power directly off of Panels?

73 Posts
Jul 14, 2008 08:41 pm
power directly off of Panels?

when using a 12v dc appliance directly off the battery bank, during the day while the sun is out, are you pulling power directly from the solar panel OR are you pulling power from the battery?

the reason I ask is that I'm trying to figure out just how much ah of battery I need for my system.

I'll be using 40w of 12v power for 6 hours a day during sunlight hours. The panel array will be 60w. So will the battery be acting in effect as a "voltage regulator" during the day and only be storing the extra 20w an hour?

thoughts?

James
 
97 Posts
Jul 14, 2008 11:05 pm
Re: power directly off of Panels?

Just my thoughts on this James J............The battery is much like a storage capacity device.  If the voltage applied is higher, it charges, and if the load applied reduces the voltage, it discharges.  You lose around 1/4 of the applied charge energy during discharge due to inefficiencies in converting electrical energy to chemical energy, and back to electrical energy again. There are additional losses in the Inverter in converting DC to AC voltage........so it is not an ideal condition where you can just put in X amount of energy and get the X back out again.  So many things are variable, and you will have to experiment with this to see what your actual gain (or loss) is?  I love learning experiences :-)  Jon C.
 
184 Posts
Jul 15, 2008 09:49 am
Re: power directly off of Panels?

I agree that there are too many variables.  Maybe your least expensive option would be to get a Marine Deep Cycle battery from your local big-box department store.  Keep in mind that for best battery performance (life), avoid discharging it below 80% of capacity. 
   
 
163 Posts
Jul 15, 2008 11:27 am
Re: power directly off of Panels?

For the purpose of this "experiment" you can consider the pv panel to be dumping its power into the battery, and the 40watt load to be taking its power from the battery.

What you need to be careful of is if you have no sunshine for an entire day then the 40w load will discharge your battery by 40/12 * 24 hours or 80AH. This would be a significant drain on a decent 100AH deep-cycle 12V battery.

On the other hand, if you have no load for a couple of days and your pv panel is pushing 60/15, or 4 amps continuous (with 15V being the charging voltage of the pv panel) into your battery. Once the battery is fully charged you could end up damaging your battery if you do not have a charge controller in the system.
 
578 Posts
Jul 15, 2008 12:40 pm
Re: power directly off of Panels?

 storing the extra 20w an hour?

there is no such thing as watts per hour.  watts is a rate = joules per second, not a quantity.  watt-hours is a quantity.  this causes much confusion and is a common mistake.

first off, with a 60w array, anything above 40w instantaneous is great, as 60w assuming stc is unlikely.

second, unless your load is designed to run pv direct, you will want a battery.  you compare your watt-hours per day of load to your watt hours per day of production.  the latter must be higher.

third you must decide how many days of autonomy you would like. your battery must be able to handle the watt-hours of load for as many days as you want your system to work in absence of sun.

there is more, but this is the basics of off grid system design.  we talk at length about these topics in the courses.

if you tell me watt-hours per day of load, and days of autonomy that you want, i can suggest a battery and array for you.

cheers,

james

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
 
73 Posts
Jul 15, 2008 04:32 pm
Re: power directly off of Panels?

Thanks all for the great info and feedback. What I'm tryng to figure out is how "small" of a battery I can get by with while not discharging it more than 80%. This is the last thing I need to figure out before I start purchasing parts and start building. I have my mini system figured out with the following  components:

PV array = (3) PowerUp BSP 20. I currently have (1) and am adding (2) more to the system

BSP 20 specifications
Maximum Power (Pmax) 20W
Voltage at Pmax (Vmp) 17.3V
Current at Pmax (Imp) 1.20A
Short-circuit current (Isc) 1.30A
Open-circuit voltage (Voc) 21.7V

Combiner Box
Midnite Solar MNPV6 Combiner Box

Breaker Box
Square D QO24L70RB Square D QO Load Center

Charge Controller
Xantrex C35 35A, 12/24V PWM Charge Controller

I'll be working in the studio for approx 6 hrs a day during daylight hours only. Hence my question about how much I really need in terms of battery storage.

