Grid interactive: what benefits make it worthwhile?

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #32
    Re: Grid interactive: what benefits make it worthwhile?

    The "in" efficiencies multiply:
    • 0.81 panel * 0.95 charge controller * 0.80 battery * 0.85 inverter = 0.52 for off grid system

    Note that cold weather will increase panel derating/efficiency (if using MPPT charge controller--cold weather raises Vmp), Charge controllers and inverter efficiencies vary based on loading, battery efficiency depend on construction (agm vs flooded cell) and how you operate them (charging at 90% state of charge is less efficient than charging at 75% state of charge), etc...

    Also, MPPT "efficiency" vs PWM "efficiency is quite a different model. MPPT are a constant power model (Pmp=Vmp*Imp*losses).

    Whereas PWM efficiency--there are still small(er) losses to run the (simple PWM) electronics, but the "power model" is Power=Vbatt*Imp - controller losses... Note that Vmp (which goes up as temperature decreases) is not in the equation. It is Imp and Battery Voltage that matters here (note, Imp does rise, but only a very small amount, as panels get hot)....

    Overall, the 0.77 derating number for charge controllers and solar panels seems to be "close enough" to use for warm climate MPPT systems, and any climate PWM systems (note that if Vmp-array gets too close to Vbatt-charging, then you will have other "losses" as the array voltage is not high enough in very hot weather to charge the battery bank).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #33
    Re: Grid interactive: what benefits make it worthwhile?
    vtmaps wrote: »
    I don't see much solution for the dedicated UPS solar panels going to 'waste' when the batteries are full. Best solution is to undersize the panels (explanation below) you only need to have 1 panel to get the tax credit.

    Now, about the UPS system... many off-grid systems use an 'inverter/charger'. A single unit that a acts as an inverter (powered by battery) or a charger (to charge that battery). These inverter/chargers always have an input for AC current (or they couldn't be chargers).

    Many folks (myself) use these off grid, in which case my AC input is from a generator. Other folks use these on grid, in which the grid is the AC source. In either case, when the AC stops the inverter/charger changes from charger to inverter quickly enough to be a good UPS.

    Therefore, I think you should hook up an inverter/charger type of UPS to the grid. In order to get the tax credits, take one panel and a cheap controller and hook it up to the UPS battery. Size your battery (which will almost always be fully charged) for a day of use. A battery that sits fully charged most of the time should be an AGM battery.

    When the grid is down for more than a day you will run your generator for general homestead use. At the same time, that same generator will be charging up your UPS battery.

    --vtMaps

    So the setup would be (and please correct me where I a wrong).
    * two inverters: a grid-tie and an inverter/charger
    * The grid-tied inverter connects to almost all the PV panels, and feeds to the main breaker panel (bidirectional).
    * The inverter/charger has input from a battery pack, and from the main breaker panel (one way), and outputs to a sub-panel containing the
    circuits I want UPS protected (call this the "UPS subpanel")
    This inverter/charger passes AC current through (to this UPS subpanel) in the 99% of the time the grid is running smoothly (so the battery is almost never being used).
    When the grid goes down, it uses batteries to power the UPS subpanel. Thus, it acts as a battery backup for these loads.
    Furthermore, this switch to batteries is quick enough that it also acts as a UPS: it protects equipment from transient events (surges, brownouts, outages) on the grid.
    * One or so PV panels are also connected to the battery pack, using a small charge controller.

    * On long outages, a generator is used to power the house . Actually, to power a "critical loads" subpanel.
    * During these outages, the "ups subpanel" is powered by the battery.
    * Actually, if the inverter/charger received its AC power via the "critical loads" subpanel, then whilst the generator is running the charger/inverter would be in charger mode and would be
    charging the battery (along with the one-or-so-pv panels).

    This is kind of interesting: since the "UPS subpanel" could also contain "super-critical" loads, that should be running even when one turns the generator off (say, when everyone is asleep; or in the afternoon when everyone is at work). Like the fridge or boiler.

