MPPT using an inverting boost converter

Hi all

I am thinking about this options for MPPT

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I have different options for my input, and this can convert up and down, so it fits me well. Also, a panel can be used over a larger scale.

Does anyone have any experience with this?

WBR Sonnich

Reply to
Sonnich Jensen
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MPPT?

Reply to
John Larkin

Den tirsdag den 26. maj 2015 kl. 16.56.39 UTC+2 skrev John Larkin:

maximum power point tracker, for example load a solar cell to where is makes most power and converts it to the voltage required to charge a battery

-Lasse

Reply to
Lasse Langwadt Christensen

What is your battery voltage and what is the solar cell output voltage?

Reply to
John S

That's a very complex question. Depends a lot on your location, cloud cover, budget, an otther requirements.

Bottom line was that it was decided to stick with standard buck MPPT and forgo panel sun tracking for a mountaintop radio system. And here's why.

Sun tracking was a matter of reliability. More power most of the time is trumped by a system that freezes solid in winter and becomes unreliable. Your mileage on that one may differ, but all that mechanical stuff exposed to the wind and rain can shake itself apart. Wind whipping up underneath a pivoted panel is a lot more stressful than if it sits flat on a roof.

The problem is winter. The system has to be scaled to give sufficient power in winter. Most of the year, power is excess and a standard buck converter works fine.

So, consider winter insolation and panel angle. Over part of the day, you get plenty of panel voltage. Problem is sunrise and sunset.

Calculate or measure the current delivered into a short over the course of the shortest day using a standard buck MPPT controller. Also measure the panel voltage under those circumstances.

Plot that curve and put a horizontal line across it at the battery voltage. The tails on left and right represent lost power. Integrate the current graph for those areas potential regained power. But that's not all extra. You'd still get some power if you just connected the cells and ran at less than MPPT under those conditions. The difference is what you can regain.

Do the experiment again with a panel configured for higher voltage and correspondingly less current.

For a given size panel, there's a peak in the voltage/current tradeoff for a given battery voltage. That's the most you'll ever get from a boost/buck controller.

In our case, the additional energy recovered barely covered the additional losses in the converter and introduced a number of additional parts to fail.

I'm sure you can easily come up with a reason to try to recover a few percent. For a hobby, it usually isn't worth the trouble. You might get most of the benefit by switching to a higher voltage panel.

Try to stick with synchronous rectifiers. Every diode you put in the circuit costs you dearly.

Anyhoo, a few things to think about.

Reply to
mike

Of course, that assumes your roof angle is ideal and sun-facing! :>

Here, roof pitch is pretty shallow -- if not flat (no snow loads). So, most PV arrays stick up like angels' wings! By (unfortunate) coincidence, we tend to get microbursts originating out of the north -- the "underbelly" of all those panels!

OTOH, here, your (household) needs drop significantly in Winter (like a factor of 2) and there's still plenty of sunshine (though at a different incident angle).

I don't think any of the PV arrays, here, track. And, many are

*thousands* of square feet (think "covered parking").

But, I'd like to make a small "sun tracker" that simply measures solar radiation (e.g., gauge transpirational losses, household heat accumulation, etc.). So, I've considered the most effective way of doing this:

- array of fixed detectors (with some math)

- wide angle lens (with some math)

- mechanism driven by foreknowledge of where the Sun *should* be etc.

Any suggestions in that regard?

Reply to
Don Y

Don't know whether I understand your question correctly, but the shown circuit may work as voltage regulator, but hardly as MPPT. You won't get maximum power out of a solar panel, if you disconnect the panel from the source during the off-time. An MPPT requires a converter with continuous input current.

Klaus

Reply to
Klaus Bahner

Lol. Is the sun ever not where it should be? The only time your collectors might do better than facing where the sun *is* could be when they are next to a large glass building nearby reflecting light onto the array. There is no reason to think this is going to be more than what the sun is providing directly and it will only be for a short time until the reflection passes by anyway.

