Breaking the thermo-mechanical coupling of thermoelectric materials : determining the viability of a thermoelectric generator

Abstract

Thermoelectric power generators (TEGs) convert a temperature difference into electricity. This temperature difference can be created from waste heat. Since up to 50% [1] of US industrial energy input is lost as waste heat, an economical means of recovering waste heat and converting it into useful electricity could represent significant energy savings. Coupled with our integrative system design which involves creating application specific thermoelectric arrays, this technology can also help enable low power generation for off-grid needs in the developing world. Although conversion efficiencies as high as 20.9% [2] (heat to electrical energy) have been predicted from simulations of TEGs systems, in practice the efficiencies are typically only a few percent. Moreover, conventional systems often require expensive components to manage heat flow through the system.

As a result of the low efficiency and high system cost, electricity generated by thermoelectric energy harvesting from waste heat is currently not competitive with conventional electricity generation on a dollars-per-watt basis. This realization has led researchers to not only focus on increasing TEG device efficiency limits but to devise cheaper manufacturing processes and methods. A system design constraint that has not been fully investigated is the coupling of thermal and mechanical properties in thermoelectric materials. The extent to which this coupling affects the performance of the TEGs will be studied. This thesis develops an approach for decoupling the thermal and mechanical properties and tests it through a variety of simulations. We propose a mechanically compliant attachment strategy which could be integrated in various waste heat recovery applications.

The strategy involves breaking the thermal and mechanical bond formed by the brittle thermoelectric elements and its substrate. Copper wire, which is more pliable, is then used to connect the thermoelectric element to the substrate. A system analysis was performed for waste heat recovery from a vehicles exhaust pipe. We found that utilizing the proposed strategy should not only lead to increased mechanical compliance but can also lead to cost savings on a dollars-per-watt basis. We found that 84% power retention could be obtained when up to 16x less material is used under most apparent conditions¹.