One MPPT controller or Two?

cwbiii

I have an older Airstream trailer. I plan to put 600W of solar panels on top. This would be 2 banks of 3x100W panels mounted on each side of the trailer. Because the trailer roof is curved these panels are biased between 15 and 25 degrees from horizontal and double that with respect to each other. Because of this situation in order to maximize the recharging capacity of these panels I believe I will need a separate MPPT controller for each bank of panels. It is the difference in solar angle that I believe would create the less than ideal charging situation, typically about a 45 degree difference in where they are pointing.
In use I would normally orient the trailer N/S and the sun would progress E to W and the power from the panels would vary with the suns position in the sky. I figure with this setup I should harvest approx. 400w per hr for a 5 hr period and an average or 200W per hr for a 4hr period on an ideal day. This equates to about 3KW-hr of energy stored on an ideal day. If I used a standard PWM controllers or a single MPPT controller the results would be less because of the imbalance between panels or less efficient energy conversion from the PWM. Cost matters, but so does efficiency... I figure the energy efficiency more than compensates for the additional cost of equipment. 600w is maximizing the available space to put panels without shading. I am new to this and this is my technical understanding of the systems involved... am I missing anything important?

Photowhit

I'd leave them on 1, it takes very little light for a panel to reach a reasonable voltage so one set shouldn't hinder the other much.

Your calculations don't take into account the various losses in charging, a lead acid battery(?) is only 80-85% efficient (input to output) to begin, and the top 10-15% will be less than that... So stored energy will be a good bit less than potential.

BB.

It is pretty easy to estimate the average harvest from your solar power system... You can use something like PV Watts and enter in some basic information. Say you have a fixed array mounted flat, and are out of Boston MA:

Month    Solar Radiation (kWh/m 2/day)
1      1.85    
2      2.79    
3      3.79    
4      4.61    
5      5.58    
6      6.03    
7      6.03    
8      5.43    
9      4.25    
10      3.15    
11      1.89    
12      1.58    
Year      3.92

The farther to the north you are, the more having a tilted array can help harvest. The same panels mounted at 42 degrees from horizontal:

Month    Solar Radiation
(kWh/m 2/day)
1      3.36    
2      4.36    
3      4.79    
4      4.92    
5      5.33    
6      5.41    
7      5.60    
8      5.64    
9      5.13    
10      4.65    
11      3.14    
12      2.98    
Year      4.61

Get quite a bit more power in winter, a bit less in summer.... Depends on what time of year and where you camp as to what is the best for you.

Then,, power wise:

600 Watts * 0.52 AC end to end system efficiency * 4.36 hours (Feb 42 degree tilt) = 1,360 Watt*Hours per day for 120 VAC loads
600 Watts * 0.61 DC end to end system efficiency * 4.36 hours (Feb 42 degree tilt) = 1,596 Watt*Hours per day for 12 VDC loads

PWM vs MPPT charge controllers... Most 100 Watt solar panels have Vmp~18 volt output--Perfectly fine to put panels in parallel to a PWM controller to charge the battery bank on an RV.

MPPT charge controllers are more expensive and allow you to use "GT Solar Panels" (grid tie) that are larger (200 watts and larger) and Vmp not 18 volts (typically Vmp~30 volts)... GT panels tend to be cheaper and may or may not be easier to mount to your RV (fewer panels, fewer electrical connections, fewer bolts, etc.). An "expensive" MPPT controller + cheap GT panels may be less expensive than a cheap PWM controller + expensive 100 Watt panels.

If you have different tilt on the panels--You should use one MPPT controller per array--So that sends up the costs with two MPPT controllers. In a smaller/basic RV system, MPPT controllers are a nice to have, but really not going to give you a lot of extra power unless you do a lot of snow camping (very cold weather will increase the array Vmp voltage--Which MPPT controllers can harvest as extra energy--typically 10% or so more in very cold conditions).

You are correct--Solar electric panels do not work well with shading. A good PWM controller should work OK (single controller with panels at different angles).

Many MPPT charge controllers use fans for cooling (particularly the larger units)--Fans running during the day may be an issue for an RV where you are "on top" of the controllers. The new MPPT controllers frequently have more option/computer communications/programmable options and can be very nice. Personally, I am not convinced that I would run two MPPT controller vs just a single charge controller for your situation (cost/mounting space/wiring for two controllers vs harvest efficiency). I would try one charge controller and see how your harvest works out.

