To go micro or hybrid, that is the question

techntrek · September 2010

Hello all, first post. I’m a newbie to solar in the sense that I don’t have a solar PV or solar thermal system. Not-so-newbie with an electronics background as a hobby, Homepower subscription for many years, I installed a backup inverter (6 kw/8.5 KVA) and battery bank (22 kw) a few years ago, and I’ve run the numbers for my dream PV system a few times over the years. Ok, more than a few. I’ve been to wind-sun.com many times in the last few years drooling over the latest dollar-per-watt pricing and looking for parts like a shunt, for my backup system. Only recently saw that they have a forum.

I was hoping to be an early Volt buyer (electric car has been another long-term want), but I feel they priced it too high so partly in protest and partly to put that kind of investment to better use, I’m now seriously considering making the jump into a PV system sooner rather than later. Solar thermal isn’t an option due to my property layout, with 200 feet minimum between the corner of my house and a location with good southern exposure. Much easier to move electrons 200 feet to my service panels than water molecules over 250 feet. With future plans for a ground-sourced heat pump that takes away some of the need for solar thermal. I also use a wood stove all winter. So that leaves me with installing a decently-sized PV system.

I’ve done a lot of wavering over the last 2 years on whether to go with micro inverters, or a hybrid inverter that will be mostly used for GT. I see many here discussing the XW line, and that’s what I would go with. When they announced that product a few years ago I fell in love right away for the reasons you all already know. However, the supposed advantages of the micros is highly appealing, too. No string shading issues, keep the system online if one panel or inverter has a problem, etc. I have a backup system already so a micro-based system could be added separately and then I’m done. I also wouldn't need to build a shack down by my clearing to house one or two XW's and the batteries. I do want GT since you get your best overall efficiency that way, and I need backup because I’m at the edge of the service area for my provider and down a winding, tree-filled country road. Outages are common, and neighbors have stories of going 2 weeks w/o power after ice storms.

But… I personally believe the public system will become unstable in the future since no headway has been made to fix its basic problems. And one good solar storm could wipe out the big transformers and then we’re all back to 3rd world status in less than a month < puts on tin foil hat >. So GT may be less and less advantageous – or completely useless – in the future. Going with micros would mean redoing half of the system and adding solar charge controllers, or hoping that they will sync up and stay online with my backup inverter (with a good chance they won’t), if my assumptions become true down the road.

Your thoughts?

icarus · September 2010

Re: To go micro or hybrid, that is the question

Just a note, off grid systems come at about twice the price per KWH with about 1/2 the efficiency, leaving a net cost of ~4 times the net/net cost.

Since you already have a battery based system, you already know the battery costs. Personally, I don't have the fear of grid failure that you have. That said, I live off grid so I have no choice.

If I had the grid, I would use it.

Good luck and welcome to the forum,

Tony

techntrek · September 2010

Re: To go micro or hybrid, that is the question

Thanks. Glad to find a place to talk PV. I admit to some doom-and-gloom with my thoughts on the grid - hopefully I'm wrong. I certainly don't shun its use as I'm fully wed to it right now and think that at least in the near future a GT system is the best use of resources. Fully understand the ramifications of a GT vs. off-grid system... I'm considering a 10 kw array, which would give me about 32.5 kw/day GT on average in my area after temperature and inverter losses (10 kw x .8 x 4.375 x .93). Or only about 16 kw/day off-grid on average after adding in charging losses, not factoring in temperature effects on the battery bank. More than enough to keep life nearly normal in a long outage without firing up my genset.

AntronX · September 2010

Re: To go micro or hybrid, that is the question

I would want to have both grid-tie for high DC-AC conversion efficiency and off-grid for battery backup. Buy one or two XW-6048 for your existing 22KWh battery bank. Grid-tie the bulk of your solar panels via SunnyBoy inverter or any other GT inverter that can trottle it's power output based on AC line frequency. Then when power fails, you can connect (via transfer switch) Sunnyboy to XW. With the latest software update, the XW can act as SunnyIsland inverter, by generating AC for SB to lock onto and absorb excess AC back into your 22KW battery bank. When batteries are full and your grid-tie power output is higher than your loads, the XW will increase AC line frequency which will cause SB inverter to reduce AC power output.

If you have shading areas, you can fill those with solar panels connected to your battery bank via MPPT charge controller, and fill best lit area with grid-tie panels. Then let the XW inverter sell excess current to the grid. This way the bulk of your solar power will flow to the grid at about 94 - 97% total PV DC to line AC conversion efficiency versus 86 - 89% conversion efficiency for XW inverter based low voltage PV current path. This way you get high efficiency of grid-tie and backup power availability with being able to utilize full power of your GT system during the power outage.

