Array charge rate and ability to harvest extra sun

beth · November 2010

In an off grid system where you will be using a generator to make up any short fall. I am wondering what the size relationship of the batterys to array should be. I have read that in general a 5-10% charge rate is recommended, but it seems like if your panels are sized to provide more that 5% you won't be able to harvest to much extra energy from your array above you daily usage.

According to the following example if you have your battery bank and array sized so that in full sun actual out put of the array is 5% of your total battery bank and you plan on using a 50% discharge max. This lets your harvest about 3.5 full sun days in a row and gives you about the same number of no sun days reserve. This seems like an ok average cloudy days- sunny days cycle that this system could handel.

Battery bank 1245 amp hours total (24v system)

1245 amp hours x 28 volts charging = 34860 watts

50% of total bank for storage = 17430 watts

34860 watts x .05 = 1743 watts of actual array out put needed for 5% charge rate

now if your average sun is 3 hours a day and your daily usage is about 5kw , on an average day you produce 3hours x 1743 w = 5229 w

imagine you start with a 50% discharged bank

lets say on average you use 1229 w during the sun shine hours
that means you need to store 4000w in the batteries

but on a max day where you get 5 hours of sun
5 hours x 1743 w = 8715w produced
that means you need to store 7486 w in the batteries
and after you use the rest of your daily 5kw you have an extra 3846 w in your bank saved.

Needing to store 4000 w every night for your usage you have an extra storage capacity of, 17430 w total bank minus 4000 w nightly usage=13430 w
so you can take advantage of about 3.5 of the above max sun days in a row.
You now have a full bank and if you have about 3.5 day of usage in your bank.

So as mentioned i am wondering about the array to charge rate relationship

in this example you only get a 5% charge rate with max out put? The example seems to work nicely except that the array will only produce a 5% charge rate at max sun? But this example seems to fit the recommended sizing rules? So i am wondering if it is correct? Or am i making a mistake some where?

Thanks for all your reply's!

BB. · November 2010

Re: Array charge rate and ability to harvest extra sun

beth wrote: »

Battery bank 1245 amp hours total (24v system)
1245 amp hours x 28 volts charging = 34860 watt*Hours Charging Energy

50% of total bank for storage = 17430 watts

1,245 AH * 24 volts (discharging voltage) = 29,880 Watt*Hours energy storage

34860 watts x .05 = 1743 watts of actual array out put needed for 5% charge rate

I like to calculate the size of the solar array vs battery bank by including ~0.77 derating factor for solar panel and charge controller losses:

1,245 AH * 29 volts charging * 1/0.77 derating * 0.05 = 2,344 Watts of panels at 5% charge rate.
1,245 AH * 29 volts charging * 1/0.77 derating * 0.13 = 6,096 Watts of panels at 5% charge rate.

28 vs 29 volts is not a big difference between yours and mine--The 0.77 is most of the difference. This accounts for the "PTC" rating of solar panels (what they really output when "hot") and ~5% charge controller losses.

now if your average sun is 3 hours a day and your daily usage is about 5kw , on an average day you produce 3hours x 1743 w = 5229 w

I would calculate the size of a system that uses 5kWatt*Hours of AC power per day (assuming 0.80 efficient battery bank and 0.85 efficient inverter) and overall efficiency 0.52 as:

5,000 WH * 1/3 days of sun * 1/0.52 = 3,205 Watts of panel minimum

The "extra losses" of "real" solar panel output, charger efficiency, battery cycling, and AC inverter losses really add up.

Note that I would suggest aiming at ~3.3 kWH per day system that runs around 9 months of they year on solar and the other 3 months you make up the short fall with generator run-time.

imagine you start with a 50% discharged bank

lets say on average you use 1229 w*hours during the sun shine hours
that means you need to store 4000w*hours in the batteries

but on a max day where you get 5 hours of sun
5 hours x 1743 w = 8715w*hours produced
that means you need to store 7486 w*hours in the batteries
and after you use the rest of your daily 5kw*hours you have an extra 3846 w in your bank saved.

