The EnOcean protocol is a good example of this. Their sensors don't use batteries at all, instead opting to charge a capacitor from small PV panels or peltier g

The EnOcean protocol is a good example of this. Their sensors don't use batteries at all, instead opting to charge a capacitor from small PV panels or peltier generators. https://www.enocean.com/en/technology/energy-harvesting-wireless/ https://www.enocean.com/en/technology/energy-harvesting-wire... https://www.enocean.com/en/technology/energy-harvesting/ https://www.enocean.com/en/technology/energy-harvesting/ This is well suited to stuff like occupancy sensors in buildings. Time resolution isn't as small as an always-powered system could be, but if your use case for the data is "Turn off the lights if we haven't seen anyone move for 30 minutes" then it doesn't much matter if you transmit every 5 minutes instead of every 5 seconds.