Hi all,
I am currently working on a 4.7kw Grid tied PV array. I would also like the option of being off-grid when necessary as I live in a hurricane prone area and sometimes power outages can be lengthy. I wouldn't want to use a conventional battery backup system due to losses from inefficiencies and the constant use of batteries which would deplete their life.
The Vmax of the array is about 400v on the coldest of days. This would be too much for any charge controller that i know of. Would I be possible to to use 2 Classic 250's connected in series and split the voltage? Getting on the roof and reconfiguring strings is more work than i would like to do during a grid outage. I figured that a DPDT transfer swtch could be used to go from a grid tied to an off grid system. I could throw the switch weekly for a short time just to keep the batteries topped off.
No, unfortunately you cannot put Classics in series on the inputs for higher voltage.
Sounds like your best bet is to wait for the XanSchneider 600V controller. I don't know
if we will be doing one of those types of high voltage controllers or not.
Another option would be to take an off the shelf HV grid tie solar inverter, like the
SMA Sunny Boy and AC couple it to the "output" of a sine-wave inverter such as
the OutBack FX or VFX series to get it to charge the batteries.
That will work right now and off the shelf. You will still have to control
the charge voltage when the batteries start to get full, with some kind
of diversion controller since the inverter running that way does not
control the voltage.
In the bulk charge stage, it will work fine though because the battery voltage
won't be high enough to worry about until it passes into Absorb.
My 2 cents worth for today.
boB
Thanks for the input boB. Sounds like Xantrex CC will be the way to go.
i understand your concerns over efficiency and not wanting batteries in the mix while operating gt, but there's but. in a scenario a year goes by while you are happily grid tied and all of a sudden your power is out due to another storm. well, ok, just drag out a battery and connect it to the cc right? wrong. after a year of sitting that battery is now probably dead. you have to have something connected to it to maintain it or your wonderful idea of connecting your gt system to the battery is for nothing. this could be a dedicated pv system for maintaining a float charge or a gt connected charger serving the same function. the method of a solar float charger will not impede your gt efficiency, but it comes at an extra expense at running a separate solar setup. the other being a gt charger is the same as running a battery backed gt system and would impede upon the efficiency.
technically both cases require extra power to maintain the batteries and you will not get around this little fact. this extra power has to come from somewhere and without it the battery or batteries will fail when pressed into service due to no maintenance float charges. the separate solar float supply will maintain the illusion of your running efficiently being it is a separate power source than the gt system, but anybody expecting to use their gt system with a high voltage controller better have a way to keep the batteries good until they are needed or there will be a very disheartening oops moment when you need that power the most and your other half, who was ready to kill you then for spending so much money, is now revisiting those thoughts while you both sit there with candles glowing. you did remember the candles didn't you? ???
In a conventional gt system w/battery backup the batteries are constantly being cycled (charged - discharged). This greatly diminishes their life. In the setup that I propose, it is my intention to keep the batteries fully charged at all times. In a gt system w/BBU everything should be sized properly to the array output. In my proposed cc system, it would be sized to only power critical loads in lieu of a generator.
Quote from: SolRay on February 15, 2011, 08:35:49 PM
In a conventional gt system w/battery backup the batteries are constantly being cycled (charged - discharged). This greatly diminishes their life. In the setup that I propose, it is my intention to keep the batteries fully charged at all times. In a gt system w/BBU everything should be sized properly to the array output. In my proposed cc system, it would be sized to only power critical loads in lieu of a generator.
Actually in a conventional GT system with batteries the batteries will be charged by the grid on initial power up of the inverter and never get cycled again until the power goes out. They will float anytime there is dc input present at a lower float setting than normal and when the inverter stops selling do to a lack of dc input (IE solar or wind) the house loads run off the grid. The next day when a dc input is present the battery's are still full so it takes just a little to get them to float again.