And lastly... it is just Watts not Watts per Hour... Watts is already a rate (like gallons per hour). And Watts*Hours is an amount (like gallons pumped). We do not need to use Watts per Hour in solar

-Bill

Photowhit

BB. wrote: »

If you have different tilt on the panels--You should use one MPPT controller per array...

Okay, I don't believe this, why would you need to worry about one being shaded, it will achieve charging voltage without direct sun. Are you worried about the power point?

I am about to fill out my rack and have panels that I can hook up in the shade and see if they limit the charging current, what types of limits would you like? I can likely do a test next weekend.

BB.

I believe that this installation has the panels mounted with 3 tilting "to the left" and 3 tilting "to the right"... Sort of a virtual tracking array.

Shaded panels and MPPT--Not really worried about that... Shaded panels (even with just a little shading) produce near zero useful current/voltage/energy.

For me, as I see the issue... It is the Vmp of the panels.... Different orientation, different cell temperatures, different Vmp point for panels. Solar Guppy who used to post a lot here over the years was very clear that each orientation needed its own MPPT controller. Since he had the equipment to accurately measure the output of lots of different controllers/combinations, he is certainly one to listen too.

From another point of view, "hot panels" are the ones directly facing the sun... As one array becomes more "off angle" and the other becomes more on angle to the sun... The off angle array is getting cooler, but producing less current--So, it becomes less "important" in the equation. More or less, that is why I am not sure a second MPPT charge controller is always worth the costs. MPPT controllers also have "tare" losses (consume power just because they are "turned on"--Switching losses)... Having two controllers running has somewhat more losses than one controller sharing both arrays--Another reason I am not sure I would be worried about a couple percentage points of increased production + increased tare losses, vs a single controller and miss-matched Vmp (due to temperature/operating points) as the sun moves across the sky.

-Bill

Photowhit

Okay, I'll hunt around and look at some of Solar Guppies old posts. I can't devise a situation where it would create a problem. Though I'm more of a touchy-feely understanding of the workings. It will be easy enough to put a string in parallel with the rest of the array, or even limit the array to a single string (run in pairs) of panels and see if it limits the current flow, even completely covered.

scrubjaysnest

cwbiii wrote: »

I have an older Airstream trailer. I plan to put 600W of solar panels on top. This would be 2 banks of 3x100W panels mounted on each side of the trailer. Because the trailer roof is curved these panels are biased between 15 and 25 degrees from horizontal and double that with respect to each other. Because of this situation in order to maximize the recharging capacity of these panels I believe I will need a separate MPPT controller for each bank of panels. It is the difference in solar angle that I believe would create the less than ideal charging situation, typically about a 45 degree difference in where they are pointing.
In use I would normally orient the trailer N/S and the sun would progress E to W and the power from the panels would vary with the suns position in the sky. I figure with this setup I should harvest approx. 400w per hr for a 5 hr period and an average or 200W per hr for a 4hr period on an ideal day. This equates to about 3KW-hr of energy stored on an ideal day. If I used a standard PWM controllers or a single MPPT controller the results would be less because of the imbalance between panels or less efficient energy conversion from the PWM. Cost matters, but so does efficiency... I figure the energy efficiency more than compensates for the additional cost of equipment. 600w is maximizing the available space to put panels without shading. I am new to this and this is my technical understanding of the systems involved... am I missing anything important?

With my 280 watt system and a pwm cc I see only a slight drop in input voltage to the CC for different facing angle or one panel in a little shade. You could make the panels tilt able with out to much problem and that would be cheaper then two mppt cc's.

Here is a link to a hinged solar install. http://s185.photobucket.com/user/Fire_Instructor/library/Rockwood%20Solar%20Install%20%202011?sort=6&page=1

BB.

PWM controllers "don't care" about different panel orientations. It only cares that Vpanel >~ 15 volts. And they arrays will simply add current from each array--according to the amount of sun hitting the panel(s).

MPPT (Maximum Power Point Tracking) controllers try to find the Vmp and Imp of the array... With panels facing different directions, Vmp-array will have several "bumps"... Once for each array orientation. Arrays facing sun head on--will be hotter and have Vmp lower (high cell temperature drops Vmp voltage by 5%-10%-20% or so--depending on exact cell temperatures)

We nominally "match" solar panel Vmp by 5-10% -- So a spread in Vmp-panel of 5-10% does not seem like the end of the world.

Having a spread of Vmp-array local peak voltages will not hurt the MPPT controller--And it is questionable which Vmp-local peak may be set by the MPPT controller (which may be programmed to find a "peak", not the maximum peak) and it could track the "wrong peak".

How each controller behaves is gong to vary between designers/products/etc.).

