General Motors (GM) is using shape memory alloys that require as little as a 10°C temperature difference to convert low-grade waste heat into mechanical energy. When a stretched wire made of shape memory alloy is heated, it shrinks back to its pre-stretched length. When the wire cools back down, it becomes more pliable and can revert to its original stretched shape. This expansion and contraction can be used directly as mechanical energy output or used to drive an electric generator. Shape memory alloy heat engines have been around for decades, but the few devices that engineers have built were too complex, required fluid baths, and had insufficient cycle life for practical use. GM is working to create a prototype that is practical for commercial applications and capable of operating with either air- or fluid-based heat sources. GM's shape memory alloy based heat engine is also designed for use in a variety of non-vehicle applications. For example, it can be used to harvest non-vehicle heat sources, such as domestic and industrial waste heat and natural geothermal heat, and in HVAC systems and generators.
If successful, GM's shape memory alloy heat engine technology would increase the fuel efficiency of U.S. vehicles up to 10%—reducing the consumption of up to 380 million barrels of oil every year.
This project would help increase the energy efficiency of power generation systems that produce waste heat, such as engines in vehicles, generators, and power plants—significantly reducing the amount of oil the U.S. needs to import each year.
If this waste-heat recovery system was applied to all of the estimated 130 million cars in the U.S., it could eliminate over 200 million metric tons of carbon dioxide emissions a year.
GM has the ability to quickly move forward with commercial integration of this technology. GM's waste-heat recovery system could also be applied to non-vehicle sources of waste heat, expanding its impact to other industries and products.