MPPT for Industry

System

Hi, I am doing research on Photovoltaic maximum power point tracking. Dose anyone know what kind of technique for maximum power point tracking is the most widely used in industry and how dose it deal with shading effect?

Thank you.

BB.

Re: MPPT for Industry

Here is a thread that contains a Technical Paper on MPPT posted by Solar Guppy. A good place to start (ignore the English Teachers' comments in the thread :roll: ).

-Bill

RCinFLA

Re: MPPT for Industry

The most significant factor in MPPT is the temperature of solar cell.

A pretty good max power panel voltage can be predicted solely by knowing the temperature and temperature coefficient of cells used in the panel. For a given panel at a given temperature there is little variation in the optimum loading voltage on the panel over illumination. There will of course be a temp drop as sunlight drops on the panel and wind cooling can also be a factor on temp but these effects have a relatively long time constant compared to illumination current rate of change due to clouds going by. Worst case temp change may be a 'sun shower' which we get quite often in Florida.

The 'special sauce' between MPPT controllers is how quickly they adapt to changing conditions such as clouds rolling by. There is some efficiency loss in the process of determining a new MPPT point. There is a bit on 'hunt and peck' to the process but using the knowledge that temp to MPPT voltage has a long time constant can be used to reduce error in MPPT tracking changes.

Running an extra set of sensor wires to a temp sensor on the panel would be undesireable. Also temp coefficients will be different between mono-crystaline and poly-crystaline panels. Because there are infinite variation possibilities in poly-crystaline mixes there can be a wide variety of slightly different temp coefficients.

The temp to MPPT voltage relationship still remains and can be used to speed readjustments. The first order of business for the MPPT switcher when the cloud goes by and illumination current drops off is to quickly back off to keep the panel voltage variation small. Fine tuning can then be done at a slower rate.

The shading problem can only be solved with more bypassing diodes within the panel to reduce effect of one or more cells losing illumination. A bypass diode could be integrated into a cell but at about 35 mA/cm^2 for the illumination current a sizable diode would be needed to handle the current from the series string of 150 or 200 mm cells. Heat dissipation also becomes a significant problem to an integrated bypass diode. Present panel construction does not provide for this kind of heat sinking. A 150 mm cell absorbs about 15 watts of heat at a full sun illumination but this is spread over the whole cell surface with contact to front panel glass. An integrated bypass diode would be a point hot spot. The bypass diode would add extra steps to the cell processing that would raise its cost.

If you have a shading problem location, the only option you have is to use a bunch of low voltage (18v) panels with bypassing diode on each and a stack total voltage that can stand taking a panel or two of the stack out of service. If you have 8-10 amps of illumination current the diodes have to dissipate 5 watts which requires a reasonable sized heat sink.