Charging battery with solar while using battery for building loads

[Jackbenimble7]

Hello all. I am working on a microgrid project where we have the following scenario. There is solar, battery storage, and a fossil-fuel generator installed on the load side of the existing switchgear. We are using a microgrid controller to island the system when the utility goes down. When the utility goes down, the battery picks up the load of the building and provides power until it discharges to a certain level. Once it hits a certain level of discharge, the battery will shut down, and the microgrid controller will send a signal to the generator to kick on and start running. The generator will run until the utility service comes back online.

Two questions.

1. When the battery is providing backup power, can the installed solar also charge the battery? I know that with the Tesla Powerwall, this is possible through the use of their Gateway, but for commercial sized solar/storage without a dedicated connection between the two, I'm not sure.
2. When the generator is running, can solar provide backup power to the building loads? I lean towards no on this, that the solar inverter will not be able to synch with the generator and the output of the solar array may damage the inverter.

Any insight would be much appreciated.

 

[winnie]

Just some musings:

In principal you can have a PV 'charge controller' that does its best to maintain the state of charge of the battery, operating at the same time as DC loads on that battery. So instead of having a common PV system with inverters to supply the AC mains, you have a PV system dedicated to keeping the battery charged, and the battery inverter then supplies the AC mains. In this way you never have to select between PV or battery, you always use both.

In general solar inverters do not play nice with generators. The problem is not synchronization, but rather that most inverters are designed to provide as much power as possible to the grid. If you try to connect such a PV system to a generator, and PV production exceeds local consumption, then the generator is not capable of absorbing the excess power.

If your generator is controlled based on the state of the battery, then PV generation will never exceed consumption. If the battery is fully charged and available PV exceeds consumption, then the generator will be off and the charge controller will be reducing PV generation. If the battery is discharged and the generator is running and more PV becomes available, then the battery will absorb more energy, increasing consumption to match available PV.

As I said, just musings. Others on this forum actually do PV installs.

-Jon

 

[gadfly56]

Just some musings:

In principal you can have a PV 'charge controller' that does its best to maintain the state of charge of the battery, operating at the same time as DC loads on that battery. So instead of having a common PV system with inverters to supply the AC mains, you have a PV system dedicated to keeping the battery charged, and the battery inverter then supplies the AC mains. In this way you never have to select between PV or battery, you always use both.

In general solar inverters do not play nice with generators. The problem is not synchronization, but rather that most inverters are designed to provide as much power as possible to the grid. If you try to connect such a PV system to a generator, and PV production exceeds local consumption, then the generator is not capable of absorbing the excess power.

If your generator is controlled based on the state of the battery, then PV generation will never exceed consumption. If the battery is fully charged and available PV exceeds consumption, then the generator will be off and the charge controller will be reducing PV generation. If the battery is discharged and the generator is running and more PV becomes available, then the battery will absorb more energy, increasing consumption to match available PV.

As I said, just musings. Others on this forum actually do PV installs.

-Jon

Why can't the controller require a manual reset once the batteries hit their low limit and power is switched to the generator? The generator and PV never see each other at the same time.

 

[winnie]

Why can't the controller require a manual reset once the batteries hit their low limit and power is switched to the generator? The generator and PV never see each other at the same time.

Ideally both generator and PV run at the same time, but this requires careful design to make it work.

-Jon

 

[Flicker Index]

What sort of facility and what is your overall goal? (kWh reduction or kW demand reduction?)
If you use complex system that lowers everyday efficiency and have considerable phantom consumption of its own, you could easily negate the point of having solar toy.

Ironically, the keep hot heaters to guarantee <10 second startup uses several magnitudes more electricity from the grid than the generator ever produce in their useful lifetime. So if you use solar power to heat the generator's heater, that can cut down on generator's electricity usage.

 

[Jackbenimble7]

Some interesting replies here. I'll add some of my own thoughts.

For smaller scale residential designs utilizing solar, Tesla Powerwall, and a backup diesel generator, you can design the system without a microgrid controller so that when the utility fails, backup power shifts to the battery and solar. The solar and battery are connected via the Tesla Gateway, which allows solar to charge the battery while the battery is discharging to local loads. Under this scenario, the diesel generator is connected to the service panel via an ATS. Once the battery discharges to a specific, pre-set level, it will shut down and cause loss of power to the whole system. The ATS senses the loss of power and flips the source from the Powerwall/solar to the backup diesel generator. The generator will then run until the utility comes back online, at which point the ATS will flip back over to normal operations.

