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.
