PWM or MPPT controller?

Dickodownunder · Guru

RE: PWM or MPPT controller?

T1 Terry · Guru

Dickodownunder wrote:
Thank you T1 Terry,

While some of the technical details goes over my head and probably others, you alway provide an understandable and interesting view.

Cheers

The most valuable information in regards to solar panels is the stuff they try not to tell you. The stuff displayed on the back of the panel is of very little value these days, but at least the Vmp (the voltage the panel makes the most power or watts and this matches the highest voltage reached before the amps start to reduce) allows you to compare one panel against another under standard test conditions (STC). The problem is that STC figures do not represent the true panel outputs as it achieved by mounting the panel over a light box and flashing the light on for a few seconds, the readings are taken while the light is on. One of the major errors in taking these figures as a useable comparison is the effect heat has on the voltage and amps, the Vmp drops and in some panels the amps also drop, in other panels it goes up.
If you pester the panel manufacturer enough they will give you the irradiance and temperature curves or the -% per degree over 25degrees C
These curves are from Tindo the South Australian solar manufacturer

Tindo Irradiance chart 2.jpg

These Charts are from another well known manufacturer Kyocera

Kyocera 140w panel compressed.jpg

Hope these come out in the post.

Even though they are different sized panels, the Tindo (270w) is roughly double the output and voltage of the Kyocera (140w) the drop in Vmp is clear as is the difference in the number of volts the drop is between 35*C and 50*C. You might also note Tindo give the 0*C voltage, very important when calculating the max open circuit voltage when connecting panels in series (voltage adds together but amps remain the same) as many MPPT controllers will let the smoke out if the voltage goes too high, others simply shut down till the problem is corrected.

Something that is also interesting, both manufacturers provide a measurement for 70*C, on another forum some argued a panel would never get that hot ..... if that was the case why would a manufacturer provide a test graph? Be assured, they can exceed that temp in full sun in the mid of summer

T1 Terry

dorian · Guru

If a panel's Vmp is chosen to suit the battery voltage, under what conditions, if any, would an MPPT controller require boost mode? AFAICT, even allowing for a drop in voltage due to temperature, a properly selected controller should never need to boost the panel voltage.

Aus-Kiwi · Guru

Again the difference DEPENDS on the quiality of of each brand . Not just type or construction . Sum thymes itâs tha simple tings that matttter !!

dorian · Guru

I'm looking at the I-V characteristic curves provided by T1 Terry. The MPP at 1000W/m2 occurs at a voltage of 29V, while at 200W/m2 it occurs at 31V. That's essentially flat. However, the MPP range from 0C to 70C appears to be 31V - 23V. Maybe there's some scope for boosting at extreme temperatures?

One other question is, what happens to the irradiant energy during a no-load condition? Wikipedia models a solar cell as a current source in parallel with a diode. AFAICT, when there is no external load, the current source would be heating the diode.

BTW, the following tutorial models the PV panel as a Thevenin voltage source:

http://www.electronics-tutorials.ws/diode/bypass-diodes.html

Surely this is wrong?

Jaahn · Guru

dorian wrote:
If a panel's Vmp is chosen to suit the battery voltage, under what conditions, if any, would an MPPT controller require boost mode? AFAICT, even allowing for a drop in voltage due to temperature, a properly selected controller should never need to boost the panel voltage.

Hi Dorian smile

Yes boosting the voltage is not normally required or done IMHO in solar reg/controllers. Except DC--DC chargers perhaps. The MPPT boost is to the current.

Looking at the graphs Terry put up. The second one shows that at lower temperatures there is more voltage available than required for charging a matched battery, and that excess voltage drops as the temperature rises. However the current available does not drop much as the temperature rises until the end. As the available power is the combination of voltage and current, if the booster can combine them to output a greater current at the minimum voltage required for the battery, then at lower temperatures there is more current available. That is the point of MPPT in one function, the other is to track that maximum power point as it varies due to clouds, angle, temperature etc and do the best it can.

