Friday, December 10, 2010

More on MPP Balancing v.s. Microinverters

 IEEE Spectrum has a short article this month discussing MPP balancers v.s. microinverters for optimizing power from solar PV systems. If you recall, in this post, I discussed the reasoning behind my decision to go with Tigo's MPP balancer solution v.s. a microinverter solution such as Enphase. Suprisingly (at least to me) the author of the article confirmed the reasoning. The author found  the MPP balancers, in particular Tigo, better than microinverters because the electronics are simpler and cheaper and there are no electrolytic capacitors to fail. The complicated part is not up on the roof, it is down next to the inverter doing calculations, so it is much easier to replace. REC Solar (the company we are having install our system) tried out both microinverters and MPP balancers at its corporate headquarters for a year before going with Tigo, something we in Silicon Valley call "eating your own dog food."

The REC sales guy also mentioned something about an infrared picture of a system with a microinverter where the microinverter was in the center of a huge red area of high temperature. I searched a bit but couldn't find it. I found a forum where there was some discussion of microinverters v.s. MPP balancers. The claim from the microinverter faction is that microinverters are mounted on the panel racking and therefore don't touch the panels, but even so, they increase the temperature by around 6F. That shouldn't be enough to cause major problems if properly installed, but a slight bit off and the panel could get cooked. Temperature increase was another one of my concerns. It can cause premature panel aging and will in any case reduce the amount of power produced by the panel. MPP balancers use DC-DC conversion to maintain the voltage at an optimal level for maximum power output. This results in less heat loss near the panel, though, of course, the centralized inverter still is not 100% efficient so the inverter can heat up. But it is easier to put the inverter in a shaded area, or an area with some wind to dissipate the heat.

Really, the only advantage I can see of microinverters is that they can be installed by someone with essentially no training, except in standard AC wiring. MPP balancers require training to install. This of course makes the labor cheaper, but - the professionalism of American construction technicians being in general what it is - you get what you pay for. If the guy installing it decides, hey, why not just clip the microinverter to the panel nobody will notice, it will end up prematurely aging the panel. Then they get to come back and replace it, for a fee. We've had plenty of experience on this job with that kind of reasoning.

I also discovered I need a CAT5 cable from the monitoring box to where my DSL router is located (technically it is a DSL modem cabled to a Linksys router, I have an older model modem). Luckily, the walls are completely open,  since the electrician will need to run the cable through the walls from the front of the garage to around the middle of the house on the second floor, where our home office is and where we keep the DSL modem. I asked Tigo about a WiFi wireless bridge. These devices are not access points, but act like a wire without the copper. Tigo discouraged me from using one. They claimed that the wireless bridges often go down and then they need to call up the homeowner and ask them to reboot it. The Tigo box sends measurements to the Internet every 10 minutes, and can buffer up to 2 days worth of data. Tigo monitors the condition of the panels and I guess lets the homeowner know if there is some problem.

I think the assumption behind the wireless bridge manufacturers is that they will be used in an office setting and therefore someone will notice if they go down because YouTube isn't coming up. Then somebody will reboot the bridge. For machine to machine communications, though, the reliability must be much higher, otherwise, you end up with an expensive "truck roll equivalent" in which some person must find the box (could be difficult if it is in an out of the way place) and reboot it. Perhaps wireless links are better for control systems, where a person is dialing temperature up or down or setting a timer on a fan. In that case, if the wireless link is faulty, it should show up on the remote and the person operating it can take some action, like replace the batteries in the remote.

So we'll go with the CAT5 cable, and, in addition, get another from the solar hot water tank so that we can at some point put Internet data recording for that too. With data on both, I can get a much better picture of how much of the house's energy is coming from our renewable energy systems.


  1. I like to thanks for the information about the Solar pv training courses..

  2. well. such programs are a good motivational and do provide more employment. besides . they promote a cleaner energy sources. thanks for sharing. do keep up posting more updates.

    Micro Inverters