Home off grid

Re: Home off grid

BoFuller wrote: »

• Outback 3600Watt 48Volt system
• 16 of the Trojan L16RE-B be sufficient?
• 15 Kyocera 235 panels?

As North Guy says--We like to start with your known loads and then design the system.

The second choice, is start with the batteries and design the system around the batteries. then predict how the system performs.

Trojan L16RE-B batteries are 6 volt at 375 AH. The charging for such a bank assuming 5% to 13% typical rule of thumb:

• 16x 7.25 volts charging * 375 AH * 1/0.77 panel+controller derating * 0.05 rate of charge = 2,825 Watt array minimum
• 16x 7.25 volts charging * 375 AH * 1/0.77 panel+controller derating * 0.10 rate of charge = 5,649 Watt array nominal
• 16x 7.25 volts charging * 375 AH * 1/0.77 panel+controller derating * 0.13 rate of charge = 7,344 Watt array "cost effective maximum"

Using 235 Watt panels, the numbers would be (note, series/parallel connections to meet controller input requirements may require a few more panels to make viable array configuration):

• 2,825 Watt array / 235 Watts = 12 panels
• 5,649 Watt array / 235 Watts = 24 panels
• 7,344 Watt array / 235 Watts = 31 panels

So, with that size battery bank, 15 panels would pretty much be the minimum recommended for a bit more than 5% rate of charge. If you plan in cycling the battery bank a lot, I would suggest more towards the 24 panel range.

From the battery bank, using rules of thumbs, the loads would look like (1-3 days of storage, 50% maximum discharge, 0.85 inverter efficiency):

• 16 batteries * 6 volt * 375 Watts * 1/2 days * 0.50 maximum discharge * 0.85 inverter eff = 7,650 WH = 7.65 kWH per day average loads

And a flooded cell battery can output ~20 hour rate (efficiently) or 8 hour rate (maximum recommended continuous draw), or C/2.5 rate for starting surges/very short discharges (note, as battery discharges at higher rates, the apparent 375 AH per battery will appear to reduce--So you will not get 8 hours to dead, but perhaps 7 hours because of the Puerket factor):

• 16 batteries * 6 volt * 375 Watts * 0.85 inverter eff * 1/20 rate = 1,530 watt nominal average load
• 16 batteries * 6 volt * 375 Watts * 0.85 inverter eff * 1/8 rate = 3,824 watt max average load
• 16 batteries * 6 volt * 375 Watts * 0.85 inverter eff * 1/2.5 rate = 12,240 watt max surge load

Using PV Watts for Las Vegas NV, fixed array tilted to latitude (assuming really sunny region of NW Arizona similar to LV NV):

Month    Solar Radiation(kWh/m 2/day)
1      5.19     
2      5.98     
3      6.57     
4      7.32     
5      7.42     
6      7.35     
7      7.37     
8      7.29     
9      7.31     
10      6.56     
11      5.75     
12      5.05     
Year      6.60      

If you pick 7.65 kWH per day as your "design point" per battery bank sizing, and toss the bottom three months of sun, we get February for 5.98 hours of sun minimum:

• 7,650 Watt*Hours per day * 1/0.52 system efficiency * 1/5.98 hours of sun per day = 2,460 Watt array minimum
• 2,460 Watt array / 235 watts per panel = 10.5 panels minimum

So--because of the huge amount of sun in the region (not much clouds, high/dry elevation), a 12-15 panel array would be a pretty good fit... Or looking at the output from a 15 panel array (3,525 watt array):

• 15 * 235 Watt per panel * 0.52 system efficiency * 1/5.98 hours of sun Feb = 10,961 Watt*Hours = 11 kWH per day February (day and night loading)

Any help? Any comments?

-Bill

Re: Home off grid

Think carefully about the crawl space for batteries... Flooded cell batteries will mist electrolyte and leave a sulfur smell--Some people don't notice, some people hate it. Ventilation will help.

The second is fire danger... A shorted battery bank is not fun.

Regarding battery bank voltage. Personally, I like to recommend a single string of batteries when possible. 2-3 parallel strings would be my maximum recommendation unless you have no other options.

Each parallel string should be fused (see above: Fire dangers), but not many people do that (extra fuse/breaker per string is not cheap). Also, maintenance for parallel strings is quite a bit more. 2-3x as many cells to check, you really need a DC Current Clamp DMM to measure the current in each string (heavy loads, heavy charging). And parallel strings do have some internal positive feed back which can cause unbalanced current flow/charging (hot batteries require lower charging voltage, etc.).

