8 ms·

Great article, lots of technical details. A simple answer to the 'Why' can be found in the middle under the 'Off Grid Without Batteries' headline. > If you hav

Great article, lots of technical details. A simple answer to the 'Why' can be found in the middle under the 'Off Grid Without Batteries' headline. > If you have a typical grid tied system (microinverters or normal string inverters, so easily 95+% of installed rooftop solar), the system is technically incapable of running off grid (without additional hardware). There's no waveform to sync with,

raverbashing 8y ago
"there's no waveform to sync with" is technically correct and a very poor excuse A residential no-break has no waveform to sync with as well. Something capable of syncing to the grid and then more or less keeping pace even if the main grid goes down should cost very little today. (And when the grid goes back it shouldn't have drifted too much unless something ridiculously big happened)
- TheSpiceIsLife 8y ago
  When the grid goes back up the system would check the grid frequency and phase, and slowly match that waveform over a few minutes by making light changes. When the two are aligned within a good-enough tolerance the system will switch back to it's regular state of mains + solar ( + batteries + wind + generator + etc / whatever). These are all solved problems with commercial off the shelf components.
  - zaarn 8y ago
    Are these commercial off the shelf components as cheap as a microinverter?
    - TheSpiceIsLife 8y ago
      I just typed microinverter in to Google and clicked on shopping. It looks to me like microinverters are a commercial off the shelf product? Will a microinverter do all of the things a multi-component phase-syncing system with automatic transfer switches do? I don't see why a microinverter can't be built with these components integrated. I can't tell you if such a unit exists as I'm not well versed in the product range. As far as a price comparison goes, I guess it only makes sense to compare a like-for-like system?
      - zaarn 8y ago
        From what I know, you will need a sync check relay, ie a relay that only closes when you're synced up. Those are generally available for 1MW and upwards (one manual notes a minimum constant load on the internal grid of 500 kW or it won't work), look like about the size of half of a car battery, at a price of "contact our sales team". Could it be made cheaper? Probably. If you get it wrong, the grid probably doesn't care but you'll briefly pump about 500W into the device that is supposed to have 500W going out of it. The reason these are big and expensive is that it requires significant safety gear so nothing explodes even in the worst case. And that safety gear is expensive. So you sell it to people who not only can afford it but also really really need it (ie, 1MW and upwards where you enter the domain of "can fry small section of grid")
        TheSpiceIsLife 8y ago
        I used to work with a few (16 odd) Siemens 200kVA UPS that did the syncing. There’s no technical reason the requisite electronics can’t be built on a much smaller scale. Home grid-tie solar inverters are clearly capable of syncing, so the electronics are already present.
        zaarn 8y ago
        grid-tie inverters are capable of syncing to a present signal, what they can't do is provide their own waveform and sync that to the grid when it comes back online.
        Dylan16807 8y ago
        Providing a 60Hz waveform and syncing it to the grid the easy part. You could make a standalone device that does it out of a twenty cent microcontroller.
        zaarn 8y ago
        And then you'd still have to switch over and you'd need a sync check relay for safety (if you don't and the microcontroller is off because you forgot a comma somewhere, your inverter explodes). Additionally producing the clean sinewave that you'd need for this is not that easy, atleast not at the quality levels you want for this (if your DAC that produces the wave is off by 1% then at a 2kW load you're going to burn up 20W somewhere that doesn't like 20W being burned up)
    - londons_explore 8y ago
      Yes. It's all pretty much the same hardware with different firmware. Manufacturers of solar hardware love to charge you an extra $1000 for that extra firmware though...
      - zaarn 8y ago
        You need more hardware when you want to sync to grid after being off grid. You need one on the inverter and a seperated output, most grid-tie inverters simply have one port for everything. If you use one port for everything you can hardly sync the house grid to the external grid since you can't both provide power and try to sync the phase.
        namibj 8y ago
        Err, you can provide the power, you do need another voltage sensing connection to the grid though, and might want an automated breaker to not have to attend the device and re-connect inverter and grid once sync is complete. If the inverter is properly fused, you should even get away with a LED or a small display that shows you whether it's safe to reconnect or even how long until sync is complete.
