Resizing my PV system

gpspowell · January 2015

We’ve had our PV system for about a year and a half and it is now apparent that it isn’t sized correctly for our usage. Because of that the battery bank (8 golf cart batteries) is now damaged and needing replacement. With the help of feedback on the Solar Beginners forum I’ve come up with a design plan and hope to get validation or constructive criticism of it here on this forum.

I live on the plains of NE Colorado with lots of good sunlight (5-6 hr/day average). The current system is: 48v, 8 panels of 250W rating, Xantrex SW4048 inverter, Midnight Classic charge controller, 8 golf cart batteries. For backup I have a Honda 2000eu generator and a Companion 2000eu that will couple to produce 3200W at this altitude (4900’) but the system is currently set up to automatically draw from the grid if needed (Not set up to FEED the grid at all). The intent of the installation was to get my feet wet in solar power with a system running "critical needs" (boiler, refrigerator, freezer, home office, and well pump) but I want the system to be "off grid" and able to be used as a primary system in case of any future emergency (Our family farm was without electricity for 5 months after a severe blizzard in the late 70s). My average daily usage is 4kW. In the summer months it goes up as I irrigate a small orchard and large garden but only during the middle of the day when solar gain is at its maximum potential. The well pump draws 1200W.
I'd like to be able to switch to grid backup (generator backup if ever the grid becomes unavailable) at no lower than 60% SOC, have roughly 2 days storage in that first 40% draw, and enough charging capacity to return to 100% SOC as often as possible.

I’m considering 16 Trojan J305P-AC batteries, for a theoretical 660Ah of storage (which would give me close to a two day storage using 40%) again with the grid support to cut in at 60% SOC. Then add 4 more 250W panels to bring my array up to 3kW for charging the enlarged battery bank and to supply more watts when irrigating in the summer time. My battery house is a detached structure with attendant temperature swings so I’ll plan on increasing the ventilation for summer with ajustable venting and insulate the building for winter. If possible I’ll add a remote temperature sensor to one of the batteries to more accurately control the Midnight Classic charge controller. Lastly, in case of grid failure I’ll look into purchasing a larger generator since my paired Honda 2000s only puts out 120v @ 3200W.

westbranch · January 2015

Re: Resizing my PV system

I would like to make a few observations that will affect the planed size/ configuration of what you would like to do....
When do you suffer power outages? If in winter why are you worried about the irrigation?
Do you want a single gen set to run everything or will purpose specific gens do the trick? Inverter sets for light loads etc...
What about an array for the pump only and a Grundfoss pump, with no batteries.
is the daily 4 Kwh including the pump? not e nomenclature Kwh for consumption, 4 Kw for potential of a gen set etc.

hth

gpspowell · January 2015

Re: Resizing my PV system

westbranch wrote: »

I would like to make a few observations that will affect the planed size/ configuration of what you would like to do....
When do you suffer power outages? If in winter why are you worried about the irrigation?

Long power outages are more common in winter but summer sees mid day rolling blackouts when the Colorado front range needs more power for air-conditioning.

westbranch wrote: »

Do you want a single gen set to run everything or will purpose specific gens do the trick? Inverter sets for light loads etc...

Single. The ultimate purpose of the system is to be off-grid, stand alone. Currently it is most convenient to use available grid for backup rather than generator.

westbranch wrote: »

What about an array for the pump only and a Grundfoss pump, with no batteries.
is the daily 4 Kwh including the pump? not e nomenclature Kwh for consumption, 4 Kw for potential of a gen set etc.

The system is already in place and splitting it wouldn't be economical. The well pump is new (and was purposely chosen over Grundfoss). The daily 4Kwh does include the pump but not the irrigation. When irrigating (4 hrs/day) consumption goes up to 8Kwh/day.