    So what would this cost. I will probably get something wrong, but using some prices from our gracious host's website:
    a) Xantrex trace 1500 W 24v, $720. Or increase to 2500W (if serious about fridge/boiler being on it) for $863.
    b) 100 AH at 24V (might be a bit small), using Universal Battery AGM battery: 4x 91= $360. (might be a bit small, but the anticipation is that the genset will be doing most of the work)
    c) charge controller (sun saver): $150

    So, about $1300. Knock off 30% fed subsidy: than for $100k bucks I have centralized UPS, and a super-critical-loads battery backup. Though one should add the cost of the panel to this.

    Do I have anything way wrong?

    I do have to wonder if that may be bending the intent of the fed subsidies; since the solar part of this is minor. Not legally bending, but could you defend this use of tax payer dollars to your skeptical neighbors. In contrast, for a grid-tied system, there are a lot of reasons why fed subsidy is good public policy.
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #34
    Re: Grid interactive: what benefits make it worthwhile?
    mtdoc wrote: »
    Just an FYI/ comparison point - My Grid interactive system using an Outback GVFX 3648 with Midnite Classic/ AGM battery backup is 80% efficient - from production as measured by the CC to output of inverter (to either loads or selling to the grid). YMMV

    My system is wired so that almost all my 120 VAC loads come through subpanel/inverter. One critical 240 VAC load (septic pump) is powered via autotransformer. My other 240 VAC loads (Heat pump, water heater, dryer) are not backed up - except by other means -- woodstove, etc.

    I have no problem with any significant delay when grid power goes out - no computer reboots, etc.
    Interesting.

    mtdoc wrote: »
    This system works well for me as back up for semi-unreliable -end of the line - grid power. Most of the time I'm selling to the grid when production exceeds the loads. My system will be unlikely to ever pay for itself but it helps that my utility pays 0.18 per KWH produced while only charging 0.065 per KWH consumed!
    Isn't that the opposite of typical (most utilities want to sell at retail and buy at wholesale, which is why net-metering is to the residential PV producer's advantage).
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #35
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    Interesting.



    Isn't that the opposite of typical (most utilities want to sell at retail and buy at wholesale, which is why net-metering is to the residential PV producer's advantage).

    Yeah - my county PUD pays extra for systems with components made in Washington state.

    Link: http://www.clallampud.net/uploadedFiles/conservation/documents/INSPIRED_Program_Overview.pdf

    Fortunately Outback is in WA. If my solar panels were from WA I'd be getting 0.54 per KWH! Unfortunately the only panels made here (Silicon Energy) are pricey and did not make sense for my application.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #36
    Re: Grid interactive: what benefits make it worthwhile?

    A "hybrid" inverter for a "whole house" UPS for "non-critical" loads--Works well...

    I have a bit of a problem with stand-by UPS (and typical Hybrid inverter/configurations) for backup of critical loads (data processing, etc.)...

    For whatever reason, I had lab gear that would do 1/2 cycle (programmable) drop outs on AC lines and every UPS passed the test just fine (and the computer plugged in never had an issue).

    However, in "real life", my guess is about 1 out of 10 power failures would "confuse" the UPS switch over frequency and cause the attached AC powered computers to reboot...

    If you have truly critical loads, I would suggest "continuous" conversion UPS's (or standard off-grid inverter supplied by battery power) instead. Then no version of AC failure will make it through the "battery charger" to battery to AC inverter to computer chain.

    Also, you should be looking at dual input/redundant power supplies for the computers too (a standard non-redundant inverter is going to have a a failure in the 5-10+ year range anyway).

    One reason I am so fond of laptops--They are already designed for low power (because they have to run from batteries), and they have their own UPS (internal battery pack which needs to be replaced every several years too).

    Having reliable computer processing with N+1 redundancy, looking at various failure modes, etc. can really be a nightmare (in design, cost, and maintenance--you need to know when the +1 redundant device has failed).

    Many times, a "simple" system with good data and power backup (and spares on shelf) is the best solution for "home" type systems.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #37
    Re: Grid interactive: what benefits make it worthwhile?
    BB. wrote: »
    A "hybrid" inverter for a "whole house" UPS for "non-critical" loads--Works well...

    However, in "real life", my guess is about 1 out of 10 power failures would "confuse" the UPS switch over frequency and cause the attached AC powered computers to reboot...

