Evaluation of Rankine cycles with mixed component working fluids

The thesis evaluates the performance of Rankine cycles with mixed component hydrocarbon working fluids, or hydrocarbon mixtures. The objective is to compare mixtures with pure fluids on the basis of the same total heat exchanger (HX) area. This is achieved through further development and application of a three-step cycle optimization model developed in the project work, which calculates the maximum work output for a pre-defined value of total HX area. The model simultaneously calculates the optimum distribution of HX area between the condenser and the heat recovery heat exchanger (HRHE). A literature review is performed that studies heat transfer and pressure drop of working fluid mixtures through horizontal smooth tubes to evaluate and implement improved correlations. The literature review demonstrates that several methods are available for predicting heat transfer coefficients (HTCs), and that a method by Bell and Ghaly (1973) is most common for condensation, and a method by Thome (1996) is most common for evaporation. The cycle optimization model is further developed though implementation of new heat transfer correlations better suited for hydrocarbons. A more comprehensive estimation of overall HTC is made, and the option of internal heat exchanger (IHX) is included. A more detailed working fluid comparison is made through the study of optimum heat exchanger designs, including the distribution of condenser and HRHE area for different values of pre-defined total HX area. The specific working fluid affects HX design in terms of pinch points and distributions between condenser and HRHE area. Moreover, HX pressure loss is determined by working fluid overall HTCs and operating pressure, and the number of tubes and tube diameter is most affected by pressure levels. Four cases are defined that represent present and future applications of Rankine cycle. Case 1 and 2 consider a heat source at 100℃, with no lower limit on heat source outlet temperature. In case 1, the heat sink outlet temperature fixed and in case 2 it is allowed to vary. In case 1 and 2, butane, ethane and ethane (0.6/0.4) are studied. Case 3 and 4 consider a heat source cooled from 200℃ to 80℃. Case 4 differs from case 3 in that the optimization tool is given the choice to include an IHX, and does so if this contributes to an increased work output. In case 3 and 4, butane and butane-propane (0.6/0.4) are studied. For all cases, work output is maximized for different values of pre-defined total