Like @Ampster if you find one let us all know -- i would be interested in at least looking at how it works ... I mean by its very nature it would need a transfer switch of some sort ... but the guy looking over my shoulder as I type says he has heard of these without transfer switches but their costs are in the 10's ... and uses software and blah blah blah - (which in my mind is till a transfer switch of some sort)
I put two simplified one-line diagrams here:
First diagram is a most common grid-tied system like I currently have with SRP. For simplicity I put jut one inverter with 3 MPT channels and skipped combiners, fuse boxes, disconnects and other equipment not important for concept understanding
Second diagram contains additional equipment shown with red. I believe it can achieve "Meter Zero" mode during peak time without transfer switch. It's DC-coupled with existing Grid-teed inverter via vacant MPT channel "C"
This is how it's all going to work:
- Current transformers on solar and utility lines duplicate existing SRP meters here. They measure solar energy production and energy flow to/from grid at any particular moment providing data to computer. Based on those parameters I can calculate grid demand at any particular second
- If time 8pm-2pm, it's non-peak time and electricity is free from solar or cheap from grid. Computer turns on relay to charge battery bank via BMS
- If time is 2-8pm (peak time), battery charging is off. Electricity is expensive
- If time is 2-8pm and Solar power >= power in grid line, there is no grid demand as it's 100% covered with solar. No action required
- If time is 2-8pm and solar power < power in grid line, this is where I'm consuming expensive peak-time electricity from grid and demand charges apply. Following sequence of events will happen:
- Computer turns on additional "off-grid" inverter to get 240VAC from battery bank
- Double-voltage rectifier gets 480VDC from 240VAC
- Computer-controlled PWM provides 90-480VDC to vacant MPT channel "C" in existing grid-tied inverter
- MPT inverter believes sun is shining again and feeds that power to grid/loads via main distribution panel like it usually does with regular solar energy
- Software in computer monitors power in grid line and sends command to PWM to increase or decrease power output from battery bank to MPT "C" in grid inverter. Its characteristics should simulate technical parameters of regular 480VDC solar array (e.g. 12 of 40V/300W panels)
- Having feedback from grid line to battery power output, software maintains "Meter Zero" mode on grid line as long as battery lasts
- If battery capacity is not sufficient to cover 100% of demand during on-peak time, software can be adjusted for some demand allowance. E.g. up to 3kW or so. Since demand charges in utility payment plan have three tiers (please see previous messages), first 3kW of demand will cause only ~$25 demand charge in monthly bill. It's pretty bearable and can be consumed
- Bigger battery bank and more powerful second inverter can achieve 100% demand mitigation and true "Zero Metering"/TOU mode
Typical grid-tied system
DIY Meter Zero system:
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