For the project that I am working on, it is a bit more complex, as we are installing all generating sources on the load side without an ATS. The microgrid controller is what we'll use to communicate with all of these devices. The controller will be used to send signals back and forth to tell these devices how they should operate.

Again, my question really comes back to: what is the harm in running solar in parallel with a diesel generator. Say the electrical load varies between 40kW and 100kW, while the solar's peak is 50kW, and the generator's peak is 100kW. Are there issues supply building power using both solar and generator? This video is the closest to an answer I have found:

Winnie - you mention that a generator and PV can run at the same but this requires careful design to make it work. What sort of careful design are you referring to? Programming settings of the microgrid controller?

Conservatively, I'm saying that none of these generating sources can run in parallel, with the exception of the battery and the solar. One thing I don't know is whether I can charge a commercial battery using solar while discharging that same battery to feed building loads. I know that you can do it using the Tesla Powerwall and their Gateway, but I don't know about other batteries.

 

[winnie]

There is one key question to answer if you want to run PV in parallel with a generator: "What happens when the PV output is greater than local consumption?"

_Residential_ PV systems are designed to be 'line interactive', to look for power from the AC side, synchronize their output to the mains, and then supply as much PV production to the mains. When connected to the grid, this is exactly what you want to do: supply as much energy as possible, some of which will be used locally, the rest which goes to the grid.

Now what happens when the same system is on a 'microgrid' with only local loading and a local generator?

If the local loading exceeds the PV output, then everything works just fine. The generator provides 'excitation' for the system, PV output reduces the load on the generator, but the generator still has some load and is working as designed.

But if the PV output exceeds the local loading, then you have a problem: the PV system is doing its best to pump out as much energy as possible, but the local load cannot absorb it, and the generator is also not capable of absorbing the energy. This is where 'bad things happen'.

If you are lucky and using well designed equipment, either the inverter or the generator will shut down on overvoltage, and everything will be fine except for having to reset the equipment. If you are not lucky then you have to replace equipment.

I do not know the details of a 'microgrid controller', however if you can control the PV system to reduce its output so that the generator is always operating at a reasonable load, then you can operate PV and a generator at the same time.

The key is to have a method of dealing with the PV power when it exceeds local loading. Since you can always operate PV panels at reduced efficiency by changing the operating point, this is strictly a matter of controller limitations. If the controller stupidly always tries to get maximum output from the PV panels, you will have problems. If the controller can dial back the output as necessary the you can parallel PV with other sources.

-Jon

 

[jaggedben]

Hello all. I am working on a microgrid project where we have the following scenario. There is solar, battery storage, and a fossil-fuel generator installed on the load side of the existing switchgear. We are using a microgrid controller to island the system when the utility goes down. When the utility goes down, the battery picks up the load of the building and provides power until it discharges to a certain level. Once it hits a certain level of discharge, the battery will shut down, and the microgrid controller will send a signal to the generator to kick on and start running. The generator will run until the utility service comes back online.

Two questions.

1. When the battery is providing backup power, can the installed solar also charge the battery? I know that with the Tesla Powerwall, this is possible through the use of their Gateway, but for commercial sized solar/storage without a dedicated connection between the two, I'm not sure.
2. When the generator is running, can solar provide backup power to the building loads? I lean towards no on this, that the solar inverter will not be able to synch with the generator and the output of the solar array may damage the inverter.

Any insight would be much appreciated.

In theory anything is possible. Especially when the details about equipment and other fundamental system design parameters are completely vague. While it's true that most solar inverters made in recent years are not designed to work with a generator, there's nothing physically impossible about it. This is an area that's seeing a lot of changes right now and in a few years we may come to be regarded as ordinary.

As a wild-guess for the general case, I would say the answer to your question 1 is probably yes, when the load is less than the solar can provide. Your battery inverter(s) (if it's not the same as your solar inverters) will have to manage that somehow, with frequency shifting or something more sophisticated. And the answer to your question 2 is probably no, unless you are either working with a solar inverter(s) specially designed for that purpose, or unless you are essentially manufacturing your own solution.

 

[GoldDigger]

At low power ratings (say 10kW or so) there are solar inverters, called hybrid inverters, which can operate both strictly standalone and strictly grid interactive, with the determination based on whether POCO power is available.
But some of these are also rated for "generator support" via a second set of AC input terminals which configure the interactive inverter to accept a wider voltage and frequency range. By monitoring the (directional) power through those AC terminals and the load power the hybrid can throttle back the inverter to maintain a set minimum power load on the inverter, or if the PV output is high enough shut the generator down and switch the inverter to standalone mode. The question is whether there is such equipment available for the power range your grid requires.