Surfpath · January 2015

Re: Resizing my PV system

gpspowell wrote: »

I’m considering 16 Trojan J305P-AC batteries, for a theoretical 660Ah of storage (which would give me close to a two day storage using 40%) again with the grid support to cut in at 60% SOC. Then add 4 more 250W panels to bring my array up to 3kW for charging the enlarged battery bank and to supply more watts when irrigating in the summer time. My battery house is a detached structure with attendant temperature swings so I’ll plan on increasing the ventilation for summer with ajustable venting and insulate the building for winter. If possible I’ll add a remote temperature sensor to one of the batteries to more accurately control the Midnight Classic charge controller. Lastly, in case of grid failure I’ll look into purchasing a larger generator since my paired Honda 2000s only puts out 120v @ 3200W.

Sounds like you are moving in the right direction in order to avoid another costly early bank change. Aside: Odd that eight 250 watt panels (2000 watts) failed to adequately charge a 48v bank of 8 golf cart batteries. Perhaps your winters were bad, ahhh now I see...your 4kw of daily use was quite high for (I'm guessing) a 200-250 amp hour bank.

OK, before someone does a panel to amp hour calculation, had you thought of Trojan RE-A's (320 amp hr per battery or 650 for your proposed bank). If you take the manufacturer's stats (click here for chart) with a pinch of salt....the brown premium line (RE-A's) vs the red standard line (J305's) - quite a few more theoretical cycles - depending on how you treat them. I checked the prices as well. about $45us more per battery for the RE-A's, but may be worth it. I have the RE-B's and (knock, knock) two years so far OK.

Photowhit · January 2015

Re: Resizing my PV system

gpspowell wrote: »

... considering 16 Trojan J305P-AC batteries, for a theoretical 660Ah of storage...

660Ah at 48v = @31Kwh

gpspowell wrote: »

....(which would give me close to a two day storage using 40%) ...

31kwh x .4 = 12.4kwh

gpspowell wrote: »

...Then add 4 more 250W panels to bring my array up to 3kW for charging the enlarged battery bank and to supply more watts when irrigating in the summer time.

Trojan recommends a minimum of 10% charge rate, for a 660 ah battery bank you would want to provide a minimum 66 amps at 48 volts. 66amps x 48v = 3.168 watts, but panels normally only produce 75% of panel rating so an array in the range of 4200 watts would be minimal...

gpspowell wrote: »

....My battery house is a detached structure with attendant temperature swings so I’ll plan on increasing the ventilation for summer with ajustable venting and insulate the building for winter. If possible I’ll add a remote temperature sensor to one of the batteries to more accurately control the Midnight Classic charge controller. Lastly, in case of grid failure I’ll look into purchasing a larger generator since my paired Honda 2000s only puts out 120v @ 3200W.

The extra summer sun should work well with your irrigation, when there is less sun there should be less need for the irrigation, so this should compliment things nicely! With a 4200 watt array facing south, you have a pretty good setup to provide 4 kwhs a day, particularly if you have regular sun. I suspect you could take this off grid after a year or so of checking your usage. the same genny might be a factor, but if you have rare times of multiple days of cloudy weather, you might well be fine. I think 4200 watt array will be a bit out of the range of a single Classic, but you might get by with a Kid, but I'd go with another classic (lite) and use follow me to have them sync up somewhat. I'm sure boB has more good stuff for us in the future.

gpspowell · January 2015

Re: Resizing my PV system

Surfpath & Photowhit: Thanks, that's just the kind of info I'm looking for. Surfpath, I had considered the L16RE-A batteries but haven't priced them yet and (I guess) I assumed the price difference would be greater. The installer of my system will be here next week to meet with me about the upgrade & battery replacement and I'll see what he says about prices.

Photowhit: I had missed the Trojan recommendation about 10% of capacity for charging. I see on their website they go on to say, "Chargers with lower ratings can be used but the charging time will be increased." Hmmm... I wonder how much actual extra time I can really count on with PV?????? Food for thought at least. I have room for 5 more panels before getting really innovative but that would only bring me to 3250W (and 13 panels for anyone being superstitious). Through the middle of the day here at 4900' my panels seem to produce at 82%(+/-) so that would give me 2665W or 2 1/2 panels short (at best). Either way, your figures with me needing an additional 9 panels, or my trying to be "cheap" leaving me needing 8 panels, I come up short on space unless I come up with a different mounting plan. I'd sure like to get this right this time.

