A recent post on MAHLE's development of a heat exchanger designed to increase the driving range of electric vehicles in cold weather drew the attention of Luc Traonvouez, a French design engineer and owner of Insula France. In 2013, he worked with RBL Plastiques on a similar project. He contacted me via LinkedIn and offered to share the unpublished details of his work.
Traonvouez’s demonstration unit uses erythritol, a phase change material with a melting point of 118° Celsius and a latent heat value of 340 joules per gram. As with the MAHLE system, the PCM is melted while the vehicle battery is charged, using electricity from the grid. The stored heat is released as the PCM solidifies during driving.
The prototype weighs 15.7 kilograms, of which 22 percent is erythritol. The PCM is stored in a rectangular aluminum container with large fins inside and out. The interior fins drive the heat out of the PCM to the aluminum container; the exterior fins transfer the heat to air. The finned assembly is placed inside a thermally insulated envelope. The insulation provides enough resistance to maintain the heat for several hours. An electric fan moves air over the fins and pushes it into the vehicle’s cabin on demand.
Erythritol is relatively cheap and shows little supercooling, according to Traonvouez. But he notes two major problems with the material:
“1. The volume change between solid and liquid states is roughly 10%, and when cooling happens the PCM crystalizes around the fins, with a void between the fins and in some areas a void between the PCM and the fins; this means that after a peak of power when releasing heat from the storage, there is a fast drop. As the need for heating is much higher when starting the cold car than in stabilized mode, this drop is less a problem as it would be if we had needed a constant output.
“2. This PCM is subject to fast degradation when over heated in air; so the best is to cap heating at 140°C and confine the PCM in an airproof volume from which oxygen has been removed. When using a PCM with 10% volume size, we cannot fill the macroencapsulation completely with liquid PCM, but rather leave a significant empty volume and put it under some vacuum before sealing.”
This chart shows the heat output of the demonstration unit over a 54-minute period:
Traonvouez concludes: “The demonstration unit must be improved, one way being for example to use another way than fins in the PCM to improve its global thermal conductivity. However, the result is interesting and can be industrialized once tested extensively and over a high number of extreme cycles.”