Moving from a 12V to 24V system. Here's the plan and some questions.

Susido

For the last 4 years I’ve had a small off-grid solar and wind setup on a rural property I spent about 1/3 of the year at. The time has come to expand it a bit and convert from 12V to 24V. Some of the components I already have and some (primarily a new inverter/charger, MPPT solar controller and an additional pair of solar panels) I wish to soon purchase. But before I buy them, I’d love to get some feedback from the experts here. I apologize for the length of my post but hope I’m supplying sufficient detail.

Currently I have 2 pairs of 12V solar panels (each pair in parallel) totalling 420W described below, a cheap PWM solar controller I wish to replace, 8 new 6V golf cart batteries (Costco brand FLA I bought last fall when they were being cleared @ $50 each!), a small wind turbine (a TLG-500 from TLG Windpower Products – it’s a pretty good generator as far as these things go), a Xantrex C40 40A charge controller dedicated to the wind generator with diversion loads, a 12V 1250W MSW Tripp Lite inverter/charger which I want to replace as it is too low powered for my needs and a pair of Honda eu2000i gas generators (with parallel kit) which compensate for any deficit of battery charging and are used to directly power high draw items (water pump, microwave, air compressor, welder, table saw, etc.). Since I need a new inverter/charger, this seems like a good time to reconfigure everything as a 24V system.

I don’t know if it would do much good to calculate my total loads in watt hours because those can vary greatly on a daily basis and I am perfectly willing to make up any shortfalls by running a gas generator or two. Suffice to say that I am typically able to use all the power I generate and I am not likely to expand my system anytime soon beyond the envisioned 700 watts of solar panels + wind generator.
The battery bank seems straightforward; the 8 batteries will be arranged as 2 parallel strings of 4 batteries in series to create (I think) about 450AH at 24 volts.

The inverter/charger I am planning to buy is a Tripp Lite APS2424 2400W 24V (4800 watts peak) http://www.tripplite.com/en/products/model.cfm?txtModelID=174. This is a modified sine wave inverter but that’s what I’ve been using for years and have been able to run apparently just fine everything I’ve tried (including a lot of electronics) with the sole exception of an electric blanket.

Having the wind generator help power a 24V system is interesting. The TLG-500 is marketed as 12V/24V but the owner of TLG tells me I will need to replace the existing 3 blade hub with a 2 blade one and discard one of my blades. He says that even though the 2 blades don’t turn until a somewhat higher wind speed is reached, once they do turn they can spin up faster than three blades due to less parasitic drag. Does this seem like a logical change for 24V charging?

What MPPT solar controller I should buy is a bit of a mystery. First I need confirmation that the collection of panels I have and hope to buy are close enough in specifications to combine. I think they are but the different panels use slightly different terms in their specifications. Here the information I have for them:

(2) 12V Sharp Modules
Max. Power 80W
Open-Circuit Voltage (Voc) 21.6V
Short-Circuit Voltage (Isc) 5.15A
Voltage @ max. power (Vpmax) 17.3V
Current @ max. power (Ipmax) 4.63A
Max. System Voltage 600V
Min. Blocking diode 10A
Series Fuse 10A

(2) 12V 130W Poly-Crystalline
Rated Voltage (Vmp) 17.6V
Rated Current 7.39A
Open-Circuit Voltage (Voc) 21.7V
Short-Circuit Voltage (Isc) 9.11A
Max. System Voltage 1000V

Additional proposed panels:
(2) 12V 140W Poly-Crystalline
Rated Power 140W
Tolerance 0...+3%
Max. power voltage, Vm 18.3V
Max. power current, Im 7.65A
Open circuit voltage (Voc) 21.95V
Open circuit current (Isc) 8.25A
Cell efficiency 16%
Max. system voltage 1000VDC

I’m proceeding on the assumption that each pair of these panels in series will produce about 35V and this will is enough to adequately charge a 24V battery bank. Does it look like I can combine these 3 different pairs of panels as 3 24V strings of 2 panels in series into one MPPT controller? I’m thinking about taking a bit of a risk and buying a fairly inexpensive Tracer-4210RN http://www.epsolarpv.com/en/index.php/Product/pro_content/id/444/am_id/136 40 Amp controller. It’s Max.PV open circuit voltage is 100VDC with a maximum of 1170W @ 24V. Would this work for the 6 panels above (700 watts total) or would they exceed the rated open circuit voltage at 24V?

