Analogy question for jflorey2

[kingofbanff]

I didn't want to hijack the thread where this first came up so I thought I would start a new one.

Jflorey2 posted this in response to my question:

"In a hybrid system, the charge controller prevents overvoltage. It takes less and less power from the panels as the batteries reach full. If the grid is connected, then the inverter pulls power out of the batteries so they never reach full, and the charge controller transfers as much power as it can into the batteries."

Here and there I see people using water and buckets for analogies. So in this case you have a certain amount of water coming from the solar panels and being put into the bucket. As The bucket starts to fill the charge controller would slow the water down to a trickle. With this analogy, where does the "extra" water coming from the solar panels go? I guess the Corollary question is when my solar panels are sitting on the roof but not connected to anything where does the "water" go?

I have some follow-up questions but I will try to understand this part first.

Thanks

[ButchDeal]

it is not collected and thus doesn't exist. Short the modules out, no production....

[kingofbanff]

Ok cool. So if I understand correctly the charge controllers are smart enough to turn down the flow of water that goes to the batteries as they fill so they don't overflow. How does the "water" get through to the loads in the house? Does all "Water" pass through the batteries?

Foe example if somebody turned on a tap in the house that uses 2 gallons per hour does the water come out of the "bucket/battery" and then the PV simeltaneously refills it? Or does the inverter just pull the "water" straight from the charge controller while the battery just sits there doing nothing? (Assuming the Modules are generating enough)

[karrak]

Here goes my attempt at the water analogy.

If you think of each individual solar cell in a solar panel as a small bucket sitting in a puddle of water, light landing on the cell gives the water in the puddle enough energy to jump into the bucket. If we don't empty any of the water out of the bucket it will eventually fill up and then overflow back into the puddle again. Now if we take the water out of the bucket and transfer it to the puddle in the next cell the bucket in the first cell will not overflow. If light lands on the second cell the water will jump into the second bucket and so on until the water reaches the top bucket. This is what happens in a solar panel. Each cell adds a bit of pressure to the water.

Connect the top bucket in the solar panel to a tap ( the solar controller) via a pipe, connect a battery below the controller tap via a T junction. The load is another tap connected to the T junction. If the load tap is turned off and the controller tap is turned on all the water from the "solar panel" will flow into the battery. When the battery is full of water the controller tap is turned off. The "solar panel" will start overflowing and form a waterfall in the solar panel. If we turn the load tap on the pressure at the T junction will drop. It takes pressure to force the water into the "battery" so the water will prefer to flow into the load tap rather than the battery. If there is not enough flow from the panels the pressure at the T junction will drop to the point where the battery will start supplying water to the load to keep the water pressure up.

I hope this has been useful.

Simon

[ButchDeal]

Ok cool. So if I understand correctly the charge controllers are smart enough to turn down the flow of water that goes to the batteries as they fill so they don't overflow. How does the "water" get through to the loads in the house? Does all "Water" pass through the batteries?

Foe example if somebody turned on a tap in the house that uses 2 gallons per hour does the water come out of the "bucket/battery" and then the PV simeltaneously refills it? Or does the inverter just pull the "water" straight from the charge controller while the battery just sits there doing nothing? (Assuming the Modules are generating enough)

if you have a hybrid inverter then the inverter and CC are connect. The inverter ramps up as the batteries are topped off.
The charge controller, battery and inverter are connect in your plumbing analogy with a T connection.
CC is putting out 5G/m, batteries take 1G/m inverter takes the rest.

Once the battery is topped off the CC all the water flows to the inverter, battery just sits there nice and topped off, like a full bucket so no more water can fit.

[kingofbanff]

Thanks to both of you for the T junction analogy. Now the next question: why couldn't you have only one battery in the system instead of a whole bank sized to the number of panels you have? The battery would be filled early in the day if it had discharged any overnight and for the rest of the day the CC would send all the power to the loads.

[sunnyguy]

Sure you can have as big or small a battery or bank as you want. The point to a battery is to have power when you need it, not just when the sun gives it. But if you can match your power needs exactly to the panel output curves then you don't need a battery. If you have a dual axis tracker the curves will be flatter and easier to do that. The minimum battery size is determined by load needs while the maximum battery size is bounded by the panel output.

[jflorey2]

Here and there I see people using water and buckets for analogies. So in this case you have a certain amount of water coming from the solar panels and being put into the bucket. As The bucket starts to fill the charge controller would slow the water down to a trickle. With this analogy, where does the "extra" water coming from the solar panels go?

It stays in the panel. To use your analogy, if you don't use the water, you lose it.

Foe example if somebody turned on a tap in the house that uses 2 gallons per hour does the water come out of the "bucket/battery" and then the PV simeltaneously refills it?

It depends on the battery state. If the battery is fully charged (i.e. your reservoir is full) then the "water" comes from the solar panel.

why couldn't you have only one battery in the system instead of a whole bank sized to the number of panels you have? The battery would be filled early in the day if it had discharged any overnight and for the rest of the day the CC would send all the power to the loads.

A few reasons.
1) There are limits on how fast you can charge a battery. It might be C/8 (8 hour charge) for a flooded cell, or C/4 for a good AGM battery (4 hour charge.)
2) A small battery may not supply enough power to supply an inverter when it tries to start up (or sustain) a large load, like a wellpump or an air conditioner. Even if the AVERAGE power for the air conditioner is 1000 watts, and you have 1000 watts of panels, the air conditioner may take 3000 watts when the compressor is running and 10 watts when only the fan is running - with the compressor running 1/3 of the time. That means the battery would have to supply 2000 watts, which is 166 amps at 12 volts - very difficult for a single 12V battery.
3) With a small battery you have to cycle it much harder (draw power at a higher C rate) which will significantly decrease its life.

