17 August 2012; 6 PM From: Daniel Hellerstein To: Kenergy It's great to be on track and moving! That said, there are a few minor issues that need resolution. That's what I discuss below. Let's resolve them quickly! I also have some miscellaneous notes at the end, that you might find informative & amusing. The minor issues are: a) Adjusting for dropping the TED monitor b) Validation of dimensions of battery enclosure c) Finalization of inverter sizing d) Incorporating a generator: the disposition of current genTran "transfer switch" BTW: I would be interested in what Scott S has to say. In fact, I am realizing that SOME load shifting may make sense (see note below). ---------------------------- The details ... 1) TED monitor. The TED monitor was dropped, but this was NOT reflected in the change order I just signed. IOW, the $300 or so that it costs was not reduced from the price. So of course I assume that you have implicitly agreed to spend this amount on an energy monitoring solution for the two Aurora inverters? If this is not correct, we need to correct the payment amount. 2) Battery enclosure. Through sheer fortune, I have been volunteered to build a battery enclosure. What follows are tentative plans. Please inform me if changes are needed. The batteries (assuming they are DEKA 245AH AGM, model # 8A8DLTP-DEKA) measure 20.75" long x 11" wide x 10.6" high. Thus, interior dimensions of the two-shelf wooden battery enclosure will be 24" deep x 48" wide x 28" high. This permits mounting 4 batteries per shelf: lengthwise: | | | | or widthwise: _ _ _ _ Notes: * The front panel will be removable (for easy access). * Only one shelf will be used; the second shelf can be for storage or for future batteries. * A 3" (or 4") vent outlet will be on top, and two cable inlets (3" diameter) on the sides. * One or two 1" holes on the bottom for air input (say, using PVC pipe) 3) GTI sizing. The PV panels will NOT have Tigo maximizers. Rather, they will go to Aurora inverters, each with two MPPT tracking inputs. It is not yet certain what size inverters will be used. You, with my concurrence, seemed to be leaning toward at least one of them being a 3000w inveter. This size would be a good match for the 245AH battery bank (i.e.; using a 1kw charging per 100AH @48v rule of thumb) See note below for more thoughts on battery sizes and inverter sizing. 4) Generator, transfer switch, critical loads sub panel The current GenTran transfer switch supports the "critical loads". Note that this transfer switch contains both an either-or (either utilty or generator) switch, and 10 circuit breakers. Thus, it serves both as a transfer switch and as a critical loads sub panel. It is essential that these loads still be powerable by a generator, say when batteries are discharged during a long outage. And that there be some means of using the generator to charge batteries. Note that the MS-PAE 4448 has only one 120/240 AC Input (http://www.wholesalesolar.com/pdf.folder/inverter%20pdf%20folder/MagnumMS_PAE.pdf). After some thought (see the notes below), I would recommend the following setup (its a change from my prior notion). Of course, if you have a better idea... do tell! a) Using the Gen Tran as the critical loads sub panel b) Install a new transfer switch. Inputs are from the generator, and from the utility (from the main circuit breaker panel). Output is to the MS-PAE 4448. This transfer switch would have to permit backfeeding. I can't see why that would be an issue -- so long as it is a MANUAL transfer switch (i.e.; please do NOT consider automatic generator startup) c) Connect the MS-PAE 4448 to a junction box. This junction box links 3 sets of 120/240 wires i) From the MS-PAE 4448 ii) From the Aurora grid-tie inverter iii) From the generator input of the GenTran This avoids issues with wasting breakers on the GenTran (to recieve the GTI's output). Thus, the generator input of the GenTran will be the pathway to the loads served by the GenTran's breakers. d) The utility input of the GenTran will not be used. One could use this "utility"input as an emergency "bypass" from the main circuit breaker panel to the critical loads subpanel; should both the MS-PAE 4448, and the bypass on the Magnum load center, fail. But let's forget I mentioned it That's all for now. I probably will think of more stuff. Please do respond to the above (especially the first two) in the near term, they do effect my planning. ---------------------------- Minor notes --------------------- * Inputs to Auroras. Just out of curiosity, do you plan on using these sets of PV's for each of the four GTI inputs (the two inputs per Aurora inverter): i) the 3 panels on the east facing roof, ii) the two rows of four (total of 8) on the flat roof, iii) the 5 on the SE facing roof, iv) the 9 on the long north facing roof * Circuits now served by the GenTran It might be useful to move the circuit wiring going to the critical loads. Right now the circuits end in the main circuit breaker box. They