720 watts of panels. Suggestions please

[cujet]

I've talked about this on other forums. But it seems the guys here are really sharp. I'm in South Florida.

I have 4ea. 170 watt panels. My reason for owning them is to power my fridge/freezer. Either full time or part time. The most I've ever seen my fridge + freeze draw is 1.3KWH/day. Most days are below 1.2KWH.

I also have a number of used, but not old, 44AH, 24V aircraft batteries. As we install new ones every year. Golf cart or "real" batteries are within the budget too.

One option is to purchase an inverter/charger/UPS type setup. When the grid dies, the solar takes over. The idea is to protect my vast and expensive food stores (ha ha).

The other option is to run the fridge off the solar panels only, all the time. With some form of grid based charger if the battery bank drops below a pre-set.

The fridge defrost heater draws 600W and runs for short period of time daily. Otherwise the draw is 260W. The freezer draws 230-250W and runs when needed.

Obviously, when I am out of town, the consumption is much lower. This is where I'd want the solar system to perform properly.

In particular, I'd like pure sine wave inverter suggestions please.

[WStayton]

cujet:

  I THINK that more a concern for you current draw is going to be the start up loads for the freezer, not the running loads.

  Can you put a kill-a-watt on it and see what the actual starting load is? If you can do this, then you are shooting at a "real" target, not somebodys guess of two or three times running current for start-up current.

  Just my thougs, which is worth EXACTLY what you paid for it!  <grin>

Regardz,

Wayne Stayton

[cujet]

Both the fridge and freezer draw about 600W +- a few during startup, according to the Kill-a-Watt. However, the numbers jump so rapidly that it's not 100% accurate. My 1500W Xantrex will start and run both, no problem. I do want to size the inverter for a touch of growth too.

Chris

[LowGear]

Hi cujet,

I recommend you plug both appliances into the kill-a-watt at the same time.  With one running when the other starts up could give you some interesting data.  If they are frost free then you might want to see what they draw when they're "drying" out the box.

Casey

[XYZER]

Could you dissable the defroster with a switch? Dissable it when things are on the edge or out of town? They arn't as nessary if the door seal is good and the door is kept closed....turn it on when power is abundant......just a thought....

[cujet]

The fridge defrost draws 600W and I want to keep it. It only runs for a short time and does not run along with the compressor.

The freezer does not have a defrost cycle.

[WStayton]

cujet:

  Ok, so you need 1.3 kW-hr/day from 740 watts of panels.

  Since you're in sunny Florida, you SHOULD get at least ten hours a day of output from the panels, but just to be safe, lets assume eight hours.

  Eight hours x 740 watts =  3,120 watt-hours.

  IF your charging efficiency is eighty (80) per cent. you will put .8 X 3,120 = 2,496 watt-hours into the batteries.

  IF your discharge efficiency is, again, eighty (80) per cent, you can get back out of the batteries .8 x 2,496 =   1,997 watt-hours

  IF your inverter is ninety (90) percent efficient, you should have available .9 x 1,997 =  1,8982 watt-hrs of 120 volt AC current.

  Since you need 1.3 and you have 1.9+, even with slightly conservative assumptions as to the efficiency of batteries/inverters/etc., you should be good to go.

  I don't know anything about inverters in the six\ze range that you need, but you obviously need something that is capable of at least 600 x 2, in case they both start at the same time, so it sounds like your Xantrex 1,500 watt unit will work.

  For the batts, you will need the ability to deliver something like 2 kW-hrs per day at 50% rate of discharge.  I know nicads can take more than this, but you are using "aged" nicads, so lets assume that they have lost some and that 50% discharge is a far as you want to go.

  44 AH at 24 volts at 50% = 44X24X.5=about 500 watt-hrs, so if the inverter is 24 volt, you need four of these batteries in parallell, feeding the inverter.

  Note, you may have problems putting four USED batteries in parallel, in as much as they won't necessarily be all of the same "quality" and some may run down sooner than others, so real close monitoring is required until you know, for sure, what is going on.  But, I assume that you've got a pile of these batteries and can change out any low/slow/tired ones for newer/fresher/less-tired ones.   And, a complete charge of ALL the batteries when ever you fiddle with the configuration would be advisable.