BB. · September 2010

Re: To go micro or hybrid, that is the question

And don't write off solar thermal either... You may need to crunch some numbers and do a bit of experimentation--But it may certainly be doable..

Solar Shed

Here is a thread with some links to other solar thermal projects too.

And here is a thread with a bunch of various solar projects, ideas, other forums:

Working Thread for Solar Beginner Post/FAQ

-Bill

techntrek · September 2010

Re: To go micro or hybrid, that is the question

AntronX, interesting option I didn't know about. I suppose to get rebate payment on all parts of the system it would have to initially be wired with the array split down the middle. Half on the SB, half on the XW. Then after "day one" most or all of what is on the XW would have to be switched over to the SB. If the initial design took this into account it may be as easy as unplugging and replugging the panels into a new string configuration. Doesn't mean it would be cheaper than just eating the cost of the XW at the start. Plus the inverters would be so oversized I'm not sure if it would qualify for rebates. MD pays per kw of solar panels so maybe they wouldn't care. I have no idea about US.

One plus of this path is I would want to double the system in the future to cover electric car useage. The XW could be brought online then - its paid for and in place - with the understanding that I won't get quite as good overall performance.

One question - excluding my reason above for payment, why put any panels on the XW at all?

Bill, I'll check out the links, thanks. I would love radiant floor heat!

BB. · September 2010

Re: To go micro or hybrid, that is the question

I am a little confused about the SB and XW statements/questions... SB = Sunny Boy Grid Tied inverter?

You do or do not have a SB already installed right now?

The usual problem with GT and Hybrid/Off-Grid/Solar Battery chargers is that, typically GT inverters are ~200-600VDC input and solar battery chargers are around 17-150 VDC input voltage--So just switching over the solar panels between the two systems is a pain in the behind... A bunch of series/parallel switching involved.

The XW + solar panels + battery bank connected to a utility in Hybrid (GT/Off-Grid Backup mode) does pretty much the same GT function as the Sunny Boy GT inverter---And there is no reconfiguration for an end of the world/moving to off-grid only solution--The XW Hybrid simply does not care (and it has an AC generator input too).

There are some other potential configurations available too--One is to connect the SB GT inverter to the XW's protected output subpanel... The SB GT operated in GT mode with utility power, and in off-grid mode can actually drive the XW Hybrid inverter "backwards" and recharge the battery bank... Note that the Hybrid Inverter has to be as large, or larger, than the connected GT inverter (absorb all power provided by GT inverter to charge battery bank if there are no AC loads turned on during the day).

The XW GT firmware has been updated to support this mode--But I have not seen any detailed configuration setup yet from the manufacturer--So some further research is required to understand the detailed requirements.

-Bill

techntrek · September 2010

Re: To go micro or hybrid, that is the question

Bill, yes, I was responding to AntronX's suggestion, so SB = SunnyBoy. No, I do not have a SunnyBoy installed. I do have a backup inverter and battery bank. I'm open to Antron's suggestion of using a XW as the backup inverter - the one I have now has its issues - and using a SunnyBoy instead of the XW as the GT inverter due to higher efficiency. An extra 8-9% is a big deal.

This leads into the question I had in my last post. Antron was suggesting putting a few of the panels on the XW to keep the battery bank topped off. Since the XW comes with an AC-sourced charger it seems redundant to attach PV to it, which would also mean adding a solar charge controller that would only be used to top off. Bulk charging would be through the AC charger when the SunnyBoy was tied in during outages. I figure why not only use the AC charger, forget the solar charge controller and put all of the PV on the SunnyBoy to get the best long-term output. Edit: I just re-read Bill's post and it sounds like he is suggestion doing this.

While there would be major reconfiguration of the PV panels if they were first wired to both inverters (to get the rebates), if it was factored in during the initial design I imagine some quick disconnect/reconnect of the MC4 connectors wouldn't be terribly hard. Go from 4 strings of 6 to one string of 24, and change the wiring on the other end to connect to the SunnyBoy instead of the XW. The larger wire between the array and inverter, first designed to handle the lower voltage going to the XW, would just help a bit with efficiency when attached to the SunnyBoy.

It sounds like my original question in my first post has been answered - stay away from the micros because a XW (hybrid) or XW (off-grid) + SunnyBoy (GT) solution has some critical advantages.

Anyone around here try the XW + SunnyBoy solution?

