Off Grid Plant for day use

debangana

Hi experts

Need your feedback on the battery sizing for an off grid system to be used only during the day. Basically the system would be used used in a small labour intensive industrial unit to light a few CFLs, Tubes and Fans during the day (Let us avoid the question why in the first place light CFLs and tubes during the day). The total KwH required is around 8 per day. (Total 1 KW load for 8 hours running)

Sizing the panel was not difficult and we have converged on a 3 KW poly crystalline module system. This seems adequate at a CUF of ~12%. Now my doubt is with the battery sizing. Since I do not require any storage for night use, the only storage required is back up for the next day use. Moreover, since the workshop is closed on sundays the battery bank can get charged in those days. Can I go ahead with a small battery bank. I was considering 24 Volts, 300 Ah of batteries.

My idea is once I install the batteries at full charge. Since the load would be required during the days, the batteries would get discharged only during small fluctuations, in the morning and in the late evening.

Am I going right?

RandomJoe

Re: Off Grid Plant for day use

What about overcast / stormy days? Any clouds at all can reduce PV output considerably, and a heavy overcast can reduce it to zero. Will you still need to keep the lights going on those days, or will you shut down operations? (Or fire up a generator.)

Adding to that, do you have extended periods of bad weather? In summer time where I am storms are generally fast-moving and it'll be sunny again the next day - if not a few hours later. In winter though it may be overcast for days on end, which can require a large battery bank - or frequently running the generator - to keep things running.

vtmaps

Re: Off Grid Plant for day use

That 300 ah rating of your proposed battery is usually at C/20 rate which is a 15 amp draw. Your kilowatt load will draw at least 45 amps from your battery bank. Therefore your battery bank capacity is significantly less than 300 ah at a 45 amp draw (Peukert law).

I think 300 ah is too small for your application. I don't think that it will get you through much more than a couple of hours of low sunlight.

--vtMaps

jcheil

Re: Off Grid Plant for day use

3kw of solar will just barely meet your loads assuming that you are getting 5 "solar" hours per day. Most people here like to see a 10% charge rate which is closer to 6wk of panels since you can't count on the sun to be perfect every day.
If you post the city you will be in the experts here can determine the average solar hours based on your location.

And based on 8kwh per day loads, with 1 day of reserve and not going below 50% soc you would likely need around 800ah of batteries at 48v. But like everyone else said, you can't "depend" on solar so a generator is a must in any system.

This is in no way a "small" system and will cost quite a bit. If you have the ability to go on grid (or even grid-tie) you would save a lot of money. Off-grid solar is 10x what it would cost to buy the electric from the power company.

NorthGuy

Re: Off Grid Plant for day use

There's no doubts that if you do not have night loads you can get away with a smaller bank, but it will take some imagination to calculate its size.

First of all, day pattern. When you day start, is it a full sunshine already. If not you'll be drawing batteries. Some for the evening. This also different in summer and in winter, so you need to make your calculations for the worst period - Christmas time

Second. Is there some emergency usage during the night possible?

Third. What are you plan for cloudy days? Avoid using power? Drawing batteries? Using generator?

YehoshuaAgapao

Re: Off Grid Plant for day use

If you live up north or if cloudy (as many people in this forum do), you will probably need to half the PV KWH/day. My KWh/day is based on 5 KWh/day per 1KW. In AZ 8 hours of sun per day is common even in the winter. Northern Wisconson, Canada, or worse, western Washington or british columbia, will not get 8 hours of sun per day. 15 60-cell panels, 16 golf cart batteries (2 strings of 8; do 1 string of L16s if you can afford it), Outback VFX inverter (48V; VFX is indoor only), 1 80A charge controller.

Do 48 volts if you can. More efficient. 3KW will prefer 48 volts. Plus 3KW of PV will not fit a single charge controller @ 24 volts. Single controller, single inverter systems are simpler, and allow brand-mixing -- no data communication is needed, and those are usually brand-proprietary (especially xantrex), though keeping everything the same brand will be more expansion friendly..