A lot of people do a hybrid system if they want a smaller battery bank. For example you could do say 1-3kw of pv on an Outback GVFX3648 and then 1-100kw of PV on any grid tie only inverter. This setup allows them to use all the pv during an outage and also allows them to have a minimal battery based system if they are doing a large PV system.
I would suspect anything around 5kw of PV or less if there is going to be batteries in it anywhere you will be better off with a simple battery backed up system like the Outback or Xantrex. Charging the battery bank once a week will be less efficient I feel than floating it 24-7 the idle discharge of the battery bank for 7 days will use more power than floating each day. The other side of this would be cost you would be buying 2 different systems with the op's proposed setup.
Please forgive my ignorance. I have not had any practical experience with battery backed up gt inverters. I was under the impression from literature that I read and from my solar schooling that these systems are similar to UPS systems. When the grid goes down and the batteries are depleted it's time for the generator and/or candles. I would like to run my critical loads for a long time with the array replenishing my batteries whenever the sun is shining. Are you telling me that this is possible using a battery backed up gt system? If so, that may be the way for me to do the setup.
Back to my proposed setup. If the inverter input and CC input were connected in parallel, a DPDT transfer switch may not even be necessary. Wouldn't the CC divert power from the inverter when the batterries needed charging and when the batteries were charged the CC would divert only what is needed for a float. When the grid goes down, 100% of the juice would go to the CC.
Quote from: SolRay on February 16, 2011, 08:01:52 PM
Please forgive my ignorance. I have not had any practical experience with battery backed up gt inverters. I was under the impression from literature that I read and from my solar schooling that these systems are similar to UPS systems. When the grid goes down and the batteries are depleted it's time for the generator and/or candles. I would like to run my critical loads for a long time with the array replenishing my batteries whenever the sun is shining. Are you telling me that this is possible using a battery backed up gt system? If so, that may be the way for me to do the setup.
Well I think you have it close the above system would indeed switch to battery power when the grid goes down but all the solar panels would be charging the batteries as well so you would in essence be off grid. You would have to manage your loads so the pv system could maintain the daily usage that's all.
If you're concerned about battery losses, with with AGM batteries. It takes a lot less power to float them than flooded batteries and they require little or no maintenance.
About the only maintenance you'd need from a GVFX3648 is a nice periodic discharge and charge. Throw in a FLEXnet DC and it will make sure the batteries hit "Absorb" every day before selling.
And if boB ever gets me into a Classic's Modbus connection, we'll have software that will show your Classic and GVFX3648 nice and pretty and right next to each other on the same screen. And boB can tell you that sometimes the display even looks pretty ;D
Just a couple of observations here.
1. You cannot put the input of the charge controller and grid tie inverter in parallel. One or both will go up in flames.
2. The relay you desribed is a difficult one to obtain. Regular relays with contacts do not work at high voltage DC.
The system you have described wants to be an AC coupled system using both a grid tie and battery based inverter. You will need an inverter capable of being backfed.
Not sure if a Flexned DC will work or not? The batteries are always full.
Robin,
A FLEXnet DC would work very nicely with a GVFX being backfed from something like a SunnyBoy or even just plain fed-fed from a Classic.
What happens is that the "selling" voltage is never quite exactly right to keep batteries perfectly full. It's either a little too high (not exactly a good thing), or a little too low (leads to gradual sulfation, potentially) and the batteries slowly discharge. What the FLEXnet DC does is prevent the G-series inverter from "selling" until the batteries have become about as full as can be. Then the Mate changes the "sell enabled" bit in the inverter and it's off to the races.
One alternative is a mixed system. I have a client who has a SB 6000U with 6KW PV and a GVFX 3648 with 2KW. They don't have the configuration I'm about to describe, but it is a possibility. Mostly pointless, I think, if one is using a high efficiency charge controller and AGM batteries, but it would work.
The idea is to construct a sort of "string combiner" using ordinary DC circuit breakers. In one configuration, strings are wired up in series, while in the opposite, the strings are wired up in parallel. This is left to the reader as an exercise.