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What Are AC Solar Panels?

9/11/2009 12:00:00 AM

Tags: solar, electric system

What are AC solar panels? I always thought solar panels had to be DC, but now I’m hearing about new AC products. How well do these work? Are there any benefits to using them?

AC (alternating current) solar panels are a promising new idea on the photovoltaic (PV) market. The panels themselves still produce DC (direct current), but each panel would come with its own attached micro-inverter, which converts the DC to AC. The inverters also synchronize the output with the grid, allowing the modules to be grid-intertied. The micro-inverters replace the need for the single, centralized inverter currently used in solar-electric systems.

AC solar panels with integrated micro-inverters are under development by several companies, but are not yet in widespread use, so there’s no performance data from the field. Bob-O Schultze of Electron Connection, an installing dealer in northern California and southern Oregon says, “No one that I’m aware of is marketing AC panels at this time. What is available is something called an Enphase inverter. These are designed for use with individual 24-volt (nominal) PVs. You parallel as many inverters as you have PV modules.”

Solar modules with preassembled micro-inverters will be easy to install once they are available. The AC panels in production are touted as “plug-and-play” products, so installation costs should be considerably less. The installation process seems easy enough, but we are not aware of plans for making them available to DIYers. The modules connect together to form an AC branch circuit, which is wired to a breaker in your house’s main panel. The micro-inverters eliminate the need to calculate inverter string sizing, but installers will need to stay within circuit breaker and service entrance limitations.

The advantages for having individual micro-inverters, whether integrated with the solar modules or not, are several.

  • Increased performance. Each individual micro-inverter performs Maximum Power Point Tracking (MPPT) to optimize its panel’s performance under conditions of varying sunlight and temperature. While centralized inverters also have MPPT capabilities, they average the performance of an entire “series string” of connected PV modules. This means that if one panel in the group is shaded, dirty or not working, it degrades the performance of the entire string. In conditions that are less than ideal, the micro-inverter may increase performance of the overall array.
  • Problems with solar module mismatches or differing orientations are eliminated. Because the micro-inverters optimize output independently, you can use mismatched panels of varying watts, ages or types. With a centralized inverter, using mismatched panels is not recommended because they limit the performance of the series string to the lowest common denominator — the smallest or lowest performing panel in the group. Likewise, placing modules in differing orientations, for example some panels facing south, and others facing southwest, will no longer be a system design issue.
  • Incremental system installation. Another great benefit to the end-user is that you can start with as many panels as you can easily afford, and add to the system over time. You won’t need to worry about finding matching panels later on or having to buy a larger centralized inverter as your system grows.
  • No single point of failure in the system. When a centralized inverter fails, your whole system shuts down. With the distributed nature of the micro-inverters, if one of them fails, the others continue to operate. The Enphase micro-inverter has a 15-year warranty, and an operating efficiency of 95.5 percent, both of which are comparable or better than traditional inverters on the market.
  • Safer to install. Having a series of micro-inverters with AC output eliminates the dangers of working with high DC voltages (up to 600 V) on the rooftop.

As with any technology, there are many potential downsides. Here are several questions to consider.

  • Do the gains outweigh the costs? The technology is too new to assess actual performance, so we don’t know if all that extra gear will be justified. You have additional costs for either a preinstalled micro-inverter or lots of extra costs to install multiple inverters instead of just one. For a well-sited system, it may not be cost effective.
  • Have you considered tare losses? All inverters have tare losses (energy consumed to operate the device itself), but now you have tare losses multiplied by the number of inverters. According to Schultze, “Inverters aren’t a free lunch. If they are powered by the grid rather than by the module, then it’s easier for the manufacturer to hide those losses but they are still there in the form of reduced net kilowatt hours produced.”
  • What happens with inverter failures? Adding another concern, Shultze says, “Looming even larger are potential (and probable) inverter failures. If a main inverter fails, that’s bad, but it’s easily accessible. Identifying a failed micro-inverter may be easy — or ridiculously hard — depending on how the manufacturer does it. Enphase assigns unique addresses to their machines so if the installer recorded the installation correctly, you can find it via the web interface.” However, getting to the failed inverter is a different matter. It requires uninstalling the array back to the point of failure, which would be costly for the customer, the contractor, or the manufacturer.
  • What do installers think of these? Most installers are cautious about embracing the technology because of the possibility for more than one inverter to go out on the same job, but at different times. Return calls are costly for contractors in terms of time and reputation, so the potential of multiple failures over time is ominous. A great many installers don’t want to advocate this technology until it has proven itself in the field as reliable and effective.
  • How much do the AC solar modules cost? The price of the panels with integrated micro-inverters will make or break the deal. It should be quite cost effective for a company to buy the micro-inverters in bulk and install them onto the solar panels in a production setting instead of on the roof. The current retail price of Enphase micro-inverters is approximately $200, so that will probably be close to the additional retail cost of solar panels with them preinstalled.

You will need to carefully weigh the potential gains versus pitfalls in the technology. Smaller or incrementally installed solar-electric systems may be cost-effective. Other situations, such as systems with some shading problems or multiple orientations of panels, may benefit. People who have perfect solar sites and who are installing larger systems will need to make a very careful analysis, comparing projected reduced labor costs versus the cost of the equipment to assess the best value, while still considering all of the risks.

— Linda Pinkham, former managing editor of Home Power magazine



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