Several types of thermoelectric generators (TEG) exist today, differing in their materials and range of operation temperatures. TEGs based on zinc antimonide and tin-doped manganese silicide (Zn4Sb3/Mg2SiSn) promise high efficiency and low cost in the temperature range relevant for trucks and other heavy automotive applications. Such TEGs can supplement and eventually replace the on board generator powered by the motor, in this way increasing overall fuel efficiency.
A TEG is made of two different types of semiconductors cut into legs and arranged alternately in a sandwich as illustrated in the figure 1. Such a sandwich is typically referred to as a thermoelectric module (TEM). In order to use the exhaust waste heat efficiently on a truck, a number of TEMs are mounted in a heat exchanger.
The TEMs mounted in the heat exchanger constitutes the thermoelectric generator, the TEG. This is further integrated into the truck design in connection with the Exhaust Gas Recycle (EGR) cooler, which is mandatory equipment in all new trucks.
TEG-modules are at present mechanically inflexible, despite heavy research in making TEGs of flexible materials. This means that TEGs cannot be fixed to movable parts. However, the EGR cooler is immovable. Also the efficiency of TEGs grows with the temperature differences between heated exhaust gasses and the cool air whistling by a TEG when a vehicle moves. The tricky part is finding space for TEGs in an actual motor design, but that is an aim of the project.
A full scale test in an operating truck with data monitoring is very costly and therefore VOLVO has designed test bench setups to bring down the expenses and also gain the benefit of repetition in the test design.
A conventional TEG module typically delivers an efficiency of 5-8% (power out/heat in), and a typical engine has an efficiency of about 30%, heavy diesel engines a bit more; 40% of the energy are lost as heat with the exhaust gas. The objective of “TEG for Energy Efficiency in Heavy Vehicles” is to integrate the TEGs with the exhaust gas manifolding and harvest some of this wasted energy.
If the TEG could generate power corresponding to 5-8% of the full amount of energy lost, the efficiency improvement would be 2-3%. However this will not be achieved due to several factors: waste heat is lost during start up, the output of the TEG has to be power conditioned, and the operating temperature window of the TEG does not have a perfect match with the exhaust gas temperature profile.
This is why the initial target of the present project is set at a system efficiency of 1%. However, work is ongoing to improve power conditioning, thermal control, materials performance and temperature tolerance in order to in time lift TEG module efficiency and give an improvement in the overall system efficiency of 2.5-4%.
However, even a system efficiency improvement of 1% will constitute a significant commercial advantage in an increasingly competitive trucking industry.
(For more information, contact Paul Egginton, pne@tegnology.dk or Kaspar Kirstein Nielsen, kaki@dtu.dk)