Off Grid During Sunny time only

[shawki]

Hi everyone:

I am a newbie and I just posted the below discussion in the newbie section by mistake.

I want to build a 5k solar system to be used during sunny day times only. The maximum load power consumption is 2900w. Taking around 1.5 factor of the DC to AC power conversion, the load consumption is always lower than what the panels produce. Since the system operation is during sunny time, I want to minimize the battery bank just enough for the system to operate without shortening battery life or damaging it.

Assuming the battery is charged , my confusion is that I am not sure how the inverter is fed from ,the battery or from the solar panel through the MPPT controller. Does the fully charged battery in the system acts as a charged capacitor and in this case the current to the inverter comes solely from the panels? Does it fed from the battery and in such a case the battery is always in charging and discharging state.

In case the battery is completely discharged for some reason like unexpected shadow on the panels , is there a way to decrease the charging current in order to protect the battery from fast charge.
The question might seems naïve to some, but I greatly appreciate your feedback and it helps me understand more how the system works.

[Sunking]

Not going to work worth a Flip, and most certainly not the way you think it does. You could not afford it. But if it could the order of operations does not change.

Panels > Controller > Battery > Inverter > Load

Hint: You are looking at a 8000 to 12000 watt panel array, 48 volt 800 AH AGM 2000 pound battery. Surprise Surprise Surprise.

[jflorey2]

Assuming the battery is charged , my confusion is that I am not sure how the inverter is fed from ,the battery or from the solar panel through the MPPT controller. Does the fully charged battery in the system acts as a charged capacitor and in this case the current to the inverter comes solely from the panels? Does it fed from the battery and in such a case the battery is always in charging and discharging state.

No. If you don't use the power, the panel voltage rises and the "extra" power is wasted.

is there a way to decrease the charging current in order to protect the battery from fast charge.

Sure - don't use batteries. Grid tied inverters are cheap, efficient and don't require expensive batteries.

[shawki]

Thanks for the feedback

There is no Grid connection on site that why grid tie is not an option.

Sunking explained the flow of current from the panel to the load. I just can’t see why the current to the inverter is fed from a fully charged battery and not from panels even though the power produced is more than the load’s need. Does that mean the battery is continuously in a small charging and discharging mode. What control this mechanism.

Since I am not looking for energy storage for night time, I thought about supercapacitor to escape the battery option since they have very high life time cycle and fast charge rate. I am not sure if this is a good option and whether there is charge controller for them or I just insert a diode between the panels and the capacitor.

[Sunking]

Sunking explained the flow of current from the panel to the load. I just can’t see why the current to the inverter is fed from a fully charged battery and not from panels even though the power produced is more than the load’s need. Does that mean the battery is continuously in a small charging and discharging mode. What control this mechanism..

Physics as it relates to the higher order of energy state. If the panels are in a higher energy state the panels supply the power. If the batteries are in a higher energy state they provide the power. You have no control. .

Since I am not looking for energy storage for night time, I thought about supercapacitor to escape the battery option since they have very high life time cycle and fast charge rate. I am not sure if this is a good option and whether there is charge controller for them or I just insert a diode between the panels and the capacitor.

Does not matter what you want, physics dictates what happens and not what you want. Physics does not give a damn about what you want or think.

Fist a solar panel does not generate rated power all day. In fact it never generate rated power. In the morning hours it starts at about 1% when the sun rises. As the sun gets higher in the sky, power level creeps up until solar noon when it peaks for those few minutes around solar noon. That a few minutes after noon the power starts to taper off to nothing when the sun sets. A little cloud passes by and all power stops. Hazy conditions power is greatly reduced. Power from the panels is dynamic and changes every second of the day. You have no control.

Any battery can only supply a limited amount of power even when fully charged. The maximum current a battery can supply depends on what type it is. FLA batteries can only handle about C/8 while AGM's can handle about C/4. Go beyond that and voltage falls off a cliff and your gizmo stops working. So 2900 watts @ 48 volts is roughly 60 amps of current. That means the battery must be at least 250 amp hours of AGM or 500 Amp Hours if FLA.

But the fun does not stop there. Batteries can only take so much charge current. It happens to be the same as it can discharge. A 2900 watt panel generates 60 amps at noon, less at any other time of day. That means you must use much larger panels of 3 to 4 times more than required. That means a much larger battery to be able to absorb all the charge current. Otherwise battery explodes.

All about physics and why your idea will not work. At least not the way you think it will. It can be done, but wil be much larger than anything you can imagine and extremely expensive. Gotta love physics.

[jflorey2]

There is no Grid connection on site that why grid tie is not an option.

Ah, OK.

Sunking explained the flow of current from the panel to the load. I just can’t see why the current to the inverter is fed from a fully charged battery and not from panels even though the power produced is more than the load’s need. Does that mean the battery is continuously in a small charging and discharging mode. What control this mechanism.

Basic physics. You can model the system as a series of voltage sources in series with resistances, all connected to a common node. Whichever voltage source is higher feeds current to whichever voltage source is lower.

At steady state, the charge controller is the highest voltage source in the system. The battery will either be lower in voltage (and thus will accept current) or will rise until it is just on par with the charger. At that point no more current will flow into the battery.

