Ejector-assisted liquid metal topping cycles

Abstract

Direct fired coal power stations exhibit a comparatively poor performance since they cannot be operated in a combined cycle. A liquid metal topping cycle could dramatically improve their overall conversion efficiency. The high temperature required poses demanding technical problems both in the primary heat exchanger and in the turbine. The present paper discusses the possibility of alleviating the turbine-related problems through the use of an ejector which transfers the energy produced in a metal vapour topping cycle to a helium-vapour mixture at much lower temperature and pressure. Ejector performance is analysed both from a mechanical and from a thermodynamic point of view. Three typical alkali metal fluids are examined: sodium, potassium and cesium. The efficiency of the energy transfer from vapour to helium significantly increases with the metal molecular weight and attains optimum values of around 90 per cent. Turbine inlet temperature reduction due to the ejector action is about 500 K. Important gains are also the reduction in turbine inlet pressure and expansion ratio. After the energy transfer to helium and the turbine expansion, metal vapour and inert gas are separated by vapour condensation at a decreasing temperature. Such cooling represents the main thermodynamic loss in the process. At a maximum temperature of 1500 K and a condensation temperature for the reference vapour cycle of 950 K, computed efficiencies of the ejector-assisted topping cycle are in the range 17–19 per cent, as against pure vapour cycle efficiencies in the range 24–26 per cent. The strong cooling of the mixture needed to separate the liquid metal implies a significant increase in heat consumption which can be mitigated by means of an internal heat regeneration, which improves cycle efficiency to 19–22 per cent. With reference to a standard steam bottoming cycle, and including electrical, mechanical and boiler losses, plant efficiencies of around 50 per cent seem achievable.