Current Status and Future Applications of Supercritical Pressures in Power Engineering

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It is well known that the electrical-power generation is the key factor for advances in any other industries, agriculture and level of living. In general, electrical energy can be produced by: 1) non-renewable sources such as coal, natural gas, oil, and nuclear; and 2) renewable sources such as hydro, wind, solar, biomass, geothermal and marine. However, the main sources for electrical-energy production are: 1) thermal - primary coal and secondary natural gas; 2) nuclear and 3) hydro. The rest of the sources might have visible impact just in some countries. Therefore, thermal and nuclear electrical-energy production as the major source is considered in the paper. From thermodynamics it is well known that higher thermal efficiencies correspond to higher temperatures and pressures. Therefore, modern SuperCritical (SC)-pressure coal-fired power plants have thermal efficiencies within 43–50% and even slightly above. Steam-generator outlet temperatures or steam-turbine inlet temperatures have reached a level of about 625°C (and even higher) at pressures of 25–30 (35–38) MPa. This is the largest application of SC pressures in industry. In spite of advances in coal-fired power-plants they are still considered as not environmental friendly due to producing a lot of carbon-dioxide emissions as a result of combustion process plus ash, slag and even acid rains. The most efficient modern thermal-power plants with thermal efficiencies within a range of 50–60%, are so-called, combined-cycle power plants, which use natural gas as a fuel. Natural gas is considered as a clean fossil fuel compared to coal and oil, but still due to combustion process emits a lot of carbon dioxide when it used for electrical generation. Therefore, a new reliable and environmental friendly source for the electrical-energy generation should be considered. Nuclear power is also a non-renewable source as the fossil fuels, but nuclear resources can be used for significantly longer time than some fossil fuels plus nuclear power does not emit carbon dioxide into atmosphere. Currently, this source of energy is considered as the most viable one for electrical generation for the next 50–100 years. Current, i.e., Generation II and III, Nuclear Power Plants (NPPs) consist of water-cooled reactors NPPs with the thermal efficiency of 30–35% (vast majority of reactors); subcritical carbon-dioxide-cooled reactors NPPs with the thermal efficiency up to 42% and liquid-sodium-cooled reactor NPP with the thermal efficiency of 40%. Therefore, the current fleet of NPPs, especially, water-cooled NPPs, are not very competitive compared to modern thermal power plants. Therefore, next generation or Generation-IV reactors with new parameters (NPPs with the thermal efficiency of 43–50% and even higher for all types of reactors) are currently under development worldwide. Generation-IV nuclear-reactor concept such as SuperCritical Water-cooled Reactor (SCWR) is intended to operate with direct or in-direct SC-“steam” Rankine cycle. Lead-cooled Fast Reactor (LFR) can be connected to SC-“steam” Rankine cycle or SC CO2 Brayton cycle through heat exchangers. In general, other Generation IV reactor concepts can be connected to either one or another cycle through heat exchangers. Therefore, this paper discusses various aspects of application of SC fluids in power engineering.