Sorry, just needed to add the *Hours for energy storage... (Watts is like MPH--a rate; and Watt*Hours is like the miles driven).

And, you really need to take efficiency/losses into account--For a real system, you will only get about 50% end to end efficiency on the best of days.

Needing to store 4000 w*hours every night for your usage you have an extra storage capacity of, 17430 wh total bank minus 4000 wh nightly usage=13430 wh so you can take advantage of about 3.5 of the above max sun days in a row.
You now have a full bank and if you have about 3.5 day of usage in your bank.

And remember, that the PVWatts numbers are 20 year average numbers--You should never plan your system on being able to run 100% of the available power every day... You are much better assuming you can run maybe 1/2 the power on average days and perhaps more on random days (like laundry/irrigation days).

That is another reason when I size a system... I suggest the 9th worst month as the number of hours of sun per day--Then for 8 months of the year, your system generates much more power than your planned loads per day--when you approach the 9-12th months, then you will need the generator on days when clouds move in/sun behind the hills/trees type situation.

So as mentioned i am wondering about the array to charge rate relationship

in this example you only get a 5% charge rate with max out put? The example seems to work nicely except that the array will only produce a 5% charge rate at max sun? But this example seems to fit the recommended sizing rules? So i am wondering if it is correct? Or am i making a mistake some where?

As you can see, when you take losses into account--you lose about 1/2 what would seem to be the daily predicted output. Add extra capacity so that you are only running at 50% of estimated available system power to carry you through on cloudy days, bad weather, and to allow for load growth--You are now down planning on 25% to less then 50% of "panel name plate" energy.

In the end--if your system produces within 10-20% of what you have predicted--that is about as good as you will get. Weather is variable by 10-20% or more, over the 20 year average. Trying to get accuracy to within 5% is not really practical and not even measurable without expensive lab equipment.

There are lots of variables too in how you operate your system... If much of your power is used during the day (water pumping, running shop tools)--You don't have the 20% losses from cycling the battery bank.

If you cycle your battery bank between 80-90% state of charge instead of 90-100% state of charge--your batteries will be much more efficient.

If you run the genset to recharge the battery bank (when needed) early in the morning and recharge to ~80-90% state of charge (when the charging current starts to fall off as the batteries enter the "absorb" charge stage)--and let the solar panels take the rest of the charge--You will use much less generator run-time and fuel... Versus running the genset late in the day and trying to recharge from 90-100% state of charge (low charging current, long genset run-time, at very low and inefficient generator loading for lots of wasted fuel).

-Bill

niel · November 2010

Re: Array charge rate and ability to harvest extra sun

i should add to this that the range of 5% to 13% is what we recommend. that does not mean that a 5% rate will suffice for everybody and i might add that some batteries are able to charge at even higher rates of charge than 13% so our recommendation of 5-13% is an arbitrary figure applied generically and is a real time actual net charge to the batteries. as for example, if one wants to use say 10a of battery power while feeding from pvs 15a and the battery ah capacity is say 200ah then the charge rate is actually 15a-10a=5a and 5a/200ah=2.5% and not 15a/200ah=7.5%.
5% rates are 20 hour rates and as i said it is not a given it will be fine for all circumstances of use, but is an acceptable charge rate in terms of what a battery will properly accept under most straight forward charges to the battery. solar is a unique circumstance as 5% can take 2-3 days to charge a battery given sunny skies and 50% dod. if you use 50% of battery capacity per day then a higher charge rate is warranted with at least 10% being minimum not counting losses in charging. what's to stop you from feeding at a 13% rate other than the need to shell out for more pvs and such? agm batteries will, without a doubt, be able to accept much higher charge rates than 13%, but you have your batteries already and i assume they are fla. in any case checking with the battery manufacturer on what their max charge rate recommendations would be is highly advisable.
i am also assuming you designed for 50% daily usage as you indicated, but many design for less and as such it changes circumstances. you don't have to use 50% of it daily.

beth · November 2010

Re: Array charge rate and ability to harvest extra sun

Thanks so much for those answers guys.

Neil in the example I wasn't inferring a 50 % discharge daily, but a 50 % max overall discharge max of the battery bank.

