OFF GRID POWER; various thoughts on...

[MountainDon]

Dave. I did not mean to imply you were picking on me. Just wanted to it clear my system got close to trouble but usually does not.


Wind generator: yes it would need it's own charge controller. One that has provision for a dump load.

FM60 and diversion. There is an Aux output. It can be prrposed for different tasks but only one at a time. The manual explains. (D/L at Outback) It could be used to trip a relay that would be sized to handle the required voltage and amperage for load diversion. I investigated that route when looking for ways to keep "things" from freezing in the cabin over winter. The conclusion I reached was that it is possible but likely not a good return on the money; not to mention the bother. I was wanting to use the excess for resistance heating. It really doesn't amount to a lot of useful heat was my conclusion. What were you thinking of using the unused PV module power for? Or were you thinking of a wind generator dump?


System planning: I get uncomfortable when discussing planning a new system and at the same time to be talking about how the system will be grown in the future. Adding a wind generator is different from increasing battery bank size or changing the system voltage. That is my opinion and may differ from that of others. Adding more PV modules in a second group as Glenn has done is fine too as those modules have their own charge controller. The absorb charge and float charge voltage points may have to be played with to keep one from interfering with the other. That's in theory; as I've never had the situation I can't say much more than that.  Adding batteries is not great practice, IMO. Sometimes not avoidable, but that is where planning comes in.


MPPT: Again, just my opinion, but I do think MPPT has advantages no matter what the system size, except for all but the smallest system where two panels in place of one may be a better buy.  Maybe the advantages are more quickly realized when the modules are wired in series making the PV array voltages much higher than otherwise needed. By that I mean 30+ VDC from the array for a 12 VDC battery, 60+ for a 24 volt battery.

In hot or cold weather I believe any system will gain in performance wired that way with MPPT. The PV output voltage always has to be higher than what is required to charge the battery. It is the PV cell temperature that matters, not the air temperature. The module temperature can be 30+% greater than the surrounding air temperature. In a system where the PV output voltage is close to the battery system voltage the module voltage may fall too low to do any charging on a hot sunny summer day. If the series voltage is higher (like 2 panels in series instead of in parallel) there will always be enough voltage and the MPPT controller will make best use of what there is.

Similarly on cloudy days. I am amazed at times that our system is still pumping out a few amps on cloudy dark days. Looking at the voltages being received at the charge controller I can see that without series and MPPT there would be no charging happening at all.


One of the things I like about what I put together is that it runs itself. I don't worry about it being cloudy anymore; I notice, but I no longer worry. My wife can "work" it too. I wrote a user guide.

[Dave Sparks]

And I have had a friend run a winter temp CC at close to 148V  waiting for Midnite to release their 200V contoller. I am glad that you don't think I am picking on you! You would know if I was... Spring is coming! Although Glen may doubt this!

[MountainDon]

Wow! 148... makes you wonder how well everything is calibrated.   

Spring is definitely here, at least where our cabin is! A year ago we had three feet of snow in places, minimum of 12 to 24 inches everywhere, no ground or grass showing anywhere. This weekend it's all dry grass and pine needles save for north facing slopes.

[Squirl]

I never thought about the advantage of going further over voltage of the battery bank with the panel array.  This way when the array puts out lower voltage and would not be charging at all, it still gives it a little charge.  That is another great factor to consider.  I wonder if there is a way to quantify that.  Also upon further reading of the subject last night I read that MPPT chargers are best to maximize power gained from the panel array, while PWM are used to maximize battery life.  There weren't a lot of articles on this.  Most of the books and articles I have read though give these as descriptions of each technology but not as a comparison.  Such as the statement "pwm maximizes battery life", but not pwm gives longer battery life than mppt. There was no quantitative data on the extended battery life of PWM over MPPT technology though.

There seemed to be a lot of "if" and "possible" caveats in a lot of the marketing and advertising of certain devices without much information as to what the parameters of the scenarios they are referring too.  I figured bringing up the topic with some like minded individuals would flesh out a the limits a little better.  Thanks don and dave.

[MountainDon]

I've seen the claims that PWM produces superior battery life too.    Claims but no data to back the claims. I have no proofs either, but I'm willing to go out on a limb and state that no matter how much good might come from a PWM charger, it is more often than not countered by batteries that...
...get discharged too deeply, too frequently,
...are not brought back to full charge pretty much every day,
...have the fluid level fall too low too often,


I don't see big name battery manufacturers giving longer or shorter warranties depending on the charging technology being used.