    If you have truly critical loads, I would suggest "continuous" conversion UPS's (or standard off-grid inverter supplied by battery power) instead. Then no version of AC failure will make it through the "battery charger" to battery to AC inverter to computer chain.

    Also, you should be looking at dual input/redundant power supplies for the computers too (a standard non-redundant inverter is going to have a a failure in the 5-10+ year range anyway).

    One reason I am so fond of laptops--They are already designed for low power (because they have to run from batteries), and they have their own UPS (internal battery pack which needs to be replaced every several years too).

    Having reliable computer processing with N+1 redundancy, looking at various failure modes, etc. can really be a nightmare (in design, cost, and maintenance--you need to know when the +1 redundant device has failed).

    Many times, a "simple" system with good data and power backup (and spares on shelf) is the best solution for "home" type systems.

    -Bill

    Nothing I have is that critical.. 90% "no reboot rate" would be just fine. And a "continuous" conversion UPS's" would not be cheap (wasn't it you who convinced me that running through a battery is highly inefficient!)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #38
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    Nothing I have is that critical.. 90% "no reboot rate" would be just fine. And a "continuous" conversion UPS's" would not be cheap (wasn't it you who convinced me that running through a battery is highly inefficient!)

    Yep--That me. :blush:

    And those "extra losses" add to your A/C cooling requirements.

    "Simple" is usually "good enough" for most people. And supplying "UPS"/generator/Solar Backup power just to the critical loads (your needs--Mine would not include A/C, yours would) usually helps keep costs down.

    And you may have two or more backup's... Run generator for A/C backup, and battery based for "quiet" nighttime lighting, computers, TV, etc..

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #39
    Re: Grid interactive: what benefits make it worthwhile?

    Daniel, The inverter/charger I mentioned is not the best route to go if you want cleaned up power. Perhaps you should consider separate inverter and charger. It won't cost any more than an inverter/charger, might even be cheaper. Bill had it right:
    If you have truly critical loads, I would suggest "continuous" conversion UPS's (or standard off-grid inverter supplied by battery power) instead. Then no version of AC failure will make it through the "battery charger" to battery to AC inverter to computer chain.

    Under this UPS system approach the battery charger is always hooked up to the grid and the battery. The inverter is always providing power from battery to your UPS loads. When the grid is down you start the generator and generator powers the battery charger as well as your general house loads. To get the tax credits, just hook up a panel to the battery through a cheap controller.
    danielh wrote:
    I do have to wonder if that may be bending the intent of the fed subsidies; since the solar part of this is minor. Not legally bending, but could you defend this use of tax payer dollars to your skeptical neighbors. In contrast, for a grid-tied system, there are a lot of reasons why fed subsidy is good public policy.

    Let's not go there or this thread will be locked by the moderators. Good policy or not, the tax laws are what they are. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #40
    Re: Grid interactive: what benefits make it worthwhile?
    vtmaps wrote: »
    Daniel, The inverter/charger I mentioned is not the best route to go if you want cleaned up power. Perhaps you should consider separate inverter and charger. It won't cost any more than an inverter/charger, might even be cheaper. Bill had it right:

    Under this UPS system approach the battery charger is always hooked up to the grid and the battery. The inverter is always providing power from battery to your UPS loads. When the grid is down you start the generator and generator powers the battery charger as well as your general house loads. To get the tax credits, just hook up a panel to the battery through a cheap controller.
    I don't have those kind of "truely critical" needs.

    But for my own edification, your suggestion means:

    1) a grid connected battery charger charges the battery. Or, a generator (perhaps selected by a transfer switch) is connected to this battery charger.
    2) a PV panel or two, connected to a second battery charge controller, is also charging the batteries.
    3) a third device, the inverter, is drawing power from the batteries for the UPS loads

    This differs from the setup you mentioned previously in that 1 and 3 are separate, so that there never is a case where AC power is flowing straight from the grid (or your generator)
    to the UPS loads. But it does mean lots of use of the batteries, with attendant losses in efficiency and quicker replacement times.
    vtmaps wrote: »
    Let's not go there or this thread will be locked by the moderators. Good policy or not, the tax laws are what they are. --vtMaps
    Okay dokay.
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #41
    Re: Grid interactive: what benefits make it worthwhile?
    BB. wrote: »
    Yep--That me. :blush:

    And those "extra losses" add to your A/C cooling requirements.