The inverter has the lowest voltage source in the system. Thus current will flow into the inverter. If the inverter needs more power (and it accomplishes this by either reducing its equivalent voltage source or decreasing its equivalent resistance) it draws more current. If it draws too much from the charge controller, then the equivalent voltage from the charge controller will decline. At that point the battery will have the highest equivalent voltage in the system, and current will flow out of it.

(You can do exactly the same analysis looking at all the devices as current sources. But since batteries prefer specific voltages, the math is a lot easier if you consider them as voltage sources.)

Since I am not looking for energy storage for night time, I thought about supercapacitor to escape the battery option since they have very high life time cycle and fast charge rate. I am not sure if this is a good option and whether there is charge controller for them or I just insert a diode between the panels and the capacitor.

A few problems with this.
1) Startup. Ultracaps start at zero volts, which looks to a charger like a dead short. Most chargers will not operate into a dead short, so you won't be able to start the system up.
2) Storage. Ultracaps have a miniscule amount of storage compared to batteries. As soon as it gets cloudy out you will be lamenting the loss of storage capacity.
3) Lost capacity. The amount of energy in a capacitor (in watt-seconds) is equal to .5 * capacitance * voltage^2. But you can't use all of that. All you can use is the difference between max and min voltage of your inverter. For example, if your inverter only works from 10 to 15 volts, then you are losing out on 45% of the energy in the capacitor.

[Sunking]

As I said this can be done, but you have not taken the first step. You have not stated how many Watt Hours you want to generate in a day, what time a year, or location. It makes no difference if it during day or night. The panels have to be able to generate the power in the span of sun rise to sunset. That is a day. Makes no difference when you use it.

So if you want to run that gizmo that uses 2900 watts for 6 hours, you need 2900 watts x 6 hours = 17,940 watt hours a day or 18 Kwh. If your shortest Sun Hour day through the year is 4 Sun Hours means you must have 6750 watt panel system. If using a 48 volt battery means the panel current is 140 amps. Minimum battery size to handle that is 500 AH for AGM and 500 AH for FLA.

Now you might think 500 AH AGM is better than 1000 AH FLA, but not so fast. They both cost the same and AGM will only last about half as long as FLA.

That is the process. Physics demands it and does not give a damn about you or what you want.

[shawki]

I read several posts’ responses and an article posted by Sunking about how to calculate the battery size.
They helped a lot and I feel comfortable calculating that now.

I have background in electronics and I was interested in understanding how the system works which Jflorey2 explained.

I thank both of you

[Sunking]

I read several posts’ responses and an article posted by Sunking about how to calculate the battery size.
They helped a lot and I feel comfortable calculating that now.

I have background in electronics and I was interested in understanding how the system works which Jflorey2 explained.

I thank both of you

That gets you on the right track. Panel wattage is whatever it is. Now you can deviate on battery size from 5 -day reserve. But you cannot deviate far especially if your short Sun Hour months fall below 4 Sun Hours Example if you have 1000 watt panels on a 12 volt battery with MPPT, the minimum battery capacity is 600 AH in order to handle 80 amps of charge current that can hit the batteries. There are a couple of ways around that by using something other than FLA batteries like using AGM or Lithium.

You can make the case for using lithium batteries because just about any of them you can hit with 1C charge current. So using the same 1000 watt 12 volt setup you can use 100 AH lithium. A 12 volt 600 AH FLA battery will hit your wallet up for $1400. A 12 volt 80 AH Lithium around $300 to $400. But a word of caution. If you were to do that, you run a good risk of over discharging your lithium battery because just 1 hour of clouds while operating will burn them up with over discharge. Does that help?

[shawki]

You can make the case for using lithium batteries because just about any of them you can hit with 1C charge current. So using the same 1000 watt 12 volt setup you can use 100 AH lithium. A 12 volt 600 AH FLA battery will hit your wallet up for $1400. A 12 volt 80 AH Lithium around $300 to $400. But a word of caution. If you were to do that, you run a good risk of over discharging your lithium battery because just 1 hour of clouds while operating will burn them up with over discharge. Does that help?

A 12V 100A lithium battery with 1C should handle the 80A charge current. Shouldn't the battery be able to discharge the 80A in one hour with no problem . Why should I be worried about one hour of cloudss. What am I missing something?

[inetdog]

A 12V 100A lithium battery with 1C should handle the 80A charge current. Shouldn't the battery be able to discharge the 80A in one hour with no problem . Why should I be worried about one hour of cloudss. What am I missing something?

As long as you do the math correctly and have a reliable way to cut off the inverter as the battery voltage drops there is no problem.
But consider this:
1. You do not want to discharge the battery below 20%, but you also do not want to charge it above 80% for best life. So you really have only 60 amp-hours to work with not 80.
2. Your load needs to be such that it will not cause you any problems to shut it down unexpectedly when the batteries are drained.
3. If you rely on the battery to make up shortfalls in solar production, you may end up actually cycling the battery many times per day instead of only once. This will reduce the useful life of the battery.

As long as you keep these in mind, you may be able to come up with a satisfactory syste.