Critique of my off grids preliminary plans

Roughneck · January 2015

Hi All, I'd just like to introduce myself around here. Long time lurker trying to get a hybrid grid tie off grid system working. I've been reading a lot for several years and I think I have an idea of what I'm trying to build but I wanted to get some thoughts and suggestions from people who have done this in case I overlooked anything (quite probable). I'm located in SE Texas (hurricane country) and the household electrical usage fluctuates between 1-2000 kWh a month winter to summer. Our heating and hot water is gas. We are looking to replace most of our electric bill with this system and be able to run indefinitely in the event of a power loss albeit without some heavy loads like AC running as much (or at all), however we plan to add a generator later to take care of that. Electrically we will move the heavy loads upstream of the inverter (220v loads i.e. dryer, AC, and stove) since total use for everything else in the house is far below 8KW the radian provides.

Inverter: outback radio GS8048 bundled with Load center (GSLC175-PV-120/240), Mate3, Hub4, and 2 FM80 charge controllers
Batteries: 8x sun brand 6V 230AH battery single string 48V 1840 Ah
Panels: 20x 435W (8700W) Sun brand laminates (Vmp=72.9, Imp=5.97, Voc=85.6, Isc=6.43) configured as 2 parallel strings (1 for each FM80 so as not to exceed the Voc limit)

I am curious about which combiner box would be recommended for this application as most of them seem to indicate they are designed for series circuits.

What are people's thoughts on the laminates? I have an irregularly shaped roof with limited space facing directly south (at 30 degrees) and so I am trying to maximize energy density.

I am using Unirac and attaching the L-feet directly to the shingles using bishop tape to keep it waterproof so no flashing, does this present any known problems?

I know I'll need some large wire to handle the amps from each array but at the cost of the laminates I think I would come out ahead.

Thanks for everyone's input!

ButchDeal · January 2015

Re: Critique of my off grids preliminary plans

Roughneck wrote: »

Batteries: 8x sun brand 6V 230AH battery single string 48V 1840 Ah

that would be a 48V 230Ah string a little small for off grid
http://www.zbattery.com/Connecting-Batteries-in-Series-or-Parallel

Those Laminates are large and probably not the best for limited space. For limited space you might consider LG 300w

Roughneck · January 2015

Re: Critique of my off grids preliminary plans

You are correct, my mistake, so that would mean I should be using larger batteries. How many Ah per kW is recommended?

As for the laminates these are not thin film and have a higher energy density/conversion than anything else I have found at 18.7W/ft2.

Cariboocoot · January 2015

Re: Critique of my off grids preliminary plans

Welcome to the forum.

Your battery bank is way undersized for either off-grid or grid-tie use. You need at least double that capacity for the Radian 8kW inverter and 4X would be better, especially if you're planning a 8+kW array. Otherwise the ripple current will be large and you probably won't be able to keep the system connected to the grid.

I don't know about those panels, but I wouldn't use them. They will be dimensionally awkward. What's more, the ideal configuration would be all in parallel, ten per controller (Vmp 72.9 is fine for a 48 Volt system). That's 4350 Watts per controller, about 70 Amps peak current each.

That means 140 Amps output, which is way over what 230 Amp hours of battery will take so we're back to that.

I'd like to suggest an alternative plan.
1). Figure out how much battery back-up power you really need in the event of an outage. Just the essentials.
2). Figure out how much PV power you need to offset your grid consumption at a practical level.
3). Design a system that has both battery back-up for essential needs and cheaper direct grid-tie to offset the utility usage.

Roughneck · January 2015

Re: Critique of my off grids preliminary plans

So if I increased the battery capacity by 4X that would result in a workable system? The panels while large I think are manageable for my application especially for the cost at ($0.39/watt) and energy density. The the panels that don't fit on the south facing area (only about 5-7) would simply be moved to the east/west side of the house. I don't see how using smaller panels (physically and wattage) would really change that, but I of course might be mistaken.

Cariboocoot · January 2015

Re: Critique of my off grids preliminary plans

Couple of things.

In order to keep the inverter working happily with the grid you need about 800 Amp hours of battery minimum.

