What happens with excess production off grid?

[Bluedog225]

I’m curious about a fundamental question. In a simple setup (panel, controller, battery), what happens with excess production? I leave my panel, little Renogy wanderer controller, and trojan deep cycle unattended for long periods. Sometimes only going up to water the battery every couple of months.

Putting aside equalization and other occasional events, what does the system do with watts produced when the battery is full (which is most of the time)? Is it simply dissipated as heat in the controller?

Thanks

 

[FilterGuy]

I’m curious about a fundamental question. In a simple setup (panel, controller, battery), what happens with excess production? I leave my panel, little Renogy wanderer controller, and trojan deep cycle unattended for long periods. Sometimes only going up to water the battery every couple of months.

Putting aside equalization and other occasional events, what does the system do with watts produced when the battery is full (which is most of the time)? Is it simply dissipated as heat in the controller?

Thanks

When the battery is full, the controller stops all current flowing through the panels. At that point the energy from the panels can be calculated as
0Amp x Voc = 0W. So the system does not do anything with excess production because there is no excess production.

 

[Bluedog225]

That makes sense. On a related note, why do micros need voltage control to shut down production?

That is, if the controller can simply shut off the current (0 amps) based on draw/need, doesn’t that achieve the same result?

Why go to all the trouble of creating a micro-grid? One could have alternating current in the breaker protected wires from the panel to the controller, but no amps flowing.

Thanks for your help understanding this basic stuff.

 

[upnorthandpersonal]

why to micros need voltage control to shut down production?

That is, if the controller can simply shut off the current (0 amps) based on draw/need, doesn’t that achieve the same result?

Think in terms of power and energy instead. Power is just the product of current and voltage. Energy is power over time. That is, current and voltage are related.
If the battery is full, it can not absorb any more energy. The voltage on the controller side and battery side is the same, and no current is flowing. However, keeping this voltage could be detrimental (e.g., keeping a LiFePO4 cell at 3.65V all the time is generally not a good idea). Therefor the controller needs to turn off (or drop to float, especially for lead acid) the voltage.

Excess energy at the solar panels is just heat. If you don't consume power by drawing a current from the panels, this will just result in the panels not generating anything and all solar will just be turned to heat like it would if there was no panel at all. Some reading:

How a Solar Cell Works - American Chemical Society

American Chemical Society: Chemistry for Life.

www.acs.org

 

[Bluedog225]

Right. I think I get that. Though I would have thought that excess energy at the panel creates a differential in the voltage of the open circuit, with no amps flowing.

I’m wondering why the same condition cannot exist with micros and alternating current? Much like a plug in my wall with nothing plugged into it.

This would eliminate any need for the logic at the micro. It would simply invert the direct current to alternating current and done. The controller could decide whether to use it (complete the circuit).

Sorry if I’m being dense. But a dead simple panel, micro, breaker protected wire to a UL listed controller seems ideal.

 

[FilterGuy]

That makes sense. On a related note, why do micros need voltage control to shut down production?

That is, if the controller can simply shut off the current (0 amps) based on draw/need, doesn’t that achieve the same result?

Why go to all the trouble of creating a micro-grid? One could have alternating current in the breaker protected wires from the panel to the controller, but no amps flowing.

Thanks for your help understanding this basic stuff.

Are you referring to micro-inverters?

Micro inverters are designed to shut themselves off if it does not see the grid power for safety reasons. If the power goes out and the micro-inverter kept pumping out energy, it could be very dangerous to anyone working to repair the grid.

 

[eabyrd]

When the battery is full, the controller stops all current flowing through the panels. At that point the energy from the panels can be calculated as
0Amp x Voc = 0W. So the system does not do anything with excess production because there is no excess production.

Do we have a new term "Opportunity Production" The production that isn't created due to a lack of storage. My economics is decades in the past. There may well be a more appropriate theory

 

[RCinFLA]

When power is not needed from panels the controller lighten load and panels go to their Voc voltage.

At Voc all the illumination generated current just shunts into the cell's inherent diode. This does not hurt panels.

 

[Bluedog225]

Yes, microinverters.

I’m trying to understand why direct current and be controlled at the controller level, and the world seems ok with this.

While alternating current is controlled at the microinverter level, with complexity and exposure of panel-level electronics.

Controlling the flow of power at the controller level for both would make sense but I’ve not seen it as a possibility.

Both present the same (?) risk to someone downstream on the grid. Right?

 

[FilterGuy]

When power is not needed from panels the controller lighten load and panels go to their Voc voltage.

At Voc all the illumination generated current just shunts into the cell's inherent diode. This does not hurt panels.

I don't understand that statement. What is 'the cell's inherent diode'?

 

[FilterGuy]

Both present the same (?) risk to someone downstream on the grid. Right?

