PWM versus MPPT- a quick analysis

NorthGuy

Re: PWM versus MPPT- a quick analysis

vtmaps wrote: »

Thus, using the same panels as in your example, consider 5 parallel strings of 2 panels per string: if there is shading on one cell there will be a disadvantage to dropping the voltage to 49 volts and allowing the bypass diode to conduct.

Of course, every situation is different. Shading pattern matters too. If you get many unshaded parallel strings and only one shaded, it doesn't make any sense to impair good strings to get lesser production from the bad strings.

I didn't expect any shading, so my configuration is about the worse - 4 parralel series of 3. So, when I did get shading, it hurts a lot.

stephendv

Re: PWM versus MPPT- a quick analysis

I've been thinking that with a PWM controller, wouldn't it be more efficient to install half the array with grid-tie panels and the other half with off-grid panels? The voltage difference between the grid-tie's Vmp and the battery charging voltage would be much less; but you might have problems reaching absorb and EQ voltages at the peak of summer, which is why you'd install the other half of the array with the higher voltage off-grid panels.

The question is then what's the best ratio of off-grid to grid-tie panels?

Photowhit

Re: PWM versus MPPT- a quick analysis

NorthGuy wrote: »

PWM: 4kW array. Vmp = 36 Imp = 111; MPPT: 3.5kW array. Vmp = 36 Imp = 97. Assume best conditions.

OK 1 time, My panels are 35VMP at my cabin, but we'll use you startup since it won't matter much

NorthGuy wrote: »

In bulk. Battery voltage 27. PWM produces 27*111 = 2997W; MPPT produces 36*97*0.95(efficiency) = 3317W (a little bit better; 3.2kW array would be the same)

There is a big advantage in Bulk. In a properly setup system, you are rarely in bulk only as your system is 'ramping up' to in the morning to reach absorption, huge advantage while producing little. Since your keeping your system in the top 20% this is a very short time and represents very little of your battery charging. This is the normal situation, by the time the sun runs around and hits the panels fully and the array starts producing large amounts of current, you have reached absorb and your system starts tapering off. So neither array is producing anywhere near panel rating during this time. .

NorthGuy wrote: »

In absorption. Battery voltage 29. Current 50. Both MPPT and PWM produce 29*50 = 1450W + whatever goes to loads (both arrays are the same)

In float. Battery voltage 27. Current 0. All production goes to loads (both arrays are the same)

I don't understand at what point 4kW array with PWM is different from 3.5kW array with MPPT. Perhaps you can tell this to me?

Perhaps you still don't understand, but it's just that simple. In my situation, my heaviest loads come with sunny days. Dang near a direct 1:1 relationship!. I can choose to have a generator for this days and run a much smaller array, or a large enough array to provide for these days.

Other great reasons to use a MPPT charge controller, not every day is perfect! Other reasons for having an MPPT CC, not the least of which will be accurate measuring of charging while under load, during the absorption stage. And starting loads once a point in charging is reached, which will pretty much eliminate my drawing my battery down below 80% by running my water heater when I won't be around in the afternoon once my charging hits float to start it. I could and would, and even might go ahead and put the heater on a timer to start at 2pm for a couple hours, if I can't get the Midnite CC to communicate with each other. Something I've not had time to deal with. Having sold my array at my cabin(received deposit last night) and getting the cabin it's self sold.

NorthGuy

Re: PWM versus MPPT- a quick analysis

stephendv wrote: »

I've been thinking that with a PWM controller, wouldn't it be more efficient to install half the array with grid-tie panels and the other half with off-grid panels? The voltage difference between the grid-tie's Vmp and the battery charging voltage would be much less; but you might have problems reaching absorb and EQ voltages at the peak of summer, which is why you'd install the other half of the array with the higher voltage off-grid panels.

The question is then what's the best ratio of off-grid to grid-tie panels?

There was a thread recently were I said that some grid-tied panels could be used with PWM, but Cariboocoot convinced me that this was a bad idea.

The penalty for putting Vmp below battery voltage is severe. Must be avoided at all costs.

NorthGuy

Re: PWM versus MPPT- a quick analysis

Photowhit wrote: »

Perhaps you still don't understand, but it's just that simple. In my situation, my heaviest loads come with sunny days. Dang near a direct 1:1 relationship!. I can choose to have a generator for this days and run a much smaller array, or a large enough array to provide for these days.

I wish I was that lucky. My heaviest loads come in the middle of winter when I produce misearble 3kWh/day. That's not enough to even begin thinking about absorptions

So, perhaps I don't understand your situation full well. At these heavy load sunny days, when your batteries are in absorptions, are you loads so big that they frequently draw the batteries below the set absorption voltage? Say, you have it set at 29, but big loads pull it down to 27 or even lower at times?

Photowhit wrote: »

Having sold my array at my cabin(received deposit last night) and getting the cabin it's self sold.

Congratulations. Did you only sell the array or the whole system?

Photowhit

Re: PWM versus MPPT- a quick analysis

While they usually don't draw the voltage down, loads prevent the charge controller to accurately charge the battery, since it's timed, and not charging enough, or end amps are confused by the loads. I hope most of this will be solved once MidNite set's up it's battery module. One of those reasons for liking MPPT CC.

