Call for Input / Opinions

Hello,
I am attempting to install a very basic solar system.
The goals are as follows:
1. first and foremost, to learn by doing
2. be able to expand and increase capacity in the future
3. Power one CFL light and one pedestal Fan no more than 4 hours/day, no more than 2 days/week

I think my power requirements are fairly low - Peaking at roughly 300wh/day and averaging just under 100wh/week. I am much more interested in learning the basics of the system than providing a certain quantity of power.

I intend to use a single ONYX 60w PV module.
I am married to this PV, mostly because I can get it locally without paying shipping.

I plan to run that to the Blue Sky SB2512ix Charge controller. I think it has the capacity to handle my modest requirements, it can do an equalizing charge, and I like the idea of the independent display.

I also plan to use a single 12v 120AH Lead acid battery.

I suspect that I need to install a Ground Fault Interrupt in the negative conductor between the Charge controller and the battery, and Circuit Breakers in all the negative conductors (in order to be able to isolate each component)

Does this sound plausable? Am I overlooking something important? Please let me know!

Thanks -
-Cale

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Call for Input / Opinions

    Hello & Welcome!
    I'll toss out a few observations in bold, just to get the ball rolling. :D
    rugburn451 wrote: »
    Hello,
    I am attempting to install a very basic solar system.
    The goals are as follows:
    1. first and foremost, to learn by doing
    Good. You'll probably learn a lot. Although maybe you won't like what you learn. :p
    2. be able to expand and increase capacity in the future
    This can be very difficult. The more different your "now" and "future" needs are, the less likely this will be feasible.
    3. Power one CFL light and one pedestal Fan no more than 4 hours/day, no more than 2 days/week.
    Here you need to get a fix on real Watt hours. The purchase of a Kill-A-Watt (or similar) meter is a good investment, as it can tell you a lot about how much energy you're using everywhere.

    I think my power requirements are fairly low - Peaking at roughly 300wh/day and averaging just under 100wh/week. I am much more interested in learning the basics of the system than providing a certain quantity of power.
    I'm going to assume you mean 1000 Watt hours per week, as 300 per day * 7 = 2100. Don't confuse maximum Watts with Watt hours: 1000 Watts used for only 6 minutes is 100 Watt hours.

    I intend to use a single ONYX 60w PV module.
    I am married to this PV, mostly because I can get it locally without paying shipping.
    Good idea: never invest mega $ on learning/educational systems. :D

    I plan to run that to the Blue Sky SB2512ix Charge controller. I think it has the capacity to handle my modest requirements, it can do an equalizing charge, and I like the idea of the independent display.
    Perfectly good charge controller, but might be over-sized.

    I also plan to use a single 12v 120AH Lead acid battery.
    Ah, but is that the right size for your needs? (See below).

    I suspect that I need to install a Ground Fault Interrupt in the negative conductor between the Charge controller and the battery, and Circuit Breakers in all the negative conductors (in order to be able to isolate each component).
    Er, no. Learning system = disconnects and fuses as needed. Keep it simple.

    Does this sound plausable? Am I overlooking something important? Please let me know!

    Thanks -
    -Cale

    Right. Let's begin with the basic design formula.
    First, determine your potential loads. This includes the maximum Watts anything & everything will draw at any one time. In your scenario, that looks like a 13 Watt CFL + a 40 Watt (?) fan = 53 Watts. That can be handled by just about any inverter. Even an MSW (with efficiency losses). The other factor here is time, as in how many hours per day each device will run. This gives you a daily Watt hours number on which you can base your battery size. Your proposed 120 Amp hour battery would supply (roughly) 60 Amp hours at maximum 50% depth of discharge or around 720 Watt hours. That would be all 53 Watts for roughly 13.5 hours.
    Second, once you've got your inverter and battery sized you can determine what's needed to recharge it. Your particular site insolation is a big factor here, but on a rule-of-thumb basis that battery would want 12 Amps peak @ 14.2 Volts or 170 Watts (after derating - 221 without typical derating). Now it could be as low as 6 Amps for the minimum recommended 5% charge rate. In a best-case scenario you're looking at 110 Watts of panel. Preferably more. Two of the panels you mention should work.
    As you can see from that, the Bluesky could be bigger than you need. It comes back to that "future expansion" plan at this point. If you've got some ideas on what your end goal is we can speculate on some paths to get there without spending too much or having to toss everything you started with for something completely different.
  • BB.BB. Posts: 27,898Super Moderators, Administrators admin
    Re: Call for Input / Opinions

    As Marc says, know your loads/requirements--and don't forget conservation FIRST.

