Design Challenge- PV, Battery, Grid, Generator all in one system

Dear All,

Every time I check this forum I learn something new, thanks to everyone involved, today was the cloud effect on PV systems :)...

I decided to take a design challenge I have and see your opinions about it:

Challenge: Optimize PV system for showroom+manufacturing facility

Grid Situation: Electricity is cheap $0.1/kWh, though intermittent in cycles of 6 hours. Which means if grid is On at midnight, it will go off at 6AM and come back at 12 Noon, go Off and 6 PM and so on.
Generator: Thus the need for a generator, the generator is relatively large (>200kVA) due to the presence of a manufacturing facility and machines and has a cost of almost $0.3/kWh when consumption is large enough.

Consumption Situation:
- 3-phase, 380V, 50Hz following EU standard grid power
- The manufacturing facility will not be run all the times thus the generator should be ON only when manufacturing facility needs it, or batteries are at low level
- Showroom consumption: approximately 10kW power (during daytime) with a simultaneousity factor of 0.7. Elevator also in the facility used not so frequently, power 10kW.
- Nighttime and weekends: some power needed for a few lights facing the streets

Analysis so Far:

- I previously developed a methodology in HOMER to simulate intermittent grids (if you are interested: and the simulation will not stop using the expensive generator and always sacrifices in the battery bank size. Even though simulating the PV/Battery system will produce lower COE's relatively.
- If we will connect all the showroom through the inverters, thus we need at least 3x7kW inverters, but the space we have for solar panels will only be for 12kW and the battery bank (size still debatable)
- I also thought of oversizing batteries relative to PV capacity and thus they can help during peak hours, in case the whole showroom was connected through the inverters.
- The other option is to have the AC from the PV/Battery system join with that of the generator, in that case we will need a synchronizer, as far as i know. To prevent backfeed and control the outputs of the generation units. Joining with the grid is easier since inverters usually come with that option.

My questions:

- Did i overlook a better method for connecting such a system?
- Do i need a synchronizer to prevent backfeed or are there other reliable and cheaper systems for that? Because i rather oversize the inverters than buy a synchronization panel

(Batteries are a must for nighttime, dual mode inverter will probably be used, generator can be turned ON to recharge batteries when/if needed)

Awaiting your opinions.

Thank You

Best Regards,
Elie AJ


  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Showroom consumption: approximately 10kW power (during daytime)

    welcome, as you know this is a ot of power for daytime use... but some/all? of it may/can be supplied by PV when there is sunshine... BUT first, have you taken any conservation measures, change to LED lighting? any lowering of this consumption will be a benefit in the long run...
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  • BB.BB. Super Moderators, Administrators Posts: 30,184 admin
    Short term answers may be:

    1. Large genset for factory
    2. small(er) genset for front office loads
    3. Battery System (possibly with solar) for security/weekend lighting

    You can build larger solar power/battery backed systems... But cost wise, it is going to be difficult to match the cost of utility power/generator power (capital costs, maintenance, batteries every 5-8 years, new electronics/inverters/chargers every ~10+ years, etc.).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MelLandryMelLandry Registered Users Posts: 5
    If the only limitations on grid power are up-time, and you've determined grid costs to be the lowest option, why wouldn't you store grid power in a battery bank?
  • eaboujaoudeheaboujaoudeh Registered Users Posts: 7
    [USER="379"]westbranch[/USER] : Yes of course, all necessary EE measures have been taken
    @BB. and [USER="16410"]MelLandry[/USER] : We have RE and EE soft loans, and to be eligible the system has to be based on a RE measure. Thus batteries will not benefit from this loan. A PV/Battery system will as long as the storage is less than 7800W.h/kWp.

    Is there a rule that allows one to connect a PV system to a generator considering the generator as a grid? I can imagine if the PV system is relatively very small compared to the generator then the risk to damage the generator with feedback would be non-existent?

  • BB.BB. Super Moderators, Administrators Posts: 30,184 admin
    I am not sure I understand your question... The question can cover a wide range of issues. First, here is a discussion on the major types of AC inverters, how they work, and are connected:

    battery-less Outback Radian (GT/Off Grid/Hybdrid Inverter options explained)

    Next, the specific question of how to connect to a genset.... A "standard" off grid battery backed AC inverter would connect to the Genset or the Utility and simply use the AC power source for either directly powering the AC loads (pass through mode) and recharge the battery bank from the AC source. Or, if the AC source "goes away", then the AC source is disconnected and battery power is used to power the AC inverter and power the loads connected to the output of the AC inverter.

    Then there is the GT inverter... The standard Grid Tied Inverter (solar panel -> GT inverter -> AC mains) takes 100% of the available energy from the solar array and "dumps" that energy into the AC mains. The actual power is either used "locally" and/or sent back upstream to the AC source to be used elsewhere (nearby utility customers, etc.). A GT Inverter does not "regulate" voltage/frequency/current--It (more or less) simply dumps energy into the AC mains and the AC source (utility generators) set your 230 VAC 50 Hz power quality. For all the world, it is very much like the alternator simply dumping energy into the car's electrical system. The car's battery "regulates" the 12 volts (absorb charging current or supplies current to power loads when the engine is not running). If the grid voltage is too high/low and/or the grid frequency is + 1.0 Hz too high/too low, the GT inverter simply turns off for 5 minutes waiting for the grid voltage/frequency to stabilize.

    There is also a "hybrid AC inverter". It is basically an Off Grid inverter that can also behave like a GT inverter and feed excess solar panel energy back to the grid.

