flexible and up to 98.5% efficient.SiC's higher voltage handling capabilities are ideal here.
SiC manufacturer Infineon created a 1500V PV string inverter reference design that uses active neutral point clamped (ANPC) SiC MOSFET technology and operates at 48kHz.This design is five to ten percent less expensive than a comparable system using IGBT-only technology at 16kHz on a cost per kW basis.While the semiconductor switches and drivers using SiC technology in such a design command a price premium over their IGBT-only counterparts, they enable sizable savings in magnetic components that can make up the maximum difference.
Potentially lower losses and increased performance in inverter applications make a SiC-based system even more attractive over the long term.Add in multiple conversions in an energy storage solution (ESS) setup and the argument for using upgraded SiC technology only becomes stronger.While what is outlined above is for a PV string inverter implementation, other topologies will see similar benefits from SiC capabilities.
Central inverter
Multiple strings of solar panels feed into a central inverter.Such topologies are appropriate for grid-level operators and can produce over 1000kW (i.e., over a gigawatt) of power.Input voltages can be up to 1500VDC, with output up to 690VAC 3-phase.This technology is the least scalable of the three topologies but offers the highest efficiency (up to 99%).SiC's increased voltage capabilities are attractive in this scenario.A central inverter allows the application of SiC's high-efficiency properties on a massive scale.Per kW, central inverters are the least expensive type of solar inverter system.
SiC benefits for solar power generation
While certain SiC characteristics are more desirable in specific scenarios, all the benefits of SiC will apply to each topology listed.The cost/benefit ratio will become even more attractive as the technology develops.Companies including Infineon, Microchip, Onsemi, STMicroelectronics and Wolfspeed continue to push SiC tech forward, and we can expect SiC to improve power efficiencies in the solar industry and beyond!
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