Re: [electricboats] Complex design issues

On solar cells in series vs parallel: If all things were equal, series strings would not be preferred, specifically because of the direct effect that shading a single cell in the string has to the output.  It's a huge price to pay---just a splat of seagull poop on one cell that drops its output to 40% will drop the entire string's maximum current output to 40% as I see it.

So, ideally, solar panels would parallel ALL of the cells to eliminate this huge penalty.  The reason panels are not made this way is because of the very low voltage (around 0.58v Voc) put out by a cell.  Boost converters just can't boost this low voltage up to 12v efficiently, so cells are put in series on panels.  And so some of the earliest panels put out maybe Voc~18v and today's panels, with 32-96 cells have Voc from 20v to around 60v.

Now, to reduce the shading penalty, solar panel manufacturers often (always?) include some number of Germanium bypass diodes such that that panel's output will drop in voltage but can still deliver an optimal high current.  It's been suggested to use panels with at least 3 such diodes.

But let's say you have panels on port and panels to the starboard.  It might make best sense to put the port panels in series, same with the starboard ones and then either parallel up the 2 strings and connect to the MPPT controller or send each string's output separately to a converter.

 

It helps to think about lithium battery configurations for EVs these days.  It used to be (when most EVs on the road were DIY---i.e. pre-2000) that no one recommending paralleling battery cells or batteries due to risks involved.  You'd see some folks paralleling up strings and others paralleling up 12v batteries, usually without incident.  Even with lithium based EV packs, we saw in 2010-11 cars being produced with 2 parallel strings of 96 cells.  And EACH of those cells in each string needed a cell voltage manager.  Tesla went a different direction.  They realized that if they massively paralleled the cells as much as possible, then they could avoid multiple series strings, multiple BMS, etc.  Sure, there's risk that a single or leaky shorted cell will cause major issues, but they took that risk.

Back to solar cells---paralleling cells does not bring any real risk.  Each cell itself is actually made up of a lot of silicon in parallel.  For a boat, with possibility of mast, humans and other objects obscuring part or all of several cells, it seems that the best solution would be massive parallelization of cells.  And yet, at some point you need that pack voltage and so ultimately cells need to be put in series.

 

-mt