laugh, but please help :) system for IL

Re: laugh, but please help system for IL

Montat,

Comments

The calculator is seriously flawed… pass on it. The fridge compressor may be a “swing” compressor. 4 A at 60% on and 40% off   = 24 hrs/day x 60% x 4 A = 57.6 Ah/day. That’s the same as 57.6 Ah/day / 24 hours/day = 2.4 A.

(Gu)estimiated Energy Requirement

Clumping the fridge, the small fan, a laptop (85 W?) and some lights together may result in a daily energy requirement of 1,500 Wh. Assuming no inverter, VRLA batteries and overall summer system efficiency of ~75%, you’ll need a system rated for 2,000 Wh/day.

Environment

Average summer insolation (April – September) for the Rockford, IL area on south-facing PV array tilted up at ~27 degrees (+ roof slope?) is 5.0 hrs/day of “full” Sun or better. This suggests that you’ll need an array rated at 2000 Wh/day / 5 hrs/day = 400 W STC.

See: http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/sum2/94822.txt

Unfortunately, winter insolation falls as low as 2.3 hrs/day average. This suggests you’ll need an array rated at 2000 Wh/day / 2.3 hrs/day = 870 W STC.

PV Module Suggestions (tentative; see below)

For a 400 W array: Two each GE 200 W modules, wired in parallel.
See: http://store.solar-electric.com/geel200wasop.html

For an 870 W array: Five each Evergreen 180 W modules, wired in parallel
See: http://store.solar-electric.com/ev180wasopa.html

Charge Controller Recommendation

Get an OutBack MX60 along with its optional remote battery temp sensor and be done with it.
See: http://store.solar-electric.com/outpowmxmp.html

Batteries

If your net energy requirement is 1,500 Wh/day, that’s 125 Ah at 12 V. Allowing for three days of autonomy and limiting discharge to 50%, you’ll need a 12 V battery bank rated at 125 Ah/day x 3 days / 50% = 750 Ah. For five days of autonomy, you’ll need 1,250 Ah.

The size 8D AGM 12 V battery is rated at ~250 Ah. You’d need three wired in parallel for 12 V x 750 Ah, and five wired in parallel for 1,250 Ah.

See: http://www.mkbattery.com/images/lagm.pdf

A 400 W STC array will struggle to recharge a 12 V x 750 Ah battery bank, but an 870 W array should be able to keep up with a 1,250 Ah battery bank.

Alternate “Balanced” Solution.

Mounting PV modules on a roof and parallel to the roof slope can result in excessive power generation capacity in the summer and insufficient capability in the winter. An alternate approach is to install the array at a steep angle (I.e., latitude + 15 degrees). This reduces summer production, but it increases winter production, resulting in a better balanced and more cost-effective system for year-‘round use.

Increasing the array tilt to ~57 degrees would improve the worst case winter month insolation by ~22% to 2.8 hrs/day. Plugging this into the discussion above, you’ll need an array rated at 2,000 Whr/day / 2.8 hrs/day = ~715 W STC. This array size would be a good match for the 12 V x 750 Ah battery bank.

Based on the 1,500 Wh/day energy assumption and installing the array at the steep angle suggested, the following configuration is worth considering:

Four each Evergreen 180 PV modules wired in parallel
One each OutBack MX60 charge controller
Three each 12 V x 250 Ah size 8D AGM batteries wired in parallel (good for ~ 3 days autonomy)
Misc fuses, breakers, wire, etc.

HTH,
Jim / crewzer

Re: laugh, but please help system for IL

The Evergreens are sometimes labeled for on-grid use due to their unusual voltage specs (~23 Vmp, as I recall). The MX60 controller doesn't care (within limits, of course). It'll take the "extra" module voltage and convert it into extra charging current.

Outback's official array power limit for the MX60 used with a 12 V battery bank is 800 W STC. However, the output current can be user set as high as 70 A, so, as a practical matter, it can unofficially handle an array of up to ~900 W STC or so for 12 V applications.

If you have to stay with the roof pitch, using five ~180 W STC PV modules instead of four should work (per the discussions above), assuming the pitch is ~correct. The "flatter" array will require more maintenance: cleaning off leaves, snow, pollen and misc. environmental debris.

HTH,
Jim / crewzer