Diy mppt?

[Xringer]

I've been looking into how I can get more PV watts into my 10 Ohm hot-water heater.

I've been looking at this pump controller.

Here: Build this simple Mini Maximiser

And thinking about the 'Knee' point (sweet spot) of a PV array.


This particular PV array seems to have a (1300w) sweet spot around 152 volts..
As you can see, the Open voltage (no load) is up around 200v.

Anyways, if you made a voltage divider similar to the Mini-Max (above),
fed the voltage to the comparator & FET, with your reference
pot pre-set to trigger the FET into conduction right at 152 volts (or higher)..

It seems like the load would only see 152v (or higher).
Once the RC time constant ran down a tad, the FET would turn off, and
the PV would take a little time to recharge the input cap, before the PV
voltage was high enough to trigger the FET again.

In weak sun, the off-on rate would slow down, as the PV struggled to keep up, charging up the input cap more slowly.

But, in bright sun, the off-on rate would zoom up and try to keep a continuous 152 volts on the load..

But, either way, while the FET was on, the load would see the highest voltage : power available from the PV.
Thus, making the fixed load, dissipate a maximum amount of watts.

Each time the FET turns on, it can hit the load with about 1300 watts.
If only for a short time.. But, the Flywheel Effect might help here.

By allowing only higher voltages to pass power on the feed lines,
allows for less line loss. (Especially with the small wire I'm using).


If I could use this type of circuit on my 10-Ohm hot-water heater,
zapping the load with full power hits, I'm sure it would put more heat into the water..


~~
Today, my 500w array shows 65.1v under load & 78.9v open (no load).
At the load, (after 75 feet of wire) the voltage is 63.3 (lost 1.8v ~ 11.4w).

63.3 volts (6.33 Amps) on the 10 ohm load dissipates about 401 watts.
With the line loss, 412 watts isn't too bad, but this is a peak reading.

The passing clouds allow the 10 ohm load to pull down the PV voltage,
making the line losses increase and the fixed load, seeing lower voltage,
can't draw as many amps.


I can't help but wonder if I could get closer to 500 watts, if the PV was
running at around 67-71 volts..?.

I have not worked with FETs for years, I hear they have some real good ones now.
What do you guys think? Good DIY, or start shopping for a pre-made?

Cheers,
Rich

[Daox]

I looked into diy mppt a while back. Someone posted a link here to a chip that does it. It is over my head on how to use it, but I remember it wasn't cheap just to buy that chip. If I recall correctly the chip was ~$25. I think the cheapest MPPT controllers I've seen are at least $70 though.

[NiHaoMike]

What about using a PIC or other microcontroller? Use a voltage divider to measure the voltage and hardware PWM to control the MOSFET.

[Xringer]

I wondered if there was a MPPT chip. Like the specialized PWM chips used in power supplies.

Using a micro controller adds a layer of complexity that I would like to avoid.

I kinda like that little pump controller (above), since it's very effective
and doesn't really cost a lot, in parts or time.

What would be the draw-backs to using that kind of circuit?

Or, maybe I could design a little 'watt meter' on the output, that would feed-back
an analog signal to the input, that would change the comparator reference voltage
to a setting where the most watts were present on the output.?.
Real-time active feed-back to keep the panels producing the
maximum amount of power under various lighting conditions.?.

Might make a more interesting project..

[Xringer]

I found a little dump-load / charge controller on Ebay.
UNIVERSAL RELAY DIVERSION CHARGE CONTROLLER REGULATOR - eBay (item 130409810996 end time Jul-20-10 04:14:47 PDT)

It seems to be made to monitor a 12V battery (tied to PV or wind) and if the battery
gets up to the 14.3v 'Charged' state, User Settable Dump/Diversion/On-Off Voltage Set-point (13.1 to 15.1 Volts)
It will turn on a load relay or SSR to discharge the battery a little.

See Solar hookup diagram here:
http://www.windandsunpower.com/Downl...RDC_Manual.pdf

Anyways, I was brainstorming and came up with the idea that I could
put four or five 12V batteries in Series. To be fed by my 65-70 volt PV..
70v of PV / 5 batts = 14volts each..

AND, the little Ebay board could monitor just one of those series batteries!
But, when it switched on a fat SSR, it would be connecting not just 12v,
but the whole series string of batteries to the load!