The 8.7kW array is going to produce 140 Amps of charging current. Usually this will be going mostly to power loads (including back-feeding the grid) but you never know: the batteries could get hit with full current under certain circumstances. That makes for a 17.5% peak charge rate potential, which is pretty high for flooded cells.

What you do not want to do is put in four strings of the 230 Amp hour batteries. Get larger capacity batteries (even forklift type) and keep it to one or two strings at most. Whereas the current sharing issues are not as prevalent in 48 Volt systems, why tempt fate? To say nothing of adding extra wiring.

You may want to purposefully split up those panels so that some face East, some South, and others West. It will not increase the maximum output but will spread it throughout the day making for more consistent power production in daylight hours. Much depends on your usage patterns and the utility's rate structure (what time of day is optimum for your system to back-feed).

Again, having it all on the battery system may not be the best choice.

BB. · January 2015

Re: Critique of my off grids preliminary plans

Are you ready to spend ~10x what you present utility based power bill is?

New batteries every 5-8 years (perhaps 15+ years if you get forklift/traction batteries). New electronics (charge controllers+AC inverters+etc.) every ~10+ years.

If you have kids/guests that leave power on, forget to check electrolyte levels, controller failure, etc. that will kill the battery bank in a couple days.

Near by lightning strike--New AC inverter. etc.

In general, if you have reasonably reliable and priced grid power--Off grid solar will almost never be a cost effective decision.

If you have power failures that last a few days or week or so--A genset+backup fuel supply is not a bad solution.

If you have power failures that last months (ice storms, huricanes, etc.)--The, perhaps a hybrid off grid system can be a solution (still expensive). And even an off grid solar power system--You still usually need a backup genset still.

Say your electric bill went up by a factor of 10x (from $0.10-$0.30 per kWH to $1.00-$2.00+ per kWH)--What would you do conservation wise around your home.

Refrigerators, well pumps, fans, etc. are usually the big cost drivers of off grid power. If you need A/C--That is another killer (usually you need lots of A/C when there is lots of sun--So that is a blessing with solar).

Not to say that off grid solar is not a good solution for you--But highly recommend measuring/reviewing your power needs, doing lots of conservation (conservation is almost always cheaper than building out a larger solar power system), and several paper designs before you purchase any hardware.

If you want to save money--Grid Tied Power (solar panels + GT Inverter connected to your main panel) can save lots of money.... However, that requires that your utility gives you a good rate plan for your solar power. And, as more people install GT solar, more and more utilities are starting to alter their rate panels to make the return on investment for GT Solar less and less (this is very political--And the utilities are in business to make money--GT solar costs them, and their customers, money).

$8,000 for 800 AH @ 48 volt fork lift battery bank, $4,000 for an 8kW inverter, $4,000 for electrical, $4-$8,000 for racking, $8,000 for solar panels, $X,XYZ for genset+fuel+fuel storage, etc...

If your power needs can be reduced (say minimum amount of power for a "near normal" electrical power existence)--Look to see if you would be able to live with ~3.3 to 4 kWH per day (100-120 kWH per month--Refrigerator, well pump, washing machine, LED lights, laptop computer+network, battery chargers for tools/cell phone/ipads/etc.)--Only a faction of cost of a system capable of 16+ kWH per day a large Radian system is capable of.

-Bill

oil pan 4 · January 2015

Re: Critique of my off grids preliminary plans

If you 2 main reasons for going off grid are "power outages" and to "save money".

Then realistically to meet those requirements of both "saving money" and immunity to "power outages" you will want 2 things.
To save money on your power bill and have power when the power goes out, with out spending $100,000 you need install a simple grid tie system for any where between $8,000 to $20,000 to give you at least some to most of the power you need day to day.
Then when the power goes out, you will want a disconnect for your solar and a simple $1200 backup generator for when the power goes out.

If you are real creative you could employ some creative switching, stand alone inverters and a few small backup up batteries so that you can have solar power during the day when the power goes out and run the generator at night if you need it.

Remember a pull start generator can sit around for 10 years until you need it and still work, batteries can't.

Critique of my off grids preliminary plans

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