    Now--Note that a GT inverter (and Hybrid Inverter) treats the AC power source (utility power) for all the world like a giant (infinite) AC Battery Bank.The "battery" (utility) can supply or accept all loads/"charging" current from the solar array (and turn the electric meter "backwards" when "recharging" the grid).

    AC generators (really "alternators") cannot usually accept "excess" (or charging current) from a GT inverter. Generators are only designed to output energy. If they receive energy, that energy would have to "go somewhere" -- Like spinning the generator faster (genset actually "spins" the diesel motor faster than 50 Hz) or into "heating" the electric windings.

    So, for a GT + Generator, the AC loads would always have to be larger than the GT Solar power so that there is never any energy "backfeeding" the generator. If backfeed does occur, either the genset spins "too fast" (GT inverter shuts down, Genset faults/shuts down) or you get damaging currents in the genset (windings overheat, etc.).

    There can be some other issues... One is that the genset needs to be pretty accurate in its frequency/RPM -- 50 Hz + 1.0 Hz. Many smaller gensets (portable generators) are not accurate enough for a GT inverter to "lock on" to the AC input power (GT inverter will never "turn on" or "connect" to AC power).

    You are talking about relatively large levels of power and you need a very good electrical engineering team with experience designing/connecting such systems. And even then, you can still have problems. For example, here is a discussion about a (relatively) large solar+genset installation for a large villa and the headaches that that owner/caretaker had:

    Having a good power engineer (check references) will be absolutely essential to make your system work. And if you are doing something that nobody has done before--You may have a lot of issues to address to get the system to work (or it may never work and you have to start over with new/different equipment).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • eaboujaoudeheaboujaoudeh Registered Users Posts: 7
    [USER="512"]BB.[/USER] : I did work with similar systems before, but the systems were 1-5 kWp connected to gensets which were in the orders of >100kVA. And most of the systems were dual mode (ON/OFF grid) but they were not set to backfeed. This system requires that all the loads to be supplied by the inverter are smaller than its capacity (priority line). The other loads are supplied direct power when the generator of grid are on.

    In this case, if I want to create the same setup and put a priority line, then my inverters will be 27kW to supply the power from 12kW of panels due to space limitations. I wanted to think of a way to save money on the design, thus put a smaller inverter and have it supply to the power line coming from the generator. This method saves fuel from the generator due to lesser energy needs from it. The only drawback is what you described earlier. To prevent this drawback you usually would use powersave or similar to make sure generator doesn't drop below its 30% performance and to ensure no backfeeding occurs. But in this case, such a unit will be very costly since it is a small PV system and thus it will not be able to return on the investment, thus the investor loses money.

    I wanted to see if there was a directive or rule that allows you to directly connect a PV system to a generator as mentioned above without having to put a controller/powersave/other...for example a 1kWp system connected to a 100kVA generator has virtually little or no risk due to the large difference in capacity. Can you treat a 12kWp system connected to a 150kVA generator the same?
    My other question was if there was another mode of connection that i was missing, but judging from your reply, I think you covered them all...


  • BB.BB. Super Moderators, Administrators Posts: 30,184 admin
    If the loads are always larger than the GT Inverter's output (i.e., 5kWatt of GT inverter and 6+ kWatts of load), then as long as the generator frequency/voltage is stable enough for the GT inverter to sync with--You should be OK.

    Of course, what happens if you pop a circuit breaker to your 6+ kWatt of base loads and the GT now starts to (try) back feed the genset--Will it be damaged or will the GT inverter "fault" due to voltage/frequency regulation issues--That I do not know.

    There is another mode for some Off Grid (/Hybrid) AC inverters--It is called generator support mode. More or less, the OG inverter behaves like a UPS (AC in, battery, AC out) but if there is excess solar energy available, the AC inverter uses less AC power from the Genset and uses Solar+Battery power to supply energy to the loads. [h=3]Demonstration of Generator Support [/h] It is a possible solution for you... Mixes Generator+AC Mains with solar.

    This can be a complex subject--And not all inverters implement generator support the same way or how you might have wanted to do it.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • eaboujaoudeheaboujaoudeh Registered Users Posts: 7
    That is a great article...thank you...btw the video is not working but i did find it on youtube...

    I have one question about the setup though, suppose we have a 6kW inverter and 6 kW generator, we would be able to supply loads of 12kW? If i understood correctly...thus the energy of the generator does not pass through the inverter but they are connected to the loads together.

  • BB.BB. Super Moderators, Administrators Posts: 30,184 admin
    In theory, yes, you could add the genset+inverter capability... In practice, the AC inverter usually has a fix/limited maximum output current. And for a 6 kWatt inverter, it would normally be limited to 6kW/kVA and a ~12kW/kVA surge (few seconds to few minutes, depending on inverter specifications).

    For many folks, it allows them to use a smaller genset (say 4kW) and run larger loads (like a well pump) for short periods of time with the larger inverter. A smaller genset running near maximum rated load (typically ~80% maximum for non-commercial rated gensets) for best fuel economy (running base loads, battery charging) and the inverter supports the high surge current/short term loads.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • eaboujaoudeheaboujaoudeh Registered Users Posts: 7
    Well then, guess this settles it, you need an inverter with a capacity to sustain all the loads in such a system. In all cases the issue was solved with the user, apparently his initial projection was wrong, and a PV system will be able to sustain his needs. He will need to turn on the generator for using heavy machinery, but he only needs them for few minutes per day since he only is interested in showcasing them.

    Thanks for the help...
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