- zaarn 8y ago
  The grid isn't a fixed frequency. While on average the grid doesn't change phase, the reality is that the grid may be out of phase by several seconds most of the time. When the grid comes back from a blackout, chances are that it browned out beforehand so your sync is to a low frequency and coming back it'll be high frequency because the grid wants to compensate for loads jumping back on. Additionally the components to generate your own waveform are cheap, yes, but not that cheap, adding them to the microinverter would increase cost quite a bit. And you'd still need a transfer switch because if you happen to be 180 degrees out of phase, which CAN HAPPEN then your panel will behave like a dead short at double grid voltage. The current flow will definitely exceed the maximum tolerances and the magic smoke goes out. You will absolutely need a transfer switch just so you don't fry all your devices the moment the grid comes back. Even then, syncing to the grid is a rather delicate maneuver since the grid will be constantly changing phase and it'll be simpler to shut down all inverters, connect back and have it all run back up on the grid itself.
  - Doxin 8y ago
    Or you have your inverters run a phase locked loop. When the grid is there it'll keep perfect sync, when it's not there it'll keep running. Switching back to grid power should be as easy as waiting to reconnect until your inverters are back in sync, which depending on the PLL design shouldn't take long.
    - zaarn 8y ago
      Waiting to be back in sync could take forever, plus you need a Sync Check Relay. Price of those is usually "contact sales team" and they don't work reliably if you don't need more than a couple dozen kW of power. (They're intended for 1MW+ installations). Just shut it down, flip the switch and restart. Everything else will just be prohibitively expensive because it needs to be very safe. If you get the phase wrong then you'll either reduce the lifetime of your components or the components explode after the nearest power plant tries to pump all available power into your poor inverter.
      - stephen_g 8y ago
- avianlyric 8y ago
  It might be worth reading the rest of the article. Which points at that the big reason why you can run off grid is because solar panels without a battery store are effectively incapable of usefully powering variable loads without: Only running at a fraction of their max available output allow enough headroom for high peak startup draw from loads, and headroom for clouds and planes passing overhead. Frequently cutting in and out as the load regularly exceeds available supply. Keeping the frequency sync is the easiest problem to solve, the article even covers what would happen if you tried to use a little generator to produce a sync signal.
  - raverbashing 8y ago
    I agree with what you wrote there, so they should justify by it not by just saying "it's a frequency issue" As you said: > Keeping the frequency sync is the easiest problem to solve
  - DoubleGlazing 8y ago
    I think that points to a bigger problem with solar and that is that it is a horribly mis-sold concept. Most solar dealers have a one-size-fits-all product that they fit to nearly all homes without much modification. The homeowner thinks they are buying something that will save them a fortune and eliminate their need for grid power. In reality they discoverer that solar is persnickety and they will get nowhere near eliminating their grid reliance. And when you do the math you realise that it might take 10+ years to recoup the high cost of installation and by that point your batteries will need to be replaced and your solar panels will have lost some of their efficiency. I know people who have gone fully off-grid in Ireland, but they don't just rely on solar. They supplement with wind turbines and in some case hydro power from streams.
    - siddboots 8y ago
      I think you have it wrong. You make it sound like homeowners are tricked into thinking they will be grid-independent, which I don't think is a fair depiction of how the product is sold (at least in Australia, that is.) The value proposition isn't getting entirely off the grid, it's just saving a bit of cash and doing some good for the planet. Rooftop solar is popular because the payback period is short enough for homeowners - well under 10 years here for a system that is built to last at least 20. Modules built now degrade about 0.25% per year. Solar farms built now are typically financed as 25-30 year projects. Source: I am a data analyst at a solar engineering firm.
jccalhoun 8y ago
I agree that there are a lot of good technical details. However, I wish the article had started off by saying, "It's Not Power Companies Being Evilly Evil - It's Homeowners Being Cheap" instead of waiting more than half way through it.