    "Simple" is usually "good enough" for most people. And supplying "UPS"/generator/Solar Backup power just to the critical loads (your needs--Mine would not include A/C, yours would) usually helps keep costs down.

    And you may have two or more backup's... Run generator for A/C backup, and battery based for "quiet" nighttime lighting, computers, TV, etc..

    -Bill
    Yep, two or more backups. And AC is not a critical load -- not at 4kw or so! One can get by without AC.

    Digressing a bit -- does anyone use Edison (NiFe) batteries? They seem like an interesting alternative (though 3x the cost).
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #42
    Re: Grid interactive: what benefits make it worthwhile?

    We do stretch the limits to allow political discussions that are related to solar/conservation/energy usage (I would suggest a separate thread so we don't take your hardware discussions off track)... But we want to keep them as much "fact based" as possible.

    In general, we aim for technical discussions mostly here... Fewer hurt feelings (we all have differing political opinions and many readers are from other countries where a US centric discussion is pretty meaningless) and we (hopefully) actually help/make a difference for more people with the technical.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #43
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    This differs from the setup you mentioned previously in that 1 and 3 are separate, so that there never is a case where AC power is flowing straight from the grid (or your generator)
    to the UPS loads. But it does mean lots of use of the batteries, with attendant losses in efficiency and quicker replacement times.

    No. As long as the grid is up the batteries remain fully charged (floating). When you turn on your loads (computers, etc) they will draw DC current (through the inverter). As you draw this current the battery voltage will try to dip, but the battery charger won't allow that (unless you exceed its capacity). If your max loads are 500 watts (for example), just buy a battery charger with 500 watt capacity. The batteries will never know when there is a load (unless the grid goes down).
    Or, a generator (perhaps selected by a transfer switch) is connected to this battery charger.
    No need for a second transfer switch. You will have a main transfer switch for you generator to power the whole house. The UPS battery charger is just another load on your house circuit and will be covered by your main transfer switch.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #44
    Re: Grid interactive: what benefits make it worthwhile?

    A couple threads:

    Nickle-Iron batteries
    Fw: 900Ah 48V NiFe Battery Bank Available for Immediate Purchase

    Poster "mike90045" has purchased a set not too long ago--Perhaps he can let us know how things are going today...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #45
    Re: Grid interactive: what benefits make it worthwhile?
    BB. wrote: »
    A couple threads:

    Nickle-Iron batteries
    Fw: 900Ah 48V NiFe Battery Bank Available for Immediate Purchase

    Poster "mike90045" has purchased a set not too long ago--Perhaps he can let us know how things are going today...

    -Bill

    Thanks (I have a friend who is intrigued with the idea)
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #46
    Re: Grid interactive: what benefits make it worthwhile?
    vtmaps wrote: »
    No. As long as the grid is up the batteries remain fully charged (floating). When you turn on your loads (computers, etc) they will draw DC current (through the inverter). As you draw this current the battery voltage will try to dip, but the battery charger won't allow that (unless you exceed its capacity). If your max loads are 500 watts (for example), just buy a battery charger with 500 watt capacity. The batteries will never know when there is a load (unless the grid goes down).
    --vtMaps

    I think I am getting it: when the battery voltage starts to dip (due to the inverter drawing power for AC loads), the charger senses the dip and starts to charge the battery. Which basically means that most of the time the inverter is pulling power from the charger, with the batteries acting as a conduit. Actually, as a buffering conduit, so failures in the grid (intermittent or longer) have no impact on the inverter -- since the inverter doesn't know whether the DC power it is receiving is coming directly from the charge controller, or out of battery storage.

    Please forgive me for asking dumbish questions, but it helps me learn. So... . what is the "no" a "no" too? Am I incorrect in visualizing 3 physically seperate units, each of which is attached to the battery pack?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #47
    Re: Grid interactive: what benefits make it worthwhile?

    Bit of an interrupted day for me so this might be redundant.