So just perform some basic calcs to come up with a fixed tracking path. It likely can't be optimized much for different paths over the seasons unless you are near one of the polar circles.

--

Rick
Reply to
rickman

you'll just need a capacitor on the input.

any switch-mode configuration that can provide the right input-output relation should work, it is just a "gearbox"

-Lasse

Reply to
Lasse Langwadt Christensen

That would not make it a *maximum* power converter. Certainly better than without a capacitor but not optimum.

Klaus

Reply to
Klaus Bahner

Some control logic, usually a uP, would have to adjust the switch duty cycle to find the max power point. Probably some dithering algorithm.

That inverting topology would work. If the solar panel output voltage is high, a buck converter would work, too.

--
John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 
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Reply to
John Larkin

Lots of ways to do that. The guys built one to test and it worked. I've never seen it cuz I'm in another state helping by remote. It's two solar cells with a vertical divider between them. Use the same perturb and observe algorithm to find where the solar outputs to the two cells are equal. That works well for a single axis tracker because it's relatively insensitive to misalignment in the N-S axis. You an use 4 detectors for two-axis sensing. I'd use two 2-axis sensors to improve convergence. You can also use one sensor at the end of a tube. That gets hard to manage when there are clouds. The panel can be thrashing all over.

Problem with tracking the sun is that you can't always tell where the sun is by looking in the visible spectrum. On an overcast day, the tracker can wander all over the place.

For the MPPT, hunting for the peak is no big deal. For a massive mechanical assembly, you don't want to do that.

I tried to get them to use time of day to aim the panel and just trim it with the sun detector if there was significant difference between the outputs of the two sensor cells.

They gave up because of mechanical considerations. The site gets a lot of snow and ice and wind. The wind generator tends to ice up, go off balance and tear itself apart. KISS ruled the day.

Switching to philosophical mode....

KISS. Never ask a question if the answer won't change your future. You may not have the means or the motivation to do what the answer suggests. Another pearl is "measure what you want to know."

I've spent a lot of time constructing measurement capability that was fun for 20 milliseconds. Lots of stuff is relatively constant. Once you have the answer, you don't really need to measure it again.

If you're having unseasonably cold weather, there's really not much you can to about it. You don't need to measure it, because it's on the evening news.

Stuff that varies can often be controlled by other means. If you want to save energy, take shorter showers. You don't need ANY data to know that.

I have two monitoring systems. A PalmPilot monitors a flapper on one of the air registers. Factor out the fixed offsets and I can tell how long the heat or the air conditioner runs. Plots a graph. That tells me everything I want to know about what the sun is doing to the house. When I turn on the computer in the morning, I can watch the heater throttle back to compensate. But, once you optimize the house, there's not much more that having the data gains you.

Second is the power monitor on the electric utility meter. Again, once you optimize the place, the data isn't very useful. Sometimes, I see power I can't account for and find I've left the lights on in the attic. But that's about it. I tried micromanaging power consumption, but the change in the utility bill was in the noise.

Diminishing returns happens much quicker than I'd thought. ;-)

Reply to
mike

you will always have some input current ripple, and afaict some controllers use it to find the maximum

-Lasse

Reply to
Lasse Langwadt Christensen

I have different options for my input, and this can convert up and down, so it fits me well. Also, a panel can be used over a larger scale.

John is correct. Whatever configuration you choose, it will require something (uC, probably) to interpret the results and make adjustments. The MPPT system I designed for a customer does just this quite well.

Is the open circuit panel (cell) voltage greater than the battery you want to charge? If so, you want a buck. Otherwise, you want a boost. In either case, you will need something with an algorithm to seek the MPP.

Your inverting topology does nothing for your cause. Make it easy on yourself. There is no topology of which I'm aware that will automatically seek MPP.

For your info, a solar cell/panel produces maximum power when its output voltage under load is somewhere around 75% of the unloaded voltage when fully illuminated. In fact, it doesn't change much with diminished sunlight over some range which I have not yet measured. Nevertheless, if you can adjust your load such that you maintain that voltage, you will get relatively close.

