Micro turbine technology is a rapid learning front. Many of these very tiny turbines can be sandwiched together and can put out alot of power per size. Some are no larger than a dime. Because of this small size, they have the potential to replace batteries. The most common types are used in model aircraft to provide alot of thrust in a small lightweight package. These are usually a single turbine design but the cost is high on a turbine. The fuel efficiency was an issue at one time but now with advancements have become more efficient than piston engines! This is why they look so promising as a small portable battery charging devices. Materials other than metals have been used to minimize weight.