    A normal hybrid GT inverter system consists of:
    Solar array that recharges the batteries through a charge controller (not necessarily a high Voltage one).
    The inverter-charger that when the grid is available feeds "surplus" solar power to the grid/AC loads or if necessary draws power from the grid to keep the batteries charged. If the grid becomes unavailable it draws power from the batteries to supply the loads. It does not converter AC to DC and back to AC the way a continuous UPS unit does, but the switchover is not noticeable for 99% of loads.

    An AC coupled system uses a standard GT set-up (array feeding GT inverter directly) to "back-feed" a conventional battery-based inverter which uses this power to keep its batteries charged. Otherwise it feeds the grid/loads as usual. In the event of the utility going down the battery-based inverter becomes a "sync source" for the GT system, allowing it to continue to supply power for loads. The tricky part is regulating the battery charging from the AC coupling. Some systems, such as SMA's Sunny Island in conjunction with their Sunny Boy GT inverters, do this very well. Xantrex's XW inverter is capable of it, but does not have the "seamlessness" of the SMA system. I believe the new Magnum inverters can also function the way the XW does, shifting frequency to drop the GT inverter input when load requirements are low. Some people have put together such systems without using inverters designed for such function and it is what most people would call "a nightmare". This is one of the reasons why I didn't recommend it: it is not for the technically faint of heart even if you do use the SMA system.

    The high-Voltage controller solution is to use the 600V Xantrex unit to switch a standard GT array to be a charge source for a 24 Volt or 48 Volt system when the power goes down. It requires some HV DC switching & wiring plus a battery bank and off-grid type inverter. Some have done a variation on this theme by switching out part of the HV array (or rewiring completely) to supply Voltage in-line with a standard MPPT type controller (<150 Volts max input).

    And then of course there's the gen + batteries + inverter solution which allows the gen to recharge the batteries which then supply the inverter for "quiet power" with the gen not running.

    Lots of different ways to go about it, each with its benefits and drawbacks. And costs. Deciding which one works best for your particular situation is how migraines start.
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #48
    Re: Grid interactive: what benefits make it worthwhile?
    Cariboocoot wrote: »
    Bit of an interrupted day for me so this might be redundant.
    Isn't redundant synonymous with robust?
    Cariboocoot wrote: »
    A normal hybrid GT inverter system consists of:
    Solar array that recharges the batteries through a charge controller (not necessarily a high Voltage one).
    The inverter-charger that when the grid is available feeds "surplus" solar power to the grid/AC loads or if necessary draws power from the grid to keep the batteries charged. If the grid becomes unavailable it draws power from the batteries to supply the loads. It does not converter AC to DC and back to AC the way a continuous UPS unit does, but the switchover is not noticeable for 99% of loads.
    So you get AC power dependably almost all the time. Not necessarily as clean as what a "continous" UPS would provide.
    Cariboocoot wrote: »
    An AC coupled system uses a standard GT set-up (array feeding GT inverter directly) to "back-feed" a conventional battery-based inverter which uses this power to keep its batteries charged. Otherwise it feeds the grid/loads as usual. In the event of the utility going down the battery-based inverter becomes a "sync source" for the GT system, allowing it to continue to supply power for loads. The tricky part is regulating the battery charging from the AC coupling. Some systems, such as SMA's Sunny Island in conjunction with their Sunny Boy GT inverters, do this very well. Xantrex's XW inverter is capable of it, but does not have the "seamlessness" of the SMA system. I believe the new Magnum inverters can also function the way the XW does, shifting frequency to drop the GT inverter input when load requirements are low. Some people have put together such systems without using inverters designed for such function and it is what most people would call "a nightmare". This is one of the reasons why I didn't recommend it: it is not for the technically faint of heart even if you do use the SMA system.
    But but but .... http://www.wholesalesolar.com/pdf.folder/inverter%20pdf%20folder/MagnumACcoupling.pdf makes it seem so simple :)

    BTW: by "Otherwise it feeds the grid/loads as usual." -- the example in the magnum article suggest that the battery-based inverter is feeding the loads, and grid (from power supplied by the GT-inverter). Is that what you meant?
    Cariboocoot wrote: »
    The high-Voltage controller solution is to use the 600V Xantrex unit to switch a standard GT array to be a charge source for a 24 Volt or 48 Volt system when the power goes down. It requires some HV DC switching & wiring plus a battery bank and off-grid type inverter. Some have done a variation on this theme by switching out part of the HV array (or rewiring completely) to supply Voltage in-line with a standard MPPT type controller (<150 Volts max input).

