Metering GT with Battery Back-Up (long)

[VTSolarGuy]

Hi Folks,

I've only installed one grid-tie with battery back-up that needed REC Metering (a.k.a. SREC's or "Green Tag" metering), and it was a nightmare.  System was done over a year ago, and now the utility is re-visiting this, and thinks something is not right.  I agree -- they put in the wrong meter (not net registering), so I'll work that out with them.

Without getting bogged down in the specifics of that one job, I think there will be a lot more GT w/back-up systems in the near future, as more people become concerned about grid stability (etc...), and more and more utilities are using a virtual feed-in-tarrif or micro-FIT as a way to meet their RPS goals.  A growing number of utilities are now going beyond simple net metering, and paying the customer more for renewable power than they charge for grid juice.  And there seems to be a growing market for all these SREC's, so there's a growing need to measure this stuff.  Some pay for every kWh produced, even if none was exported to the grid.  This creates some real challanges in accurately metering every kWh produced, while excluding all grid power passing through to backed-up loads or used in recharging batteries.

Here's my request to the RE world:  Inverter manufacturers, metering companies, and utilities really need to get together and figure this stuff out.  Outback has only been slightly helpful, and Xantrex tech support is a joke.  I have a few diagrams form Blue Mountain Solar in Oregon which have been very helpful for Outback GVFX inverters, and that very sophisticated and elegant Tall Girl helped me quite a bit, but now a colleague is asking about metering an XW inverter, and then what about an AC-Coupled system?

If people on this forum have already figured this out, maybe it's time to offer a book, a pamphlet, or an on-line course or webinar.  I still need to get to that "ah-HA!" moment.

Ideas?

Clear skies,

Brad

[Robin]

We have a dual outback system on our wiring diagram section of our website. Here is a link to it. http://www.midnitesolar.com/pdfs/Dual_OutBack_grid_tie_with_dual_meters2.pdf
We will be uploading an XW version very shortly. IT is just not simple to do this today. We have some thoughts on this subject, but our total solution will have to wait until our inverters are on the market.

[Mike M]

Hi Brad!
I am running into similar problems.  I am working with a utility to get approval for incorporating battery backup into their grid-tied systems.  It's a bear of an effort. 

[VTSolarGuy]

Hey Mike,

The part the utilities might not understand is that measuring all the kWh that comes out of the inverter won't work, since some will, at times, come from the batteries.  The assumption is that the batteries are charged by the solar panels, right?  However, after an extended outage, the first priority of the inverter is (or should be) to re-charge those batteries in preparation for the next outage.  Some of that power will come from the PV array, but most will often come from grid juice because there's just more of it and it gets the job done faster.  So, that means some of the power coming out of the inverter came from the grid in the first place.  Maybe splitting hairs, but in areas with frequent outages, it could be a significant factor.

The 5th jaw meter can and form 12-S meter strategy seems to work fine, but with a dual inverter 240-volt system, there are two meters, and the correct one needs to be "net registering".  The meter reader also has to juggle three numbers, summing two and subtracting the third.  My utility put in two identical meters, so they are aren't getting the enlightenment they seek.

I think -- though I can't be sure this is true -- this would be a whole mess easier if Outback just made a 240-volt output without having to stack inverters.  That may be the solar wimp in me speaking (no, more like screaming), but I'm getting cozier with Magnum, and getting more and more annoyed with Outback and the %$*^# MATE menus.

In fact, a great idea just struck me...  Maybe we can sponsor a new field event at solar gatherings like the "Tossin' o the Caber" at Scottish festivals... something like the "Tossing of the MATE", which we could just call "G-day, MATE!" (or Good Life, MATE)... We'd attach a MATE to the communications cable and with a wild flailing swing of the body and arms and a tremendous grunt we'd heave the little piece o'junk in a high arc over hill and dale with cable a-trailin', and try to set a distance record, or an impressive splashdown in a local pond.....  Aaaah, but I'm just dreaming again...

Clear skies,

Brad

[tallgirl]

Uh, I support the Shark 100, which is a serial number traceable, revenue grade, bi-directional watt-hour meter.  It uses CTs to measure current and the CTs it supports can handle systems up into the megawatts.

As regards the problem with green tags and battery-backed systems -- life is like that.  The meter has no way to tell the difference between a kWh that is going to batteries, only to be sucked back out later, and a kWh that came magically from the sky.  The only solution to this problem is for utilities to =require= that any customer selling RECs from such a system buy green power to start with.  That way the customer can't fill a bank with dirty electrons and sell them as clean later.