Energy harvesting seems to be the rage these days. All kinds of devices are being
used to capture a tiny bit of power from ambient light, heat, vibrations, sound and
stray RF. I often wondered if the beta type radiation of tritium gas could be used
as an energy source, when no other form of energy can be harvested. Small plastic
vials containing a bit of gas and some visible light emitting phosphor have been used
for many years as low power lights. These are often sold as key chain lights, lights on
wristwatches and sighting lights on some weapons. The half-life of tritium is about 12
years, so these devices would glow for a long time. My idea is to use a special
infrared light emitting phosphor inside a plastic vial containing the tritium gas.
The infrared light would be a better match to the response curve of a typical silicon
solar cell than visible light. A long skinny plastic assembly would house the gas,
phosphor and the solar cells as shown in the sketch below. A higher gas pressure
inside the assembly would increase the available power. The solar cells would be
wired in series to produce a suitable voltage. The current generated by the solar
cells would be fed to a super capacitor, so a higher peak current would be available for
low duty cycle devices such as an RF transmitter. I estimate that twenty microamps
of current could be produce using this method with a voltage of perhaps 5v. That
would yield a power of about 100 microwatts. That may not sound like much but
considering it would continue cracking out that power for 12 years it adds up to 38,000
joules (watt-seconds) of energy. |