Making the case for microinverters in larger solar installations

11/11/2012

An Enphase microinverter Traditionally, photovoltaic arrays have been arranged in series of strings, with one or two centralized inverters changing the module’s DC electricity to AC electricity.

An Enphase microinverter Traditionally, photovoltaic arrays have been arranged in series of strings, with one or two centralized inverters changing the module’s DC electricity to AC electricity. More recently, with the advent of microinverters this is changing—at least in some, usually smaller applications. But now microinverters are being used in larger applications, because they can offer advantages, like the ability wire in parallel without conversion efficiency loss and ease of changing out damaged units.

One of the leading microinverter companies, Enphase, which sponsored a recent GTM Research Webinar, said microinverters can now effectively be used on systems up to roughly 1 megawatt. The company said the microinverters which are placed on each module in an array offer number of advantages over other central inverter PV arrays. Still one of the issues surrounding microinverters is that they cost more upfront than a centralized inverter system.

Microinverters for a 250 kilowatt DC system would cost $149,760. For the same sized system with an inverter, the centralized inverter cost would be $67,275, according to Microgrid Energy President Marc Lopata, who spoke during the Webinar. But operations costs are less with an inverter system. “From the operations side we find that we can reduce overhead by as much as a third with Enphase making the project more cost-competitive. This is thanks to factors in development, design, inventory, monitoring set-up and commissioning,” Lopata said.

For instance, when a microinverter breaks, just one module needs replacing while the rest of the array can still produce. A centralized inverter can bring down an entire array until the defective unit is replaced.

Since microinverters convert DC current to AC current at a module level as opposed to a system level, they can be arranged in parallel with no loss if one module is producing electricity at a lower level than others and they can accommodate PV modules with different voltages. In a centralized inverter system one bad module or module string could bring down the production of the entire array. “From the design side it’s easy to match different voltages and panel capacities,” Lopata said. It’s easy to switch modules depending on availability and the market.”

Microinverters add about 30 cents per watt more to a project’s up-front costs, explained Teff Reed Enphase’s director of Field Applications Engineering. But that’s offset by other factors like lowered costs of PV modules. “Panels have come down significantly, so panels are now a smaller part of projects than they used to be, we're assuming panels and racking at a about $1.15 per watt,” he said.

Other advantages of a microinverter system include electrical engineering. “There's no DC engineering required for the microinverter system. There's no sizing of strings. There's no choosing inverters on the voltage window,” Reed said. “It's all going to come down to AC design work that is very familiar to almost any engineering firm in the country.” Ultimately simplifying the installation and engineering process can make projects with microinverters more profitable for the installer and integrator, he contended.

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