Hybrid inverters are commonly used in the developing world, but they are starting to make their way into daily use in certain areas of the U.S due to their ability to stabilize energy availability.
A solar inverter’s main job is to convert DC power generated from the array into usable AC power. Hybrid inverters go a step further and work with batteries to store excess power as well. This type of system solves issues renewable energy variability and unreliable grid structures.
“Inverters for grid-tied applications can only provide power based on what the array can immediately generate from the sun,” explained Bryan Whitton, product manager at Darfon. “Hybrid inverters can store power in batteries and then drawn upon it as needed for energy stabilization.”
Hybrid inverters can vary in size, performance and features. But Mara White, product manager for OutBack Power, said most models usually operate bi-directionally, meaning they can convert DC power from modules to usable AC power and then convert stored AC from the batteries to power loads when needed. “Hybrids can also remain grid-connected and use a mix of renewable and non-renewable energy to charge batteries and offset loads,” White added.
Some contractors have used hybrid inverters in the residential, remote home applications for the past decade or two. But Allan Gregg, VP of applications engineering at GreatWall—which manufactures Satcon inverters—said the range of applications has expanded over the past few years to include large capacity microgrids as well as grid-connected systems.
Historically, hybrid inverters have been used more frequently in developing countries that do not have access to a reliable power grid.
“In North America and Europe, hybrid inverter-based systems are usually elective,” White explained. “Users choose to use them for storing energy for self-consumption or provide back-up power during emergencies. But in the developing world, hybrids are more of a necessity to compensate for weak or intermittent grids or a lack of grid electricity all together. Microgrids in places such as India, Asia and Africa are also driving hybrid inverter adaptation.”
Still, Whitton said hybrid models are beginning to be used on a more daily basis in areas of the U.S. where the grid is unpredictable, such as Hawaii, or in states where net-metering has been widely supported. “Applications with less than ideal solar characteristic are also good for hybrid-based systems because they can store power and redistribute it during peak times, improving payback,” he added. “Basically, if the site has the potential for losing the grid frequently, you should consider a hybrid for off-grid operation.”
Having the flexibility of a hybrid system can add initial cost to a project, though experts say this can be offset by the ability to self-consume all of one’s available PV electricity.
There are also important design considerations when using hybrid inverters. For example, Gregg warned that the battery bank voltage should be compatible with the DC input requirements of the inverter, and there should be enough solar capacity to supply the load as well as charge the batteries.
Wiring can also be more complex when using hybrid inverters, especially when panels are dedicated for critical backed-up loads. “And as with any device that does several jobs at once, a hybrid inverter is usually slightly less efficient,” White added, “although, improvements in other balance-of-system components can compensate for that slight loss easily.”
There are also specific electrical safety issues with any type of energy storage, so White recommended getting specialized training in energy storage techniques and design. “Most available training is focused on simple grid-tied systems because they have been the majority of U.S. solar installations until now,” she said. “But with incentives changing and the surge in energy storage interest and applications, it’s important to get ahead of the curve and get advanced training quickly.”
Andrew McCalla of Austin, Texas-based Meridian Solar, a Solar Power World top contractor, said he commonly used hybrids in the mid to late ’90s when the now standard grid-tie inverter sector was just a glimmer. “I can imagine that, when regulatory hurdles are fabricated to limit the consumer and societal benefits of bi-directional power flow from distributed generation, these battery-based platforms will become far more common. What is old is new again!”
Another breed of hybrids
Another segment of hybrid inverters includes inverters that can use two energy sources. For example, Ginlong offers a PV / wind hybrid inverter that has inputs for both sources, instead of having to use two inverters. In much of the United States, wind speeds are low in the summer when the sun shines brightest and longest. The wind is strong in the winter when less sunlight is available. Therefore, because the peak operating times for wind and solar systems occur at different times of the day and year, such hybrid systems have the potential to produce power when it’s needed, and reach a higher return on investment.