08-17-2014, 09:05 AM
quote:
Originally posted by kaialoha
WOW!!! MarkP’s post makes very practical sense, but I still have questions.
I’m building in Fern Forest and want to install a solar array that is functional without spending more money than is necessary or practical.
My hale on Maui gets by with just 2 panels and 2 batteries for lights and PC and I have a propane fridge so is not part of the solar equation though it would be nice to have the reliability of an electric fridge. The system is definitely minimal and I have to use the generator on occasion.
I appreciate MarkP’s post because it seemed to speak to me in language I (somewhat) understand.
So, MarkP, can I ask, with your 2,100 watts, what are you powering? Are you running a refrigerator or other such larger appliance? I assume the idea is to be selective when running anything so that the draw on the batteries is minimized and spread out over a variety of appliances, lights, PC’s, etc. Is that a reasonable assumption?
I have lights, a fan, a dehumidifier, a washing machine, a microwave, a TV, and a 12 volt battery charger that charges the battery that runs my water pump. And a small refrigerator. If it is sunny the led on my charge controller starts flashing by around 10 am indicating that it is throttling juice to the batteries so I do laundry then. Rainy nights is when I am most troubled by humidity so I often run the generator when I turn on the dehumidifier but sometimes I run the dehumidifier off of the batteries alone. It depends on how sunny a day it was earlier and how lucky I feel about tomorrow. The rest of the appliances I run as needed with no special regard for anything. The 12 volt battery charger is on a timer to run during the sunny part of the day.
I think I understand the %’s for matching panels to batteries and please pardon me being dumb, BUT, what does the ‘210’ represent? Is that total amps of the 9x240 watt panels?
Each battery has a rating of 210 amp-hours when discharged at 20 amps. I think this is an industry standard for expressing amp-hours. Multiply this rating by .05 or .13 and you get 10.5 amp-hours or 27.3 amp-hours. Leave off the -hours and you get the amp ratings I quoted. This is a rule of thumb for estimating the proper charge rate for your battery bank. Since the same current goes through each battery in a series configuration, my battery bank has a rating of only the 210 amp-hours of an individual battery but the volts are added up to get a nominal bank voltage of 8 x 6 = 48 volts. If I had made two strings of four batteries each the bank would have 420 amp-hour rating at 24 volts. 840 amp-hours at 12 volts is also an option. I have a 2,700 watt 48 volt inverter. At peak load it will draw close to 60 amps. A 1,000 watt 12 volt inverter will draw close to 85 amps. That means lots bigger wire, switches, fuses, etc and correspondingly greater expense for those items all for way less power.
You mention 9 panels in 3 strings. Why not all 9 panels in a single string?
The panels are nominally 24 volt panels. They have an open circuit voltage of well over 30 volts and even when pumping out power the Vmp (max power) is still often over 30 volts. I must put at least two in series to get voltage high enough to overcome the voltage of the battery bank. Because I use an MPPT (Maximum Power Point Tracking) charge controller I can string several panels in series up to the input voltage limit of the controller. In my case this is around 150 volts so I have to be careful. I might be able to run 4 panels in series. Anyway I chose 3 panels in series. The MPPT DC to DC transformer technology takes the resulting 90 to 100 volts and makes more amps at 50 to 60 volts required to charge the batteries. The simpler PWM (Pulse Width Modulation) technology would simply chop the current as needed without making greater amps and turn the rest into heat. Imagine a roofer carrying shingles up a ladder. The ladder must be tall enough (voltage high enough). If too tall he must still carry the shingles to the top of the ladder then drop them from there (not really I know but work with me here). The MPPT technology is like having an automatically adjustable ladder. As long as you select one that can go high enough it will automatically adjust to deliver the proper height/voltage for any lesser height. On cloudy days or during marginal morning/evening times the panels stay productive longer. In contrast, panels/strings perfectly sized for a PWM controller on a sunny day at noon wouldn't have enough oomph at any other time and are therefor over-sized resulting in a 50 volts needed/60 volts delivered situation most of the time, analogous to carrying the shingles higher than the roof and dropping them.
Assumption: It appears you’re using close to one battery per panel and I wonder if that’s coincidence or a good general rule of thumb or just simply happenstance.
Coincidence.
Another assumption: I think what you’re saying about battery life as it relates to charging and discharging is that it makes more sense to use the batteries rather than let them sit with no draw on them BUT at the same time keeping the batteries from discharging TOO much. Fair assumption?
Exactly or as close to it as possible when describing the black magic that is maintaining such a battery bank.
Anyway, GREAT comments and I hope to learn more as I go ~ ~ ~
Mahalo for everyone’s time and consideration ~ ~ ~
Aloha
Punanny