got some pv´s , now what?

doggyjumper

Hi all,

My head is spinning from all the info and numbers I have been reading while at the same time local solar installers (big hype due to government fundings) and campercenters (RV centers in the US) barely have a clue what they are selling here (Belgium/EU) except the info they have from their catalogs :-(

Anyway I have been given for free a couple of pv´s (3 in fact but one has a damaged frame) and it has started my interest into solar power.
Maybe my luck to be able to see some light (maybe litterally too) has changed when I stumbled on this forum.

Maybe a silly angle, rather then trying to figure out the sizing of panels and batteries based on the needs I am more wondering what I am capable of doing with these panels. Some thoughts are 1 or 2 on my van often used for weekend camping or only 1 and the second to make some type of portable powerplant to run a computer/laptop and video camera on when no mains is available or combine the two



The panels are :

200Wp
Imp 7.50 A
Isc 8.25 A
Voc 32.8 V
Vmp 26.7 V
online info

Any use would be for 12v system

Being that "normal" 12v panels generate around 18V in order to be able to charge batteries and these panels in fact consist of 3 "modules" (with bypass diodes) does it make sence to consider to connect 2 panels like they would be 3 individual 12v panels in series ?

is one complete panel (27v) as such more suitable on its own to charge a battery (or both in parrallel)

when looking for charge controlers what data on them do I really have to watch out for ?

As far as I understand Amps is drawn , not given so basically its the loads that dictate the controlers and pv amps ?

Hope I am not to silly but need to start somewhere and I have the panels so why not try to put them to some good use.

icarus

Re: got some pv´s , now what?

Probably the best thing you can do is to ship them to me,,,,LOL

Barring that, I would suggest that you spend some time reading up (here and elsewhere) to learn a bit about PV ect.

Avoid the "ready, fire, aim" syndrome and don't buy anything until you know what it is your are trying to do. Remember two basic rules,, First, people over estimate the amount of power they can get from PV and they under estimate their loads.

Gotta run, more later,

Tony

BB.

Re: got some pv´s , now what?

Not a silly question at all--frequently we have limited space and/or budget and want to know how much power we can get from a given set of conditions.

If you live near one of the places listed in the PV Watts website--then it is very easy to get a lot of information.

So lets assume you are near Brussels -- that you have 2x200=400 watts of panels, you are using a flooded cell battery (80% efficient) and an 85% efficient inverter. The overall panel+charge controller ~77%--Overall eff = 0.77*0.80*0.85=0.52 derating/overall eff): We will have to use "1.0" kWatt for panel size (minimum accepted by program). You will have to take the power numbers and multiply *0.4 to get the amount useful 230 VAC 50 Hz out the inverter for your setup (assume tilted to ~50 degrees above flat):

"Station Identification"
"City:","Brussels"
"State:","BEL"
"Lat (deg N):", 50.90
"Long (deg W):", 4.53
"Elev (m): ", 58
"Weather Data:","IWEC"

"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 0.8 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 50.9"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 0.1 euro/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (euro)"
1, 1.03, 21, 0.03
2, 1.80, 36, 0.05
3, 2.26, 50, 0.07
4, 3.28, 72, 0.10
5, 4.01, 87, 0.12
6, 3.76, 79, 0.11
7, 4.31, 92, 0.13
8, 4.00, 86, 0.12
9, 3.03, 64, 0.09
10, 2.28, 50, 0.07
11, 1.45, 30, 0.04
12, 0.65, 11, 0.02
"Year", 2.66, 677, 0.96

So, from 1,000 watts of panels you get 11 kWhrs per month (December) to 87 kWhrs (May). or

• 0.4kW (of panel * 11 kWhrs per mnth / 30 days per month=0.147 kWhrs per day or 147 Watt*Hours per day for December
• 0.4kW (of panel * 87 kWhrs per mnth / 30 days per month=1.16 kWhrs per day or 1,160 Watt*Hours per day for December

To be honest, if the numbers are correct--you have a lot of problems with cloudy weather, especially in winter.

