You were right about the tea light powered lamp being expensive. Still super cool.
The technology you were describing where they harness the river water is also interesting. It is very similar to normal heat pump technology although they are creating electricity instead of heat I guess.
My brother occasionally installs heat pumps in more expensive houses to make heating using electricity more efficient. The heat pumps are basically air conditioners that work in reverse and pump heat in rather then out. You can generate almost 3x the heat using a heat pump as you could just running a normal heater.