The computer system I am powering with DC directly off the battery will consume approx 40w an hour DC (40wx6=240w) per day.

I really only need 1 of 2 days of autonomy for so, if you could figure out how much storage I need that would be great! I'd prefer to have only (1) battery and not mess wiring a battery bank.

Now all this talk has got me thinking...maybe I should purchase just (1) larger panel (say an 80w) panel? and then I can sell my existing 20w panel. This would probably end up being more cost effective in terms of cost per watt and mounting just 1 panel would be much easier than managing a string of panels?

So, I guess I need to still nail down the battery and Panel size...

James

 
163 Posts
Jul 16, 2008 10:41 am
Re: power directly off of Panels?

The computer system I am powering with DC directly off the battery will consume approx 40w an hour DC (40wx6=240w) per day.

I really only need 1 of 2 days of autonomy for so, if you could figure out how much storage I need that would be great! I'd prefer to have only (1) battery and not mess wiring a battery bank.

Now all this talk has got me thinking...maybe I should purchase just (1) larger panel (say an 80w) panel? and then I can sell my existing 20w panel. This would probably end up being more cost effective in terms of cost per watt and mounting just 1 panel would be much easier than managing a string of panels?

So, I guess I need to still nail down the battery and Panel size...

James


For a 12V system your math is easy. Just divide your watt-hours by 12 to get that will be taken in amp-hours from the battery. In your case, 240/12 is 20AH.

One small caveat is that the efficiency of a lead-acid battery can be anywhere from 50 to 80 percent. So in order to recharge a lead-acid battery that you have taken 20AH from, you may have to put back the equivalent of anywhere from 25AH to 40AH from your charging source.

As for the PV panels, I would recommend (without having used them myself!) a couple of CIGS modules such as this one that will work well in low light conditions, and is attractively priced at $5 per watt.
http://store.altenergystore.com/Solar-Panels/51-to-99-Watt-Solar-Panels/Global-Solar-60W-12V-Framed-Solar-Panel/p5574/
 
578 Posts
Jul 16, 2008 11:57 am
Re: power directly off of Panels?

240 w-h of energy per day.

50% depth of discharge for battery health.

240 x 2 = 480 watt-hours of battery capacity

15% battery charge discharge inefficiency

480 w-h x 1.15 = 552 watt hours of battery capacity

we will assume room temp for batteries for this case.

2 days of autonomy

552 watt hours x 2 = 1104 watt hours of battery capacity

1104 watt hours / 12 volts = 92amp-hours of battery capacity which works out to be a group 27 or group 31 12v battery.

---

pv system

for 2 days of actual autonomy to be possible, our array must have the ability to produce 1104 watt-hours in a day, so that after one cloudy day, the next sunny day the array is able to cover the loads as well as make up for the lost day.

without knowing your location, we will assume 4 sun hours per day.

1104 watt hours / 4 sun hours = 276 watts of pv at STC

276w of pv x 1.2 for difference between STC and Reality = 331w of pv

it is important to understand you can do less pv than this, but that the system design will not handle bad weather without another charging source.  just matching the loads alone in good weather would put you at 72w of pv power, so your 80 example could work, but when it is not a great sunny day, your system will not function at its best or for more than another day, maybe, without sunshine.

john's math is correct, but i wanted to spell out how i would do it if it were my situation, i am a little more cautious.

james
Alt-E staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
 
73 Posts
Jul 16, 2008 05:25 pm
Re: power directly off of Panels?


For a 12V system your math is easy. Just divide your watt-hours by 12 to get that will be taken in amp-hours from the battery. In your case, 240/12 is 20AH.

One small caveat is that the efficiency of a lead-acid battery can be anywhere from 50 to 80 percent. So in order to recharge a lead-acid battery that you have taken 20AH from, you may have to put back the equivalent of anywhere from 25AH to 40AH from your charging source.

