New 6.3kw solar system installed now planning battery backup!

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90cummins
90cummins Solar Expert Posts: 57 ✭✭
in Solar Beginners Corner #1
I recently had a system installed by Solarcity on 6/18/14 and it has generated 1576/kw after 41 days of operation and I currently have 479kw credits.
I'm recording data daily so I can plot performance.
Now that I have Solar I’m planning the next part of the system.
I have some questions pertaining to the addition of a separate battery backup charging system if the grid is down.
I’m currently planning my system and my next purchase will be a 24v AIMS 6kw split phase inverter charger.
Recently I performed a fuel consumption test on two 4kw Generac gas generators and one 3.7kw Yanmar Diesel generator so I can determine the fuel consumption/kw in order to plan my fuel needs.
The results surprised me; at full load 3.7kw the fuel cost for the Yanmar was $0.41/kwh.
At a very light load 30w the fuel cost per per/kW jumped to $22.89
It’s obviously inefficient to run a generator gas or diesel at 3600 rpm to power a 120v battery charger to recharge a 24v system as it would place a relatively light 2kw load on the high speed generator.
That has been bothering me so I've been thinking of using a 10hp Yanmar diesel operating at approximately 2000 rpm to direct drive a 28v 450a brushless generator that has the potential to generate 350A (9.8kw) at 2000rpm. It is essentially an oversize automotive generator in a military application.
This generator is temperature compensated and has a setting for tropic & artic conditions.
During operation the generator should be operating well within the engines hp curve.
Looking forward to your input.
90cummins
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: New 6.3kw solar system installed now planning battery backup!

    Welcome to the forum.

    Do not buy an AIMS inverter. Search this forum for that brand and you will see why. You would be better off with an Outback Radian or Xantrex/Schneider/Conext XW. Much better off.

    Using a big gen to supply a small amount of charging is always a waste of fuel. But batteries can only take so much current. Having a huge amount of battery that is rarely used doesn't help either.

    I'd say the first thing to do is determine what size battery bank you actually do need for back-up power purposes. This is best accomplished by measuring with a Kill-A-Watt those loads which must be kept running in an outage. Once you figure that out it will be mush easier to determine the most economic way of recharging the bank (when solar is not available either).
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  • 90cummins
    90cummins Solar Expert Posts: 57 ✭✭
    #3
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    Re: New 6.3kw solar system installed now planning battery backup!
    Cariboocoot wrote: »
    Welcome to the forum.

    Do not buy an AIMS inverter. Search this forum for that brand and you will see why. You would be better off with an Outback Radian or Xantrex/Schneider/Conext XW. Much better off.

    Using a big gen to supply a small amount of charging is always a waste of fuel. But batteries can only take so much current. Having a huge amount of battery that is rarely used doesn't help either.

    I'd say the first thing to do is determine what size battery bank you actually do need for back-up power purposes. This is best accomplished by measuring with a Kill-A-Watt those loads which must be kept running in an outage. Once you figure that out it will be mush easier to determine the most economic way of recharging the bank (when solar is not available either).

    Well you discouraged me from buying Aims so I did the research & agree.
    Magnum looks to have a good reputation and is stackable. Any others in the 24v 4~6kw range you'd recommend?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: New 6.3kw solar system installed now planning battery backup!

    Have you determined if you need 4kW or 6kW or 8kW? It affects the choices.
    For one thing in that power range you would be better off with 48 Volts.
    Outback's Radian comes in 4kW and 8kW sizes. Good reputation.
    Xantrex/Schneider/Conext XW series comes is 4kW and 6kW sizes. Some known software bugs and operational issues.
    Magnum has a 4kW 48 Volt inverter. They have had a good reputation but there have been some recent reports of failure. Also the company has just been bought out.
    Victron has a good reputation but not much of a North American "presence" so I don't know if they have an inverter in the range you want.
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  • 90cummins
    90cummins Solar Expert Posts: 57 ✭✭
    #5
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    Re: New 6.3kw solar system installed now planning battery backup!