Alternatively, there are interactive inverters which can be throttled back by a command signal rather than frequency shift. Look at what is offered specifically for the Hawaiian market.

 

[Jackbenimble7]

In theory anything is possible. Especially when the details about equipment and other fundamental system design parameters are completely vague. While it's true that most solar inverters made in recent years are not designed to work with a generator, there's nothing physically impossible about it. This is an area that's seeing a lot of changes right now and in a few years we may come to be regarded as ordinary.

As a wild-guess for the general case, I would say the answer to your question 1 is probably yes, when the load is less than the solar can provide. Your battery inverter(s) (if it's not the same as your solar inverters) will have to manage that somehow, with frequency shifting or something more sophisticated. And the answer to your question 2 is probably no, unless you are either working with a solar inverter(s) specially designed for that purpose, or unless you are essentially manufacturing your own solution.

I would add detail about the equipment, sequence of operations, and other system design parameters, but I don't think it would help. What I am looking for is someone who has done this installation before and knows what problems may arise. This sort of microgrid stuff is brand new to everyone, so I know there isn't a lot of experience out there yet.

 

[winnie]

This site has a mix of active and retired electricians, folk from the manufacturing side of things, theory guys, etc.

I am a theory guy and have no direct experience with the sort of install you contemplate. But I have the physics of the system _down_.

I think you will get more useful information from this group if you do provide more details. But be warned: discussion threads have a way to wander and lose sight of the original question. You might be even better served if you post a couple of related threads with specific questions, such as this thread where you are specifically looking for someone with actual experience in this scale of installation, and a separate thread discussing the specifics of the PV to battery connection, etc.

Good luck, and please post what the final layout of your system is, so that we can all learn how such systems go together.

-Jon

 

[jaggedben]

I would add detail about the equipment, sequence of operations, and other system design parameters, but I don't think it would help. What I am looking for is someone who has done this installation before and knows what problems may arise. This sort of microgrid stuff is brand new to everyone, so I know there isn't a lot of experience out there yet.

Yeah I don't really agree with your premises. There were some manufacturers supporting your type of installation over a decade ago and even then I think the answer to your question #2 was different between them. (Actually, perhaps only SMA's multicluster supported a >20kW system, but the point stands.) Now there are a bunch of others in field. So this isn't that new and people have come up with different answers.

Mind you I've only worked on single phase systems under 20kW when it comes to solar+storage, and I haven't integrated generators (yet). My impression (from industry literature and such) is that for larger systems people either scale up smaller designs or they custom engineer something (and probably have it UL field listed). So which do you think you will do? If the former, consult the product literature. If the latter, the world is your oyster, if you know what your doing. But my guess is that other people who've gone that route aren't going to share their trade secrets here.

 

[Jackbenimble7]

This site has a mix of active and retired electricians, folk from the manufacturing side of things, theory guys, etc.

I am a theory guy and have no direct experience with the sort of install you contemplate. But I have the physics of the system _down_.

I think you will get more useful information from this group if you do provide more details. But be warned: discussion threads have a way to wander and lose sight of the original question. You might be even better served if you post a couple of related threads with specific questions, such as this thread where you are specifically looking for someone with actual experience in this scale of installation, and a separate thread discussing the specifics of the PV to battery connection, etc.

Good luck, and please post what the final layout of your system is, so that we can all learn how such systems go together.

-Jon

Thanks Jon. Some of my frustration probably leaked out there. This is a design I've spent a lot of time thinking about (and of which, after speaking with multiple vendors and design engineers, not a lot of information/previous projects exist), and to hear others say what they believe is possible rather than what they have seen installed is a little difficult to hear.

There are many ways to skin the cat here. As I mentioned with the Tesla Powerwall, there is a solid sequence of operations/design in place to allow for solar, storage, and a fossil fuel generator to coexist. Granted, the elegance and simplicity of the Powerwall design is that it uses an ATS to physically separate the solar/storage from the fossil fuel generator. It guarantees that the gen will never run in parallel with the solar/storage. Once you are asked to mix a generator with solar/storage, that is where problems arise.

JaggedBen - you raise a good point about scaling up something small vs. custom engineering. That is a question that the person funding this operation will need to answer. Like you say, anything is possible, just depends on how much you want to pay and how much irritation you're willing to endure.

I will keep this post updated as I learn more.