Throttling Power Output from PV Solar Panels

Thread Starter

Schrimpieman

Joined Mar 22, 2017
17
Hello; I'm new to the forum. I have a hobby-project idea where I need to "throtle" power output from PV solar panels & need advice on circuit design. Concept: I have 2 panels which have approx 10 amps output (each) with full sunlight. I have only one panel hooked up at the moment to a MPPT charge controller, equally rated at 10 amps. Naturally, as sunlight intensity varies thru-out the day, so does panel output. My idea is to hook up both panels in parallel, then as combined power output (from both paralleled panels) increases beyond 9.5 amps, excess power (beyond 9.5) is diverted to a branch circuit (as not to overload the MPPT). Therefore, with increased sunlight intensity, the MPPT will be supplied with a maximum sustained 9.5 amps, and the excess power is diverted to another circuit where variable power is not a crucial factor. I need a way of "throttling" the excess amperage, without the use of PWM. I was thinking about using a shunt (where the paralleled panels join) to measure the combined amperage, and using the voltage drop as a reference voltage to "throttle" a power transistor's base, inevitably allowing the excess current to flow thru its Emmiter-Collector junction to the branch circuit. (The branch circuit will be connected "upstream" from the MPPT).
Now, before members suggest me simply getting another MPPT which can handle 20 amps, this is a hobby experiment, a learning experience, & for reasons I could write a short-story beyond the Forum's scope. …I just need to "throttle" excess amperage beyond 9.5 to a branch, as the sunlight intensity varies thru-out day. Any suggestions?
My background is 20 years of automotive diagnostics & some simple circuit design. I'm a novice... but have enough background in Ohm's + Kirchoff's Laws, voltage drop, resistance, etc. Thanks in advance. Schrimpieman.
 

Kermit2

Joined Feb 5, 2010
4,162
Feedback varied active shunt loading of a PV array is a bit of a leap for beginners. Are you sure about this course of action or would you be open to alternatives offered by members that might be more suited to your experience level?
 

Thread Starter

Schrimpieman

Joined Mar 22, 2017
17
Modifying your charge controller to handle the increased power is my suggestion of a more attainable goal.
Pardon, but as I hinted with my introductory post, I do not want to modify or replace the MPPT with a more powerful unit. The purpose of my experiment is to throttle a non-critical resistive load off the branch circuit, while maintaining a constant 9.5 max output to the MPPT (any time the PV panels output, combined, reaches/exceeds 9.5.
 

Thread Starter

Schrimpieman

Joined Mar 22, 2017
17
What is your load? It might be easier to control current at the load than at the supply.
At this time, since this is simply an experiment, my branch load is a resistive array of light bulbs hooked up in a paeallel-series array simply to consume the branch outpit as it fluctuates. I realize that as my battery bank charged by the MPPT reaches their full charge, the MPPT itself will throttle back as to not overcharge the batteries. The extra current has to go someplace, so my branch resistive load can handle roughly 18 amps at 20+ volts. And incidently yes, controlling current flow to the light bulbs to offset the excess from 9.5 was my original idea. I'm not trying to throttle "back" output to the MPPT and have current "forced" thru branch. Instead, I'm trying to "open" current flow to the branch in order to offset the excess of 9.5.
 

Thread Starter

Schrimpieman

Joined Mar 22, 2017
17
Feedback varied active shunt loading of a PV array is a bit of a leap for beginners. Are you sure about this course of action or would you be open to alternatives offered by members that might be more suited to your experience level?
I'm a humble newbie. I'll take any constructive criticism or suggestions.
 

Thread Starter

Schrimpieman

Joined Mar 22, 2017
17
Two more items I've considered: just by opening a branch circuit will not encourage all current flow towards it. Rules of parallel current flow vs resistance still prevail. So, I know some resistance has to be in circuit flowing to MPPT to "force" excess current into branch. The only way to divert current to the branch withOUT restricting the MPPT would be just shy of a dead short at branch. But this will offer no useful work nor power.
Also, I've considered a comparator circuit (shunts), one after the MPPT and one at the paralleled PV panels, because when the MPPT throttles down current to battery bank, all that available power from PV can be used elsewhere.
For example, right now, it's bright sunshine & my panels are at "max" output, but my batteries are being charged only at about 3 watts. (They're fully charged). What's the MPPT doing with all the excess energy pumping out by the PV's? If I can harness that energy, that'll serve my ultimate goal.
 