In the example what I was trying to say is that if the actual full sun max out put of your array, after losses, gives you a 5% charge rate to your batteries, then it seemed with that ratio of panels to battery bank size you could take advantage of about three good sun days in a row.

And it seemed that under this scenario your average charge rate would be lower than 5%, so I thought this might be to low, but I thought if you added more panels to boost your charge rate up then your battery bank would not be able to take advantage of as many full sun days in a row and store the power for a string of cloudy days.

Bill I see what you are saying about losses but if you just look at actual watts coming out of your array, and OK you will have some loss in the charge controller and wires to the batteries, but if your actual max array out put produces a 10% charge rate, lets say your panels are out putting 2500 watts at max sun, if there were no losses in the charging process ,then your battery bank would be 25,000 watts , and if you only want to use half of that (as to not go below a 50% discharge). This gives you five hours of full sun storage, which I thought was not that much, but if you increase the size of you battery bank as in my original example, then at max sun you only had a 5% charge rate which I also thought was may be to low..........

?????????????

Sorry if I am being redundant and didn't completely get your previous answers.

Thanks again for the help!

BB. · November 2010

Re: Array charge rate and ability to harvest extra sun

beth wrote: »

Bill I see what you are saying about losses but if you just look at actual watts coming out of your array, and OK you will have some loss in the charge controller and wires to the batteries, but if your actual max array out put produces a 10% charge rate, lets say your panels are out putting 2500 watts at max sun, if there were no losses in the charging process ,then your battery bank would be 25,000 watts , and if you only want to use half of that (as to not go below a 50% discharge). This gives you five hours of full sun storage, which I thought was not that much, but if you increase the size of you battery bank as in my original example, then at max sun you only had a 5% charge rate which I also thought was may be to low.

That is why we aim to balance charging current against panel rating against loads...

There is the issue of the absolute current rating (below 5% and may have issues "mixing" battery when charging, difficult to recharge fast enough to reduce sulfation, and hard to keep up with self discharge as the bank gets old).

And there is the size of the panel vs how much energy you draw out of the battery every day. You can have a large enough panel to meet the 5% minimum charge rate--but not enough sun (say 2-3 hours of full sun) to replace all of the energy you use in a day--Then you would need even more solar panels to make up for all of the power used.

Also, we have the issues of drawing energy during they (shop loads, business hours) vs a home (night, cooking, cleaning, audio/visual, etc.).

Pretty much, my first assumption is that most of the energy is used at night and most of the charging is done during the day (light loads).

If you have heavy daytime loads--you could use less battery as the panels supply most of the energy. However you still end up with days of no-sun (we get a week of heavy clouds/rain during winter where we get 5% of the rated energy)--so you still may want the 3 day no sun/50% maximum discharge rule.

Then we look at the minimum charging current and typical load profile during the day--and, perhaps, you want more solar panels to make up for the 5% minimum charge rate + loads.

In the end, many times it really does not matter that much. The sun is up for 6+ hours and your loads may be for 10-12 hours per day--so the Peak Sun output of the array, just to keep up with daily loads still has to be >> than the average loads of a "day time" business.

You can drive your self nuts trying to "get the details right"--when, in the end, rules of thumb plus a generous safety factor generally will workout OK anyway.

You can start with a "slightly on the large sized battery bank" and a "on the small size" array--Monitor your batteries (specific gravity, water use per month, Battery Monitor, etc.) and make up underproduction with a genset--then after a 1/2 year (couple seasons of use) decide if you need more solar PV panels or if you are OK with the generator use as is...

Usually, we like to avoid adding batteries to an existing bank--but adding solar panels is relatively easy (however it is not cheap

).

In the end, it is usually unrealistic for a 12 month a year operation to expect to run on solar PV only 12 months of the year. Most locations have a 3 month or so season of very poor sunlight. And it is usually better to eat the cost of fuel and a genset than to build out a system with enough solar panels to operate the whole year round (much of the year will probably end up with overproduction--just a waste of energy and money).

-Bill

Array charge rate and ability to harvest extra sun

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