    I would like 6kw with 4kw minimum to cover for 2 small freezers, 1 fridge/freezer with 2 compressors plus an 80' well with 70' of water.
    24v would be preferred because I could use my (still in transit) 28v 450a generator if & when I need more capacity than my batteries will provide.
    I have powered my entire house on a 4kw generator with no issues.
    My solar panels develop 377vdc, is there an inverter that will use that voltage input?
    I ask that because my leased system is grid tied and I would like the option of utilizing the solar panel output if there was a prolonged outage.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: New 6.3kw solar system installed now planning battery backup!

    Off-grid inverters don't run at 377 Volts. However, there are some high-Voltage charge controllers which could be switched in for the GTI to recharge batteries when the grid goes down. There are also some inverters that can AC couple to the GTI and provide power that way. The XW is one of those, another is the SMA Sunny Island. The latter is 120 VAC so needs to be used either in tandem (two 5kW units) or with an autotransformer.

    One problem with 4kW+ on 24 Volts is the current involved. 167 Amps for nominal Voltage and 4kW output. By the time you get up to 6kW the current would be 250 Amps. That's why 48 Volts is better at those power levels. Another reason is storage capacity: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    This 28 VDC generator you are getting: does it have adjustable output? Because to charge a 24 Volt bank you may need to ramp that up a bit. They usually charge at 28.8, not including temp compensation. It would at least manage to bulk charge. At 450 Amps it has much more capacity than most battery banks would take, and somehow that would need to be addressed. Don't expect it to run the inverters 'directly' without sufficient battery to filter between the two. Remember that a DC generator is really AC rectified.
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  • 90cummins
    90cummins Solar Expert Posts: 57 ✭✭
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    Re: New 6.3kw solar system installed now planning battery backup!
    Cariboocoot wrote: »
    Off-grid inverters don't run at 377 Volts. However, there are some high-Voltage charge controllers which could be switched in for the GTI to recharge batteries when the grid goes down. There are also some inverters that can AC couple to the GTI and provide power that way. The XW is one of those, another is the SMA Sunny Island. The latter is 120 VAC so needs to be used either in tandem (two 5kW units) or with an autotransformer.

    One problem with 4kW+ on 24 Volts is the current involved. 167 Amps for nominal Voltage and 4kW output. By the time you get up to 6kW the current would be 250 Amps. That's why 48 Volts is better at those power levels. Another reason is storage capacity: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    This 28 VDC generator you are getting: does it have adjustable output? Because to charge a 24 Volt bank you may need to ramp that up a bit. They usually charge at 28.8, not including temp compensation. It would at least manage to bulk charge. At 450 Amps it has much more capacity than most battery banks would take, and somehow that would need to be addressed. Don't expect it to run the inverters 'directly' without sufficient battery to filter between the two. Remember that a DC generator is really AC rectified.

    The generator is actually rated at 28 volts 450 amps max and is temperature compensated.
    My generator should arrive at the terminal Friday so I can start working on mounting & driving it for some testing.
    I realize that 48 volts is more efficient and there is more surge capacity but the generator output of (28v) eliminates that option.
    I have additional questions.
    • Is there was a way to charge a 48v battery bank with a 24v generator by isolating the batteries into (2) 24v groups for the purposes of charging while still allowing the inverter to operate on 48V?
    • Could a very large blocking diode be inserted between the two 24v battery groups to allow this?
    • Also would a DC step down transformer work to reduce the 377vdc to a more usable voltage??
    • Is it possible to use an inverter like the magnum or others to energize the grid side of the system (after disconnecting it from the Grid) to make my solar system operational?
    I have main breakers on both sides of the solar system so I can isolate it from the grid.
    I will look into your suggestions/options.
    I realized the short comings of Grid tied and now realize what it can and can’t do.
    I don’t regret my decision, I’m just looking to adapt, modify, and improve if possible to prepare for a long term outage.
    Would be a shame to have that potential on the roof and not be able to utilize it.
    Sorry for the barrage of questions I’m trying to gather all the information I can in order to purchase the proper equipment (once) and be Prepared!
    Thank you for your input, no doubt more questions will follow.
    90cummins
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: New 6.3kw solar system installed now planning battery backup!

