Friends:
I am getting asked more often to provide design for grid-tied battery backup systems. I generally try to talk clients out of battery backup because I have always felt that the grid is the greenest battery and that the grid reliability is better than one might remember. However, with climate change and media attention, the need and market pressure is becoming stronger. I am really interested in what others have created in the way of solutions to this question. I have thought about it a lot. If its ok, I’d like to free-associate about some of my concerns and conclusions then ask if any of you are willing to share your ideas on how best to fill this need. Here goes: I am a firm believer in segregate load delivery for these reasons: 1. To back up an entire grid service you have to, in good conscience, perform a thorough load study and provide enough through-transfer to provide for maximum load conditions during grid up-time. The backup system cannot be a supply bottle-neck. 2. I don’t accept manual load-shedding for one moment. What if the AC and the dryer are running when the grid goes out? The system crashes. 3. In a home with whole-house backup, it is difficult for the consumer to know there is an outage and to conserve. Cellphone notification features are now more common, but cell phone batteries die and phones get left in the other room with the ringer off, so it is possible for homeowners to leave consumptive loads on after the grid goes down. Conclusion: With segregated loads, the transfer, inverter and storage can be scaled down and the reliability increases. Less cost, better performance. So given that the system sizing will be modest, has anyone come up with a reasonable design using standard, grid-sell capable battery inverters? Here is how I am imagining such a system: I see a modest battery system with a 4 to 8 KW inverter, DC coupled PV and a matching no-maintenance battery system. It would have these components: Inverter: Must have a dedicated generator and grid input and good remote monitoring, robust web interface and email notification. Outback Radian. Batteries: Gel or lithium. I have a client that has been grid-floating a set of MK Powered gel batteries since 1999. Or a Blue Planet or Fortress cabineted system. Generator: Small pad-mount natural gas or propane fueled generator or a portable gas powered unit with stabilized gas. If the generator is portable, provide an AC flanged inlet and sturdy cord long enough so generator can be 10 feet from any opening in the home. Provide durable signage on the generator indicating CO safety practices. PV: DC coupled for simplicity. Sized based on annual energy needs, not on critical loads. Grid power is likely to go down in the stormy winter months or smoky fire season, so PV is not a reliable energy source during grid outages. AC Distribution: Segregated loads with at least one commonly used lighting circuit not on the critical loads panel so the homeowner soon realizes the power is out. Unless you are a Tesla dealer, I am sure many of you have gone through the same mental acrobatics trying to figure out the best solution to this design request. I am very interested in what the rest of you may have come up with. Thanks in advance. William Miller Miller Solar 17395 Oak Road, Atascadero, CA 93422 805-438-5600 www.millersolar.com CA Lic. 773985
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