    Actual physical switches one throws to directly connect the PV arrays to the Xantrex, rather than the GT-inverter?
    Cariboocoot wrote: »
    And then of course there's the gen + batteries + inverter solution which allows the gen to recharge the batteries which then supply the inverter for "quiet power" with the gen not running.

    Lots of different ways to go about it, each with its benefits and drawbacks. And costs. Deciding which one works best for your particular situation is how migraines start.

    No kidding.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #49
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    Isn't redundant synonymous with robust?

    No, it's synonymous with "government operation".

    So you get AC power dependably almost all the time. Not necessarily as clean as what a "continous" UPS would provide.

    Yes. But then utility power isn't all that clean much of the time. Again the issue of how critical (and particular) the load is.

    But but but .... http://www.wholesalesolar.com/pdf.folder/inverter%20pdf%20folder/MagnumACcoupling.pdf makes it seem so simple :)

    Yes. Nothing to it. Easy as Pi. Carried out to the last decimal place. :roll:
    Only my opinion, but the SMA system works the best for this.
    BTW: by "Otherwise it feeds the grid/loads as usual." -- the example in the magnum article suggest that the battery-based inverter is feeding the loads, and grid (from power supplied by the GT-inverter). Is that what you meant?

    That would be the hybrid inverter function: feeding grid and loads from battery (actually from PV when battery is full). What I was trying to explain is that the AC coupled system uses standard GT inverter which feeds the grid/loads the way it would in any other application, but there is now an added load of back-feeding the off-grid inverter to keep the batteries charged.


    Actual physical switches one throws to directly connect the PV arrays to the Xantrex, rather than the GT-inverter?

    Yes. Has to be able to handle the 600 Volt maximum @ the current rating of the array. Not the easiest order to fill.
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #50
    Re: Grid interactive: what benefits make it worthwhile?
    Cariboocoot wrote: »
    BTW: by "Otherwise it feeds the grid/loads as usual." -- the example in the magnum article suggest that the battery-based inverter is feeding the loads, and grid (from power supplied by the GT-inverter). Is that what you meant?

    That would be the hybrid inverter function: feeding grid and loads from battery (actually from PV when battery is full). What I was trying to explain is that the AC coupled system uses standard GT inverter which feeds the grid/loads the way it would in any other application, but there is now an added load of back-feeding the off-grid inverter to keep the batteries charged.
    Hmm, seriously the magnum article suggested to me that their battery based inverter is the heart of the system-- which is why it can "fool" the GT inverter into continuing to produce power during
    grid outages, said power used for local loads. So the magnum MUST also have "transfer switch" logic (hmm, the "GT" inverter could get away without it ???!?). I might be missing something :(

    Are you saying that there are other setups, wherein an off-grid inverter keeps batteries charged and also uses such batteries during outages for local loads -- but during these outages the PV panel power is unusable (the GT inverter isn't fooled into producing power).



    BTW: I greatly appreciate all of your answers. This has been real informative for me

    Hopefully others might learn from this discussion, which is one reason I am being pedantic.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #51
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    Hmm, seriously the magnum article suggested to me that their battery based inverter is the heart of the system-- which is why it can "fool" the GT inverter into continuing to produce power during
    grid outages, said power used for local loads and isolated from the grid (so the magnum has grid tie logic).

    Magnum does not have an exclusive on AC coupling. The Xantrex XW works much the same way, and the SMA Sunny Island is designed to work specifically with their GT inverters (but will work with others).

    As I said before: normally the standard GT inverter feed loads/grid as they would without the battery-based inverter available. This includes keeping the batteries charged by back-feeding. If the grid goes down, the battery inverter turns on and provides the Voltage and frequency for the GT inverter to sync to.

    You can even make off-grid inverters that aren't designed to do this function this way, although regulating the battery charging and controlling the power from the GT is where the word "nightmare" enters into it.
    Are you saying that there are other setups, wherein an off-grid inverter keeps batteries charged and also uses such batteries during outages for local loads -- but during these outages the PV panel power is unusable (the GT inverter isn't fooled into producing power).

