Why is “enough power to charge your phone” your line of practicality? Phones contain an enormous amount of power hungry electronics, bright screens, speakers, multiple GHZ CPUs and radios for transmitting across different spectrum. With 40uW, you’d have to strip most of that away, but you could do something like: power up, take a temperature/CO2/RH/etc reading, transmit to a base station, sleep and repeat every 5 minutes. Sensor networks like these are extremely practical, especially in an industrial setting. Being able to accumulate some power, eg. with an integrated capacitor could also increase your max power draw for when your device is doing work.

It'd be great for spy devices though