Prospects of Mixtures as Working Fluids in Real-Gas Brayton Cycles
This paper discusses the thermodynamic characteristics of the closed Brayton cycles in which the compression is placed near the critical point of the working fluid. Under these conditions, the specific volumes of the fluid during the compression are a fraction of the corresponding values under ideal gas conditions, and the cycle performances improve significantly, mainly at moderate top temperatures. As the heat is discharged at about the critical temperature, the choice of the correct working fluid is strictly correlated with the environmental temperature or with the temperature of potential heat users. To resort to mixtures greatly extend the choice of the right working fluid, allowing a continuous variation of the critical temperature. These cycles have a high power density, and the use of ordinary turbomachinery is accompanied by high capacities (tens of megawatts). In the low power range, microturbines or reciprocating engines are required. One important constraint on the choice of the right working fluid is its thermochemical stability that restricts the operative temperatures. Among the organic compounds, the maximum safe temperatures are limited to about 400 °C and, forecasting high temperature applications, it could be interesting to explore the potentiality of the inorganic compounds as secondary fluids in binary mixtures.
CO2 mixtures as innovative working fluid in power cycles applied to solar plants. Techno-economic assessment
Abstract This paper discusses the adoption of CO2 mixtures for improving the thermal-to-power efficiency conversion in solar tower plants and reducing the Levelized Cost of Electricity. Two different fluids are considered for blending the CO2: N2O4 and TiCl4. The main advantage of the innovative mixtures relies in a higher critical temperature with respect to pure CO2, which allows condensing cycles even at relatively high ambient temperatures typical of solar plants locations. Thermodynamic results show that the innovative cycles can achieve conversion efficiencies as high as 43% and 50% at 550 °C and 700 °C maximum temperature respectively, outperforming the reference CO2 cycle by 2 points percent. In addition, the simpler lay-out and the liquid compression reduce the power block capital costs below 700 $/kW. Detailed solar plant annual simulation is performed to assess the overall solar to electricity efficiency which can be around 21% for the innovative fluid, corresponding to 10% increase with respect to state-of-the-art solar plant. The higher performance and lower costs lead to a Levelized Cost of Electricity reduction of 10% with respect to conventional steam cycle power block.
Conceptual design of an HTGR system for a total energy application.
Thesis. 1975. Nucl.E.--Massachusetts Institute of Technology. Dept. of Nuclear Engineering.