100W Paper Design

asteroidsasteroids Registered Users Posts: 17
As a paper exercise, what would it take to run a 100W light bulb indefinitely?

Lets assume south facing sunny Arizona where every day is cloud free.

We want a highly reliable system with no single point of failure.
A remote unattended system with as little maintenance as possible.
Let's also assume a 50% overnight discharge on batteries.

Are PWM controllers more robust and reliable that MPPT?
Would a dual 12V be the way to go? Assuming so....

My back of the envelope calculations show approx two 800W panels feeding a 400AH (AGM?) battery bank.

I am little bit puzzled as to what to do at the inverter stage.


  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: 100W Paper Design
    asteroids wrote: »
    Lets assume south facing sunny Arizona where every day is cloud free.

    We want a highly reliable system with no single point of failure.

    Let's also assume a 50% overnight discharge on batteries.

    Too much contradiction... if you discharge 50% overnight, one cloudy morning means you will over discharge your battery. That is a single point of failure.

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • LandKurtLandKurt Solar Expert Posts: 41
    Re: 100W Paper Design

    A rule of thumb 50% system efficiency would mean your 2400 Whr load would need 4800 Whr panel rating. Assuming 4 hours of sun, that’s 1200 watts of panels. But since you are assuming cloudless Arizona sky you could cut that in half and just assume 8 hours of sun on 600 watts of panels. Without looking up the actual average hours of sun in Arizona I’m going guess reality will require close to 1000 watts of panel for that load.

    How serious are you about the “no single point of failure”? You could go with two separate arrays of panels on separate controllers. Your battery bank could use two strings of batteries.

    Did you want to go as far as two inverters on separate battery banks fed by separate controllers and arrays? Having them feed the same AC circuit would be complicated. Possibly some of the smarter inverters can cooperatively power an AC feed panel. Someone else will have to answer that. But then the circuit breaker on your AC circuit is a single point of failure. I will note the some enterprise computer equipment come with dual power supplies that could be fed by separate circuits.

    Maybe you should just use two “light bulbs” on completely separate solar power systems. While you’re at it why not use different manufacturers for the components of the two systems: panels, charge controller, batteries, and inverter. Then your single point of failure is the sun, or more reasonably the weather.
  • asteroidsasteroids Registered Users Posts: 17
    Re: 100W Paper Design

    This is a paper study, I want to understand certain issues and tradeoffs. Separate solar panels and batteries combining at the "stacking" inverters that feed the mythical 100W light bulb. Does anyone have any experience with PWM controllers? Are they bulletproof? They may not be as efficient as MPPT but they seem to be very cost effective. Of course the real magic (and expense) comes at the inverters that provide proper AC without blowing up the light bulb.
  • BB.BB. Super Moderators, Administrators Posts: 29,350 admin
    Re: 100W Paper Design

    Nothing is bullet proof.

    A 100 watt filament lamp will last from 800-2,500 hours or so.

    A battery bank with "golf cart" type batteries will last ~3-5 years.

    Solar Charge Controllers and AC Inverters will last around 10+ years (but there can be early life failures too)

    Solar panels can last 20+ years, but can also fail in less than 6 years too (ask me).

    Chances of failures multiply-- 99% up-time * 99% * 99% * 99% * 99% = 0.95% up-time

    You double the number of components, your chances of failures double.

    You run 100% of the time from "A" -- then switch to "B" and find "B" has already failed--So you need to ensure the backup is always working when in standby.

    One way to do this is to run "A" for one day, then run "B" for the second day--So both systems have equal use. However, eventually one system will fail as it "powers up" (running 24x7 systems such as voice mail--Our techs quickly figured out the power supplies are much more likely to fail when they power cycle the system--Power surges from starting stress already weakened components).

    Temperature is another important element of longevity... First there is absolute temperature. If you "do the math" you will find the activation energy of most materials have a factor of 2 increase in aging for every 10C increase in temperature. Raise operating temperature by 20C, life is cut to 1/4 as long. Likewise, running 10C cooler with lengthen life by 2x.

    And there is temperature cycling/thermal stresses. Keeping even temperatures will reduce thermal cycling stresses.

    There is a qualification test that some electronics companies use... "HALT" Highly Accelerated Life Testing--Run at high and low temperatures and cycle the temperatures quickly (some will use liquid nitrogen).

    So--Not only components, configuration, diagnostics, but environmental has significant affects on system reliability.

    The deeper you go into high reliability systems--The more chasing of your tail you do.

    Unless you are doing aerospace or medical (life critical) equipment--Frequently simpler is better. And accepting some level of failure with quick recovery may be "good enough" with significant savings in costs and complexity.

    For example, if you are powering a 100 Watt bulb--You can power that with DC, so you don't even need any AC inverters at all (remove a complex point of failure). etc. etc. etc.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LandKurtLandKurt Solar Expert Posts: 41
    Re: 100W Paper Design

    Let’s consider the failure modes you could tackle. Since you raise cost effectiveness as an issue you may want to consider the likelihood of each and the cost to mitigate it in your study.

    1. Panel failure

    Solar panels are generally wired in series as a string of panels then combined in parallel at a combiner box. A single panel failure or even a leaf blowing onto a panel can take out the production of the entire string. So plan on an extra string beyond what is necessary. For example: 1200 watts of panels in four 300 watt strings if you can get by with 900 watts of panels from three functioning strings.

    2. Charge controller failure

    Use multiple charge controllers in case one fails. You’d need separate banks of panels feeding each controller, and each bank might need an extra string if you wanted to survive a string failure combined with a controller failure. Multiple chargers can feed the same battery bank if you are leaving that as single point of failure.

    3. Inverter failure.

    This one gets tricky. You can’t just hook the output of multiple inverters into the same AC circuit unless they are designed for it. They exist, like the Outback VFX-3524, but they'll be a lot more expensive than the sort of inverter you’d normally buy to run a 100 watt “light bulb”. Just running a pair of big stacking inverters like that is going to add a standby load of 20 - 60 watts or so to your system. Which is not at all insignificant to a 100 watt load.

    4. Battery failure

    You’d have to research battery failure modes to decide whether multiple strings of batteries in a single bank would provide enough redundancy. But if you've got separate controllers and inverters already you might as will go with separate battery banks for them as well.
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