MICRO INVERTER OR THE AC-MODULE INVERTER: TRUTHS VS. MYTHS

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MICRO INVERTER OR THE AC-MODULE INVERTER: TRUTHS VS. MYTHS KACO new energy, San Francisco, CA, 94129: USA Telephone: +1 (415) 931 - 2046, Fax: +1 (415) 931 - 1688, Email: kacoinfo@kaco-newenergy.com In the last few years, the solar industry has changed significantly. Most importantly the products have evolved. Now, when it’s time to choose an inverter for a PV system, the system owner and the installer have the choice to use a string inverter or micro inverters for the installation. It is important to realistically assess the viability of this technology. KACO feels that some important issues have been overlooked and need to be discussed in depth before a decision on which inverter technology to buy takes place. First let’s take a brief look at micro inverter history. Some folks overlook the fact that the micro inverter concept was first introduced in the 1970’s and that real world products were launched in the 1990’s. Many of the concepts used today in the design of the micro inverter are based on research done by Professor Werner Kleinkauf from the Institut für Solar Energie-versorgungstechnik (ISET). i His work in the development of micro inverters or “module integrated converters” prompted the manufacturing of AC products all over the world. Most companies like ZSW manufactured a range of micro inverters from 100 to 400 watts. Due to the small size of the PV market and a lack of financial backing however ZSW and companies like it were defunct or acquired before the PV market grew in the early 2000’s. Other companies like AES (United States), OKE-Services (Netherlands; recently acquired by SMA), and Ascension Technology gave the micro inverter a try in the 90’s. Let’s take a closer look at some of the issues that were present then and still today have not quite been answered. Truth or Myth? Residential performance monitoring per module for PV monitoring helps homeowners. This is an interesting feature of the micro inverter options in the market today, and at first glance may seem desirable. Nonetheless, when we look closer it becomes clear that an inherent flaw in this feature has been overlooked. A system owner who carefully watches their PV system performance and does not understand the mechanics of a PV system is liable to over react to the multiple data points. For example, if one panel produces less than expected, this not so tech savvy homeowner is immediately alerted. The overall performance of the system is still at an acceptable level, however, this system owner will contact the installer to complain about their “under performing” system. It is also important for thorough training of the user as to how to use the monitoring capabilities of this system fully. Therefore, with increased maintenance calls, training costs and the upfront cost of the hardware and software the lifecycle costs associated with maintaining this owners’ system could then be much higher than with a string inverter. It is apparent that the ability to monitor the performance of each single module at all times can present both convenient and inconvenient scenarios for the system owner and the party responsible for maintenance. At the other end of the spectrum are system owners who prefer the convenience of a built in inverter display. This system owner may not be as technically interested in the day to day functionality of their PV system and will want to check if their inverter works properly without additional accessories and monitoring software. String inverters, from most manufacturers are sold with a display that shows the most pertinent performance data. This is not possible with micro


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