    Yes. You can have an off-grid inverter & batteries for critical loads without any PV at all. Normally it would be kept charged by the grid, and would then instantly switch to invert mode when the grid goes down and keep the loads running. The grid needs to come back up at some point to recharge the batteries, or a generator can be substituted for grid power when recharging is needed. This can be entirely separate from a GT system.
    BTW: I greatly appreciate all of your answers. This has been real informative for me

    Hopefully others might learn from this discussion, which is one reason I am being pedantic.

    Ever since they discontinued the free donut program here we have searched for a raison d'être. Answering the questions seems to be it. :p
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #52
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    So... . what is the "no" a "no" to? Am I incorrect in visualizing 3 physically separate units, each of which is attached to the battery pack?

    You asked: "But it does mean lots of use of the batteries, with attendant losses in efficiency and quicker replacement times."
    I responded: "No. As long as the grid is up the batteries remain fully charged (floating). When you.... "

    Three separate units attached to the battery? I see only an inverter and a battery charger.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #53
    Re: Grid interactive: what benefits make it worthwhile?
    Cariboocoot wrote: »
    Magnum does not have an exclusive on AC coupling. The Xantrex XW works much the same way, and the SMA Sunny Island is designed to work specifically with their GT inverters but will work with others).

    As I said before: normally the standard GT inverter feed loads/grid as they would without the battery-based inverter available. This includes keeping the batteries charged by back-feeding. If the grid goes down, the battery inverter turns on and provides the Voltage and frequency for the GT inverter to sync to.

    You can even make off-grid inverters that aren't designed to do this function this way, although regulating the battery charging and controlling the power from the GT is where the word "nightmare" enters into it.
    So ...
    AC coupling capable inverter/chargers go into standby mode when useable AC power is detected from the grid. In this state the inverter/charger is acting like a simple wire:
    power can flow FROM the GT inverter, through a sub panel, through the inverter/charger, into to the main cb box and hence the grid.

    When AC-from-the-grid is not detected, the inverter/charger switches to invert mode, AND disconnects from the grid.

    The nightmare part, of what to do with excess PV power during invert mode, seems to be where AC coupling isn't quite ready for prime time.


    Where does one send the gift certificates for Krispy Kremes (or are Dunkin's preferred)?
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #54
    Re: Grid interactive: what benefits make it worthwhile?
    vtmaps wrote: »
    You asked: "But it does mean lots of use of the batteries, with attendant losses in efficiency and quicker replacement times."
    I responded: "No. As long as the grid is up the batteries remain fully charged (floating). When you.... "

    Three separate units attached to the battery? I see only an inverter and a battery charger.

    --vtMaps

    What about the charge controller (connecting the PV panels to the batteries)?
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #55
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    What about the charge controller (connecting the PV panels to the batteries)?

    oops... Now I see three! -vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #56
    Re: Grid interactive: what benefits make it worthwhile?
    vtmaps wrote: »
    oops... Now I see three! -vtMaps

    Phew, I wasn't hallucinating!
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #57
    Re: Grid interactive: what benefits make it worthwhile?

    Are there inverter/chargers that will use DC power if available (if the battery is fully charged) rather than AC power?
    Something like a hybrid WITHOUT the grid-tie ability. This feature could be useful in a small system with a few panels that is dedicated to a few
    critical loads The use of DC power avoids wasting panel output keeping a rarely used battery fully charged..
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #58
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    So ...
    AC coupling capable inverter/chargers go into standby mode when useable AC power is detected from the grid. In this state the inverter/charger is acting like a simple wire:
    power can flow FROM the GT inverter, through a sub panel, through the inverter/charger, into to the main cb box and hence the grid.

    Correct. Except it's not technically in standby mode, it's still on, just not inverting. If the batteries are not 100% charged, then it will be charging the batts with the power from the GT inverter.
    danielh wrote: »
    When AC-from-the-grid is not detected, the inverter/charger switches to invert mode, AND disconnects from the grid.

