Grid Tied W/generator Without Batteries

[T.M.Haja Sahib]

I would like to suggest a truce ( and also a solution?).

Let the grid tied inverter be fed through the client's generator after rectification to a D.C voltage corresponding to the voltage of the battery bank it is designed for.

 

[jaggedben]

I would like to suggest a truce ( and also a solution?).

Let the grid tied inverter be fed through the client's generator after rectification to a D.C voltage corresponding to the voltage of the battery bank it is designed for.

I don't understand what you're proposing, but regardless you're forgetting that the whole point of this thread is whether it is possible to eliminate the batteries. Most inverters designed for batteries already have a solution for interaction with a generator. Inverters not designed for batteries require interaction with an AC sine-wave and could not possibly operate on a rectified circuit.

 

[Besoeker]

Does the governor limit the output to the required load in the case where the grid is out?

The governor regulates speed.
The generator will provide output up to its rated capacity.

 

[ggunn]

The governor regulates speed.
The generator will provide output up to its rated capacity.

How does it regulate current output? It supplies current on demand, right? <Prissy voice>I don't know nuthin' 'bout no mechanical generators.

As a rule I don't put PV inverters and mechanical generators on the same circuit if there is the smallest chance that the load on that circuit will ever demand less current than the PV's nameplate output. Make that never.

 

[jaggedben]

The generator will provide output up to its rated capacity.

Maybe my question needs to be more precise. After all, you said this generator is connected to an inverter.

How does the inverter determine the voltage and current to output when connected to the grid?
How does it determine the voltage and current to output when it is not connected to the grid?
How does it determine the power to output if it is connected in parallel to other generators on the site, but the load is less than the capacity of the generators?

(Can the answers all possibly be the same? I don't think so.)

 

[Besoeker]

Maybe my question needs to be more precise. After all, you said this generator is connected to an inverter.

I'll try to be concise. Answers in blue.

How does the inverter determine the voltage and current to output when connected to the grid?
The grid determines the voltage. The inverter has to match both voltage and frequency.

How does it determine the voltage and current to output when it is not connected to the grid?
It was never not part of a grid. Either national or local to the mill. It was not a stand alone system

How does it determine the power to output if it is connected in parallel to other generators on the site, but the load is less than the capacity of the generators?
The required power is determined by the load, not the generating capacity.

And PV cells can operate with no load. Their output voltage varies somewhat with load but any decent inverter can accommodate that variation.
We make some fairly big drive systems that take a varying input voltage and frequency, convert it to fixed voltage DC, stuff it through an inverter and output fixed voltage and frequency.
It's not only doable, its been done.

 

[ggunn]

And PV cells can operate with no load. Their output voltage varies somewhat with load but any decent inverter can accommodate that variation.
We make some fairly big drive systems that take a varying input voltage and frequency, convert it to fixed voltage DC, stuff it through an inverter and output fixed voltage and frequency.
It's not only doable, its been done.

PV cells/modules/panels can operate (produce a voltage) without a load, but a grid tied PV inverter cannot.

 

[jaggedben]

How does it determine the voltage and current to output when it is not connected to the grid?
It was never not part of a grid. Either national or local to the mill. It was not a stand alone system

If it is only part of a local to the mill grid, how does it avoid backfeeding the other generators if load does not exceed its capacity?

The required power is determined by the load, not the generating capacity.

HOW? Generating capacity has to be regulated to match the load on all grids. How is this done on this inverter?

It's not only doable, its been done.

I'm not sure whether what you're saying has been done is similar to the topic of this thread.

Let me put it this way: If you know of a cheap and simple method of building a solar inverter without batteries that does the following:
-outputs max capacity when tied to a grid whose minimum load is guaranteed to never be less than that capacity
-follows the load automatically when connected to another generator but not connected to a grid such as above
...then, I would like you to document the method and PM it to me, so I can patent it and make a fortune.
Let's keep the cost under 25cents a watt.

 

[Besoeker]

If it is only part of a local to the mill grid, how does it avoid backfeeding the other generators if load does not exceed its capacity?
HOW? Generating capacity has to be regulated to match the load on all grids. How is this done on this inverter?

It doesn't have to run at rated capacity. The output is fixed frequency and fixed voltage. Think of it as a generator. If you buy or hire a portable generator, it doesn't have to run at maximum rated output.

Let me put it this way: If you know of a cheap and simple method of building a solar inverter without batteries that does the following:
-outputs max capacity when tied to a grid whose minimum load is guaranteed to never be less than that capacity
-follows the load automatically when connected to another generator but not connected to a grid such as above
...then, I would like you to document the method and PM it to me, so I can patent it and make a fortune.
Let's keep the cost under 25cents a watt.