-Bill

cwbiii

Thank you for all the replies in such short order. I understand a lot about the difference between PWM and MPPT... my job kind of depends upon it. I work engineering battery management chips. Pairing them up with solar, not so much. I understand the value of efficiency and storing energy into a battery bank. This whole system is quite expensive when you add up all of the components... 4-6 batteries, a 2kw charger/inverter, 6 solar panels, wiring, and either 1 or 2 solar charge converters. But... the delta between 1 or 2 charge converters is only a relatively small cost adder to the system and appears to increase its efficiency by about 10-20% overall and even quoted out to 40% but I suspect that is for a poorly matched condition. I'm planning on using blue-top Optima's for my storage because of the cost, weight, and ruggedness advantage though I've had recommendations not to. My history with them has been stellar for mobile use, andTesla has an interesting solution on the horizon that may make sense later. I am very interested in what people have to say and I like to understand a number of points of view. There are many ways to solve a problem and often several of them are outstanding even though they might be quite different.( That's the engineer in me. )
The bottom line is that I'm investing serious money and another $300 for a 10-20% increase in overall system performance seems like a wise investment overall... it's still a very good investment in the 5-10% range... Inverter/charger $1350, 4-6 batteries $1000-$1500, 6 solar panels $1800... the system is in the $4200-$4700 without the controller(s). (I might be able to do slightly better on price, but marginally so)
If I use the median value, then I'm paying $4500 for this system without the controller(s) and if the charging capability can increase by 10-20% with MPPT vs PWM and neither solution is going to deliver better than 90% efficiency then; 4500/90% = 5000, or a 10% delta in charging efficiency is worth approx. $500 . It only gets better as the max efficiency goes down.

Thanks again for your inputs!!!

BB.

the 10-15% or so increase in charging efficiency is due to increased Vmp--Which is due to reduced cell temperature. More or less, the substantially reduced cell temperature is going to occur in sub freezing weather. And you may only have 2 hours of sun per day vs the 5-6 hours of sun in summer. So a 10% increase in 2 hours of sun per day is 2.2 hours of sun in winter.

On hot days, you will get 10% to 20% reduction in Vmp, and an equal amount of reduction in output power for MPPT. Or, roughly equal performance with a PWM controller matched to the correct panel (Vmp-stc of ~17.5 to 18.6 volts or so). So, for 9 months of the year, there is little reason to expect significant harvest from MPPT controllers (and their 5% switching losses) vs PWM controllers (little operational losses, their losses are due to miss-match solar panel+battery bank--like Vmp 30 volts and a 12 volt battery bank--PWM miss match will cost you ~1/2 of the array's wattage).

I do like MPPT controllers (running higher Vmp-array voltages allows smaller awg wiring from array to charge controller, and more extreme conditions--such as a cold battery bank and hot weather/equalization were a PWM controller + 17.5 volt Vmp panels many not output optimum power in deserts/temperature extremes)

But I would not count on 10%-15% more power in winter being even noticeable for most people.

I always try to be conservative in my predictions... If you get a bit more--Great.

-Bill

Photowhit

cwbiii wrote: »

The bottom line is that I'm investing serious money and another $300 for a 10-20% increase in overall system performance seems like a wise investment overall... it's still a very good investment in the 5-10% range...

I suspect you will see a 5-10% increase in charger efficiency between a PWM and a MPPT charge controller with panels that have a VMP of 17.5 volts, I question weather you will have any increase in a single MPPT over 2 MPPT charge controllers.

cwbiii wrote: »

Inverter/charger $1350, 4-6 batteries $1000-$1500, 6 solar panels $1800... the system is in the $4200-$4700 without the controller(s). (I might be able to do slightly better on price, but marginally so)

I think the Inverter charger is on the cheap side of things, do you have one picked out? Is it a true sine wave inverter? Magnum makes some Modified sine wave inverter/chargers in the range.

Have you considered regular flooded batteries? Good deep cycle 6volt golf cart batteries can be had for $80 + core at Sams Club or Costco.

600 watts of panels for $1800!!!!!!!!!! Ouch! There are a few places where 12 volt nominal panels can be had for near $1 a watt.
Here's some at $1 a watt ETL
Some here for $1.25 a watt
Some for $1.23 a watt
Amazon $1.50 a watt free delivery

cwbiii wrote: »

If I use the median value, then I'm paying $4500 for this system without the controller(s) and if the charging capability can increase by 10-20% with MPPT vs PWM and neither solution is going to deliver better than 90% efficiency then; 4500/90% = 5000, or a 10% delta in charging efficiency is worth approx. $500 . It only gets better as the max efficiency goes down.

