Rankine Cycle Using Supercritical CO₂ And Heat Transfer Simulation

The improvement and optimization of Rankine cycle and cycle efficieny is one of the hotspots in pyrology field. And how to improve its circulation efficiency is increasingly becoming the most important part. This paper introduced a solar enegyed Rankine cycle using supercritical CO2. The cycle using the medium pump and solar collector to transform CO2 into supercritical state, consequently, this system pushed the power components to work while utilizing heat recovery system for recovering heat.Then assessed its cycle performance from both theoretical and practical aspects, analyzed each thermodynamic parameter that affected the circulation efficiency, and simulated an important component which influenced its cycle properties.Selected three representative mediums such as CO2, water and tetrafluoroethane, and then calculated their cycle parameters in the same conditions. By comparing their circulation rate, it was found that Rankine system working with the medium of supercritical CO2 is almost 2-5 times the efficiency of that working with water and tetrafluoroethane.In order to make a further study on the Rankine cycle system working with CO2, this paper analyzed its cycle performance from these aspects:the heating of collector, the pressure drop of turbine, the high-side pressure and so on. The result showed that the cycle performance could be greatly increased through improving the heating temperature. Therefore the best cycle performance can be gained at the noon when there had the largest solar radiation. The smaller pressure drop and higher-side pressure can lead to higher total circulation efficiency. But at the same time the corresponding net work outputted would decrease. And the pressure drop affected the net work significantly more than that high-side pressure did. In addition, the consumption of pump power can be reduced by the smaller pressure drop and the higher-side pressure.Built a Rankine cycle system working with the medium of supercritical CO2, tested the thermodynamic parameters that affected the circulation performance, and found that the theoretical result is in concordance with experimental result. In the condition of fulfilling the net work, the smaller pressure drop and higher-side pressure could lead to a higher total efficiency.This paper got the results of a kind of typical working conditions:the temperature is heated to 100℃and the pressure of the power components is from 10 Mpa drop to 6.5 MPa, the total cycle efficiency is 20.58%. Thus suppose that the surface area of the heat collector is 10 m2 it could be assessed that the oil consumption is saved by nearly 379.88 1 annually and the emission of CO2 can be reduced approximately 1276.73 kg, which indicates that this circulation has a quite broad prospect.The heat conducting of supercritical CO2 in solar energy absorber is an important part of the circulation as its heat transfer efficiency influences greatly the cycle performance. Making a numerical simulation on supercritical CO2 conducting in horizontal tube by CFD and comparing the result of simulation with related heat transfer correlation, this paper made a study on the mechanism how the heat transfer performance was influenced. It was finally found that to increase the mass flow of CO2, improve the pressure of CO2 and add solar radiation are all beneficial to heat conducting of supercritical CO2. And the main factor influencing the character of heat-transfer was attributed to the dramatically physical changing of supercritical CO2.