Grid independent solar setup advice

[rethin]

I currently have a large array (10kw)and a couple of grid dependent inverters for my house. It works just fine but I'd like to change this setup for one that is grid independent w/ a small battery backup.

I'd like only a small battery setup for powering the essentials (freezer, well pump, a few lights etc) at night or when the sun isn't shining.

I'd then like to power everything else(dishwasher, tv, lights computers etc) during the day when the sun is shining.

One more issue is I'd like to be able to use a large 6kw, 220v electric motor (for an oil seed press) off the non battery side of this setup. I'd only be running this for a few hours in the summer afternoon.

Is it possible to have such a two tied solar setup and does anyone have any recommendations  as to where to start?

Thanks

Ed Lentz

[dnix71]

What voltage do the inverters use? It would seem easier to set up a battery bank with a charge controller in between the pv cells feeding one of the inverters and then run what you want protected from that inverter's output panel. That's how it's usually done commercially. One of my co-workers has a 5.5kw array set up to run the whole house (except air condition) that way. Her batteries will run the house for three days with no sun.

The battery bank feeding the inverter has to match the pv cells' voltage and be large enough to handle the load for whatever cloudy time you expect.

[ghurd]

"Small" is in the eye of the beholder.

Anything big enough to start a fridge or refrigerator (you will get better search results with those) is pretty good sized.  That is Big, IMHO.

Well pump is nothing to sneeze at, and has it's own issues.

Lights are trivial compared to that stuff.  Hardly worth calculating.

Oil press.  What is "the non battery side"?  Direct from solar?  

I don't think that is possible and practicle, at the same time.  

No hope it is worth the cost.

Start with what actually needs done.  One at a time.

Work out from there.

I expect it will be easiest and cheapest to...

Have a grid charged (from a switching regulated battery chrger, just because they use less power, and so I don't get flamed) 100AH battery and inverter for a couple lights.

Use a gas gennie for the freezer and pump. Only run it for water, and every few hours for the freezer.

G-

[jonas302]

maybe running a small deisel on your veggie oil would be a good idea to run the press

[zeusmorg]

 I'm not sure of your motivation for switching your system. You can have a grid tied system with battery backup, and some inverters support this. Determining how much backup you'd need is dependent on many factors, like how long your grid goes down, and how much you wish to run off just batteries alone.

 Splitting the system having part of your A/C system on grid, and part on off grid could get expensive, depending on how much you can do yourself. However doing this does limit what is run off what.

 Maybe more explanation of why you wish to go off grid could be helpful.

[rethin]

"Maybe more explanation of why you wish to go off grid could be helpful."

I'm the only house on the end of a long line out in the country. I am worried in the future that the electric company will find it un-economical to keep this single line for a single customer in good repair.

I also don't like the idea that my large and expensive solar panels will become large and expensive lawn ornaments exactly at the time I need them the most.

My goal is to utilize my solar panels while minimizing my battery bank. I've measured my usage and sized a battery bank to run the stuff I want to run for 3 days. It seems expensive to size a large battery bank for all my electricity uses. I have no problems waiting for a sunny day to run the washing machine or my oil press.

My idea so far is to have one inverter feeding a battery bank that runs my essentials. Then to have a second inverter for all my other usages.

What I'm unclear about is how to work a 220v motor into this mess.

[rethin]

I'm considering this. But it seems wasteful and inefficient. Especially when I have a 10kw solar array shading my lawn.

An electric motor seems a much more elegant solution.

[rethin]

Yes, I am aware that the well pump and freezer are pretty big draws. I've measured my usage and sized out a battery bank that will run them for 2-3 days.

And yes, I'd like to run the oil press directly from the solar. It seems very wasteful to increase the batteries just for a motor I'd only run 2-3 hours a couple of days in the summer.

Your advice to take it a step at at time is sound. But I'm just hoping for a general idea of where I'm going to end up.

[rethin]

The problem with my current inverters is they are grid tied. I was working under the assumption that to go grid independent I'd have to replace them entirely.

[electronbaby]

just replace one of the inverters. Im assuming you have 2 or 3 grid tie inverters. Replace one of them, rewire the dc side to meet your new requirements and replace with a battery based grid tie inverter A little more expensive after the fact, but it will meet your needs.

[ghurd]

Keep in mind the batteries will need replaced every so often, if they are used or not.

Disconnecting "X" amount of solar from the grid tie to charge rarely used batteries will reduce what is fed back into the grid.

The cost of the inverter, connections, etc.

The 220V motor, only used a couple days and a couple hours a day? Can't it wait until the grid come back online?  Or a gallon of gas in a gennie?

Seems like a lot of money for something that will probably never be worth the trouble and cost.

I'm not sure they are allowed to shut off your utility service.

G-

[DanG]

I am looking at the 220VAC motor and wondering if that can be upgraded to a more RE friendly motor. Since you are grid tied do you have a figure for the present grid contribution on motor start up? Have you recorded the true amperage - voltage drop when the motor starts? Is this motor a crude Chinese clone bundled with press or is it a discrete component easily swapped out? Can motor be started free-wheeling with minimum drag or is it locked into turning screw drive from the beginning?

As I read it now we must start using the exact motor name plate data ie: frame style to allow estimating the ideal starting surge current but 5 or 7 times running amperage for startup surge will be a pricey retrofit for sure. (your 6000w figure equals 27~amps @ 220V)

Forgive me this, I am having a hard time with 10,000 watts solar as the system voltages must be high, 450VDC or more. Having a dedicated panel for daylight hours local usage tap into the current grid-tie solar feed circuits is going to be an irritation for every approval authority in your region, if they know about it.

A pair of the Xantrex XW-6048 6000w inverters could conceivably throw 24,000 watts surge out, they claim 104 amps surge for 7 seconds per unit but the reserve may not be there from PV generated power where from grid it would try and brown out everyone on your grid circuit, town, county...

Using the Outback FlexMax charger set for 60V output gives 4800 watts MPPT feed into a single XW-6048 if you can stay under their inverter 64V maximum input and so not need batteries while the sunshines - but rewiring to have alternate taps for 12 or 18 PV strings to feed the FlexMax to stay under 140V and stay within inspection code may mean a pardon from the Governor or something : )

[DanG]

about the RE friendly motor - having grid-dependent inverters may not be bad if you can adapt a high voltage DC motor to direct drive the oil press. Many grid-tie inverters use voltages up to 600VDC, with your split system using two inverters that may be a good source of two 450VDC (insert your system MPP #'s) at 10-12 amps that will sure spin up a healthy DC motor if it's got a starting circuit. See http://www.reliance.com/mtr/mtrthrmn.htm for a decent overview of DC motor types...

[carlb23]

What is going to happen to you motor when you are running right from the PV's and there isn't enough voltage or amperage like when a cloud goes by.

I have a 10kw grid tied system and i have a slow speed diesel running a small 3kw generator for backup when the grid goes down.  If you run the engine on WVO it can be much less inexpensive then using Diesel.

Carl

[wpowokal]

Apart from looking at the existing motor for best suitability, you need to look at a soft starter for running it direct off the solar panels.

allan down under