14th December 2011 09:23 AM
Constant Voltage Solar MPPT
I'm trying to do a project on Solar Maximum Power Point Tracking as part of my final semester. I did some basic reading on the types of MPPT and found an interesting type called Constant Voltage MPPT.
It says that the problem associated with this type is that there is a frequent need to disconnect the circuit in order to measure the output voltage and then set the optimum voltage to 0.76 times the actual voltage of the Solar Array.
My question is if i use a timer circuit or a microcontroller with a power semiconductor device like an IGBT or a MOSFET and set the duty cycle to 0.76, will it suffice? And if yes wouldn't this eliminate the need for opening the circuit for measuring?
1st January 2012 04:05 PM
No, because of variable illumination current, Rs of cells and switcher circuitry, the switcher duty cycle must adjust to account for these changes. The 0.76 has nothing to do with switcher duty cycle.
The switcher regulates based on input voltage sense instead of conventional switcher output voltage sense.
The "0.76" factor between Voc and Vmp depends on series resistance of cells and their illumination current output. I would say 0.76 is too low. At 25 degs C Vmp is about 0.52v where Voc is about 0.66 vdc. Mono-crystaline cells have lower Rs and the factor would be more like 0.81. Poly-crystaline would be more like 0.79.
You want to avoid going too high in voltage where inherent cell diode conduction goes up exponentially so the 0.76 is likely to ensure you always stay down on the more constant current region. You will loose some available power by doing this over a more efficient MPPT algorythm.
If charging a battery the simplest and most effective MPPT algorythm is to just maximize the battery charging current.
Last edited by RCinFLA; 2nd January 2012 at 12:25 AM.
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