  If you have the batteries, Iwould be inclined to add a couple, in  parallel, to the "string" just to be sure that you have covered mom and the underwriters, and I don't think there is a real down side to using more nicads than you need.

  That's my take on it -and I'm sticking to it! <grin>

Regardz,

Wayne Stayton

[cujet]

I neglected to add that these are lead acid AGM batteries. I have 2 older NiCad's too, but they are awful. I come across the lead acid (Concorde brand) batteries, because we change them every year in our fleet. Plus neighbor flight departments do the same.

I don't mind purchasing quality batteries either.

I actually had not considered using NiCads. I have a possible source for a large number of NiCads. It would take some work and many of the batteries would have bad cells that would need to be swapped out. That sounds like too much work.

[mike90045]

Can you put a kill-a-watt on it and see what the actual starting load is? If you can do this, then you are shooting at a "real" target, not somebodys guess of two or three times running current for start-up current.   

kill-a-watt can't read fast enough to record the spike     figure it's at least 5x the first large reading you get off it. 

[cujet]

Can you put a kill-a-watt on it and see what the actual starting load is? If you can do this, then you are shooting at a "real" target, not somebodys guess of two or three times running current for start-up current.   

kill-a-watt can't read fast enough to record the spike     figure it's at least 5x the first large reading you get off it. 

This I understand. Also, a 4 amp compressor can draw 15 amps with a locked rotor. So, the inverter size cannot be too small. As I mentioned, I would like to be oversized for growth and so I can use the inverter power for other uses if necessary.

Maybe 2500 watts?

Are there inverters that "do it all". Grid tie, charge, battery operation. Might be worth it?

Chris

[bschwartz]

This is what I'm using.  I think it has all the functions you are looking for.

http://www.wholesalesolar.com/products.folder/inverter-folder/outbackGVFX3648.html

(I didn't get it from here)

[LowGear]

http://www.wholesalesolar.com/products.folder/inverter-folder/outbackGVFX3648.html

Grid-intertied solar systems are great but the big drawback has always been that when the grid has a blackout so do you! Without batteries there is nowhere for all that solar power to go so it shuts down until the grid comes back. Now you can have your meter turn backwards AND power when the neighbors are without!

Huh?  How do you run your meter backwards while the grid is down?

Casey

[bschwartz]

HaHa..... the meter doesn't turn backwards when the grid is down.

This unit is grid tie when the grid is up, but will invert (to a dedicated circuit) when the grid is down.
It does need some batteries to work though.  I use an Outback FX60 charge controller to bring the panel voltage to 52 ish volts.   The controller is connected to 4 el-cheapo walmart 'deep-cycle' batteries.  They are not really used for storage, but more as a buffer between the charge controller and inverter. 

When the grid went down here for 8 hours a few months ago, and I was in the middle of rebuilding the listeroid, I simply connected the output of the dedicated output of the inverter to one branch of my breaker panel.  It provided power to my refrigerator, freezer, tv and some lights until the grid came back up.
Power was limited to what the sun was providing at the time + whatever little the small batteries could provide.

[WStayton]

cujet;

  Have you considered latting both units, the reefer and the freezer, sit "unplugged" for a couple of hours, unitl you're sure they will both want to start, and then plugging both of them into your 1500 watt inverter and turning it on to see if it will start them?  VERY quick and dirty, but it will tell yes or no for at least one inverter!  <grin>

  Since your batteries are lead-acid, I would be inclined to use eight of them, in parallel, so you won't draw them down below 25% DOD.  It WILL make your balance (i.e. not all bats exactly the same) problems more problematic, but I think it is worth the PITA to monitor them real good at first and change as necessary, just to try and use a smaller DOD and hence not wear them out so soon.

  My opinion - YMMV.

Regardz,

Wayne Stayton

[cujet]

The Xantrex 1500 watt inverter is not staying. The Outback linked above sounds great. With plenty of room for growth and will work with my generators.

Thanks!