24vdc to 12vdc

jACKr

Aloha!
I'm new to this forum.
I'm doing some renovations to my residential (off grid) small/medium sized Outback FX2812, MPPT60, system
I've recently updated my 10 flooded cell batteries to 10 sealed. It's working fine for past 2 months since.
Now, I need to add PV panels after recent roof repairs. I have 2 new REC320NP 24VDC. Which are to replace several 25 year old lower watt (12vdc) panels.
Because system is 12VDC, I need to convert the 2 REC320s to 12VDC before the Outback combiner.
Can I simply use a 24 to 12 Buck Converter? Would my 15Amp breakers be adequate? Will the 320 watts be compromised?
What do I need to consider, in a Buck converter?
There are 4 older 120watt PV panels (staying) and 2 -60 watt panels (which may retire). I retired a bank of old 48 watts.
I'd greatly appreciate any feedback! 
jACKr

Photowhit

Never understand why people ask how to best use equipment they already bought. If you bought it you should have a plan on how to use it.... 

Okay soap box over, We would need specs on equipment before trying to figure out, so VMP VOC of panels needed.

The Outback(?) MPPT60 would be limited to 60 amps output, so about 1000 watts of array (1000 watts array usually outputs about 800 watts, 800 watts divided but 14.5 volts charging voltage for a 12 volt system = 55 amps...

Series rated breakers for combiner box will be on the solar panels info on back of panels or in info on panels.

jACKr

Aloha Photowhit,
   I appreciate your response! The Soapbox sarcasm aside.
   To help you understand, us DYIers, need to start somewhere. Inevitably, we encounter hurdles in various forms.
   Trusting the local vendor, I asked for PV panels to upgrade to. Being more of a profit driven vendor vs the PV technology
   guru he perceives himself to be. He neglected to indicate the PV panels he offered, were 24VDC, despite knowing I have a 12VDC system. When I discovered such, weeks later, he immediately tried to convince and sell me a 24VDC inverter. Which isn't that bad of an idea. But, that would entail upgrading all my PV panels, or rewiring them. I can safely assume the rest.
  I also purchased a bank of 10 - 6vdc sealed batteries from him. Which are now,1 bigger 12VDC battery. 
 I can stay under the 1000 watts of my 60amp CC.
 Until I can afford a 24VDC Inverter and more 24VDC panels, and possibly an 80ampCC, I need to use whats on hand.
 To do so, apparently I can step the 24VDC down to 12VDC. I'm not clear if its only via a Buck Converter, or if by wiring the panels, like can be done on a battery bank?
  I have no confidence in the guy now! Yesterday, i asked if he had any Buss Bars to hook PV Panels 10aw to my 6aw. 
  He claims to have never seen wire that big used? DUH! 
  25-30 years ago, that was common. As evidenced by the physics of DC over copper vs distance and what the PV panels and Combiner Breakers, etc... accept, it was obvious his experience didn't go back that far.
  Its becoming apparent, I need a affordable solution to regulate the 24 to 12.
  The cost of 1 Buck converter looks to be more than a new 24vdc inverter, and several PV panels. Albeit, a wiring reconfiguration on battery bank.
   Mahalo, for you time!

BB.

Welcome to the forum Jackr,

Regarding Photowhit, he is probably the winner here for DIY solar system that has the lowest $/WH for cost of generated & energy used for an off grid solar power. So he both feels your pain and has the experience about finding hardware for the best price and how to put it together.

As we suggest--Don't buy any more hardware until you have a plan on paper to make sure it all plays together nicely.

For me/us, I like to start with the basics. Can you tell us a bit more about your system?

• Battery bank: voltage, AH, and battery type (i.e., 12 volt @ 50 AH AGM, or what) configure as 10x parallel for 12 volts @ YYY AH
• Solar Charge controller: Brand/model/PWM or MPPT/etc.
• Solar Panels: This may be a list of your several types of panels--Very least need Vmp (voltage maximum power) and Imp (current maximum power) and how many panels of each
• AC inverter: i.e., 1,000 Watt @ 12 volts @ TSW or PSW
• Rough location: Nearest major city--And is your array in open sun most of the day or do you have shading (trees, buildings, mountains, overhead power lines, etc.).

And what are your daily loads... In Amp*Hours or Watt*Hours per day (i.e, 100 AH @ 12 volts or 1,200 WH per day) for lights, laptop, RV Water pump, washing machine a couple times a week, etc.

I know that this is a lot of information--But if you wish to mix/match hardware together, we kind of need to know the specifics. With solar panels and PWM OR MPPT charge controllers, there is a "dance" of mixing/matching solar panel ratings and have the Array Vmp-array / Imp-array match the requirements for your specific charge controller.

