Wind & Sun Grid Tie, Now Want to add Battery Backup

mnittlermnittler Solar Expert Posts: 63 ✭✭✭
:confused:I currently have a 5.1kw Solar grid tie system (SMA SB5000US) and am in the middle of installing a 2.5kw A.R.E. grid tie Wind Turbine (SMA WB3000US). I am curently exporting 4-20 kwh/day to the grid. I would like to now capture the exported power and save to batteries for use at night since my current power provider does not buy it back. I would also like to be able to generate power with the Solar & Wind as a off grid system if required (trick GT to produce power if grid is down for an extended period of time using a transfer switch and other devices as required?). If tricking the GT inverters to work off grid, how would one deal with the problem of making more power than use. How could I charge the batteries? Since I am working with limited funds, 2 or more SMA Sunny Islands are totally out of the picture for a 240 Volt Split phase system. What would charge the batteries and how would a person manage the power? Buyers remourse on direct GT vs. battery GT.
19.76kw Solar/GT Enphase IQ7+ MicroInverters
 5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
 6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

Comments

  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    You would need an off-grid inverter/charger, but the size doesn't have to match your panel output, it only has to match your average power draw from the batteries.
    You're looking at 2 use cases: night time draw from bats, and complete off-grid draw (day and night).

    1. Night time with grid support
    Let's say your average night time power draw is 2kW and your peak is 5kW, you can get away with installing a 2kW off-grid inverter and simply using the grid for those few infrequent 5kW peaks when you do some midnight vacuum cleaning and ironing.

    2. Complete off-gird
    During the day your PV array can be supporting the off-grid inverter, so if you had a 2kW off-grid inverter and 4kW from the PV, then you have 6kW of usable power.
    At night you'll be limited entirely by the off-grid inverter, so 2kW max (unless you buy a generator too).

    If you did the above with a sunny island then there's no need to manage battery overcharging, 'cause it will do that for you automatically. You WILL however have to install a transfer switch to take your mini-grid off-grid when the real grid goes down.
    Victron have an auto-switch which will do this automatically: http://www.victronenergy.com/solar/solar-switch/

    If you decide to use a different inverter/charger and backfeed it from the GTI then you'd need to manage battery overcharging. A dump load controlled by the relays in most off-grid inverter/chargers could work.
  • mnittlermnittler Solar Expert Posts: 63 ✭✭✭
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    So would I need to install a charge controller (or something?) between the inverter/charger (maybe a Xantrex XW plus a C60) and the batteries to prevent overcharging?

    Poor Boy wild hair idea: Would it be practical to install a 48vdc battery charger/conditioner and feed the battery charger right off the grid via a Intermatic type mechanical timer during peak power production hours? Would this just make the power go around in circles as the batteries come up in charge and the inverter kicks on which discharges the batteries and they need charging again?
    I could put another Intermatic mechanical timer on the output of this same GT inverter to only be connected to the grid at night. Or make inverter connection never be connected while battery charger is on (work opposite to battery charger Normally OFF/ Normally ON contacts)?

    Since the Sunny Island is only 120VAC? Would you need 2 each of them to make the system work as 240vac split phase? How can you make one each of them 240vac split phase. (20kw/day * $0.12 $/kw = $2.40/day maximum or $876/year maximum potential savings if every kw squeezed out of excess) More realistically $438/year or half of the max savings. I could probably stomach one each Sunny Island with 48vdc of batteries (I was thinking of 6 each 8vdc 100 amp 4JC-100 C&D batteries), but 2 Sunny Islands is a real big stretch.

    Another thought is to take 1 of my 3 solar panel strings and rewire them for battery charging and add another inverter.

    Is there a device that can close a relay contact if you are exporting to the grid? (To turn on other loads or charge batteries)

    I know this is a lot of questions/rambling but just grab and answer the ones you can answer.

