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  • Min/max for solar panels and batteries

    While looking at everything, I have a lot of general questions after reading posts in the forums and other articles to be sure I'm thinking about solar in the right way.

    A solar panel and inverter seems like a generator that takes less effort and more space to make/use with a varying output because of the sun being it's fuel source and buffers could be used to steady the electricity they produce. Right?

    Is an output at 12.5% of the rated watts on a solar panel with an inverter a good way to think about the minimum electricity produced and 80% as the max during daylight hours?
    I'm asking because I was reading that solar panels could produce 15% of their rated watts in perpetually cloudy areas and the inverters can reduce the amount of watts that go in then out of them by 20%. The amount of hours in a year is about 8,765, but the calculated times for daylight and averaged cloudy (greater than 30% cloud cover) or not cloudy (less than 30% cloud cover) hours varied by location.

    Is an output at 60% of the watt hours that go into a battery a general way of thinking about the minimum electricity that will be coming out and 90% as the max?
    I'm asking this because I was reading about batteries reducing the amount of watt hours that go in then out of them depending on how they were charged and type.

    Is there a way to charge a battery to 80% of it's rated watt hours in the same amount of time it would take to quick charge it to 100%?
    I'm asking this because I was reading about how quick charging caused more wear/tear on batteries and charging at 80% of the rated watt hours takes about the same amount of time to charge the remaining 20%.

  • #2
    I think you are overthinking it

    PV - Gridtie inverter is, overall, about 75% of the PV nameplate. Only for sunny days. Clouds drop it dramatically.

    PV - Batteries - inverter is about 50% efficiency of the PV nameplate, because of battery recharge losses. Clouds drop it dramatically.

    Is there a way to charge a battery to 80% of it's rated watt hours in the same amount of time it would take to quick charge it to 100%?
    When most batteries are low, they can accept a lot of amps, till they get to about 80% full. Then the amps must taper off as the battery begins to gas and slowly absorb the final 20% No quick charge to 100% Sorry.
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

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    • #3
      more questions

      Thank you. Good to know the overall grid tie inverter and inverter-battery percentages of the PV nameplate. Knowing the overall, min, and max modifiers for each thing and the reasoning is helping me to make sense out of it all for sure. I think that I could be considered as an over thinker sometimes while thinking about the technical things. Thank you for the clarification about quick charging. I see now that the large amount of amps, close to the rated amp hours if depleted enough, at a given voltage that's higher than the current battery voltage could be used at the beginning of the charge. Then the amps would need to be progressively lower to avoid dangerous levels of heating and gases. Trying to keep the temperature of the electrolyte below 115F after starting the charge at 85F seems important as well. It looks like a grid tie inverter puts as much power that the solar panel can produce towards the house, then, if less power is being used in the house, the grid tie inverter feeds the grid.

      I think this is going to sound weird, is there an overall formula to take in all the power from a solar panel through a whole day, on any day of the year and keep a battery between 20-80% while putting out a certain number of watts, or a range of watts that I could choose perhaps, through a grid tie inverter or inverter for off grid?

      Would it be better to angle a solar panel towards the afternoon sun during winter, summer, somewhere in between, or does it matter to have the most power output through out the year? I'm seeing that a stationary solar panel increases from a min of 0% watts at sunrise, to a max of 100% in the afternoon, then 0% at sunset again in the shape of a bell type curve.

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      • #4
        Originally posted by kb3 View Post
        I think this is going to sound weird, is there an overall formula to take in all the power from a solar panel through a whole day, on any day of the year, and keep a battery between 20-80% while putting out a certain number of watts, or a range of watts that I could choose perhaps, though a grid tie inverter or inverter for off grid?
        Sure there is or else it would be useless. Once you know the formulas and the cost you might conclude it is still useless.
        MSEE, PE