    It costs more to generate a watt with solar PV than to conserve a watt. Do everything you can to reduce your loads (LED vs filament lamps, small laptop computer vs big desktop with screen, small DC ceiling fan vs 120 VAC box fan, etc.).

    Second, here are a couple threads that you may find interesting. The first starts out from ground zero... The second shows the issues of installing a solar array in a small RV trailer.
    Both are small systems around the size you are looking at--so they should be right up your alley.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Call for Input / Opinions

    Thanks for the speedy replies!

    @ Marc:
    I used the kill-a-watt and found 18Wh for the CFL and 56Wh for the fan. I then multiplied this by the daily usage (4 hours per day) for a total of 296Wh per day. This is where I got my peak usage number.
    For my average usage (no more than 2 days per week) I multiplied the Peak usage by 2/7 for 85Wh per day. I did however misrepresent that in the previous post as just under 100/Wh per week.
    That should have read more like "average just under 100/Wh per day for 2 days/week: 200Wh/week."
    Thanks for catching that, after the changes, does It seem like I have done these calculations correctly?
    Based on these (hopefully) more accurate usage numbers, do you still think I will need a second PV module?

    @ Bill:
    Thanks for those links! They do appear to be right up my alley.

    Once again, thanks for the help!
    -Cale
  • BB.BB. Posts: 27,898Super Moderators, Administrators admin
    Re: Call for Input / Opinions

    There are many ways to address the question... A big issue is the Lead Acid battery. They don't like to be operated below 20% state of charge--so you never want to take the battery dead. Try to avoid going below 50% state of charge often (reduces cycle life). And try to avoid spending days/weeks below 75% state of charge to reduce sulphation.

    Adding more batteries, a typical suggestion, is almost always a bad idea. With a large battery and a small solar panel, the battery tends to never get fully charged and/or the battery sets for long periods below 75% state of charge (sulfate crystallization). For flooded cell batteries, too small of charging current does not "mix" the electrolyte well and you get dense electrolyte at the bottom of the cell and mostly water at the top (stratification). So we end up with the 5% to 13% rate of charge rule of thumb (for "tall batteries" 10% is usually the minimum rate of charge).

    What this all comes down to, is most people have way fewer solar panels that they really need for good battery life. But batteries are cheap and for some applications (RV, weekend cabin) it is sometimes hard to justify (or even fit) a lot of solar panels and replacing the battery bank every few years is just part of the costs (or carrying a small AC genset and battery charger to charge during the morning and help the solar panels keep up).

    Ok, now some numbers. You did not say where the system is located (Illinois?) or what time the system will be used... Let's assume that this is a three season system and that you have (conservatively) 4 hours of sun per day minimum (winter, you may be down to 2 hours, summer can be 6 hours for many places).

    296 Watt*Hours per day, 85% efficient inverter, 2 days of no sun, 50% maximum battery discharge (for long cycle life):
    • 296 WH per day * 1/12 volt batt * 1/0.85 invtr eff * 2 days of no sun * 1/0.50 max discharge = 116 AH battery bank @ 12 volts
    So, your battery choice (assuming it is a deep cycle battery) is a good fit.

    Based on your power needs, assuming total system efficiency of 0.52 (panels, charge controller, battery, inverter) the minimum solar array should be:
    • 296 Watt*Hours * 1/0.52 system eff * 1/4 hours of sun per day = 142 watts of solar panel (Vmp~17.6 volts)
    So, two to three solar panels would be my recommendation--Really three (180 watts minimum). You cannot use 100% of your "predicted power" every day--142 watts of solar panel is about the power level where you may need to use a genset based on 4 hour per day sun if you have a consistent 300 watt per day load.

