battery bank

robocop

renesola 300w  vmp 36,6  8.2A  eff  15.46%

Raj174

Robert you are going to need a 48 volt battery, just to get the maximum amount of power through the charge controller.
The issue is that the battery size is based on the load (A/C units wattage over time of use) You have stated 2900 watts for 3 to 4 hours. However A/C units cycle or if inverter, can run at a reduced speed lowering power usage. It would help to have an idea of how much of the time they are consuming less power. Are the A/Cs installed yet? Is there any way to measure the actual power consumption over a typical 3 or 4 hour period? The difference could be a 3000 dollar battery bank verses a 6000 dollar bank. The A/C may run at full power for the first hour and cycle at 50% for the next three. It depends on insulation and whether the unit was sized correctly for the space being cooled. Once a battery size is nailed down then panel configuration can be determined.

Raj174

Ok, I looked at the charge controller specs and your best bet is to max out the array. Although it states a max of 3000 watts, you could over panel with 3600 watts because they never output their nominal wattage when it's hot and it never really gets cold there, right? They will make more power when it's cloudy. Can you get 12 volt 250 amp hour true deep cycle AGM batteries? I'm not talking about deep cycle marine batteries, they will not last as long.   

robocop

Hi Rick  For now I would like to know how many panels of that 300w renesola do I nned to charge a 48v/400-500 ah battery bank, need to jnow how many to begin to order them, It is always sunny here.

Raj174

Ok, you will need 12 panels, there may be some debate as to whether the configuration will be 6 strings of 2 or 4 strings of 3 panels, but it will be 12 panels either way. The battery bank will need to be AGM batteries configured to at least 48V 500AH.

mcgivor

robocop said:

renesola 300w  vmp 36,6  8.2A  eff  15.46%

Do you have a model number, with that VMP  it looks like they are 72 cell panels, you could use 60 cell panels, since the inverter charger is MPPT, 60 cell panels generally tend to be cheaper.

Raj174

I agree with mcgivor, If you can find 60 cell panels cheaper, they will work just as well. Probably better, it will give the MPPT charger more headroom to down convert to the charging voltage.

Estragon

72 cell panels probably make sense for 2 panel strings in the heat if array is reasonably close. If going further or 60 cell panels are cheaper, 3 panel strings makes sense.

The charge controller would run a bit cooler with 72c 2p strings with Vmp ~70v. 60c 2p strings in the heat may be a bit low Vmp for EQ.

I just bought 3 72cell panels for $250CAD (~190USD) each. They were remains of pallet qty used in a larger installation and they wanted to get rid of them.

BB.

For a 5% to 13% rate of charge:

• 500 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,916 Watt array (weekend/seasonal)
  
• 500 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 3,831 Watt array nominal
  
• 500 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 4,981 Watt array "cost effective" Maximum
  

For full time off grid/taking loads during the day (which reduces charging current to battery bank), I would suggest 3,831 Watt array minimum for full time off grid.

And a 3,831 Watt array would produce (fixed array):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Willemstad
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 78° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
6.65	6.95	6.92	6.23	6.58	6.78
Jul	Aug	Sep	Oct	Nov	Dec
6.74	6.61	6.42	6.41	6.28	6.32

Lots of sun... Say 6.23 hours per day (Apr). If you have a minimum load requirement--I would suggest that you use only 65% to 75% of the "predicted levels" of output--Allows for variations in weather and temperature. A 3,831 Watt array (3.8 kW+/- 10% is "close enough for solar"--I just copy all of the digits so you can follow the numbers as I use them))--Numbers are long term averages and may not account for local weather patterns (like marine layer):

• 3,831 Watt array * 0.52 system derating (all losses) * 6.23 hours of sun * 75% (variations in weather) = 9,308 WH ~ 9.3 kWH per day minimum per day available energy

The above numbers should be pretty conservative. You might do upwards of 20% "better" because most of your energy usage is when the sun is up... You do not have the 20% losses that flooded cell lead acid batteries get from cycling.

If your energy usage can vary (less sunny day, less A/C usage)--Without the 75% derating number:

• 3,831 Watt array * 0.52 system derating (all losses) * 6.23 hours of sun = 12,441 WH per day nominal (April)
  

All numbers are beginning estimates. Local conditions are not always "ideal". For example, if you have trees/buildings/power poles/power lines that can shade your panels--That will dramatically reduce your output while the shade is present (50% or more losses for a solar array/string for minor shading is not unusual).

-Bill

Photowhit

Raj174 said:

Ok, you will need 12 panels, there may be some debate as to whether the configuration will be 6 strings of 2 or 4 strings of 3 panels, but it will be 12 panels either way. The battery bank will need to be AGM batteries configured to at least 48V 500AH.

Check the needs of your charge controller, It's likely with a MPPT type charge controller, that 2 - 72 cell panels will not have high enough in coming voltage for the MPPT system to work properly! Also check that 3 don't have too high a VOC. We've found some situations where 72 cell panels did not 'fit' with some charge controllers for 48 volt charging.

robocop

To Raj74 thnks for your reaccion

Do you mean that I can charge the battery bank with 2 or 3  72 cell panel 300w   .

robocop

PH1800 MPK Plus MPPT Series (1-5KVA   from MUST company  this is the inverter model . plse let me know thx

Raj174

robocop said:

To Raj74 thnks for your reaccion

Do you mean that I can charge the battery bank with 2 or 3  72 cell panel 300w   .

The charger specs indicate a maximum PV voltage of 130 volts, so 4 strings of three panels is not possible because the VOC would be 134 volts. So 6 strings of 2 panels per string would work. Are you able to get 60 cell panels? They would be in the 250 watt range and usually cheaper per watt. The reason I ask is that although the VMP voltage of 2 300 watt panels is 73.2, a heavy load, either charging or supporting the A/C, could drag the PV voltage down to the mid 60s volts. AGM batteries usually charge to absorb at about 58 volts. The MPPT controller needs 5 or 6 volts above the battery voltage in order to down convert the PV voltage to charging voltage. I think that you would be ok with each string being 2 panels in series, however it will be right on the edge the controller's capabilities. If you can get 250 or 260 watt panels, they can be installed in strings of 3 making the PV voltage around 90 volts leaving plenty of room for the MPPT charger to work. You would still need 12 panels.  
Rick

Photowhit

Raj174 said:

... So 6 strings of 2 panels per string would work.

Be sure to check about this! boB has posted here that MPPT type charge controller need a VMP of about 30% above the battery charging voltage to work properly! boB is an engineer at Midnite and formerly Outback, so should know his business. 2 - 72 cell panels in series, would have a working voltage of around 70 volts or less, 48 volt battery banks charge at 58-60 volts

mcgivor

My thoughts are 60 cell panels in strings of 3 would  be the best fit from the standpoint of sufficient headroom and not pushing the envelope, at around 90-96 volts

jonr

Be sure to de-rate VMP for temperature (perhaps -4V).  One source says to take the charging voltage needed plus a 2V margin for the controller and 2V for possible wire loss.   So I get something like 73.2 - 4 -2 -2  = 65 which is greater than 59. 

I think it is OK to run close to the limit - as I understand it, if the charging voltage is slightly low, the batteries won't accept as much charge.  So the current output from the panels will drop and the panel voltage will rise to above VMP.  Ie, the situation is partially self correcting, resulting in slightly slower charging, not no charging.  Plus, high temperatures tend to happen when there it lots of sun.

Estragon

Best bet would be to check around to see what local pricing and availability is like and go from that. Shipping partial pallet qty likely to be costly.