    There are a couple of ways you can charge a 48 Volt battery bank from a 24 Volt (nominal) source. Both have drawbacks. One is to charge half at a time: negative to +24 Volts, then +24 Volts to +48 Volts segments. This will be very uneven charging and certainly not suitable for a bank in use. The other is a DC-to-DC converter that can step the Voltage up from 24 to 48. This has problems of losing current (they will not be available in very large current capacity) and it is a regulated Voltage supply not a charger so some charging regulation must be provided at the output. This is a buck-booster circuit and could be custom designed to meet the need but of course that would come with a hefty price.

    There's no such thing as a DC transformer, despite what you may read some places. DC does not work with transformers. To reduce DC you need some form of circuit, and if you want to keep relatively the same amount of power it will be a buck converter (like mentioned above only operating the other way). MPPT charge controllers do this, for example. There are even a couple that will take up to 600 Voc on input and reduce it to charging levels for 24 or 48 Volt batteries.

    It is also possible to use the right kind of battery-based inverter to AC couple to grid-tie inverter(s) and regulate their output for charging that way. Two that will do this are the Xantrex XW series and the SMA Sunny Island.

    Considering what you have already (including the 28 Volt gen) it looks like your best bet maybe be the Xantrex/Conext XW 4024 http://www.solar-electric.com/inverters-controllers-accessories/inverters/xain/nexaxwseinan/xaxwhyin.html It's 4kW output, runs from 24 Volts, and could be AC coupled to your existing GT system to provide solar charging. The exact details of the installation would have to be worked out carefully, but from what I see now it would be the least difficult and least expensive way to achieve your back-up power goal.
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  • 90cummins
    90cummins Solar Expert Posts: 57 ✭✭
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    Re: New 6.3kw solar system installed now planning battery backup!

    Before I order an Grid-tie off grid inverter it may be worth mentioning that my solar inverter is a SolarEdge SE6000A-US.
    I thought it may be pertinent to my purchase in the event of compatibility.
    I like the Schneider Conext XW4548 and XW4024 120/240-60 Hybrid. It looks to have support that is second to none and it can be stacked if needed. I’m leaning towards the 4548 because the higher voltage gives me a bit more output and I can most likely avoid a parallel string.
    I’ll order another 24v generator if needed, having a spare is a good thing.
    A friend was over looking at the generator and we were talking about running two generators to charge the 48v system and he said that I may need to electrically isolate them from each other to avoid (sparks)
    Is going from 24v to 48v as significant as going from 12v to 24v? I’m planning and looking towards the future because things change and I would rather take it in stride than have regrets.
    Thanks again for the advice;
    90cummins
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: New 6.3kw solar system installed now planning battery backup!

    Your friend is correct: you'd be running the two gens "stacked" in series so they would have to be electrically isolated. The negative of one is connected to the same point as the positive of the other, and if the negatives are shared any other way (such as through grounding) then one gen has a dead short across it.

    The upgrade from a 24 Volt system to a 48 Volt one has the same effect as going from 12 to 24: double the Voltage, halve the current, increase the efficiency.

    You may not be too keen on running two generators of that capacity to supply the relatively small amount of power needed to recharge a suitable battery bank. 28 Volts * 450 Amps = 12.6 kW. A battery bank on a 4kW 24 Volt system would use less than 1/10 that at peak: 400 Amp hours charging at 40 Amps * 28 Volts +/-. Generators don't like to be lightly loaded.
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