AntronX · September 2010

Re: To go micro or hybrid, that is the question

Since you don't mind rewiring your array during power outage, you can get away without XW, unless the inverter you have now needs replacement. You can even make a complex switch that will rewire panels for you. Just make sure Voc of 6 panels in series will not be too much for MPPT charge controller. 140V is when Outback FM-60 shuts down and 150V is max limit when you lose your warranty. If your panel has Vmp = 18V, then 6 * (18 / 0.806) = 134 Voc. That's too high, unless you can build voltage clamp circuit that will dissipate any current above 135V. Also, getting the panel swich approved by inspector may be a challenge.

Since you wanted microinverters due to shading issues, I suggested placing those shaded panels via XW and allowing XW to sell that power into the grid. Then wire unshaded panels to SB inverter. Another two great benefits of XW is a whole house UPS effect and ability to charge your batteries from the generator. You can adjust charging current of XW to find fuel efficiency sweet spot of your generator. And you are even better off with cheaper non-inverter based generator for this purpose.

techntrek · September 2010

Re: To go micro or hybrid, that is the question

I do have a 12kw LPG genset, but I want to get away from relying on it due to operating costs and noise. That's the biggest reason I installed the backup inverter and battery bank. 90% of my outages are handled by the inverter only, and the other 10% let me keep the genset off 2/3 of the day. With hindsight 20/20 I shouldn't have purchased the genset and should have started building a PV-based system 6 years ago. Oh well, still nice to have multiple backup options. The automatic genset handling is one reason I like the XW.

I'm not talking about rewiring for each outage, just once after signoff. And I didn't say I had a shading problem. Sorry if I gave that impression. I will get shadows in early morning and later afternoon due to lines of trees on either side of my "meadow". I was hoping to squeak out a little more production with the micros since only one panel at a time would go offline as the shadows creep in. With two or large strings attached to a SunnyBoy this will be a bigger problem. One panel gets shading at 4:30 pm and 23 others that could be producing for another 30 minutes, aren't.

Bill's comment about the XW needing to be greater than or equal to the SB throws a wrench in the new plan. I might have to get two smaller SBs and only tie one of them to the XW during an outage. A big problem since that would cut my daily off-grid average from 16 to 8 kw. Still doable but 50% below optimal. I guess an alternative would be to stack two XWs.

mike95490 · September 2010

Re: To go micro or hybrid, that is the question

Just a couple things you may be overlooking.
1) XW uses a seperate charge controller for PV. They have a 60A model

2) not sure what voltage your battery bank is, but say 48V, which is 5, 12V panels. Paralleling 4 banks will give you good wattage without going to microinverters.

stephendv · September 2010

Re: To go micro or hybrid, that is the question

As far as I know there's no reason to choose the Sunny Boy over the microinverters for AC backfeeding. I believe the XW just pushes the line frequency above a certain limit when the batteries are charged, which will mean the GTIs will disconnect.
The only advantage of the SunnyBoy is that it offers variable output based on frequency, but I don't think the XW supports this - the GTIs are either connected or not. (Hearsay purely, would be good to find out from Xantrex themselves).

techntrek · September 2010

Re: To go micro or hybrid, that is the question

Mike, yes. As I understand it, with the suggested "frankensystem" I wouldn't need it. All PV would be on GT duty, and the batteries would get charged from the XW's AC charger.

Stephen, or anyone, what is the reason to throttle the GT output? I don't see why since power sources will only supply amps to meet the load on them.

AntronX · September 2010

Re: To go micro or hybrid, that is the question

techntrek wrote: »

..With two or large strings attached to a SunnyBoy this will be a bigger problem. One panel gets shading at 4:30 pm and 23 others that could be producing for another 30 minutes, aren't.

It's not supposed to be like that. PV panels have series bypass diodes, usually 3 per panel which keep the current flowing, bypassing shaded section. What will happen, your Vmp will fall as if instead of 24 panels in series you now have 23.7 - 23 depending on how many sections of that panel are shaded. You need to account for how many panels will be shaded at the same time. Vmp of remaining unshaded panels should be above minimum MPPT cutoff voltage of your GT inverter. You need to size the amount of panels in series with that consern as well.

techntrek wrote: »

Bill's comment about the XW needing to be greater than or equal to the SB throws a wrench in the new plan.

I want to hear from Solar Guppy on this. I don't think what Bill wrote is accurate. I think you should be able to use larger than 6 KWac GT inverter with XW6048. You just will not capture all of potential GT output into your batteries. I am not sure if single XW will be able to govern larger GT inverter.

