Power flow in solar hybrid system

[lamagra]

I tried looking all over the internet for an answer on the power flow in DC-coupled grid-tie systems with battery back-up. All sources claim that if PV production > load then the surplus will go to charge the battery. I'd like to know why it is like that and not the opposite, meaning why isn't the battery primarily fed and the surplus sent to the loads.

[inetdog]

I tried looking all over the internet for an answer on the power flow in DC-coupled grid-tie systems with battery back-up. All sources claim that if PV production > load then the surplus will go to charge the battery. I'd like to know why it is like that and not the opposite, meaning why isn't the battery primarily fed and the surplus sent to the loads.

The real answer is that it does not matter. The electricity will divide by the voltages and resistances involved and where the loads are, including the battery charger as just another load.

The CC will normally send all the power it can get to the battery as long as the battery can accept that current.
If some of the current goes to a load (DC load or via inverter), most CCs will not be able to tell whether it is going to the battery or a load or some combination.
Once the CC output it at maximum, and additional loads will end up drawing current from the battery.

If you have a more sophisticated CC it will be able to tell via a shunt what the actual battery current it, but it will still not have any way to tell the current to go one way or the other.

When the the CC is integrated into a hybrid inverter, then it does have the capability to control the loads more directly, but it still cannot control how much goes to the battery and how much goes to the load.
If the panel output is greater than the total non-charging load, the extra current will go into the battery. The only way to preferentially charge the battery requires the ability to actually shut down some or all loads. Most systems do not have that functionality.

[lamagra]

From what I understand, the charge controller comes directly after the PV array and is not considered a load, but rather the batteries are. The CC basically just dictates the voltage at which the batteries should be set depending on their state of charge. I think it does matter how the power flows in that system since according to most sources, PV primarily feeds the loads and any excess is used to recharge the batteries. What I'd like to know is how they came up with that statement. Could it be something between the CC and the Inverter or maybe something related to basic electric circuits theory that I must have missed?

The real answer is that it does not matter. The electricity will divide by the voltages and resistances involved and where the loads are, including the battery charger as just another load.

The CC will normally send all the power it can get to the battery as long as the battery can accept that current.
If some of the current goes to a load (DC load or via inverter), most CCs will not be able to tell whether it is going to the battery or a load or some combination.
Once the CC output it at maximum, and additional loads will end up drawing current from the battery.

If you have a more sophisticated CC it will be able to tell via a shunt what the actual battery current it, but it will still not have any way to tell the current to go one way or the other.

When the the CC is integrated into a hybrid inverter, then it does have the capability to control the loads more directly, but it still cannot control how much goes to the battery and how much goes to the load.
If the panel output is greater than the total non-charging load, the extra current will go into the battery. The only way to preferentially charge the battery requires the ability to actually shut down some or all loads. Most systems do not have that functionality.

[jflorey2]

From what I understand, the charge controller comes directly after the PV array and is not considered a load, but rather the batteries are. The CC basically just dictates the voltage at which the batteries should be set depending on their state of charge. I think it does matter how the power flows in that system since according to most sources, PV primarily feeds the loads and any excess is used to recharge the batteries. What I'd like to know is how they came up with that statement.

It's just how the system works. Current flows are dictated by Kirchoff's Current Law and Ohm's Law. The charge controller sets a voltage that is good for the battery; the system then takes current as needed from whatever has a higher potential. During the day that means that power comes from the panels, and depending on the battery's state of charge, goes to either the batteries or the load. (In some cases coming from the battery AND the charge controller.) At night all the current comes from the batteries. There is no switching to make this happen - just the two previously stated laws.

[ButchDeal]

I tried looking all over the internet for an answer on the power flow in DC-coupled grid-tie systems with battery back-up. All sources claim that if PV production > load then the surplus will go to charge the battery. I'd like to know why it is like that and not the opposite, meaning why isn't the battery primarily fed and the surplus sent to the loads.

I don't know what sources you have but in a GRID tie system with any monitoring like with the outback and Snyder integrated systems. They charge the batteries first, extra power during charging goes to the inverter which send the power to load and grid. The batteries are generally charged in a hybrid system like this. Also if the solar is not enough it will pull power from the grid to charge batteries. These are default settings.

[lamagra]

This is getting confusing. Most online sources say that the solar panels give priority to feeding the loads and use any surplus to charge the batteries. If the batteries are full, the surplus is fed to the grid. That's how I understand it at least. The question is how is that topology assured?

[ButchDeal]

This is getting confusing. Most online sources say that the solar panels give priority to feeding the loads and use any surplus to charge the batteries. If the batteries are full, the surplus is fed to the grid. That's how I understand it at least. The question is how is that topology assured?

There is a DC Bus. the Charge controller, batteries, and inverter are on that bus. In a smart monitored system the sensor determine the state of charge and the charge controller drives the voltage up to a charge level or whatever level is required. The inverter is turned on to draw what excess is available off. There is nothing manually switching

[lamagra]

Do you have any documentation that supports that?

There is a DC Bus. the Charge controller, batteries, and inverter are on that bus. In a smart monitored system the sensor determine the state of charge and the charge controller drives the voltage up to a charge level or whatever level is required. The inverter is turned on to draw what excess is available off. There is nothing manually switching

[jflorey2]

This is getting confusing. Most online sources say that the solar panels give priority to feeding the loads and use any surplus to charge the batteries. If the batteries are full, the surplus is fed to the grid. That's how I understand it at least. The question is how is that topology assured?

