Solar system design questions

vdub411
vdub411 Registered Users Posts: 3
New to the solar world and have some questions.  I've been collecting bits and pieces for many years and finally have enough to do something with.  Thus far I have collected the following:
4x250w 24v Renogy Solar panels
Renogy Commander 40A MPPT Charge controller
26 x 115ah AGM batteries
and tons of UL PV wire, MC4 connectors, usage monitors, bus bars and almost unlimited amounts of any gauge wire.

A little background on myself and my project.  I've been in the Telecommunications industry for nearly 20yrs and tend to keep things headed to the dumpster.  I purchased a property next to my current one with a house and detached 2 car garage (was a shop for restoring corvettes).  The house was tore down and everything disconnected at the road but the garage remains.  I would like to make the garage a "In-Law retreat" for when visitors come to stay and a place to work on extra toys.  

Now for my dilemma.  So far nothing is hooked up.  I'm wanting to tie into the existing panel so all outlets work as if they were tied to the gird.  I'm not sure on my energy consumption as I have not had power out there but would like to keep it somewhat basic.  It's heated with wood.  The 4ft florescent lights have LED replacements waiting.  No stove and the fridge is from an RV so 110, 12v or propane.  

The inverters I've looked at are split phase 24v 240 by Samlex, GoPower and Aims.  I'm wanting to start out with 2000w and go from there.  Is that a good starting point?  I won't be running any 220 stuff to start off but would like to do so in the future.  Should I go 48v?  Is it possible to tie the solar panels in series and accomplish this?  

 This is just my experiment until I can figure everything out, then I will expand as necessary.  Any input would be greatly appreciated.  Thanks

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Welcome ot the forum Vdub411.

    Ok--Understand you have lots of material--But until you have figured out how much load you have, we really cannot do much yet.

    The only other option is to design the system around a major component (such as the solar panels you already have) and then estimate how much power would be available from such a system.

    And, the other "warning", Off Grid Solar power is expensive. If you have Grid Power available, off grid solar power will never save you money. In some cases, such as for folks that have very unstable utility power (long outages after ice storms, afternoon power failures, etc.), off grid solar power can make sense as a backup.

    Note that some items, like your RV refrigerator, are not really even good candidates for solar (or even grid tied power) for long term use. The RV fridge is probably ammonia/propane based unit. While running on propane, they are not bad (perhaps a 1/2 lb of propane per day). However, on 12 VDC or 120 VAC power, they are very inefficient--They just use the electricity to provide heat instead of a propane flame (using solar electric power to provide heat is usually inefficient).

    At this point, I do not understand enough about your needs to even begin to design a system. If you want to power some LED lights, charge a cell phone, run a laptop, charge some battery powered tools... We could make a guess and estimate your needs at ~1 to 3.3 kWH per day.

    A 1,000 Watt*Hour (1 kWH) per day system is pretty small and can be a good system to experiment with. A 3.3 kWH per day system, can be enough to run a full, very efficient, off grid cabin/home (lights, fridge, well pump, washing machine, laptop computer, etc.) and have a "near normal" electrical life. But a 3.3 kWH per day system is not small, and not cheap.

    Getting batteries for "free" can be interesting--But that gets us into questions about exactly what kind of batteries you have. Telecom batteries are typically designed for "float" operation and may not sustain daily deep cycle operations. And "small batteries" (such as your 115 AH @ 12 volt AGM batteries), are not a great solution for larger/deep cycling operation (lots of parallel battery strings, and especially AGM types, can make for a "unstable' battery bank--Difficulties with equal current flow between strings, lots of electrical connections, finding the occasional cell that goes back--shorted or open, etc.).

    And your solar panels are 60 Cell Vmp~30 volt panels... They are just fine, however, they do not "map well" to use a PWM (simple, inexpensive) charge controller to charge a 12 volt battery bank (really need Vmp~18 volts) or a 24 volt battery bank (Vmp~36 volts). So, you should use a MPPT type charge controller... These are very nice controllers, but they are not cheap (~$100 for PWM, ~$600 for MPPT soalr charge controller--Although, you can find less expensive import MPPT controllers of mixed quality).