Less expensive charge controllers, "PWM Type", would take these panels in parallel just fine.

If the panels are mounted farther from the battery bank (10 meters or more), you would mount the panels in series and use a "MPPT Type" charge controller (more expensive, can take high voltage/low current panels and efficiently to low voltage/high current battery bank voltage.

All About Charge Controllers
Read this page about power tracking controllers

If you were doing a lot of winter camping in your van--it may not be worth it. Just get a small 1kW Honda eu1000i genset. Fuel efficient, quiet, and small.

If you are doing summer/southern camping--then you can get a good amount of power...

Questions?

-Bill

doggyjumper

Re: got some pv´s , now what?

Tony; thanks for making me win the bet that the first answer would be to ship them ... make me an offer ;-)

Gues I am ok with the ready,fire, aim. seems this is how the are selling home installations here popping up like mushrooms due to goverment insentives and no adaptations to consumption etc. so bad installers havent been able to keep up with the deadlines

Bill; sigh barely seen the sun in the last couple of months even if my data might be a bit better (closer to Ostend) but then again Belgium is so small the sun can shine accros the street while you stand in the rain

Less expensive charge controllers, "PWM Type", would take these panels in parallel just fine.

so if I understand correctly the idea of splitting up a panels connections and as such creating 3 * 18V panels in parallel would be ok (favourable ? ) using PWM for battery charging ?

btw a local installer claimed that then I would have 600 W :roll:

when I mentioned before a portable setup for cam and laptop (used at events) the use would be more in summer as winter events are indoors hence mains power available)

Guy

BB.

Re: got some pv´s , now what?

I said 400 watts, because I thought that one of the panels had a bad frame--So I only used 2x200=400 watts... Use whatever numbers of panels/watts is correct (why I always "show my work" ).

The big issue is that the amount of sun times 52% efficiency is a surprise to pretty much everyone... Basically, that 1/2 the panel's rated output is "lost" in the average off-grid installation is hard for most people to accept (basically, you have to buy 2x the number of panels to get the output "you want").

MPPT controllers are very nice and can give you 10-15% more power with identical batteries and solar panels vs a standard PWM setup (especially in very cold weather). But MPPT is expensive and may not be a "great investment" for small systems.

Where MPPT is really useful is when you have a long wire run from the solar panels to where the charge controller+batteries are located. Also, because MPPT controllers can down convert power efficiently, they are useful for non-standard solar panel voltages. Because MPPT are, basically, switching power supplies, they can efficiently "transform" (like an AC transformer for DC circuits) from high voltage to low voltage. The basic equation is:

• Power in from panels - 5-10% losses = Power out to battery
• Pin panel -5% = Imp*Vmp - 5% = Pout batt = Ibatt *Vbatt

Now, sorry I did not have a lot of time for my first post--you rightly talked about Vmp of the solar panels being 26.7 volts.

That is a "problem". PWM controllers just need Vmp>Vbatt charging + ~2 volts for controller and wiring drop. For a "12 volt batter", Vbatt-charging=~14.5 volts at 25C. For a 24 volt battery bank, Vbatt-charging=~29 volts.

If you used a PWM controller, your panels are much higher than needed for 12 volt bank (Vmp>~14.5+2=16.5 volts minimum). And not high enough for 24 volt bank (Vmp>33 volts).

So, for your 600 watts of solar panels on a PWM controller assuming Zero loses for the moment:

200Wp
Imp 7.50 A *3 panels in parallel = 22.5 amps at 26.7 volts
Isc 8.25 A
Voc 32.8 V
Vmp 26.7 V

• Pbatt-charging = 22.5 amps * 14.5 volts batt charging = 326 watts

If you did the same thing with an MPPT controller:

• Pbatt-charging = Imp * Vmp of panels = Power to Battery
• Pbatt-charging = 22.5 amps * 26.7 volts = 601 watts to battery bank

So, because you are using "non-standard" Vmp panels for battery charging application, an MPPT charge controller would give you almost 84% more power with the same panels. (your panels are probably designed for "grid tied" inverters used on homes and businesses for power AC utility feeds--they can use much higher voltage panels for those applications).

