To mppt or not to mppt. That is the question.

[luxint]

Hi

From reading this article

http://www.wholesalesolar.com/Windy/MPPT-article.html

it seems like the author is implying that the main benefits of using an MPPT controller are best realized in climates that have cooler temperatures. I live in the tropics and it NEVER gets cold Is it worth me making the extra investment or would I be ok to just go with a normal PWM controller.

Also ultimately my set up will be 300w with 18amps. Would I need a 30a controller or would something closer to 18 be acceptable (assuming such a controller exists of course)? Can I stack controllers? So two 10a would give me a 20a controller? If that's possible how would I do it?

Thanks

Lux

[Sunking]

Basically the link you show, does not know squat about their products they sell. A MPPT charge controller is a true dc to dc voltage converter. MPPT controllers conversion efficiency is as high as 98 % vs 66 to 75 % for a shunt or PWM controller.

The biggest advantage of the MPPT Charge Controller has is it allows the operator to run the panel voltage much higher (up to 150 VDC) to gain even more efficiency.

As to the current question, you select the current closest to your Isc rating of the panels without being lower. So for example if your Isc = 18 amps, you would selct one of 18 amps or more. In this case 20 or 25 amps.

[Mike90250]

MPPT controllers down-convert the panel voltage, to the battery voltage. This accomplishes 2 things.

1) Higher array voltages have less wire loss, or copper expense, for same wattage.

2) Adds the temperature advantage of cooler panels produce slightly higher voltage, and therefor, more power in cold weather.

A third thing , could be, for equal systems, you can get a bit higher wattage, into a low battery with MPPT, than with PWM. As Battery voltage rises to normal full charge, the MPPT advantage fades, as it throttles back

[luxint]

Thank you both for your responses.

Mike you mentioned the 2, possibly 3, main advantages but I'm still left wondering whether in my particular situation the added expense is justified.

I won't have a large array (I assume you mean panels here) and won't be creating significant voltage (18 max). Since the distance is not too far from battery to panels I'm not too concerned about the cost of wire.

Point 2 seems to be validating what the guy in the article said. Correct me if I've misunderstood.

Not sure about your point 3 but it seems like it may be related to point 1 and only be of value if you're intending to have a large system.

Is there ever a circumstance where it's not actually necessary to go the MPPT route? A small system in a tropical climate with a relatively short distance between panels and batteries (30 ft)?

Thanks again

Lux

[Sunking]

Maybe a little math fun will help you out. Let’s consider 2 systems. One with a MPPT controller, and another with PWM or linear controller. Both will use a 18 volt 100 watt panel which produces 5.55 amps at peak power output.

System 1 uses a linear controller. With a linear controller the input current = output current. So 5.5 amps in and 5.5 amps out. This is the key difference you want to note. So assuming it is mid day sun your panel is producing 18 volts at 5.55 amps. At the controller we loose 1 volt on the wiring so the input to the controller is 17 volts at 5.55 amps or 93.5 watts input. On the output on the battery side the output is 13 volts at 5.55 amps, or 72 watts output to the battery. You just lost 28% of your power.

Ok same scenario using a MPPT controller. Solar panel is generating 18 volts at 5.55 amps or 100 watts. At the input the controller sees 17 volts @ 5.55 amps or the same as the PWM at 93.5 watts. Now comes the difference, the MPPT controller output is at the same 13 volts but the current is at 7 amps or 91 watts. With a MPPT controller you only loose 9 % compared to 28% using a linear controller.

Ok now another trick with MPPT we are not stuck to using a inefficient low voltage panels, we can step the voltage up to say 100 volts, so at 100 watts means 1 amp of current on the same length of wire, So now at the input of the MPPT controller running a much lower input current the input is now 98 watts, and the output is 95 watts. Only a 5% total loss compared to 9 and 28% loss.

Sure the temperature has a little to do with it, but insignificant compared to the heat loss in the controller. If that is the authors main selling point he missed the boat.