are then spliced to leads coming from the GenTran (that go to the breakers in the GenTran). Would it be best to move these circuits outside of the new circuit breaker box (maybe not, given the lengths of the wiring currently terminating in the main circuit breaker box). * Battery size. I suspect some would comment that a 4.4kw inverter should have 440AH of battery. At an extra $2000, this ain't happening Rather, the system will rely on the robustness of AGM batteries, and the fact that battery usage will be infrequent. This usage pattern should minimize issued associated with accelerated battery degradation that such a mismatch may lead to (i.e.; microcyling and ripple) * Thoughts on incorporating the generator: using GenTran transfer switch? One approach is to fully use the GenTran transfer switch; * the GenTran's "utility" leads connected to the inverter output (or to the "junction box" noted above). That is, they would NOT be connected to the main circuit breaker panel. * the "generator" leads would remain unchanged. * The MS-PAE 4448 would be wired directly to the main circuit breaker panel (more precisely, via the Magnum power center). The drawback to this approach is the need to provide some means of charging the batteries from the genset. One approach is to add a 20A AC battery charger ($250) attached to the genset (say, via a receptacle that is one of the critical loads). Alternatively, a new transfer switch could be installed that would provide either utility power or generator power to the MS-PAE 4448. * As above, the GenTran would be used as the critical loads panel, with only one of its inputs live: either the utility inputs or the generator inputs. And this "live" input would be wired to the AC out of the MS-PAE 4448. In this setup, under outages the "new" transfer switch is set to generator, and the Magnum powers both the loads and charges the batteries. Note that with my current generator, it is unlikely that AC coupling will occur (not very clean power). This is probably a good thing, as it avoids possible AC coupling and GTI-to-generator feedback issues. Actually, perhaps more precautions are needed (I recollect reading something about that in the AC coupling article). Note that under this setup the GenTran will NOT be used as a transfer switch: only one set of inputs will be live. It might be convenient to use the generator input, rather than the utility input, to connect to the MS-PAE 4448. Advantages of using generator input: * probably easier to wire (use a stub of the current cable-to-the-generator) * in generator mode, amperage meters work (so nice monitoring of power use) Disadvantages: * max of 3500 amp per leg. Right now the max, from utility, is 6000w per leg (50w x 240v). The 3500 max applies to generator input. Now, these circuits don't get heavy use, so it is very unlikely that there will more than 7000w of instantaneous demand (more precisely, 3500W of demand on either leg). But the MS-PAE 4448 is rated at 4.5kw, with 5 second surge at 8000w. So the magnitude of power production would be close to the magnitude of power distribution. Thus: this disadvantage is tolerable (and if not, I can always rewire in the future). Hence the conclusion noted above: install a new transfer switch that choses whether utiltiy or generator power goes to the MS-PAE 4448, etc. * On load shifting. The main reason for the AC coupling/battery backup is to provide most-of-house-UPS services, and support during extended outages, to a set of critical loads. These events are relatively rare: as a rough guess, they will require about 100 charge cycles over the next 10 years. Load shifting is also a usage of batteries in a grid-tied environment. However, load shifting does mean extra battery cycles. Which means quicker battery depreciation. This means that the magnitude of any time of use pricing differential would have to be quite high to financially justify load shifting (some estimate it to be over $1.00/kwh). In other words, load shifting doesn't seem to make financial sense. However, if the batteries need replacing every decade, EVEN if they aren't used at all, then my expected usage can be greatly expanded without adverse consequences. In other words, a good guess is the batteries could cycle 500 times (@ 50% DOD) over the next decade with little loss of capacity. If I am only cycling them 100 times, then 400 cycles are going to waste! Of course, this is a rough analysis, but full accounting for aging curves, capacity losses over cycles, etc. is not likely to change the overall conclusion: that a moderate amount of load shifting could be done at no real additional cost. Thus, it is worthwhile considering some kind of time-of-use pricing, or other "on demand" load shifting. Something that would not entail a lot of usage: say, around 50 events per year. I wonder if such an opportunity exists. I even wrote PEPCO asking this. I kind of doubt there is, and I don't know the Magnum is smart enough to do this. But wtf.