So you are doing away with the constant 28VDC output?

What are you driving off the panel?

 

So you are doing away with the constant 28VDC output?

What are you driving off the panel?

Just a battery for now.

I'm kind of stuck in a maze. I really don't know what to do. I've gotten something fundamentally wrong about all this. I just can't figure out what it is.

Thanks a million for your help and everyone else too!

 

I too am having a problem understanding how an MPPT controller can work with charging a battery. Somebody please tell me where I am wrong:

1. Given constant temperature and illumination, the voltage produced by a solar panel depends inversely (though not linearly) on the current drawn.
2. The only way to control the current going into a battery you are charging is to control the voltage across it's terminals.
3. Assuming a solar panel with output voltage at MPPT greater than the battery voltage at the MPPT current, you must convert the voltage down to be at this operating point.
4. A linear converter will simply waste some of the power in the ratio of the battery charging voltage to the MPPT voltage.
5. A switching converter will also waste power because it is off for some portion of the duty cycle and therefore the panel is producing no power during the off time. This waste is even worse that that of the linear converter because the same ratio becomes the duty cycle only if the converter is 100% efficient.
6. The same sort of logic would apply if the MPPT voltage is lower than the battery voltage. A switching converter would not be drawing the MPPT current continuously.

So how do you get the panel to produce the MPPT power continuously?

Bob

 

Post # 16 = Yes.

 

Post # 16 = Yes.

Great! Thanks a lot, Bernard!

 

Lets approach this from a different direction and break it up into logical parts.

1. A MPPT has no useful function if the power produced by the solar array is greater than the demand of the charging system.
2. A MPPT becomes useful when the power produced by the solar array is less than the power required by the charging system.
3. When abundant power is not available from the solar array, the MPPT will adjust the charging parameters to maximize the efficiency of the system.
4. The main purpose of a MPPT is to maximize the efficiency of a system with varying solar levels and varying battery charge requirements. (You defeat the purpose of a MPPT when you begin with the statement, "with all things being constant".)

In other words, if a solar array has the capability of producing 20 watts, but the battery charger only needs 18 watts, then the MPPT function is not needed. If the solar array has the capacity to produce 18 watts, but 20 watts is needed by the battery charger (ideally), then the MPPT will adjust (lower) the battery charger output (current) to use the available 18 watts most efficiently. It will then search for that "sweet spot" that gets the most power out of the array.

Battery chargers go through different charge phases and require different amounts of power throughout the charge cycle. The initial "bulk" charge requires a lot of power. The "finishing" charge requires much less power. The solar panels go from the capacity of producing no power, (darkness), to max power, (full brightness). A MPPT tries to maximize these varying factors.

Remember, a switching buck regulator is not just a regulator, it converts one power form to another power form (minus some loss).

 

Simply put,
MPPT is applied within the limits of output voltage and output current.

 

From post # 13, " by applying MPPT", does not say where to apply it, so might we use a fixed 28 V BC & apply MPPT to control a load? Re: post # 15.
It might be helpfull if you could give the exact wording of the assignment.

 

I'm not exactly sure myself.

Right now, I just have to develop a Simulink model of MPPT but I can't figure out how to control SP's output current and voltage for a given temperature and irradiance, if that's even possible.

 

Re read post # 26 step # 4.
To opperate, MPPT needs SP V & SP I, & a device to do it's bidding, ie a controllable DC- DC converter.
Try to find: EDN December 5, 2008, pgs 51-52, designideas,Solar-array controller needs no multiplier to maximise power.

 

https://courses.cit.cornell.edu/ee4...jps89_cdq2_mam584/jps89_cdq2_mam584/pvSim.pdf

 

Still, no one has answered my question. Assuming a solar panel and a buck converter in a battery charging application. The buck converter is controlled to produce the MPPT current into the battery. But a buck converter is switching the solar panel current on and off at some duty cycle. The rest of the duty cycle is wasted power, no? Or is this wasted power somehow captured, by diverting it to a capacitor for instance?

Bob

 

Still, no one has answered my question. Assuming a solar panel and a buck converter in a battery charging application. The buck converter is controlled to produce the MPPT current into the battery. But a buck converter is switching the solar panel current on and off at some duty cycle. The rest of the duty cycle is wasted power, no? Or is this wasted power somehow captured, by diverting it to a capacitor for instance?

You last statement is what is normally done. The input to the converter has a capacitor large enough so that the current from the panel is basically steady even though the converter is taking current in discrete chunks.

 

There are a lot of cheap charge controllers that are simple switches, (not switching converters.) These devices simply close the switch connecting the solar array to the battery when the voltage is below a threshold and open the switch when the voltage rises through an upper threshold. This is not a power converter function. They are no where as efficient as a power converter, or a power converter with a MPPT controller.

