MPPT output

Re: MPPT output

Gunner,

OK. A quick look at the Sandia form suggests the Ah/day required is a gross figure that contains all efficiency factors. I’ll assume this is for a “12 V” nominal system.

Average insolation from May through September on a south-facing array tilted up at ~33-1/2 degrees (latitude minus 15 degrees) for International Falls, MN, ranges from 4.3 hrs/day to 5.9 hrs/day.

Using the lower number suggests that you’ll need an array rated at 105 Ah/day / 4.3 hrs/day = 24.4 A. Assuming an STC Vmp spec of ~17.4 V, the array will need to be rated at 17.4 V x 24.4 A = 425 W STC.

Average insolation from May through August on a south-facing array tilted up at ~33-1/2 degrees (latitude minus 15 degrees) for International Falls, MN, ranges from 5.4 hrs/day to 5.9 hrs/day.

Using the lower number suggests that you’ll need an array rated at 105 Ah/day / 5.4 hrs/day = 19.4 A. Assuming an STC Vmp spec of ~17.4 V, the array will need to be rated at 17.4 V x 19.4 A = 338 W STC. This might be a more cost effective way to go, and then use the generator for backup in September.

One option would be an array of three ~125 W modules (~375 W STC total) wired in parallel through a Morningstar Tristar 45 PWM controller with a remote battery temp sensor. Candidate “12 V” modules of ~ that size include products from Mitsubishi, Sharp, and Kyocera. See: http://store.solar-electric.com/hiposopa.html and http://store.solar-electric.com/mochco.html

Another option would be a pair of larger, higher voltage modules of ~175 W STC each (~350 W to 380 W STC total) wired in parallel along with an OutBack MX60 MPPT controller and a remote BTS. Candidate modules include products from Sharp, Mitsubishi, Kyocera, and Evergreen. See: http://store.solar-electric.com/hiposopa.html and http://store.solar-electric.com/outpowmxmp.html or http://store.solar-electric.com/xaxwmp60amps.html

A cost effective 12 V battery bank would be three Interstate SRM-4D flooded-cell lead acid batteries wired in parallel for ~600 Ah. Assuming the batteries are located in the cabin, they’d need to be vented to the outside while being charged. See: http://www.solarseller.com/battery_box_power_vent_by_zephyr_industries.htm

An alternative that I’d recommend would be three side 4D VRLA (AGM or gel) batteries wired in parallel for 12 V X ~600 Ah. East Penn’s Deka and MK brands would be good candidates.

You’ll also need wire, conduit, an array combiner box, circuit breakers, etc. What about an inverter to provide 120 VAC?

HTH,
Jim / crewzer

Re: MPPT output

For systems with arrays less than ~400 W to 500 W STC, the MX60 may not be the best choice, as its higher cost and its internal power losses take their toll. It’s worth running the numbers…

Also, if you find that you have to expand your system, it would be less costly to add another 125 W module to the system with the TS-60 controller than to add another large module to the MX controller. However, the latter would be able to deliver more power...

Choices, choices...

The systems I’ve described will deliver ~20 A peak battery charge current at mid-day. That’s about the miminum for a 600 Ah battery bank allowing for light use and battery aging. An 800 Ah bank would probably work initially, but, depending on day-time loads, may be come difficult to recharge as the bank ages.

Another option might be to go with the 800 Ah bank initially, and later add another PV module to the array.

Tip: try to postpone some daytime loads until after the controller has been in absorb stage for ~30 minutes or so. The controller operates in current limit mode when in the absorb- and float stages, so there’s unused current available from the array.

Other “small” true sine wave inverters to consider are the XP’s from Exeltech. If possible, try to turn off the inverter at night to reduce idling power drain. You’d need a separate charger.

See: http://store.solar-electric.com/exsiwain.html
And: http://www.exeltech.com/xpspecs.htm
And: http://store.solar-electric.com/bach2.html

HTH,
Jim / crewzer