I've already mentioned that we make systems that convert variable voltage and variable frequency input sources to fixed voltage DC, put it through an inverter stage and subsequently back into the grid. We've don this at fairly high powers - up to several megawatts. Getting it wrong at that sort of power level is unthinkable - not to mention hugely costly. The techniques are not really very complex.
And could be readily applied to PV inverters. You have a somewhat variable DC voltage source that can easily be regulated to be a fixed DC voltage. The inverter technology is pretty well proven in the UPS and variable speed drives areas. And you don't have to load the inverter to make it work. Nor do you have to load PV cells to make them work.

So what's the problem?

 

[T.M.Haja Sahib]

I don't understand what you're proposing, but regardless you're forgetting that the whole point of this thread is whether it is possible to eliminate the batteries. Most inverters designed for batteries already have a solution for interaction with a generator. Inverters not designed for batteries require interaction with an AC sine-wave and could not possibly operate on a rectified circuit.

Thanks for your clarification.

 

[T.M.Haja Sahib]

what's the problem?

The problem is when when generators run in parallel, their speeds and voltages need to be regulated by changing settings on their individual governors/AVR's to match with the loads in the grid.

 

[T.M.Haja Sahib]

a solar inverter without batteries that does the following:
-outputs max capacity when tied to a grid whose minimum load is guaranteed to never be less than that capacity
-follows the load automatically when connected to another generator but not connected to a grid such as above

Any more details on grid tied solar inverter by web reference or otherwise? Thanks.

 

[Besoeker]

The problem is when when generators run in parallel, their speeds and voltages need to be regulated by changing settings on their individual governors/AVR's to match with the loads in the grid.

Whilst I appreciate your efforts to enlighten me on the matter, we've been designing and manufacturing AVRs for decades, so maybe, just maybe I might know just a little bit about them. As a matter of fact, we made the static AVRs I mentioned in post #14.
So, no I don't think running generators in parallel is a problem - it's done quite routinely.
And I see no fundamental obstacle to taking the output of a PV array, and converting it to a fixed voltage, fixed frequency supply.

So, my question remains.
What's the problem?

If there is a fundamental reason why operation without a battery is not feasible, maybe someone explain.
What is it that stops the inverter inverting?

 

[jaggedben]

If there is a fundamental reason why operation without a battery is not feasible, maybe someone explain.
What is it that stops the inverter inverting?

I don't think the reason is "fundamental." I think the reasons are economic and practical.

A grid-tie-only inverter is designed to do one thing: push the max power capacity of a solar array into the grid via the customers electrical system. It's configured as a current source, not as a constant voltage generator. What stops the inverter from inverting is if the grid disappears or goes out of an acceptable voltage and frequency range.

The reason we want a grid-tie-only inverter to always output max power is because that's how the customer saves money on their electric bill. Anything that reduces that maximum raises the cost of solar per kWh and makes it harder to sell. For that reason grid-tie inverters are based around maximum-power-point-tracking, which is pretty integral to the inverter.

Now what you're proposing is to put a charge controller between the PV array and the inverter. That reduces the efficiency of the system and thus raises the cost per kWh exported. That's why it is only done in solar when battery storage is desired (for totally off-grid operation or emergency backup), because then it makes sense to connect the PV and batteries in parallel to the inverter at the same voltage. What you're proposing is already standard for that situation, but not for applications without batteries.

Obviously what I said above about you giving me a patentable idea was tongue-in-cheek, but it was not totally a joke. The situation that we are interested in is where the customer already has a generic backup generator on an ATS. We want the inverter to be able to positively get a signal when the grid goes out and the ATS switches. Based on that signal it would start inverting in a different mode that follows the load, safely, with no chance of backfeeding the other generator(s). If you think it's simple and easy to develop such a multi-mode inverter without adding more than, say, 10cents a watt to the cost of the inverter, then you should be developing that product, because there would be a market for it. The solar industry would love to be able to tell customers about such a product.

 

[Besoeker]

I don't think the reason is "fundamental."

Which is pretty much was I said in post #8:

I can't see any fundamental reason why it couldn't be done.

I think the reasons are economic and practical
A grid-tie-only inverter is designed to do one thing: push the max power capacity of a solar array into the grid via the customers electrical system. It's configured as a current source, not as a constant voltage generator. What stops the inverter from inverting is if the grid disappears or goes out of an acceptable voltage and frequency range.
The reason we want a grid-tie-only inverter to always output max power is because that's how the customer saves money on their electric bill. Anything that reduces that maximum raises the cost of solar per kWh and makes it harder to sell. For that reason grid-tie inverters are based around maximum-power-point-tracking, which is pretty integral to the inverter.

Wouldn't the maximum power be that which you can get from the PV array? It would be reasonable to assume that the inverter stage would be rated to handle this.

