MPPT for 60W modules

Romes1

Dear community,

I am trying to measure the maximum power point of two 60 Watt panels ( max. voltage: 16.9 V, max. current: 3.7 A) for a duration of 6 hours for an experiment.

My problem is, that the commercial MPPT inversors that are available do not work with this low voltage. So I thought I would just try a small MPPT charge module (for example one that is based on the LT3652 chip). However, these small ready-made modules are made to be connected directly to a battery.

Would such a module still draw the maximum power from the solar panel after the battery is completely charged?

Or could I even just connect a load to the MPPT module, either directly without battery or in parallel to the battery to prevent the battery to fully charge during the 6 hour cycle?

Kind regards,

Romes1

BB.

Welcome to the forum Romes1,

To answer your questions as asked...

You are correct, you need at least 2 volts Vpanel-input higher than Vbatt-output for "MPPT" to function for a typical solar charge controller.

The standard solution is to put the two 60 Watt panels in series for Vmp~33.8 volts and Imp-array 1.85 amps (if I guessed correctly for your solar panel specs). That would take care of the "low Vmp" issue with an MPPT controller (rated for >45 volts Voc or so).

You are correct in that most solar charge controller are of the "series regulator" type. When the battery approaches 100% charge (however the controller defines that), the charge controller will hold (for example) 14.75 volts and the battery (assuming some sort of Lead Acid battery) will slowly accept less and less charging current. At 1% rate of charge (50 Amp*Hour battery, 1% is ~0.5 amps) is near 100% full and the charge controllers will typically fall back to "float charging" (around 13.6 to 13.8 volts Float) to keep the battery fully charged and avoid damaging the battery--And will support any DC loads on the battery bank while the sun is shining.

Yes, you could discharge the battery bank--Typically with a "dump controller" of some sort... As the battery reaches 14.75 volts, the "dump controller" turns on and (usually) is connected to a resistor bank to Burn Off excess charging current. Usually these dump controllers are PWM type (pulse width modulation) and will keep the average battery voltage around 14.4 to 14.8 volts.

There are various "active" electronic loads out there that could be configured to keep your battery voltage at a (relatively) fixed amount.

Now comes my question. Why? Are you trying to see if the panels are good, or are you trying to see how many Hours of Sun you receive in a day? Or what?

-Bill

Romes1

Dear Bill,

Thanks a lot for your quick and thorough response and for welcoming me to the forum in such a kind way!

To give some more context about the project: I am building a cooling system for these 60W panels as part of a small research project. My goal is to then measure the maximum power output of one panel with the cooling system and one panel without the cooling system.

So the objective is just to pull the maximum power out of the panels for 6 hours straight and during that time measure the panels' output voltage and current with seperate sensors to calculate the power output of each panel. This way I want to measure the effect the cooling system has on the PV performance.

I would not be able to put them in series for that reason and have to do MPPT with each panel separately. Using a "dump controller" or "active" load is a good point to avoid that the battery charges up to 100%, I will look into that, thanks!

Just two more questions to understand this correctly:

- If I connect a small MPPT charge controller to a solar panel without a battery/load connected to the charge controller, will the panel still operate in maximum power conditions (so that I could measure the max. output voltage & current of the panel)?

- And if a battery connected to the MPPT charge controller would be fully charged, would the power from the panel still flow to the controller and dissipate somehow or would the panel just have zero power output in that case?

Kind regards and all the best,

Romes1

BB.

Genesun is the only company I am aware of (there may be others) that does a boost type MPPT solar charge controller. Designed for smaller systems, but the controllers are not cheap:

https://genasun.com/

Technically, if you could find a good MPPT controller + load/battery setup, you could put the two panels in series. Measure the voltage across each panel (cool panel will have higher Vmp voltage--And each panel will have the same current Imp--More or less Imp rises a slight bit as panels get hot).

Take the output of the data logger and do some math: 

• Eone = Vone * Iseries * Time (fraction of an hour
• Etwo = Vtwo * Iseries * Time

Some each (for example) 5 minute Watt*Hour segment up to 6 hours total. Not pretty--But a good first pass result (assumes Vmp changes with temperature and Imp is fixed with respect to temperature--In real live, Vmp charges around 5-10x (falls with rising temperature) vs Imp (rises slightly with increased temperature):

Some solar panel Current/Voltage graphs:

https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

My suggestion is to do some modeling in a spread sheet first. For example here is a website that you can output to a spreadsheet the power from a solar array in one hour steps. You can start here and simply derate by a fixed amount (a very hot panel is ~81% of the Vmp of a "cool panel" @ 77F/25C).

https://pvwatts.nrel.gov/

Or you can measure the temperature profile of your solar panels (measure actual temperatures, or measure Voc--Which also drops as cell temperature rises).

VOC Temperature Coefficient C° (Default is -0.33%)
VMP Temperature Coefficient C° (Default is -0.45%)

Essentially (using big round numbers) to get roughly 1.25 more power (Vmp) from your arrray, you have to cool it to at least ~25C (assuming ~40C rise on hot, still air day).

So your plumbing, water, water pump, solar panel cooking jacket (unless you choose to spray water on panel--Different issues with possible hard water deposits, corrosion, thermal cycling) is "worth" a 25% increase in "real world" increase in harvest (vs 80% loss with Hot Panels). 

There is also the cost of running a MPPT type charge controller vs "forgetting" MPPT and using a much cheaper/simpler PWM controller.

There have been folks that have tried water cooling of solar panels (including companies)--But the issues of plumbing, pumping, water, and cooling the water--Seem to make it unprofitable. Vs just buying a 25% larger solar panel/array (and for smaller systems, a PWM type charge controller)....

There is (was?) a company that mixed solar PV and Thermal collectors on one roof/installation (video from ~10 years ago):

https://www.sundrumsolar.com/products-services.html

A 12 page thread on another forum... Lots of ideas, few (if any?) were cost effective (I did not read whole thread--On a slow Internet link):

https://www.solarpaneltalk.com/forum/solar/solar-energy-facts/18035-water-cooled-solar-panels-for-significant-output-boost

-Bill

Photowhit

Romes1 said:

- And if a battery connected to the MPPT charge controller would be fully charged, would the power from the panel still flow to the controller and dissipate somehow or would the panel just have zero power output in that case?

I suspect Bill covered it and I just missed it, Solar panels don't 'have to' make power. Unlike wind generators, the charge controller can just shut them off.

In reality they maintain a trace amount of current going to the battery and 'watch' the voltage. This is called the 'float' stage. If the voltage drops it will increase the current from the solar array.

Here are the basics of charging a lead acid battery bank;

The voltage you are seeing is the system voltage and not the battery voltage. If you are connected to charging or a load it will effect the system voltage.

During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

BULK;

First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

ABSORB;

Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.

On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries. 

The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

FLOAT;

Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

The system voltage drop you see at night when the sun goes down is the charge controller moving into a resting mode with no energy to contribute to the system.

NANOcontrol

Regardless of what you read, that chip you mentioned is not MPPT.  It is a fixed power point controller. It does no I V calculation. Pseudo tracking can be made by obtained by adding an external temperature sensor.  What accuracy are you seeking?  A control keeping the panel at a fixed voltage is quite close and give reasonable evaluation of local conditions.   The most accurate test is a momentary scan as shown below.  If you have microprocessor experience this is easy to do by just calculating I and V to find the peak. Data can be collected all day with this method. Red is power, blue is current and yellow is voltage.