So--for your setup, yes, an MPPT controller would work much better.

The other advantage to an MPPT controller I spoke of was the ability to move panels farther away from the charge controller / battery bank.

For example, say you setup to camp at the edge of a forest. The van is in the shade, and you want to put your three solar panels 20 meters away in the sun... The total current would be for a parallel installation would be:

• Imp 7.50 A *3 panels in parallel= 22.5 amps at 26.7 volts

You would need a pretty heavy 20 meter power cable to carry the current of the solar panels to your van/charge controller.

However, you could wire the three panels in series and now:

• Imp 7.50 A 3 panels in series = 7.50 amps at 3*26.7 volts = 80.1 Volts

Now, that 20 meter cable needs to only carry 7.5 amps and can be 1/3 as heavy (less copper = less cost = less to weight to carry around).

The short answer is you can wire your panels either in series or parallel with the (correctly sized) MPPT Charge Controller.

The slightly longer answer is that charge controllers are not quite as efficient with higher Vmp voltages--but I would not worry about it here (only usually a percent or two difference). Also, you should have a series protection fuse with each parallel panel to protect against shorts (the three panels in series don't need any fuses).

Lastly, you need to measure the average energy used by your AC loads...

We usually use Watt*Hours as the unit of energy. For example, a laptop may use between 20 to 60+ watts. For 10 hours of use:

• 20 watts * 10 hours = 200 Watt*Hours
• 60 watts * 10 hours = 600 Watt*Hours

Or, assuming 600 watts of panels:

• 0.6kW (of panel * 11 kWhrs per mnth / 30 days per month=0.221 kWhrs per day or 221 Watt*Hours per day for December
• 0.6kW (of panel * 87 kWhrs per mnth / 30 days per month=1.74 kWhrs per day or 1,740 Watt*Hours per day for May

And, for your peak summer production, the "average laptop" would run:

• 1,740 WH / 20 watts = 87 hours of use per sunny day
• 1,740 WH / 60 watts = 29 hours of use per sunny day

Obviously, you have other loads too--so just add in your loads and month of use to estimate your available energy.

Also, you need to size your storage battery--For off-grid solar, we generally recommend that you use 3 days of no sun and 50% maximum discharge for long battery life... For a portable application, that may give you too heavy of battery bank, so you could use 1.5 day or even 1 day of no sun and still be pretty happy:

• 1,740 WH * 3 days * 1/0.50 max discharge =10,440 Watt*hours of battery bank
• AH of 12 volt batteries = 10,440 / 12 volt bank = 8780 AH at 12 volts
• 1,740 WH * 1 days * 1/0.50 max discharge =3,480 Watt*Hours
• AH of 12 volt batteries = 10,440 / 12 volt bank =290 AH at 12 volts

I am working the numbers backwards here--I am assuming the maximum energy on a sunny summer day and sizing your system.

You could also size your system based on your needed loads--so that may result in much smaller battery bank requirements (and even less solar panels required too).

-Bill

doggyjumper

Re: got some pv´s , now what?

Bill, Guess There is a slight misunderstanding or I misexplained (english isnt my first language)

I said 400 watts, because I thought that one of the panels had a bad frame--So I only used 2x200=400 watts... Use whatever numbers of panels/watts is correct (why I always "show my work" ).

- I have 3 panels of which one has a bad frame so I intend to use only 2
- each panel is a 54 cell panel (totaling 200wp , 27 V)

- I thought that because each panel consists of 3 tracks of cells connected in series in the junction box including bypass diodes that splitting up these lines would create the equivalent of 3 panels of 18 V each (thats when the installer said I would have then 600Watts) ie 2 tracks in series parralleling those 3 two tracks.

- I understand by now that in standard configuration (panels original state) the 27V would b usefull with MPPT controlers but I like for now to do it the basic way (basic battery charge controller).