As for the PV panels, I would recommend (without having used them myself!) a couple of CIGS modules such as this one that will work well in low light conditions, and is attractively priced at $5 per watt.
http://store.altenergystore.com/Solar-Panels/51-to-99-Watt-Solar-Panels/Global-Solar-60W-12V-Framed-Solar-Panel/p5574/
[/quote]

Wasn't even aware of battery inefficiencies, so thanks for that knowledge.

So if I base my calcs on cycling the battery only 20% deep AND want 2 days of autonomy at 20ah per day. Then I would need a 200ah battery based on my usage of 20ah a day.

To be on the very safe side, I would then need to make sure to put back 40ah a day, just to be safe.

AGAIN, my question is, what is really happening when I am the computer system is using 3.33ah of DC off the battery, when at the same time, the battery would be receiving 7.0ah of solar. Is ALL of the 7.0amps from the panels going into the battery? OR is my battery usage somewhat offset, by the solar panels, thus requiring a smaller battery?

thanks so much for your help...just about there..

James
 
163 Posts
Jul 16, 2008 09:26 pm
Re: power directly off of Panels?

James,

I hope we haven't put you off by giving you the worst case scenario, but you should know it and you can decide for yourself what amount of risk you can live with.

Typical battery efficiency is usually given between 70 and 80 percent, but I was quite surprised to read a report on the Sandia Labs site somewhere that under certain conditions it can be as low as 50 percent. It's a very good site for research.
http://www.sandia.gov/ess/

Now on to a practical answer to your question. The first thing that a charge controller does is take whatever energy it has available and put the battery through a charging cycle. Once the battery is fully charged, the charge controller will actually turn "off" the pv panel so that no current is flowing. Now if you put a 40watt load on the battery then you interrupt the charge controller in its single-minded pursuit of charging the battery, and knocks it off its bulk/absorb/float cycle.

So if you have a pv panel capable of delivering 7 amps (84 watts for easy math) at 12v, and say your fully-charged battery requires 1 amp to maintain the float, then your 40 watt load would be drawing its 3.3 amps directly from the charge controller. The charge controller would also be delivering 1 amp to maintain the float voltage, so it will effectively be "de-rating" your 84w panel down to about 50w.

Now if you add a second 40w load, the charge controller would turn the panel power back up to 72w and service both loads while still maintaining the 1 amp battery float current. Add a third 40w load and suddenly you are drawing more current than the pv panel can deliver so you start taking current out of the battery and the battery voltage drops. The charge controller "sees" the drop in battery voltage and says "hey, I'm not in float mode any longer. It's time to re-evaluate the situation".

For practical purposes you can think of the charge controller as dumping everything that it gets from the pv panel into the battery, and the load as drawing all of its current from the battery, but in reality it is slightly more complicated than that.
 
Jul 18, 2008 06:29 am
Re: power directly off of Panels?

What an amazing coincidence.
http://medievalwriting.50megs.com/scripts/letters/historyj.htm

I have got to say it if nobody else will.
"an attractive price of $5.00 a watt."
Are you sure you feel ok J.B.? Do you want me to call you a doctor or something? I am kidding, of course.
I know of a place selling PV at $7.00 a watt. Now they make $5 seem attractive. $5 would seem to be the going rate.
The PV industry could do better, you know?
I was thinking maybe, more like, $2 or $3 per watt.

(How about it PV manufactures? Don't you want to gives us "end users" a break for a change? Maybe instead of using both your hands to "rake" in your profits you could use just one for a while? Your arms must be getting tired by now.

Oh! I forgot. Thats not how the world works, is it? I mean, its always been the "end user" that suffers the burden of industry. In more ways than one. Why should RE be any different? Just following the trail left by those before you and all that sort of stuff hey?)

J.J., did you know that your human body uses 2 metals; Sodium (Na) and Potassium (K) along with body fluids which is mostly water (H2O) to produce bioelectricity for, among other things, your brains thought processes?
http://www.medicinenet.com/electrolytes/article.htm

Do you really need your computer? Oh sure, a computer can perform a lot faster but... Wait a second, has the company given you deadlines to meet or something? Have to make production do we? Well your not alone.

