Exploration and Analysis of CO2 + Hydrocarbons Mixtures as Working Fluids for Trans-critical ORC

Abstract

ORCs (Organic Rankine Cycles) are of great significance for energy conservation and CO2 emission reductions owing to their usage in converting low grade waste heat to electrical power. The working fluid properties are the key to getting good cycle performance. The advantages of CO2 and hydrocarbon working fluids for use in ORCs are investigated here for six binary mixtures: CO2+propane/n-butane/iso-butane/n-pentane/iso-pentane/neo-pentane. A transcritical ORC simulation program was used to predict the cycle performance for various hydrocarbon mass fraction working fluids for a turbine inlet temperature of 453.15 K and inlet pressure of 1.3pc. The predicted cycle performance is used to divide the six working fluids into three categories relative to the thermal efficiency of an ORC using pure CO2. The first working fluid type (CO2+propane) has increasing efficiencies with increasing hydrocarbon mass fraction while the second type have maximum thermal efficiencies at hydrocarbon mass fractions of 0.3, and the third type has maximums at a mass fraction of 0.2. In addition, the thermodynamics relative efficiency, ξ, is highest for CO2+propane and then decreases for CO2+iso-butane, CO2+ n-butane, CO2+neo-pentane, CO2+iso-pentane, and CO2+n-pentane with the lowest. Then, the cycle efficiencies of the three working fluid types were analyzed from the perspective of the hydrocarbon molecular structure. The results show that longer main carbon chains and branched carbon chains have larger pc and Tc, with this critical property leading to higher efficiencies at low hydrocarbon mass fractions. Lower p′c and T′c lead to higher efficiencies at high hydrocarbon concentrations. Thus, there is a trade-off between the cycle efficiency and the hydrocarbon mass fraction. Finally, the three best working fluid mixtures, 0.3CO2/0.7propane, 0.7CO2/0.3neo-pentane and 0.8CO2/0.2pentane, were further analyzed according to their cycle characteristics. The 0.3CO2/0.7propane mixture working fluid had the best cycle efficiency for these conditions.