Off-Grid: AC-what usable % of DC energy, now with Lithium!

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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Privatteer » Sat Mar 30, 2013 2:28 pm

For your last question one of the great things about logging my main array is the ability to review output.
I have had 4 days that did not even achieve 1kWh in 388 days. This link should take you to it sorted by generation.

There is a few gaps were the logging stops due to the SMA explorer program randomly deciding not to open.
If your lucky you might have a system near you in PVoutput that will give an idea for your local area.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby davidg » Sat Mar 30, 2013 2:45 pm

Privatteer wrote:For your last question one of the great things about logging my main array is the ability to PVoutput that will give an idea for your local area.

Thanks I have looked at PVoutput a bit eventually I'll get around to linking up the all the inverters to it. or the metering or something. :D

The reason I was asking actually had nothing to do with where I am or will be, it is more to understand the cycling and the comparison rate they are being discharged at, not really relevent to area more about bank size D.O.D, rate power is being consumed, etc. Will give me a proper comparison to PB, I see generalised reports on whirlpool, I'm working on getting details.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sat Mar 30, 2013 5:40 pm

davidg wrote:What size bank do you have


800AH @ 26V, actually discharge mostly happens around 26.5V, so I call it 21kWh of energy storage in my DOD calculations.

what would the typical number of kWh per day used


A bit tricky to answer, as it varies so much! I have used over 24kWh DC (~21kWh AC) on a big day in summer when preserving fruit and generally using lots of power, and when trying to minimise power usage, that can be <6kWh AC. Typical use is around 8-10kWhAC, but 10-12kWh is also fairly common.

what is the number of no sun days you target or have acheived,


The low sun days are generally no more than 4 in a row here- see the graphs I posted on the previous page of this thread (but winter is worse with the shorter possible total of sun hours), or below are 2 graphs from a week ago I prepared earlier today, these ones account for all loads (the earlier ones did not include some small DC loads), and the true charging efficiency works out to be just over 96%.

In the below graphs I had 4kW of PV operating, 1kW of which was fixed, pointing a bit E of N, the rest tracking the sun. The energy used was a bit above average, no efforts were made to limit consumption this time.

Gross Power and Energy Use
20120321-24Gross.gif
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Net Power and SOC + Voltage
20130321-24Net.gif
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what would you consider to be the worst amount of energy you have derived from a no sun day and how many days in a row would that likely to be. :?:


I think the worst day I've had was about 0.3kWh from a just under 2kW array, but I have had 2 days in a row with just over 1kWh from a 2.19kW array. At the moment I'm running 4kw of PV, so could probably go about 4 very gloomy days straight, without needing the generator if I conserved energy. I've never had 4 days of completely gloomy weather in a row though... but then, my comprehensive records only go back a few years. The high efficiency of the LiFePO4s mean you can climb out of a big DOD hole pretty quickly :)

Anyway, enough of this for now, its BBQ and beer time :D
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby davidg » Sat Mar 30, 2013 5:53 pm

Gordon thanks now thats really handy information.
BTW how many cycles total and to what avrage depth do you think the bank has been through for the time you have had it? Really hard to answer I know. Thanks in advance.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sun Mar 31, 2013 10:51 am

1067kWh AC between 18/12/2012 and 31/3/2013 (103 days) = 10.36kWh/day, or 12kWhDC/day @ 86% inverter efficiency, determined in January as previously posted.

It depends on how you want to define a cycle, as SOC can go up and down a bit in a day due to loads, cloud and night, but an approximation would be 103 less 11 days on which 100%SOC was not reached, so lets say 92 cycles back to 100%SOC, and typically 15-20%DOD overnight, rarely as low as 10%, 70% once (January graphs), and 30%-50% a few times. No generator use in that period.

As an example of a situation that would probably call for generator use if I had Pb-acid batteries and wanted to stay above 80%SOC:

We've (2 of us) just had visitors- 2 adults with 3 young children for a couple of days, so lots of dishwasher use, lots of cooking with induction cook top and electric oven, and today is heavily overcast and there's a fair bit of washing machine work underway.
100%SOC at 5pm yesterday when the cloud was becoming fairly extensive and sun about to set behind the ridge, 3kWh net decrease (14%) in SOC to midnight, and another 3.4kW net SOC decrease to 9:30am due to lots of toaster and induction cook top use, so 30.5%DOD is the minimum so far today. Since then it has been up and down a bit between 26 and 27%, but I don't count that as a cycle ;) Even if it stays heavily overcast all day I wont need to think about using the generator.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby cameronp » Fri Apr 05, 2013 10:58 pm

Hey Gordon, this thread has been great, and has opened my eyes to the possibilities of lithium, so I've been researching the last couple of days now.

