Grid Tied W/generator Without Batteries

[Besoeker]

With a charge controller. The inverter also must sense the battery voltage and charge or discharge as appropriate.

OK. It what way does charge controller differ from a voltage regulator?
Would the voltage regulator necessarily be more expensive?

 

[ggunn]

Interesting. Which ones are those?

I don't know the model number and they might not yet be UL listed for availability in the US, but I saw one in the SMA booth at SPI.

 

[jaggedben]

OK. It what way does charge controller differ from a voltage regulator?
Would the voltage regulator necessarily be more expensive?

I'm not sure where you're going with this. I think the issue is with how the inverter responds to conditions on the AC side of things.

 

[texie]

I don't know of a PV inverter which can supply AC current on demand at a fixed voltage without batteries*. Is it possible? I don't know the answer to that; I am not an inverter designer. But there is plenty of demand for something like that and it seems to me that if it were possible then someone would be doing it, even if it were a lot more expensive and a lot less efficient

* Actually that's not entirely true. There is a new line of SMA grid tied inverters that have an AC outlet that remains energized when the grid is down as long as there is insolation on the array, but it provides only a trickle of energy - a very small percentage of the output of the system when the grid is up.

I attended the SMA dog and pony show at the SMA assembly plant in Denver back in August and this product was annouced to us. At that time they were unable or unwilling to share the details on how this works and at what capacity. As you can guess, I had lots of questions, but details were a little thin at that time.

 

[Besoeker]

I'm not sure where you're going with this. I think the issue is with how the inverter responds to conditions on the AC side of things.

Load it more and it outputs more up to the rating of the inverter. At least those I've dealt with do.
Where I'm going........
In post #29, I mentioned a circuit arrangement we use to take variable voltage variable frequency and convert it into fixed voltage fixed frequency.
There are three principle parts to this.
An input rectifier
A voltage regulator
An inverter
For PV, you obviously wouldn't need an input rectifier and if you swapped the charge controller for a voltage regulator, you would have a system that would work without batteries.
The circuitry we use for the voltage regulator is fairly simple comprising of an inductor, a switching device, and a capacitor.
So doable and I don't think at a great cost premium, if any.

Maybe the absence of such systems is because there is no great demand for them. A battery can level out input fluctuations. And maybe the storage can allow the rating of the inverter stage to be reduced by converting the available input energy over a longer period.

 

[ggunn]

Maybe the absence of such systems is because there is no great demand for them. A battery can level out input fluctuations. And maybe the storage can allow the rating of the inverter stage to be reduced by converting the available input energy over a longer period.

On the solar fora in which I participate one of the most common questions posted is "why can't I use my PV array for power when the grid goes down?" People are constantly floating schemes whereby they would try to use a tiny battery to fool a grid tied inverter into thinking the grid is up. There is ample demand for a system which will provide constant voltage current on demand from a standalone PV system without batteries. To my knowledge there is only that SMA inverter I mentioned, and it only powers a single outlet at very low output. Load matching to power delivery curves is definitely one reason for batteries, but that's not the whole story, I'll wager. But I don't design inverters, I just use them in systems design. I can't say that it can't be done, only that it isn't being done, and it's not just because no one wants it.

 

[Besoeker]

On the solar fora in which I participate one of the most common questions posted is "why can't I use my PV array for power when the grid goes down?" People are constantly floating schemes whereby they would try to use a tiny battery to fool a grid tied inverter into thinking the grid is up.

Do you know what the usual inverter power circuit configuration is for t grid tied inverters?

 

[ggunn]

Do you know what the usual inverter power circuit configuration is for t grid tied inverters?

No, I don't. I deal with the guzintas and guzoutas, but they are black boxes to me.

 

[jaggedben]

Load it more and it outputs more up to the rating of the inverter. At least those I've dealt with do.

I think perhaps the inverters you have dealt with have fundamentally different designs. I'm not an engineer or inverter designer any more than ggunn (less actually). But my understanding is that solar inverters generally have a digitally controlled H-bridge IC.

Do you know what the usual inverter power circuit configuration is for t grid tied inverters?

This one isn't exactly 'usual' because it has two MPPT inputs. Ignore one of them and I think you have pretty much the "usual."
http://www.power-one.com/sites/powe...energy/datasheet/pvi-3.8_4.6-i-outd_us_en.pdf

Here's one that's "transformerless."
http://www.power-one.com/sites/powe...nergy/datasheet/pvi-3.0_3.6_4.2-outd_us_0.pdf

 

[Besoeker]

I think perhaps the inverters you have dealt with have fundamentally different designs. I'm not an engineer or inverter designer any more than ggunn (less actually). But my understanding is that solar inverters generally have a digitally controlled H-bridge IC.

This one isn't exactly 'usual' because it has two MPPT inputs. Ignore one of them and I think you have pretty much the "usual."
http://www.power-one.com/sites/powe...energy/datasheet/pvi-3.8_4.6-i-outd_us_en.pdf

Here's one that's "transformerless."
http://www.power-one.com/sites/powe...nergy/datasheet/pvi-3.0_3.6_4.2-outd_us_0.pdf

Thank you for the links. Much appreciated.
The block diagrams, particularly the second, is pretty close to the system I described in post #29 - minus the input rectifier, of course.
The DC/DC converter, that which I called a voltage regulator, is there to get from PV voltage to inverter input voltage and employs a switching device, the capacitor following it and an IGBT inverter stage.

 

[jaggedben]

The DC/DC converter, that which I called a voltage regulator, is there to get from PV voltage to inverter input voltage and employs a switching device, the capacitor following it and an IGBT inverter stage.