 

Vorador
Maybe this will help you with your project.
For a P+O to work you need something to control and something to measure. You could measure voltage and current of the array and calculate power for your measurement. Then you could control the battery charge current. You would change (Perturb) the battery charge current and then see if the wattage went up or down. Up is good, down is not good.

 

Still, no one has answered my question. Assuming a solar panel and a buck converter in a battery charging application. The buck converter is controlled to produce the MPPT current into the battery. But a buck converter is switching the solar panel current on and off at some duty cycle. The rest of the duty cycle is wasted power, no? Or is this wasted power somehow captured, by diverting it to a capacitor for instance?

Bob

Oh, I see what you're asking. I'm doing something similar on my project. I also put a capacitor (3000 uF in my case) across the solar panel. So maybe the panel puts out 10 amps, but my switching converter draws 40 amps peak (actually a current ramp as an inductor charges up) for, say, 25% of the time. For the other 75% of the time, the solar panel is busy putting its current into the capacitor. Because of the capacitor, and because I'm switching fast enough, the circuit appears to the solar panel as a rather smooth draw of 10 amps, and the solar panel voltage adjusts itself to whatever it will be at 10 amps.

Note that I can monitor both current and voltage of the solar panel, and of the battery.

When I'm just charging the battery, I limit the duty cycle of the switching converter, so that I don't exceed maximum charging current on the battery. When the battery is at fully charged voltage, the duty cycle goes WAY down, to just provide a tiny maintenance current to the battery. MPPT never comes into play, unless the panel can't provide the recommended charging current. In that case, I use MPPT to provide best-effort charging.

If a load switches in, the controller tries to compensate by increasing power output until output voltage is up to the battery maximum (28 volts in your case). In the likely event that the panel cannot provide all the power the load needs, the controller will provide best-effort power by using MPPT. I'm lucky, in that my load is heavy, so it's a sure bet that when the load kicks in, I'll always go to MPPT.

As you can see, there are a lot of decisions to be made in the controller, not the least of them the MPPT itself.

 

Roderick Young
Well said!

 

If I was trying to design a MPPT converter, I would start with a simulation: I show the I vs V curve of a typical panel that has a short-circuit current of ~2A and an open-circuit voltage of 19.5V (Red trace) in full sun. I show a battery load, and to get the I vs V plot of the panel, I sweep the battery voltage from 0V to 21V (independent variable).

I also plot the power delivered by the panel [the expression V(p)*I(V1)], the green trace. If V1 was a lead-acid battery, its voltage would be going from ~12V t0 about 14.5V during the bulk-charging phase (when the battery takes as much current as a panel can deliver).

Note that if the panel is connected directly to the battery (as simulated), if the battery voltage is 12.0V, the power delivered to the battery is 23W. If the panel voltage could go to 17.42V (a lead acid battery should never go above ~14.8V), the panel would be delivering the peak power of which it is capable (at full sun) of 31.66W.

The goal of a MPPT tracking algorithm is to interpose a special buck converter between the panel and the battery which operates this simulated panel with a voltage of 17.39V at its input, which results in the panel producing peak output power at a panel current of ~1.8A. Obviously, the output voltage from the converter is the battery voltage, which ranges from ~12V to ~14.5V during the time the battery is accepting bulk charge.

The battery current would be initially be 17.42*1.8/12.0*eff, where eff is the efficiency of the buck converter. A typical eff is ~85%

Note that the improvement in power deliver to the battery is 0.85*31.7/23 = 1.17, or 17%, which is hardly worth the trouble for a small panel into a single battery...

If you want to try some sims yourself, ask me to post the panel subcircuit...

 

Still, no one has answered my question. Assuming a solar panel and a buck converter in a battery charging application. The buck converter is controlled to produce the MPPT current into the battery. But a buck converter is switching the solar panel current on and off at some duty cycle. The rest of the duty cycle is wasted power, no? Or is this wasted power somehow captured, by diverting it to a capacitor for instance?

My intuition agrees with you: if I am only drawing current for a portion of the time then even if I am maxing out the power during that time I am still leaving the remainder unused inside the panel.

Of course, who said you are limited to only a buck converter to do all the heavy lifting?

 

Oh, I see what you're asking. I'm doing something similar on my project. I also put a capacitor (3000 uF in my case) across the solar panel. So maybe the panel puts out 10 amps, but my switching converter draws 40 amps peak (actually a current ramp as an inductor charges up) for, say, 25% of the time. For the other 75% of the time, the solar panel is busy putting its current into the capacitor. Because of the capacitor, and because I'm switching fast enough, the circuit appears to the solar panel as a rather smooth draw of 10 amps, and the solar panel voltage adjusts itself to whatever it will be at 10 amps.

Okay, this explains it. Thanks.

Bob