AntronX · September 2010

Re: To go micro or hybrid, that is the question

stephendv wrote: »

As far as I know there's no reason to choose the Sunny Boy over the microinverters for AC backfeeding. I believe the XW just pushes the line frequency above a certain limit when the batteries are charged, which will mean the GTIs will disconnect. The only advantage of the SunnyBoy is that it offers variable output based on frequency, but I don't think the XW supports this - the GTIs are either connected or not...

SunnyBoy's variable power output based on AC frequency is the main feature that allows this scheme to function well with SunnyIsland inverter. When Solar Guppy announced that XW6048 now can replace SI inverter, I assumed XW can completely replicate the behavior of SI. This was great news since 5KW SI costs $5,200 and XW6048 costs $3,400 on Wind-Sun. Although as soon as this news came out, certain other well known solar parts retailer started selling 5KW SI inverters for $3,750. So I think SMA kept SI price high due to no competition on the market.

If GT inverter cannot vary power output and simply disconnects when frequency is high, this whole setup will not work. The beauty of this system, is that you can simultaneously have AC loads connected to both SB and SI. When SB output is higher than AC load, SI will absorb excess AC into the battery bank. And when the cloud blocks sunlight, SI will turn around and supply AC deficit to AC loads, thus keeping steady AC voltage.

BB. · September 2010

Re: To go micro or hybrid, that is the question

The original XW vs GT limitation was that the XW (and most off grid inverters) had no sort of controls that monitored the reverse current flow... Perhaps the XW/OG inverter would fault if the battery voltage went too high--But that was about it.

A GT inverter is designed to output as much current (power) as possible regardless of line voltage and frequency--As long as both are within specifications...

So, you could put a 10kW GT inverter on a XW or other off grid inverter and it would try and dump 10 kW into the 6 kW rated off grid inverter. Not a good thing for long life.

Perhaps, the XW can now monitor and control the frequency--and for Sunny Island compatible equipment, that would work fine. For standard GT inverters, they probably have too states--Off or Full Available Solar Power, and that is it.

That would still mean that XW would have to fault the GT inverter (run the frequency outside the 60Hz +/- 1% range) to stop the GT from over powering the XW--But that would not be a very satisfactory solution (GT would be "off/waiting 5 minutes for stable power" then start the whole thing over again).

I think even the Sunny Boy inverters need to be configured for operation in the "Sunny Island" power grid--I am not sure that the allowed frequency spread in a Sunny Island system is within the requirements for grid frequency range. So--you may end up with issues trying to do the "Sunny Island" Frequency control with a mixed Grid/Off-Grid/Sunny Island/XW-frequency control enabled system... The Sunny GT would have to "reconfigure" on the fly for Grid Tied and Off-Grid operations... Certainly possible--but I am not sure that the regulatory powers that be would like that.

-Bill

AntronX · September 2010

Re: To go micro or hybrid, that is the question

BB. wrote: »

The original XW vs GT limitation was that the XW (and most off grid inverters) had no sort of controls that monitored the reverse current flow... Perhaps the XW/OG inverter would fault if the battery voltage went too high--But that was about it.

Doesn't XW sense reverse line current when charging battery? They gotta have the circuitry to do this. Where is Solar Guppy when you need him.

BB. · September 2010

Re: To go micro or hybrid, that is the question

Even if it did sense the over current condition somehow, either the XW would fault or the generic GT inverter (non-sunny island or compatible) would fault. And it would not not harvest power very well during the middle of the day (where GT Output>>XW rated power).

The above is all based on my limited understanding and guesswork on my part--These are points I would investigate if I where going to install/configure such a system in my home.

-Bill

techntrek · September 2010

Re: To go micro or hybrid, that is the question

Antron, where did you get your GT efficiency numbers for the XW (86%-89%)? I can't find efficiency curves for the XW, either GT or off-grid. I see the max rating (95% low-load) and CEC weighted (92.5%) numbers in the specs. Sounds like a XW + SB system might be so bleeding-edge that if the XW is actually in the 90's then system cost and complexity could be kept down by going back to only using an XW. Don't get me wrong, I've been bleeding-edge my whole life, but its one thing to drop $1000 on a phone and another to drop $50k on this. 8)

BB. · September 2010

Re: To go micro or hybrid, that is the question

I can see using a pure GT inverter because 98% of the time the system will be running from the grid and 5% increase in efficiency/output is difficult to ignore.

But the added cost complexity of such a setup can be an issue too.

-Bill

AntronX · September 2010

Re: To go micro or hybrid, that is the question

techntrek wrote: »

Antron, where did you get your GT efficiency numbers for the XW (86%-89%)? I can't find efficiency curves for the XW...

