There is an Eco friendly device called Hydro Powered Calculator, this water powered calculator has three hole on above of the calculator screen. These holes is used for pairing with three tube that used as the calculator batteries.
How it works? What you have to do is just fill up the tube with water that consists of two electrodes made of special alloys, afterward you plug them into the holes, which, when immersed in water the device will produce electricity through a chemical reaction and your water powered calculator now are ready to used.
As you know water will evaporates when the environment temperature is higher than the water temperature, so this water powered battery is not make exception, so you need to check the battery around two to three month after your last time filled it. You can buy one at perpetualkid for only 10.99