I had a fan go out on one of my home inverters, so I took the 1800watt Prosine out of the 'system', though I am giving them a 1000 watt true sine inverter that is a cheap one I haven't used. So a complete system sorta... I also only have 4 golf cart batteries on it now so only have 1640(?) watts of array up, though I'm selling with another 340Watts of panels. They got a pretty good deal, I though in several things that people starting out would not think of... and I'll help them move it and set it up. $2700. Panels are 4-6 years old, with a power center that has all the breakers, and a combiner box, throwing in a clamp meter, SS clips to clean up the system, frame work, battery box and power center exterior weather box.

NorthGuy

Re: PWM versus MPPT- a quick analysis

Photowhit wrote: »

While they usually don't draw the voltage down

Perhaps having MPPT is helping with that. If you had PWM, you could get bigger/more frequent drawdowns.

Photowhit wrote: »

loads prevent the charge controller to accurately charge the battery, since it's timed, and not charging enough, or end amps are confused by the loads. I hope most of this will be solved once MidNite set's up it's battery module. One of those reasons for liking MPPT CC.

End amps are built-in in my Xantrex XW system. It doesn't measure it, but calculates from the data provided by networked devices. So, it takes the loads into account. It doesn't do that very precisely however. It works fine most of the time. But when loads get bigger, the error increaes. If a water pump and microwave, and pehaps dishwasger, are turned on together, it underestimates the amps going to the battery, which causes absorbtions to end prematurely. On the other hand, when I'm not home and there's no loads at all, it overestimates the amps and absorptions get longer. Perhaps, that's Ok, but I'm taking the control into my hands and will make it more consistent.

It was time-based termination before then, but I didn't like it. Absorbtion starts, but then clouds come on, voltage drops and you're not really getting any absorption, but it does count that in as an absorption time. So, I had to increase absorption time, which was already very long. End amps work much better.

Photowhit wrote: »

I had a fan go out on one of my home inverters, so I took the 1800watt Prosine out of the 'system', though I am giving them a 1000 watt true sine inverter that is a cheap one I haven't used. So a complete system sorta... I also only have 4 golf cart batteries on it now so only have 1640(?) watts of array up, though I'm selling with another 340Watts of panels. They got a pretty good deal, I though in several things that people starting out would not think of... and I'll help them move it and set it up. $2700. Panels are 4-6 years old, with a power center that has all the breakers, and a combiner box, throwing in a clamp meter, SS clips to clean up the system, frame work, battery box and power center exterior weather box.

Wow! It is a good deal.

Cariboocoot

Re: PWM versus MPPT- a quick analysis

There's a slight flaw in the reasoning being used here.

Bulk mode: maximum current from array to batteries as Voltage rises. MPPT has an advantage in being able to produce slightly more current from the same size array. (This effect can be greater with a higher Voltage system if the V-drop through the same size wires over the same distance is a fixed amount. But that is assuming a lot.)

Absorb mode: Voltage is fixed at one point and current diminishes over time. Assuming no loads and both controllers have sufficient PV on them there is no difference in performance to the battery as some power will be lost. MPPT merely has greater current potential, so you could say more power is 'lost' (not realized). If there are loads then MPPT may be advantageous for maintaining Absorb level while also supplying load demand due to this current potential difference between the two controller types.

Float mode: Voltage is fixed and current varies to maintain this V set point on batteries (there will never be zero current at Float as it is above battery resting Voltage). Again the greater current potential of the MPPT may be advantageous in supplying any loads.

For charging batteries only, with all other factors equal, the MPPT charging advantage is found only in Bulk mode where maximum current is desired.

Over-all the MPPT advantage is in its flexibility for array design. The claims of greater power are an exaggeration and occur only under certain circumstances. Notice it is usually stated "up to" 30% more power, and "up to" begins at zero.

Comprenez-vous?

stephendv

Re: PWM versus MPPT- a quick analysis

NorthGuy wrote: »

There was a thread recently were I said that some grid-tied panels could be used with PWM, but Cariboocoot convinced me that this was a bad idea.
The penalty for putting Vmp below battery voltage is severe. Must be avoided at all costs.

Yes, that's why I suggested only using half of the array with grid panels, and the other half with off-grid, so that the average voltage would be enough to EQ in summer. You would never have panel voltage below battery voltage with 60 cell grid tie panels. They might not reach EQ, but they'll definitely be higher than the battery voltage.

Photowhit

Re: PWM versus MPPT- a quick analysis

Cariboocoot wrote: »

Comprenez-vous?

Sí mi buen amigo.

Of course all those cloudy fall and winter days, for people who's heaviest demands are in winter, you will see a boost with a MPPT CC. Of course once things get squared away at MidNite and they integrate the battery module, everyone will quit talking about a production advantage and start talking about the only (or one of the only) battery chargers that actually measures battery charging rather than wattage in and wattage out of the charger.

I'd have loved to just leave my A/C on all day, but I'm not sure what the charge controller will think when it has a nearly steady close to 700 watt load (450 A/C, 150 fridge in the heat + assorted other things and inverter losses)