Inverters serve as the vital link between the DC Electricity that is generated by a PV solar system, and the Grid AC Electricity that it transmits is used throughout the world, and in each of our homes. Even though all inverters perform this same vital task, the way in which they do so varies based on the DC source from which they are drawing the electricity.
String-Line Inverters connect directly to a PV array in order to convert the DC electricity to AC, as soon as it is generated. Multiple solar panels (or modules) are connected together in strings, before they are connected to the inverter; this allows for all of the modules in that string to share the same conductor (wire), thus cutting down on the amount and size of the wire required to connect the system safely. The advantage of this inverter type is price, simplicity and high efficiency.
The primary disadvantage to this system is that if even one module in the string becomes shaded, the entire string becomes limited by the shaded module. Furthermore, since all of the modules are connected together, the inverter “sees” the string voltage equal to that single module’s, and as such reduces the performance of the string to that shaded module’s output. To learn more about how this affects system performance and so much more, come visit us at US Solar Institute today!
DC Optimized products overcome shading, and module mismatch losses that are associated with string-line inverters by installing a “power optimizer” on the back of each and every PV module. This power optimizer performs to maximize and power point track (MPPT) each module in real time, and is able to adjust the output characteristics of each optimizer to ensure that the string is producing as much power as possible, across the string as a whole.
Solar Edge is the unchallenged leader in this market, due to their outstanding product record, and understanding of the real challenges installers face. As another added advantage, since the Optimizers are tracking the power production of each module, they are able to report that production back to the inverter, which can then be forwarded on to the Solar Edge monitoring portal; Installers and customers can then view exactly how much power each module, and the system as a whole, is producing at any given time, from their computer or smart phone. Additionally, installers can set up alerts so that they are notified if a module fails or if it significantly under-performs (due to shading or physical damage).
Micro-Inverter Systems, such as the EnPhase product, consist of a small inverter, which is placed on the back of each and every module; this means that the output at the Panel level is AC, and immediately available for interconnection, without any other power conditioning equipment. The Micro-inverter systems also allow for module level MPPT, however they are not as reliable as comparable DC optimized systems. This is because these micro-inverters require that several complex power transformations are forced to happen in a small metal box, out in the hot sun and all of the elements; the excessive heat tends to lead to increased failure.
The product is warranted for 25 years, and covers the cost of the equipment, however it does not cover the cost of labor required to climb up on the roof, remove the old inverter, and replace it.
Module level monitoring is also available from en-phase, but it requires additional hardware, and an annual fee after the first 10 years of operation. All in all, micro-inverters are simple to design and install, and great for newcomers to the industry, who don’t yet understand how to design a better system.
Battery based inverters draw energy from a battery bank. Batteries do not generate electricity, they simply store it for later use, so there must be a power source to replenish the electricity taken out of them; common power sources include a combination of solar power via a charge controller, a gasoline / natural gas generator, the utility grid or even another renewable energy source such as a wind turbine, or a hydroelectric generator.
Some battery based inverters have an integrated charger, and an “AC Pass-through” function. This allows an ac power source, such as the utility grid, to be connected to this input, and as long as this source is present it will directly power any loads connected to the output of the inverter, as well as simultaneously recharge the batteries; then if the utility grid should fail, and the power source is removed, the inverter will immediately begin to power the loads at it’s output, with energy that was previously stored in the batteries. Some of these inverters, such as the Outback GTFX line, even have the ability to push power back onto that grid input, once the batteries are completely full, allowing the PV system to further offset electrical usage.