BB. · January 2015

Re: Resizing my PV system

In general, if you use power at night and recharge during the day, 5% or so rate of charge can be OK. Of course, if your loads are every day loads (work, tools, etc.) or weekends/seasonal, 5% to 10% rate of charge can work OK.

If you use power during the daytime (and/or day and night), then 10%+ makes managing the system easier (i.e., daytime loads reduce available battery charging current).

More or less, 5% (without any other loads) is probably the least amount of "useful" charging current for a cycling battery system (need somewhere around 2.5 to 5% to equalize). 1% rate of charge for "float charging" (no equalization) can work OK (although as batteries age, their self discharge rate goes up).

Anyway, those are how the numbers seem to shake out from the various battery mfg. and practical usage.

-Bill

Photowhit · January 2015

Re: Resizing my PV system

gpspowell wrote: »

I had missed the Trojan recommendation about 10% of capacity for charging. I see on their website they go on to say, "Chargers with lower ratings can be used but the charging time will be increased."

Even Trojan has gotten goofy, after reducing the SG for off grid batteries for a couple years they switched back, 4-5 years ago. Then after repeated statements about 'Don't equalize' unless the cells are out of balance, they post a video stating you should. and that they recommend it every 30 days, which hasn't been in their literature for many years. They even get confused in the different types of equalizing in the video, stating that SG should be checked every hour, then saying some chargers take care of that for you!

gpspowell wrote: »

Hmmm... I wonder how much actual extra time I can really count on with PV?????? Food for thought at least. I have room for 5 more panels before getting really innovative but that would only bring me to 3250W (and 13 panels for anyone being superstitious). Through the middle of the day here at 4900' my panels seem to produce at 82%(+/-) so that would give me 2665W or 2 1/2 panels short (at best). Either way, your figures with me needing an additional 9 panels, or my trying to be "cheap" leaving me needing 8 panels, I come up short on space unless I come up with a different mounting plan. I'd sure like to get this right this time.

Elevation and cooler temps help, I would prefer 'over paneling' since panels are relatively cheap... but if you're in the desert area of Colo, and have those great long periods of sun, I suspect you could get by with a bit less. I just had an hour of direct sun in 11 days, worse I've seen in Missouri, glad I was over paneled, and wish I had more up and running.

gpspowell · January 2015

Re: Resizing my PV system

Let’s see if I’m putting this together: Assuming I go with the L16RE-A battery bank @ 650Ah, add 5 panels to fill my available space giving me 3250W (2600 @ 80%). Trojan recommends 10% of battery capacity as a minimum charging potential (65A in this case) but I would have 55A or 8.5%. Close….
In winter I probably average 4 good solar hours daily for 10.4 kW/h potential with a household usage of 4kW/day. Since I want to be prepared for the occasional two days of clouds interrupting my recharge it sounds like, “so far so good”.
In summer my usage with irrigation goes up 3-4 days/week to 8-10 kW/day. Daily solar hours probably average 8 for a potential of 20.8kW/day. Still doing okay?

Photowhit · January 2015

Re: Resizing my PV system

Pretty much, with a fixed array, it will be very hard to realize the 8 hour potential, might use 6 or 6 1/2 hours, you will only realize close to full potential the hour and a half on either side of solar noon. But you should be covered. Might setup your charge controller to replace any draw down after you reach float. So if the high usage of the day time loads, such as the irrigation pump and household draw from the batteries during the day, the charge controller will kick back to absorb and keep the batteries topped off. If the load is less than the production, you will basically be running from the array as the charge controller will pass more current to maintain float voltage.

...less the NOCT value, Normal Operating Cell Temperature, This will reduce the % of wattage during summer as cell temperatures rise, closer to that 75% likely. Though perhaps you measured your wattage during summer. Manufacturers are more likely to include the NOCT values in the information either on the back of the panels or in the panel specs on their website. As the panels heat up their production goes down...

