Offgrid system with backup generator question

[Engy]

I have a project to design a 40KWp system for a school that doesn't have grid connection. They do not need storage, they are planning to relay on solar + generator. I m kinda confused what type of inverter to use? I know that ongrid should be connected to a grid to synchronize with its frequency. But the fact that we do not need batteries make me think that the offgrid inverter is not a right decision. I m planning to use SMA. So what inverters are needed u think? Also it is expected that grid will be connected in a couple of years.
Thanks!

[ButchDeal]

I have a project to design a 40KWp system for a school that doesn't have grid connection. They do not need storage, they are planning to relay on solar + generator. I m kinda confused what type of inverter to use? I know that ongrid should be connected to a grid to synchronize with its frequency. But the fact that we do not need batteries make me think that the offgrid inverter is not a right decision. I m planning to use SMA. So what inverters are needed u think? Also it is expected that grid will be connected in a couple of years.
Thanks!

The "FACT" that you do not need batteries is FALSE!
any off grid system is going to need batteries other than small direct use systems (like a pump directly tied to solar).
You can not use any grid tie system.
If the grid will be there soon then you could use a bimodal system to be able to utilise the grid when it shows up.
You could plan on using AGM batteries and a minimal bank with the generator though.

[Engy]

so how can i decide on the size of the minimal bank? do the exact calculations i do while designing offgrid system or does the generator presence make a difference in that?

[ButchDeal]

The generator absolutely makes a difference. I suggested AGM as well as they can take a higher charge rate (allowing a smaller bank).
The key is getting a decent usage idea to come up with the battery plan. The batteries need to be able to handle the full charge from the solar panels, thus the reason for the AGM batteries. AGM are less maintenance and can handle a higher rate of charge. If the generator is on auto start then the batteries need not be able to run the school all day, just handle the charge rate. It is still going to be a large battery. Particularly if the school is only used during the day hours.
The batteries would be cycled quite a bit and would likely need to be replaced in 4 years or so. But if the grid is there then you can replace the batteries at that time and reconfigure the system to use grid power.
You want to make sure that the inverter/charger is bimodal so you can connect to the grid latter.

I would suggest getting a SMALL bimodal system that can handle some lights, security, whatever at night, morning; and running the school on generator till grid shows up.

[Engy]

ok, so reading again i m trying to understand WHY i have to have batteries? Is it to avoid having extra charge from solar panels running into the generator? what happens if the batteries are already fully charged?

[adoublee]

PV paralleled with a generator is not going to work out well for you. Assuming a grid-interactive inverter would even accept the generator as source it will parallel with (probably not), the generator would be damaged if any PV current is pushed back into it. Only way to avoid that is to be able to perfectly ensure the load is always equal to or greater than what is available from the PV - not practical if you want the PV to be of any significant size in relation to the generator.

A "microgrid" where a battery sized large enough to be a buffer is conceptually the solution. Orchestrating this may not cheap or uncomplicated. Simple and cheap method is probably a source selection between to two, with PV using some level of battery to enable off grid operation (the better the load matched relatively well to PV production, the smaller they battery capacity can be depending on what type of equipment is used).

Ideal Power's 3-Port converter may be something you want to look at.

[SunEagle]

I have a project to design a 40KWp system for a school that doesn't have grid connection. They do not need storage, they are planning to relay on solar + generator. I m kinda confused what type of inverter to use? I know that ongrid should be connected to a grid to synchronize with its frequency. But the fact that we do not need batteries make me think that the offgrid inverter is not a right decision. I m planning to use SMA. So what inverters are needed u think? Also it is expected that grid will be connected in a couple of years.
Thanks!

Unfortunately you still need some type of battery system to use solar for off grid applications. Especially with loads that exceed a few hundred watts.

Having a generator will help keep the battery system small but you still need a battery that is matched to your panel wattage and charge controllers.

[Sunking]

You have to have batteries period. If you demand 40 Kw peak demand is going to be a very large expensive battery. No way around it. So dig real deep in those pockets, and remember the battery used the way you intend to use it will need replaced every couple of years.

Here is a clue. A battery that can deliver 40 Kw peak demand is going to weigh in around 19,000 pounds and cost $300,000 to $600,000 USD. Enough for most people to live their whole life time. You will be replacing that beast every 2 or 3 years the way you will abuse it. .

[Engy]

so the battery bank has to be 40KW? i thought i wouldn't need the battery size to be large since there is a generator. no?

[RedDenver]

You could run entirely off a generator until grid power arrives and then add grid-tied solar at that point.

[Engy]

ya that seems like an option