Now back to your question. MPPT is justified on larger systems. On a small system it is less costly to add a few more panel watts to get the desired production, than to spend the extra on a MPPT Controller. The balance point varies associated with the regional cost differences A good designer can tell you instantly which is the least expensive.

[luxint]

Thanks for that SK. Very educational and informative.

Now this were I show my true 'newbie' status and total lack of knowledge when it comes to batteries/voltage/amps

I follow your examples right up to the point where you're pumping 100 volts. The battery is 12v so won't pumping 100 volts into cause a slight problem. Forgive me if this is a dumb question but like my dear old mother always said "if you don't ask you don't find out".

How would this pumping of 100 volts occur? A different panel set up, parallel maybe to increase the volts? Is the benefit of increasing the volts so we can send it through a smaller wire? I have a feeling it is but just figured I'd ask for 100% clarity. Again if we're concerned about reducing the size of wire I guess we're talking about large installations producing major power and having to travel some distance from panel to battery? What size of installation and what kind of distances?

Thanks again and sorry for the dumb questions. You guys are very patient.

Lux

PS: Can you stack controllers? If so how? A simple case of daisy chaining or something more?

[Sunking]

Thanks for that SK. Very educational and informative.

You are welcome

I follow your examples right up to the point where you're pumping 100 volts. The battery is 12v so won't pumping 100 volts into cause a slight problem. Forgive me if this is a dumb question but like my dear old mother always said "if you don't ask you don't find out".

Well I am not going to go into great detail because it is not important that you understand.

A MPPT controller is a true DC-To-DC Switch mode converter. It takes the DC voltage from the panels, changes it to AC power, and then back to DC power. The magic or mystery to you is what happens in the transformer in the controller, where current and voltage a inversely proportional to each other. What this means as you step the voltage down using a transformer, you are stepping up the current by the same ratio. So to make the math simple lets say you input 100 volts a 1 amp (100 watts) on the primary of a 10:1 ratio voltage transformer, the secondary or output is 10 volts at 10 amps (100 watts). So we stepped down the voltage by a factor of 10, and stepped up the current by a factor of 10.

[luxint]

Thanks for taking the time to try to answer my question.

You're right it's probably not crucial I understand, I was just curious. You're response didn't really make it any clearer to me to be honest but never mind

Thanks again!

Lux

[Sunking]

Thanks for taking the time to try to answer my question.

You're right it's probably not crucial I understand, I was just curious. You're response didn't really make it any clearer to me to be honest but never mind

Thanks again!

Lux

Well here is what you need to know. A linear or PWM controller has an efficiency around 50 to 70 % meaning for every watt you put in, you only get out around .5 to .7 watt. With a MPPT controller you get about 95 to 98% percent efficiency out of it, .95 to .98 watt out of 1 watt.

[MarineLiner]

..... pumping 100 volts. The battery is 12v so won't pumping 100 volts into ...
How would this pumping of 100 volts occur?

[Mike90250]

[QUOTE=MarineLiner;8990]

[MarineLiner]

The Solar DC side, uses a high voltage PV array, and a MPPT controller downconverts the voltage to your battery voltage.

Then your inverter takes the battery voltage, and converts it to 120 or 240 VAC for regular household appliance use.

[e_techno]

Comparing the MPPT charger which converts 95% of solar panel input to output vs 70% for PWM charger, Can we replace a system using PWM charger of 100W panels to system with MPPT charger and 80W panel only?
Since 80Wx95% is > 100Wx70%

[Mike90250]

Comparing the MPPT charger which converts 95% of solar panel input to output vs 70% for PWM charger, Can we replace a system using PWM charger of 100W panels to system with MPPT charger and 80W panel only?
Since 80Wx95% is > 100Wx70%

Maybe.

The price difference in a 100W system leans heavily toward PWM, when you get to 300w or more, then the prices even out. But if you have limited space, go with a high voltage panel and MPPT. (a 60V 100W panel)