[OlJarhead]

Ok so here is a question:  If a piece of electronic equipment does not list watts but rather lists kwh's (I'm sure amps was there somewhere but without my reading glasses I couldn't read it) could you not do the following to get average wattage:

Given:
24x365=8760 hours in a year
1000 watt hours equals a kilowatt hour
a 100 watt bulb running 10 hours = 1 kilowatt hour or 1000 watt hours.
Freezer uses 242kwh per year with an energy star tag.

Then:
1000 watt hours divided by the wattage of the device equals the amount of hours it takes to make one thousand watt hours or:  1000/27.65=36.16 and 8760/36.16=242

This would mean that a 27.65watt Freezer would use 242kwh per year assuming it ran 24x7x365 at that rate.

Reality though tells me that it draws much less then that most of the time and some number higher then that periodically as it cycles.  SO I'm unsure if calculating this way really helps.  What I'm trying to ascertain is both if I can use this kind of conversion to convert kwh to watts for the purpose of the solar calculator and if in fact I can use this as a base for a system or components of said system?

Mostly I'm just trying to see if my 3 panels and 8 batteries will handle the freezer running 24x7 in the spring, summer and fall when outside temps range above freezing most of the time (it can get as cool as 40 at night in the summer but mostly runs in the high 40's at night and 80's during the day.

I also plan to put the Freezer on the North side of the cabin to keep it cooler when it gets hot out rather then have it in the porch where the greenhouse effect will make the porch VERY warm during the day.

Thoughts?
Erik

[glenn kangiser]

I haven't seen one that small.

Seems a lot of my stuff uses around 125 or more watts but runs from 8 (Old efficient GE) to 40 or so minutes per hour (new fridge or freezer).

[glenn kangiser]

Offhand I don't recall the size panels and batteries - Total watts panels - Golf Cart Batts?  Hours sun per day?

[glenn kangiser]

242/365=.66 kwh per day -

660 watts of panels for 1 hour - 330 for 2 hours 165 for 4 hours sun plus another third for losses and inefficiencies and ability to store about half of that for night plus draw the batteries down only about half way.  A backup generator for cloudy days or several of them, will likely be necessary, as it is even at my place.

[Windpower]

I have found this calculator to be very handy
you can plug in various Kw-Hrs per month and get an idea of the system size needed adjusted for your average hours of sun for your location

for mine I plugged in 300 Kw-hrs per month  (about 10 Kw-hrs per day) in zone 5 (4.5 hours sun per day) and got 2222 watts of panel required (pretty close to my 12 evergreen 205 watt panels (I hope to get them operational this year)


So if your freezer uses 242 Kw-hrs per year, then divide by 12 months gives 20 Kw-hrs per month

plug that in, find your average sun  (zone 2 = 5.5 ?)  gives 121 watts of panels required minimum

http://www.wholesalesolar.com/StartHere/OFFGRID/OFFGRIDCalculator.html

[Squirl]

I think your calculations are a good start.  I don't know what the test conditions they use for determining power usage of an appliance.  If it is with normal usage and you plan on only going in it every few days, I would say that the power consumption stated is above what is needed.  If the test condition was no one going in it, and you will go in it infrequently, I would go with the assumption that you will use slightly greater power.  Also don't forget the losses to the inverter and I believe there is a slight extra inefficiency if running a msw inverter as opposed to a true-sine. (not 100% on that) Also if your inverter doesn't have sleep mode, it can drain as much power from the batteries as the freezer.
I tend to average usage across a day.  You will be lucky that the time you want to use it is when the sun stays out longer.  IIRC you are in zone 4, so you will have slightly less than zone 2.  I assume you panels are around 600 watts total, so an average of around 2KW a day in spring? So it seems like you would get enough power per day just based on the energy star rating.  I would be more concerned with the DOD of you batteries.  What are you comfortable with?  Are they top quality batteries? My assumption is you have about 5 Kwh capacity.  I have been shopping for inverters lately and most seem to be rated for 90% efficiency.  Add in the power usage if your inverter doesn't go into sleep mode, and the freezer, you could easily be at 1kw a day based on your calculations.  So 2 days of cloudy weather could bring you below .25% DOD.  So it all depends on the level of discharge you are comfortable with.  My assumption is if you will be there during times of use so you can monitor your batteries and recharge them when necessary.
If it still has its energy star tag, I assume it is new?  Maybe buy it from a store with a generous return policy.  Put it in on Kill-A-Watt meter and see if it performs as advertised.  This would also be a good way to check your inverters usage too.  If it doesn't perform as advertised, return it.  The only thing you would be out is the effort to move it.

[MountainDon]

I look at those annual energy star numbers like this. The label says 242 kWh or 242000 watt hours.
8760 hours in a year
242000 / 8760 = 668 watts used in an average day, Call it 700 watts a day for good measure.
7 days a week = 4900 watts a week.

