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Why are liquid-metal MHD systems limited to lower temperatures than plasma MHD systems?

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magnetohydrodynamic power generator Comparison of the operating principles of (A) a turbogenerator and (B) an MHD generator.

Liquid-metal magnetohydrodynamic (MHD) systems are limited to lower operating temperatures—approximately 1,250 K (about 975 °C, or 1,800 °F)—primarily due to the inherent physical properties of the liquid metals used as the conducting medium.1

In contrast, plasma MHD systems utilize ionized gases that can operate at significantly higher temperatures, often exceeding 1,800 K, to achieve the necessary electrical conductivity for efficient power generation. While liquid metals are attractive for their high electrical conductivity, they cannot function directly as a thermodynamic working fluid in the same manner as plasma; they must be combined with a driving gas or vapor, or accelerated by a pump. Consequently, the performance of liquid-metal systems is constrained by the temperature limits of the metals themselves, whereas plasma-based systems are designed to leverage the high-temperature capabilities of ionized gases to maximize thermodynamic efficiency.231