    Yes. The SMA system behaves differently because the Sunny Island (SI) and Sunny boy (SB) are connected with a comms cable. It does the following when the grid disappears:
    - Opens the internal transfer switch to disconnect the grid
    - The SI reprograms the SB into a special "off-grid" mode (the SB is the only GT inverter on the market that has this). In off-grid mode the GT will regulate it's output based on the line frequency.
    - The SI now inverts and controls the line frequency. If there is too much power available, then the SI increases the frequency in exact proportion to the power. The SB detects this frequency and adjusts it's output accordingly.
    - When the grid is detected again, the SI reprograms the SB again to be in UL grid feeding mode and then closes the relay to reconnect to the grid.
    danielh wrote: »
    The nightmare part, of what to do with excess PV power during invert mode, seems to be where AC coupling isn't quite ready for prime time.

    I'd say this is true for other systems, but not the SMA system. AC coupling is SMA's standard off-grid solution and has been used for years in many large projects. Same background info here: http://files.sma.de/dl/10040/INSELVERSOR-AEN101410.pdf
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #59
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    Are there inverter/chargers that will use DC power if available (if the battery is fully charged) rather than AC power?
    None that I know of. I don't understand why you would want to. As mentioned, doing this would cycle the batteries (shorten life) and be less efficient. Some batteries (especially flooded) need to be cycled from time to time. Just disconnect the AC once in a while to exercise the batteries. In a UPS system the idea is to always keep the batteries fully charged and ready for loss of AC power.
    danielh wrote: »
    The use of DC power avoids wasting panel output keeping a rarely used battery fully charged..
    All of us off-grid are faced with 'wasted' power when our batteries are full. Some of us do opportunity loads (laundry in the afternoon when the batteries are full), or diversion loads (make hot water when the batteries are full).

    There is a bit of a generation gap among off gridders. Twenty years ago batteries were cheap and panels were dear. Now its just the opposite. Old designs had more battery and less panel (rarely any 'wasted' power). New designs have more panel. I am getting ready to expand my panels with the idea of reducing my generator time a bit (not that I run it very much anyway). The old way of reducing generator time is to buy more battery (= more autonomy). The new way is to buy more panel (make power on overcast days). In the summer I already 'waste' nearly half the power I could produce, and the ratio will go up after I add more panels.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #60
    Re: Grid interactive: what benefits make it worthwhile?
    stephendv wrote: »


    Yes. The SMA system behaves differently because the Sunny Island (SI) and Sunny boy (SB) are connected with a comms cable. It does the following when the grid disappears:
    - Opens the internal transfer switch to disconnect the grid
    - The SI reprograms the SB into a special "off-grid" mode (the SB is the only GT inverter on the market that has this). In off-grid mode the GT will regulate it's output based on the line frequency.
    - The SI now inverts and controls the line frequency. If there is too much power available, then the SI increases the frequency in exact proportion to the power. The SB detects this frequency and adjusts it's output accordingly.
    - When the grid is detected again, the SI reprograms the SB again to be in UL grid feeding mode and then closes the relay to reconnect to the grid.

    So line frequency is how the SI tells the SB "stop sending me power, by batteries are full". Or something like that.
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  • danielh
    danielh Solar Expert Posts: 69 ✭✭✭✭
    #61
    Re: Grid interactive: what benefits make it worthwhile?
    vtmaps wrote: »
    "The use of DC power avoids wasting panel output keeping a rarely used battery fully charged.."

    All of us off-grid are faced with 'wasted' power when our batteries are full. Some of us do opportunity loads (laundry in the afternoon when the batteries are full), or diversion loads (make hot water when the batteries are full).

    There is a bit of a generation gap among off gridders. Twenty years ago batteries were cheap and panels were dear. Now its just the opposite. Old designs had more battery and less panel (rarely any 'wasted' power). New designs have more panel. I am getting ready to expand my panels with the idea of reducing my generator time a bit (not that I run it very much anyway). The old way of reducing generator time is to buy more battery (= more autonomy). The new way is to buy more panel (make power on overcast days). In the summer I already 'waste' nearly half the power I could produce, and the ratio will go up after I add more panels.
    --vtMaps

    That is interesting. On-grid (grid-tie) people like me might face a different calculus, but the notion of batteries as the most scarce resource is useful to keep in mind.


    And yes, I am an economist
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