Physics and chemistry.

The charge controller provides a voltage, backed up by the current it can produce from the array. If more current than that is drawn from the controller than the array can support, the voltage droops. This reduces charging to the battery, or causes a discharge from the battery.

The battery itself is basically a voltage source, with the voltage set by the state of charge. The battery's chemistry determines what the relationship between voltage and state of charge is, and also what the equivalent resistance is.

Additional DC loads can be attached to the battery. The loads draw power from the system. Since the charge controller is a "stiffer" voltage source than the battery, it supplies most or all of the current due to Ohm's Law.

Some inverters will sell extra power to the grid. They do this by setting a voltage level. If the system voltage goes over this level, it is assumed that the battery is adequately charged, and the additional current (that would ordinarily go towards pushing the system voltage up and/or being unused) is used to run the inverter to feed the grid.

A lot of people get confused because they don't see the "switch" that "decides" to send power to the battery or the loads. That's not how it works. The routing of current happens entirely autonomously, based on Ohm's Law.

[Sunking]

Could it be something between the CC and the Inverter or maybe something related to basic electric circuits theory that I must have missed?

Nope you just have no idea how electricity works or anything about theory. I could explain it, but you would not understand.

[ButchDeal]

Do you have any documentation that supports that?

supports what? that there is a DC bus that everything is attached to?
that everything is controlled by the computer (in outback called Mate3)?


try the documentation:

http://www.outbackpower.com/outback-products/integrated-systems/item/flexpower-one?category_id=521

[lamagra]

You're so kind. Thank you.

Nope you just have no idea how electricity works or anything about theory. I could explain it, but you would not understand.

[lamagra]

Thank you that's very helpful. I was not looking for that switch though i just wanted to understand how it happens and according to you it's all ohm's law. One more thing: you do confirm that dc loads and/or the inverter are primarily fed by the array and the surplus charges thr batteries?

Physics and chemistry.

The charge controller provides a voltage, backed up by the current it can produce from the array. If more current than that is drawn from the controller than the array can support, the voltage droops. This reduces charging to the battery, or causes a discharge from the battery.

The battery itself is basically a voltage source, with the voltage set by the state of charge. The battery's chemistry determines what the relationship between voltage and state of charge is, and also what the equivalent resistance is.

Additional DC loads can be attached to the battery. The loads draw power from the system. Since the charge controller is a "stiffer" voltage source than the battery, it supplies most or all of the current due to Ohm's Law.

Some inverters will sell extra power to the grid. They do this by setting a voltage level. If the system voltage goes over this level, it is assumed that the battery is adequately charged, and the additional current (that would ordinarily go towards pushing the system voltage up and/or being unused) is used to run the inverter to feed the grid.

A lot of people get confused because they don't see the "switch" that "decides" to send power to the battery or the loads. That's not how it works. The routing of current happens entirely autonomously, based on Ohm's Law.

[jflorey2]

Thank you that's very helpful. I was not looking for that switch though i just wanted to understand how it happens and according to you it's all ohm's law. One more thing: you do confirm that dc loads and/or the inverter are primarily fed by the array and the surplus charges thr batteries?

That's the effect, although the cause isn't as deterministic as that.

The inverter is "smart" - it will draw as much current as it needs to satisfy its loads. In other words, if it is generating X watts at 120 volts, it might draw 10 amps at 14 volts on the DC side. If the voltage drops it will draw more current. If voltage drops to 10 volts, for example, it would then draw ~14 amps to support the same load.

Batteries have the opposite behavior. If a battery is taking 10 amps at 14 volts, as the voltage drops so will the charge current. At 13 volts it might take 2 amps. At 12 volts it might take nothing. At 10 volts it is going to be _sourcing_ power back to the loads.

So as the load on the inverter increases, the inverter will take more and more power and will pull the voltage down. The battery will take less and less power as the voltage drops, and at some point might even be _sourcing_ power.

That's also the reason that off grid systems almost always need batteries, so that the transient changes in AC loads don't pull the DC voltage down too far. The battery keeps the voltage within a reasonable range, which makes it easier to operate the inverter and the rest of the system. (And of course provides power for times of no sun.)

[lamagra]

Fair enough but this is the behavior of an inverter acting as a voltage source and not a current source as the grid tied is supposed to act. Does this mean that the hybrid inverter acts as a voltage source even when the inverter is grid tied?

That's the effect, although the cause isn't as deterministic as that.

The inverter is "smart" - it will draw as much current as it needs to satisfy its loads. In other words, if it is generating X watts at 120 volts, it might draw 10 amps at 14 volts on the DC side. If the voltage drops it will draw more current. If voltage drops to 10 volts, for example, it would then draw ~14 amps to support the same load.

Batteries have the opposite behavior. If a battery is taking 10 amps at 14 volts, as the voltage drops so will the charge current. At 13 volts it might take 2 amps. At 12 volts it might take nothing. At 10 volts it is going to be _sourcing_ power back to the loads.

So as the load on the inverter increases, the inverter will take more and more power and will pull the voltage down. The battery will take less and less power as the voltage drops, and at some point might even be _sourcing_ power.

That's also the reason that off grid systems almost always need batteries, so that the transient changes in AC loads don't pull the DC voltage down too far. The battery keeps the voltage within a reasonable range, which makes it easier to operate the inverter and the rest of the system. (And of course provides power for times of no sun.)