    If I had to do it over again I would make the entire system a grid tie battery system but I was just too cheap to do that at the beginning. Come to think of it the cost would have probably been prohibitive and nothing would have been done. Trying to fix the mistake as cheaply as possible and not burn anything down or blow it up. Trying to keep the lights on/or lower the electric bill.
    19.76kw Solar/GT Enphase IQ7+ MicroInverters
     5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
     6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
    18kw Kohler Propane Generator

  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    I'm not familiar with how split phase works, but it would seem that you'd need 2 x sunny islands. The setup would look something like the diagram.

    Except you'd have 2 x SI instead of the multiplus and you'd have a manual switch instead of the "solar switch".
    I've just been perusing SMA America's site and seen that they only offer the 4 and 5kW sunny islands, which I can understand is quite expensive if you have to install 2 of them. Furthermore the 4kW version is a much older product that doesn't have all the features of the 5kw version.

    Do you absolutely have to have split phase? Would a transformer work instead?
  • mnittlermnittler Solar Expert Posts: 63 ✭✭✭
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    Yea, 2 each Sunny Islands at $5,200 each plus batteries is a little hard to justify (pay back). I agree that would be the correct way to approach the problem. If only SMA had made the Sunny Island with a 240VAC split phase option like residential power in the U.S. I could also just bite the bullet and install a Xantrex XW6048, batteries, more solar panels, and rewire one string of my existing 3 strings panels for the same $10,400 but I don't know how to handle any extra power if the existing GT inverters generate more than the required load. How do you protect the batteries from over charging with backfeeding charge?
    19.76kw Solar/GT Enphase IQ7+ MicroInverters
     5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
     6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
    18kw Kohler Propane Generator

  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    You can use a dump load to prevent overcharging, e.g. an immersion hot water heater which you could control with a relay in the XW based on the battery voltage.

    1 XW definitely makes a lot more sense than 2 x SI! It's a pity that SMA have a limited range available in the US. 2 x 2kW SI would have been a nice fit for your situation.

    Note: I don't know much about the XW system, I _think_ you can backfeed it, but hopefully others with more knowledge will chime in.
  • BB.BB. Super Moderators, Administrators Posts: 32,212 admin
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    SephenDV,

    In the US... For residential (and low voltage commercial) power, we have a center tapped 240 VAC transformer on the pole. The "center tap" is grounded at the pole and the home/service entry. So, Leg A to Leg B is 240 VAC. And Leg A to neutral or Leg B to neutral is 120 VAC. If Leg A and Leg B to neutral have roughly equal 120 VAC loads, then the neutral carries almost no current.

    It is similar for 3 phase commercial power--except we have other options (star or "Y", and "Delta" mounted transformers). Which may give us 208 VAC / 120 VAC, or 240 VAC / 120 VAC)... Gets confusing quickly.

    Mnittler,

    Yes, you can "back drive" a XW Hybrid Inverter with 5kW of GT inverter to charge your battery bank.

    But, because the backdriven XW Inverter has no battery charge control in this setup--you would need to add a pair of diversion controllers (for NEC and safety). Or you could set up an AC relay and/or a GT Inverter Inhibit Line (if available in the GT inverter) to shut down the GT Inverter when the batteries are full.

    The advantage to doing this--you don't have to rewire your solar array and can leave it running at 200-600 VDC (or whatever your current GT setup is).

    If you did it the way the XW Hybrid Inverter system is designed--You would need to get a pair of XW 60 Amp MPPT Solar Charge Controllers (or equivalent) and rewire the PV Array to a maximum of ~100 Vmp--which can be a pain as you will need to add a bunch more copper from the array to the charge controllers in order to manage the lower Vmp / higher current configured array. Also, this way the charge state in the battery bank is better maintained and you don't have the dump loads to setup and maintain (dump loads are a crude way of managing battery state of charge).

    Also, when setting up parallel charge controller systems (the AC charger of the XW Hybrid inverter and Dump controllers)--you have to make sure that the dump controllers do not turn on when your XW charger (and other charging sources) are charging your battery bank... A fairly complex configuration setup.