[kingofbanff]

I don't understand 1) speed of charging. I assume you are talking about charging it too quickly not too slowly. Is the charge controller not capable of throttling the incoming "water" to a slow enough Trekell that it doesn't attempt to charge the battery to quickly?

I am not concerned with items two or three because the goal is to never use the battery. The only reason I would have the battery in the system is so that when the grid is down and the sun is shining and I can still generate and use power. I have yet to see a system which allows you to use your modules when the grid is down in less there are batteries in the system. During the night or cloudy days we would just use camping lights etc.
thanks as always.

[jflorey2]

I don't understand 1) speed of charging. I assume you are talking about charging it too quickly not too slowly.

Correct.

Is the charge controller not capable of throttling the incoming "water" to a slow enough Trekell that it doesn't attempt to charge the battery to quickly?

Good MPPT controllers are indeed capable of throttling the charge to a safe level. In that case you are not using most of your panel's capacity. In addition, since the load must be connected to the battery, that trickle (let's say it's 10 amps) is all the current that will be delivered to the battery AND the load. You can't set it up so it delivers only 10 amps to the battery but allows the "rest" of the current to drive your load.

I am not concerned with items two or three because the goal is to never use the battery. The only reason I would have the battery in the system is so that when the grid is down and the sun is shining and I can still generate and use power. I have yet to see a system which allows you to use your modules when the grid is down in less there are batteries in the system.

There are several ways to do that.

The easiest is with direct power. Get a 30 cell panel and use it to charge/operate 12 volt devices directly. Note that the voltage can be as high as 18 volts, so make sure your 12 volt devices can handle this.

The next easiest is with an SMA inverter with the SP option. This will give you ~1500 watts of 120VAC to a separate backed up outlet.

I have also recently heard rumors that the SolarEdge inverters with the StorEdge functionality can do no-battery backup for as long as there is solar power. This would require a 120/240 volt autotransformer but would be then able to power both 120 and 240 volt systems.

[ButchDeal]

I am getting confirmation from SolarEdge that the StorEdge with auto transformer and now battery can provide 5kw of power to the emergency panel. Still trying to get some documentation on this. This is far better than the SMA SPS, due to:
automatic
higher wattage
wired in.
upgrade able to battery solution

[ButchDeal]

now the answer is no backup without battery.

[kingofbanff]

Ok I think you just said something that is finally clear this matter up for me!
"You can't set it up so it delivers only 10 amps to the battery but allows the "rest" of the current to drive your load."

This is the point I was missing. I was always thinking that we could have 2 ounces per hour trickling into the battery and 20 gallons per hour going straight to the inverter and then to the loads and to the grid.

So let's assume for a moment that I have an absolutely massive battery bank that is fully charged. At that point does the inverter not just send a trickle to the batteries and a large amount through to the inverter and loads and grid?

[ButchDeal]

So let's assume for a moment that I have an absolutely massive battery bank that is fully charged. At that point does the inverter not just send a trickle to the batteries and a large amount through to the inverter and loads and grid?

well remember there is no control of the flow. The Charge controller has one output. The inverter has one DC connection and the battery has one connection. the three are tied together with nothing controlling the flow except the CC controls how much it puts in and the voltage level, and the inverter controls how much it takes out.
So if the battery (regardless of size) is full and the monitoring computer knows it then the CC drops the voltage down to the level of the battery (no more charging) and the inverter pulls that same level of power off. (This would be for an outback system which I am most familiar with but believe that Xantrex can do something similar).

The issues come up when the grid is down, battery is full, and not much load... CC then needs to stop making power....

[kingofbanff]

"So if the battery (regardless of size) is full and the monitoring computer knows it then the CC drops the voltage down to the level of the battery (no more charging) and the inverter pulls that same level of power off"

So when I am selling to the grid is the grid drawing half the current from the batteries and half the current straight through the inverter from the CC? As I think about your above statement I would think I could only sell as much as was trickling into the battery if the grid was drawing only from the inverter from the CC.

I feel like I'm getting very close to understanding all this!

[jflorey2]

So let's assume for a moment that I have an absolutely massive battery bank that is fully charged. At that point does the inverter not just send a trickle to the batteries and a large amount through to the inverter and loads and grid?

Sort of. (Note that in a conventional off-grid system the inverter is a load, not a source, of power.) Once the battery is in its constant-voltage charge phase (either absorb or float) then the battery will not accept any more current at that voltage. If you had a sudden load that did not exceed the solar's output, AND you increased the current limit, then all that power would go to the load and not the battery - because in that state, at that voltage, it could not absorb any more power.

So if you could ensure that the battery remained always in either absorb or float, you could increase the current limit and not overcurrent the battery. That would be a pretty severe constraint on your loads, though, which means you wouldn't get the full benefit of increasing the current limit.

If you are really looking for a way to use a seriously small battery you might want to consider the more exotic battery technologies, like LiFePO4 (higher charge rate and higher energy density) or even something like ultracaps (nearly infinite charge rate, very low energy density.) Both are expensive in terms of $/kwhr. But if you are looking for a very small battery they might be the ticket.

[ButchDeal]

No. My system is grid tied bimodal and the batteries are fully charged most of the time. The inverter never pulls from batteries to send to the grid. If the cc is providing 2.3kw then the inverter is inverting no more than 2.3 kW.