There are lyrics in a song I heard. A sort of prayer. I will not soon forget them. "I can see the concrete, slowly creeping, Lord take me and mine before that comes" (sung by the late Ronnie Van Zant)
Pity about Earth.
 
184 Posts
Jul 18, 2008 09:28 am
Re: power directly off of Panels?

James,

John B has done a good job in answering your question.  Others have provided you with some good design and system hardware information.  I would just like to add that using power directly from the solar panels to power a load is much more efficient than storing and retrieving it with batteries.  So, IMHO, you should make sure that you have enough PV to fully power the load, with enough left over to fully charge your battery each day. Considering your smallish load, you're not going to be spending that much on batteries anyway.  Personally, I would start with one 80 to 100ah battery, see how the system performs for a month or two, and add another battery if necessary.  But hey, that's just me.   

Thomas,

Yes solar panels are expensive, but once you've bought and installed them they should last for 25 years or more. I prefer to build my system now, rather than to wait and hope for a price reduction.  With this crazy run-up in gasoline prices, you just never know what will happen in the future.  I don't want to be in a situation where PV panels are in such great demand that they're hard to get at any price.  People sometimes talk about the payback of solar PV, which IMHO is absurd.  In addition to cutting your costs for electricity, putting up PV gives you protection from grid power outages.  It can prevent your food from spoiling, protect your property, keep your communications working, etc.  It's hard to put a price on those things.

Sure, I wish the cost of solar panels would go down, but I won't let their high cost prevent me from achieving my goals.
 
73 Posts
Jul 20, 2008 05:45 pm
Re: power directly off of Panels?

James,
I would just like to add that using power directly from the solar panels to power a load is much more efficient than storing and retrieving it with batteries.  Personally, I would start with one 80 to 100ah battery...

John D - you hit the nail on the head here..my idea is to be as efficient as possible hence the DC powered studio..good ot know I'm on the right track. As for battery size..again I think you nailed it..I'll get an 100ah battery and see how the system does..more than likely I'll get a battery monitor as well that handles multiple shunts, so I can monitor the pv array coming in and the battery power coming out..

John B - you're explanation below seems to spell it out perfectly for me..which is what I thought all along...


"...So if you have a pv panel capable of delivering 7 amps (84 watts for easy math) at 12v, and say your fully-charged battery requires 1 amp to maintain the float, then your 40 watt load would be drawing its 3.3 amps directly from the charge controller. The charge controller would also be delivering 1 amp to maintain the float voltage, so it will effectively be "de-rating" your 84w panel down to about 50w.

Now if you add a second 40w load, the charge controller would turn the panel power back up to 72w and service both loads while still maintaining the 1 amp battery float current. Add a third 40w load and suddenly you are drawing more current than the pv panel can deliver so you start taking current out of the battery....."

If the battery only needs 1amp to maintain float and the array is generating much more than the 1amp..the excess power will then be available for my DC use until I run over the array's  amp output, which will then switch over to battery power. Needless to say, I'm sure this will fluctuate a quite a bit during daylight hours as the sun dances in and out of the clouds. So the Gobal Solar Panels might very well be the best choice for optimum "averageing" amp performance during the day...thanks again everyone..I'll post some photos as the system goes together..

James
 

Disclaimer and Disclosure

The Alternative Energy Store, Inc reserves the right, within its sole discretion, to refuse or delete any posting or portion thereof, or terminate or block the access to this forum.

The opinions and statements posted on this forum are the opinions and statements of the person posting same, and do not constitute the opinion or act of the Alternative Energy Store, Inc (AltE). The Alternative Energy Store, Inc does not endorse or subscribe to any particular posting. No posting shall be construed as the act or opinion of the Alternative Energy Store, Inc.

Click here for BBB Business Review

McAfee SECURE sites help keep you safe from identity theft, credit card fraud, spyware, spam, viruses and online scams
Desktop Website | Mobile Website

Share

Click on an icon to share! If you don't see the method you want, hover over the orange "+".

Feedback

What can we do to help you?

Please enter a summary
Sorry, the copyright must be in the template.
Please notify this forum's administrator that this site is using an ILLEGAL copy of SMF!
Copyright removed!!