BB. · January 2015

Re: Resizing my PV system

Using PV Watts for Cheyenne WO (looks a bit closer to you), fixed array, tilted to 41 degrees from horizontal:

Month
Solar Radiation
(kWh/m²/day)

1
4.14

2
4.93

3
5.67

4
5.86

5
5.82

6
6.24

7
5.96

8
6.12

9
6.20

10
5.40

11
4.46

12
3.79

Year
5.38

And here is Boulder CO--:

Month
Solar Radiation
(kWh/m²/day)

1
4.43

2
4.89

3
6.05

4
6.09

5
5.99

6
6.08

7
6.06

8
6.24

9
6.25

10
5.67

11
4.60

12
4.29

Year
5.56

4 hours of noon time equivalent sun may be possible (long term average) for winter--But summer looks much closer to 6 hours rather than 8 unless you use a tracker or something.

Boulder CO with 2 axis tracker:

Month
Solar Radiation
(kWh/m²/day)

1
5.59

2
6.16

3
7.83

4
8.51

5
8.95

6
9.39

7
8.92

8
8.75

9
8.25

10
7.24

11
5.84

12
5.46

Year
7.58

In general, you do not want to plan on using 100% of available energy every day... You should plan on using around 65 to 75% of your predicted power for your minimum loads needed to live, and on sunny/clear days, run the washer, vacuum, pump extra water, etc...

If much of your power usage is after ~4pm--Then 8% will be OK. However, if much of your loads are in the middle of the day--Those loads will reduce the amount of power available for charging the battery bank--Which reduces the effective rate of charge.

And then there is the genset--You can run fewer panels, but will need to run the genset more (unless you can reduce power usage for predicted bad weather).

These days, panels are about the cheapest part of the system... Is there some reason you don't/can't get an another 25%+ more panels for a higher charge rate?

Not to say that an 8% rate of charge won't work for you... But it will require you (and your spouse/kids) to pay more attention to weather and battery bank state of charge.

-Bill

gpspowell · January 2015

Re: Resizing my PV system

Ah! Temperature and actual gain on fixed panels = fly in the ointment. Bill, Cheyenne is probably an excellent comparison as it is 40 miles straight north of me. The discrepancy between the charts you included and my estimates is really enough to throw serious doubt on my trying to fudge on panel requirement. It looks like I better be looking at how to mount three more panels. That would bring me up to 4000W (3200W @ 80%) which should be 66.7A potential charging capacity. That would satisfy the 10% (barely) and I'm pretty sure my Midnight Classic 200 will handle that (again barely). Better?

westbranch · January 2015

Re: Resizing my PV system

Cariboocoot just posted this in another current thread about charging at less than 10%

You may have seen me write a few hundred times "peak charge current". It is exactly that: the maximum current you will see. It is not necessary nor even likely to have that maximum throughout the whole Bulk charge stage. Panels start out producing lower in the morning and increase as the sun becomes more direct on them.

What matter is: are the batteries getting charged? If so, the rate is not particularly relevant. It is a shortcut to assuring you can put enough energy back into them to complete a charge on a good, sunny day.

BB. · January 2015

Re: Resizing my PV system

In the end, it is how the batteries are doing... If they need some water every ~two months and you get them >90% SOC once or twice a week--You are probably OK.

If you find yourself watching the batteries like a hawk and trying to get the family to reduce power because the clouds are coming in tomorrow, etc... And/or are spending more on gasoline for the genset than you would like--Then adding more panels can help reduce the stress in your life.

We use rules of thumbs to get systems "close" to paper based needs (I really believe in a "balanced system"--Loads drive battery bank, battery bank & loads & sun drive solar array, etc.).

Once you have the system installed--Read all you can about your batteries and keep an eye on them. If they appear to be "happy" -- Don't sweat the small stuff to reach some magic XX% number. Bumping from 8% to 10% is not a huge difference--If you don't have the need for that extra 2% points--You don't need it.

However--When that streak of bad weather rolls in a couple times a year--The extra panels can be a help--Or be no help at all (dark clouds, you may get 5% or less from your array)--A 2x larger array is still only 10% of your normal output--cutting power or running the genset are really the only options--Not very many folks are going to put a 10 to 20x larger array for a few days/weeks of bad weather every year.

-Bill

BB. · January 2015

Re: Resizing my PV system

In this thread are a lot of likes/posts about batteries/conservation and alternative solar RE projects...

