Temperature Controlled Exhaust Heat Thermoelectric Generation 2012-01-1214

The amount of energy wasted through the exhaust of an Internal Combustion Engine (ICE) vehicle is roughly the same as the mechanical power output of the engine. The high temperature of these gases (up to 1000°C) makes them intrinsically apt for energy recovery. The gains in efficiency for the vehicle could be relevant, even if a small percentage of this waste energy could be regenerated into electric power and used to charge the battery pack of a Hybrid or Extended Range Electric Vehicle, or prevent the actuation of a conventional vehicle's alternator.

This may be achieved by the use of thermodynamic cycles, such as Stirling engines or Organic Rankine Cycles (ORC). However, these systems are difficult to downsize to the power levels typical of light-vehicle exhaust systems and are usually bulky. The direct conversion of thermal energy into electricity, using Thermoelectric Generators (TEG) is very attractive in terms of minimal complexity. However, current commercial thermoelectric modules based on Seebeck effect are temperature-limited, so they are unable to be in direct contact with the exhaust gases. A way to downgrade the temperature levels without significantly reducing the regeneration potential is to interpose Heat Pipes (HP) between the exhaust gas and the Seebeck modules in a controlled way. This control of maximum permissible temperature at the modules is achieved by regulating the pressure of phase change of the service fluid of the HP. In this way the system will be failsafe against overheating and will be able to operate efficiently under both low and high thermal loads. Such is the case of the range extender unit being developed by the team, which has a low (15 kW) and a high (40 kW) power mode of operation.

Various designs concepts were evaluated by simulation, design and test. Although efficiencies were still moderate, it was possible to demonstrate the potential of this system for optimizing the output of commercially available temperature-limited TEGs.