ronv

Joined Nov 12, 2008
3,770
Two more items I've considered: just by opening a branch circuit will not encourage all current flow towards it. Rules of parallel current flow vs resistance still prevail. So, I know some resistance has to be in circuit flowing to MPPT to "force" excess current into branch. The only way to divert current to the branch withOUT restricting the MPPT would be just shy of a dead short at branch. But this will offer no useful work nor power.
Also, I've considered a comparator circuit (shunts), one after the MPPT and one at the paralleled PV panels, because when the MPPT throttles down current to battery bank, all that available power from PV can be used elsewhere.
For example, right now, it's bright sunshine & my panels are at "max" output, but my batteries are being charged only at about 3 watts. (They're fully charged). What's the MPPT doing with all the excess energy pumping out by the PV's? If I can harness that energy, that'll serve my ultimate goal.
You only have 2 choices.
1- you can shunt the excess current to your lamps.
2- you can reduce the voltage to the charge controller.
Since you don't want to use PWM that would force you into some analog design. But 20 amps at 12-18 volts is a lot of power.
Maybe you could use a digital design where you turn on ever increasing numbers of bulbs based on the current.
Say 1 ant 9.5 amps, 2 at 9.55 amps, 3 at 9.6 amps and so on.
Lot of "stuff.":(
If you have specs on the lamps we could have a look at the power in the analog switching device.
 

Kermit2

Joined Feb 5, 2010
4,162
The PV panels are providing the exact amount of power the load is asking for. The power is not wasted, only the opportunity to more fully utilize the capabilities of the panels has been wasted.
 

BR-549

Joined Sep 22, 2013
4,928
You believe that you need......or you want.....to use and consume all current available from panels.......right?

1. you can increase load on batteries. or add more batteries and controllers.
2. divert and waste in lamps.
3. nothing.
4. invert and supply grid.
 

tcmtech

Joined Nov 4, 2013
2,867
I realize that as my battery bank charged by the MPPT reaches their full charge, the MPPT itself will throttle back as to not overcharge the batteries. The extra current has to go someplace,
Unless it's a really cheap unit that dumps the excess power into making heat in itself by a simple voltage clamp and shunt load it does not have to do anything with the incoming supplied power once the battery reaches its desired state of charge. It simply increases its equivalent series resistance between the solar panel and the battery to keep the battery voltage where it needs to be.

How many amps the solar panel is capable of is largely irrelevant just as is the potential wattage your home electrical power is capable of. Just because your alarm clock needs .05 amps to run it doesn't mean it shunting the other 100 - 200 amps your main service point is capable off to someplace else.

Personally if you have that much extra solar power available so often it may be worth considering stepping into the world of grid tie co-generation (pirate DIY built or legal of the shelf unit) and sending the extra solar power you have available into your home electrical system where your other utility power operated stuff can make use of it.
 

Thread Starter

Schrimpieman

Joined Mar 22, 2017
17
No it doesn't! Reducing the load on the panel merely causes the voltage to rise a little. At the extreme you remove all load and the panel approaches the open circuit voltage.
You are correct. Perhaps I should have stated ...reducing load on panel allows for potential to use available power elsewhere. Thanks.
 

Thread Starter

Schrimpieman

Joined Mar 22, 2017
17
Unless it's a really cheap unit that dumps the excess power into making heat in itself by a simple voltage clamp and shunt load it does not have to do anything with the incoming supplied power once the battery reaches its desired state of charge. It simply increases its equivalent series resistance between the solar panel and the battery to keep the battery voltage where it needs to be.

How many amps the solar panel is capable of is largely irrelevant just as is the potential wattage your home electrical power is capable of. Just because your alarm clock needs .05 amps to run it doesn't mean it shunting the other 100 - 200 amps your main service point is capable off to someplace else.

Personally if you have that much extra solar power available so often it may be worth considering stepping into the world of grid tie co-generation (pirate DIY built or legal of the shelf unit) and sending the extra solar power you have available into your home electrical system where your other utility power operated stuff can make use of it.
Thanks for everyone's responses. You're right about the MPPT. It's not creating heat and the incoming voltage is rising towards PV open circuit voltage when batteries are fully charged. So it's not a cheap unit dumping the extra energy into a resistor. Just for the record, no this won't be a grid-tie experiment. As indicated, I have only 2 panels making a maximum of roughly 250 watts in ideal conditions. This is simply a hobby experiment; should it prove useful, I'll apply it to my stand-alone shed. Energy production 250 watts is hardly enough to supply average person's daily energy consumption. But let's not go there. I'll open another post to describe my ultimate goal & we can see where this is headed.
 

