There is also a new technology under reseach called Reverse Electro Dialysis.
Basically they can draw power from the chemical reaction that occurs when saltwater meets freshwater.
http://www.redstack.nl/RS-Pres01/RS-pres_bestanden/frame.htmhttp://www.onderzoekinformatie.nl/nl/oi/nod/onderzoek/OND1309967/
