Alongside their core function of converting AC to DC and optimizing the output of a PV or solar-plus-storage system, inverters must also be fire-resistant, support electrical safety, and withstand outdoor conditions. Environmental resistance is critical because inverters may be installed in areas that are hot, wet, humid or where there is salt air.
Longevity is also important because inverters need to provide reliable performance for more than 20 years. In addition, product designers want protective materials that support high-volume manufacturing and come with a choice of features for application-specific challenges.
Silicone advantage
Silicones are used as encapsulant materials and for other purposes in inverters because they resist higher service temperatures than typical alternatives like epoxies or polyurethanes. As power electronics trend toward higher operating voltages, greater amounts of heat are produced. Higher voltage systems can improve power conversion efficiency and reduce energy losses, but the inverters that they use tend to be more expensive and require greater levels of thermal management. Higher voltages also require greater electrical isolation.
Among their advantages, silicones are available in formulations that meet various UL flame ratings, making them suitable for applications requiring enhanced fire safety. This is especially relevant for systems using lithium-ion batteries, which can catch fire in rare situations.
Beyond fire resistance, silicones also offer good environmental resistance, maintain their mechanical properties, resist cracking caused by thermal cycling, and enable electrical isolation. They come in a range of products that support high-volume manufacturing.
With their strong environmental resistance, silicones can withstand the moisture, humidity and wide range of temperatures that are associated with outdoor installations. They can also withstand corrosion from saltwater environments. Because they are soft and stress-relieving, they offer protection against mechanical shock and vibration. These stresses can occur during transportation or installation, due to environmental factors such as high winds, or because of the constant operation and switching of power within a PV inverter.
Normally, silicones are thermally insulating. However, the addition of thermally conductive fillers also enables efficient heat dissipation. Compared to the air that would otherwise fill the gaps between heat sources and heat sinks, silicone-based materials have a higher thermal conductivity, a measure of the ability to dissipate heat. They also have low thermal resistance, a measure of a material’s ability to resist the flow of heat.
Thermal management
Thermally conductive silicones for PV inverters include silicone encapsulants, thermal interface materials (TIMs), thermal greases and thermal gels. These thermal management materials are also used in battery energy storage systems (BESS) but using silicones that are thermally insulating instead of thermally conductive.
Thermally conductive silicone encapsulants flow readily to fill complex geometries in a PV inverter’s inductance module. With their low viscosity, they are easy to dispense with automated equipment for high-volume production. After silicone encapsulants are applied to printed circuit boards (PCBs) and their components, curing is required. Some products can cure at room temperature instead of with energy-intensive ovens, but ovens may still be used to accelerate the process.
Silicone TIMs have higher thermal conductivities than silicone encapsulants. In inverters, they are applied between PCBs and the insulated gate bipolar transistors (IGBTs) that are used for high voltage switching. In a solar string, silicone TIMs can support the use of 1,500 V DC/AC inverters that handle up to 300 kW of power. Among their advantages, these materials can be applied as pads with automated equipment or either screen-printed or stencil-printed for faster assembly than can be achieved manually.
Silicone thermal greases support thinner bond lines than silicone TIMs. This is important for thermal management because there is less distance for heat to travel from a heat source to a heat sink. Silicone thermal greases are also easy to rework and have viscosities that range from non-flowable to semi-flowable. Because they are non-curable, they remain in a paste-like state that helps optimize surface wetting for lower thermal interface resistance.
Thermally conductive silicone gels support a range of bond line thicknesses to provide design flexibility. They are less expensive than fabricated thermal pads and can be used to protect IGBTs. Because they are flowable, silicone thermal gels can be applied as printable pads or dispensed like a liquid gap filler. These soft, compressible and stress-relieving materials support rework and are suitable for electronic designs with small shapes and intricate geometries. While they require curing, some formulations can do so at room temperature.
While BESS use many of the same thermally conductive silicones found in PV inverters, they also incorporate fire-resistant silicones that are thermally insulating. For example, silicone foams offer a lightweight alternative to encapsulants, and potting foams are used to fill the spaces between individual battery cells.
BESS designs may also incorporate silicone adhesives to enhance mechanical stability. Available in both thermally conductive and thermally insulating formulations, these adhesives support component staking for fewer vibrations and less pressure on component leads.
As solar energy continues to reshape power generation, transmission and distribution, material selection is playing a pivotal role in driving progress. Importantly, silicone-based materials for inverters have highly tunable properties and can meet application-specific requirements for performance and processing. Designing the technology of tomorrow isn’t a one-size-fits-all approach, however. It is therefore essential for designers to select advanced silicones that are thermally conductive, support PV performance, and promote safety.
Cody Schoener
About the author
Cody A. Schoener is senior marketing manager for Dow Performance Silicones at Dow Chemical Company. His responsibilities include short- and long-term strategy development, innovation portfolio management, and leading a global team specifically for industrial electronic strategy and global accounts. Prior to his role in marketing, he spent eight years as a technical and developmental scientist for Dow Chemical Company servicing polyurethane and cellulosic chemistries. He earned his PhD in chemical engineering from the University of Texas at Austin in 2012.
The views and opinions expressed in this article are the author’s own, and do not necessarily reflect those held by pv magazine.
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