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        • #5
          Originally posted by kb3 View Post
          Thank you. Good to know the overall grid tie inverter and inverter-battery percentages of the PV nameplate. Knowing the overall, min, and max modifiers for each thing and the reasoning is helping me to make sense out of it all for sure. I think that I could be considered as an over thinker sometimes while thinking about the technical things. Thank you for the clarification about quick charging. I see now that the large amount of amps, close to the rated amp hours if depleted enough, at a given voltage that's higher than the current battery voltage could be used at the beginning of the charge. Then the amps would need to be progressively lower to avoid dangerous levels of heating and gases. Trying to keep the temperature of the electrolyte below 115F after starting the charge at 85F seems important as well. It looks like a grid tie inverter puts as much power that the solar panel can produce towards the house, then, if less power is being used in the house, the grid tie inverter feeds the grid.

          I think this is going to sound weird, is there an overall formula to take in all the power from a solar panel through a whole day, on any day of the year and keep a battery between 20-80% while putting out a certain number of watts, or a range of watts that I could choose perhaps, through a grid tie inverter or inverter for off grid?

          Would it be better to angle a solar panel towards the afternoon sun during winter, summer, somewhere in between, or does it matter to have the most power output through out the year? I'm seeing that a stationary solar panel increases from a min of 0% watts at sunrise, to a max of 100% in the afternoon, then 0% at sunset again in the shape of a bell type curve.
          The cost of a "tracking system" to keep your panels perfectly aimed at the sun all day will yield a small additional percentage of power generation but will easily cost more due to the expense of building and maintaining the "tracking system".

          The math works out that just adding a few more panels to "fixed" system will give you more output than what the "tracking" system gives you as long as you have them aimed South at the correct angle (depends on where you live).

          And a solar grid tie system will have a much lower $/kWh cost then a solar off grid battery system no matter how you aim your panels.

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          • #6
            The PVWatts site will allow you to enter the same panel information for a fixed array, a single-axis tracker, and a dual axis tracker so that you can see for yourself exactly what difference it makes at different times of the year for your latitude.
            Just keep in mind that when you use a tracker you cannot put the panels as close together (on separate tracker units) as you could for a flat fixed array where you did not have to worry about one unit shading another.
            For low latitudes the single horizontal axis array of many panels on one pivot rod can be an interesting option, and is often used for large scale commercial arrays. The array just rotates from east facing to west facing during the course of the day while never actually facing directly toward the sun on the north-south axis.
            SunnyBoy 3000 US, 18 BP Solar 175B panels.

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            • #7
              Thank you for the clarifications and tools Sunking, SunEagle, and inetdog. I see now that trying to keep the batteries at 20-80% on any day of the year while drawing power from only the batteries would take a bigger battery bank, efficiency loss, and last about 5 years or less. I'm trying to think of a general formula, or part of a bigger formula, that could calculate the min and max power that may be expected from the solar panels to keep a buffer, like batteries, at 20-80% through out the year at a constant drain, or a range of drains. I like the idea of not using the power directly from the solar panels for distribution because of the power fluctuations but it's more expensive and I agree, after seeing the costs, I might conclude that it is still useless.

              Thank you for the information about adding a few more panels instead of using a tracking system, the $/kWh being much better with a grid tie versus the off grid system, different types of solar trackers, and about the PVWatts site.

              I can see how adding a few more panels would be better than maintaining a solar tracking system and the money spent per kWh on a grid tie inverter is much less than the off grid system mostly because of the expense of needing a buffer, like batteries, with the off grid system.

              I really liked the PVWatts site and seeing how close I was to the other calculations being made. The solar trackers seem like something nice to have. I vaguely remember a solar tracker from a long time ago that passively detected the difference in power output between the solar panels to slightly adjust the array directly at the sun, where ever it was, then return to it's original position when there was no sun. It only used power when needed and seems like it wouldn't need me to adjust the timing for anything. Things like clouds and maintenance did seem like they would be an issue.

              Would a flywheel be better than batteries for a solar system?
              I was asking this because I saw a flywheel, about as big as a refrigerator but shorter, being developed that said it was going to be able to store 15 kWh per day. It seems like it would last longer, be less expensive, and more efficient than batteries in certain ways.

