Tag: "string inverters"


  01:14:00 pm, by Jim Jenal - Founder & CEO   , 414 words  
Categories: Commercial Solar, Safety, Shortcut Solar

FIRE! Walmart Sues SolarCity/Tesla Over Rash of Solar Fires

On August 20th, Walmart sued Tesla Inc, alleging “widespread negligence” in the installation and maintenance of systems on something like 240 Walmart stores across the country, resulting in 7 fires!  Is this a uniquely SolarCity/Tesla problem?  Are rooftop solar installations invariably unsafe?  Or is there a design difference that can make systems safer, particularly for residential solar clients?  Here’s our take…

Fire at Walmart store allegedly caused by Tesla solar installation

Fire damage at Walmart store allegedly caused by Tesla solar installation.

The 114-page complaint is a pretty damning set of accusations, saying that the installations were rushed, that faulty materials were used during the installation, and that the maintenance provided by Tesla did not meet “Prudent Industry Practices."  For example, after one Tesla maintenance team left a Walmart site, a DC combiner box, which could involve DC voltages of as much as 1,000 volts, was found left with the cover off!

Other problems involved multiple solar modules with “hot spots” possibly caused by micro-fractures of the cells, as well as mismatching cabling connectors (connecting MC4 connectors to Amphenol connectors) such that excessive resistance in the connections could occur, resulting in overheating, and potentially fires.

While the lawsuit is specific to SolarCity, and its now parent, Tesla, the types of conditions described are going to be potentially present in any string inverter system - which all of these were.  Since you are dealing with strings of solar panels, you are dealing with higher string operating voltages, with more power running through those strings.  If you use mismatched connectors, or stand on solar modules (one of the best pictures in the complaint shows the foot of a Tesla maintenance inspector standing on a solar module!) you can have the potential for fires.

A Safer Way…

DC arcing at 240 volts

DC arc at 240 volts.
Video by John Ward
6:20 into the video.

Which leads us to yet another reason to prefer the Enphase microinverter approach - no high DC voltages involved!  When a DC circuit opens under load, it is possible to get significant arcing, like you see at the left - ouch! 

But since each solar module plugs directly into the Enphase microinverter, there is no additive effect leading to those crazy high DC voltages.  Open a DC circuit with a voltage of 40 volts or so and guess what? No arcing!

While human error is never going to be eliminated in the solar industry – those are human beings doing the work after all – the Enphase microinverter system is inherently safer.  And if you are going to put solar on your home, school or business, isn’t safer what you want?



  09:45:00 am, by Jim Jenal - Founder & CEO   , 691 words  
Categories: Solar News, Commercial Solar, Residential Solar, Energy Storage

Solar Trends: Storage & Lower Cost

Predicting the future is always fraught with hazards but a recent survey caught our eye with a bold prediction: nearly one-third of solar installers/integrators world wide expect to utilize storage in more than 40% of their installs by 2015!  Oh and they need microinverters to come down in cost by 50% before they are willing to adopt them!!!  Numbers like that beg for some analysis, so stay with us.

Booming Market for Storage

The survey in question was conducted by IMS Research (h/t to SolarIndustryMag.com) and included global installers, integrators and wholesalers.  Among respondents from the U.K., Germany and Italy, the addition of storage capability was cited as the most important requirement for inverters over the next two years.

We aren’t surprised by this result.  As the penetration of PV systems onto local grids increases, grid operators and utilities start to complain about solar undermining grid reliability.  (Well, ok, utilities are actually complaining about losing market share, but that is not the concern of the ISO.) Likewise for many commercial solar clients, PV alone may not be a complete solution if the client’s electric bills fall under a demand-charge rate structure and their peak demand falls outside of the peak output for their PV array.  (Add Feed-in Tariff clients with time-of-delivery factors and you have yet another potential market for cheap storage.)

According to the survey, respondents were looking to add storage for 10-30% additional cost, but is that a realistic expectation?  Not from any of the products we’ve seen so far.  (The survey noted that 30% of respondents were willing to pay even more - which from what we can tell, they will certainly need to do!)

Ultimately, local storage for distributed generation will boom if the economics justify it - whether because net metering becomes less desirable (or even unavailable), or if demand shifting or time-of-delivery factors boost the ROI enough to overcome initial costs.

How Low Can You Go?

SMA's fabled microinverterMeanwhile, we were baffled by the survey’s contention that the “high cost of microinverters” was seriously limiting their growth.  Indeed, according to the report, a majority of survey respondents who are not presently using microinverters said that they would need to see the price drop by over 50% before they would consider using them!  (Interestingly, the illustration that accompanied the article showed the much discussed, but yet-to-be-sold, SMA microinverter instead of the best-selling Enphase product.)

How does such reluctance make sense?  Let’s look at a typical 5kW residential project using either Enphase microinverters or SMA string inverters.  Assume that we are using twenty 250 Watt modules to power both inverter types.  So we would need 20 microinverters and one string inverter.  Enphase cost - around $3,500 for microinverters plus cable.  String inverter cost - about $2,800.  Labor?  Well someone has to hang that SMA 5000 on a wall downstairs (preferably somewhere in the shade!) while someone else is mounting microinverters to the rails on the roof.  Pretty much a push.  (We actually have lower labor rates for microinverters than we do string inverters, but that’s just us.)  Which makes the microinverter about 25% more expensive going in.

So for the installer it might look like they have a better deal with the string inverter, but what about the client?  Our testing shows a 15% improvement in yield and that goes even higher depending on the amount of shading at the site.  Beyond that, the microinverter comes with a 25-year warranty, the string inverter only 10.  Which means for the client, they are going to have to spend an additional $2,800 or so ten years down the road when their string inverter dies (sooner if it is baking in the sun).  All of which argues quite convincingly that the microinverter is the better deal for the installer’s client - which last time we checked was supposed to be the point of the exercise.

We are all for price reductions, but we suspect that folks who say microinverter prices must fall 50% before they will consider using them have already decided that they will never use them.  Of course, installers are entitled to run their business as they see fit, but it certainly isn’t a lack of value to the client that is keeping these installers from taking the plunge with micros.

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Jim Jenal is the Founder & CEO of Run on Sun, Pasadena's premier installer and integrator of top-of-the-line solar power installations.
Run on Sun also offers solar consulting services, working with consumers, utilities, and municipalities to help them make solar power affordable and reliable.

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