Working Thread for Solar Beginner Post/FAQ

The basic Battery Information:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

After you read the above links... You will be convinced that Lead Acid Batteries will never work for solar off grid power

... But they do. You just need to follow a few rules to keep them happy.

-Bill

gpspowell · January 2015

Re: Resizing my PV system

Dang! Just when I thought I'd grasped it. So, Bill, is it then safe to assume (there's that word again) that if I replace the battery bank as planned and add the 5 panels I have room for, AND monitor everything carefully using genset or grid if any weaknesses are found, I MAY find that to be adequate. AND if not, further tweaking of the system (more panels?) could be done later on without having caused any damage to the battery bank? Or should I just say, "damn the checkbook" and do it all now?

BB. · January 2015

Re: Resizing my PV system

You monitor the battery bank state of charge and make up for "bad days" with a genset--You are fine.

Add panels when you want to (spend less on the "noise maker").

-Bill

Surfpath · January 2015

Re: Resizing my PV system

BB. wrote: »

If you find yourself watching the batteries like a hawk and trying to get the family to reduce power because the clouds are coming in tomorrow, etc... And/or are spending more on gasoline for the genset than you would like--Then adding more panels can help reduce the stress in your life....Bumping from 8% to 10% is not a huge difference--If you don't have the need for that extra 2% points--You don't need it. However--When that streak of bad weather rolls in a couple times a year--The extra panels can be a help--Or be no help at all (dark clouds, you may get 5% or less from your array--A 2x larger array is still only 10% of your normal output--cutting power or running the genset are really the only options--Not very many folks are going to put a 10 to 20x larger array for a few days/weeks of bad weather every year.

Gpspowell, After 5 days of rain and clouds I'm dealing with that however right now and let me tell you I wish my present 8.5% charge rate was 10% (or 12%, really).

Bill you say it perfectly here and I wish I understood this advice 2 years ago....."Not to say that an 8% rate of charge won't work for you... But it will require you (and your spouse/kids) to [periodically] pay more attention to weather and battery bank state of charge".

So, I am looking forward to my additional 750 watts of PV which will bring me up to 10+%. GPS, I advise you consider 10% as a minimum as well. Less stress. No need for genny. Let it rain/snow/cloud.

gpspowell · January 2015

Re: Resizing my PV system

Thanks everybody for the feedback. Here's where I've ended up: I'm adding 8, 255W ET Solar panels which will bring my array up to 4040W (After losses that's more realistically around 3200WA). In theory that gives me about 67A into a Midnight Classic 200 charge controller that has an expected output (at 48V) of 67A. So far, so good. I decided against the bank of 16 Trojan L16s (660Ah) and am installing a new GB Industrial 48V fork lift battery (804Ah). Now 67A into a 804Ah bank is only 8.3% (less than the ideal 10%) Seeing how the 8 additional panels are taking up 100% of the mounting room I have without some very involved construction I'll be watching the new system closely for shortcomings and hope I can avoid an elaborate panel mounting project in the future.

gww1 · January 2015

Re: Resizing my PV system

I am on grid and have your same battery and 5600 watts of solar. I can run some loads during the day while charging but don't find I have to much solar. I do however take my batteries down to 65% soc nightly so start from a pretty low spot when I start my charge. I have to interceed about every three days or more regaurdless of sun, Maby every 5 days if all is sunny. For me interceeding means something as small as dropping daytime loads while charging to actually using the grid to charge. If you have a lower discharge rate say 20 to 25% dod and watch your day time loads so that your charge rate stays above 5% you may be ok. I do think you can only do this with higher then normal absorb voltages. My advice is when you first install you need to give it a good charge and eq with a hydrometer in your hand and get those sg up where they are supposed to be and record them all at their highest per cell. number the cells 1-24 and write the results down and call bbi if you can't get the sg high enough and get a charging voltage recomendation from them to help you in this. The absorb would be 57.2 for them, I am running 61.4 in a cold garage. I am not telling you to run this voltage, just that it is important to get it correct in the beginning. Get the sg correct before you start using them daily. Then you have something to refer back to as your batteries age.
Good luck
gww

Photowhit · January 2015

Re: Resizing my PV system

FWIW - I have roughly the same array and half the battery, and still have to be careful, had 10 days in Dec with 1 hour of sunlight... worst I can remember. Off grid solar isn't a spectator sport!

