Detailed and simplified plasma models in combined-cycle magnetohydrodynamic power systems

Abstract

Magnetohydrodynamics (MHD) is a subject concerned with the dynamics of electrically conducting fluids (plasma) and can be applied in electric power generation. As a unique technology for producing direct-current electricity without moving parts, it can be utilized within a high-temperature topping power cycle to be combined with a traditional bottoming power cycle, forming a combined-cycle MHD system. This study presents governing equations for the electric field and current density field within a moving plasma subject to an applied magnetic field. The modeling equations are described at four descending levels of complexity. Starting with the first level of modeling, which is the most general case, where no assumptions are made regarding the electric field, plasma velocity field, applied magnetic field, or electrode geometry. In the second level of modeling, the magnetic field is treated as one-dimensional. In the third level of modeling, a specific Faraday-type magnetohydrodynamics plasma generator channel is considered, having two continuous electrodes acting as parallel constant-voltage terminals. In the fourth (and simplest) level of modeling, an additional approximation is made by setting the Hall parameter to zero and replacing all vector fields with scalar quantities. For that simplest model, a representative set of operation conditions (electric conductivity 20 S/m, temperature 2800 K, supersonic plasma gas speed 2000 m/s with Mach 2.134, and magnetic flux density 5 T) shows that the optimum idealized electric power that can be extracted from a unit volume of plasma is estimated as 500 MW/m3. This is a much larger volumetric power density than typical values encountered in reciprocating piston-type engines (0.2 MW/m3) or rotary gas turbine engines (0.5 MW/m3). Such an extremely high power density enables very compact power generation units.

Estimated electric conductivities of thermal plasma for air-fuel combustion and oxy-fuel combustion with potassium or cesium seeding

“…The Coulomb scattering of electrons (their interaction with positively-charged or negatively-charged particles, being ions and other electrons) is quantified into a respective collision frequency ν 1 , through a series of steps. First, the absolute temperature of electrons (thus, the absolute temperature of the equilibrium plasma) is expressed as an energy term in eV [139][140][141], which is referred to here as the "thermal energy", described in Equation (31) below…”

“…Direct power extraction (DPE) from the flowing high-speed plasma gas within a magnetohydrodynamic (MHD) channel [ [25] , [26] , [27] , [28] , [29] , [30] , [31] ] is a legacy concept that was explored for about three decades after the energy crisis in 1973–1974 [ [32] , [33] , [34] , [35] , [36] ], and even before that in the 1960s. These endeavors proved that MHD power generation is viable, but also is economically non-competitive and technologically challenging.…”