24 Volt Single Charge Controller Systems

#PV     StrSize #PVStr  PV Watts    PV KWh/day  BatVolt #MPPT CC    Inverter (Min)      Batteries           #BatStr BattAH  BattKWh PV Charge Rate
2       2       1       450-520     2.250-2.600 24      1 (20A)     600W Pure Sine      2x Group 31         1       130     3.12    9.35-10.81 hrs - Nominal
3       3       1       675-780     3.375-3.900 24      1 (30A)     1000W Pure Sine     4x Golf Cart GC2    1       225     5.4     10.79-12.47 hrs - Low-Nominal
3       3       1       675-780     3.375-3.900 24      1 (30A)     1000W Pure Sine     4x Group 31         2       260     6.24    12.46-14.41 hrs - Low
4       2       2       900-1040    4.500-5.200 24      1 (45A)     1000W Pure Sine     4x Golf Cart GC2    1       225     5.4     8.09-9.35 hrs - Near Cost Effective Maximum
6       2 or 3  3 or 2  1350-1560   6.750-7.800 24      1 (60A)     Outback FX/VFX      4x L16              1       400     9.6     9.59-11.08 hrs - Nominal
6       2 or 3  3 or 2  1350-1560   6.750-7.800 24      1 (60A)     Outback FX/VFX      8x Golf Cart GC2    2       450     10.8    10.79-12.47 hrs - Low-Nominal
8       2       4       1800-2080   9.00-10.400 24      1 (80A)     Outback FX/VFX      4x L16              1       400     9.6     7.19-8.31 hrs - At Cost Effective Maximum
8       2       4       1800-2080   9.00-10.400 24      1 (80A)     Outback FX/VFX      8x Golf Cart GC2    2       450     10.8    8.09-9.35 hrs - Near Cost Effective Maximum

48 Volt Single Charge Controller Systems

#PV     StrSize #PVStr  PV Watts    PV KWh/day  BatVolt #MPPT CC    Inverter (Min)      Batteries           #BatStr BattAH  BattKWh PV Charge Rate
3       3       1       675-780     3.375-3.900 48      1 (15A)     1000W Pure Sine     4x Group 31         1       130     6.24    12.46-14.41 hrs - Low
6       3       2       1350-1560   3.375-3.900 48      1 (30A)     Outback FX/VFX      8x Golf Cart GC2    1       225     10.8    10.79-12.47 hrs - Low-Nominal
6       3       2       1350-1560   6.750-7.800 48      1 (30A)     Outback FX/VFX      8x Group 31         2       260     12.48   12.46-14.41 hrs - Low
9       3       3       2025-2340   10.125-11.7 48      1 (45A)     Outback FX/VFX      8x Golf Cart GC2    1       225     10.8    7.19-8.31 hrs - At Cost Effective Maximum
9       3       3       2025-2340   10.125-11.7 48      1 (45A)     Outback FX/VFX      8x Group 31         2       260     12.48   8.31-9.60 hrs - Near Cost Effective Maximum
12      3       4       2700-3120   13.50-15.60 48      1 (60A)     Outback FX/VFX      8x L16              1       400     19.2    9.59-11.08 hrs - Nominal
12      3       4       2700-3120   13.50-15.60 48      1 (60A)     Outback FX/VFX      16x Golf Cart GC2   2       450     21.6    10.79-12.47 hrs - Low-Nominal
15      3       5       3375-3900   16.875-19.5 48      1 (80A)     Outback VFX         8x L16              1       400     19.2    7.67-8.87 hrs - At Cost Effective Maximum
15      3       5       3375-3900   16.875-19.5 48      1 (80A)     Outback VFX         16x Golf Cart GC2   2       450     21.6    8.63-9.97 hrs - Near Cost Effective Maximum