GM to Research Shape Memory Alloy for Heat Recovery Systems

Even though GM’s position on the market is very unstable now after the bankruptcy announcement earlier this year, the U.S. Department of Energy’s ARPA-E granted the R&D department a financing of about $2.7 million for an interesting project. GM’s R&D has to build a prototype of a thermo-mechanical waste heat recovery system using a Shape Memory Alloy (SMA) heat engine to generate electricity from the heat in exhaust. Noticeable is the fact that GM was between the 37 other candidates that received the ARPA-E award.

The R&D Team in Warren, Michigan, has calculated that the system will be able to get a 15% fuel economy by using as little as 15-20 °C difference between hot and cold in the system.

GM will do the thermo-mechanical heat recovery system working with HRL Laboratories (co-owned by GM and Boeing), Dynalloy Inc., a Tustin, CA manufacturer of shape memory alloys (Flexinol) made to be used as actuators, and the GM/University of Michigan Smart Materials Collaborative Research Lab to develop new low-hysteresis SMA materials and the prototype system.

If a stretched SMA wire is heated, it shrinks back to its pre-stretched length. While it cools down, it becomes less stiff and can revert to the original shape. The behavior of the SMA could be used to enable a heat engine which will convert heat energy into mechanical or electrical energy. In the beginning of 1970s there was a huge interest in the potential of the SMA heat engines. But they were too large and too inefficient to have a chance on the market.

The SMA material have been used for a long time as actuators, even in other type of applications such a latches. But the SMA heat engine for waste heat recovery has been brought in discussion for less than a year said Jan Aase, Director of the Vehicle Development Research Laboratory at GM R&D.

The problem with the SMA is that it tends to exhibit hysteresis, which is not at all good for this application. It can work with actuators but not on the thermo-mechanical heat recovery system. The development team will have a hard job to make a material for the particular temperature range of the application which could also have a low hysteresis. From the Thermodynamic point of view the SMA thermo-mechanical waste heat recovery system is not so efficient as the thermoelectric generator the GM is developing. “Per unit mass, we believe it will be very effective. And in a transportation application, the mass efficiency is very important” said Aase.

The project is still at its beginning stages as the cooling mechanism is not yet defined. There were some rumors about an air cooling system but as liquid (engine coolant) has better heat transfer there is the chance the designers will go for this option: “The cooling is the tricky part of the heat cycle. The material trick is to get the low hysteresis material, and the packaging and design is what we [the collaborative team] will work on together. We assume there will be durability and other issues that come up. We have to go over rollers [in our concept]..bending stresses tend to be rather high. Up to now, all the applications I know of have been with straight pieces of wire. And of course there is also the designing of the heat exchanger portion of it and the mechanical packaging” said Jan Aase.

If the design will be a success, it will translate in the possibility to charge the battery of a hybrid car, while in conventional engines it could mean the replacement of the alternator without any load on the engine: ” his award is significant for the gains in energy efficiency it could bring, and because it signifies how GM is doing business though collaboration and partnership. The days are gone when we would do this kind of ground breaking work on our own. We need to continue to find ways to combine our deep technical knowledge with others who can help take our ideas from concept to commercialization” proudly noted Jan Aase.