Alt-BEAM Archive

Message #01072



To: Justin jaf60@student.canterbury.ac.nz
From: Steven Bolt sbolt@xs4all.nl
Date: Sun, 28 Feb 1999 15:33:21 +0100 (CET)
Subject: [alt-beam] Re: Solar cell question


On Mon, 1 Mar 1999, Justin wrote:

> While we're discussing the finer points of solar cell use, I have a
> question.
>
> Imagine that I have a cell that gives out sufficient current for my
> task, but not sufficient voltage. Traditionally, you raise the voltage
> by adding extra cells in series. Are there drawbacks to doing this with
> much smaller cells?
> (Extreme example: I want 2.8V, I have a _large_ panel giving me 30mA -
> heaps of current - at 2.5V, and I raise it to 2.9V with the addition of
> a _tiny_ 1/8th" photo-diode solar cell (1.4mA) in series. What are the
> drawbacks in doing this? (I have my guestimates, but I'd like to have
> them confirmed or debunked :-)

Depending on load and conditions, you may waste the best of your
two panels. Here is some actual data.

Condition: indoor shade, away from window. Load: 22K resistor.

BP378234 2.9V 131uA 382uW (panel consists of 8 large cells)
BP243318 1.4V 64uA 89uW (panel consists of 5 small cells)
The above
in series 1.8V 82uA 147uW

Condition: indoor near window, shade. Load: 10K resistor.

BP378234 4.4V 440uA 1940uW
BP243318 2.5V 250uA 625uW
The above
in series 3.6V 360uA 1300uW

You'll have to look at your own design point load and conditions,
and the performance of the cells on and off panel to find out how
bad this idea actually is :)
Note that even if all cells are equal, spreading them over two
panels makes it more difficult to guarantee that all see the same
light - also a requirement for efficiency.

Best,

Steve

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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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