Thoughts on Solar

Our recent heatwave is a potent reminder of a sad solar fact: generally speaking, if you have a solar power system and the grid goes down, or even just drops really low as it did in Altadena this past weekend, your solar power will also go out, leaving you sweltering in the heat with everyone else. 

But why?  And what can you do about it?

Anti-Islanding

Every system that Run on Sun has installed is what is known as grid-tied.  Those systems are designed to shut down when the grid goes departs from a fairly narrow range of voltage and/or frequency.  The reason for this is simple - safety.  Imagine this scenario: a tree snaps in the wind and takes down a power line.  What does the utility do?  They shut off power in that area - causing any grid-tied solar systems to shut off -  and then they send a crew out to restring the line.  Once that is done, they restore power to the area and all is well.  The grid-tied systems sense the restored grid and turn back on automagically.

But now consider this - what if your solar system didn’t shut off when the grid failed?  Well you might be happy because your A/C would still be running, but what about that excess energy that your system is feeding back to the grid?  It is possible that you would energize the very line that the utility workers are coming to repair.  Your solar system is now its own “island” of energy production, and it could pose an extreme hazard to the unwitting linespeople.  And that would be bad.

Thus the need for “anti-islanding” - the intelligence built into your inverter to keep workers safe.

Comfort is just a Microgrid Away!

So what can you do about it?  How can you keep your solar investment running even when the grid fails?  The answer is in a microgrid which requires two key features: isolation and self-starting.  The isolation follows from the anti-islanding discussion above - you need to make sure that your system cannot export power back to the grid.  This is generally handled by installing a “transfer switch” which can be either manual or automatic.  

The second step is harder - you need something to emulate the grid.  In off-grid systems that involves a bank of batteries and a special battery inverter that can use the power of the battery bank to start-up and create what appears to be a grid.  Now the solar system “sees” what looks like a stable grid and can come back online.  That sounds pretty easy, but there are complications.  In particular, the inverter that forms the grid must also be able to match the output of the solar system precisely to the needs of the house.  Remember, there is no grid out there to absorb excess energy, so you need a way to throttle the output of the array up and down to avoid over production.  

Storage is generally a key component here, as it can absorb excess power (at least until the battery is full) and help smooth out the energy flow.  All of which has historically made for an expensive addition to a solar system just to hedge against an infrequent occurrence.

Perhaps this past weekend’s outages will cause some folks to reconsider.

Cue Intersolar

Which makes the timing of this year’s Intersolar trade show ideal.  Running Tuesday-Thursday of this week (in cool San Francisco, thank you!), Intersolar is bringing together solar and storage manufacturers as they demonstrate their latest and greatest gear.  Finding a cost-effective microgrid solution is our number one mission this week, and we will be pressing our friends at Enphase for as many details as possible about their approach to solving this problem - watch this space!