New Paradigms in Defining Optimum Cost Conditions in Thermoelectric Energy Recovery Designs

Abstract

Cost is just as important as power density or efficiency for the adoption of waste heat recovery (WHR) thermoelectric generators (TEG). Prior work [1] has shown that the system design that minimizes cost (e.g., the G [$/W] value) can be close to the designs that maximize power, but these design regimes are not necessarily aligned with high system conversion efficiency or power density. Recent work [2–5] further explores the impact of heat exchanger conditions on the optimum TEG fill factors and cost scaling of a waste heat recovery thermoelectric generator with a detailed treatment of the hot side exhaust heat exchanger. The cost scaling analysis considers the overall TEG system costs, including TE material costs, manufacturing costs, and specifically heat exchanger costs; and the performance analysis accounts for heat lost to the environment and updated relationships between the optimum hot-side and cold-side conductances that maximize power output. The optimum fill factor, Fopt, to minimize TEG energy recovery system costs is strongly dependent on the heat leakage fraction, σ, the mass flow rate of the exhaust, the hot-side heat exchanger effectiveness, heat exchanger UAu, and interfacial heat flux [1]. A more rigorous analysis and conclusion for the optimum fill factor was recently introduced and characterized at the 14th European Conference on Thermoelectrics [5] in 2016. A new, more comprehensive relationship for Fopt has created an updated, more rigorous relationship for the optimum cost, Gopt, relationship for thermoelectric energy recovery systems. These new [Gopt, Fopt] relationships are explored and characterized to show the inherent design complexities. The heat exchanger costs can often dominate the TEG cost equation and it is critical to fully understand the tradeoff between heat exchanger performance, TEG manufacturing and material costs, optimum TEG fill factors, and TE performance to establish potentially optimum design points within the cost-performance design space. This work explores and characterizes new design tradeoffs and new cost minimization relationships within this cost-performance design space for a typical thermoelectric energy recovery system application where the interplay between optimum TEG fill factors, TE device design parameters, and heat exchanger design parameters can impact system footprint, volume, and mass in weight-sensitive applications. This work shows how heat exchanger UAu, interfacial heat flux, and Fopt govern cost minimization and create and control cost regimes within the overall cost-performance design domain. This shift of emphasis acknowledging the interdependence of optimum TE fill factors, heat exchanger performance, and interfacial heat flux has significant implications on thermoelectric WHR system designs, their performance, and their operation.