Every year when I dig out the Christmas lights I wonder how many of them are going to work and how long it will take me to find the one light that doesn’t, so I can replace that one light in order to get the entire strand to light up. That is frustrating to say the least, but fortunately a new strand of lights only costs a couple dollars and my frustrations are relieved, except for the feeling of waste when I throw away a strand of lights that mostly works.
What does this have to do with photo-voltaic energy (PV)? Typically, PV systems are installed with a “string” inverter which inverts electricity from direct current (DC) to alternating current (AC) for multiple solar modules at once in a Zero Energy Home. However, just like a set of Christmas lights is affected when one light goes out, so is a string of modules affected when there is a problem with only one of them.
The most common cause for decreased production is shading. The Black River Produce solar project in Springfield, Vermont, had a lot of roof space but also had some small shading problems across the roof. This posed a problem when designing the system to use string inverters because we would have had to plan around valuable roof space that had shading for only a couple hours a day. The solution? Microinverters. Microinverters invert electricity for one module at a time, which provides many advantages.
First, shading no longer reduces the output of multiple modules at once. If there is shading on one PV module connected with a string inverter it will reduce the output of all the modules connected together. With a microinverter installed, only the output of the panel that has shading on it will be reduced. This is a big advantage on this particular roof because of the different roof elevations and equipment that we have had to work around.
Second, the inverters come with a 25-year warranty. They are completely enclosed and have no moving parts. That’s pretty impressive if you ask me!
Third, and the coolest feature of all, is that since there is an inverter for each PV module, every single module can be monitored! The monitoring package provides a map of the array that shows the output of each module in five minute increments, the past seven days, and the entire history. You can visually see production increase and decrease as the sun comes up, clouds pass over, or even when equipment is shaded.
This detailed level of monitoring will decrease the amount of time the system is down for maintenance. If service is required on a system that is using string inverters, all panels connected to a particular inverter have to be shut down in order to service it. On a small residential system that is not really a big deal, but on a large commercial system, hundreds of panels (or more) might have to be “turned off” on a sunny day to complete service.
With the monitoring capabilities of the microinverters we will be able to pinpoint the exact module or inverter in a field of 1,490 modules that needs servicing. Not only does that save time for the service technician, it is a huge savings for the customer because only one PV module will need to be disconnected for service, not hundreds, and the rest can keep on working while the one needing service is repaired.
Besides the fact that the precise monitoring capabilities are fascinating and will be fun to watch, they also add a lot of value to a PV system. There will be less down time, increased production, quick reporting of problems, and a long warranty. For me, this all adds up to an impressive system that is definitely on the cutting edge of PV technology.