Performance of N 2 O 4 Gas Cycles for Solar Power Applications

The use of N₂O₄ as the working fluid in gas turbine power cycles is recognized as a potential instrument for improving cycle efficiency at moderate top temperatures while maintaining the technical advantages connected with the waste heat rejection at a comparatively high average temperature. Solar central receiver power stations, whose economic effectiveness is very sensitive to cycle efficiency and which must often reject their waste heat into the atmosphere, could usefully adopt this fluid. The thermodynamic reasons which explain the peculiar behaviour of N₂O₄ as the Brayton cycle working fluid are discussed. With respect to inert gas cycles, N₂O₄ permits, for a given efficiency, a reduction in turbine inlet temperature by 200-250°C. At a given turbine inlet temperature, the dissociating character of N₂O₄ allows overall efficiencies similar to those of steam cycles (at least for moderate plant capacities and provided N₂O₄ and steam cycles reject their waste heat at comparable temperatures). The relatively long relaxation time of the second step of the N₂O₄ dissociation can represent a problem mainly for the regenerator. A cycle is presented where regeneration at a pressure higher than the compressor inlet pressure can alleviate this problem.