Design considerations and estimated onvehicle performance for a compressioncouple based thermoelectric generator.

摘要

Approximately 55% percent of the energy produced from conventional vehicle resources is lost in the form of heat. An efficient waste heat recovery process will undoubtedly lead to improved fuel efficiency, reduced greenhouse gas emissions and increased profit. Thermoelectric generators (TEGs) are one of the most viable waste heat recovery approaches that are being widely studied among energy-intensive industries which focus on the ways to convert waste heat energy to electrical energy. With the rising cost of fuel and increasing demand for clean energy, solid-state thermoelectric (TE) devices are good candidates to reduce fuel consumption and CO2 emissions in an automobile. Although they are reliable energy converters, there are several barriers that have limited their implementation into wide market acceptance for automotive applications. These barriers include: the unsuitability of conventional thermoelectric materials for the automotive waste heat recovery temperature range; the rarity and toxicity of some otherwise suitable materials; and the limited ability to mass-manufacture thermoelectric devices from certain materials. However, skutterudite is one class of material that has demonstrated significant promise in the transportation waste heat recovery temperature domain. These materials have little toxicity, relatively abundant, and have been studied and developed by NASA-JPL and others for the past 20 years. The converted electrical energy can be used to recharge batteries, run auxiliary electrical accessories, support heating system, and etc. However, durability and reliability of the thermoelectric generators are the most significant concerns in the product development process. Cracking of the skutterudite materials at hot-side interface is found to be a major failure mechanism of thermoelectric generators under thermal cyclic loading. Cracking affects not only the structural integrity but also the energy conversion and overall performance of the system. In this project, cracking of thermoelectric material as observed in performance testing is analyzed using numerical simulations and analytic experiments. With the help of finite element analysis, the detailed distribution of stress, strain, and temperature are obtained for each design. Finite element based simulations show the tensile stresses as the main reason causing radial and circumferential cracks in the skutterudite. For thermoelectric generator design, loading conditions, closed-form analytical solutions of stress/strain distributions are derived and scenarios with minimum tensile stresses are sought. All these approaches yield a minimum stress/strain necessary to produce any cracks. Finally, based on FE and computational fluid dynamic (CFD) analysis, strategies in tensile stress reduction and failure prevention are proposed followed by the reasons to change the thermoelectric couple design for having a reliable thermoelectric generator. Using a modified compression couple technology, a 15-watt thermoelectric generator prototype was designed, built and tested. Experimental results of the TEG are presented. This prototype was analyzed using 1-D engine simulation and computational fluid dynamics (CFD), and the resulting analysis is presented. In a model configuration utilizing eight of these 15-watt TEGs, each having a 4% conversion efficiency, an estimated 136 watts of electricity could be produced at an operating point of 2000 RPM and 3 bar engine load in a 4.7L V6 gasoline engine.