No, there is not way to naturally, without heat,
straighten your hair.
You can do roller sets, however, to give yourself loose curls:
MahoganyCurls' Roller Set How-to | Curly Nikki | Natural Hair Styles and Curly Hair Care
And wrapping your hair after taking out the rollers will give you a straighter look, but if you're looking for pin straight hair, as far as I know, it's impossible to achieve naturally.