If you understand how battery charging works, then you understand that how much a MPPT charge controller helps depends on how you use your system. Batteries that are regularly drawn down to 50% SOC are helps a good bit more by the use of MPPT type charge controllers. MPPT type charge controller help most when batteries are bulk charging, Once the charge controller hits the absorption stage, about 80-85% full, they only help a little since the battery begins to accept less current

Dave Angelini

Photowhit wrote: »

Okay, I'll hunt around and look at some of Solar Guppies old posts. I can't devise a situation where it would create a problem. Though I'm more of a touchy-feely understanding of the workings. It will be easy enough to put a string in parallel with the rest of the array, or even limit the array to a single string (run in pairs) of panels and see if it limits the current flow, even completely covered.

Be sure and do your testing with full loading on the arrays, with clouds and log over several days. It is not easy to do! With 2 mppt controllers there will be a much smoother constant power output. With one controller you will see erratic power as the mpv will be close to correct and then quite a ways off in voltage. The clouds really can fool the controller as it forces more calculations and some of the older mppt controllers really were on the edge to begin with. As Bill says, and "dear old" Mr. Guppy would back up.
Only you can determine if the cost works out for having 2 controllers. The engineering definitely does!

cwbiii

I was off on my memory of the price of the panels... $300 was for the 100w kit. The individual panels are $180. so that knocks $720 off of the cost total. The inverter/charger is a ProSine 2000. I have 4 Trojan 6v from my last RV but they are too big and too heavy for this one. I guess I need to do the research for 2 20a MPPT vs 1 45a MPPT vs 1 45a PWM to make a fair comparison of cost vs efficiency. I will do that shortly. I need to make up my mind and order this stuff shortly or I won't finish this project in time for our planned camping adventures. I don't do much winter camping and since I have a new job I won't be able to go away
for longer periods. Summer boon-docking is our favorite pastime. I have a small AC unit I can run in the heat of the day and this will consume considerable energy (650w max) Everything else has ben updated to be energy efficient. Fans will suffice most of the time.

Thanks again for all of your input,

Chuck

scrubjaysnest

Our camping primarily is boondocking. We use fans but no ac during the day and a fan in the bedroom at night and get by fine with our system.

cwbiii

Well I've pretty much made up my mind... I can purchase 2 MPPT 12-24v 20a (Tracer) controllers for about $125 ea. The total system cost is about $4000... 250/4000 = 6.5% and I should easily see that improvement so the cost effectiveness proves out. Thanks again for all of your input... it helped me to do my homework and make the correct decision!

600w of panel, 2kw inverter/charger, 4 blue top Optima's, 100' of 0 gauge welding wire (50 red/50 black) for the 12V power backbone, 6 0 gauge connector blocks with boots for connection hard points. Ring connectors with crimping tool for 0 gauge wire, silver solder, 10 gauge wire from controller to backbone, standard connectors from controller to PV panels. I'm mounting the MPPT controllers to an aluminum vent I built for the refrigerator... it should provide a good heat sink and they are easily accessible inside an overhead cabinet. Moreover the wires from the PV panels come in through the refrigerator vent so the wiring should be short.

I still have to figure out a monitoring system... I want to put an interface on all of my sensors (tank levels, battery voltage/usage monitoring, solar sensors, etc) that will interface with an ipad that I keep inside the trailer and can additionally access with a iphone to be able to monitor/configure/troubleshoot the system. This is a pretty aggressive project so I don't know what kind of success I'm going to have... though if I can work it all out I should be able to market it.

Chuck

zoneblue

Something else to factor in the number and type of controllers is idle tare. Too many people overlook the fact that charge controllers (and inverters) can have significant self consumption. A 600W system is not big enough to throw power away. Me personally id be looking at a single midnte brat to see if that will work. Youll need 36/72 cell panels for PWM. But the 0.2W tare is best in class. Midnite KId takes a step up to 0.5W, midnite classic lite, 3W, full midnite 5W. To take this to one extreme, you can see what a single classic would do on a 600Wp system. Lets say you produce 2Wh/Wp per day. Thats 1200Wh per day total production on an average good day. The charge controller uses 5W * 24 = 120Wh/day, a full 10% of your production. The inverter uses how much? Well for a best in class (outback VFX) thats 20W * 24. 480W/day, thats 40% of production.

Tracer? Well, all i can say is that having two might be a good idea for cheap gear!
You get what you pay for in this game. Be careful.