Vic · January 2015

Re: Resizing my PV system

Hi gpspowell,

Am a bit late to this Thread ... did look at most of your other posts, and did not find anything on your string configurations, string Vmp, etc.

So, are you planning on using a single Classic 200 as the CC for the entire array?

Have you used that great MidNite Classic String Sizer (?):
http://www.midnitesolar.com/sizingTool/displaySizing.php

You MIGHT be expecting a bit too much from a single Classic 200, if you plan to use it as your only CC. I did make some guesses at your system, and perhaps they were incorrect, but looks a bit too tight to me. And, it is best to not run electronic power producing/regulating systems at their very maximums, IMO.

From another of your posts, are you using the Classic 200 now? Do you have the Battery Temperature Sensor attached to one of your GC batteries?

Guessing done for now. Thanks, Vic

gpspowell · January 2015

Re: Resizing my PV system

Thanks for the continuing feedback.

gww1: It sounds like you use much more of your battery than I intend to. My average daily usage is 4kW. In the summer months it goes up as high as 10kW as I irrigate a small orchard and large garden but only during the middle of the day when solar gain is at its maximum potential. The well pump draws 1200W and runs about three hours of a four hour irrigation cycle on the largest set. I live on the plains of NE Colorado with lots of good sunlight (5+ hr/day average). So I'm hoping to draw down no more than 20% per day. As an aside; the fellow at GB told me that the expected life of the battery as expressed in charge cycles was pretty much based on a charge cycle being each time the battery is charge fully to the state of "gassing". He recommended that my CC be set up to daily charge only to 2.30-2.34 V/cell and EQ weekly to fully charge. His contention being that charging in that manner would extend the life of the battery and would be essentially one cycle per week. Solid advice?

Vic: I am hoping to use my single Classic CC and I DO have a temp. sensor on one of the GC batteries. The current system is: 48v, 8 panels of 250W rating, Xantrex SW4048 inverter, Midnight Classic 200 charge controller, 8 golf cart batteries. For backup I have a Honda 2000eu generator and a Companion 2000eu that will couple to produce 3200W at this altitude (4900’) but the system is currently set up to automatically draw from the grid if needed (Not set up to FEED the grid at all). The intent of the installation was to get my feet wet in solar power with a system running "critical needs" (boiler, refrigerator, freezer, home office, and well pump) but I want the system to be able to be used "off grid" as a primary system in case of any future emergency (Our family farm was without electricity for 5 months after a severe blizzard in the late 70s). I'd like to be able to switch to grid backup (generator backup if ever the grid becomes unavailable) at no lower than 60% SOC, have roughly 2 days storage in that first 40% draw, and enough charging capacity to return to 100% SOC as often as possible.

gww1 · January 2015

Re: Resizing my PV system

gps

as expressed in charge cycles was pretty much based on a charge cycle being each time the battery is charge fully to the state of "gassing". He recommended that my CC be set up to daily charge only to 2.30-2.34 V/cell and EQ weekly to fully charge. His contention being that charging in that manner would extend the life of the battery and would be essentially one cycle per week. Solid advice?

The web site says every time you charge is considered a cycle and if you have a low discharge rate you should discharge over more then one day before charging.

I don't nessisarily believe the web site cause like all web site it gives conflicting info.

I like the ideal of the advice you were given however it does cause one issue in my mind. Charging with out boiling is going to leave you in the position of not being able to relie on anything but voltage all the time except once a week. The acid is heavier then the water and if you don't get mixing you don't really know wheather you got above 80% charge daily and I here that leaving a battery below that is not good. Lastly you won't know how it is working till the first time you equalize. If it takes 20 hours to charge full, you are not going to like it.

I got pretty simular advice when I bought my battery. I was told to absorb at 57.2 volts (2.38 per cell) and then eq till the battery got warm once a month. I eq at 62 volt for 20 hours to get the sg up and it takes that long to heat the battery.

