Newbie PV System using MPPT

Re: Newbie PV System using MPPT

Welcome to the forum.

Your need of 13.5 kW hours per day is quite large for an off-grid system. I do not believe you will be able to achieve this with a 3kW array. Normally we would expect:
3000 Watts * hours of good sun (say 6 where you are) * 0.52 over-all system efficiency = 9.36 kW hours per day AC.
You would probably need 5kW to 6kW of array to achieve that much daily 'harvest'. Keep in mind that hot panels will not produce as much power as cool panels, and Guatemala is hot!

The battery bank would also need to be substantial for that much power, even on 48 Volts. 13.5 kW hours AC is going to be 15 kW hours DC due to conversion losses, and then you need to add in the inverter consumption. You are very close to needing 16 kW hours DC capacity. On 48 Volts that is about 334 Amp hours. At 25% depth of discharge that would be a very large 1336 Amp hour 48 Volt battery bank. It would need about 134 Amps of peak charge current potential. This would mean around 8kW of array and two large charge controllers such as the Outback FM80 or MidNite Classic 150.

Now about those panels. You have fourteen of these 215 Watt Evergreens. Their Vmp is too low to work on a 48 Volt system with just two in series: Vmp 29 * 2 = 58 Volts; a 48 Volt system needs a Vmp around 70 to work properly, especially in a hot climate because it is the Voltage that will fall as the temperatures rise. If you put three in series this problem is solved, but you will not be able to use all fourteen panels; you either have to drop two or buy another one (I'd opt for the latter). Likewise more than three in series and you will have trouble with the array Voc being too high for the controller unless you opt for one of the MidNite Classic 200 or 250 models. Again, though, the number of panels you have does not divide up evenly into this configuration.

So you have two options: reduce the load demand considerably or increase the array size significantly. If I were you I'd try for the first option before looking at the second.

Re: Newbie PV System using MPPT

Let's do some basic math and see what happens.
Three of the panels in series would give you: 645 Watts, 87 Vmp, 106.8 Voc, 7.43 Imp, 8.12 Isc per string.
If you had seven of these strings in parallel: 4515 Watts, 87 Vmp, 106.8 Voc, 52.01 Imp, 56.84 Isc for the whole array.

That array on a 48 Volt system using an MPPT controller has an output current potential of about: 4515 Watts * 0.77 (panel & controller efficiency) / 48 (minimum system Voltage) = 72 Amps. One 60 Amp charge controller would be stressed with that much current; it would 'clip' the 12 Amps above its output maximum and that power would be lost.

The best way to set up a system is to size the battery bank first, based on the daily load demands. Then you calculate the amount of panel & charge controller capacity necessary to recharge that bank. The good "middle-of-the-road" rule-of-thumb number is to try for a peak charge current of 10% of the battery bank's Amp hour capacity (based on the "20 hour" rate). This is how I arrived at the 1336 Amp hour 48 Volt battery bank in the previous post, its need for 134 Amps peak charging current, and the resulting 8kW array (134 Amps * 48 Volts / 0.77 efficiency = 8353 Watts).

Frankly I do not think the Morningstar MPPT 60 is going to work in this application. The power demands are quite large and would have to be reduced by more than 50% before a single 60 Amp controller becomes viable. You should look at the MidNite Classic line: http://www.solar-electric.com/misocl.html One of these can handle 80 Amps @ 48 Volts. For the previously mentioned set-up you would still need two such controllers as the combined current is so high.

By the way, it is also sometimes possible to split systems into multiple smaller set-ups which can handle the total power requirements without having to rely on any heavy current situations as above. This would depend on what kind of loads there are and whether some can be grouped together separate from others.