Of course there are different conditions if the panel voltage is not matched to the battery but higher voltage panel or series panels are used. Then the MPPT unit can work even at lower voltages than normal, low solar output or high temperatures, to produce some current for charging a battery. EG if those panels shown on the graphs were charging a 12V battery. I add this for people who are trying to get their head around the MPPT mode of action. I realise others already know it.

Jaahn

oldtrack123 · Guru

Hi

I would suggest that a buck/ boost MPPT would be most useful in locations where either/both panel temp & light intensity could vary greatly throughout the year , such as nearer the poles or high altitudes or extreme weather conditions.

i.e where panel output voltage could vary greatly.

It's function would automatically change from buck to boost

The charge current going into the battery is determined by the charging volts

MPPTs tracking function basically converts the available power ,at any point in time , to match the battery apparent impedance @ that SOC,

They would work even in Australia especially with panels'whose MPP was only just above that required to charge the battery but the cost would need to be considered .

One of the problems with panels whose MPP volts is on the low side compare to the battery, is that they must cease charging earlier than panels with a higher MPP when using a PWM reg. even more so with a standard MPPT but a boost MPPT could still

charge

All horses for courses biggrin

-- Edited by oldtrack123 on Saturday 9th of December 2017 12:54:24 PM

T1 Terry · Guru

dorian wrote:
If a panel's Vmp is chosen to suit the battery voltage, under what conditions, if any, would an MPPT controller require boost mode? AFAICT, even allowing for a drop in voltage due to temperature, a properly selected controller should never need to boost the panel voltage.

The catch is the closer the panels Vmp to the batteries max required voltage the more efficient the whole PWM control becomes, if the panel temp increases to the point the Vmp drops so far below the max required battery voltage the current would be minimal then a voltage boosting device would come into its own. The device could pull the panel output voltage slightly below the Vmp allowing it to harvest the full current flow possible and then boost the voltage to suit the battery charging stage, thus ensuring max current at the required voltage. Once the Vmp again matched the battery voltage due to panel cooling or the battery moving to a charging stage that did not require a voltage higher than the Vmp, then the controller could revert to the more efficient PWM control mode.

Could such a smart device be sold as being better than the current MPPT devices that have more hype and rather questionable outcomes than a political campaign? Sadly, probably not, the spin regarding the current technology MPPT controllers has already saturated the market place to the extent no one wants to accept they could have been mislead by it.

T1 Terry

T1 Terry · Guru

Something I missed mentioning regarding the 2 charts and the Vmp temperature effect, the Tindo panels are polycrystalline where the Kyocera are monocrystalline, from these 2 examples it appears polycrystalline better suits Aust conditions and this is a point the engineers from Tindo like to stress, they chose a construction type better suited to the extreme Australian environment.

Something that is also worth looking at is the difference in the panel outputs when exposed to the sun rather than a light box. Sadly engineers that design these things get tied to results obtained during STC testing rather that actually getting outside the lab and seeing if the STC results match real world outputs. They end up building panels that get the best STC figures regardless of the real world outputs and then other engineers build devices that produce the max gains based on the STC figures.
The speed of change is certainly faster these days than back when large field data collecting and analysing was the bases for testing if the results were positive or negative, sadly these changes based on lab tests don't always result in improved field data results..... but that method is now out dated and rather costly so it is never used to back up the lab finding ........ maybe they actually don't want to know for what ever reasons

T1 Terry

AdoraKalb · Newbie

Hi...i am a new user here. In my case a cheap PWM it did not work from day one ,the manufacturers sent an updated reg am mppt ,but after reading tons of reviews and doing a lot of research I decided to go with a good brand from Victron, putting them side by side the el cheapo weighs nothing ,the Victron is a lot heavier in weight it has a 20amp fuse on the outside of the unit for easy replacement if needed.

Ron-D · Guru

AdoraKalb wrote:
Hi...i am a new user here. In my case a cheap PWM it did not work from day one ,the manufacturers sent an updated reg am mppt ,but after reading tons of reviews and doing a lot of research I decided to go with a good brand from Victron, putting them side by side the el cheapo weighs nothing ,the Victron is a lot heavier in weight it has a 20amp fuse on the outside of the unit for easy replacement if needed.

You cant go wrong with it I have the same one mounted near the battery its perfect ....