raverbashing 8y ago

"there's no waveform to sync with" is technically correct and a very poor excuse A residential no-break has no waveform to sync with as well. Something capable of syncing to the grid and then more or less keeping pace even if the main grid goes down should cost very little today. (And when the grid goes back it shouldn't have drifted too much unless something ridiculously big happened)

TheSpiceIsLife 8y ago
When the grid goes back up the system would check the grid frequency and phase, and slowly match that waveform over a few minutes by making light changes. When the two are aligned within a good-enough tolerance the system will switch back to it's regular state of mains + solar ( + batteries + wind + generator + etc / whatever). These are all solved problems with commercial off the shelf components.
- zaarn 8y ago
  Are these commercial off the shelf components as cheap as a microinverter?
  - TheSpiceIsLife 8y ago
    I just typed microinverter in to Google and clicked on shopping. It looks to me like microinverters are a commercial off the shelf product? Will a microinverter do all of the things a multi-component phase-syncing system with automatic transfer switches do? I don't see why a microinverter can't be built with these components integrated. I can't tell you if such a unit exists as I'm not well versed in the product range. As far as a price comparison goes, I guess it only makes sense to compare a like-for-like system?
    - zaarn 8y ago
      From what I know, you will need a sync check relay, ie a relay that only closes when you're synced up. Those are generally available for 1MW and upwards (one manual notes a minimum constant load on the internal grid of 500 kW or it won't work), look like about the size of half of a car battery, at a price of "contact our sales team". Could it be made cheaper? Probably. If you get it wrong, the grid probably doesn't care but you'll briefly pump about 500W into the device that is supposed to have 500W going out of it. The reason these are big and expensive is that it requires significant safety gear so nothing explodes even in the worst case. And that safety gear is expensive. So you sell it to people who not only can afford it but also really really need it (ie, 1MW and upwards where you enter the domain of "can fry small section of grid")
      - TheSpiceIsLife 8y ago
        I used to work with a few (16 odd) Siemens 200kVA UPS that did the syncing. There’s no technical reason the requisite electronics can’t be built on a much smaller scale. Home grid-tie solar inverters are clearly capable of syncing, so the electronics are already present.
        zaarn 8y ago
        grid-tie inverters are capable of syncing to a present signal, what they can't do is provide their own waveform and sync that to the grid when it comes back online.
        Dylan16807 8y ago
        Providing a 60Hz waveform and syncing it to the grid the easy part. You could make a standalone device that does it out of a twenty cent microcontroller.
        zaarn 8y ago
        And then you'd still have to switch over and you'd need a sync check relay for safety (if you don't and the microcontroller is off because you forgot a comma somewhere, your inverter explodes). Additionally producing the clean sinewave that you'd need for this is not that easy, atleast not at the quality levels you want for this (if your DAC that produces the wave is off by 1% then at a 2kW load you're going to burn up 20W somewhere that doesn't like 20W being burned up)
  - londons_explore 8y ago
    Yes. It's all pretty much the same hardware with different firmware. Manufacturers of solar hardware love to charge you an extra $1000 for that extra firmware though...
    - zaarn 8y ago
      You need more hardware when you want to sync to grid after being off grid. You need one on the inverter and a seperated output, most grid-tie inverters simply have one port for everything. If you use one port for everything you can hardly sync the house grid to the external grid since you can't both provide power and try to sync the phase.
      - namibj 8y ago
        Err, you can provide the power, you do need another voltage sensing connection to the grid though, and might want an automated breaker to not have to attend the device and re-connect inverter and grid once sync is complete. If the inverter is properly fused, you should even get away with a LED or a small display that shows you whether it's safe to reconnect or even how long until sync is complete.
zaarn 8y ago
