Charging battery pack with AC diesel generator

schmekschmek Posts: 34Registered Users ✭✭

I'm currently designing a large hybrid energy system with a battery pack, solar array and diesel generator. I am using the program Homer Energy for simulations, where I use a control strategy allowing the generator to run at full power and charge the battery with excess power. 

What is the best way to convert the AC power comming from the generator to DC power into the battery?

I've read about rectifiers, battery chargers and bi-directional inverters, but I'm not really sure how the most optimal way of connecting the system will be. Should I have a seperate inverter for the solar array and battery, and use a rectifier/battery charger for charging the batteries with the generator? Or should I use a common bi-directional inverter that will convert both DC power from the solar array to AC, and AC power from the generator to DC charging the battery?

Kind regards, 


  • mcgivormcgivor Posts: 1,761Registered Users ✭✭✭✭
    An inverter charger would probably be the best solution, it incorporates a charger designed to charge a battery proportional to the inverter's needs, or a complete system. When factoring in the costs of  buying separate components as apposed to purchasing a system, there is little gain and the components would be mismatched 
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • schmekschmek Posts: 34Registered Users ✭✭
    Okay thanks a lot for your answer!

     Just to be sure, if I use string inverters or microinverters, charging the batteries with the generator will not be possible without a seperate rectifier? In other words, rectifiers aren't/can't be added into the standard inverters?
  • BB.BB. Posts: 27,444Super Moderators admin
    How big is "big" system? Roughly, where located, daily power usage (kWH/Watt*Hours), size of battery bank, size of solar array, size/type of loads, etc...

    Just as an FYI--This thread is about a relatively large off grid hybrid power system and the problems they had that where (primarily?) caused by having the "wrong engineer" configuring the system.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • schmekschmek Posts: 34Registered Users ✭✭
    It is for a project where I'm designing a floating solar power platform and batteries to help deliver electricity to a fish farm. The excisting energy supply is diesel generators, and I want to integrate a solar system and batteries into the existing system. Daily power demand is around 1000 kWh with a peak of 100 kW during the day. I am currently working on sizing the solar array and battery bank with the simulation program Homer Energy. 

    The problem is that I am not sure how to most efficiently connect the system in real life. I want the AC generators to be able to charge the batteries, so they can run on optimal loading and charge the batteries with excess energy. For example, do I want to have an inverter charger on land and have DC cables going from the platform to the inverter, or do I want to use microinverters and have AC cables going from the platform to the inverter? The last option requires a rectifier for charging the batteries with the generators.

    Any tips are much appreciated.
  • schmekschmek Posts: 34Registered Users ✭✭
    An important notice here is that the generator should be able to charge the batteries and serve a load at the same time. I also want the solar power to serve the load at the same time. Maybe that will be problematic with an inverter charger?
  • BB.BB. Posts: 27,444Super Moderators admin
    Where will the system be installed--Somewhere in Norway? What is the area like... Deep fjords that get little winter sun? Lots of Marine Layer?

    Do you want 100% of the power to be solar+batteries for ~9 months of the year?

    Or do you want to just reduce diesel fuel and maintenance costs--But still plan on running the diesels a few hours a day or every other day (and near constantly in winter)?

    More or less, you can have solar=>GT Inverters=>Diesel AC and cut back on daytime fuel usage...

    Or, for 4x more expensive solar system, Solar=>charge controllers=>battery banks=>AC inverters=>AC powered pumps (water/air). With Gensets connecting through hybrid solar AC inverters (support both GT and Off Grid operations and have integrated AC to DC battery chargers).

    Looking at pumps and motors--Have you picked motors that are efficient (permanent magnet motors+VFD--Variable Frequency Drives tend to be more efficient than standard AC induction motors). And different pumps have different efficiencies too (water pumps that have their pump body below the water line--positive inlet pressure--tend to be more efficient than pumps that "draw/suck" water into pumps above the water level (negative inlet pressures).

    The basics can be done--But you are talking about some pretty large battery banks, gensets, and inverters.

    We can possibly help with the basics, but you would need to get some serious power engineers involved for a safe and reliable system (in my humble opinion).

    For example--Just a simple 48 volt lead acid battery bank. 2 days of stored energy and 50% maximum discharge... Basically the battery bank is 4x the daily load (and tends to be a closer to "optimum" design for charging and daily use system):
    • 1,000,000 WH per day * 1/0.85 AC inverter eff * 1/48 volt battery bank * 2 days storage * 1/0.50 max discharge = 98,039 AH @ 48 volt flooded lead acid battery bank
    That is a pretty large battery bank... And if we go with sizing for 13% rate of charge (full time off grid) for a "happy" battery bank:
    • 98,039 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 976,570 Watt array "nominal"

    And based on amount of sun in Norway with a south facing, adjustable tilt array:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel where the angle is adjusted each month to get optimum sunlight.
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec

    Toss the bottom 3 months, gives us February with 2.06 hours of sun per day:
    • 1,000,000 WH per day * 1/0.52 off grid AC system eff * 1/2.06 hours of sun = 933,532 Watt array for "break even" February
    Sizing the genset... Assume 100 kW load + 10% minimum charging and run at 80% of rated output:
    • 100,000 Watts AC load + (98,039 AH * 59 volts * 1/0.80 AC battery charger eff * 0.10 rate of charge minimum) = 823,038 Watts total load (note the AC battery charger may have power factor issues---Watts /= VA ratings)
    • 823,038 Watt load * 1/0.80 genset nominal rated load = 1,028,796 Watts ~ 1 MWatt genset
    My guess is that you do not need all of this power in one location/for one application... Say you have 20 "fish pens", then design 20 separate systems so you can move them around where needed plus a few for backup/spares/servicing back on shore....

    Also--Inverter-Chargers are (more or less) synchronous rectifiers for charging (more efficient than a standard AC to DC rectifier/battery charger). And tend to be "cheaper" because the synchronous switching is used both for inverting (DC to AC power), and charging (AC to DC power)...

    Basically, the inverter-charger is "bi-directional". It supports power flow in both directions depending on how the electronics are configured and what "mode" is needed at the time. And they can do things like "AC Support"--Where you have (for example) a 5 kWatt load that takes 2.5 kWatts from the genset and 2.5 kWatts from the battery bank (+solar if available).

    Or, take 10 kWatts from the genset and supply 5 kWatts to the AC load and 5 kWatts to charging the battery bank...

    In the end, if I did not get the math wrong (always a possibility that I slipped a decimal point)--This is a large system that will need a power engineer and very careful system design and seeing what equipment (inverters and such) are available for your scale of system (one large system or 20x smaller independent systems).

    And there are other ways of possibly solving the problems... For example, setup DC solar system and DC powered pumps (typically a version of VFD--Variable Frequency Drive--That takes Solar DC in and outputs, typically 3 phase power to Induction or Permanent Magnet motors). VFDs are another "magical" electronic device--Can support "soft start", efficient PM motors, and also vary frequency based on amount of sunlight and loads required (i.e., runs slower RPM in morning and evenings, and rated RPM during day). And run diesel at night (skip the whole battery thing). Some VFDs have been designed to take both AC and DC inputs to their VFD... See for one mfg.

    Or hybrid systems... Lots of power during the day, and a limited amount of battery power needed at night. I try to reduce the usage of batteries as much as practicable... They tend to be the "weak and expensive" point of most system designs.

    Good Luck!
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • schmekschmek Posts: 34Registered Users ✭✭
    Thanks a lot for your detailed answer :smiley:
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