Thread Starter

Schrimpieman

Joined Mar 22, 2017
17
Pls allow me to explain the reasoning for me opening this experiment to begin with; maybe it'll put the discussion closer on-track.
First, I like to tinker with stuff. That's one way we learn & keep the brain healthy.
Now for the real objective: I have an electrical load in my shed which consumes near 9 amps, 12 volts, which I'd like to keep "on" as long as possible, daytime. During the time the panel(s) are not producing power, the load shuts off. (This is the load currently handled by my MPPT & battery bank.) Solar panels work their "strongest" only under ideal conditions & bright sunlight. The duty cycle of full output (sunny days, no clouds) to support my load is approx between 10 AM to 4-ish in afternoon. Add clouds to the mixture & energy production nose-dives, but I'd like to maintain a constant 9-amp load as long (time) as possible during daylight hours. So instead of purchasing more batteries to increase Amp-Hour capacity and a stronger MPPT, my idea was this:
Add on a second solar panel, pump that energy into the battery bank at roughly 9.5 amps (allow for parasitic losses), and any extra energy offered by the panels could be diverted into a separate circuit (a water heater) when production exceeds 9.5 amps. Furthermore, during cloudy conditions, 2 panels are more powerful than one. So, I have a better chance to harness maximum power at all times, up to 9-ish amps, especially during cloudy periods.
The excess energy beyond 9 amps can be pumped into a water pre-heater before the water flow enters the principal one. That's why I stated, the branch load is not critical. It can turn on/off as conditions allow. No big deal. For now I just have a string of cheap light bulbs before I invest $$ into the water heater. (?)
 

tcmtech

Joined Nov 4, 2013
2,867
Just for the record, no this won't be a grid-tie experiment. As indicated, I have only 2 panels making a maximum of roughly 250 watts in ideal conditions.
Too bad. I like doing pirate grid tie Inverter design. I did write the "Basic GTI design for dummies' book on it after all. :p

Pls allow me to explain the reasoning for me opening this experiment to begin with; maybe it'll put the discussion closer on-track.
First, I like to tinker with stuff. That's one way we learn & keep the brain healthy.
Now for the real objective: I have an electrical load in my shed which consumes near 9 amps, 12 volts, which I'd like to keep "on" as long as possible, daytime. During the time the panel(s) are not producing power, the load shuts off. (This is the load currently handled by my MPPT & battery bank.) Solar panels work their "strongest" only under ideal conditions & bright sunlight. The duty cycle of full output (sunny days, no clouds) to support my load is approx between 10 AM to 4-ish in afternoon. Add clouds to the mixture & energy production nose-dives, but I'd like to maintain a constant 9-amp load as long (time) as possible during daylight hours. So instead of purchasing more batteries to increase Amp-Hour capacity and a stronger MPPT, my idea was this:
Add on a second solar panel, pump that energy into the battery bank at roughly 9.5 amps (allow for parasitic losses), and any extra energy offered by the panels could be diverted into a separate circuit (a water heater) when production exceeds 9.5 amps. Furthermore, during cloudy conditions, 2 panels are more powerful than one. So, I have a better chance to harness maximum power at all times, up to 9-ish amps, especially during cloudy periods.
The excess energy beyond 9 amps can be pumped into a water pre-heater before the water flow enters the principal one. That's why I stated, the branch load is not critical. It can turn on/off as conditions allow. No big deal. For now I just have a string of cheap light bulbs before I invest $$ into the water heater. (?)
I see. What I would be looking at is setting the systems up so that once the panel voltage goes above whatever it normally does when the MPPT unit is running at full draw it starts dumping the excess power above that into your secondary load.

Simple voltage limit based load dump transfer of sorts. Without knowing your level of skill that dumping systems could be a simple analog voltage clamping circuit (considerable amount of heat to be lost to the switching device doing the load transfer) or a more complex PWM based one. (little losses in the load transfer switching device.)

You have lots of options and ways to go about it depending on what you main goal is.
 

ronv

Joined Nov 12, 2008
3,770
Pls allow me to explain the reasoning for me opening this experiment to begin with; maybe it'll put the discussion closer on-track.
First, I like to tinker with stuff. That's one way we learn & keep the brain healthy.
Now for the real objective: I have an electrical load in my shed which consumes near 9 amps, 12 volts, which I'd like to keep "on" as long as possible, daytime. During the time the panel(s) are not producing power, the load shuts off. (This is the load currently handled by my MPPT & battery bank.) Solar panels work their "strongest" only under ideal conditions & bright sunlight. The duty cycle of full output (sunny days, no clouds) to support my load is approx between 10 AM to 4-ish in afternoon. Add clouds to the mixture & energy production nose-dives, but I'd like to maintain a constant 9-amp load as long (time) as possible during daylight hours. So instead of purchasing more batteries to increase Amp-Hour capacity and a stronger MPPT, my idea was this:
Add on a second solar panel, pump that energy into the battery bank at roughly 9.5 amps (allow for parasitic losses), and any extra energy offered by the panels could be diverted into a separate circuit (a water heater) when production exceeds 9.5 amps. Furthermore, during cloudy conditions, 2 panels are more powerful than one. So, I have a better chance to harness maximum power at all times, up to 9-ish amps, especially during cloudy periods.
The excess energy beyond 9 amps can be pumped into a water pre-heater before the water flow enters the principal one. That's why I stated, the branch load is not critical. It can turn on/off as conditions allow. No big deal. For now I just have a string of cheap light bulbs before I invest $$ into the water heater. (?)
If you can find a 1 ohm heating element the one in post 17 should work.
 
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