Power Density of Hydrogen Magnetohydrodynamic (H2MHD) Generators at Different Pressures, Seed Types, Seed Levels, and Oxidizers

Hydrogen and some of its derivatives (such as e-methanol, e-methane, and e-ammonia) are promising energy carriers that have the potential to replace conventional fuels, thereby eliminating their harmful environmental impacts. An innovative use of hydrogen as a zero-emission fuel is being a source of weakly-ionized plasma through seeding its combustion products with a small amount of an alkali metal vapor (cesium or potassium). This plasma can be used as the working fluid in supersonic open-cycle magnetohydrodynamic (OCMHD) power generators. In such OCMHD generators, direct-current (DC) electricity is generated directly without moving turbogenerators. In the current study, we quantitatively and qualitatively explore the levels of electric conductivity and the resultant volumetric electric output power density in a typical OCMHD supersonic channel, where thermal equilibrium plasma is accelerated at a Mach number of two (Mach 2) while being subject to a strong applied magnetic field (applied magnetic-field flux density) of five teslas (5 T), and with the assumption of an isothermal temperature of 2,300 K (2,026.85 °C). We varied the total pressure of the pre-ionization seeded gas mixture between 1/16 atm and 16 atm. We varied the seed level between 0.0625% and 16% (pre-ionization mole fraction). We varied the seed type between cesium and potassium. We varied the oxidizer type between air (oxygen-nitrogen mixture, 21%-79% by mole) and pure oxygen. Our results suggest that the ideal power density can reach exceptional levels beyond 1,000 MW/m3 (or 1 kW/cm3). The power density can be enhanced using any of the following techniques: (1) lower total pressures, (2) using cesium instead of potassium for seeding, and (3) using air instead of oxygen as an oxidizer (if the temperature is unchanged). A seed level between 1% and 4% (pre-ionization mole fraction) is recommended (much lower or much higher seed levels are harmful to the OCMHD performance). The seed level that maximizes the electric power is not necessarily the same seed level that maximizes the electric conductivity due to additional thermochemical changes caused by the additive seed.