Whether or not cars use alternative fuel engines or regular gasoline ones, their overall manufacture isn't really very green at all. But that may only be until Toyota's latest idea becomes a reality, because the car giant is suggesting an adjustment to the material they make cars from--the company is talking about going from metal to reconstituted seaweed.
Yup, you read that right: seaweed, specifically a kelp-based bioplastic. The vehicle is a variant of the company's 1/X hybrid concept design--a car that's already very green indeed with a super-light body (incorporating carbon fiber and plastic) and a tiny engine that mixes 500cc gasoline power with lithium-ion batteries for electric-only power. As Toyota project manager Tetsuya Kaida notes, the existing 1/X uses "lightweight carbon-fiber reinforced plastic throughout the body and frame for its superior collision safety" but that product still uses oil-derived plastic.
Bioplastics are an emerging technology that converts biomass into an oil-plastic alternative--typically crops like corn starch, or vegetable oil are used with "plasticizing" chemicals to create a durable material. As alternatives to "traditional" plastics, they have the advantages of not using up a precious non-renewable resource and, depending on the particular chemistry involved, they can be biodegradeable so they don't consume waste-dump space or leach unfriendly chemicals into the environment at the end of their life. Seaweed-based plastic is an alternative, and brings the benefit of not consuming farm resources that may be better suited for food production.
Toyota's car is, of course, a concept: But it certainly points toward a potentially green future for cars a decade hence that will have a vanishingly small eco-footprint compared to current ones. Interestingly, the plant-based car has an unusual historical precedent: East Germany's Trabant car. Designed and built during the communist era, the tiny vehicle made use of a steel frame and body parts fashioned from "duroplast." This material was a reinforced plastic where the reinforcing fibers were recycled cotton, wool, and sometimes even paper. A slightly greener alternative than metal, but the material couldn't be recycled further, and released toxic fumes when burnt. A kelp car almost sounds crazier, but it's eco-credentials mean you shouldn't take the idea with a pinch of salt.