    Just to give you an idea of what your 4x 250 Watt solar panels can do for you. In general, I would suggest a 5% to 13% rate of charge for your system... Weekend/seasonal use, you can use 5% rate of charge. For full time off grid daily use (9+ months a year), I would suggest 10%+ rate of charge.
    • 1,000 Watt array * 0.77 panel+controller derating * 1/0.05 rate of charge * 1/29.0 volts charging = 531 AH @ 24 volts maximum
    • 1,000 Watt array * 0.77 panel+controller derating * 1/0.10 rate of charge * 1/29.0 volts charging = 266 AH @ 24 volts nominal
    • 1,000 Watt array * 0.77 panel+controller derating * 1/0.13 rate of charge * 1/29.0 volts charging = 204 AH @ 24 volts minimum
    Lets say you go for 2x12 volt batteries in series, and 3x parallel strings for 345 AH @ 24 volt battery bank (I would recommend avoiding lots of parallel battery strings--1x strings is "ideal", and 3x parallel strings maximum (unless you have no other options)).

    A 345 AH battery bank, operated for 2 days of storage and 50% maximum discharge would give you around:
    • 345 AH * 24 volts * 1/2 days storage * 0.50 maximum discharge * 0.85 AC inverter eff = 1,760 WH per day (or over night) use
    Note, that a 2,000 Watt AC inverter would run for less than 1 hour per day if you followed our rules of thumbs.

    And normally for a 345 AH @ 24 volt battery bank, I would be suggesting a typical maximum of wattage of Solar Array and AC inverter of (345 AH / 200 AH per 1kW=) ~1.73kW or 1,730 Watt array and AC inverter typical maximum. However, for AGM Batteries, they support much higher surge/continuous loads than flooded cell lead acid batteries, so, in theory, you could support your 2kWatt (or even larger) AC inverter).

    However, you bank would still not support a 2kWatt inverter very long--Taking a battery dead (which ruins lead Acid/AGM batteries):
    • 345 AH * 24 volts = 8,280 Watt*Hours of storage (until dead)
    • 8,280 Watt*Hours * 0.85 inverter eff * 1/2,000 Watt inverter-load = 3.5 hours maximum operation (or less)
    For various reasons, recommend that you use only 1/4 to 1/2 of your stored battery power per day... And avoid taking your battery bank below 50% state of charge very often, and never below 20% State of Charge (deep cycles will "use up" a lead acid battery pretty quickly).

    And then, how much energy will a 1,000 Watt array produce... Lets guess you are near Olympia Washington, using a fixed tilt 1,000 Watt array:
    http://solarelectricityhandbook.com/solar-irradiance.html

    Olympia
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 43° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    1.68
     
    2.79
     
    3.52
     
    4.15
     
    4.55
     
    4.76
     
    Jul Aug Sep Oct Nov Dec
    5.21
     
    5.18
     
    4.68
     
    3.13
     
    1.95
     
    1.51
     
    Toss the bottom three months (use genset, cut back on energy usage):
    • 1,000 Watt array * 0.52 system eff * 2.79 hours of sun (Feb) = 1,451 Watt*Hours per day of 120/240 VAC power (February "break even" month).
    The above is what we call a "balanced" system design. And close to optimum for most people. If you have different needs (say use the system on weekends during summer days--We can do something different like a larger solar array). Or if you have "free" batteries, perhaps you want to discharge them to 50% state of charge every day--But then you would need a larger solar array to get them recharged by the end of the next day, etc...

    However, for a garage where you plan on using power tools/larger amounts of power (air compressor, etc.), I would highly suggest you think about reconnecting to the grid for the property, or bring power from your own home next door.

    If your power needs are random (a few weekends a month) a genset (plus gasoline powered air compressor, etc.) may be a more cost effective solution--And, if you want, use a 1,000 Watt*Hour per day system for "quiet power" and experimenting with solar (lights, laptop, radio, small tool charger).

    Your thoughts?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vdub411
    vdub411 Registered Users Posts: 3
    WOW!!!!  Thank you sir for all this great info.  I'll try to answer as much as I can.

    My needs for this property aren't much.  Just wanting to use it for my testing and understanding of solar before I go bigger.  Right now it's just used for storage but I would like the lights to work when I go out there.  It will probably be used a few times over the summer for guests to have a place to stay.  Currently the parcel is for my goats, chickens, garden and swim pond.  I have a hand pump setup for water and a composting toilet.  If i can run some lights, a radio and charge a phone or drill batteries it would be perfect.