I finaly called the battery builders and thats where my very high charging voltages come from. I am not an expert and I may be killing my batteries but I absorb at 61.4 volts and eq at 63.2 volts once a month. I do not temp comp during charging and sometimes if the battery starts at 45-50 degrees f I will absorb for one hour and then use the eq voltage for the next 2-3 hours till the sun goes down. I usually only do this every three or forth day to get my amps zeroed out and more as read on the trimetric. It would be nice if someone who has charged as you plan to and has did it for a while, would chime in and give you guidence. My batteries are only one and a half years old and if I kill them in five years I could tell you that but I haven't owned them long enough to know what I am really doing. I couldn't get the sg up before in the time the sun gave me and now I am doing better.

Take out of my comments what makes sense to you and if you go a differrent rout please keep us informed.
Good luck
gww

Ps I would like to see 1.270 sg daily and 1.285 1.295 during monthly eq, and maby higher then 1.270 every 5 to 10 days.

Vic · January 2015

Re: Resizing my PV system

gpspowell wrote: »

Thanks for the continuing feedback ...

Vic: I am hoping to use my single Classic CC and I DO have a temp. sensor on one of the GC batteries. The current system is: 48v, 8 panels of 250W rating, Xantrex SW4048 inverter, Midnight Classic 200 charge controller, 8 golf cart batteries. For backup I have a Honda 2000eu generator and a Companion 2000eu that will couple to produce 3200W at this altitude (4900’) but the system is currently set up to automatically draw from the grid if needed (Not set up to FEED the grid at all). The intent of the installation was to get my feet wet in solar power with a system running "critical needs" (boiler, refrigerator, freezer, home office, and well pump) but I want the system to be able to be used "off grid" as a primary system in case of any future emergency (Our family farm was without electricity for 5 months after a severe blizzard in the late 70s). I'd like to be able to switch to grid backup (generator backup if ever the grid becomes unavailable) at no lower than 60% SOC, have roughly 2 days storage in that first 40% draw, and enough charging capacity to return to 100% SOC as often as possible.

1. OK, so, are you planning on using a single Classic 200 as the CC for the entire array. And the String Vmps are fairly close to the same for the original PVs, compared to the new ETs?

2. Have you used that great MidNite Classic String Sizer (?):
http://www.midnitesolar.com/sizingTo...playSizing.php

3. You MIGHT be expecting a bit too much from a single Classic 200, if you plan to use it as your only CC. I did make some guesses at your system, and perhaps they were incorrect, but looks a bit too tight to me. And, it is best to not run electronic power producing/regulating systems at their very maximums, IMO. How do you feel about the MN String Sizer's results?

4. What is the configuration of the PV strings and what PVs are being used (manufacturer/model number) for the previously installed system? What is the String Vmp? What is the calculated Maximum string Voc for your coldest temperatures?

5. What string configuration are you planning to use for the PVs that will be added? With only one CC, and 8 PVs that you plan to add, you really must be planning on two strings of four of the ET PVs, but perhaps you plan to mix the existing PVs from the older part of the system, which might allow you to run strings of three PVs ... or something, possibly ...

With your probable cold temps for the coldest time of year, am wondering about the String Voc or Vin to the Classic 200. The Linked String Sizer will help you determine what you can get away with. I plugged some real data from the Data Sheet for the ET PVs, and guessed about your coldest days, and the results were flagged as EXCESSIVE (believe that this was the term). If you are relying on the Classic's Hyper VOC ability to give you some cushion, this can mean that at all other times of the year, the Vin is high, and makes the CC run hot from the high Vin reducing CC efficiency, based on some experience. AND, you are planning on running the Classic at its maximum output current capability, at least for some part of the charge cycle, and perhaps when your pump or other loads are active, at other charge stages.

