In an age of diminishing fuel resources and increasing gasoline costs, auto manufacturers are doing everything they can to make cars more energy efficient. But there's one obvious place where energy is simply being thrown away while a car is running: in the heat coming out of the tailpipe.
The First Law of Thermodynamics tells us that energy can neither be created nor destroyed, but that doesn't mean a certain amount of it can't become simply useless. Generally, useless energy leaves a system in the form of heat, and in a car much of that wasted heat energy can be found in the exhaust. But what if we could convert some of that heat energy back into a useful form — like, say, electricity? General Motors has developed a prototype device that does precisely this using shape memory alloys (SMAs). An SMA is an alloy that can be made to have two different shapes depending on temperature, a so-called home state shape when it's hot and a second state when it's cool. Move a piece of shape memory alloy back and forth between hot and cold environments and it will change back and forth between these two shapes. Scientists have realized for quite a few years that SMAs extruded into wires could be a way of producing useable energy from heat, but GM's system is the first to show genuine promise as a way of generating electricity in a car.
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To understand how GM's device works, imagine a system with three rotating pulleys arranged at the corners of a triangle. Connecting the pulleys is a continuous band made out of shape memory alloy wires. If one corner of this pulley arrangement is placed near a car's hot exhaust pipe and the other two corners in a cooler, better-ventilated area, the band will contract into its shorter home state as it nears the pulley in the hot corner. This contraction will pull the rest of the band after it, causing the pulleys to rotate and moving more of the wire toward the heated pulley. The rotating pulleys will then pull the heated SMA wire back toward the cool region where it will expand again in a continuing cycle. The circular motion produced by the pulleys can be used to drive a generator that produces electricity, which in turn can run the car's radio, air conditioner, electric outlet — whatever needs a relatively low level source of electric power.
According to GM's researchers, it should take about ten years to move this device from the prototype stage to practical application, but there are a lot of problems that still stand in the way. Imbuing the SMA wire with its home-state shape is a time-consuming process — at present it takes about three months — and metal fatigue makes SMAs susceptible to breaking after they've been used for a while. GM's existing prototype only generates about two watts of electricity, but the research team responsible for it believes this can be scaled up for practical applications. Of course, the applications for electricity generated by shape metal alloys go way beyond automobiles, but SMAs probably won't come into general use as a means of recycling heat energy for several years to come.
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