              Comment


              • #8
                Originally posted by kb3 View Post
                Thank you for the clarifications and tools Sunking, SunEagle, and inetdog. I see now that trying to keep the batteries at 20-80% on any day of the year while drawing power from only the batteries would take a bigger battery bank, efficiency loss, and last about 5 years or less. I'm trying to think of a general formula, or part of a bigger formula, that could calculate the min and max power that may be expected from the solar panels to keep a buffer, like batteries, at 20-80% through out the year at a constant drain, or a range of drains. I like the idea of not using the power directly from the solar panels for distribution because of the power fluctuations but it's more expensive and I agree, after seeing the costs, I might conclude that it is still useless.

                Thank you for the information about adding a few more panels instead of using a tracking system, the $/kWh being much better with a grid tie versus the off grid system, different types of solar trackers, and about the PVWatts site.

                I can see how adding a few more panels would be better than maintaining a solar tracking system and the money spent per kWh on a grid tie inverter is much less than the off grid system mostly because of the expense of needing a buffer, like batteries, with the off grid system.

                I really liked the PVWatts site and seeing how close I was to the other calculations being made. The solar trackers seem like something nice to have. I vaguely remember a solar tracker from a long time ago that passively detected the difference in power output between the solar panels to slightly adjust the array directly at the sun, where ever it was, then return to it's original position when there was no sun. It only used power when needed and seems like it wouldn't need me to adjust the timing for anything. Things like clouds and maintenance did seem like they would be an issue.

                Would a flywheel be better than batteries for a solar system?
                I was asking this because I saw a flywheel, about as big as a refrigerator but shorter, being developed that said it was going to be able to store 15 kWh per day. It seems like it would last longer, be less expensive, and more efficient than batteries.
                Some of the big Utility size pv arrays use a tracking system. Apparently they have determined that the additional output justifies the expense of keeping the tracking system working. They are on the scale of Megawatts so maybe there is a break point when you get a lot bigger than a home sized system.

                As for the flywheel. We have discussed that technology in length in other postings on this forum. It has been around a long time and someone is always looking for a way to improve it's "storage" capabilities. Again like the "holy grail battery" that will solve our energy storage problem the flywheel technology hasn't been found yet and may never be. There are too many physical limitations (gravity and friction to name 2) to make a rotating mass either efficiency useful or economic to store a large amounts of energy. Flywheels just aren't that good yet.

                Comment


                • #9
                  Originally posted by kb3 View Post
                  Would a flywheel be better than batteries for a solar system?
                  I was asking this because I saw a flywheel, about as big as a refrigerator but shorter, being developed that said it was going to be able to store 15 kWh per day. It seems like it would last longer, be less expensive, and more efficient than batteries in certain ways.
                  Flywheels have been around for a long time just for this use. They have never been first choice or even 10th choice for various reasons.

                  There are new materials on the market all the time but the mass is what makes a flywheel work.

                  What you see blogged about is mainly either someone tilting windmills or a huckster fishing for suckers.
                  [SIGPIC][/SIGPIC]

                  Comment


                  • #10
                    Originally posted by russ View Post
                    What you see blogged about is mainly either someone tilting windmills or a huckster fishing for suckers.
                    Tilting windmills usually reduces their output when the prevailing wind is nearly horizontal.
                    What Don Quixote did was tilting (jousting) at windmills.
                    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                    • #11
                      Originally posted by inetdog View Post
                      Tilting windmills usually reduces their output when the prevailing wind is nearly horizontal.
                      What Don Quixote did was tilting (jousting) at windmills.
                      But with horizontal tilted windmills all that hot air talking about flywheels will rise and make them go faster.

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                      • #12
                        Originally posted by inetdog View Post
                        Tilting windmills usually reduces their output when the prevailing wind is nearly horizontal.
                        What Don Quixote did was tilting (jousting) at windmills.
                        These were VAWT types
                        [SIGPIC][/SIGPIC]

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