Perhaps your existing strings have a similar max string Voc, as the ETs you are considering, and you have run the system in the coldest time of year, and the Summer, as well ... All of this is really guessing based on very little real data. FWIW.
Guessing done, Good Luck, Vic

softdown · January 2015

Re: Resizing my PV system

Vic and others; the Midnite string sizing tool is invaluable. It has also moved to a new location: http://www.midnitesolar.com/sizingTool/index.php

According to it, I can add another six panels very easily. Given todays low prices, this seems like a can't hardly miss investment. Thinking I should stick with identical Grape Solar panels. http://www.solar-electric.com/grape-solar-185-watt-monocrystalline-module.html Yes.....they do cost significantly more. But there are benefits to uniformity and consistency.

I have seen these panels rated at 180, 185, and 190 watts with no discernible difference?

gpspowell · January 2015

Re: Resizing my PV system

ARGGGG! Eighteen months of living with this system and I am still WAY behind the learning curve. Vic: The link doesn't work but I found the sizing calculator you recommended and the results are a bit tight. As to the panels: The string amps are nearly the same; The existing panels are ET 250s and the new ones are their replacement models (ET 255s). I'm not sure about your question re: cold temp VOC, the calculator says I will enter "Hyper VOC" at -104.8ºF. The panels will be configured into 4 strings of 4. The calculator concludes that I need 1.1 CCs (their recommendation is that a second CC is required at 1.2). Okay, it looks like I'm pushing it unless I add a second CC. I don't want to be pushing so hard as to burn up components. I guess I dig out the checkbook again.

BB. · January 2015

Re: Resizing my PV system

We use 0.77 (1/0.77 = 1.30) as the "break point" for upsizing a charge controller. Dirty panels with a little age on working in hot sun (lower Vmp, there for lower panel wattage) means that is the around the typical maximum power for a sea level installation in warm weather.

A MPPT charge controller will limit its current output by design (it will not move more current than it is rated for), and most will reduce current when they get hot.

On the other hand, you are correct that electronics running hot do not last as long. If you can justify running at less than 1.0 of rated current (two or more parallel controllers), it would not be a bad thing (other than for your wallet).

Current heating is Power = I²R -- So if you have 2x the current, you have 4x the heating... Reducing current flow will reduce heating very significantly.

-Bill

gww1 · January 2015

Re: Resizing my PV system

gps
What is the voc per panel? You are at the tipping point as far as two cc. I intend to eventually go a bit over the max of my cc cause to get high enough volts I have to run in threes and and that will over max the cc. The cc are over $500 each and it is hard not to fill them. It does require faith that you will get by with it and it might be a loser. You might midigate the risk slightly by adding a exterior fan on them. My mx60 ran for a bit while the fan didn't work and I don;t know how long it was before I noticed it. It has 2820 watts solar on it. I have added a computer power source fan insted of fixing the internal fan. It is ten years old and just keeps going. I could be wrong and don't want to advise that I am correct but I personally have faith that it will handle the maxing of panels. I do know that my cc is on the edge of overvoltage with only 3 panels in series in a warmer place then colorado. (missouri) My voc is 37 volts. Four panels seems to much, but you are working with 200 volts not like mine which is 150 volts, so did you use the voc voltage when you used the string sizing tool?

In your position and expesially if you may expand some day. Even if you were to try and run with one cc, You might put a combiner box that would allow you to do another one later and if you are trenching are your wires are hard to run, run two feeds, one to use if needed. Then after watching it for a bit, it would be easier to add another cc.

My solar durring summer would still, most times, produce under the max even if over paneled. I think vic does know of which he speaks and my route would be harder on equiptment, but I keep thinking a ten year old cc should be dead after running a bit without a fan but it aint. It might die today.

Just my thoughts.
gww

westbranch · January 2015

Re: Resizing my PV system

I have seen these panels rated at 180, 185, and 190 watts with no discernible difference?

If the physical dimensions are the same, they have come from the same production run, in all likelihood, and then they are STC tested and sorted on output (Watts)...

softdown · January 2015

Re: Resizing my PV system

They may have averaged something like 182, 184, and 188 watts. Would be pretty easy considering that every batch of everything varies. Could be the glass, could be the wires, could be the testing equipment, could be the temperature etc.

westbranch wrote: »

If the physical dimensions are the same, they have come from the same production run, in all likelihood, and then they are STC tested and sorted on output (Watts)...

Resizing my PV system

Comments

Categories