When people talk about cars being powered by electricity and hydrogen and other types of renewable energy, they forget one thing: cars, great, but what about planes? As much as we've been working on alternative energy to power cars- to whatever degree of success- we haven't even really started looking at what to do about powering planes with alternative fuel. Jet fuel needs to be denser with its energy output, so the problems there are different than with cars.
Well, until KLM, national airline of the Netherlands, stepped in, that is. They've been using a mixture of 75% jet fuel and 25% cooking oil imported from Louisiana to power some of their flights, though on a somewhat irregular basis starting in 2009. They'll be making their first regular usage, on their Thursday New York-Amsterdam flights. Cooking oil isn't ideal as alternative fuel, but it is energy-dense enough to where the planes are able to use it. So it's a good place to get the train of thought going. You will probably be paying to go green, though; cooking oil in the form seen here runs the airlines about three times what it costs to use jet fuel.
'But you can get cooking oil for free! The restaurants just want to be rid of it!' True for you, perhaps, but again, you're not flying an airplane. All you have to do to make cooking oil work in a diesel car is strain out the food residue. You have to refine the stuff to make it work in an airplane.
Like I said. Still a long way to go.