Note that it's not just any voltage regulator. It's a DSP controlled maximum power point tracking device, meaning it's seeking an input voltage that delivers the maximum power from the solar array. (In an off-grid system there will be a similar MPPT device in the charge controller.) I have no idea what that means internally (trade secret anyway, probably). But to repeat, I don't think it has anything to do with responding to different conditions on the AC side of things.

 

[Besoeker]

Note that it's not just any voltage regulator. It's a DSP controlled maximum power point tracking device, meaning it's seeking an input voltage that delivers the maximum power from the solar array.

Yes, but the point is that it's there. So it isn't an additional bit of hardware.
And, as far as I could tell, no batteries were shown.

 

[Haji]

Yes, but the point is that it's there. So it isn't an additional bit of hardware.
And, as far as I could tell, no batteries were shown.

It is possible to build a solar panel without a MPPT circuit in which case no batteries are required. But the system would be very inefficient.

 

[ggunn]

It is possible to build a solar panel without a MPPT circuit in which case no batteries are required. But the system would be very inefficient.

I am not sure what this means. By "solar panel" most people are referring to a solar module, and solar modules do not contain MPPT circuitry. I don't think that whether or not an inverter has MPPT tracking circuitry has anything to do with whether or not it requires batteries to operate in the absence of an active grid tie.

 

[Haji]

I am not sure what this means. By "solar panel" most people are referring to a solar module, and solar modules do not contain MPPT circuitry. I don't think that whether or not an inverter has MPPT tracking circuitry has anything to do with whether or not it requires batteries to operate in the absence of an active grid tie.

Sorry for confusion.

Take a 'plain' solar panel with no battery back up and no MPPT circuit in the inverter and no grid tie as well. Such a panel is still capable supplying power to a home. But maximum power to the house can not be assured due to absence of MPPT circuitry.

But if MPPT circuitry is added to it, maximum power is assured with one condition that load be present across the inverter terminals. The condition is always fulfilled by providing batteries.

 

[ggunn]

Sorry for confusion.

Take a 'plain' solar panel with no battery back up and no MPPT circuit in the inverter and no grid tie as well. Such a panel is still capable supplying power to a home. But maximum power to the house can not be assured due to absence of MPPT circuitry.

But if MPPT circuitry is added to it, maximum power is assured with one condition that load be present across the inverter terminals. The condition is always fulfilled by providing batteries.

I don't see why this would be true. The MPPT circuit works on the DC side (input) of the inverter; why would the AC section care if there is MPPT on the input or not? And a battery is a source, not a load; which terminals are you talking about? It can't be on the output (obviously), and battery inverters never have MPPT. The MPPT in an offgrid PV system is in the charge controller, not the inverter.

I design PV systems, not inverters, but this sounds like nonsense to me.

 

[Haji]

I don't see why this would be true. The MPPT circuit works on the DC side (input) of the inverter; why would the AC section care if there is MPPT on the input or not? And a battery is a source, not a load;

With MPPT, power supplied is maximum irrespective of sufficient is load is there to absorb it or not. When sufficient load is not there, system voltage rises to account for maximum power. To prevent it, the inverter is provided with batteries to absorb excess power

battery inverters never have MPPT. The MPPT in an offgrid PV system is in the charge controller, not the inverter.

It means that battery inverters are so designed and a standalone solar panel may supply power to a load even without any inverter battery.

this sounds like nonsense to me.

This sounds like rude to me.

 

[ggunn]

With MPPT, power supplied is maximum irrespective of sufficient is load is there to absorb it or not. When sufficient load is not there, system voltage rises to account for maximum power. To prevent it, the inverter is provided with batteries to absorb excess power


It means that battery inverters are so designed and a standalone solar panel may supply power to a load even without any inverter battery.

This sounds like rude to me.

Sorry, I didn't mean to cast aspersions, but truly, this does not make any sense to me. I have designed and built PV systems with and without batteries, and all GT inverters I have used (and all that I know of) have MPPT whereas no battery inverters have it. Most off grid systems of any size do have MPPT, but that function is performed by the charge controller, not the inverter. In an off grid PV system the only purpose of the PV is to charge the batteries; the batteries power the loads. The PV is not connected directly to the inverter, so having MPPT on the inverter would make no sense.

There is plenty of demand for PV to power AC loads directly with no grid tie and no batteries; on many solar forums this topic is the single most often posted question/complaint about GT systems. If it were possible for PV inverters to do this merely by leaving off the MPPT function on the DC side, someone would be doing it. Furthermore, there is one line of GT inverters that can indeed power a single outlet of very low output from the PV in the absence of grid power (some of the new SMA Sunny Boys, not yet UL listed for distribution in the US), and they DO have MPPT functionality.

If you can explain how not having MPPT would enable an off line GT inverter to power AC loads rather than merely stating that it is so, I would love to hear it. I'm an engineer; I can take it and I am used to having to change my position in the face of solid evidence.

For now, I remain unconvinced.

 

[jaggedben]

Haji, ggunn is right. The presence of an MPPT component makes no difference to the subject being discussed here. An MPPT is just a DC-to-DC conversion that maximizes the efficiency of the PV circuit. The MPPT has essentially no effect on the output after it passes through a charge controller or inverter. It is the charge controller or inverter that will have to respond to any change in load or power supply.

 

[Haji]

Haji, ggunn is right. The presence of an MPPT component makes no difference to the subject being discussed here. An MPPT is just a DC-to-DC conversion that maximizes the efficiency of the PV circuit. The MPPT has essentially no effect on the output after it passes through a charge controller or inverter. It is the charge controller or inverter that will have to respond to any change in load or power supply.

Sorry, you both seem to be wrong in that an MPPT circuit changes the switching duty cycle of the inverter so that the output power to the load is maximum by making the source resistance equal to the load resistance. But if the load can not accept all this power, the voltage would rise and to prevent this, the inverter is provided with battery to absorb this excess power also,