By passing very-hot ionized combustion gas through a strong magnetic field a magneto hydrodynamic (MHD) generator can convert heat to electric power, without any rotating or moving parts. This makes it possible to reduce mechanical losses and operate at elevated temperatures using a ―topping cycle to increase the overall cycle thermal efficiency above what is possible for more conventional Brayton and Rankine cycles—thereby effectively increasing the idealised Carnot efficiency.
For decades Russia has devoted considerable resources to develop a light compact propulsion as well as power system for future space ships. Over the years Russia has developed a number of different types of MHD generators. Some were intended for ground use and thus were not limited by weight. In the eighties the Kurchatov Institute in collaboration with Energia corporation developed and successfully launched a superconducting magnet for a sub orbital test flight to prove that it was possible to shield plasma with magnetic field and in this way maintain radio communication when spacecraft has being going trough the dense layers of the atmosphere
An actual working ground based non-nuclear 10 MW MHD generator using a superconducting magnet
The results of calculations of the characteristics and development of a scheme and technical make-up of an open-cycle space power facility based on a high-temperature nuclear reactor for a nuclear rocket motor and a 20 MW Faraday MHD generator are presented. A heterogeneous channel-vessel IVG-1 reactor, which heated hydrogen to 3100 K, with the pressure at the exit from the reactor core up to 5 MPa, burn rate 5 kg/sec, and thermal power up to 220 MW is examined. The main parameters of the MHD generator are determined: Cs seed fraction 20%, stopping pressure at the entrance 2 MPa, electric conductivity ≈ 30 S/m, Mach number ≈ 0.7, magnetic induction 6 T, electric power 20 MW, specific energy extraction ∼4 MJ/kg. The construction of the scheme of a MHD facility with zero-moment exhaust of the working body and its main characteristics are presented.
Case for High Efficiency Ground Based MHD generators
MHD generators are now practical for fossil fuels, but have been overtaken by other, less expensive technologies, such as combined cycles in which a gas turbine’s or molten carbonate fuel cell’s exhaust heats steam for steam turbine. The unique value of MHD is that it permits an older single-cycle fossil-fuel power plant to be upgraded to high efficiency.
State-of-the-art combined-cycle power plants are based on the Brayton cycle and have only recently reached 60% (LHV) efficiency. This was an increment of 2%-point achieved following one decade of development work that cost the major companies (GE, ABB and MHI) around $2 to $3 billion each. After 40-years of commercial development the combined Brayton with Rankine cycles appear to be asymptotically approaching an efficiency ceiling of ~64%.
When fully integrated with post-combustion CO2 capture these cycles would be penalised with an estimated 8%-point while capturing ~90% of the CO2 (and emitting NOx). The development cost for such integration has evidently an additional $billion price tag that would at best result in a cycle efficiency of ~56%.
Given similar state-of-the-art technology for oxy-turbines the oxyfuel cycle has a targeted efficiency of ~60%. In this preliminary Study we suggest that inclusion of Oxy-MHD could further improve this efficiency by a factor of 1.15, inferring a significant commercial advantage for Oxy-MHD compared with competing zero emission cycles.
The underlying principle of MHD power generation is therefore simple. Furthermore, if the conductor is an electrically conducting gas, it will expand and the MHD system constitutes a heat engine involving expansion similar to that of a gas turbine.
According to Kesseler and Hals (1992), the first generation commercial MHD plants would need to be sized between 250 to 500 MWe power output and would be expected to attain efficiency of 40 to 42% by using oxygen-enriched air with mod-erate (1200 °F) pre-heat temperature for the oxidant. To achieve 55 to 60% efficiency there would need to be significant technology development regarding super-conducting magnets, materials and plasma fluid dynamics.
The recognized technical issues that remained for coal-fired MHD technology and that needed to be resolved were identified as being; High-temperature heat exchangers to pre-heat the combustion air to over 2,500 °F (1,370 °C).
Cost-effective seed regeneration and recycling process.
Durable (e.g. 8,000 hours) high-temperature electrodes for the MHD channel.
Removal of at least 50 to 70% of the slag from the combustor in order to maintain stable power generation.
Optimised size of plant and reduced capex with improved plant integration.
The zero emission oxygen fueled MHD concept effectively addresses several of these issues