Energy harvesting from solar panel

Hi all! I'm doing some work about solar panels, power point tracking, batteries charging etc. Let's say - solar energy harvesting system . I do not understand load maching principle with PWM and buck converter...How buck changes the load depending on PWM ?

 

Yes, you right here. PWM and buck, maybe buck-boost changes the voltage. Maybe can you give simple example with numbers? I mean... PV output for example 6V 300mA, load is 50Ohm. How should dc-dc converter convert voltage for better power ?

 

I don't have any circuit yet. I'm collecting info and then will choose components, write program, make pcb etc.. Like in picture, in my final project i want to build as simple as can autonomous wireless sensor transmitter (black squared), and wired receiver for checking what i get. Low power consumption of course. So now i'm only wondering how to regulate power point with dc-dc converter.

 

Hi steave . At first - big thanks to you for the answer ! Yes, i understand voltage vs current curves for a solar panel. I think I not everythink understand about the buck converter. You say output power is equal as input power for dc dc? So..if solar output Uoc=7V, Isc=0.33A , maximum power is about ~1,28W when the matched load is ~30Ohm. If solar intensivity changes lets say 6V 0,25A, now 30Ohm load is not matched. Power P=U*I, U=P/I , I=P/U . If dc-dc buck is lowering voltage using pwm , current is growing. And thats the way how is optimized new current ant voltage levels for the load ?

 

Due to poor my english language i can't explain everything as i understand. And you may think, that i do not understand, like you did about mine V vs I curves understanding. Voc is in the right of x axis, where I=0, and Isc is on the top of y axis. That is the current when circuit is shorted, and voltage is equal to zero. The numbers i wrote (7V, 0,33A are not real, only examples). Look at the attached pictures. Made some calculations and graphs in matlab with real numbers. It is right now, when I use real calculation not random numbers from my head ? Or i do not know something more about V I curve ? Thanks !

 

After lot of reading I realize, that is better buy microchip with integrated mppt algorithm and dc-dc converter. Because if i want to get low power consumtion, it is better to have integrated solution, like cymbet cbc915 . If i will try to make my own dc-dc and mppt , lets say, with low power atmega 328, in final result i will have bigger energy consumption. It is better to use atmega328 in deep sleep mode, and wake up only for sensors data transmitting.

 

Now i'm think that I understand solar cell curves and output characteristics. If load is too high, output is near Voc, if very low, then current is near Isc. My earlier post about IV curves was tottaly wrong.. . Buck converter, as i understand now controls output current, and output characteristics are V_out=V_in / Duty, I_out=I_in*Duty .Know I'm reading about MPPT now, and i'm thinkink : do MPP tracking needed only for solar panels, because they have varying impedance depending on light, or MPPT is needed for all energy harvesting methods such as thermo, wind, piezo etc. ?

 

Nice ! Example for understanding : So i connected MCU as a solar panel load. MCU wake up from sleep and measure sensors values, the current consumption will increase, and load to solar panel increase, but load resistance decrease assuming R=U/I?

So, if there are DC-DC buck between solar cell and mcu, what it should do ? It should increase current equal to load consumed current at that moment? But then voltage drops?

So it can be used for every energy harvester ? Because pjezo ar TEG's are constant output impedance, MPP will be at 50% from Voc and Isc ?

 

There is storm in my head ! Step by step something is coming (i hope it will be understanding... ).. Tried to do some calculations. Please look at attached file and comment or this time i'm thinking correctly.

 

No, i think that 4.4W will be in perfect conditions, when irradiance is very high, and load is matched ... In my example, increased load is show as work point changes to the left (current increases, load resistance decreases)..

After you answer now i'm thinking...I always thought that MPPT matches load RESISTANCE. But is not ?

I collect information about energy harvesting, but i write everything in Lithuanian language...

One of better source about MPPT i found here http://www.solar-electric.com/mppt-solar-charge-controllers.html

 

In that site about MPPT are example with battery. So if solar panel gives 5V and 0.2amps and i connected 3.6V battery straight, then solar panel voltage drops to the battery level and power is P=3.6*0.2=0.72W. 5-3.6=1.4V is wasted. So if between solar panel and battery is connected buck converter with 72% duty cycle, than output will be 3.6V and 0,277 Amps, and we have power equal to 0,927W. Very difficult to understand when my english skills are not very good, and there almost no litherature in my language..