- still crunching on the numbers you explained. but would my thoughts be reasonable ? would 2//27V not be to much of overkill for charging 12V batteries ( weather isnt always this grey here :-) ) in contrast to the 3//18V ?

depending on I could always use the third panel (fix the frame) to add or give a seperate use ( if depending on the weather this one panel might not be enough to balance the load consumption it would at least prolong the use before having to swithc the battery when used as a standalone in the field powersource fro some loads while another battery is being charged in the van for example)

I know generators are common use in the US (lived there for a couple of years) but here ... might get away with in in the van or camper (rv) but in the field ... everyone it too used to having a plugin nearby. I just try to be a bit greener

Maybe its the Amps that are playing tricks on me

Guy

BB.

Re: got some pv´s , now what?

doggyjumper wrote: »

Bill, Guess There is a slight misunderstanding or I misexplained (english isnt my first language)

Heck--English is my only language--and I don't do so well in that... :roll:

- I have 3 panels of which one has a bad frame so I intend to use only 2
- each panel is a 54 cell panel (totaling 200wp , 27 V)

- I thought that because each panel consists of 3 tracks of cells connected in series in the junction box including bypass diodes that splitting up these lines would create the equivalent of 3 panels of 18 V each (that's when the installer said I would have then 600Watts) i.e., 2 tracks in series paralleling those 3 two tracks.

Assuming you could rewire your panels to and get 32 cells in series for 16-18 volts and 1 string of 16 cells you could put in series with 1/3 of the other panel could work.

HOWEVER--You cannot just disconnect the bypass diodes--You would have to cut a wire/bus connection to isolate those 1/2 strings so they can be wired in series for ~18 volts/36 cells.

Now, remember that Power is:

• Power=Voltage*Current

When you cut the panels apart--assuming 200 watt panels, you would get approximately (really closer to 0.494 volts per cell):

• String A = 36 cells * 0.494 volt per cell * 7.5 amps per string = 133 watts
• String B = 18 cells * 0.494 volt per cell * 7.5 amps per string = 67 watts
• Original Panel = 54 cells * 0.494 volt per cell * 7.5 amps per string = 200 watts for complete panel

So, two of these panels would still only add up to 400 watts---Not 600 watts

- I understand by now that in standard configuration (panels original state) the 27V would b useful with MPPT controllers but I like for now to do it the basic way (basic battery charge controller).

Lets compare two setups... One with a 200 watt 17.8 volt / 11.25 amp panel, and the other 200 watt 26.7 volt / 7.5 amp panel, both charging a 12 volt battery at 14.5 volts (assume the rest are losses such as controller voltage drop and running panels below rated Vmp) on a "simple" PWM controller (maximum charge rate -- controller is just "on" 100% of the time):

• 200 watt Vmp=17.8 volts => 11.25 amps * 14.5 volts = 163 watts into 12 volt battery
• 200 watt Vmp=26.7 volts => 7.5 amps * 14.5 volts = 109 watts into 12 volt battery

- still crunching on the numbers you explained. but would my thoughts be reasonable ? would 2//27V not be to much of overkill for charging 12V batteries ( weather isn't always this gray here :-) ) in contrast to the 3//18V ?

See above--a 200 watt "17.8 volt Vmp panel will charge a 12 volt battery with PWM controller at 163 watts (under ideal conditions).

The same panel wired as 200 watts and 26.7 volt will only charge a 12 volt battery at 109 watt (again, under ideal sunlight conditions).

So, you can work out the price to figure out if you should buy/rewire panels, or buy a MPPT type charge controller.

depending on I could always use the third panel (fix the frame) to add or give a separate use ( if depending on the weather this one panel might not be enough to balance the load consumption it would at least prolong the use before having to switch the battery when used as a standalone in the field power source fro some loads while another battery is being charged in the van for example)

Fixing the frame--do it "correctly"--glass solar panels break pretty easily if not well supported (thin tempered/hardened glass).