My PV calc has fields to enter the watts and the hours per week of use and is set up to calc an weeks worth of power to help average things out.
Enter 700 watts in the watts column and 7 in the Hrs/Week column and that produces the 4900.


So the first thing is to estimate how the batteries will stack up and then the array.

[MountainDon]

Squirl, in case you haven't looked the calculator/spreadsheet has fields to enter correction factors for inverter efficiency, battery cold weather performance, depth of discharge and autonomous days.

I do have to work on the PV array section at the end though.

[MountainDon]

From oljarhead in another thread. Copied this part to here as it fits here better.


- I see voltage in another way   Running under 48vdc from the panels to the charge controller limits range on 10awg wire to under 50 feet (not sure the distance) where as 48vdc will run 50 feet on 10AWG according to xantrex.  In my case I'm pushing 60vdc from the panels which gives me a little better (I think) performance at 50 feet over 10awg.

I'm no electrician mind you, I'm just thinking that running 18vdc from the panels would cost me in wire size (read $$$).  

So, question:  and I should post this in the other thread, but here goes anyway:  Am I losing something by putting the panels in series and pushing out 60vdc to the controller?

Another question is this:  If I put 3 batteries in series I don't change the AH rating just the voltage so if I put 3 panels in series I'm wondering if all I'm doing is increasing voltage and not wattage?  I figured 3 205watt panels would give me 615watts of charging power.

Anyway, good topic!

I'll have some response later. Dinner prep beckons....

[MountainDon]

I'm no electrician mind you, I'm just thinking that running 18vdc from the panels would cost me in wire size (read $$$).

I'm not sure of the exact numbers for your 205 watt panels, but if we say 18.5 volts and 11 amps that's probably close.
so... using 50 feet for the 10 AWG copper wire length...
55 volts 11 amps computes to a voltage drop of 2.4%
60 volts 11 amps = 2.2% voltage drop

18.3 volts 33 amps = 21.8% drop
20 volts 33 amps = 20% drop

Number 1 AWG wire would be needed to get below 3% with 18 to 20 volts.

So yes series wiring PV modules is a very good thing. Not only is the smaller gauge less expensive it is much easier to handle and make connections to. I know from my experience with the 2 AWG I used in our system.

[MountainDon]

If I put 3 batteries in series I don't change the AH rating just the voltage so if I put 3 panels in series I'm wondering if all I'm doing is increasing voltage and not wattage?

You are increasing voltage by a factor of three and the current flow remains the same.
Using the example of a module rated at 18.5 volts, 11.1 amps, 205 watts...
Place three panels in series you get 55.5 volts at 11.1 amps, 615 watts
In parallel you get 18.5 volts at 33.3 amps, 615 watts

[OlJarhead]

You are increasing voltage by a factor of three and the current flow remains the same.
Using the example of a module rated at 18.5 volts, 11.1 amps, 205 watts...
Place three panels in series you get 55.5 volts at 11.1 amps, 615 watts
In parallel you get 18.5 volts at 33.3 amps, 615 watts

Awesome clarification Don!  Thanks!

[Squirl]

From oljarhead in another thread. Copied this part to here as it fits here better.


- I see voltage in another way   Running under 48vdc from the panels to the charge controller limits range on 10awg wire to under 50 feet (not sure the distance) where as 48vdc will run 50 feet on 10AWG according to xantrex.  In my case I'm pushing 60vdc from the panels which gives me a little better (I think) performance at 50 feet over 10awg.

I'm no electrician mind you, I'm just thinking that running 18vdc from the panels would cost me in wire size (read $$$).  

So, question:  and I should post this in the other thread, but here goes anyway:  Am I losing something by putting the panels in series and pushing out 60vdc to the controller?

Another question is this:  If I put 3 batteries in series I don't change the AH rating just the voltage so if I put 3 panels in series I'm wondering if all I'm doing is increasing voltage and not wattage?  I figured 3 205watt panels would give me 615watts of charging power.

Anyway, good topic!

I'll have some response later. Dinner prep beckons....

Yes, the C40 is a PWM charge controller, not MPPT so everything over 18v is lost power.  That is probably why you didn't fry the controller. 615 watts exceeds the recommendation of 40 amps and a 12v battery array (40x12=480 watts).  But with your configuration you are probably only charging at 200 watts. Also check the maximum Voc of the controller, you may be exceeding that too. (could fry it)  Many of Don's calculations work for the controller he has the Outback Flex60 with MPPT.  See my discussion above of tradeoffs of the $400 extra spent on an MPPT charge controller.  Although, I have spent the past week researching this issue through books, all the old issues of home power magazine, and internet articles since I started that discussion.  I was able to find a Morningstar mppt-45(great reviews) for $350.   This puts the price difference much closer to the PWM controllers and pushes me to the category that MPPT is worth it at any panel array over 1 panel.