Most people seem to recommend a Battery Management System for each cell - Did you set yours up like that at all? Also I guess you cancelled the float and equalization stages in your charge controller? Did the manufacturer recommend the 20 mins in absorption or you just did a bit of experimenting and found that best?
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sat Apr 06, 2013 7:46 am

Yes, you do need a BMS for a large off-grid system Cameron, in addition to the cell equalisation modules- one on each cell.
Definitely no EQ for LiFePO4s! As for Float- that's where my batteries spend much of the day if there's a bit of sun- over 6 hours yesterday :) I found the best duration for Absorb by experimentation- I wanted enough time for the cells to get within 0.03V (but it is often less than that by the end of Absorb), and for the end amps to come down to under 1% , ie 8A. As I have posted previously, the end amps will get down to <3A with a bit more time in Absorb, but I only do that after a day or 2 when cell balance was not acheived. ATM I'm running 18 mins Absorb time, it is adjustable by 0.1hr increments on the FM80. On days when there is a fair bit of cloud and I run a large load, the voltage may drop down to 26.3V, which is where I have set the Rebulk voltage- so another Absorb stage will start, to bring the cells back up to 100%SOC. However there is some more testing I plan to do -I might reduce that to 26.2V or 26.1V and see how close it gets to 100%SOC whilst in Float, without going to Absorb. MPPT is used in float, so the CC will make the most of the available radiation to get the battery back to Vfloat.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby cameronp » Sat Apr 06, 2013 11:20 am

Interesting :) So you have 8 cells @ 800ah each or 16 @400ah in 2 strings?
Would you mind explaining the BMS setup a bit more if you dont mind? Even some pics? :) Does the BMS's purpose simply allow you to record/view the voltage in each cell? I assume it has no way of charging individual cells?

Also did you do anything particular with the first charge of the lithiums - supposedly they need an extra big initial charge up to 4 v?

Inverter questions :)
I'm looking at using a Sunny Island with this type of setup, for a new system that I'm researching, and after reading through the manual, I know I can set times for the absorption/float so i was thinking in theory of setting the float to 0 hrs or similar. I can disable the equalisation and set a minute value for absorption.

The thing that I'm not too certain about is when the boost charge will kick in. Sounds like with yours you could set a voltage, which is perfect - on the sunny island the manual states it will kick in at 70% soc. So I don't know if that's really an issue or not, it would be nice to be able to manually specify this :(.
Heres a link i found talking about it, it would be nice to set it lower based on this info no?

http://gwl-power.tumblr.com/post/26696964746/depth-of-discharge-dod-all-battery

Reason I'm leaning towards SMA is because I want to set up a ac coupled for off grid, sma is perfect for that and really easy to set up, and fairly cost effective in my situation.

I should probably ring SMA and target my questions towards them :/
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sat Apr 06, 2013 12:40 pm

16 cells @ 400AH each, in a single string of cell pairs, I've posted some pics of it in various threads over recent months, such as here showing balancing modules and fuses: solar-wind-gear/topic5147.html?hilit=modules#p32548

The BMS doesn't record anything, it is just the controller that connects or disconnects the cells if there is a problem. Individual cell balancing modules do the cell balancing work, and the single wire circuit connected to the BMS passes through each of them. If one cell goes high or low voltage, it opens the circuit, causing the BMS to disconnect the battery. If the whole battery voltage goes too high or low, it also disconnects the battery. It also has a "fuel gauge", although it only works for up to 511AH apparently, so only gives an indication of the "top half" of my SOC. However, I monitor my SOC much more precisely with a data logger and software, so that is no problem.

No, I did not give them a high voltage initial charge, they have never been over 3.8V- which was only briefly for a couple of cells during the initial balancing.

I dont think the SMA CCs can be used with Lithium, my reading of the manual found no suitable voltage settings for LiFePO4s.

One problem with the thoughts on that link is that if you are only using small percentage DODs each night, they recommend not recharging the battery to anywhere near full- that means you dont have full capacity in reserve when you wake up in the morning to find the weather is going to be overcast for 3 days. It also means that the batteries will rarely be balanced, and lack of balance is bad news for LiFePO4 batteries. The result of that may well be the BMS disconnecting batteries more often that is desired near full charge or low DOD. I make sure I stay way from the limits, so have had no disconnects.

The LV and HVDs of the BMS should only be used as a backup to your primary disconnects- the BMS is the failsafe in case your other devices dont do their job.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sat Apr 13, 2013 5:33 pm

I've been doing some more testing of voltage set points recently and have discovered that going too low on the Absorb voltage- down to 28.9 (=3.61V/cell) for a few days results in some battery imbalance that took over 3/4 hour at 29.3V to bring back to within 0.05V range for all the cell pairs. There just isn't enough current differential to bring lower cells up to the set point, so i wont be setting it that low, so more testing is in order to see how low I can go without the balance drifting out.

Regarding Float, 27.34V (3.4175V/cell) seems to be spot on, at this set point today the current required to maintain it was just a few milliamps, essentially zero, as can be seen in the below graph. The red line just above zero shows the mean current for this period of float.
Early in the period shown in the graph there were larger loads, so volt drop through shunts etc was a little higher- meaning battery voltage will be fractionally lower- by a couple of hundreths of a volt. Loads in the later parts were a bit under 150W.

Float27.34V.gif
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