Thermoelectric generator systems require high-performance hot-side and cold-side heat exchangers to provide the temperature differential needed to transfer thermal energy while withstanding temperatures up to 650 °C. Because the hot-side heat exchangers must have a high heat flux, they are often made of metals such as stainless steel or Inconel alloys.
Although these materials can operate at high temperatures, resist corrosion, and are chemically stable, they also have several drawbacks: (1) their lower thermal conductivity negatively affects their thermal performance, (2) their higher thermal expansion leads to stresses that compromise system structural integrity, and (3) their high mass/volume reduces the power density of generator systems into which they are integrated. As a result, they are difficult to integrate into viable energy recovery systems. They also make the systems unreliable, non-durable, and susceptible to failures caused by thermal-structural expansion.
Researchers at JPL have developed, built, and tested an innovative heat exchanger that offers reduced thermal expansion, increased structural strength, low pressure drop, and improved thermal performance while lowering the weight associated with typical heat exchangers. Unlike typical metal heat exchangers that suffer from high thermal expansion and high density, JPL’s innovation offers several improved properties. Its lightweight, high-heat-flux design offers a low coefficient of thermal expansion (CTE) — and therefore low expansion characteristics — and reduces the pressure drop during heat transfer. The design can handle high-temperature gases (up to 650 °C).
JPL researchers chose to replace the metal in traditional heat exchangers with graphite, which offers an improved conductivity-to-density ratio in thermal applications as well as a low coefficient of thermal expansion. In addition, they used a mini-channel design to further increase thermal performance. Combining more advanced materials with the innovative thermal design has yielded significant improvements in performance.
This technology has undergone successful preliminary testing. Developed for an aircraft exhaust energy recovery application, the system meets the requirements of high-temperature, high-specific-power thermoelectric generators and other energy recovery systems for industrial, automotive, military, and space applications.