[Squirl]

Jarhead, correct me where I make mistakes, but I took a look at your set up the other day.

You have 8 batteries at (6v?)  Wired to have a 12v battery bank.
You have a 12v msw inverter.
You have 615w panel array wired to 60v.
You have a C40 charge controller.

To get maximum power out of your setup you can change the panels to 18v, buy new wire, and get rid of a panel for a total of 410 watts.  (least expensive)
You could get a new charge controller to handle the higher voltage and amperage.  MPPT preferable and 60 amps if you are sticking with a 12v battery bank. As you saw in my calculation earlier even if you are limiting it to under 40 amps coming in, it exceeds the controller rating for amps coming out (12x40=480 watts).  So you would need 60 to get over 615 watts out. (12x60= 720 watts)  You would still need new wire for a 12v bank with a non-mppt charge controller. ($200-$360 estimate)
You could change you battery bank to 24v but you could have to get a new inverter, charge controller, and maybe an extra panel. (most expensive)

This is just off the top of my head.  I hope it helps.

[OlJarhead]

Squirrel the C40 is designed to allow up to 120vdc in and will convert to 12, 24 or 48vdc for the bank so I am in no danger there I beleive.  I'm running half what the input voltage it can take limits me to.

As Don pointed out 11.27imp and 54.60vmp should give me 615watts of power...I think I'm fine.

[Squirl]

Squirrel the C40 is designed to allow up to 120vdc in and will convert to 12, 24 or 48vdc for the bank so I am in no danger there I beleive.  I'm running half what the input voltage it can take limits me to.

As Don pointed out 11.27imp and 54.60vmp should give me 615watts of power...I think I'm fine.

Actually that is what I was trying to point out that his calculations were for his MPPT controller, not the C40.  The C40 does not convert the 54.60vmp to your 12v battery pack.  PWM charge controllers do not convert any power. It only uses approximatly 14.4v of the 54.6v, so 40.2 x 11.27 amps or 453 watts is wasted.  If it did convert it as you suggested, it would push the push the amperage to 51.25 amps (615w/12v), well over the rating of the controller.  Morningstar put out a good manual about the difference between the type of charger you have (all Xantrex C - series are PWM)and the type Don has (Outback Flex60 MPPT).
It can be found here:
http://www.morningstarcorp.com/en/support/library/MS.WP.MPPTvPWM.01.EN.pdf

I

[OlJarhead]

OK, I guess I had no idea that this is what was going on.  I saw the max input power of the C40 (125vdc) but I guess I missed the whole thing about not using the full power of the panels (or even 70%) because it can't use all the current.  I'll give Xantrex a call and confirm and then I guess I better start looking for an MPPT controller that can give me all 615watts (minus losses) instead of only 200!  That might explain some things I've been wondering about.

Thanks

[Dave Sparks]

I would just add to what Don has said about picking a battery voltage and then the contoller. You guy's are starting out and so pick an MPPT controller as it will give you flexibility down the road. You have to choose from the XW contoller, the Outbacks, the Midnite and the Morningstars. The rest should be avoided as they will not network. Actually, in my opinion it is Xantrex or Outback as they will interface well as a system with the inverter/charger. I have to do this for my customers but you folks can mix and match as long as you know the short side of having a system that will not integrate over time. Good Luck!

[OlJarhead]

OK spoke to Xantrex's tech support and they concur.  They advised that getting a 20amp MPPT controller would do the trick as I'm only running about 11.23imp off the panels (15amps would even work but 20 gives a little room to play) and suggested I could find one around $100-$150 if I did some searching.

Has something to do with a DC to DC converter -- way beyond me now -- and that the C40, as squirrel indicated, isn't designed to utilize the power of the panels when running them in series as it's only using about 14.2vdc in bulk charging mode leaving the remaining 40+vdc unused and all that charging current 

Thanks Squirrel!  I'm off to find a new controller!  DOH! 

[OlJarhead]

Trying to find a 20amp MPPT controller that can take the 615watts of power coming in seems hard.  Each 20amp controller I find seems to be limited to 200-300 watts of charging power for 12vdc battery banks.

Any help would be appreciated -- also, I want to be sure I get a controller that will not blow up when I kick on the Iota 750watt charger running off the Generator.  The C40 has no issue with it but cheap Chinese made controllers might start to smoke