    The other issue is that the addition of doing it the "right way" with low voltage array and MPPT charge controllers--Is that this is "legal" with the NEC and building codes. Back driving a TSW off-grid inverter with a GT inverter is not. The O-G inverters have never been tested/listed in this mode.

    Lots of issues.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,212 admin
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    I should also add--I am not convinced that you would come out ahead by using your GT Hybrid Inverter and battery bank to offset grid loads from day to night using the battery bank...

    Look at your battery specifications and figure out their life... How many years in float service with occasional power failures vs daily cycling to XX state of charge...

    Say they last 10 years in float vs 5 years in cycling service. What does the equation look like?:
    • Cost of replacement bank every 5 years / kWHrs offset over 5 years
    • Vs the cost of Peak vs Off-Peak kWhrs savings.
    Just pulling numbers from the air (or other places):
    So, just battery wear and tear will cost your $0.14 per kWhr (assuming replacement bank every 5 years to support cycling). If you assume that you have the bank anyway and would replace it every 10 years, 1 extra bank replacement every 10 years would reduce your "extra out of pocket costs" by ~2 == or $0.07 per kWHr to offset your power...

    So, what is the spread in your power costs? Mine are ~$0.27 vs $0.09 in Summer and $0.12 vs $0.09 in winter...

    Also remember if you are using Grid Power to do the offset--you have to account for charge controller and battery cycling losses (0.85 * 0.80=) 0.68 overall energy efficiency... So that drives the AC cost spread down to:
    • $0.27 * 0.68 = $0.18 worth of "useful power" time shifted in summer
    • Vs $0.09 night power + $0.07 of extra cycling costs = $0.16 per kWhr
    In summer, I am looking at a $0.02 per kWhrs savings to offset power (using grid power as the offset).

    In winter:
    • $0.12 * 0.68 = $0.08 worth of "useful power" time shifted in summer
    • Vs $0.09 night power + $0.07 of extra cycling costs = $0.16 per kWhr
    So, during the winter, I am paying $0.08-$0.16= -$0.08 to offset the nigh-time power shifted for use during the day.

    Now, if you already have the solar array, the GT inverter setup, a battery bank, and a utility that does not support Net Metering--then, if I did my math correctly (which, lately, is not a good bet :roll: ), then you are paying $0.07 per kWH for using your own power...

    If you put in the system for $45,000 (before any tax rebates) to avoid utility charges (and no net metering) using a 6kW array, PV Watts, south Texas (Corpus Cristi), 6kW panels, 0.52 end to end derating generates, on average 4,943 kWhrs per year.
    • $48,000 + 3,700 extra battery / (20 years * 4,943 kWH per year) = $0.52 / kWhrs
    So--to install such a system to "save money" on a grid tied home, spreading costs over 20 years would cost you, at lesat $0.52 per kWHr (excluding taxes, if an inverter/charge controller fails, etc.). You could reduce your cost of operation by the 30% tax rebates (plus any local ones that may be available).

    This assumes you use 100% of your system's output and pay for extra power from grid or a genset to make up for extra power used (cloudy weather, etc.).

    So, what are your utility costs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mnittlermnittler Solar Expert Posts: 63 ✭✭✭
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    Well, I guess the best bang for the buck is a Honda EU generator for those times that the grid is down for long extended periods of time. I already own a 9.5kw diesel generator but have noticed that the computer UPS in the house stays on battery off and on while it is running.
    Then how would one deal with excess power from the Solar & Wind grid tied inverters/ (in this case not grid tied grid tied inverters) when using the Honda EU? How does the EU deal with any excess incoming power? I would still want power from Solar and Wind if available during extended outages when possible without batteries. Possible? Impossible? Of course this would only be done with the main disconnect to the grid locked open. I also do not want to let the trapped smoke out of the EU or any of the existing components.
    19.76kw Solar/GT Enphase IQ7+ MicroInverters
     5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
     6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
    18kw Kohler Propane Generator