-Bill

jACKr

Aloha BB,
   I appreciate your level head!
   I hope not to bore you with the length and following scenarios?
   I've lived in central Puna, on Hawaii Island for 39 years, on the slope of Kilauea Volcano. At 1450', We have a horizon view for Sunrise. In Hawaiian, this home is call Haleokalahiki, House of the Rising Sun. Often, that's the only time it reveals. My use of photovoltaics, is one of my primary sources to run the home. Cooling is via Tradewinds. Heating is via Tropics!
   Last 2 months has been a dry time and clear sky. If climate change affects us, it could last longer.
   It's considered a Sub Tropical Rainforest. Rain catchment is guaranteed 100%. Temperatures are pretty steady. Historically we rarely reach upper 80s, it may be trending to more upper range with climate change. Winter can see upper 40s, at night, 70's by day.
   Off Grid is a different beast from most other Off Grid living scenarios. I have never been on grid until Fiber Optic Internet, and quite recently, (Wireless Broadband from 10 miles away for past 15 years until.) Exception was a landline telephone with Dial Up, until cellular.
     This home will run itself when gone, (with CCTV {web when home, hard drive server when gone} and Security cellular monitored).  But once the loads of living start, vigilance/diligence rules.
   Photovoltaic is 50% average. Honda EU7000is the rest, ($50-$60 per week, last fuel hike/spike was felt).
   By day, even with light cloud coverage, and no major electrical loads, all is a smooth operation. Twice a week, laundry is a 10:am -2:pm event. Liquid Propane, is used for refrigeration, hot water, dryer, and cooking. (So Yes, Life here is hostage to fossil fuels).
   Electrical load by night, is therefore, is basically lighting, and the always on, UV water filter, and Modem/Router.  Loads by day, are Central Vacuum in AM for a short time (I have tile floors with Sweep To Dustpan wall ports, 2 minutes max). No television is watched here, especially with Fiber web speeds. Night time charging time from 6PM -10PM, 5PMish when its been a cloud covered day. And/Or, when I've been using power tools. Bulk of power tools are battery, and charged in evening.
   Water is catchment gravity (Ferro Cement 10K tank), and lifted via 80 gallon pressure tank and a 12VDC FloLight Booster pump to 1st and 2nd floors.
   The evolution of PV started here in early 80s, in a very rudimentary form. (I had a plug to my truck battery for 12vdc TV). The needs were very small. As I built a FarmStead HomeStead, power requirements grew as well.
   The current home I built (2001-02-03), was fully designed and engineered by myself. My Drafting skills benefited the process. My Electrical Plan, (Engineer Approved and Stamped) originally used a Trace SW2512 and C-30 CC with 10- Trojan L16s, a Honda 4500, and 6 - 48watt PV panels. While the project was still in progress (2003), a lightning strike hit my (yet to be sidewalk covered) Ufer ground. Which fried the Trace system, amongst other electronics. Immediately I went to the Outback product with the current FX2812 (currently Mate port non functioning), MPPT60 CC (currently the only Mate port giving system status to the house), Mate1 (now a Mate2), and the 10- L16s, and 4 lightning arrestors, 2 on AC side, 2 on DC side. I had also moved up to 4 UniSolar US64 60watt panels then. Because of the constant threat of lightning, all power comes 90' underground, from the Power Shed. My Breaker Panel is actually only a 60 Amp service. With no 220VAC appliances, its absolutely fine. I use the Gen if cement mixer, welder, or table saw is needed. Which is rarely.  
   PV panels are roof mounted on a stand alone Power Shed. Which is actively getting a new roof, as weather allows. The declinations here are 4° Summer, 17° Spring/Fall, 34° Winter. Which I physically tilt myself when paying attention.
   Over time, Battery Banks and Generators were upgraded. PV went to the current 4 Kyocera KC120. And kept 3 UniSolar.
   The newest Battery Bank is 10 Centennial CB6-400 AGM deep cycle DT(6V400AH/10HR) configured as 1-12VDC . I'm actually using only 6 at moment, testing charging ability. 4 are Virgin. Until I add more 24VDC PV, ASAP. 
   2 new REC320NP PV panels are installed, and waiting to learn best wiring choice,( see questions below).
   My next add on upgrade plans, are migrating to 24VDC inverter, more PV, and a Mate Hub. I'll be attaching my original Engineer approve plan. What has morphed, is the Inverter change and PV array changes.
   After starting this Forum discussion, I've learned the Outback MX60 has a built in Buck Converter. I'll have the oldest PV panels on a separate CC if I use them.
   The new knowledge about the MX60 actually resolves my issue/concerns about the newer  REC320NP 24VDC PV panels.
   My current questions would now be.
   Do you perceive or suggest any valuable improvements?
   Such as best PV for this environment? (lower Sun exposure). I assume the REC320NP?
   Will any brand Battery Temp Sensor work on any brand/style of batteries? (I have a Trace sensor).
   Whats the best PV configuration for 2 REC320NP 24vdc panels with a MX60 (soon to have in voltage changed to    24vdc),to a  FX2812, All parallel, or in series? 
   (More compatible panels to be added, depending on any limitations?)
   If I use them, where is a 2nd CC, for older PV panels (probably my unused Trace C30) ideally installed? Before and shared to the same system disconnect breakers, via appropriate buss, as MX60?
   I'm now semi retired, so all expenses are scrutinized.
   But, Like everything learned, new questions shall arise!
   Mahalo
   jACK