    Thanks for the heads up on the RV fridge.  It was also free and works good in the pole barn.  Looks like that's where it will be staying.

    As far as the batteries.  I have access to some forklift batteries that are in the 400-450ah range.  The smaller telecom batteries were just something I had already.  Right now I charge them one at a time with 1 Renogy 100w panel and a PWM charge controller.  I have a 40A Renogy Commander MPPT charge controller for the larger array.  I keep a couple of the batteries in a yard cart with a 2000w junk Harbor Freight inverter that I drag around the yard when I need electricity.  They seem to work well for everything I've needed.

    I guess my main concern was the inverter for the new system.  I was looking at pure sine wave split phase inverters.  The only reason was so I could run my main electrical box like it was connected to the grid.  If I use a regular inverter I can only run one leg of the box.  Is that correct?  Trying to keep it is easy possible to begin with. 

    Not sure I answered it all, but will try to keep up.  Thanks again for the help.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    vdub411 said:
    I was looking at pure sine wave split phase inverters.  The only reason was so I could run my main electrical box like it was connected to the grid.  If I use a regular inverter I can only run one leg of the box.  Is that correct? 
    Not necessarily.  You may be able to run both legs... Depends on how the house is wired, in particular, whether the two legs share a neutral.  If they do, it could be overloaded.   Of course, if your inverter is small enough it won't be able to overload the wiring.  (i.e. a 1000 watt inverter cannot overload 14 gauge wire or even trip a 15 amp breaker).

    If your box has any 240 volt circuits, they should be disconnected.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    I will humbly suggest that off grid solar power systems do operate pretty consistently. If you have shade (trees, power lines, vent stacks, chimney, etc.), almost any shade will cut an array's output by 50% or more.

    Lead acid batteries are heavy, and flooded cell batteries need (at least) mo thy checking for water levels (deep cycle batteries are even a bit more inefficient and need more water, plus they tend to be heavier--12 or 24 volt modules that can weigh many hundreds or even a thousand pounds ore more).

    Off grid power systems require monitoring--And if you are not into checking/physically servicing it 1-4 times per month, then other options may work better.

    I am not quite sure by what you say when run your main electrical box like it was connected to the grid... As vtmaps says, you first have to understand if your loads are 120 or 240 VAC (and choose an inverter appropriate to those loads).

    Further on--When you start talking about main panel--Is this a standalone off grid power system, or is this your home and you currently have grid power? While it is possible to easily put together a DIY manual transfer switch setup--Non UL/NRTL/NEC rated transfer switches (no interlocks) can be quite dangerous (i.e., somebody flips both AC mains and AC off grid inverter on at the same time. Looking at lots of smoke and possibly fire/injury.

    Your present 1,000 Watt array would map very nicely to 4x 6 volt @ 200 AH "golf cart" batteries in series for a 24 volt battery bus, or 2x series + 2x parallel string for a 12 volt @ 400 AH battery bank. And golf cat batteries are ~$100 +/- a piece (and will last, typically, 3-5 years). Note that 12 volt @ 400 AH vs 24 volt @ 200 AH is exactly the same amount of energy storage (P=V*I, 2xV or 2xI does not matter, P is still the same). More or less, for various reasons I would suggest an 800 AH battery bank is about the maximum you want to aim for (800 AH @ 12 volts, think about 400 AH @ 24 volts instead).

    If you can get a 12 volt @ 400 AH fork lift battery (and can move it)--That would also be a good fit.

    You can use the telecom batteries (2x series + 2 parallel strings) to try and see how that works for you... I would suggest avoiding 4x parallel @ 12 volt batteries--That is just asking for learning why "we" suggest avoiding parallel strings (basically, keeping charge/discharge current equal, if cell fails in one battery how to find it, etc.). Not that it won't work--But it will make you work harder (cannot check specific gravity with sealed AGM batteries--Need to get a DC clamp meter to monitor current flow, etc.).

    If you want to really get a good understanding of off grid power (it really is very similar to a car or boat electrical system--The alternator replaced by solar panels for basic charging)--Flooded cell batteries are a better starting point. Mostly because you can monitor the specific gravity (gold standard for checking state of charge), and they tend to be more rugged (can withstand "over charging" better than AGM batteries).