Some additional questions: Is there a chart somewhere that lists the optimum angle (slope) to mount solar panels at a given latitude and time of year? I primarily need the power between the months of April and October and am at a latitude of about 50 degrees north.

Is there a general rule for determining if a circuit breaker is required for each string of solar panels? Does it depend on the number of panels per string (eg. >2) or the number of strings going to the charge controller (eg. >2)?

Thanks for your help.

Cariboocoot

Re: Moving from a 12V to 24V system. Here's the plan and some questions.

Welcome to the forum.

Let's check something basic first: eight golf cart batteries configured as two parallel strings of four to achieve 440 Amp hours at 24 Volts, correct?

Well if you want to recharge that from solar you're going to need about 1400 Watts of panel.
You've got two 80's and two 130's for 420 Watts now, right? And you want to add 280 Watts more for 700 Watts. That's about half what you need. Don't count on the turbine as it will not produce all the time. It is best to consider wind as supplemental for those times when the sun doesn't shine but the wind does blow.

Considering how much equipment you're already planning on changing, why not leave the 12 Volt system as-is and dedicate it to small loads like lighting and then build a new 24 Volt system designed for the heavier loads?

I run a 24 Volt system with 700 Watts charging 232 Amp hours and I have to say the charging is about right on a good day. Your proposed battery bank is about 2X the size of mine, so it would be best with twice as much panel too.

westbranch

Re: Moving from a 12V to 24V system. Here's the plan and some questions.

I concur with 'Coot, leave what you have as is and go 24 volt with a balanced battery and PV setup. Recommend you look at the MidNite 150CC, good value for features and very versatile...

here is a thread that might interest you since you are well stocked in the generator department.
http://forum.solar-electric.com/showthread.php?18459-Demonstration-of-Generator-Support. there is a good result about an Outback Inverter discussed too.

hth

Susido

Re: Moving from a 12V to 24V system. Here's the plan and some questions.

Thank you Cariboocoot and westbranch for your feedback. Yes the amp hours and solar panel figures are correct.

I realize the proposed 700 watts of solar panels is not adequate to fully charge the 440 amp hour battery bank but, as I've stated, I've got no problem with using a generator to supply the balance. I’m finding my Honda generators to be very fuel efficient when using one or both of them on the Eco-mode that they are almost always left on. And, yeah, I don’t take for granted that anything useful is going to be provided by the wind turbine, it’s just for bonus power on windy days. Now I could add more (2X) solar capacity (provided I have a much higher capacity charge controller), I just have my doubts it would ever pay for itself.

I really don’t think two systems, the existing 12V along with a new 24V system, would work for me. The way my AC runs to different buildings is not conducive to having dedicated low power loads and the need to maintain two separate battery banks with two separate inverters would be a pretty big issue. My existing 12V inverter/charger is old and flakey, replacing my cheap solar controller is no big deal and my old battery bank (four 6V batteries @ 12V) still seems surprisingly good but it has to be near the end of its life span.

But why the recommendation really? Is it because my existing solar panels are not optimal for charging a 24V battery bank and I should start from scratch with more suitable panels?

The Generator Support Inverter is certainly a neat idea. But I don’t think automatic generator support offers very much for my situation, especially when you consider the generators I already have do not support automatic start and I have the generator capacity to power anything I currently need to power and to do so at relative high efficiency. It's not all that convenient but I am very used to it. And manufacturer support still seems really lacking.

No doubt the Midnight 150 charge controller would be a better choice but that controller seems difficult to procure in Canada and would cost 4X that of the Tracer MPPT controller. Admittedly though, if I do eventually add more solar panels I would need a higher capacity controller anyway. I'll look into it.

Still wondering if I will need circuit breakers and combiner box? Are the specs for that Tracer CC adequate for the 700 watts of proposed solar panels? Optimal slope angle for panel installation? Oh yeah, are those 3 pairs of solar panels even sufficiently compatible to combine into one MPPT controller? Thanks again.

westbranch

Re: Moving from a 12V to 24V system. Here's the plan and some questions.

there are lots of Canadian sellers of MidNite products. Just bought a CL150 locally for ~ $20 more than I would have paid for one by courier, so I kept my money local... just google and narrow your search to Canada.