Need help for small system

Re: Need help for small system

Lets see if I can help a bit... I am guessing you are from around Abu Dhabi, UAE?

And from a little bit of searching I found this document about how much sun you get per day on a monthly average:

From Table #1 on Page 2 for for UAE/Abu Dhabi (PDF download)

Table 1
Monthly mean daily values of global solar radiation for Abu Dhabi
Months Global radiation, H (MJ/m2/day)
Present measurement: NASA SSE model[17] (22-year average): Abdalla et al.[2] (Average 1971–1980)
January 14.08 14.72 12.17
February 17.42 17.28 15.52
March 20.63 18.83 17.78
April 22.79 22.75 21.46
May 25.06 25.85 23.58
June 24.62 26.03 22.97
July 24.52 23.22 21.74
August 23.51 22.75 20.41
September 22.32 21.78 19.44
October 19.40 19.40 17.17
November 15.26 16.31 14.62
December 11.30 13.61 12.3

The last last two columns are averages over 22 years or 9 years--Use the last column since it is a bit more conservative (lower numbers).

The units are MJ/m2/day--Which to converter to Hours of Noon Time equvialent hours of sun per day (what I am more used to), need to convert MJ into Watt*Hours (should divide by 3,600 seconds per hour):

I toss out the lowest 3 months (assume backup generator for application--but you can use other numbers if you don't want to use a genset). That would give us February at 15.52 MJ/m2/day:

• 15,520,000 J/m²/day * 1/3,600 seconds/hour * 1/1,000 Watts per m² noon time sun = 4.3 hours of noon time equivalent sun per day

Ok... Now that I know how many hours of sun per day you get for at least 9 months of the year, we can start from the beginning.

First size the battery bank. 50 watts * 24 hours per day with 1-3 days of "no sun" and 50% maximum discharge for maximum life (assume 2 days of "no sun" for this calculation):

• 50 watts * 24 hours * 1/12 volt battery * 1/0.52 system losses = 192 AH @ 12 volts recommended

Next, charging the battery bank... We recommend 5% to 13% rate of charge for the average Lead Acid battery... However, more GEL batteries are recommended 5% maximum rate of charge (and usually we do not recommend GEL batteries because of this). The recommended solar array size would be:

• 192 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.05 rate of charge = 181 Watts minimum rate of charge solar array
• 192 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 362 Watts "healthy nominal" solar array
• 192 AH * 14.5 volts charging * 1/0.77 panel+charger derating * 0.13 rate of charge = 470 Watts "maximum cost effective" solar array

We also need to look at how many hours of "noon time" equivalent sun per day you have for your location. Above, I used 4.3 hours of sun per day for February... Depending on your location and your needs, you may need to adjust this number:

• 50 watts * 24 hours * 1/0.51 system derating for DC power * 1/4.3 hours of sun = 547 Watt minimum solar array

And here is where we hit some issues... The maximum size array you can use to "safely" charge a GEL type battery is around 181 watts (for 5% rate of charge). If you charge at higher than 5% rate of charge, you run the risk of gas bubbles forming in the GEL electrolyte--Which will permanently damage the battery.

Next, the amount of solar array to recharge 50 Watts * 24 hours worth of load in February requires a 547 Watt array... That is a bit larger than the recommended 470 watt "cost effective" maximum array... You have a choice of using AGM batteries (which can take higher recharging current), using a slightly larger lead acid battery (too much current could cause battery to overheat in some conditions), or just use the size suggested and recharge at a higher rate (recommend a charge controller with a remote battery temperature sensor option to reduce risk of thermal run-a-way).

It is a close call (470 Watts vs 547 Watts is not a huge difference here) as to size of battery bank vs size of array vs how much sun you actually receive per day (and what months you need to run the system).

Also, picking the correct Inverter will be important... Too large of inverter will waste more power (larger inverters can consume 20 watts or more just being "turned on" which adds to your 50 watt load--and makes you system requirements even larger).

Questions? More information?

-Bill