In this Kickstarter campain Tryst Energy, spinoff from TWTG, promised they could power LoRa and BLE applications from indoor light or even light from underneath a desk using few square cm solar panel. Now my circuit is similar. I am going to calculate how much it can harvests indoors.
In the solar cell datasheet is a chart that shows indoor light is about 10 W/m2, that is about 1/100th of outdoor light (500-1000W/m2)
It also shows that solar cell open circuit voltage at 10 W/m2 is 0.458V per cell. Since I have 2 in series it means that the minimum voltage required for cold start of the AEM10941 harvesting IC (0.38V) is easily exceeded. Now I need to calculate the amount of power harvested with my cells. Since 1000W/m2 is same as 100mW/cm2 and my solar cells are 2.2 x 0.7 = 1.54 cm2 each, I have about 3 cm2 in total. So at 1000W/m2 my two solar cells can generate 300mW. And at 10W/m2 (indoors) 3mW. The AEM10941 harvesting IC requires minimum 3uW input power for cold start so that is easily exceeded. So how much current is actually going into the battery?
Well, solar cell voltage is about 70% of the open circuit voltage (2*0.458=0.92) so 0.64V. At that voltage the boost efficiency is about 75% according to the datasheet.
1/100th * 44mA * 75% boost efficiency * 95% charge efficiency * 0.64V/4.12V = 50uA. The harvesting IC itself uses only 0.5uA so that is insignificant. So if we have 50uA for 10 hours a day then 0.5mAh is stored in the battery every day. So for an application that runs 24 hours a day the average current should be less than 20uA.
Can TSEM power a simple BLE application from indoor light?
Three years ago I made a BLE beacon with average 15uA current. These days they are much more efficient. So YES, it seems simple BLE applications can run from the TSEM.
Can TSEM power a simple LoRa application from indoor light?
Sending a LoRa message is about 30mA for 1 second. To bring average current down to 15uA you should send once every 2000s or roughly once per 30 mins, which is still a acceptable interval for a simple application. So YES, from indoor light TSEM can power simple LoRa applications.
All in all I need to conclude that based on my calculations Tryst Energy was right that in an office you can run a very simple BLE or LoRa application from a few square cm solar panel. However I doubt enough power can be harvested from underneath a desk, especially when the application requires a bit processing power.