Computing stuff tied to the physical world

Solar fingernails

In Hardware on Feb 11, 2013 at 00:01

A couple of weeks ago, I described a little indoor solar energy test setup, based on a couple of fingernail-sized CPC1824 solar cell chips, with 0.47 Farad supercaps for storage:


It has been an interesting setup to keep running for a while, and now that 20 days have passed, it’s sort of intriguing to see how well, or badly, these solar energy cells performed. To summarise, here is what the four different test were about:

  1. One CPC1824, very lightly loaded with 20 MΩ, i.e. a 0.15..0.20 µA current draw
  2. One CPC1824, lightly loaded with 2 MΩ, i.e. a 1.5..2.0 µA current draw
  3. One CPC1824, moderately loaded with 0.2 MΩ, i.e. a 15..20 µA current draw
  4. Two CPC1824’s in parallel, moderate load with 0.2 MΩ, again 15..20 µA current draw

In addition, there is a top-up voltage cycling once per ≈ 2 hours, to prevent any the supercaps from discharging much further than about 2.7V. This is not a realistic situation if this setup were to be used for real, but I wanted to prevent a total discharge of the supercaps, to try and find out whether the supercaps would get better (i.e. less leaky) over time, as the constant charge conditions those membranes full of useable electrons.

Here are the results of 20 days, superimposed one upon the other:


(click two times to see it in full scale – X axis is in minutes since midnight UTC)

  • #1 = blue, #2 = bright green, #3 = orange (dotted), #4 = red (longer dots)
  • the last day is marked in black for all four cases
  • around noon, all cells fall into the shadow, so energy generation almost stops
  • note how only the blue lines stay above 3.0V (except in the first 2..3 days)

This was a wintery mix of sunny and cloudy days, which has a huge effect on the charge / discharge curves. Looks like indoor solar with these tiny cells is not going to work…

One conclusion is that only the lightest load of 0.15..0.20 µA allows the supercap to retain a decent amount of charge through each night. On the other end of the range: with two solar cells in parallel, a 15..20 µA load might be able to stay alive for a few hours each day.

Ehm, well, just maybe – this will really depend on the startup power consumption curves!