I attached two PDFs with inverter efficiency curves. To estimate more accurate total PV to AC loss, include losses from wires, MPPT charge controller, charge controller to XW wire loss, inverter to electrical panel wire loss. In XW circuit, apart from extra 3 - 4% loss in MPPT charge controller you also have higher loss due to lower PV string voltage and higher current. For example, you will have 24 x 200W 18Vmp panels.

200/18 = 11.1A; 18*24 = 432V.
Voltage drop in 300 feet of 8 AWG at 11.1A = 5V.
5V/432V = (0.012 - 1) * 100% = 98.8% efficiency to carry 4800W to SB6000US inverter.

Now, lets try get same power same distance to XW via two Outback FM-60's. Lets use 6 panels in series, 2 strings in parallel per each FM-60.

18*6 = 108V; 11.1A*2 = 22.2A.
Voltage drop in 300 feet of 8AWG at 22.2A = 10.1V
10.1V/108V = (0.0935 - 1) * 100% = 90.65% efficiency to carry 2400W to FM-60. Since you need two FM-60's, you need second 8 AWG conductor pair, and in example above I assumed each FM-60 has it's own wire run to PV array. Now add efficiency of FM-60 itself. I attached an image of eff. graph of FM-80 which should be about the same as FM-60.

2400W*90.65% = 2176W (at PV input terminals of FM-60)
2176W*95.4% = 2075W (at Battery output terminals of FM-60)
2075 / 2400 = 86.46% efficiency just to get power from PV to XW over two separate 8 AWG pairs, versus 98.8% for SB6000US over one 8 AWG conductor pair.

Now, how much additional loss will be inside XW6048? From the PDF, 4150W/6000W = 70% rated power efficiency will be about 92%. I ignored FM-60 to XW losses because you can make the wires really short.

86.46% * 92% = 79.54% total PV to AC efficiency via 2x FM-60 and XW inverter system. This result is lower than my initial guess. I underestimated wire loss.

For SB6000US, things are looking a lot better. 98.8% * 95% = 93.86% PV DC to AC ouput total efficiency. 4800W * 93.86% = 4505 W(ac).

With XW setup AC output will be: 4800W * 79.54% = 3818 W(ac).

techntrek · September 2010

Re: To go micro or hybrid, that is the question

Thanks for the charts. Here are some additional details that will improve the numbers above. The run from the closest point of the array to the location of my service panels is 200 feet one-way, so that cuts 1/3 of the distance off. That increases the array-to-SB efficiency to 99.2%, and to 99.5% if I move up to 6 gauge.

If I went with the XB-only solution I already figured I would have to build an inverter/battery shack about 20 feet from the closest point of the array to keep the low-voltage runs short. That would make the run to the service panels 180 feet @ 240 volts, 20 amps (4800 watts to make the comparison equal). Making the XW-to-service panel efficiency 97.7% with 8 gauge, 98.5% with 6 gauge, and 99.1 % with 4 gauge.

That makes things much more equal - excluding the ~5% boost from using a SB inverter over the XW.

AntronX · September 2010

Re: To go micro or hybrid, that is the question

Don't forget to factor in 95.4% efficiency of MPPT charge controllers that you will need for XW. SB6000US is both MPPT and inverter in one box. It is also much better to run 432Vdc 200 feet versus 180 feet of 240Vac. Not just due to lower power loss, but also due to AC voltage drop. There may be times when your utility voltage is higher than usual, added with 5.5Vac drop across 180' of 8AWG will cause your inverter to disconnect from the grid due to high voltage cutoff limit, while your GT inverter placed next to the utility entrance would have stayed connected and kept selling power to the grid. Research maximum grid sell mode voltage of both XW and SB inverters. This problem did happen to someone on this forum. Also before you buy XW inverter, make sure you can get that software update and that XW is really capable of replacing Sunny Island inverter. Contact Xantrex before buying anything.

techntrek · September 2010

Re: To go micro or hybrid, that is the question

Definitely will talk to Xantrex first, good things to think about before plunking down some big change. Still hoping to get input from the other person ya'll mentioned earlier (Solar Guppy?).

techntrek · October 2010

Re: To go micro or hybrid, that is the question

I just saw the announcement about the Xantrex high voltage charge controller.

http://wind-sun.com/ForumVB/showthread.php?t=9275

That might make my final decision much easier. High-voltage DC to a central GT inverter, which also solves my 200+ foot home run problem without building a power shed. Add that high-voltage charge controller and I'm good for backup power, too. Those micros are still very tempting though...

To go micro or hybrid, that is the question

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