I know generators are common use in the US (lived there for a couple of years) but here ... might get away with in in the van or camper (rv) but in the field ... everyone it too used to having a plugin nearby. I just try to be a bit greener

Green is really working towards minimum power use. Solar, gensets, etc., all cost similar amounts of money to run over the long term ($$$/kWhr). For much of the world, centrally supplied electricity is very cheap in comparison.

Maybe its the Amps that are playing tricks on me

Probably--Always remember that Power is:

• Power = Volts * Amperes

If you have two panels (or batteries)... If you put them in series, you double the voltage and double the power.

If you put two panels (or batteries) in parallel, you double the Amperage (current) instead and still double the power.

Of course--the loads matter (for example; supplying 24 volts to a 12 volt load is not good).

The problem is that there are many ways of "solving" the Solar PV Array / Battery issues... Some are more expensive, some are less expensive but "waste" power.

For Solar / Off-Grid power--wasting power is very expensive--and you normally want to waste as little as possible (conserve).

But sometimes, the more efficient solution (MPPT vs PWM charge controller) may be way to expensive--especially for smaller systems.

-Bill

doggyjumper

Re: got some pv´s , now what?

HOWEVER--You cannot just disconnect the bypass diodes--You would have to cut a wire/bus connection to isolate those 1/2 strings so they can be wired in series for ~18 volts/36 cells.

correct or if I play with only 1 panel I can without cutting connect at a 2 string point ie waisting 1/3 panel like a voltage divider (I know not efficient but trying to understand all/most angels ) and you meant parallel right (18v/36 cells)

Lets compare two setups... One with a 200 watt 17.8 volt / 11.25 amp panel, and the other 200 watt 26.7 volt / 7.5 amp panel, both charging a 12 volt battery at 14.5 volts (assume the rest are losses such as controller voltage drop and running panels below rated Vmp) on a "simple" PWM controller (maximum charge rate -- controller is just "on" 100% of the time):

* 200 watt Vmp=17.8 volts => 11.25 amps * 14.5 volts = 163 watts into 12 volt battery
* 200 watt Vmp=26.7 volts => 7.5 amps * 14.5 volts = 109 watts into 12 volt battery

11.25 amps... is a totally different panel right ?

Guy

BB.

Re: got some pv´s , now what?

Right, just showing how the same wattage panels with two different Vmp/Imp values have different efficiency when charging a battery through a PWM charge controller. I.e., the higher Vmp is away from Vbatt, the less power gets into the battery to charge with a PWM controller.

If you had an MPPT controller (and everything was correctly configured)--then Vmp differences would be almost zero and most of the energy would be going into battery charging.

-Bill

doggyjumper

Re: got some pv´s , now what?

Thanks for your patience Bill

Guy

BB.

Re: got some pv´s , now what?

In your drawing from post #8, energy wise, one panel would charge the battery almost most as well as the three Vmp=15volt panels in series. I.e., they are completely wasted.

The caveat is that Vmp=15 volts is very low for charging a 12 volt battery bank... Vbatt-charging would be around 14.5 volts plus 0.2-1.0 volt drop through the blocking diode.

It is fine however for "trickle charging" though where you only need >~13.2 volts or so to "float" the bank. (again 1 panel putting almost the same current into the bank as three in series--the two extra panels doing no useful work).

I hope I am making sense to you... It can be confusing. Solar panels output near constant current over their operating voltage range--which is very different than a "true battery" which outputs near constant voltage over a wide range of current. So, when you look at the "power equation":

• Power= Voltage * Current

As the panel current is near constant (fixed value for fixed amount of sunlight), the output power is directly proportional to output voltage at the load.

-Bill

doggyjumper

Re: got some pv´s , now what?

oh sorry
the drawing isn't representing my panel !! (its an example from http://www.polarpowerinc.com/info/operation20/operation23.htm#2.3.4 )

in my case I should have changed 15-30-45 V to 9-18-27V and 3A to7.5A

then technically I would waste 1/3 panel

BB.

Re: got some pv´s , now what?

Guy,

Absolutely correct!

-Bill