    And get a few tools to help (links are starting points--You pick tools and website to use)...
    • Some sort of decent hydrometer (for flooded cell batteries)
    • An AC/DC Current Clamp DMM (for general debugging)
    • Kill-a-Watt type meter (for measuring/understanding AC power usage--Great for conservation efforts in your home)
    • DC AH/WH type meter (for measuring "smaller" DC load energy usage)
    • Battery Monitor (nice to have but not cheap, almost a requirement for AGM/Sealed Battery banks--Still need to "understand" how a Battery Monitor works)
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vdub411
    vdub411 Registered Users Posts: 3
    A little more info I can add to help you understand.  I've owned this location for 3yrs and have been tracking the sun year round since then. I have several locations in the yard where I get year round horizon to horizon sun. The property is surrounded by farm fields and the trees on the property are on the opposite side as the building and does not hinder my sunlight. (It's 3 acres so I have some wiggle room)

    What I mean by "as if still tied to the grid", imagine this.  You have a garage in the middle of farm field.  It was connected to grid power since the early 70's and was up to code.  You then have the feed line from the road removed, leaving behind a structure that is ready to be connected to grid power but would like to do so with solar power.  When I say "main box" I am speaking of my electrical panel which is still connected to all internal outlets and lighting but no possibility of electrical load since my power line was removed at the road. 

    I do understand battery maintenance fairly well as I was responsible for it at 60+ cell phone towers.  I've installed 10,000 amp DC power plants and bus bars, but they were designed already and I just followed a blueprint.  This of course was all -48v power but you get the idea.

    I have the following stuff I didn't mention:
    3 Fluke clamp meters
    5+ Fluke multimeters
    Fluke Thermal imager for checking crimps, H taps, C taps, etc
    5+ Kill-a-Watt meters (I use to monitor my on-grid property not the property I am installing this system on)
    2 Honda EU2000 gensets
    Several old military surplus gensets
    4 DC 6.5-100V 0-100A LCD Display Digital Current Voltage Power Energy Meter Multimeter Ammeter Voltmeter with 100A Shunt
    Several Battery racks with mounting for 16 of my AGM batteries each.
    And tons more stuff sitting around my other properties.

    The cost is not really an issue.  All of my properties, toys and vehicles are paid off.  My only bills are electric, gas, water, sewer and property taxes.  I don't really want to dump a ton into this setup as it's just my experiment until I go larger.  For my 40th birthday I have my eyes set on a 5k Grid tied system to offset electricity costs at my business building and would like to have a little better understanding before that time.  (Gives me 2 yrs to play with this other setup).  Sure I could pay someone to come and do all the work for me but then I don't learn anything.  Yes I realize a grid tied system and an off grid system are 2 very different beasts but if I don't do this stuff hands on it will never make sense to me.

    I'm sure as with most things, I'm making this far more complicated than it should be.  
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    With the main panel in the barn... If you only have 120 VAC appliances, you can wire up both black and red to the same 120 VAC Hot lead from the AC inverter... One warning is that if you have Black+Red hot + sharing a common white return... The common 120 VAC hot (black+red) will over current the white return wire (15 amp + 15 amp = 30 amp on the return). With 120/240 VAC split phase power, the black + red are 180 degrees out phase, and will "add up to zero" current in the common white lead (15 amps + (-) 15 amps = zero amps in common lead).

    Otherwise, some of other issues... If you choose MSW (modified square wave inverter) over PSW (pure or true sine wave inverter), there are some issues. MSW inverters tend not to be isolated AC output (if you ground the DC battery bank negative to system ground) and one of the MSW output wires (typical common White Neutral to System Ground), you will short out the AC MSW inverter.

    PSW (or TSW--true sine wave--same thing) inverters are isolated and you can earth bond the "common" AC output just like standard utility power.

    Otherwise--I think you have pretty much everything down... Just a matter of understanding your loads/power requirements and how much battery and solar panels you will need to power them... Or, picking (as I did with your 1,000 watts of solar panels), and building a system around that which will power those loads.

    Overall, it is very difficult to "grow" an off grid solar power system--If you want grow by more than 2x, then you usually need to look at a higher voltage battery bank (and AC inverter DC input rating), and such... So--In general, I suggest that you "be happy" with your basic system and build out a second one a few years down the road (keep the old one for the barn, sell off to somebody else, or even put on a RV, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset