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20kWp domestic installation with a twist

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  • #16
    Since you are trying to also charge a battery bank, why not use a grid-tied, battery back up inverter with a charge controller instead of micro-inverters? Something like the Schneider XW+ or Outback Radian. You can program them when and how much to sell back to the grid. Once the battery is full and you've reached your limit to the grid, the charge controller will turn off the solar. Sounds like you are trying to reinvent the wheel, there is already standard equipment available to do what you are trying to do.
    Solar Queen
    altE Store

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    • #17
      @ Amy@altE

      Thanks for your input!

      I know about the Schneider XW+, but I really like the benefits that comes with using micro inverters:

      - Longer warranty (typical 25 years) and they are expected to last longer even
      - Higher efficiency on the module level (though connecting several panels to one micro inverter might go a bit against this)
      - Not so sensitive to shading
      - Lower voltage on the roof

      It is not my intention to reinvent the wheel. I'm really hoping for an available solution out there that I've missed

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      • #18
        [QUOTE=Krisharley;140706 The limiting function could be done by doing rapid connect/disconnect ala PWM + DC/DC conversion to the panel array and then deliver maximum 250 W to the microinverter.[/QUOTE]
        The current output of a panel depends on sunlight only.
        What happen to the excessive current produced by the panel after the limited current sent through the microinverter???
        The excessive current has to go somewhere to do something, like a water heater load.

        Instead of the "PWM + DC/DC conversion" idea,
        another idea would be "monitoring the current input to the microinverter and using a chopper circuit to divert the excessive current to a dummy load (water heater)".
        The chopper control signal may be a PWM signal.
        8.2KW 32x(PVmodule+inverter)+online monitoring

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        • #19
          A couple of problems you will need to solve.

          1) Thermal expansion / contraction of the panel roof. The design must allow for the panels to grow and shrink as the temperature changes, without stress on the glass or the seal on the edge. The rack / sub roof will have to allow for this flex and still be water/ice proof. As long as your roof designer is sure to take this and the electrical considerations into account.

          2) Custom designed panels.
          a. Do not even consider Amorphous style panels. They degrade rapidly and then you get to replace them.
          b. Hermetically sealing the PV sandwich is fiendishly difficult. Be sure the designer warranty's them with a
          independent insurance policy that remains in effect after he is out of business.
          c. Electrical/Fire Certification. UL / CE / CCC or some other certification valid in your country will be needed to have both property insurance coverage and Utility safety approval to Inter-Tie to the Grid. Generally all components of a PV system, panels, racking, mounting hardware, wire interconnects, inverters, fuses & fuse boxes require approval. If the PV is not certified, it's possible the Utility will not allow.

          3) Micro inverters.
          a. Are ONLY designed to work with PV panels and the Grid. Batteries do NOT work with Micro Inverters. You would have to use a Mains powered charger to charge the battery bank, AND use a battery power inverter to power the house when the micro's shut down from low light. (Low solar light being dawn - 10am & 2pm - sunset depending on weather and panel angle)
          b. If they are made for ~250 w panels. they may not even "activate" on smaller panels. There are 2 electrical factors to calculate "wattage". Panel Volts and Panel Amps. If the panels do not produce the proper voltage, the inverter does not turn on. If the panel amps are insufficient, the inverter may just turn on, but not sell efficiently.
          c. The only way to "limit" micro inverters, is to switch them off, either on the DC side, or the AC side (at the circuit breaker)

          4) Wiring & cooling
          The roof/attic space must allow for cooling of the inverters, which expect to be in open air, shaded by the panel they serve. Seal them in the insulated attic and they may not be happy. Wiring (AC & DC) most likely must meet local code, in the USA, solar shingle systems have not been able to meet code wiring safety requirements because of exposed wires in the attic.

          5) Electrical.
          What you are asking for, to my knowledge, has never been done. So it's going to be very expensive. Expensive to the point of foolish. I'll not mention it again. There are existing commercially produced systems (Amy mentioned one) that can be programmed to keep batteries charged, not sell to the grid, and only use grid power when the batteries get low.

          6) Generator. Large battery banks are expensive. When there are several days of low light, and the grid fails, the batteries will need to be recharged, or they will be damaged. Nearly all battery installations require backup power.

          Sorry I don't have more positive info for you, but you have planned something that will be difficult. As long as you can address all the issues, you will be OK, unless some neighbor says your solar electric makes the fur on their cat fall off and you must cover the solar array with a tarp.
          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

          Comment


          • #20
            Interesting problem from the OP. As others have stated there is existing hardware that could perform along the line of logic proposed but from what I have read and heard from some experts, the technology to do the same using micro inverters is not here yet.

            While it may not be related to the solar electrical part of question, I still feel the roofing issue must be resolved first and fully identified before an electrical hardware solution can be found using either existing or newly discovered technology.

            Without knowing exactly how the solar panels are mounted or incorporated into the roofing system, any hardware selection now would be a guess and more than likely not meet the final requirements.

            I would suggest that the OP provide more detail about the "roof" and then we can provide options to the solar pv system hardware.

            Comment


            • #21
              The panels need to be separated from the roof - up off it to keep them cooler.

              As Mike said - expansion is a big consideration.

              The panels will NOT be the roof - sealing is next to impossible.

              All the connections need to be accessible in the event of a unit failure.
              [SIGPIC][/SIGPIC]

              Comment


              • #22
                Originally posted by russ View Post
                The panels need to be separated from the roof - up off it to keep them cooler.

                As Mike said - expansion is a big consideration.

                The panels will NOT be the roof - sealing is next to impossible.

                All the connections need to be accessible in the event of a unit failure.
                I remember someone posting here in the Forum about a new technology that he designed using "solar tiles". He was from Europe and thought he had a good design to make a "roof" into a solar pv system.

                The biggest problem that people pointed out to him was the lack of a secure roof to keep the rain out. There was also the issue of not having a way to keep the electrical contacts on each tile connected properly for a secure DC wiring circuit. I wonder if he was able to improve his design?

                Comment


                • #23
                  Originally posted by SunEagle View Post
                  I remember someone posting here in the Forum about a new technology that he designed using "solar tiles". He was from Europe and thought he had a good design to make a "roof" into a solar pv system.

                  The biggest problem that people pointed out to him was the lack of a secure roof to keep the rain out. There was also the issue of not having a way to keep the electrical contacts on each tile connected properly for a secure DC wiring circuit. I wonder if he was able to improve his design?
                  A typical European style roof is often an A frame to protect a flat roof which is the real weather protection for the dwelling it is easier. I have seen many roofs where the inclined roof is just a support for tile. My has steel tube rafters, a radiant barrier, 1" particle board, 6" of foam and then the tile - can't even start to get to the back of the tile.
                  [SIGPIC][/SIGPIC]

                  Comment


                  • #24
                    Originally posted by russ View Post
                    A typical European style roof is often an A frame to protect a flat roof which is the real weather protection for the dwelling it is easier. I have seen many roofs where the inclined roof is just a support for tile. My has steel tube rafters, a radiant barrier, 1" particle board, 6" of foam and then the tile - can't even start to get to the back of the tile.
                    Go it. I didn't realize that their roofing was a totally different building style then the standard ones in the West.

                    So with that type of "roof over roof" the solar pv covering would not have to be water proof but would have to meet wind and possible snow loading codes.

                    Comment


                    • #25
                      Originally posted by SunEagle View Post
                      So with that type of "roof over roof" the solar pv covering would not have to be water proof but would have to meet wind and possible snow loading codes.
                      Except here where codes are only general guidelines - like traffic lights and stop signs - everyone considers them to be "advisory" only and totally ignores them.
                      [SIGPIC][/SIGPIC]

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                      • #26
                        Hello again

                        Thank you all for the feedback so far. I appreciate all your input.

                        Just to be clear about my background:

                        I'm an engineer and have worked with solar thermal for the better part of a decade where I have developed roof integrated solar thermal solutions. I'm also the co-founder of a company developing new waste heat recovery technologies based on solar thermal. On hobby level I have years of experience with photovoltaics and battery technology where I have built several functioning prototypes.

                        Right now the PV roofing concept is a hobby project, but I hope to take it further if I feel that it will make sense.
                        I have already had the first prototypes of the PV panel manufactured. The company that manufactures them also handles the certification process.
                        I can promise you that thermal expansion has been considered and that it won't have problems with water tightness, ventilation, replacement of panels etc. As it already was suggested the PV panels aren't functioning as the watertight barrier. The design is not yet protected in any way so I hope it is ok, if I don't go into detail with it just yet? My question was solely regarding the management of the oversized PV capacity.
                        That being said there are plenty of other PV roofing solutions out there - just google it

                        A dump load is certainly an option, but it just don't seem like an elegant solution.
                        My idea regarding the PWM approach is basically this: When a PV panel is disconnected it generates no current. For current to flow anywhere the circuit has to be completed. Therefore if you disconnect the panel for 50% of the time only 50% of the power will be passed on (to the micro inverter). There is no need to get rid of any excess power because it wasn't produced in the first place.

                        A micro inverter can easily be integrated with a battery solution. One example:
                        http://gwl-power.tumblr.com/post/398...note-this-is-a
                        Another: http://gwl-power.tumblr.com/tagged/MicroInv

                        Charge/discharge of batteries is handled by a good charge controller + BMS. This way the batteries will never get damaged by too low or high voltage. Modern battery technologies such as LiFePO4 are much more robust and compact than the traditional lead acid or gel ones. Soon we will see LTO batteries introduced to the market which will handle up to or even more than 20.000 charge/discharge cycles. I'm really looking forward to those

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                        • #27
                          1) inverter/battery link you supplied
                          Micro Energy Storage Setup
                          NOTE: this is a grid-tied system setup with a energy backup to the batteries. This system is grid-tied only. The micro-inverter works only when the grid is present. If the grid is down, the system does not work. It is NOT an off-grid system.
                          I'm not sure you are understanding it correctly. It WILL NOT do what you want.

                          Regarding your thoughts about batteries:
                          This way the batteries will never get damaged by too low or high voltage. Modern battery technologies such as LiFePO4 are much more robust and compact than the traditional lead acid or gel ones.
                          There are BMS systems designed for cars. None for household-solar use. When one channel goes bad, it frys the pack. If anything glitches, it frys the pack. Dead. Gone. Lead Acid, you refill with distilled and go on. LFP, you buy new batteries. LFP cannot be recharged below freezing, or you buy new batteries. (but oddly, you can discharge them while cold). The sales pitch sounds good, but when you comprehend the tech data, it's a whole new business. If you make a mistake, you buy new batteries. (Have I said it enough, likely not)

                          With solar PV, you dont worry about dump loads, you just disconnect and let them idle. The microinverters are easiest to just disconnect from the grid and they shut themselves down. You can use a SSR (solid state relay) controlled by battery voltage.
                          BUT WITH LFP, you can't use battery voltage, because is does not change enough, and when it hits the knee, it's almost too late. So you have to develop a watt counter, 500w output discharge allows 500w recharge, then kill the solar. Or just 25% of the solar. And you will have to have a timer to recharge late afternoon to be sure you have enough battery to last the night and the next cloudy day. And develop software for all this, without bugs, or you buy new batteries.

                          So this is going to be cutting edge, mistakes will happen, and you will learn. Knowing this, you can continue with a big fat wallet to spend lots of money. (and make a battery seller very happy)


                          Please read this long, 2 part blogish article from a boating/cruising LFP adopter.
                          http://www.pbase.com/mainecruising/l...n_boats&page=1
                          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

                          Comment


                          • #28
                            Dream land
                            [SIGPIC][/SIGPIC]

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                            • #29
                              @ Mike:

                              Thanks for the link.

                              BMS for household application right here: http://www.ev-power.eu/BMS123-System/
                              I have been using several other LiFePO4 BMS systems and I have yet to "fry" a pack. I have a 8S LiFePO4 pack with BMS/balancing next to me right now that has been running for a year now without problems. I use a charger + charge controller and the maximum voltage is controlled with +/- 0.1V accuracy. I haven't disconnected the setup from the power outlet for about 6 months.

                              Ideally the battery pack would last at least as long as the rest of the system. LTO batteries might be a valid solution in that regard: http://www.aertc.org/conference2010/...iesSECURED.pdf
                              Fingers crossed that they work this well in real life.

                              I completely agree with you regarding PV and dump loads. Your idea of disconnecting the whole array during mid day would be nice and simple. Just a shame that one cannot use the capacity of the PV array if the battery would drain completely during the day. I suspect this solution would require a rather large battery pack.

                              @ russ: Thank you for contributing with that insightful and non-sarcastic comment to this thread.

                              Comment


                              • #30
                                Originally posted by Krisharley View Post
                                @ russ: Thank you for contributing with that insightful and non-sarcastic comment to this thread.
                                When someone comes blowing smoke it is our right to say so. This will never hit the market - 100% for sure.

                                The roofs you say are out there are not there - they are losers to the last one - as of today - maybe someone will come up with a better idea but this does not sound like it.
                                [SIGPIC][/SIGPIC]

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