  • BB.BB. Super Moderators, Administrators Posts: 32,212 admin
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    Practically speaking today... There is pure GT inverter that can "share" nicely with a small Honda eu2000i type genset. What may happen:
    1. GT inverter never turns on (does not like/voltage/frequency of generator--more likely with a non-inverter/generator that the frequency will not be stable enough).
    2. Many GT inverters are 240 VAC...You would have to go with a larger eu3000i genset (or other inverter/generator brand) that supports 240 VAC--Or use a setup transformer from 120 vac.
    3. If you get everything connected and working the the euX000i genset, if the attached electrical load is greater than the GT's output (say GT is putting out 2kW and you have a >2kW load (water heater, A/C, etc.)--the system may actually stay connected and your genset will take up the slack between the GT and the load (clouds, surges from load, etc.).
    4. If you get everything connected (GT+euX000i genset)--And your load is less than the GT's output--the GT inverter will see the voltage rise to maximum allowed (approximately 260 VAC or 130 VAC)--And the GT will shut down (should not damage load or genset) and wait 5 minutes to reconnect.
    The above is based on my understanding of the components--and a bit of SWAG.

    Of course, none of this is tested or listed for this type of functional connection and you will be the one responsible to ensure that it worked and was safe (you should be adding fuses to the wiring to the genset to limit short circuit current, if needed, etc.).

    Or--just store a few more gallons of gasoline that you recycle back into the car once a year (with fuel stabilizer).

    Make sense?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mnittlermnittler Solar Expert Posts: 63 ✭✭✭
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    Problem #1: Excess Export:
    Switch to Green Mountain Energy and bank exports and problem solved if consumption is within reason to make it economically feasible conmared to existing power rates.

    Problem #2: Produce power with grid down using GT inverter:
    Disconnect from Grid and install Honda EU 240vac generator. This might work but this still does not solve the problem of producing more Solar & Wind power than consumed? Maybe?
    Has anybody out there actually done this?

    Is there a device out there that would close a contact if over producing, or approaching excess power production? (More power produced than consumed)
    This contact closure could be used to disconnect solar panel string/strings or send staged inhibit signals to GT inverters. What the SMA Sunny Island does is to send a minor frequency shift that turns off the GT inverter for 5 minutes then it tries again. i guess if you do this enough times the sun starts going down and the production is less.

    I just thought that the solution would be easier than it appears.
    19.76kw Solar/GT Enphase IQ7+ MicroInverters
     5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
     6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
    18kw Kohler Propane Generator

  • BB.BB. Super Moderators, Administrators Posts: 32,212 admin
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup

    You are trying to do this with off-the-shelf items not designed for that function--does make it difficult.

    It becomes difficult to float and control the currents when >54 volts, you turn on a 3kW load, and <54 volts you turn the load off...

    What you would like is a PWM controller that is 0% duty cycle at 54 volts, 50% duty cycle at 54.5 volts, and 100% duty cycle at 55 volts... Connect output to your 6kW water heater element (for example).

    Easy to do in concept... Sort of difficult to do exactly (allowing for time and temperature drift common in electronics).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenerPowerGreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Wind & Sun Grid Tie, Now Want to add Battery Backup
    mnittler wrote: »
    Problem #2: Produce power with grid down using GT inverter:
    Disconnect from Grid and install Honda EU 240vac generator. This might work but this still does not solve the problem of producing more Solar & Wind power than consumed? Maybe?
    Has anybody out there actually done this?
    Guess not, this thread is discussing a closely similar situation. As far as "over back-feed protection", you can have 2 AC current sensors and compare their DC outputs, that could be used to trigger "a circuit" when the AC exporting current "approaching" the AC load current, to drive a relay to disconnect the SMA (or adding dummy AC loads). This "circuit" should have sufficient hysteresis to avoid the relay to trip back&forth too quickly.

    On another note, compare your typical AC usage vs. generation, if a small % are "sell", it might not be cost effective to cycle this small % through battery. If it a large percentage, it might be worth it. But if your daily PV production generally exceeding your daily AC loads, you're basically running "off-grid", any "extra" would be wasted i.e. not harvested (no where useful to dump)

    GP
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