Otherwise, the big reason to pick 12/24/48 volt bank (besides paralleling issues). If you have a lot of 12 or 24 volt native loads, you may have a reason to pick that voltage. If you have all native 120/240 VAC loads--Then it based on the choices of charge controllers/inverters. Higher voltage inverters tend also to have higher wattage ratings and are more expensive (and many times, have more options like networking, AC1/AC2/Autostart/etc. options, etc.).

I like to use the 1,200 watt limit for 12 volts, 2,400 watt for 24 volt systems, and over ~2,400 watts go to 48 volt battery bank. A 1,200 watt AC inverter+load needs >100 amps of DC current and a >~168 amp branch circuit+DC breaker/fuse -- That is not a small amount of copper.

Running at 48 volts cuts the current (vs 12 volts) to 1/4 on the DC side--And makes it much easier to send DC power longer distances (less copper, lower voltage drops, etc.).

And for Solar Charge Controllers, they are rated on their DC output maximum... A 60 amps charger running at 12/24/48 volts can manage:

60 amps * 59 volts charging * 1/0.77 panel+controller deratings = 4,597 watt array typical maximum
60 amps * 29.5 volts charging * 1/0.77 panel+controller deratings = 2,299 watt array typical maximum
60 amps * 14.75 volts charging * 1/0.77 panel+controller deratings = 1,148 volt array typical maximum

So--A higher voltage battery bank can reduce the number of solar charge controllers needed too...

-Bill

Re: Home off grid

Panel tilt is helpful for up North--And where you have significant snow fall and want the panels near vertical to keep them clear of snow.

You can play with PV Watts and see what works for you (more tilt in winter, less in summer)... This is a 70 degree tilt (from horizontal) for LV NV and fixed array:

Month    Solar Radiation (kWh/m 2/day)
1      5.61     
2      5.96     
3      5.81     
4      5.58     
5      4.93     
6      4.54     
7      4.71     
8      5.27     
9      6.16     
10      6.34     
11      6.13     
12      5.61     
Year      5.55      

You would get ~10% more hours of sun in the winter with 70 degree tilt. Or install at ~36 degrees with 10% more panels and never bother with tilting.

Me, I am lazy. And I worry a bit about high winds with an extreme tilt if I did not otherwise need to tilt.

Regarding more panels--If they are within ~10% or better on Vmp--Then you can connect them in parallel. If the current matches within 10% or better on Imp, then you can connect in series.

You have to look up the specs. for your current/new panels and see... Many times, the 10 watt different panels are really identical--The specs +/- specs have been tightened up and allow them to claim a few more watts. But sometimes, there is a major model change and the Vmp/Imp are quite different.

In some cases, Vmp/Imp are different enough where you have to get a second MPPT charge controller to add more panels to an existing system. Just the way the electrical works out.

-Billh

Re: Home off grid

You can also get Water Miser caps too... I believe there are even "taller caps" that may capture/retain more of the electrolyte mist (NAWS does not list those on their website--But another person here got a tall set and was very happy).

If you are going to store food and other important stuff in the semi-basement, then you probably want to wall off the battery area to keep the fumes from getting in the rest of the home.

Remember, you are going to be checking electrolyte levels, wiring, replacing a set every ~10 years, etc... So, you will want easy access to the batteries and an ability to wrestle them in/out of the installation.

-Bill

Re: Home off grid

BoFuller wrote: »

So if I go with 48 volt and 16 batteries, that's two strings of 8, correct? And I should fuse both strings? I can do that. With 16 batteries in the crawl space (I'll put in a ventilation fan), what size opening, and how many, do I need the concrete guy to leave in the stem wall before he finishes it (for the cables going to the unit mounted outside on the wall)?

Regarding fusing per string... If you have three or more battery strings, definitely... With two strings in parallel--It is less clear that you need two fuses/breakers for the bank.

I would still suggest it though--There is nothing like having a dead short somewhere and watching the battery wires glowing red with nothing handy (ax or bolt cutters) to open the circuit.

You want the system as safe as you can design it... It will be powered 24x7 for the next 10-20+ years and you don't want something to happen (especially when people are sleeping or nobody is there).

-Bill