The board could be adjusted to trigger at 13.5 (67.5v of PV w/ 5 batteries),
which should be near the sweet spot and maybe give a quick 455w jolt to the 10 ohm load.

And then, I snapped out of it and said, 'I can't afford 5 batteries right now'!


Then, I started thinking about 5 big capacitors in series!!
If they were matched, wouldn't they act like a voltage divider-battery string??

Then, I said.. Do I really need 5 caps? Wouldn't one big cap work?
If I put a pot across it? With the output tap set to 13.5v, when the cap
was at 67.5v?

I'm liking this idea, because caps will charge up Faster than batts..
And, the SSR will be cycling faster, delivering the jolts of watts/BTUs,
as fast as the PV can supply them. (Depending on the clouds)..

So, what do you guys think of my crazy ideas?

Cheers,
Rich

[NiHaoMike]

You could use a comparator as a MPPT controller. Find the voltage at which you get maximum power output and have a comparator switch the MOSFET based on hysteresis. Main disadvantage is that you'll have to retune the circuit for temperature and aging.

[Xringer]

Maybe the IRFP150N driven by an LM339N ?

I was just looking at the pump circuit,


And, I wonder if you added an LDR (photocell) across that top left resistor (3k3),
Then exposed it to bright sun (beside the PV) while setting the set-point
trim-pot for maximum power to the load (with good sun).
(The 3k3 resistors might need to be changed to other vaules, to compensate for the parallel LDR).



My theory is, when some clouds appear, the resistance of the LDR would
increase, which would lower the set-point voltage and keep the circuit pulsing..
(Otherwise, those clouds could turn the gizmo off)..

Heck with some careful tweaking, this could be a close approximation of actual MPPT!!

Comments please:

Thanks,
Rich

[Xringer]

I'm thinking maybe 300 uF on the input to get a faster charge.. Like 2 150uf caps..

2 X 150uF 450V Radial Electrolytic Capacitor 22*41 - eBay (item 330447531609 end time Jul-30-10 05:51:15 PDT)
2 X 150uF 450V Radial Electrolytic Capacitor

Also thinking about these..
IRFP150N, 100V, 42A, N-Channel, Power MOSFET, Qty 10 - eBay (item 390206596636 end time Aug-07-10 11:27:09 PDT)
IRFP150N, 100V, 42A, N-Channel, Power MOSFET

And these too!
10 x LM339N LM339 IC LOW POWER QUAD VOLTAGE COMPARATORS - eBay (item 320521805666 end time Jul-22-10 00:15:53 PDT)


Of course, I've got to build this thing to work with 40 to 80 volts from the PV.
I'm thinking the logic parts should have their own little regulated power supply.

~~~

Maybe the pulsed output of the pump circuit would be suitable to drive
a Buck circuit, to boost the voltage up a bit higher.



A higher voltage (without much loss in power), could really help with the line loss.
And, higher voltage into the 10 ohm load, means it's going to work better..
Since it will draw more current and get hotter..

[Solar Mike]

What would happen if you placed your panels in series, get an old 500 watt computer power supply, hack it to remove the AC input section and fed the high voltage DC from your panels into the inverter high voltage DC input. The 12 volt DC output would be fairly constant into a fixed load for varying panel voltages. The psu has all the switching components. may be easy to replace (re-wind) the output windings from the low voltage outputs to something more suitable.

Cheers
Mike

[Xringer]

The panels are already in series. Almost 80 volts w/o a load. About 63 volts with a load.
I've got a fixed 10 ohm load (water heater) and I would like to hit it with as much voltage as possible (within reason).
And, the least amount of conversions done, is likely to be better.

IIRC, the voltage coming off the full-wave bridge of a PC PS is pretty high,
in 230 mode, they turn off the voltage doubling of the bridge.

Here you go.. Found it again.
200W ATX PC POWER SUPPLY
"C5 and C6 together for about 300V"

They had a very similar PS in the NEC 5500 series printers.
I was pointing out the unsafe location of the live terminals
to the printer engineer, telling him that the off-on switch
wasn't in the AC primary, leaving the exposed terminals hot,
when he reached in, and said "these aren't hot" and took hold of 300vdc.

That was a shocking experience, to say the least..