    Looking at the amount of solar panel to keep a battery happy--5% is the minimum rate of charge and 13% is around the cost effective maximum rate of charge for a solar panel system (you can go higher, but the battery will begin to limit current and the extra panel capacity may not be used).
    • 120 AH * 14.5 volts charging * 0.05 * 1/0.77 panel+chrgr eff = 113 watts of panels
    • 120 AH * 14.5 volts charging * 0.10 * 1/0.77 panel+chrgr eff = 226 watts of panels
    • 120 AH * 14.5 volts charging * 0.13 * 1/0.77 panel+chrgr eff = 294 watts of panels
    So, from the size of battery you have--The optimum amount of solar panel for charging a flooded cell battery is around 230-300 watts.

    And that is what usually ends up happening--The amount of solar panels to keep a large battery bank "happy" is larger than the amount of solar panels to keep the load going during the sunny months of the year.

    So--the long way around--I would suggest the optimum setup for a weekend cabin would be around 180-230 watts of solar panels--or three to four panels total.

    If you are trying to keep the panels to a minimum and won't need the fan during poor weather and accept possible lesser battery life and/or using a genset once in a while to keep the battery charged, 120 watts or two 60 watt panels will work.

    Was the above helpful or confusing...:roll::confused:;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Call for Input / Opinions

    Bill,
    That was incredibly helpful; I had completely neglected charge controller efficiency.

    I am starting to get the impression that another PV module might be necessary.
    I will revise my plans to include a second 60W panel identical to the first.

    I dont think I can get up to the 300W ballpark while staying in budget - but the ability to expand is important to the project. I think I will consider the first 2-module fixture an experiment and eventually add a second 2-module fixture.

    Thanks for the advice!
    -Cale
  • BB.BB. Posts: 27,898Super Moderators, Administrators admin
    Re: Call for Input / Opinions

    Cale,

    The the ~95% efficient charge controller number that is typically thrown about sounds like something that can be ignored... But when you take every little over estimation (like solar panel performance) and each conversion loss (inverter losses, inverter idling losses, battery cycling, wire drop, voltage/current mismatches, etc.)--The actual end to end efficiency from panel rating to useful AC power out the inverter is pretty much, in the best of times, 50%

    Add that long term weather (year over year) variations can be easily 10-20% more or less sun--doing calculations to 3 decimal points is sort of silly.

    And using conservative estimates/rules of thumb will, generally, result in a much more happy user :D and a much more expensive system :cry:.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Posts: 5,108Solar Expert ✭✭✭✭
    Re: Call for Input / Opinions

    A fairly representative back of the napkin calc that I like to use is the 50%*4 rule.

    That is if you take 50% of nameplate rating of a Pv panel or array, then multiply that number by 4 to represent the average hours of good sun one might expect per day over the course of the year you will get a fairly reliable number.

    The 50% represents all cumulative system loses including PV/charge controller/wiring/basic battery charging ef/ inverter loses etc.

    The 4 represents good sunshine, averaged with clouds, seasons, nominal shading etc.

    So your 60 watt panel might net/net produce something like this.

    60/2*4=120 watt/hours/day.

    Tony
  • System2System2 Posts: 6,290 admin
    Re: Call for Input / Opinions

    @icarus and @bill:
    I appreciate the napkin calculations. I get the impression that the system-wide efficiency coefficient of .5 is the one to use.
    Using this, in December (2.9 sun hours) I should generate roughly 90Wh per day.
    Enough to run the light for 4 hours, but no fan - which is ok because It will be too cold to warrant the use of the fan.
    In June I should generate roughly 180Wh per day - enough to run the light and fan for about half of the desired time.
    However, I only require 2 days per week of usage. For non-consecutive days, I think I should be able to expect near-complete replenishment for the 4hour/day usage scenario.

    Sound right?
    Thanks!
    -Cale
  • BB.BB. Posts: 27,898Super Moderators, Administrators admin
    Re: Call for Input / Opinions

    The thing to watch out for is that if you take quite a while to recharge back over 75% state of charge--you can get sulfate crystallization and early loss of battery capacity.

    If you stick with a single 60 watt panel--you are in the area where the battery is not getting enough charging current and self discharge can become a significant load in its own right.

    If you only want a 60 watt panel for now and less loads--Your next battery should be about 1/2 as large too.

    The 5% minimum rate of charge does cover a multitude of sins.

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