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.
20 y/o lifelong natural {4a-O ; spongy ; porous ; coarse ; dense ; ~MBL stretched (2 inches from WL)}

products {'poo: TJTTT shampoo | RO: TJTTT Condish | LI/style: TJTTT Condish, SM Manuka Conditioner, homemade cocoa butter cream | DC: condish + ayurvedic powder(s), SM Manuka Mask | PT: condish + NPF | oils: castor oil, grapeseed oil | other treatments: probiotics, tea rinses, AVG rinse/spritz, oil rinse | sealing method: LOCO}

CG-friendly Products List!