Yes... and no. The DC circuits are all locally managed so the designer and operator can set up a set of safe procedures. Only the AC goes out to the grid and out of control of the owner of the system. That is why *all* inverters that are qualified for back-feeding the grid must shut off power if the grid goes down.

 

[FilterGuy]

Do we have a new term "Opportunity Production" The production that isn't created due to a lack of storage. My economics is decades in the past. There may well be a more appropriate theory

Funny you should ask. I am doing a write-up about over-paneling and struggled with what the un-captured energy should be called. I don't know of a commonly used term so I just called it 'lost energy' but that is not a good term either.

 

[Bluedog225]

I guess this leads me to the conclusion that all microinverters are intended for on-grid systems. Right?

 

[FilterGuy]

Yes, microinverters.

I’m trying to understand why direct current and be controlled at the controller level, and the world seems ok with this.

While alternating current is controlled at the microinverter level, with complexity and exposure of panel-level electronics.

Controlling the flow of power at the controller level for both would make sense but I’ve not seen it as a possibility.

Both present the same (?) risk to someone downstream on the grid. Right?

What you are asking for is an 'offgrid micro-inverter' that is not intended to be tied to the grid. I can't think of any technical reason that would not be possible.... but almost by definition there would be multiple off-grid inverters and there would need to be a way to sync up the phase between them. (DC has no phase that has to be synced).

 

[FilterGuy]

I guess this leads me to the conclusion that all microinverters are intended for on-grid systems. Right?

All of them that I am aware of..... but see my previous post.

 

[Bluedog225]

That’s what I was missing. Thanks!

 

[FilterGuy]

What you are asking for is an 'offgrid micro-inverter' that is not intended to be tied to the grid. I can't think of any technical reason that would not be possible.... but almost by definition there would be multiple off-grid inverters and there would need to be a way to sync up the phase between them. (DC has no phase that has to be synced).

You could 'sync' them by having one of the inverters as a master and the others as slaves that follow the phase and frequency of the master..... but that is no different than using an off-grid inverter to fake out the on-grid inverters....which is what some people do today.

 

[RCinFLA]

PV cells are illumination based current source that is clamped in maximum voltage by cell's inherent diode. Think of it as a forward biased diode in parallel with illumination level based current source.

For a current source, the higher the voltage it is allowed to get to, the more power produced. Unfortuately the shunt silicon cell diode limits the maximum voltage. Vmp point is the maximum voltage that allows just a slight amount, about 5%, of the illumination current to leak down the cell's inherent diode and maximizes the power delivered to external load on panel. You lose a little illumination current but get a slightly higher cell voltage so net power externally delivered is the maximum. This is why Imp is about 95% of Isc and Vmp is about 85% of Voc.

The actual illumination current is Isc when panel is shorted. Voc is when there is no external load so all the generated illumination current just gets shunted down the cell's inherent diode, with voltage of one diode drop.. Since cells are silicon, the Voc per cell is about 0.65v at 25 degs C. Vmp is about 0.5v where just a little bit of the illumination generated current is shunted down the cell's inherent internal diode.

The total Voc and Vmp per cell is multiplied by the number of series connected cells in a given panel.

Un-needed PV power just goes to a little extra heating to PV cells. Since cells are around 20% efficient the worse case additional panel heating power is 20%.

 

[FilterGuy]

I understand that a solar cell acts as a diode if it is reverse biased, but I still don't know what "shunted down the cells inherent diode" means. If the cell is open circuit there's no external current flowing. There will be a very tiny and short lived internal current till the Voc is reached and then everything goes static and no current is flowing internal or external to the cell

 

[wattmatters]

I am doing a write-up about over-paneling and struggled with what the un-captured energy should be called. I don't know of a commonly used term so I just called it 'lost energy' but that is not a good term either.

Unrealised capacity.

 

[FilterGuy]

Unrealised capacity.

That sounds official and complicated enough to be from the government!! ?
?

 

[wattmatters]

That sounds official and complicated enough to be from the government!!

I'm here to help.

 

[wattmatters]

I have unrealised capacity in my 2.22kW off-grid PV array. This is a typical (nice) day's PV output and load:



All it does is run the pool pump, some sundry electronics and keep the batteries topped up ready for outage backup.

Over the month of September my unrealised capacity will be ~160kWh. Over a year I estimate it'll be >1.5MWh.

 

[Bluedog225]

I know all about unrealized capacity….

 

[FilterGuy]

This thread (and others) prompted me to write a resource about over-paneling an MPPT controller.

Overview of Over-paneling an MPPT controller

To get to the paper, click on the orange button at the top of the screen. There is a misconception that putting more panels on an MPPT controller than the controller is able to use will damage the controller. However, in almost all cases it is...

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