Now what you're proposing is to put a charge controller between the PV array and the inverter.

A voltage regulator would describe it better if there are no batteries to be charged.

That reduces the efficiency of the system and thus raises the cost per kWh exported. That's why it is only done in solar when battery storage is desired (for totally off-grid operation or emergency backup), because then it makes sense to connect the PV and batteries in parallel to the inverter at the same voltage.

What you're proposing is already standard for that situation, but not for applications without batteries.

A couple or three points in no particular order.

Yes, the additional circuitry, if required would increase the capital cost of the equipment a bit and also reduce overall conversion efficiency a bit.
On the additional cost of kWh produced, I suppose it would depend on the period over which you amortised that capital cost and the cost of the capital required. I don't know for sure but my guess is that the extra cost of the circuitry required would be relatively minor compared to the cost of PV panels. And I don't see how it would affect the price per kWh you get for what you export.

Efficiency loss? Well, for what we do there are three stages in the system I mentioned. A rectifier to get from the variable frequency and voltage to DC, a voltage regulator to get fixed voltage DC, and an inverter to produce fixed voltage and frequency AC. Generally, the losses are below one percent of the total system installation for all three combined. Different field, I know.

If you do have batteries I think you'd need some kind of regulator to match PV voltage with battery voltage rather than just connecting them in parallel to cope with the varying input conditions. I think that concurs with the point you made on that.

Obviously what I said above about you giving me a patentable idea was tongue-in-cheek, but it was not totally a joke. The situation that we are interested in is where the customer already has a generic backup generator on an ATS. We want the inverter to be able to positively get a signal when the grid goes out and the ATS switches. Based on that signal it would start inverting in a different mode that follows the load, safely, with no chance of backfeeding the other generator(s). If you think it's simple and easy to develop such a multi-mode inverter without adding more than, say, 10cents a watt to the cost of the inverter, then you should be developing that product, because there would be a market for it. The solar industry would love to be able to tell customers about such a product.

Our market is for bespoke projects rather than standard off-the-peg products that you might buy from an electrical distributor.
So back to my original point.
I can't see any fundamental reason why it couldn't be done.
And maybe it has.

 

[jaggedben]

Yes, the additional circuitry, if required would increase the capital cost of the equipment a bit and also reduce overall conversion efficiency a bit. On the additional cost of kWh produced, I suppose it would depend on the period over which you amortised that capital cost and the cost of the capital required.

Yes, that's the kind of calculation that everyone who buys and sells solar makes.

I don't know for sure but my guess is that the extra cost of the circuitry required would be relatively minor compared to the cost of PV panels.

That might have been true a few years ago, but not now. Inverter costs have gone from being about a quarter of panels to more than half. And solar is still on the outside of most people's ability to pay for upfront. So we are really looking for a quite modest increase in cost to appeal to people who might have one of these but want their grid tied PV system to save them fuel.

And I don't see how it would affect the price per kWh you get for what you export.

It affects cost, not price. You still get paid back at the same price from the utility, but it costs you more to get paid back the same. So your payback period is longer and/or your payback is less.

Generally, the losses are below one percent of the total system installation for all three combined. Different field, I know.

In PV inverters, transformerless grid-tie-only can get up to 98% or so efficient, isolated grid-tie-only about 96%, and off-grid DC coupled about 90%.

I can't see any fundamental reason why it couldn't be done. And maybe it has.

Nope, it hasn't, at least not in a commercial product. The closest is Outback and Xantrex inverters, which can both operate off-grid and sell to the grid. But they require batteries. Here is what one Outback manual has to say about why batteries are required.

The inverter requires batteries to operate. Other sources may not maintain DC voltages that are
consistent enough for the inverter to operate reliably.
CAUTION: Equipment Damage
Do not substitute other DC sources in place of the batteries. High or irregular voltages
may damage the inverter. It is normal to use other DC sources in conjunction with the
batteries and the inverter, but not in place of the batteries.

 

[Besoeker]

Nope, it hasn't, at least not in a commercial product. The closest is Outback and Xantrex inverters, which can both operate off-grid and sell to the grid. But they require batteries. Here is what one Outback manual has to say about why batteries are required.

The inverter requires batteries to operate. Other sources may not maintain DC voltages that are
consistent enough for the inverter to operate reliably.

How is the PV output voltage matched to the battery voltage for varying insolation?

 

[ggunn]

Our market is for bespoke projects rather than standard off-the-peg products that you might buy from an electrical distributor.
So back to my original point.
I can't see any fundamental reason why it couldn't be done.
And maybe it has.

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.

 

[jaggedben]

How is the PV output voltage matched to the battery voltage for varying insolation?

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

 

[jaggedben]

* 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.

Interesting. Which ones are those?