The grid isn't a fixed frequency. While on average the grid doesn't change phase, the reality is that the grid may be out of phase by several seconds most of the time. When the grid comes back from a blackout, chances are that it browned out beforehand so your sync is to a low frequency and coming back it'll be high frequency because the grid wants to compensate for loads jumping back on. Additionally the components to generate your own waveform are cheap, yes, but not that cheap, adding them to the microinverter would increase cost quite a bit. And you'd still need a transfer switch because if you happen to be 180 degrees out of phase, which CAN HAPPEN then your panel will behave like a dead short at double grid voltage. The current flow will definitely exceed the maximum tolerances and the magic smoke goes out. You will absolutely need a transfer switch just so you don't fry all your devices the moment the grid comes back. Even then, syncing to the grid is a rather delicate maneuver since the grid will be constantly changing phase and it'll be simpler to shut down all inverters, connect back and have it all run back up on the grid itself.
- Doxin 8y ago
  Or you have your inverters run a phase locked loop. When the grid is there it'll keep perfect sync, when it's not there it'll keep running. Switching back to grid power should be as easy as waiting to reconnect until your inverters are back in sync, which depending on the PLL design shouldn't take long.
  - zaarn 8y ago
    Waiting to be back in sync could take forever, plus you need a Sync Check Relay. Price of those is usually "contact sales team" and they don't work reliably if you don't need more than a couple dozen kW of power. (They're intended for 1MW+ installations). Just shut it down, flip the switch and restart. Everything else will just be prohibitively expensive because it needs to be very safe. If you get the phase wrong then you'll either reduce the lifetime of your components or the components explode after the nearest power plant tries to pump all available power into your poor inverter.
    - stephen_g 8y ago
avianlyric 8y ago
It might be worth reading the rest of the article. Which points at that the big reason why you can run off grid is because solar panels without a battery store are effectively incapable of usefully powering variable loads without: Only running at a fraction of their max available output allow enough headroom for high peak startup draw from loads, and headroom for clouds and planes passing overhead. Frequently cutting in and out as the load regularly exceeds available supply. Keeping the frequency sync is the easiest problem to solve, the article even covers what would happen if you tried to use a little generator to produce a sync signal.
- raverbashing 8y ago
  I agree with what you wrote there, so they should justify by it not by just saying "it's a frequency issue" As you said: > Keeping the frequency sync is the easiest problem to solve
- DoubleGlazing 8y ago
  I think that points to a bigger problem with solar and that is that it is a horribly mis-sold concept. Most solar dealers have a one-size-fits-all product that they fit to nearly all homes without much modification. The homeowner thinks they are buying something that will save them a fortune and eliminate their need for grid power. In reality they discoverer that solar is persnickety and they will get nowhere near eliminating their grid reliance. And when you do the math you realise that it might take 10+ years to recoup the high cost of installation and by that point your batteries will need to be replaced and your solar panels will have lost some of their efficiency. I know people who have gone fully off-grid in Ireland, but they don't just rely on solar. They supplement with wind turbines and in some case hydro power from streams.
  - siddboots 8y ago
    I think you have it wrong. You make it sound like homeowners are tricked into thinking they will be grid-independent, which I don't think is a fair depiction of how the product is sold (at least in Australia, that is.) The value proposition isn't getting entirely off the grid, it's just saving a bit of cash and doing some good for the planet. Rooftop solar is popular because the payback period is short enough for homeowners - well under 10 years here for a system that is built to last at least 20. Modules built now degrade about 0.25% per year. Solar farms built now are typically financed as 25-30 year projects. Source: I am a data analyst at a solar engineering firm.

jccalhoun 8y ago

I agree that there are a lot of good technical details. However, I wish the article had started off by saying, "It's Not Power Companies Being Evilly Evil - It's Homeowners Being Cheap" instead of waiting more than half way through it.