In the aftermath of Superstorm Sandy in 2013, emergency preparedness has become an important part of the solar energy discourse. Laurie Reilly of Sustainable CUNY notes that “the 600 solar installations in New York City did just fine with virtually no damage [but] the inverters did their job and shut down when the grid went down.” Reilly refers to the fact that these solar systems are designed to shut down when the electricity grid is damaged, despite the fact that the systems themselves remain functional.
As 50% of the solar systems in New York City were located in neighborhoods that lost power, 201 systems with a combined 5.5 megawatts of generation capacity were out of commission for the duration of the outages. Each day, this represents 6500 kilowatt-hours of lost energy. Those who need a refresher course on kilowatt-hours can see this post, but suffice to say, that electricity could have charged a lot of cell phones!
So the question is, how can we harness that electricity during an outage? There are two basic solutions, both of which fool the solar inverters into thinking that the grid is still functioning. Those wishing for a more technical description can read on below, but the main point here is that, with a bit of extra work, solar systems can provide critical energy during power failures. It’s almost a no-brainer–why wouldn’t you add emergency preparedness to your solar system?
Technical Discussion of Solar Emergency Backup Systems:
Note: Inverters are the key components that change the DC current from the solar panels into AC current for building use. Inverters constantly monitor the grid for voltage and frequency. They need to “see” stable AC current in an electric service in order to send their new AC into that service.
AC coupling with variable speed generator
This is a solution to supply electricity during the day in the event of an outage. The design is elegant, but does require access to a variable speed fossil fuel generator. This design worked flawlessly at a large site in northern New Jersey that stayed lit throughout the entire post-Sandy outages.
* If you have an existing solar system, you will need a second inverter, an emergency service panel supplying critical load appliances, and a variable speed generator (gas, propane or diesel).
* Your solar system’s existing inverter is pushing electricity to your main panel. The new second inverter will route solar or generator electricity to the new emergency panel.
* You will plug your diesel generator into the new emergency panel for nighttime power and to supply at least a minimal AC current 24/7. In other words, your generator will run in slave mode at a low “idle” setting most of the day just to show the inverters some AC voltage and frequency.
* That signal will tell the second inverter to let the solar electricity keep flowing to the critical appliances.
This method of AC coupling fools your inverters into thinking the grid is still up–in your building at least–while disconnecting from the grid at large as required under UL 1741 rules.
AC coupling with batteries
If you are planning a new solar system, you can achieve AC coupling by adding electric storage (a.k.a. solar + batteries).
* You will need the same electronics as above (second inverter, etc) and but the variable speed generator is optional and is replaced by a battery bank.
* Under normal conditions, the grid supplies the emergency subpanel with AC supply. When the grid drops out, the batteries supply that AC voltage (via the second inverter).
* The solar + battery system’s electronics disconnect your service from the grid and main panel and inverts DC power from the PV array and batteries to AC power and sends that to the emergency panel.
* The solar + battery system charges the batteries with power from the PV array during the day.
* The solar + battery system can charge the batteries from a fossil fuel generator, which will only need to run for limited time to do so.
* The system monitors the grid for reconnection.
As above, this solar + battery method of AC coupling keeps the needed AC voltage available for the solar system’s inverters to operate. This is quite very competitive. The price for PV plus batteries plus electronics today is about the same as is was for PV alone just 5 years ago in 2008. Batteries add between $2 to $3 per watt. A diesel or natural gas generator–depending on size–adds $0.50 to $2 per watt. And the electronics–such as inverters that are off-grid capable and battery compatible–have a negligible cost difference between 2008 and today.
Both these solar emergency preparedness methods–Solar + variable speed generator or Solar + batteries–are fully compliant with anti-islanding regulations of UL 1741 and will keep your building’s critical load on during the next outage.