The panel temperature plays an important part but I'm not prepared to get into that.

Reply to
John S

I'm assuming that the idea is that the inverting buck-boost can provide output both higher and lower than the input with a single switch so it could work for different combination of cells and batteries

-LAsse

Reply to
Lasse Langwadt Christensen

I build a MPPT for a satellite going into deep space. The algorithm was like this:

The MPPT samples the voltage, lets the converter draw more current, so the voltage decreases. When the voltage has decreased 3%, the converter changes direction, samples the current. When the current has descreased 3%, it starts over

The control circuit was incredible simple

Cheers

Klaus

Reply to
Klaus Kragelund

But, the image you posted is not a buck-boost. It is an inverting boost as John L said.

Reply to
John S

Note that I *know* where the sun should be. I'm really only interested ion measuring its output by ensuring a detector is pointed directly *at* it, throughout the day. So, no need to "hunt".

The question is, whether it is better to build a sort of "phased array" of detectors that can be stationary and yet determine the solar output (with foreknowledge of which detector(s) will be exposed and to what degree at any given time of day); a single "sky view" detector with suitable optics (that cares not where the sun really is); or, a mechanized scheme that keeps a narrow beam detector pointed *exactly* at the Sun's theoretical location (even if it isn't the brightest spot in the sky!)

Exactly. As you know where the Sun *should* be (regardless of cloud cover) it should be possible to just point *a* (focused) detector in that exact direction -- instead of telling it to hunt ("Hey, it's late afternoon... what are you doing looking Eastward??")

But it tells you nothing about the irrigation needs of the flora in your yard!

E.g., the rose bushes (RIP) could *use* watering twice a day in the hottest, driest part of the year (which is why they've been removed). Or, once every week or TWO while dormant!

Similarly, the citrus trees get watered heavily when hot, dry, windy. But, early in the season, once a week is adequate. Watering too little leads to dried out fruit (i.e., any effort that you've expended is wasted as you can't "recover" from this). Watering too much leads to fruit that "ruptures" (i.e., the flesh grows faster than the encasing peel) -- again, the fruit is lost. You can't "suck" the extra water out of the soil once you notice this has started to happen!

[Unlike the California gluttons, we, long ago, realized water is a scarce resource so don't waste it on lawns, swimming pools, washing off driveways, etc. The citrus trees are a luxury -- but, at least we consume everything they produce.]

There are considerably larger savings at stake, here. E.g., it's approaching June and we've yet to turn on the ACbrrrr -- because we actively use the cooler outdoor air, overnight, to lower the internal temperature of the house *and* the outdoor temps haven't been persistently high enough to heat the home's masonry.

Were it not for seasonal allergies, we could leverage the swamp cooler to keep the house downright *cold* even into June (July requires refrigeration as the humidity climbs from ~10% to ~60+%)

Reply to
Don Y

it is also called a buck-boost, because the output can be higher or lower than the input

-Lasse

Reply to
Lasse Langwadt Christensen

A panel doesn't put out a constant voltage. That is the point of MPPT. The current and voltage are controlled for maximum power which at some times may be higher than battery voltage and other times may be lower.

I would expect the voltage on the battery to adjust automatically. The MPPT circuit will set the duty cycle to get the highest power. If the output voltage is too low, the current will be too low for the MPPT and the circuit will adjust. Likewise if the output voltage goes up too much, the current will increase beyond the MPPT and the circuit will lower it.

The only trick will be to observe the limits on the battery current and voltage so it isn't damaged.

The topology is not a control loop, so of course it won't. That depends on the controller. One nice thing about a switcher is that it provides dither in the input voltage and current which allows the power to be measured over a range around the average point. This should be enough to let a controller keep it at MPP.

This sounds like an interesting circuit to explore.

--

Rick
Reply to
rickman

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