Fundamentals of mobile, indoor energy harvesting

This is a quick list of essential facts, without links or references.  I learned these working on my application: solar powered Calder mobiles (trademarked Solabiles.)

Energy harvesting:

  • not connected to the power grid
  • years, or forever, between any battery changes
  • harvests ambient or waste energy
  • harvests light, heat, or vibration

Light has the most energy, for mobile or ‘anywhere’ harvesting.  Only constrained points, such as on a hot surface (a body?) or on a vibrating motor, might have more harvestable energy.

Energy harvested is proportional to the light level.

Light levels indoors are exponentially smaller than outdoors.  Roughly:

  • full sun: 100k lux
  • shade but with full sky exposure: 10k lux
  • bright indoor: 1k lux
  • average home lighting: 100 lux

The lighting within a room can vary exponentially:

  • near a south facing window, overcast winter day: 1k lux
  • other side of the room: <100 lux

Lighting in a room can depend on the direction windows face:

  • near a south facing window, overcast winter day: 1k lux
  • near a north facing window, overcast winter day: 100 lux
  • other side of the north room: 10 lux

The human eye and brain hides the above facts.  It doesn’t seem that the lighting is so different in a room, but a luxometer tells the story.

Solar cells produce energy proportional to their area.

Solar cell technology probably is not going to get much better soon.  They might double in efficiency, but they probably won’t reach 100% efficiency.  You must make improvements elsewhere.

At low light levels (indoors), amorphous silicon solar cells produce more energy than crystalline silicon solar cells.

Solar cells and panels produce the most power at their maximum power point (see MPPT.)  In other words, when they ‘see into a load’ of the voltage of their MPP.  (I don’t really understand why, and the best water analogy I can come up with is:  a very thin sheet of water is a lot of water but has little pressure, and can’t do much useful work.)

You don’t necessarily need an “energy harvesting” chip.  They provide:

  • voltage boosting
  • MPPT
  • battery management (using a battery might bring its own set of problems to a design.)

If you don’t need either of those, you do your own MPPT: insure the voltage the solar cell sees remains near the MPP (whenever conditions allow.)

Ordinary capacitors, supercapacitors, and batteries use different technologies that each bring their own design problems.

At very low light levels, say <50 lux, the leakage of circuits is important to a design.  See my next post.

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